Proceedings of the 20th International Conference on Fluidized Bed Combustion

The circulating fluidized bed (CFB) coal-fired boiler has being rapidly developed in China since 1980s and becomes a key clean coal technology used in thermal and power generation. In this paper, the development history and development status of the CFB boiler in China are introduced. The development history of the CFB boiler in China is divided into four periods and the important features of each period are given. Some latest research activities and important results on CFB boilers, and the typical achievements and newest development of the CFB boiler in China are also introduced. In addition, a few challenges and development directions including the capacity scaling up, SO

2

removal and energy saving are discussed.

Modelling of biomass gasification in bubbling and circulating fluidised bed (FB) is reviewed. The focus is on comprehensive fluidisation models, where semi-empirical correlations are employed to simplify the fluid-dynamics of the FB. The conversion of single fuel particles, char and gas reaction kinetics are dealt with, outlining the key phenomena that should be included in gasification models. An assessment of published models is presented and the need of further investigation is identified.

The present work provides a survey on the potentials of co-combustion of biomass and biogenic wastes in large-scale coal-fired power plants. This allows an energetic utilization at a high level of efficiency which is not obtainable in small-scale dedicated biomass combustors. Co-firing at low percentages of the thermal power (typically below 5–10 %) avoids the characteristic operating problems of biomass combustion, i.e. ash sintering and fouling of heat transfer surfaces. Co-firing of biogenic wastes is already widely practiced in Germany, non-waste biomass like forest residues are for subsidy reasons combusted in small dedicated mono-combustion plants. A future increase of co-combustion may be associated with the upgrading of biogenic wastes with high water content to biofuels by drying. Such biofuels could substitute more expensive coal and save on CO

2

emission certificates. In the more distant future biomass co-combustion may help in the CO

2

scrubbing process by lowering the target level of CO

2

absorption efficiency.

Due to the advantages of fluidized bed combustors a wide range of different fuels is utilized. The fuels range from anthracite, medium and low rank coals to peat, wood residues, biomass waste, sewage sludge and other sludges to plastics and municipal solid waste. Because of this wide range of fuels pollutants such as heavy metals, particulates, alkali, NO, NO

2

, N

2

O, SO

2

, SO

3

and HCI may be formed during the fuel conversion process depending on the fuel and operating conditions.

These pollutants may lead to difficulties in operation of the fluidized bed combustor e.g. because of slagging and fouling the heat transfer may decrease or the efficiency of the SCR catalyst. High concentrations in the flue gas may lead to health problems and pollution of the environment.

This work investigates the conversion routes of the pollutants from the fuel to the final flue gas. It is discussing the different chemistry of the pollutants and their possible interactions inside the combustor. Primary measures to avoid pollutant formation and reduction paths are demonstrated. Areas for further research are recommended.

O

2

/CO

2

combustion technology is considered as one of the most promising method to mitigate the greenhouse effect, and the O

2

/CO

2

CFB combustion technology which combines O

2

/CO

2

combustion technology with circulating fluidized bed (CFB) combustion technology will extend both their advantages. The latest research findings on O

2

/CO

2

CFB combustion technology are reviewed, the combustion and pollutant emission characteristics are expatiated, its effects on the boiler design are analyzed, the techno-economic assessment are reported and the key issues are indicated in the paper. O

2

/CO

2

CFB combustion technology has no insolvable bottleneck in its development and due to its economic superiority; it is one of the most important clean coal technologies.

During recent years, once-through supercritical (OTSC) CFB technology has been developed, enabling the CFB technology to proceed to medium-scale (500 MWe) utility projects such as Łagisza Power Plant in Poland owned by Poludniowy Koncern Energetyczny SA. (PKE), with net efficiency nearly 44%. Łagisza power plant is currently under commissioning and has reached full load operation in March 2009. The initial operation shows very good performance and confirms, that the CFB process has no problems with the scaling up to this size. Also the once-through steam cycle utilizing Siemens’ vertical tube Benson technology has performed as predicted in the CFB process. Foster Wheeler has developed the CFB design further up to 800 MWe with net efficiency of ≥45%.

Foster Wheeler’s circulating fluidized bed (CFB) boiler technology meets today’s market demand for utility-size boilers with capability to fire broad range of fuel qualities from low-grade high-ash fuels, good quality bituminous and anthracite coals to various biomass and waste fuels. The high efficiency CFB boilers are designed for firm emission performance and high reliability.

The proven high efficiency circulating fluidized-bed (CFB) technology offers a good solution for CO

2

reduction both in repowering of coal fired power plants and in greenfield power plants. CFB technology with its excellent fuel flexibility offers the opportunity to further reduce CO

2

emissions by co-combusting coal with biomass. FW has also developed CFB gasification technology for biomass applications. An example of this technology is gasification of biomass with a pressurized gasifier to produce syngas, which can be used for biodiesel production to address the reduction of CO

2

emissions from vehicles.

Carbon capture and storage (CCS) offers the potential for major cuts in CO

2

emissions of fossil fuel-based power generation in the fairly short term, provided that it gains public acceptance, the required regulatory framework is created and emission trading mechanisms or other incentives can provide solid return on the major investments required. Oxy-fuel combustion is one of the identified main CCS technology options, and Foster Wheeler is developing its CFB combustion technology for oxy fuel combustion to provide a CCS-ready solution. Foster Wheeler is currently developing Flexi-Burn™ CFB concept, which enables the plant to be operated either with or without carbon capture. This paper presents the existing status of CFB technology in respect of boiler efficiency and fuel flexibility. The main advantages of the CFB technology for oxyfuel combustion and a development plan for the Flexi-Burn™ CFB will be presented.

The use of liquid fuels for fluidized bed (FB) combustion is gaining importance for co-firing, waste incineration, switch to renewable energy sources or during plant start-up.

The design of a suitable liquid injection system is a key requirement to ensure that all of heat is released within the bed of a bubbling fluidized combustor or the riser of a circulating system.

The scientific literature has revealed around 100 papers dealing with FB combustion of liquid fuels since the pioneering work in 1975. This paper conveys an effort to review the relevant aspects of FB combustion of liquid fuels, with the exclusion of mixtures of solid fuels in liquids, which were left out of this work. Following a logical path that goes from early investigations toward a more sound knowledge, it is organized in four sections: Fuels, Fuel Feeding, Combustion Results and Emissions. The Conclusions summarize the main aspects and draw prospective for future research and application of liquid fuel FB combustion.

We have conducted a DNS study of dilute turbulent particulate flow in a vertical plane channel, considering up to 8192 finite-size rigid particles with numerically resolved phase interfaces. The particle diameter corresponds to approximately 9 wall units and their terminal Reynolds number is set to 136. The fluid flow with bulk Reynolds number 2700 is directed upward, which maintains the particles suspended upon average. Two different density ratios were simulated, varying by a factor of 4.5. The corresponding Stokes numbers of the two particles were O(10) in the near-wall region and O(1) in the outer flow. We have observed the formation of large-scale elongated streak-like structures with streamwise dimensions of the order of 8 channel half-widths and cross-stream dimensions of the order of one half-width. At the same time, we have found no evidence of significant formation of particle clusters, which suggests that the large structures are due to an mtxinsic instability of the flow, triggered by the presence of the particles. It was found that the mean flow velocity profile tends towards a concave shape, and the turbulence intensity as well as the normal stress anisotropy are strongly increased. The effect of varying the Stokes number while keeping the buoyancy, particle size and volume fraction constant was relatively weak. More details about part of this work can be found in (

2008

).

Biomass has made great in-roads in its use in energy and chemical industries. Gasification is one of the major means for its conversion. For thermo-chemical conversion of biomass three major gas-solid contacting processes, fixed bed, entrained bed and fluidized bed are used. Various versions of fixed bed gasifier (up-draft, down-draft, and side-draft) proved successful but primarily in small capacity units while entrained bed reactors found favour in very large capacity units. Fluidized bed gasifier fills the important intermediate size range. A review of the current commercial use of fluidized bed gasifier shows that it is yet to take the centre stage in the gasification market. This paper examines the issues preventing wider scale use of fluidized bed gasifier and what is the current state of research in those issues.

This paper presents an overview of advancements of circulating fluidized bed (CFB) technology in Thermal Power Research Institute (TPRI),including technologies and configuration and progress of scaling up. For devoloping large CFB boiler, the CFB combustion test facilities have been established, the key technologies of large capacity CFB boiler have been research systematically, the 100MW ∼330MW CFB boiler have been developed and manufactured. The first domestically designed 100MW and 210MW CFB boiler have been put into commericial operation and have good operating performance. Domestic 330MW CFB boiler demonstration project also has been put into commericial operation,which is H type CFB boiler with Compact heat exchanger. This boiler is China’s largest CFB boiler. The technical plan of domestic 600MW supercritical CFB boiler are also briefly introduced.

This paper will describe the background and current status of the 300MWe CFB Demonstration Project located at the Baima Power Plant in Sichuan Province. This project was the first 300MWe class CFB in China and the first project built under the Transfer of Technology from Alstom. The plant entered commercial operation in early 2006. The fuel is a high-ash anthracite which has presented significant challenges in terms of higher-than-expected ash content and top size. While this fuel has been problematic for the adjacent suspension-fired boilers, performance in the CFB boiler has been excellent, with low carbon content in the ash, low turndown and low emissions. Key boiler performance parameters will be described along with a comparison of design and actual performance and the operational experience will be addressed. Finally, the paper will describe Alstom’s process for scaling the CFB technology from 300MWe to 600MWe, and our supercritical CFB design.

As a worldwide supplier of fluidized bed combustion plants the reference list of Austrian Energy & Environment comprises more than 80 fluidised bed boilers over a wide capacitiy range with different kinds of fuel such as coal, biomass, sludges, RDF or various industrial wastes.

Actually two types of CFB Boilers were supplied by AE&E: On the one hand the “classical”, conventional

Power Fluid

®

CFB with cooled furnace, cyclone, cross-over duct or vertical convection pass to be employed for combustion of conventional fuels such as hard coal, lignite, biomass and sludge et al. The chlorine content of these fuels is as a rule lower than 0.1% in dry matter.

On the other hand the especially developed CFB for the combustion of refuse derived fuel (RDF) with the special features of an adiabatic primary loop with controlled heat extraction in an external bed material cooler, a toroid shaped adiabatic post combustion chamber downstream the primary cyclone. This combustion part is followed by a well known boiler concept of a classical waste incineration consisting of two radiation passes and a horizontal convection pass with hanging superheater and evaporation bundles.

Live steam temperatures up to 470°C (450°C) with fuel chlorine contents of 1.2% (1.5%) in dry matter can be accepted with this concept.

Market development in the last years showed a limited access to a particular segment with these two CFB concepts. This market segment consists of co — combustion plants firing biomass and / or conventional fuels with addition of RDF or production residues — in the majority of cases in the pulp and paper industry. The chlorine content in these fuel mixtures is typical around 0.2% in dry matter, with often missing sulphur content and the required live steam temperatures are between 500°C and 520°C.

Out of this AE&E developed a “combined” CFB-concept with the brand name

Power Fluid

®

Hybrid.

The presentation will provide a detailed description of the special features of the Hybrid technology by means of the Maxau plant.

Concern about air emissions and the effect on global warming is one of the key factors for developing and implementing new advanced energy production solutions today. One state-of-the-art solution is circulating fluidized bed (CFB) combustion technology combined with a high efficiency once-through steam cycle. Due to this extremely high efficiency, the proven CFB technology offers a good solution for CO

2

reduction. Its excellent fuel flexibility further reduces CO

2

emissions by co-firing coal with biomass. Development work is under way to offer CFB technology up to 800MWe capacities with ultra-supercritical (USC) steam parameters. In 2009 a 460MWe once-through supercritical (OTSC) CFB boiler designed and constructed by Foster Wheeler will start up. However, scaling up the technology further to 600-800MWe with net efficiency of 45–50% is needed to meet the future requirements of utility operators. To support the move to these larger sizes, an 800MWe CFB boiler conceptual design study was conducted and is reported on herein. The use of USC conditions (∼11 00°F steam) was studied and then the changes, that would enable the unit to generate 1300°F steam, were identified. The study has shown that by using INTREX™ heat exchangers in a unique internal-external solids circulation arrangement, Foster Wheeler’s CFB boiler configuration can easily accommodate 1300°F steam and will not require a major increase in heat transfer surface areas.

The circulating fluidized bed (CFB) combustion technology is one of the approved clean combustion technologies, and the power supply efficiency can be improved combining with the supercritical technology. A 600MWe supercritical CFB boiler is introduced in this paper. This boiler is designed based on the success of 300 MWe CFB boilers, which has a single furnace with three cyclones without external heat exchangers. There are twin furnaces and twin air distributors in the boiler. The water walls of the twin furnace above dense bed combines to a common fence wall with some channels to balance the pressure of the two furnaces. The smooth tubes are adopted in membrane water wall with mixing header. Six cyclones are located beside the furnace as well as six loopseals and six external heat exchangers. The hydrodynamic characteristic of water wall is available with the modeling prediction. And the performance of the 600MWe supercritical CFB boiler is also investigated.

The first 100MW CFB boiler, designed by the Thermal Power Research Institute and manufactured by Harbin Boiler Company Limited, has been successfully running in Jiangxi Fenyi Power Plant since 2003. Local high ash content anthracite and lean coal that are very difficult to burn out are used in the 100 MW CFB boiler. The results of the 100MW CFB boiler shows that the CFB boiler can run in 30% MCR and startup with two under bed burners, and the boiler efficiency higher than 88% can be got after the combustion modification test. The CFB boiler can be operated with full load and reaches design parameters. The emissions of NO, N

2

O and CO are less than 7Omg/m

3

, 30mg/m

3

, and 125mg/m

3

, respectively, and SO

2

less than 400mg/m

3

after limestone injection. The bottom ash temperature from bed ash coolers is less than 120°C after its modification. Coal blockage at the coal storage silo is the main problem influencing the CFB boiler continuous operation. The running experiences for 5 years proved that the CFB boiler performance is successful, and the results were applied in 210 MW and 330 MW CFB Boiler design of Fenyi Power Plant.

The way which burns slurry and gangue to generate electricity and provide heat has been always desired. If mixture of slurry and gangue are burnt by conventional combustion technology, it is difficult to be satisfied., but for circulating fluidized bed(CFB) boiler, it is flexible for fuels and it is easy to desulfurize and DeNox in the furnace of the boiler. There are lots of advantages to burning the mixture of slurry and gangue in CFB boiler. This technology has been researched and practiced for many years, it is mature now and has been used widely, by now, 50MW, 135MW and 300MW CFB boiler which burn the mixture of slurry and gangue have already been operated in China. In the paper, slurry characteristic and conveying is described, the design and operation of boilers mentioned above will be also introduced in detail.

This article presents the results of studies on the operational performance of the first 235 MWe with hot cyclones and 260 MWe second-generation Compact circulating fluidised bed (CFB) boiler installed as part of the repowering of the Turow power plant in Poland with close to 1,500 MWe of new capacity based on CFB technology. The main characteristic distinguishing the second generation type of boiler is the use of an integrated solids separator rather than an external cyclone. The analysis of flue gas emissions showed that the standards relating to permissible concentrations of pollutants such as SO

2

, NO

x

and CO, have been met. SO

2

levels have ranged from 157 mg/m

n

3

to 400 mg/mm

n

3

, levels of NO

x

’ from 120 mg/mm

n

3

to 342 mg/mm

n

3

, and levels of CO from 7 to 45 mg/mm

n

3

. The second-generation CFB boilers at the Turow power plant represent a major advance in CFB technology at this size. Studies on actual emissions indicated that the present standard related to permissible concentration of gas emissions and dust in flue gases have been met. The paper addresses also some performance improvements of the CFB boilers. In the boilers with external cyclones, erosion problems emerged after a time of 50 thousand hours. The problems have been quite precisely diagnosed recently and will be resolved on the occasion of the nearest shutdowns for major repairs.

The paper provides an update on B&W Internal Recirculation (IR) CFB boiler operating experience, new commercial projects, and developments in boiler design and process. Availability data for two projects in the U.S. will be presented, as well as data collected from two additional projects by a B&W licensee in India. Two new commercial projects are currently going through commissioning. Yet another unit, Great River Energy (being erected), is described in detail. To achieve the required amount of in-furnace heat absorption in higher capacity CFB boilers, a new B&W development is its patented in-bed heat exchanger (IBHX). The IBHX allows control of the bed temperature in the furnace as well as steam temperature in the superheater and reheater surfaces, thus accommodating higher capacities.

The article presents the results of experimental studies conducted on a large-scale 235 MWe CFB (Circulating Fluidized Bed) boiler, in which the primary air distribution system was modified. The modification was connected with the change of internal geometry of primary air channels as well as internal space of plenum chamber. The obtained results have shown, that the optimization of primary air flow has a great influence on the intensity of the combustion process and the temperature distribution along the height of combustion chamber. As a result, the NO

x

emission has been reduced by up to ten percent and the temperature profile in the combustion chamber has been revealed to be more uniform.

Properly understanding and calculating the distributions of heat flux and heat transfer coefficient (α) in the furnace is important in designing a circulating fluidized bed (CFB) boiler, especially with supercritical parameters. Experimental study on the heat transfer in a commercial 300MWe CFB boiler was conducted. The α from the bed to the water wall was measured by the finite element method (FEM), at five different heights. The influence of suspension density and bed temperature on α was analyzed. It was found that the pressure difference between the inlet and exit of the three cyclones, and the chamber pressure of the corresponding loop seal were not equal. The results indicated the suspension solid density was non-uniform in the cross section at a certain height. Consequently, the distributions of heat flux and α in the horizontal plane in the furnace was non-uniform. The furnace can divided into three sections according to the arrangement of the platen superheaters hanging in the upper CFB furnace. In each section, the heat flux near the center showed increasing trend.

A new way for the circulating fluidized bed (CFB) boiler research is proposed by the supervisory information system (SIS) in power plant level. The heat transfer coefficient in CFB boiler furnace is calculated and analyzed by the SIS calculation analysis in a commercial CFB boiler, the way how to calculate the heat transfer coefficient in SIS is introduced, and the heat transfer coefficient is accurately received by calculating a large amount of data from database. The relation about the heat transfer coefficient to unit load, bed temperature, bed velocity, and suspension density is analyzed; the linear relation could be accepted for the commercial CFB design. A new calculating and simple way for the heat transfer coefficient of CFB boiler is proposed for CFB boiler design. Using this research result, the reheat spray water flux larger than the design value in lots of commercial CFB boilers is analyzed; the main reason is the designed heat transfer coefficient smaller than the actual value.

Water wall design is a key technology of supercritical pressure CFB boiler. On account of the low heat flux and its homogeneous distribution in furnace, internally ribbed tubes with low mass flux and smooth tubes with moderate mass flux can be applied in the water wall of supercritical CFB boilers. An experimental research on the flow and heat transfer characteristics of internally ribbed tubes and smooth tubes with different inner diameters was conducted in the range of sub-critical and supercritical pressure. The departure from nucleate boiling (DNB) and dry-out boundary and the empirical correlations of heat transfer are obtained from the experiments. The mathematical model for predicting the circuit mass flux and grid pressure of a flow network system is based on the mass and momentum conservation equations and an iterative method is used to solve the nonlinear equations. Harbin Boiler Works has developed a 600MW supercritical CFB boiler with external heat exchangers and 6 separators. The mass flux and metal temperature distribution of water wall in the CFB boiler are calculated and the results show that the water wall can operate safely at BMCR and 75%BMCR load.

Measurements of emission are carried out in a 1025t/h CFB boiler. The effect of some factors including coal properties, bed temperature, unit load, excess air on the emission of NO

x

are investigated. The measurement results show that the N concentration in the coal is dominant parameter to predict the NO

x

emission from a large-scale CFB boiler. NO

x

emission from the 1025t/h CFB boiler increases with cyclone temperature and upper pressure drop due to post combustion and external cycle.

To evaluate characteristic of the mercury emission and removal from a circulating fluidized bed (CFB) boiler, a representative 135 MW CFB utility boiler was selected to take the onsite measurement of mercury concentrations in feeding coal, bottom ash, fly ash and flue gas using the US EPA recommended Ontario Hydro Method (OHM). The results show that particulate mercury is of majority in flue gas of the CFB boiler. Mercury removal rate of the electrostatic precipitator (ESP) reaches 98%. Mercury emission concentration in stack is only 0.062μg/Nm

3

, and the mass proportion of mercury in bottom ash is less than 1%. It was found that the fly ashes were highly adsorptive to flue gas mercury because of its higher unburned carbon content. Adsorption effect is related to carbon pore structural properties of fly ash and temperature of flue gas. However mercury adsorption capacity by fly ash can not be improved any more when unburned carbon content in fly ash increases further.

Compared with a conventional pulverized coal fired boiler, the combustion efficiency of a CFB boiler is lower while the self-consumed service power is 1–2% higher. The solution of these problems is the key research topic for researchers and manufacturers of CFB boilers. Based on the State Specification Design Theory of CFB boilers, Tsinghua University proposed a novel CFB technology by reconstruction of the fluidization state in the furnace by adjusting the bed inventory and bed quality. Theoretical analyses show that there is an optimal bed pressure drop, around which the boiler operation can achieve the maximal combustion efficiency and with significant reduction of the wear of the heating surface and fan power consumption. The proposed novel process was implemented in a 75t/h CFB boiler. The results of field tests on this boiler validated the theoretical analyses.

Carbon-dioxide capture and storage (CCS) offers the potential for major reductions in carbon- dioxide emissions of fossil fuel-based power generation in the fairly short term, and oxyfuel combustion is one of the identified CCS technology options. Foster Wheeler (FW) is working on reduction of carbon-dioxide with its integrated Flexi-Burn™ CFB technology. The proven high efficiency circulating fluidized-bed (CFB) technology, when coupled with air separation units and carbon purification units, offers a solution for carbon dioxide reduction both in re-powering and in greenfield power plants. CFB technology has the advantages over pulverized coal technology of a more uniform furnace heat flux, increased fuel flexibility and offers the opportunity to further reduce carbon dioxide emissions by co-firing coal with bio-fuels. Development and design of an integrated Flexi-Bum™ CFB steam generator and balance of plant system was conducted for both air mode and oxyfuel mode. Through proper configuration and design, the same steam generator can be switched from air mode to oxyfuel mode without the need for unit shutdown for modifications. The Flexi-Burn™ CFB system incorporates features to maximize plant efficiency and power output when operating in the oxy-firing mode through firing more fuel in the same boiler.

Existing boiler design tools are being modified to incorporate the features of oxy-combustion, so that various design options can be evaluated. The 460 MWe supercritical CFB power plant (recently constructed by Foster Wheeler) has been used as the basis for an integrated Flexi-Burn™ CFB study.

The vertical circulating fluidized bed (CFB) boiler has been found wide application in power generation and tends to be enlarged in capacity. Because CFB is one of environment friendly and high efficiency combustion technologies, the CFB boiler has also been expected to be used in the industrial area, such as textile mill, region heating, brewery, seed drying and so on. However, the necessary height of furnace is hard to be implemented for CFB with especially small capacity. Thereby, a novel horizontal circulating fluidized bed boiler has been proposed and developed. The horizontal CFB is composed of primary combustion chamber, secondary combustion chamber, burnout chamber, cyclone, loop seal, heat recovery area. The primary combustion chamber is a riser like as that in vertical CFB, and the secondary combustion chamber is a downward passage that is a natural extension of the primary riser, which can reduce the overall height of the boiler. In some extent, the burnout chamber is also the extension of primary riser. The capacity of horizontal CFB is about 4.2–24.5MWth (6–35t/h) steam output or equivalent hot water supply. The hot water boiler of 7MWth and steam boilers of 4.2MWth (6t/h) and 10.5MWth (15t/h) are all designed and working well now. The three units of hot water horizontal CFB boiler were erected in the Neimenggu Autonomous Region, Huhehaote city for region heating. The three units of steam horizontal CFB has been installed in Yunnan, Jiang Xi and Guangdong provinces, respectively. The basic principle for horizontal CFB and experiences for designing and operating are presented in this paper. Some discussions are also given to demonstrate the promising future of horizontal CFB.

In this paper, the challenges and problems of the circulating fluidized bed (CFB) combustion technology are summarized and analyzed. In order to resolve the problems in CFB boiler application, design principals and method of CFB boiler with the low bed inventory is proposed by Taiyuan Boiler Group Co. Ltd., cooperated with Tsinghua University. And a full set of design code of new CFB boiler with low bed inventory has been developed, to design the product structure of new generation CFB boilers. The first CFB boiler product with the low inventory was placed and has been operated for two years in Datuhe Thermal Power Plant at Lishi, Shanxi province. The operation practice and measurement data show that the CFB boiler with the low bed inventory stands for the newest trend of the development of CFB boiler in the future. It is expected that more technical advantages will be shown when this technology is applied to the CFB boiler with larger capacity in the near future.

CFB boiler technology has been rapidly developed in China and China has the largest installed number and capacity of CFB boiler in the world. The number of 135MWe CFB boilers is over 150. The operational performance of 135MWe CFB boilers was summarized in the article. And the reliability, economy, environmental protection index are also discussed in the article. It not only provides information of the development status of CFB industry, but also provides some experience and guidance to improve the operation and to further develop CFB combustion technology.

This work considers some of the questions in burning high ash Brazilian coal-dolomite mixtures in a bench-scale circulating fluidized bed combustor (CFBC). Experimental tests were performed with the CE4500 coal from Santa Catarina State, in southern Brazil, with a Sauter mean diameter

d

p

=43 μm. The coal particles were mixed with dolomite particles of

d

p

= 111 μm and this fuel mixture was fed into the circulating fluidized reactor, previously loaded with quartz sand particles of

d

p

=353 μm. This inert material was previously heated by the combustion of liquefied petroleum gas up to the ignition temperature of the fuel mixture. The CFBC unit has a 100mm internal diameter riser, 4.0m high, as well as a 62.8mm internal diameter downcomer. The loop has a cyclone, a sampling valve to collect particles and a 62.8mm internal diameter L-valve to recirculate the particles in the loop. A screw feeder with a rotation control system was used to feed the fuel mixture to the reactor. The operational conditions were monitored by pressure taps and thermocouples installed along the loop. A data acquisition system showed the main operational conditions to control. Experimental tests performed put in evidence the problems found during bed operation, with special attention to the solids feed device, to the L-valve operation, to particle size, solids inventory, fluidized gas velocity, fuel mixture and recirculated solids feeding positions.

As a key auxiliary device of CFB boiler, the bottom ash cooler (BAC) has a direct influence on secure and economic operation of the boiler. The operating situation of domestic CFB power plant is complex and changeable with a bad coal-fired condition. The principle for designing BAC suitable for the bad coal-fired condition and high parameter CFB boilers was summarized in this paper. Meanwhile, a new-type mixed-flow fluidized bed bottom ash cooler was successfully designed on the basis of the comprehensive investigation on the existing BAC s merits and drawbacks. Using coarse/fine slag separation technology and micro-bubbling fluidization are the significant characteristics of this new BAC. This paper also puts great emphasis on its industrial test in a 460t/h CFB boiler. The results indicate that it achieves significant separation of the coarse/fine slag, an obvious cooling effect, no slag block and coking phenomenon, and continuous stable operation.

Figs 7

,

Tabs 4

and

Refs 11

.

In this paper, general layout, design, operational experience and performance of the first 300MWe circulating fluidized bed (CFB) boiler that developed by Dongfang Boiler Group Co., Ltd China, are introduced. The furnace was with large width-depth ratio. The problems occurred during in commissioning were analyzed and the corresponding modifications were presented. Cold-state experiment and operation experience showed that both fluidization quality and circulating flow rate meet the designated value in the frunace. The imbalance of circulating material flow caused by asymmetric layout of three cyclones was very limited. Heating surfaces were safe except wing wall superheater located in upper part of the furnace was overheated at low load. After commissioning, the boiler was correspondingly modified and its performance was excellent.

China has the largest number of 300MW circulating fluidized bed (CFB) boilers in the word within, built in a short period of several years. Now, there are 13 CFB units in operating and other about 50 CFB boilers under construction or will be under construction soon. Summarization and analysis of the operational performance of the 300MWe CFB units are conducted. The reliability, economy and emission of these unite are introduced. The problems in these units are summarized. Some advices for improvement and future research are given.

The circulating fluidized bed is playing more and more vital role in the electric power field. Cyclone separator as the heart of the circulating fluidized bed combustion boiler, the technology of fire-resistant anti-wear layer in cyclone separator is the guarantee for the long-life and working safety of CFB unit. Based on the comparison of insulation-cyclone and water-cooled cyclone, a novel technology of Anchor bricks use in the cyclone separator was put forward. The temperature of Anchor brick surface is just 80∼90°C, much lower than conventional technology. The problems caused by conventional Y-shaped hook welding were overcome, such as fire-resistant anti-wear layer burned, fire-resistant anti-wear layer rupture, the high temperature of separator’s surface, a large number of heat dissipation and etc. Simultaneously, three types of Anchor brick were designed for the different parts of cyclone. It was applied successfully in Thermal Power Plant in Shaoguan Iron and Steel Company. The life time and the operation cycle of the fire-resistant anti-wear layer of the cyclone were prolonged, and the number of boiler off was reduced greatly. Hence, the continuous operation time of boiler was extended. It played a key role in improving the overall economic efficiency of power plant. It is great for the utilization and development of CFB technology.

In recent years, CFB boilers (CFBB) have been widely used in the commercial power plants due to its environmental benefits, high combustion efficiency, wide coal flexibility, and some other advantages. At the same time, the abrasion problem, the greatest weakness of this kind of boiler, has been gradually exposed in its application process. The abrasion, particularly on key parts such as the heating surface of water-cooled wall, furnace corners, separator entrance, seriously restricts the long-period operation ability of the CFBB. This article discusses current development status for various abrasion resistant refractory materials used in a CFBB. Some comments are provided for developing new high-performance abrasion resistant refractory materials and rapid-repaired materials according to the abrasion principle and the abrasion on different parts, as well as the economical and environmental requirements for the material. The abrasion solution and operation period of CFBB can be better improved given realization.

Mercury emissions between a circulating fluidized bed (CFB) utility boiler and two pulverized coal (PC) boilers equipped with electrostatic precipitators (ESP) were in situ measured and compared. The standard Ontario Hydro Method (OHM) was used to sample the flue gas before and after the ESP. Various mercury speciations such as Hg

0

, Hg

2+

and Hg

p

in flue gas and total mercury in fly ashes were analyzed. The results showed that the mercury removal rate of the CFB boiler is nearly 100%; the mercury emission in stack is only 0.028 g/h. However, the mercury removal rates of the two PC boilers are 27.56% and 33.59% respectively, the mercury emissions in stack are 0.80 and 51.78 g/h respectively. It concluded that components of the ESP fly ashes especially their unburnt carbons have remarkable influence on mercury capture. Pore configurations of fine fly ash particles have non-ignored impacts on mercury emissions.

Combustion of Refuse derived fuel (RDF) is considered as a priority solution to energy recovery from municipal solid waste (MSW). The co-combustion characteristics of anthracite coals with RDF were determined in the commercial scale Tonghae CFB Power Plant. As the feeding ratio of the RDF to the anthracites increased to 5%, temperature and pressure were not changed in comparison with firing only anthracites. The amount of the required air was reduced due to high O

2

content in RDF relative to the anthracites. The emissions of NO

x

, SO

x

, HCl and Dioxin were also measured. According to higher mixing ratio of the RDF to the anthracites, SO

x

, NO

x

emissions slightly decreased and HCl emissions increased, because RDF has relatively smaller S, N and higher CI than the anthracites. Heavy metals of the fly ash and bottom ash and the dioxin emissions were far below Korean maximum permissible concentration level at incinerator. The results showed that it is of great use and technically possible to co-combustion of RDF with the anthracites by 5% in the form of fuel recovery and energy production in commercial scale CFB boiler.

Poultry farming generates large quantities of waste. Current disposal practice is to spread the poultry wastes onto farmland as fertilizer. However, as the factory farms for poultry grow both in numbers and size, the amount of poultry wastes generated has increased significandy in recent years. In consequence, excessive application of poultry wastes on farmland is resulting in more and more contaminants entering the surface water. One of the options being considered is the use of poultry waste as power plant fuel. Since poultry-derived fuel (PDF) is biomass, its co-firing will have the added advantage of reducing greenhouse gas emissions from power generation. To evaluate the combustion characteristics of co-firing PDF with coal, combustion tests of mixtures of coal and PDF were conducted in CanmetENERGY’s pilot-scale CFBC. The goal of the tests was to verify that PDF can be co-fired with coal and, more importantly, that emissions from the combustion process are not adversely affected by the presence of PDF in the fuel feed. The test results were very promising and support the view that co-firing in an existing coal-fired CFBC is an effective method of utilizing this potential fuel, both resolving a potential waste disposal problem and reducing the amount of CO

2

released by the boiler.

It is well known that during firing of oil shale fuel the amount of heat released during its combustion per kg of fuel is significantly affected by the endothermic and exothermic processes taking place in mineral matter. These thermal effects are calcite and dolomite decomposing, marcasite FeS

2

oxidising, CaO sulphation and formation of the new minerals. The given paper deals with the experimental study of the influence of these thermal effects of oil shale fuel having different heating value on total amount of heat released during combustion in calorimetric bomb, circulating fluidized bed (CFB) and pulverized-firing boiler (PFB). The large-scale (250 MW

th

) experiments were performed in the K11-1 CFB boiler of the Balti Power Plant. During experiments low heating value of a fuel varied within the range 8.5–11 MJ/kg. At the end some conclusions were drawn.

According to the investment programmes of Russian electricity generating companies increased attention is paid to reconstruction and building new coal TPP. The typical projects are 225, 330 and 600 MW blocks for combustion of different domestic coals. VTI had made technical and economical comparison of CFB and PC boilers for existing and perspective (European) standards of particles, NOx and SOx emissions, according to the data of the prehminary designs and investments in new power plants of 225 and 330 MW. As the basis for technical and economical evaluations was used comparison data of metal-capacity of PC and boilers, emissions-control systems and material-handling systems, with paying attention to the exact suggestions of the boiler producers. The results of the comparisons (capital costs and O&M costs) are discussed in the paper. The most perspective fuels for combustion in CFB boilers are: anthracite culm, coals of the Pechora area, lean coals of Kuznetsk, brown coals of near Moscow, brown coals of Urals and Far East, and also the wastes of coal preparations, peat, shells and biomass. A good composition could be made from Kuznetsk coals and coals of Pechora area. Brown coals are combusted very good in suitable conditions for firing biomass and peat. Also allowed co-combustion with the wastes of coal preparations for the low reactivity fuels such as anthracite culm and lean coals. The diversification of the fuel supply is an essential advantage of CFB boilers. The CFB boiler installations are rather new for the conditions of Russian Federation. For decreasing the technical risks, first installations should be supplied by engineering or license of leading foreign companies with rather big part of their participation. One of important tasks is development of the typical projects, which would allow decreasing not only capital costs, but also decrease time of project realization. The project of the new Block #9 330MW with OTU boiler of Novocherkassk TPP is the determining for the wide use of CFB technology in Russia. The block is designed for combustion of the Anthracite culm, Kuznetsk lean coals and coal slurry and should be established in 2011 year.

Fine powders are used in many applications and across many industries such as powdered paints and pigments, ceramics, petrochemicals, plastics, pharmaceuticals, and bulk and fine chemicals, to name a few. In addition, fine powders must often be handled as a waste by-product, such as ash generated in combustion and gasification processes. In order to correctly design a process and process equipment for application and handling of powders, especially fine powders, it is essential to understand how the powder would behave. Many characterization techniques are available for determining the flow properties of powders; however, care must be taken in selecting the most appropriate technique(s).

In this work, sample fine powders with a mean particle size between 22 and 31 urn were characterized using a variety of techniques that tested powders under different stress states, ranging from static to dynamic. In the powder coating process, for example, powders must exhibit good fluidization and resist agglomeration in order to produce a smooth paint coating. It was found that dynamic characterization techniques such as fluidized bed expansion were best suited for predicting the fluidization performance, while static characterization techniques such as cohesion were better for predicting agglomeration. It was also found that results from static and dynamic characterization techniques do not necessarily agree, where fine powders that showed good fluidization performance also displayed increased agglomeration, and vice versa. This suggests that flow properties are dependent upon the stress state and that no single technique is suitable for fully characterizing a powder. Extending this example to other industries, both static and dynamic characterization techniques must be employed to completely understand the flow properties of a powder and predict how it will behave under all process conditions.

Using the principle of hydrodynamic equivalence, the procedure of calculating a velocity of complete fluidization of different-density polydisperse granular materials has been developed with account for changes in their initial size distribution. It is shown that coarse particles in composition of polydisperse mixtures can be fluidized when the velocity

u

cf

is substantially smaller than their minimum fluidization velocity (

u

mf

)

max

.

Prior to operating a dual fluidized bed gasifier (DFBG) using a riser (0.078 m-I.D x 8.5 m-high) as a combustor and a bubbling bed as a gasifier (0.2 m-I.D. × 2.1 m-high), the hydrodynamic properties such as minimum flmdizing velocity and transport velocity of silica sand (270μm) as a bed material in a DFBG were determined at different temperature (25-800°C). The minimum fluidization velocity decreases from 0.065 m/s to 0.036m/s with increasing temperature. But the transport velocity, a boundary velocity between the turbulent and fast fluidization flow regimes, increases from 2.55 m/s at 25°C to 4.47 m/s at 600°C. The effect of particle size on the minimum fluidization and transport velocities was determined and correlations are proposed to predict both velocities at different temperature. The relationship between the operating variables and solid circulation rate was also determined. The obtained data can be utilized to determine the operating conditions in the hot DFBG system.

Experimental investigation on fluidization characteristics of biomass alone and sand-biomass mixture has been carried out in a 3D fluidized bed by means of visual observation and pressure fluctuation analysis. The biomass material employed is cotton stalk, and the sands used are of three different sizes. They together make up of three kinds of mixtures, in which the percentage of biomass particle in each mixture (

X

w0

are 1.79, 4.36, 8.36, 15.43 and 31.33 by weight. Additionally, the experiment is conducted over a wide range of gas velocity. It is found that regardless of gas velocities, the biomass alone can not archive good fluidization, whereas small additive of sand substantially improves the fluidization quality. The results also show that the fluidization regime is more sensitive to

X

w0

as compared with sand particle size. The transition velocity (

U

c

) from bubbling to turbulent fluidization regime decreases with

X

w0

for the same mixture, while increasing the size of sand component leads to a delay transition in different mixture.

The recycled solids fed to the circulating fluidized bed (CFB) riser on one wall have a limited lateral dispersion rate, which results in an uneven solids distribution in the bottom zone of the bed. Using a reflective optical fiber solids concentration measuring system and visual observation, the flow properties in the bottom zone of a fluidized bed was investigated. The cold model CFB riser has a square cross-section of 0.25m×0.25m and a height of 6.07m with quartz sand as bed material (

dp

=276μm). At the condition of

u

0

=4m/s,

Gs

=21kg/m

2

s, the back feeding particles can penetrate the gas-solid flow in the riser and get to the opposite wall. It makes a denser region near the front wall of the bed. When air-staging is adopted, solids holdup increases below the secondary air (SA) injection ports with the increase of the rate of the secondary air to the total air (SAR). The solids holdup increase also exists above the secondary air injection ports but the incremental amount decreases quickly along the riser. At SAR=0.3, a denser bottom zone limits the back feeding particles’ penetration and a relatively even solids distribution is formed in the bottom bed. A W-shaped solids concentration profile is formed just above the SA injection level. At SAR=0.5, uneven solids concentration profile exists in the bottom bed and no W-shaped solids concentration profile is formed for the deeper penetration of SA jets.

The mixing behaviors of the feeding particles in the bottom zone of a circulating fluidized bed with a rectangular cross-section of 0.3 m × 0.2 m and a height of 2 m were investigated by monitoring transient pressure drop and thermal tracing method. The effects of the feeding particle size, feeding port location (relative to dense bed surface), feeding gas velocity and fluidization gas velocity were examined. The bed collapse technique was also used to examine the lateral distribution of the feeding particles for four typical cases to give reference results. Results show that the mixing behaviors are greatly dependent on particle size. Under the condition that the feeding port is fully submerged in the dense bed, a hot spot occurs as the feeding gas velocity is no more than 0.24m/s. Increase in feeding gas velocity and fluidization gas velocity can both promote the mixing of the feeding particles with bed material.

The fragmentation of coal plays a significant role in combustion in fluidized-bed boilers because it accelerates combustion and influences the distribution of particle sizes in the bed. Fine char particles produced by comminution can be carried out and increase the heat loss due to incomplete carbon conversion. The thermal fragmentation depends on the initial structure of the coal and how this structure changes with an increase in temperature when the particle is dropped into the combustion chamber. Experinments with nine Polish coals were carried out to quantify the fragmentation of burning coal particles. A bench-scale bubbling fluidized bed combustor was used to determine the degree of fragmentation for spherical coal particles during devolatilization and combustion. The effects of bed temperature, particle size and coal properties on the extent of primary and secondary fragmentation have been determined. It has been found that the combination of low porosity and high volatile content is the main factor responsible for the observed extent of fragmentation.

Hydrodynamics plays a crucial role in defining the performance of gas-solid circulating fluidized beds (CFB). A two dimensional model was developed considering the hydrodynamic behavior of CFB gasifiers. In the modeling, the CFB riser was divided into two regions: a dense region at the bottom and a dilute region at the top of the riser. Radial distributions of bed voidage were taken into account in the upper zone by using (

1991

)’s correlation. For model validation purposes, a cold model CFB was employed, in which sawdust was transported with air as fluidizing agent. The column was 10m in height and 280 mm in diameter, and was equipped with pressure transducers to measure axial pressure profile and with a reflective optical fiber probe to measure local solids holdup. A satisfactory agreement between the model predictions and experimental data was found.

Maldistribution of gas-solid tow-phase flow field in circulating fluidized bed (CFB) can cause a series of problems, such as thermal deviation, wear of water walls, etc. In this study, a cold model CFB facility, which was scaled down from a commercial 300MWe CFB boiler with three cyclones placed in an array, was built up and a series of experiments were conducted the flow non-uniformity. The results showed that in CFB boiler with multiple cyclones, the distribution of bed material in the circulation loops is different and uncertain. The gas-solid two-phase flow in the furnace is unbiased, even the circulating rates in the circulation loops are different. The circulating rate in the middle loop is larger than that in the side loops. The difference is less than 10%.

Flow regime study was conducted in a 0.3 m diameter, 15.5 m height circulating fluidized bed (CFB) riser with an abrupt exit at the National Energy Technology Laboratory of the U. S. Department of Energy. Solids fraction and particle velocity data for both the high and low density suspension in the riser were analyzed to investigate how the gas-solids flow could reach a fully developed condition in the riser. A fully developed condition in the riser is defined by (

2001a

) as:

The radial solids distribution in the riser no longer changes with its axial location

. Results of the data analysis are discussed in the following sections.

Radial solid fraction and particle velocity data generated from the optical fiber probe (the Vector probe) for the 200 micron glass beads during the test series were used to examine the solids flow development in the riser. The test series was a statistical designed factorial experiment included four (4) operating set points and a duplicated center point (therefore a total of 6 operating set points). The riser gas velocity for this RPB test series is to be operated above the upper transport velocity. The operating conditions for the five experiments were riser gas velocity,

U

g

=5.6 m/s, 6.5 m/s, and 7.7 m/s and solid mass flux,

G

s

=87 kg/m

2

· s, 194 kg/m

2

, s, and 303 kg/m

2

· s. The conditions for the duplicated center point were

U

g

=6.5 m/s, and

G

s

=194 kg/m

2

· s. However, only the results obtained for the higher solids flux,

G

s

=303 kg/m

2

· s and riser gas velocity,

U

g

=5.6 and 7.6 m/s are presented in this paper.

In the present work, heat transfer measurements are reported in a 100mm square, 5.5 m tall, cold CFB. The test section is a 19 mm OD electrically heated heat transfer tube, 4.64 m tall (covering more than 80% of the CFB height), sandwiched between two equally tall dummy tubes of 19mm OD, thus simulating a water wall geometry, forming one wall of the CFB. Narrow cut sand particles of mean diameters 156, 256, and 362 micrometers, and a wide cut sample of mean diameter 265 micrometer were used as the bed material. The superficial gas velocity ranged from 4.2 to 8.2 m/s, and the solids recycle flux varied from 17 to 110 kg/m

2

s. Local heat transfer coefficient at the simulated water wall varies, as expected from a low value at the top of the riser to a high value at the bottom, with an interesting increasing and decreasing trend in between. The average heat transfer coefficients were compared with those available in open literature. Correlations for average heat transfer coefficient are presented, both in terms of an average suspension density and also in terms of important nondimensional numbers, namely, Froude number, relative solids flux and velocity ratio. Comparisons are also made with predictions of relevant heat transfer models. Based on the present fifty-five experimental data points, the following correlation was presented with a correlation coefficient of 0.862 and maximum error is ± 15 %.

Boilers running at supercritical pressure with large capacity have been widely used in power generation technology. How to keep the safety of heat transfer in the riser system is an important issue. However, it is difficult to study heat transfer for the vertical smooth tubes in a boiler at sub-critical or supercritical pressure since the thermodynamic properties of water is very complex and sensitive to temperature and pressure near the critical point. Hot-state experiment of heat transfer for the tubes of diameter used in a boiler is of great value for the design of membrane wall. In order to study the heat transfer of vertical smooth tubes running at low heat flux and low mass flux for sub-critical CFB boilers, a near-supercritical pressure water test bed was built in Tsinghua University. The designed pressure is 21MPa, the mass flux of water is 550kg/m

2

s, and the heat flux is 136kW/m. In this paper, the design and structure of the test bed is introduced. The experimental result is analyzed, more experimental work is needed in future research.

The nitrogen as drying medium and the fluidized bed were used to study the drying characteristics of two kinds of lignite. The results show that the drying process of the lignite could be divided into three stages: a pre-heating period, a constant heating period, and a quick-down heating period. For the same lignite sample, the drying rate of lignite samples increases with rising drying temperature or flow rate of drying medium. For the different lignite samples, the composition structure of lignite and the modes of occurrence of moisture in the lignite influence the drying characteristics of lignite, besides drying medium temperature and flow rate influence lignite drying. According to the heat transfer rule equation of wet-ball in the forced flow, the drying rate formula of lignite particle in the fluidized bed has been got based on the heat transfer theory and mass transfer theory, and the formula can well interpret the effect of the experimental parameters on the lignite drying.

A new type scaling-up scheme of CFB boiler that takes separator as center and furnaces are laid around was put forward in this paper. In the recycle system, a new type heat exchanger device with double outlets was designed for this disposal scheme. As we know, the external heat exchanger is very important for the CFB, which be able no only to adjust the steam temperature, but also to adjust the bed temperature. In this paper, through the adjustment of air speed in different room of the heat exchanger, the adjusting performance of the new type heat exchanger was analyzed. Moreover, the test of the pressure in the whole recycle system was analyzed. The pressure balance system of the circulating circuit with this new arrangement scheme was realized. Through this test research, the main conclusions were got as follows: The external heat exchanger, which has two recycled solid outlets, could run flexibly and stably and could successfully discharge the materials from the standpipe into either of the furnaces. This test device has a good pressure and material balance system.

Using the external heat exchanger in large-scale CFB boilers can control combustion and heat transfer separately, make the adjustments of bed temperature and steam temperature convenient. The state of gas-solid two phase flow in the external heat exchanger is bubbling fluidized bed, but differs from the regular one as there is a directional flow in it. Consequently, the temperature distribution changes along the flow direction. In order to study the heat transfer characteristics of the water cooled tubes in the bubbling fluidized bed and ensure the uniformity of heat transfer in the external heat exchanger, a physical model was set up according to the similarity principle and at the geometric ratio of 1∶28 to an external heat exchanger of a 300MW CFB boiler. The model was connected with an electrically heated CFB test-bed which provides the circulating particles. The influencing factors and the distribution rule of the particles’ heat transfer coefficient in the external heat exchanger were assessed by measuring the temperature changes of the water in the tubes and different parts of particles flow along the flow direction. At the end, an empirical correlation of particles’ heat transfer coefficient in external heat exchanger was given by modifying the Veedendery empirical correlation.

An innovative horizontal circulating divisional fluidized bed (HCDFB) with three fluidized-zones, one plug-zone, and a special solid material recirculating device (SMRD) located in the plug zone, was developed to generate bed materials horizontal circulating in the dense zone. The characteristics of mass transfer and heat transfer in a cold-state HCDFB model were carried out with helps of tracer technique and fast response heat transfer probes, respectively. The effects of fluidizing velocity, particle size of bed materials, initial static bed height, baffle’s height, SMRD injection flowrate, and probe orientation on mass and heat transfer were analyzed. Results indicate that the amount of particle mass transfer increase with increasing of fluidizing velocity, initial static bed height, and SMRD injection flowrate. The amount of particle transfer of the low baffle fluidized-zone is higher than that of the high baffle fluidized-zone at the same fluidizing air velocity. Heat transfer coefficients increase with increasing of fluidizing velocity, and reaches their maximum value at 1.5∼2.2 times of minimum fluidizing velocity. Heat transfer coefficients on the leeward side of the probe are larger than that on the windward side of the probe. Heat transfer coefficients decrease with increasing of tested bed height. It will be helpful to increase heat transfer coefficient of around 30% in a HCDFB by horizontal migration of bed materials.

Gas solid flow characteristics in cyclone’s inlet duct of a 300MW CFB boiler were studied in a cold circulating fluidized bed (CFB) experimental setup according to a 410t/h CFB boiler with a scale of 10∶1. Tracer particles were adopted in the experiment and their motion trajectories in the two kinds of cyclone’s inlet ducts were photographed by a high-speed camera. By analyzing the motion trajectories of tracer particles, acceleration performance of particle phases in the two inlet ducts was obtained. Results indicate that the acceleration performance of particles in the long inlet duct is better than that in the short inlet duct, but the pressure drop of the long inlet duct is higher. Meanwhile, under the same operating conditions, both the separation efficiency and the pressure drop of the cyclone are higher when the cyclone is connected with the long inlet duct.

Figs 11

,

Tabs 4

and refs 10.

A square cyclone separator with double inlets was developed for a new type Circulating Fluidized Bed (CFB) boiler arrangement scheme including two furnaces. Experiments on the performance and gas-solid flow recorded by a high-speed photography have been conducted in a cold test rig with a separator cross section 400mm×400mm. Experimental results indicated that with the inlet velocity of 22.4m/s and the inlet solids concentration of 4.9g/m

3

, the cut size is 15 μm, the critical size is 75μm, and the pressure drop coefficient is 1.7. The performance is also affected by the inlet velocity and solids concentration. The trajectory of particles shows that the particles swirl in the region near the wall and are easily separated. Especially, the instantaneous separation occurred at the corner is very significant for the improvement of the collection efficiency with the high inlet solids concentration for CFB boiler.

The prediction of propane autoignition in the freeboard of a fluidized bed is complicated by the presence of solids, intermediate products and non-homogeneities (solids, temperature and species gradients) that should be accounted for in a reaction model. However, the simultaneous characterization of these parameters during combustion is very challenging. An experimental study of propane combustion inside the freeboard (I.D.=0.2 m) of a fluidized bed of sand (U

g

=290 μm) was performed at a low superficial gas velocity (U

g

=0.24 m/s). Propane was injected inside the fluidized bed (T

Bed

=650°C) through a downward-facing sparger. Also, solids flux and species volume fractions were measured using a non-isokinetic sampling probe. The results showed an exponential decrease with height of the upward solids flux (G

SU

)−G

SU

was zero at 0.17 m above the bed surface, which was taken as the inflection point of the Gsu curve. G

SUo

measurements were significantly higher than the values given by the correlation of (

1982

). The bed surface (boundary condition) and freeboard were characterized by measuring pressure, solids flux, species volume fractions and temperature at several radial and axial positions. During the experiments, the fluidized bed achieved a pseudo steady-state operation that ensured that the measured temperature profile corresponded to the solids flux and species fractions. Partial propane combustion in the fluidized bed (71%) produced CO and cracking species that were transported in the freeboard. Complete combustion occurred within 0.15 m of the bed surface and the propane induction time in the freeboard (<0.25 s) was on the same order as the values given by three induction time correlations for homogeneous systems.

Coal is generally combusted or gasified directly to destroy completely the chemical structures, such as aromatic rings containing in volatile coals including bituminite and lignite. Coal topping refers to a process that extracts chemicals with aromatic rings from such volatile coals in advance of combustion or gasification and thereby takes advantage of the value of coal as a kind of chemical structure resource. CFB boiler is the coal utilization facility that can be easily retrofitted to implement coal topping. A critical issue for performing coal topping is the choice of the pyrolytic reactor that can be different types. The present study concerns fluidized bed reactor that has rarely been tested for use in coal topping. Two different types of coals, one being Xiaolongtan (XLT) lignite and the other Shanxi (SX) bituminous, were tested to clarify the yield and composition of pyrolysis liquid and gas under conditions simulating actual operations. The results showed that XLT lignite coals had the maximum tar yield in 823–873K and SX bituminite realized its highest tar yield in 873–923K. Overall, lignite produced lower tar yield than bituminous coal. The pyrolysis gas from lignite coals contained more CO and CO

2

and less CH

4

, H

2

and C

2

+C

3

(C

2

H

4

, C

2

H

6

, C

3

H

6

, C

3

H

8

) components comparing to that from bituminous coal. TG-FTIR analysis of tars demonstrated that for different coals there are different amounts of typical chemical species. Using coal ash of CFB boiler, instead of quartz sand, as the fluidized particles decreased the yields of both tar and gas for all the tested coals. Besides, pyrolysis in a reaction atmosphere simulating the pyrolysis gas (instead of N

2

) resulted also in higher production of pyrolysis liquid.

The use of biomass fuels for energy production through combustion has a growing application worldwide mainly for two reasons: first, the utilization of biomass for energy contributes to mitigate emission of green house gases; second, its use decreases the dependence of imported fossil fuels in Europe. The objective of this work was to study the combustion behaviour of two endogenous biomass species: cardoon (cynara cardunculus) and arundo (arundo donax), which were specially produced in energy crops plantations. Mixtures of cardoon and a forestry biomass specie (eucalyptus) were also studied to evaluate potential benefits from synergies between both biomass fuel types. The results showed that the utilization of cardoon, in pelletized form, and loose arundo as feedstock, did not give rise to any operational problems related with the feeding system. It was verified that the mono combustion of cardoon could pose problems at industrial scale in fluidised bed systems, considering the high levels of HCl and NO

x

emissions obtained and tendency to sinter the bed sand material. The addition of the forestry biomass to cardoon appeared to prevent the bed agglomeration problem. Furthermore, both the NO

x

and SO

2

emissions were found to decrease at the same time suggesting potential synergy of blending different types of biomass regarding pollutant emissions and in bed agglomeration problems.

Prediction of agglomeration, fouling, and corrosion tendency of fuels is essential to the design of any CFB boiler. During the years, tools have been successfully developed at Foster Wheeler to help with such predictions for the most commercial fuels. However, changes in fuel market and the ever-growing demand for co-combustion capabilities pose a continuous need for development. This paper presents results from recently upgraded models used at Foster Wheeler to predict agglomeration, fouling, and corrosion tendency of a variety of fuels and mixtures. The models, subject of this paper, are semi-empirical computer tools that combine the theoretical basics of agglomeration/fouling/corrosion phenomena with empirical correlations. Correlations are derived from Foster Wheeler’s experience in fluidized beds, including nearly 10,000 fuel samples and over 1,000 tests in about 150 CFB units. In these models, fuels are evaluated based on their classification, their chemical and physical properties by standard analyses (proximate, ultimate, fuel ash composition, etc.;.) alongside with Foster Wheeler own characterization methods. Mixtures are then evaluated taking into account the component fuels. This paper presents the predictive capabilities of the agglomeration/fouling/corrosion probability models for selected fuels and mixtures fired in full-scale. The selected fuels include coals and different types of biomass. The models are capable to predict the behavior of most fuels and mixtures, but also offer possibilities for further improvements.

Oil shale based power production has been the basement of Estonia’s energetical independency and economy for over 60 years. At the same time oil shale power plants emissions still give the biggest share of Estonian stationary source pollution, having significant impact to the environment. Thanks to the introduction of oil shale large scale CFB firing, reduction of the total environmental impact was achieved in last years.

Detailed information about emissions from CFB power units was collected during several research projects in last years. The paper reviews this new data and compares it with former information. Analysis and estimation of changes of charges of all main polluting components at CFB firing compared to former pulverized firing (PF) are presented. It concerns mass balance of trace metals in initial fuel and in formed ash, emissions of PCDDIF, PCB, PAH, PM 2,5/10, plus conventional air emissions as NO

x

, SO

2

, CO

2

, CO, HCl, TSP.

Eg.it was found out, that the relative share of very small (<2,5 μm) particulates is higher in case of oil shale CFB firing, what could be explained with very efficient milling effect of relatively soft minerals of oil shale at CFB furnace.

In relation of emission studies also mass distribution of CFB boiler ash between different separation points along the flue gas duct (ash mass balance) was determined and analyses of sampled ashes were provided.

This information is important from the point of view of better understanding of thermochemical processes and emission formation in CFB boiler, wider reuse of the ashes ofCFB unit.

The shortage of coal promotes the lignite utility in power plant because of the rapid economy development recently. However, lignite is high in moisture content as well as volatile content and low in calorific value. It is very difficult to burn in traditional pulverized coal fired boiler. Circulating fluidized bed (CFB) boiler is an alternative with low pollutant emission. Some CFB boilers are built and put into commercial operation in Northeast China and East Inner Mongolia where lignite is abundant. The operation experiences of these boilers are introduced in this paper. The effect of coal particle size on bottom ash ratio, combustion efficiency, thermal efficiency, pollution emission, and ash deposits in convective heating surface were investigated. It was found that for the lignite fired CFB boiler, the largest coal particle size should be 20 to 40mm to maintain bed material balance. But the bottom ash only shares less than 10% of the total ash. Due to high volatile content in the lignite, the combustion efficiency could achieve more than 99%. Meanwhile, NO

x

emission was relative low and satisfied national environment protection requirement. It is suggested that flue gas velocity in convective heating surface should be ranged in a certain scope to prevent ash deposit and erosion.

The adhesion of heavy metals as they flow to the cooling surface of FBC (fluidized bed combustor) has been investigated under laboratory conditions. Model wastes spiked with mixture of Pb, Zn and Cu were used. The vaporizing ratio of these metals was kept as constant by fixing the reaction gas composition under 1223 K. Then the condensation behavior of these heavy metals was investigated by varying the tail gas composition and operational parameters such as Cl/S ratio and quenching rate. The results indicate that Cl/S ratio remarkably influence the partition and chemistry of the heavy metals. Increasing H

2

O concentration in the tail gas makes heavy metals condense in the form of sulfate instead of chloride at relatively low temperature section (573 K). In addition, quenching rate of tail gas also plays an important role on the redistribution of heavy metals between sulfate and chloride. The unavailability of equilibrium calculation for the quenching mode and some differences between the experimental and calculation results imply the complexity of the adhesion process of heavy metals.

Fluidized bed combustion of biogenic fuels can be recognized as an attractive option for an ecologically sustainable use of biofuels in residential applications. Nevertheless, biomass combustion in fluidized bed reactors presents some drawbacks that are mainly related to mixing/segregation of fuel particles/volatile matter during devolatilization inside the bed and in the freeboard or to bed agglomeration. A prototype of a 30–50 kWth fluidized bed boiler for residential heating has been designed to burn either a gaseous combustible or a solid biomass fuel or both fuels at the same time. The prototype has been equipped with a gas burner located in the wind-box to optimize the start-up stage of the boiler and with a fluidized bed characterized by a conical geometry (“Gulf Stream” circulation) to improve the mixing of the fuel particles during both devolatilization and char burn-out. The operation of the combustor adopting wood pellets as fuel has been investigated to evaluate their use in residential combustion applications. Steady-state thermally stable regimes of operation have been recognized analyzing both boiler temperatures and gaseous emissions. The optimization of the steady-state operation of the boiler in terms of gaseous emissions has been achieved by varying the nominal thermal power and air excess. An

ad-hoc

experimental campaign has been carried out to analyze the dynamic performance of the prototype as a response to changes of the demanded thermal power. On the basis of the experimental data, an interpretation of the dynamic behavior of the fluidized bed boiler has been proposed.

Understanding the height effect on the gas-solid flow characteristics in a CFB riser is important as more and more large capacity CFB boilers are used and to be developed. In this study, a cold CFB test rig with a riser of 240mm in LD. and 38m and 54m in height was built. The influences of operating conditions, such as solid inventory and fluidizing gas velocity, on the axial voidage profile along the riser were assessed. When the gas velocity exceeds the transport velocity, the S-shaped profile of voidage in the riser was established. At the same time, the voidage in top-dilute section reached the saturation carrying capacity, and the solids circulation rate did not vary with the height of the riser nor the solids inventory. It was also found the critical solids inventory for the saturation carrying capacity increases as the riser height increases. When the height was changed from 38m to 54m, the critical solids inventory increased about 25% from about 40kg to about 50kg, and pressure drop in the furnace also increased about 25%.

This work presents experimental and numerical simulation results on secondary air jet penetration into a dense phase of a 2-D fluidized bed. Velocity measuring method and non-intrusive methods based on images were used in the experiments. Effects of secondary air nozzle size and angles, secondary air jet flow velocity and suspension density of the fluidized bed material on the air jet penetration were tested. The results show that with increasing of secondary air jet velocity, the jet range increases exponentially. Secondary air jet range decreases exponentially with increasing of average bed suspension density. The size of secondary air nozzle does not have significant impact on jet range. However, larger nozzle size may result in a lower velocity decay coefficient. It was also found the secondary air nozzle angle is not an independent parameter affecting jet range. The jet range may penetrate the most depth point into the fluidized bed with the angle of −30°. As a comparison, the numerical simulation was done with same parameters in the experiments. Comparison of experimental and numerical results shows good agreements.

In the literature, the values of lateral solids dispersion coefficient (

D

sr

) in dense fluidized beds determined by different authors are scattered, ranging from 0.0001 to 0.2m

2

/s. The present work is aimed to investigate the reasons for the divergence through CFD-DEM simulations. Results indicate that the particle trajectory shows multi-scale characteristics: macro-scale movement following the gross circulation of emulsion phase, and local-scale movement induced by bubbles closing to it. The effects of fluidizing velocity and bed width on

D

sr

are examined. It is deduced that

D

sr

embodies multi-scale mixing mechanisms, which makes

D

sr

difficult to predict or scale up.

By using a high-speed video camera and particle image velocimetry (PIV) technique, the local flow properties of the solid-gas two phases flow were studied in a plexiglass rectangular CFB cold model with the a riser of 1.5×0.864×4.9m

3

. Measurements were carried out with transparent spherical glass bead between 0.1–0.425mm as bed materials and cold air as flow medium. The experimental results showed that the secondary air has an important influence on the particle velocity distribution. Because of the secondary air penetrating effect, the particle lateral movement was acute. In the dilute region, the outlet and the comer effect induced the defluxion of the particles movement and the core-annular distribution was broken. The closer to the outlet, the stronger the lateral velocity is. The obstruct of hanging screen reduced the furnace outlet effects between the hanging screen and the front wall, where the particle movement in the area was controlled by the gas flow and the constrain of the wall. High particle concentration areas were formed in the junction between the screen and the front wall and in the comer between the left wall and the front wall.

In the CFB boiler technology, improving the steam parameters can lead to the improvement of power plant efficiency. However, during the process of large scale, there exist some key problems, such as uniformity of coal feeding, it is a major factor which showed great influence on the boiler efficiency, temperature distribution, etc. In the paper, based on the structure of commercial 600MW CFB boiler unit and similarity principle, the experiment-rig was set-up; using hot trace particle injection and thermocouple temperature collection system, the three-dimensional diffusing property of hot trace particle was analyzed by measuring the temperature distribution at different positions under different operating conditions of variant bed material height and fluidized air velocity.

Design and operation of boilers using biomass or waste present a number of challenges. It is also well known that the flue gas emissions are strongly dependent on the fuel. Consequently, it is a major challenge to be able to control and maintain all emissions and combustion behavior under their designated limits for all fuel combinations required. Lately, the constant substantial rise in the price of fossil fuels has resulted with RDF (refuse derived fuel) technology becoming more valuable for generating heat in various types of boilers. A small-scale bubbling fluidized bed (BFB) RDF-fired boiler with a steam capacity of 4 ton/hr was developed by ITRI. In this paper, heat release in the fluidized bed region was calculated and the performance testing for this demonstration boiler including the items of bed temperature distribution, flue gas emissions, and the ash characteristics is analyzed and discussed. Finally, a series fuel flexibility tests were conducted in the RDF-5 BFBB.

The plant unit which consists of a fluidized bed retort and CFB furnace for burning the by-products of retorting (semicoke and semicoke gas) is presented in this paper. The oil shale retort consists of a fast fluidized bed shaft, coarse semicoke bit, semicoke separation chamber and cyclone for the separation of fine semicoke particles. The crashed oil shale and hot ash from the CFB ash separator are fed concurrently into the fast fluidized bed shaft. For fluidizing the mixture of oil shale and hot ash particles, the recycle semicoke gas is used. The pyrolysis of oil shale begins in fluidized bed and is completed in the semicoke separation chamber. The coarse semicoke particles are separated from fluidized bed directly while the medium size particles are separated from the gases in the semicoke separation chamber and the finest semicoke particles in the cyclone. All the fractions of semicoke from the fluidized bed retort and semicoke gas from the oil fractionator are burnt in the CFB furnace. The semicoke ash is separated from flue gases in the CFB ash separator. A part of separated hot ash is fed into the fluidized bed retort as a solid heat carrier material and the rest into the furnace through the ash cooler or separated from the process. The retention of sulphur dioxide formed during the semicoke and semicoke gas combustion, is guaranteed for about 99 % due to the high CaO content in the semicoke ash and convenient temperature (about 850°C) in the CFB furnace. The described plant unit is useful for retorting oil shale and other solid hydrocarbon-containing fuels. The advantages of the present retorting process and system are: improved oil yield, greater throughput, lower retorting time, avoidance of moving parts in the retorting zones, reduced downtime, etc. A new plant unit for oil shale oil production has been elaborated and defended by the Estonian Utility Model EE 200700671 UI.

The extent of attrition associated with impact loading was studied for five different limestones pre-processed in fluidized bed under different reaction conditions. The experimental procedure was based on the measurement of the amount and the particle size distribution of the debris generated upon impact of sorbent samples against a target at velocities between 10 and 45 m/s. The effect of calcination, sulfation and calcination/re-carbonation on impact damage was assessed. Fragmentation by impact loading of the limestones was significant and increased with the impact velocity. Lime samples displayed the largest propensity to undergo impact damage, followed by sulfated, re-carbonated and raw limestones. Fragmentation of the sulfated samples followed a partem typical of the failure of brittle materials. On the other hand, the behavior of lime samples better conformed to a disintegration failure mode, with extensive generation of very fine fragments. Raw limestone and re-carbonated lime samples followed either of the two patterns depending on the sorbent nature. The extent of particle fragmentation increased after multiple impacts, but the incremental amount of fragments generated upon one impact decreased with the number of successive impacts.

One of the most important factors for trouble free operation of CFB boilers is the pressure drop of the gas distributor. The pressure drop characteristic of the gas distributor depends on the nozzles used. A modified bell jar nozzle was designed and developed for use with large-scale industrial circulating fluidized bed (CFB) boilers. The nozzle consists of a vertically tapered tube with a larger end at the top, a float which is free to move within the tube and a cover with holes. The pressure drop characteristics of the nozzle were measured experimentally by using different floats and moving out the float respectively. The gas distributor equipped with the modified bell jar nozzle has a unique pressure drop characteristic. It has a higher resistance than other nozzles which results in the formation of an effective barrier against backflow at low boiler loads, which results from the pressure fluctuation caused by bubble burst and solids coming from the recycle system. In addition, it has a relatively low pressure drop at high or full boiler loads, which can greatly reduce the energy cost of the primary air fan.

By analyzing the 336-hour performance testing period operation parameters and the actual measurement data at the scene, this paper took a study of the heat balance on Baima’s 300MWe CFB boiler. Through calculating and by the use of DL/T964-2005 standard in China, the efficiency of this boiler under the Ml load was 91.9089%. The result was very close to the manufacture’s assurance efficiency. In all the heat loss, the value of dissipated heat loss calculated in this paper was about 0.63%, which was higher than the manufacture’s provided value 0.25%. Besides analyzed the reason, it was discussed about the influence factors of CFB boiler’s dissipated heat loss in this paper, including the radiation areas and the out-surface temperature. Additionally, when calculated the heat loss due to the physical heat of bottom ash, the selection of output ash temperature was important. It was discussed in paper about how to choose different output temperatures for different types of ash coolers.

Chemical looping combustion (CLC) is a means of combusting carbonaceous fuels, which inherently separates the greenhouse gas carbon dioxide from the remaining combustion products, and has the potential to be used for the production of high-purity hydrogen. Iron-based oxygen carriers for CLC have been subject to considerable work; however, there are issues regarding the lifespan of iron-based oxygen carriers over repeated cycles. In this work, haematite (Fe

2

O

3

) was reduced in an N

2

+CO+CO

2

mixture within a fluidised bed at 850°C, and oxidised back to magnetite (Fe

3

O

4

) in a H

2

O+N

2

mixture, with the subsequent yield of hydrogen during oxidation being of interest. Subsequent cycles started from Fe

3

O

4

and two transition regimes were studied; Fe

3

O

4

↔Fe

0.947

O and Fe

3

O

4

↔Fe. Particles were produced by mechanical mixing and co-precipitation. In the case of co-precipitated particles, Al was added such that the ratio of Fe:Al by weight was 9:1, and the final pH of the particles during precipitation was investigated for its subsequent effect on reactivity. This paper shows that co-precipitated particles containing additives such as Al may be able to achieve consistently high H

2

yields when cycling between Fe

3

O

4

and Fe, and that these yields are a function of the ratio of [CO

2

] to [CO] during reduction, where thermodynamic arguments suggest that the yield should be independent of this ratio. A striking feature with our materials was that particles made by mechanical mixing performed much better than those made by co-precipitation when cycling between Fe

3

O

4

and Fe

0.947

O, but much worse than co-precipitated particles when cycling between Fe

3

O

4

and Fe.

A co-precipitated mixture of CuO and Al

2

O

3

is a good oxygen-carrier for chemical-looping combustion. The kinetics of regeneration of this reduced oxygen-carrier (355 – 500 urn) were measured from 300 to 750°C when reacting it with O

2

. Care was taken to ensure these measurements were not affected by interphase mass transfer. Efforts were also made to minimise sampling problems by using a rapid-response mass spectrometer for reactions lasting for 45 s or less; otherwise, a paramagnetic analyser for O

2

was used, since the mass spectrometer drifted with time. The order of reaction with respect to O2 was found to be ∼ unity at 300 to 750°C. Below 600°C, the reduced oxygen-carrier was incompletely oxidised to a mixture of CU

2

O and Al

2

O

3

. Above 600°C, regeneration was completely to CuO and Al

2

O

3

and was controlled to a considerable extent by external mass transfer. At these higher temperatures, regeneration involved a shrinking core mechanism and the two consecutive steps:

(1)

$$ 2Cu + 1/2O_2 \to Cu_2 O, $$

(2)

$$ Cu_2 O + 1/2O_2 \to 2CuO. $$

The activation energies and pre-exponential factors for both reactions were measured from initial rates. The kinetics in the first cycle of operations were found to be similar to those in the subsequent cycles.

The CaSO

4

based oxygen carrier has been proposed as an alternative low cost oxygen carrier for Chemical-looping combustion (CLC) of coal. The reduction of CaSO

4

to CaS is an important step for the cyclic process of reduction/oxidation in CLC of coal with CaSO

4

based oxygen carrier. Thermodynamic analysis of CaSO

4

oxygen carrier with CO based on the principle of Gibbs free energy minimization show that the essentially high purity of CO

2

can be obtained, while the solid product is CaS instead of CaO. The intrinsic reduction kinetics of a CaSO

4

based oxygen carrier with CO was investigated in a differential fixed bed reactor. The effects of gas partial pressure (20%–70%) and temperature (880–950°C) on the reduction were investigated. The reduction was described with shrinking unreacted core model. Experimental results of CO partial pressure on the solid conversion show that the reduction of fresh oxygen carriers is of first order with respect to the CO partial pressure. Both chemical reaction control and product layer diffusion control determine the reduction rate. The dependences of reaction rate constant and effective diffusivity with temperature were both obtained. The kinetic equation well predicted the experimental data.

Chemical-looping combustion (CLC) is a novel combustion technique with CO

2

separation. Magnetite (Fe

3

O

4

) was selected as the oxygen carrier and Shenhua coal (Inner Mongolia, China) as the fuel for this study. The influences of operation temperatures, and coal to Fe

3

O

4

mass ratios on the reduction characteristics of the oxygen carrier were investigated using an atmosphere TGA. The sample, comprised of 2.25mg coal and 12.75mg Fe

3

O

4

, was heated to 1000°C. Experimental results show that the reaction between the coal volatile and Fe

3

O

4

began at 700°C while the reaction between the coal char and Fe

3

O

4

occurred at 800°C and reached a peak at 900°C. Fe

3

O

4

was fully reduced into FeO, while some FeO was further reduced to Fe. As the operation temperature rises, the reduction conversion rate increases. At the temperatures of 850°C, 900°C, and 950°C, the reduction conversion rates were 37.1%, 46.5%, and 54.1% respectively. When the mass ratios of coal to Fe

3

O

4

were 5/95, 10/90, 15/85, and 20/80, the reduction conversion rates were 29.5%,40.8%,46.5%, and 46.6% respectively. With the increase of coal to Fe

3

O

4

mass ratio, the conversion rate increases first and then changes no more. There exists an optimal coal to Fe

3

O

4

mass ratio.

The dual fluidized bed reactors are the key technology to fulfill the multiple CCR (calcination/carbonation reactions) cycles for CO

2

capture from the flue gases. Firstly, the dual bubbling fluidized bed reactors were selected in this work based on analyzing different types of dual fluidized bed reactors. Secondly, the design method of dual fluidized bed reactors for CO

2

capture with CCR concept was proposed. Thirdly, with the designed results, a cold mode of the dual bubbling fluidized bed reactors was built. The long-term stable operation and the continuous solid circulation between two reactors could be achieved successfully. The experimental results indicated that the solid circulation rate was increased with an increase of bed height, diameter of solid injection nozzle, and diameter of holes on the solid injection nozzle.

The work in this paper aims at determining the effect of gasification syngas on the carbonation reaction and conversion for several naturally occurring calcium-based sorbents. Experiments were performed

via

the use of a thermogravimetric analyzer (TGA) and it was observed that the presence of CO and H

2

caused an increase in initial rate of approximately 70.6%. The increase in reaction rate was attributed to the CaO surface sites catalyzing the water-gas shift reaction; as well, the shift reaction was assumed to be responsible for the increase in activation energy for limestone based on the formation of intermediate complexes.

A pilot-scale dual fluidized bed reactor system was applied to further investigate the effect of the shift reaction on CO

2

capture and sorbent conversion for two limestones (Polish and Cadomin limestone). During carbonation with steam present in the feed gas, it was observed that the high CO

2

capture period was significantly extended as compared to carbonation with only CO

2

present. This resulted in an increase in CaO conversion from approximately 16.1 to 29.7% for the initial carbonation cycle and reinforced the conclusions drawn

via

TGA experimentation. Based on the outlet gas analysis, it was confirmed that the CaO particles were in fact catalyzing the water-gas shift reaction, increasing the overall sorbent conversion to approximately 46.9% for the first cycle. In terms of sorbent regeneration, the oxy-fuel combustion conditions employed (high CO

2

and O

2

atmosphere), resulted in enhanced sorbent sintering, thus producing the negative effect on carbonation conversion.

There is an emerging postcombustion capture technology that uses CaO to capture CO

2

from combustion flue gases in a circulating fluidized bed reactor. This paper summarizes recent work conducted at CSIC to understand and develop this technology. The paper includes experimental results at conditions close to those expected in the real system, carried out in continuous mode in a 30kW test facility made up of two interconnected circulating fluidized bed reactors. In one of the reactors, CO

2

is captured from the gas phase by the CaO continuously circulating from a calciner. In the second reactor, the CaCO

3

formed in the carbonator is regenerated to CaO and CO

2

by calcination. Modeling of the system at process level, at reactor level (in particular the CFB carbonator), and at particle level (decay in capture capability of CaO) is also outlined. The work carried out so far confirms that the carbonator reactors can be designed to attain capture efficiencies between 70–90%, operating at fluid dynamic conditions close to those present in circulating fluidized bed combustors.

Combusting a solid fuel in the presence of a metal oxide rather than air, chemical looping combustion, generates CO

2

suitable for sequestration and the reduced metal. For the case of iron, the reduced oxide can be re-oxidized with steam to produce high-purity hydrogen. The reduction reactions of iron oxide in carbon monoxide and carbon dioxide mixtures were investigated in a fluidized bed. Activation energies and pre-exponential factors for the reactions (i) 3 Fe

2

O

3

+CO⇌2 Fe

3

O

4

+CO

2

and⇌(ii)0.947 Fe

3

O

4

+0.788 CO⇌3 Fe

0.947

O+0.788 CO

2

⇌were determined. The reaction order was verified to be unity, and the change in rate with conversion was examined.

A bubbling fluidized-bed reactor was used to study the CO

2

capture characteristics of dry potassium-based sorbents. Potassium-based sorbentswere prepared by impregnation with potassium carbonate on supports such as cocoanut activated charcoal (AC#1), coal active carbon (AC#2), activated alumina (Al

2

O

3

), silica gel (SG) and diatomite. Sorbents such as K

2

CO

3

/AC#1, K

2

CO

3

/AC#2, and K

2

CO

3

/A1

2

O

3

showed excellent carbonation capacity, the conversion rates of those sorbentswere 92.5%, 85.1%, and 88.2%, respectively. The regenerationrates of those sorbents were 87.1%, 83.9% and 86.7%, respectively. SEM and N

2

adsorption tests showed that the surface area and pore volume of those sorbents were 669 m

2

g

−1

, 390 m

2

g

−1

, 139 m

2

/g, and 0.08 cm

3

/g, 0.17 cm

3

/g, 0.34 cm

3

/g, respectively. The particle morphologies of those sorbents are propitious to the carbonation reaction. However, K

2

CO

3

/diatomite andK

2

CO

3

/SG showed poor carbonation capacity, the conversion conversionswere only 2.0% and 1.9%, respectively.The surface area and pore volume of two sorbents were 1.1 m

2

/g, 39 m

2

/g and 0.004 cm

3

, 0.33 cm

3

/g, respectively. The carbonation capacity is affectedby the particle morphology of those sorbents. The present work has provided data base for designing and operating a large scale CO

2

capture process with two fluidized-bed reactors.

Steamhydrationandpelletizationoflimestone wereinvestigatedusinga thermogravimetric analyzer (TGA) toimprove the sorbent utilization for

in-situ

CO

2

capture under typicalfluidized bed combustion (FBC)operating conditions. Steamhydration of CaO improves carbonation capacity but the hydratedsorbent is very fragile, which will be a problem for FBC applications. Similar sorbent improvements in terms of maintaining/enhancing reactivity were observed by sorbent fine grinding and pelletization, whichappears to be a method of using hydrated sorbent in fluidized bed applications.

This paper aims to understand the promotional effect of ultrasonic treatment on catalytic performance of Ni/γ-Al

2

O

3

catalyst for CO

2

reforming of benzene as a model compound of tar derived from biomass gasification, and the catalytic cracking mechanism was also discussed. Firstly, three Ni/γ-Al

2

O

3

catalysts were prepared by ultrasonic impregnation as the ultrasonic power variantat0, 120 and 500W, andthephysicochemical property of catalysts were characterized using N

2

-adsorption, SEM and XRD, etc. Then the catalytic performance of three catalysts for CO

2

reforming of benzene wastestedin amicro-reactor.Theoutlet gaswas measured using a Micro-GC. Finally, the coking amount on the catalyst surface was measured by thermogravimetry (TG). The results showed that ultrasonic treatment significanüy modified the pore size distribution of the catalysts especiallyin theporesize range of 10–50nm andalso improved the capability of the coke resistance. It’s beneficial to increase the lifetime of the catalyst. Meanwhile, lower ultrasonic power(120W) was more favorableto improve the coke resistance of the catalyst in the power range tested (120 and 500W). The main surface reactions over Ni/γ-Al

2

O

3

catalysts included two steps: Firstly, benzeneadsorbed on the catalyst surface, the metal active sites dehydrogenation took place, and the residual molecule fragments (coke precursor) wouldcondense further which led to coke formation.Then, CO

2

reacted with coke precursor and coke for coke elimination. The first step carriedout very quickly, and the second step was the rate-determining step.To reduce the cokedeposition on the catalyst surface, the performance of CO

2

adsorption and activation and surface oxygen transmission capacity should be improved further.

Carbonate Looping is a promising option for the capture of CO

2

from power plant flue gases. The Carbonate Looping process is carried out in a Dual Fluidized Bed (DFB) system with continuous looping of CaO, which is the CO

2

carrier, between the two beds. The system consists of a carbonator, where CO

2

is absorbed by CaO, operating at 600–700°C and a regenerator operating at temperatures above 900 °C. Experiments was carried out in a 10 kW DFB facility so as to investigate critical parameters affecting the CO

2

capture efficiency of the process which are the CaO looping rate, the carbonator space time and thecarbonator temperature. The rate of sorbent attrition was found to be 2% wt./h and is below the required sorbent make-up rate. Steady state CO

2

capture efficiencies of 90% are demonstratedunder realistic process conditions.

Two modified Chemical Looping Combustion (CLC) schemes were investigated: (a) CLC with in situ gasification of a solid carbonaceous fuel in the fuel reactor, and (b) CLC for the production of high purity hydrogen from low grade syngas. A comparison between the performance of the two modified cycles using (i) syngas from cylinders and (ii) syngas derived from the gasification of various solid fuels was made. Preliminary results indicate that both processes can be operated with sufficient conversions using low and high-rank coals. However, agglomeration of the oxygen carrier was observed if wood was used in process (a), probably owing to the formation of low-melting eutectics between the oxygen carrier and metals from the wood ash.

In the present work the thermogravimetric characterization of the sorption of carbon dioxide on polymer-modifiedmesoporous materials (MCM-41) from fly ashes is described. In order to obtain MCM-41 materials from three different types fly ashes,(including CFB fly ash) hydrothermal processesusing the supernatantsof coal fly ashes and surfactantsas the structure-directing agents,have been carried out. The obtained mesoporous materials were subjected to polyethylenimine (PEI) modification by their impregnation to obtain samples with PEl contents of 30, 50 and 70%, respectively. CO

2

sorption/desorption tests on loaded PEl samples were carried out in a flow of a mixture of gasses (CO

2

-1O%, O

2

-10%, N

2

-80%) at different temperatures: 25 and 75°C. The highest CO

2

sorption value was obtained for the sample that contained the best-quality MCM-41 and was impregnatedwith PEI in the amount of 50%. This sample at a temperatureof 75°C can take CO

2

in an amount equivalent to 111.7 mgCO

2

/g sample weight. Under the same conditions, but without PEI impregnation, this sample can take CO

2

in an amount equivalent to 3.2 mgCO

2

/g sample weight, thus 35 times less. The research of CO

2

adsorption on polymer-modified mesoporous materials from fly ashes carried out within this work has shown that these materials are characterized by high CO

2

adsorption capacity under conditions typical of coal combustionboiler flue gas and have the chance of becoming an efficient adsorbent for application to post-combustion CO

2

separation. For PEI impregnated samples, a different behaviour of adsorption/desorption profiles has also been observed (both sorption and desorptionprogressesvery rapidly).

Chemical looping with selective oxygen transport allows two step combustion or autothermal reforming without mixing of fuel and air. The reactor system consists of two reactors, an air reactor and a fuel reactor with a suitable oxygen carrier that transports the necessary oxygen for operation. In the present study, a highly active nickel based oxygen carrier is tested in a novel dual circulating fluidized bed (DCFB) system at a scale of 120 kW fuel power. The mean particle size of the oxygen carrier is 120 μm and the pilot rig is fueled with natural gas. For the investigated oxygen carrier high CH

4

conversion is achieved. Air/fuel ratio is varied at three different fuel reactor temperatures. For chemical looping reforming one can observe synthesis gas composition close to thermodynamic equilibrium. In spite of the fact that no additional steam has been added to the fuel besides the one present through steam fluidization of the loop seals, coke formation does not occur at global stoichiometric air/fuel ratios above 0.46.

Chemical-looping combustion (CLC) is a combustion technology where an oxygen carrier is used to transfer oxygen from the combustion air to the fuel in order to avoid direct contact between air and fuel. Thus, the CO

2

is inherently separated from the flue gases with a potential for considerably lower energy penalty and cost compared to other techniques for CO

2

separation. The oxygen carrier is circulated between two reactors, a fuel and an air reactor, where the flue gas from the air reactor contains oxygen depleted air and the flue gas from the fuel reactor contains mainly CO

2

and H

2

O. The water can easily be condensed and the remaining CO

2

can be transported for underground storage. Most of the prior work with CLC has focused on using natural gas and syngas as fuel and oxygen carrying material normally produced from pure chemicals. However, recent work on adapting the CLC process for solid fuels with ores and natural minerals as oxygen carrier shows promising results. This paper will present results from reactivity investigations in a laboratory fluidized-bed reactor system using previously investigated natural mineral ilmenite as oxygen carrier and a bituminous Colombian coal as fuel. Experiments were conducted at a temperature of 970°C with N

2

, steam, and/or CO

2

in the fluidizing gas. Synergy effects between steam and CO

2

on fuel conversion was noted. The results show that the fuel conversion was a roughly a factor 5 faster with steam as compared to CO

2

in the fluidizing gas.

Sewage sludge is the by-products of sewage treatment, and it is a fuel of high moisture, high ash and low caloric. Oxygen-enriched combustion technology is one of the new and clean coal combustion technologies that can control pollutant emission, which makes CO

2

separation, SO

2

treatment become easier, and NO

x

emission reduced. In this paper, we carried out the experimental research on the advantages of oxygen-enriched combustion and the characteristics of sewage sludge in a CFB incinerator that the diameter of the furnace is 100 mm, It is an important foundation for the industrialized application of the oxygen-enriched combustion of sewage sludge and coal in CFB. Experimental analyzed on the combustion characteristics of three conditions in the oxygen concentration of 21%∼35%, which were the weight ratio of coal and sludge were 1∶1, 1∶2 and also the coal was given. Furthermore, the change of gas composition along with the change of oxygen content and the temperature of dense phase region was analyzed. The results showed that the combustion characteristics differ from the different mixing rate between coal and sludge in different oxygen atmosphere, when the fluidized air velocity was 1.56 m/s∼1.88 m/s, the combustion stability; When the amount of the fuel was constant, as the increase of the oxygen contents in the experimental atmosphere, the total air volume decreased, the furnace temperature increased gradually, the concentration of SO

2

and NO

x

showed increasing trend, which is beneficial to the removal of SO

2

; The concentration of NO

x

was increased gradually as temperature of the fluidized bed increased.

The influence of oxygen-enriched gaseous atmosphere on coal char combustion was studied. Two different coals, i.e. lignite and bituminous coal, were used as a basic fuel and the reacting gases of oxygen & CO

2

were used to simulate flue gas recirculation. Moreover, a broad range of in-furnace conditions, i.e. five temperatures of 873, 973, 1073, 1173, 1273K and five oxygen concentrations of 20, 40, 60, 80, 100%vol., was investigated. Thermogravimetric method of measurement was employed to obtain the processing data on fuel conversion rate under foregoing investigated conditions. For further calculations, simplified Shrinking-Core Model was introduced. Finally, fundamental kinetic parameters, i.e. pre-exponential factor, activation energy and reaction order, were established and then on the basis of their values reaction-controlling regime for coal char combustion in oxygen-enriched environment was predicted. The investigations, financially supported by Polish Government, are a part of Framework Project “Supercritical Coal-fired Power Units”.

In this work combustion of single coal char particles was studied at 850°C in a lab-scale fluidized bed under simulated oxyfiring conditions. The burning rate of the particles was followed as a function of time by continuously measuring the outlet CO and O

2

concentrations. Some preliminary evaluations on the significance of homogeneous CO oxidation in the reactor and of carbon gasification by CO

2

in the char were also carried out. Results showed that the carbon burning rate increases with oxygen concentration and char particle size. The particle temperature is approximately equal to the bed one up to an oxygen concentration of 2%, but it is considerably higher for larger oxygen concentrations. Both CO

2

gasification of char and homogeneous CO oxidation are not negligible. The gasification reaction rate is slow and it is likely to be controlled by intrinsic kinetics. During purely gasification conditions the extent of carbon loss due to particle attrition by abrasion (estimated from the carbon mass balance) appears to be more important than under combustion conditions.

The characteristics of biomass steam gasification were investigated to make an optimum syngas for Fischer Tropsch (F-T) synthesis of bio-diesel. Korean pine wood chip was used as a fuel and the experiment was conducted in a lab scale bubbling fluidized bed (0.1m LD. x 3.Omheight). Gas composition was evaluated by changing operating parameters such as gasifier temperature, and steam to fuel ratio. Major syngas was monitored by on-line gas analyzer (ND-IR spectroscopy) and gas chromatography (GC). As the temperature of gasifier increases hydrogen in the syngas increases while CO in the product gas decreases. The low concentration of sulfur compound and nitrogen in the product gas shows the potential advantages in the purification process of the syngas for F-T process. Optimum operating condition of the gasifier was found concerning the following gas cleaning and F-T process; H

2

-CO ratio and total gas yield increase while decreasing methane and CO

2

concentrations in the syngas.

The research presents an innovative idea of developing a continuous H

2

production process employing fluidized bed technology from agricultural biomass with in-situ CO

2

capture and catalyst regeneration. Novelty of the process lies in the generation of relatively pure H

2

from biomass with CO

2

as a by-product using steam as the gasifying agent. Another unique feature of the process is internal regeneration of the catalyst, fouled in the gasifier. Thus, the technology will serve the twin purpose of regenerating the catalyst, and generation of N

2

free H

2

and CO

2

. The work also reports the experimental results conducted in a batch type fluidized bed steam gasifier using CaO as the catalyst. A 71% concentration of H

2

and nearly 0 concentration of CO

2

were achieved in the product gas when sawdust was used as the feedstock. In a separate test using a circulating fluidized bed reactor as the regenerator, a 40 % regeneration of CaO was also achieved at a calcination temperature of 800°C.

the tar from rice hull gasification for power generation which is cracked in high temperature is studied in this paper, the results reveal the part of compositions which have smaller RT in tar is first cracked into H

2

, CO

2

and carbon, and then carbon react with H

2

and CO

2

, and CH

4

, CO are formed; the cracked efficiency of tar can reach 28.66%, the carbon deposit among cracked tar can reach 12.76%, the results of the GC-MS analysis showed the aromatic extent of with tar cracking reaction carrying out in high temperature.

The influences of zeolites and metal oxides on the pyrolysis of cotton straw have been studied by thermogravimetric analysis

(TGA)

. And performance of different catalysts to volatile and yield of ash was compared. The results showed that the catalytic pyrolysis experienced three stages: water losing stage, activated pyrolysis stage(APS), passive pyrolysis stage(pPS). The passive pyrolysis stage for pure cotton straw was small. The addition of the catalysts decreased the percent of volatile conversion for activated pyrolysis stage, but with the decreasing temperature for passive pyrolysis stage and increasing the percent of volatile conversion for passive pyrolysis stage. These catalysts promoted the pyrolysis of lignin, but showed less performance to the pyrolysis of cellulose and hemicellulose. The yield of ash increased in the presence of catalysts,and the yield was higher when catalyzed by metal oxide than by zeolite.

In an attempt to improve the gasification efficiency and decrease the carbon content in fly ash of atmospheric air CFB gasifiers, an innovatory equipment by name ash-returned reactor is put forward by SKLCC. Ash-returned reactor is an ash-returned apparatus on line of ash circulation, typically like “U” type valve in CFB boilers, with additional function of some extent combustion of residual carbon and increase the furnace inlet temperature of returning ash, and hence the coal conversion of gasifiers is enhanced. As to its configuration compared to conventional “U” type valve, ash-returned rector has two distinguished features of several times of height scale of fluidizing transportation region to meet the combustion reaction time need and appropriate heat transfer tube bundles arranged in the region to moderate the local temperature so as to avoid slagging. And hence, corresponding to the structure renovation, the material transportation and regulation performance of ash-returned reactor is primarily investigated through a series of experiments in a cold lab-scale facility in this paper. The heat transfer characteristic of the tube bundles is then researched and its influential factors are further discussed. These works lay a foundation on the following study of hot state experiments and industrial applications.

During summer 2007 a 2–6 MWth indirect gasification section was integrated into the loop of the existing 82➀2 MWth circulating fluidized bed boiler at Chalmers University. With help of a particle distributor the gasification unit is connected to the loop after the cyclone. Hot bed material entrained from the boiler is so transferred to the gasifier providing the heat for the production of a nearly nitrogen free product gas. Non-gasified char is returned together with the bed material into the boiler and converted. Biomass can be fed into both sections; the boiler and the gasifier. The gasification is separated from the boiler via two loop seals and a particle distributer, directing particles either back to the boiler or into the gasification section. For that reason the CFB boiler can be operated even after the retrofit independently, just like before, or in combined combustion/gasification mode. This possibility keeps the risk for a retrofit low. As, furthermore, the investment costs for the integration are considerably lower than standalone gasification units of that size, the retrofit is an easy way to extend the potential of a CFB Boiler towards bi- and tri-generation (heat, power, fuel) and enter new markets.

The first experimental season with the installation proved stable operation of both the boiler and the gasification. Furthermore, the full functionality in combustion only mode was shown. The gasifier was operated for 60 h with wood pellets and wood chips. First analysis of the producer gas composition shows high contents of methane, making the installation a good match for SNG production. The heating value of the gas is about 14.4 MJ/Nm

3

.

A cold model dual fluidized bed (DFB) reactor, consisting of two parallel interconnected bubbling and fast fluidized beds, was designed for developing an auto-thermal biomass gasifier. The combustor of this system burns the rest char of the gasification process and provides heat to the gasifier by circulating solids inventory. To find an optimal mixing and circulation of heavy solid inventory and light biomass and char materials, we investigate two types of DFB reactors which have different configuration of distributor and way-out location of the solid inventory and char materials in the gasifier. To determine appropriate operating conditions, we measured minimum fluidization velocity, solid circulation rate, axial solid holdup and gas bypassing between the lower loop seal and the gasifier.

The tar from rice husk gasification for power generation was taken as an example to be catalytically cracked by CaO catalyst. The experimental results showed the cracking efficiency of tar greatly increased from 28.66% by thermal cracking to 65.6% by catalytic cracking, the gas compositions from tar being cracked were H

2

, CO, CH

4

, and CO

2

, and the H

2

was a majority of them. The DSC and XRT analysis revealed that the deposit carbon could be found after tar was catalytically cracked and the deposit carbon efficiency could reach 30.51%. The SEM photographs of CaO catalyst used as catalyst showed that the CaO catalyst was enwrapped by the deposit carbon and decreased its catalytic activity, at the same time, the pressure drop of gas passing through catalyst bed increased because of the deposit carbon, it was different for us to operate the cracking reactor of tar and CaO catalyst in the cracking reactor must be regenerated for its stable operation.

With the depleting of fossil fuel and environmental polluting increasing, the utilization of biomass resources caught increasing concern. Biomass gasification in fluidized bed, as one promising technology, developed quickly. However, serious agglomeration was displayed as biomass ash reacted with bed material (silica sand) at higher temperature. It hindered the wide utilization of CFB gasifier. The objective ofthis work is to investigate the agglomeration behavior between biomass ash and silica sand, and catch the inherent mechanism. Firstly, the influence of ash compounds on the agglomeration behavior was analyzed with biomass ash and synthesis ash compounds addition in fixed bed as ash sample mixed with bed material evenly before every trial. The reaction temperature was set 850°C that is the operated temperature for many fluidized bed gasificated biomass fuels. Then the influence of reaction time was analyzed. The characteristics of the agglomerated silica sand particles were analyzed by the XRD. Finally, it was simulated with HSC computer mode based on thermodynamic equilibrium. It was observed that when the ratio of the biomass ash to the silica sand was above 0.2, the agglomeration was observed. With the increase of the reaction time, more silica sand particles agglomerated with the biomass ash. There are two kinds of silicate eutecticum investigated by the XRD. It is of great significance for the running ofCFB biomass gasifier and the development ofbiomass utilization technology.

Pressure fluctuation is often used to analyze the dynamic changes of the gas-solid fluidized beds. In this paper, cold tests were carried out to study the gasification system of circulating fluidized beds gasifier, to find out the change law of the solid circulation rate Gs and the holdup by altering such parameters as air velocity, particle size and integrated particle, and recognize the flow regime in the boiler through analysis of the differential pressure fluctuation signals with wavelet transform modulus maximum method. Results indicate that Gs increases with the increase of the superficial gas velocity. When the superficial gas velocity is higher than 2m/s, the three kinds of particles with higher distribution concentration of particle size exhibit characteristics of fast fluidization at the bottom of the riser, and the number of modulus maxima lines is almost the same as that in the top area. Change the distribution range of particle sizes of bed materials, and the larger particles in the riser after particle separation exhibit characteristics of both the turbulence region at the bottom and the fast region on the top, prolonging their residence time in the bed. There are more modulus maxima lines for the differential pressure fluctuation signals at the bottom than on the top. The regime recognition method based on modulus maximum and its experimental results will help us learn more about the design, enlargement and operation of circulating fluidized beds.

In theory, an integrated biomass gasification and fuel cell system has a higher overall plant efficiency when compared to the efficiency of biomass gasification combined with simple combustion systems and gas engines. In order to develop a prototype of this new concept of power plant operating in the range of l50kW to 5MW, several institutes of the Max Planck Society and the Fraunhofer-Gesellschaft in Germany have been working on the ProBio project with focus on the theoretical and experimental investigation of an integrated 1–2kW

e

system. The paper will firstly describe the gasification unit of the system: a lab-scale atmospheric bubbling fluidized bed gasifier. Wood gasification experiments were conducted and the influence of operation parameters, i.e. gasification agents, equivalence ratio ER and steam to biomass ratio SIB on gas yield and gas composition was analyzed. In parallel with the experimental work, chemical kinetics of wood gasification was studied and simulated. Furthermore, simulation of bubbling fluidized bed hydrodynamics at high temperature, using commercial computational fluid dynamics (CFD) software FLUENT, was also conducted to better understand the phenomenon of fluidization inside the bed.

In this work we compare two different modelling strategies for the simulation of wood gasification in a dense fluidized bed. In the first method we adopt an Euler-Lagrange approach where the fluid phase is modelled as a continuum and the particulate phase is modelled on the individual particle level using the Discrete Element Method (DEM). In the second method we use an Euler-Euler or continuum approach where the multiple solid phases for wood and char particles are treated as a continuum. Both models allow the time-dependent prediction of velocity, temperature, and composition fields. Both models have been run to a statistically stationary state. We compare the numerical results of both methods against each other and with experimental data obtained at a laboratory scale bubbling fluidized bed reactor.

A pressurized gas at high temperatures with low impurities often is a basic requirement for applications for biomass gasification. Therefore, the Vienna University of Technology, in cooperation with the Austrian Bioenergy Centre, operates a pressurized gasification pilot plant in order to investigate thepressurized gasification process and estimate its potential. Within the scope of this paper this test facility as well as its operation behavioris described. Furthermore the parameters pressure, gasification temperature, lambda value and gasification agent have been investigated regarding to their influenceon the producer gas composition and arepresented and discussed in the following.

We developed a comprehensive process model in ASPEN Plus to simulate the energy and mass balances of a lignite-fueled atmospheric circulating fluidized bed (CFB) boiler integrated with coal predrying and pyrolysis topping. In this model, it is assumed that the heat from exhausted flue gas was employed for coal predrying, and the sensible heat derived from circulated bed material was used for the pyrolysis topping (endothermic process). The simulation was conducted with respectto the Yunnan Kaiyuan CFB boiler, and two representative lignite coals from Xiao Long Tan (XLT) and Xin Shao (XS) were considered. The result shows that the predrying of coal with the sensible heat of above 363 K from flue gas, the amount of coal consumed in the boiler can be reduced by 3.5% and 5.3% for XLT lignite and XS lignite, respectively. It was also found that integration of pyrolysis topping with the boiler increased the coal consumption of the boiler, and the extent of consumption-increase varies with the yields of tar and gas in the pyrolysis topping process. For agas yield of 5.2% and a tar yield of 5–6%, the consumption of XS lignite increased by about 20% comparing to that in the case without topping.

Gasification of biomass is a suitable option for decentralized energy supply based on renewable sources in the range of up to 50 MW fuel input. The paper presents the dual fluidized bed (DFB) steam gasification process, which is applied to generate high quality and nitrogen-free product gas. Essential part of the DFB process is the bed material used in the fluidized reactors, which has significant impact on the product gas quality. By the use of catalytically active bed materials the performance of the overall process is increased, since the bed material favors reactions of the steam gasification. In particular, tar reforming reactions are favored. Within the paper, the pilot plant based on the DFB process with 100kW fuel input at Vienna University of Technology, Austria is presented. Actual investigations with focus on CaO-based bed materials (limestone) as well as with natural olivine as bed material were carried out at the pilot plant. The application of CaO-based bed material shows mainly decreased tar content in the product gas in contrast to experiments with olivine as bed material. The paper presents the results of steam gasification experiments with limestone and olivine, whereby the product gas composition as well as the tar content and the tar composition are outlined.

Solid biomass can be converted into liquid fuel through fast pyrolysis, which is convenient to be stored and transported with potential to be used as a fossil oil substitute. In China, agricultural wastes are the main biomass materials, whose pyrolysis process has not been researched adequately compared to forestry wastes. As the representative agricultural wastes in China, peanut shell and maize stalk were involved in this paper and pine wood sawdust was considered for comparing the different pyrolysis behaviors of agricultural wastes and forestry wastes. Fast pyrolysis experiments were carried out in a bench-scale fluidized-bed reactor. The bio-oil yieldsof peanut shell and maize stalk were obviously lower than that ofpine sawdust. Compared with pine sawdust, the char yields of peanut shell and maize stalk were higher but the heating value of uncondensable gaswas lower. This means that the bio-oil cost will be higher for agricultural wastes if taking the conventional pyrolysis technique. And the characteristic and component analysis resultsof bio-oil revealed that the quality of bio-oil from agricultural wastes, especially maize stalk, was worse than that from pine wood. Therefore, it is important to take some methods to improve the quality of bio-oilfrom agricultural wastes, which should promote the exploitation of Chinese biomass resources through fast pyrolysis in afluidized bed reactor.

Cyclic calcination-carbonation (CC) reactions have important applications, such as carbon dioxide (CO

2

) capture from power plant flue gases and zero emission systems for hydrogen production. A critical challenge is the severe activity loss of CaO-based sorbent with increasing cycle numbers. In present study, A Thermax 500 high pressure thermogravimetric analyzer (pTGA) and the Scanning Electron Microscopy (SEM) technology were used to examine hydration reactivation on CaO-based sorbents. Two hydration methods, liquid hydration and steam hydration, were both investigated. It was found that CaO sorbents calcined under lower CO

2

partial pressurelose activity moreslowly with cycle number. Both the two hydration methods efficiently improved sorbent activity during cyclic CC reactions. The mean values of activity increase for liquid hydration and steam hydration after 6 cycles were ∼22% and ∼27% respectively. SEM images showed that the hydrated CaO particles both exhibited surface area and porosity more favorable for CO

2

diffusion through the CaCO

3

product layer. It seemed that hydration frequency could affect sorbent cyclic performance and a relatively high frequency should be beneficial. Moreover, comparison with previous study confirmed that hydration reactivation couldbe efficient at a wide range of condition. Results of the present study demonstrate that hydration is a promising method toimprove long performance of CaO sorbents in cyclic CC reactions, which provide consultations for process designing in flue gases CO

2

capture and zero emissions systems.

The experimental investigation on gasification characteristics of natural coke from Peicheng, Jiangsu with steam were conducted in a fluidized bed gasifier setup. The effects of several parameters, in terms of the catalyst type, the catalyst mixed manner and the dosage of catalyst over coke on the yield, the components, the heating value of fuel gas and the carbon conversion rate were examined. Results indicate that the fluidized bed gasification technology could overcome the shortcomings of natural coke. Ca-, Fe- and Cu-based nitrates could improve the gasification reaction effectively with a little difference, they could be listed in a descending sequence as follows: Cu-based>Fe-based>Ca-based according to their catalytic effect. The influences of Fe/Ca ratio and Cu/Ca ratio on gasification are similar, gas yield, carbon conversion rate and gas heating value per hour increase as Fe/Ca ratio or Cu/Ca ratio increases, but all of them go up first and then drop with decrease in Fe/Cu ratio. When the dosage of Ca-, Fe- and Cu-based nitrates mixed with the ratio of Ca/Fe/Cu= 10/35/55 is 3%, the best catalytic effect is achieved.

The nickel-based catalyst was exposed to the raw gas from gasification of woody biomass with air in a fluidized-bed. After dust removal on a barrier filter and sulphur compounds capture, namely H2S, on an active sorbent made of CuO and ZnO, higher hydrocarbons as tar components were decomposed/reformed on aNi-catalyst. Steam reforming reactions led to decomposition of tar and all hydrocarbons higher than CH

4

into mainly H

2

and CO which further underwent reaction with steam via the water gas shift reaction to CO

2

. The reforming reactions caused approximately 10–20 % decrease in the lower heating values of the producer gas from the inlet values 5.0–6.5 MJ m

−3

. The gas yield increased fromvalues 2.4–2.6 m

3

kg

−1

to values 2.8–3.0 m

3

kg

−1

on dry biomass basis. The chosen tar removal concept based on combination of dolomite in the fluidized-bed with the secondary catalytic reactor was proved by 20 hours long experiment in which the finaltar content below 30 mg m

−3

was attained corresponding to more than 97 % tar conversion. H2S content in producer gas was expected to be below 100 vol. ppm, bulk of which was captured on the sorbent. Only limited deactivation of thecatalyst by sulphur compounds was found in the front of the catalyst bed where sulphur content was determined as high as 173 wt. ppm compared to 22 wt. ppm in the fresh sample.

Biomass gasification in an Internally Circulating Fluidized-bed Gasifier (ICFG) using Ni/Ah03 as tar cracking catalyst is studied at low temperature. Reaction conditions of the catalyst bed are discussed, including catalytic temperature and steam ratio. High energy efficiency and hydrogen-rich, low-tar product gas can be achieved in a properly designed multi-stage gasification process, together with high-performance catalyst. In addition, considering the economical feasibility, a newly-developed Ni-loaded brown coal char is developed and evaluated as catalyst in a lab-scale fluidized bed gasifier with catalyst fixed bed. The new catalyst shows a good ability and a hopeful prospect oftar decomposition, gas quality improvement and catalytic stability.

At Foster Wheeler, a three-dimensional CFB furnace model is essential part of knowledge development of CFB furnace process regarding solid mixing, combustion, emission formation and heat transfer. Results of laboratory and pilot scale phenomenon research are utilized in development of sub-models. Analyses of field-test results in industrial-scale CFB boilers including furnace profile measurements are simultaneously carried out with development of 3-dimensional process modeling, which provides a chain of knowledge that is utilized as feedback for phenomenon research. Knowledge gathered by model validation studies and up-to-date parameter databases are utilized in performance prediction and design development of CFB boiler furnaces. This paper reports recent development steps related to modeling of combustion and formation of char and volatiles of various fuel types in CFB conditions. Also a new model for predicting the formation of nitrogen oxides is presented. Validation of mixing and combustion parameters for solids and gases are based on test balances at several large-scale CFB boilers combusting coal, peat and bio-fuels. Field-tests including lateral and vertical furnace profile measurements and characterization of solid materials provides a window for characterization of fuel specific mixing and combustion behavior in CFB furnace at different loads and operation conditions. Measured horizontal gas profiles are projection of balance between fuel mixing and reactions at lower part of furnace and are used together with both lateral temperature profiles at bed and upper parts of furnace for determination of solid mixing and combustion model parameters. Modeling of char and volatile based formation of NO profiles is followed by analysis of oxidizing and reducing regions formed due lower furnace design and mixing characteristics of fuel and combustion airs effecting to formation ofNO furnace profile by reduction and volatile-nitrogen reactions. This paper presents CFB process analysis focused on combustion and NO profiles in pilot and industrial scale bituminous coal combustion.

A model for the behavior of biomass particles in the splashing zone of a bubbling bed has been developed. The model is intended for use in CFD studies of bubbling beds, where it provides a way of modeling particle dispersion in the splashing zone. In the model, particles landing on the bed surface are assumed to reenter the splashing zone. Two initial velocities are used for the reentering particles: one represents particles landing on bursting bubbles and one for the emulsion phase. The fraction of the bed consisting of bubbles is calculated using standard expressions from the literature. The re-entering velocity of fuel particles from the bubbles is set such that the flight trajectory reaches the typical height of the splashing zone. The velocity from the emulsion phase is assumed to be of the order of the fluidization velocity. In both cases the initial direction of the trajectory is allowed to take random values. Using these simple assumptions an approximation of the logarithmic material distribution in the splashing zone is achieved.

A hydrodynamic model with binary particle diameters was developed to better predict axial voidage profile in a CFB combustor. In the model, the CFB is regarded as a superposition of two sub-beds, a fast fluidized bed in the upper riser with a characteristic particle diameter of O.2mm and a bubbling fluidized bed or turbulent bed in the bottom riser with a characteristic particle diameter of 2mm. Furthermore, a variable critical particle diameter whose terminal velocity equals to the superficial gas velocity was employed to determine which flow regime the particle belongs to. The results show that binary particle diameter model has the advantages in describing wide particle diameter distribution while reducing the complexity of computation. The model was verified by the field data of voidage profile in a 300MW CFB boiler.

With increasing size of modern CFB combustors the lateral mixing of fuels and secondary air gains more and more importance. Strong concentration gradients, which result from improper lateral mixing, can lead to operational problems, high flue gas emissions and lower boiler efficiencies. A 3D-model for the simulation of local gas and solids concentrations inside industrial-sized CFB boilers has been developed. The model is based on a macroscopic approach and considers all major mechanisms during fuel spreading and subsequent combustion of char and volatiles. Typical characteristics of modern boilers like staged combustion, a smaller cross-sectional area in the lower section of the combustion chamber and the co-combustion of additional fuels with coal can be considered. The 252 MWth combustor of Stadtwerke Duisburg AG is used for the validation of the model. A comprehensive picture of the local conditions inside the combustion chamber is achieved by the combination of local gas measurements and the three-dimensional simulation of concentration distributions.

Circulating fluidized bed combustors (CFBC) of industrial scale have sometinIes diameters of more than 10m and heights exceeding 40 m. Depending on the intensity of the reactions and the location of the insertion of reactants into the bed, temperature effects can not be neglected. A simulation tool for the investigation of these large-scale systems including sub-models for fluid dynamics, dispersion, reactions, mass balances and an enthalpy balance has been implemented. The simulation results provide three-dimensional distributions of temperatures inside the combustion chamber. The simulation results have been validated with results of local measurements in a 235 MWe CFBC of Elektrownia Turow in Poland.

The water wall heat transfer and heat flux distribution in a 600MWe supercritical CFB boiler with water-cooled panel in the furnace was numerically studied. The water wall was made of smooth tube membrane. The solid suspension density ρ, heat flux

q

and heat transfer coefficient

K

distribution in the furnace were predicted at rated boiler load (100%BMCR), 75% of turbine heat acceptance load (75% THA) and 50% THA. The results show that for a large-scale CFB boiler, the convection is the main part of heat transfer in the lower furnace, and radiation is the main in the upper lower. The ρ,

q

and

K

have the similar axial and radial distributions. Their radial distributions depend on the position of the water wall and boiler load, and show a peak value in the corner. The ρ is a sensitive to

K.

With increasing of the height in furnace, ρ decreases, thereby the

q

and

K

decrease. The radial distributions of

q

and

K

are similar at different height. The study shows that the three-dimensional model is valid to predict the heat transfer in the furnace of the 600MWe supercritical CFB boiler.

With the currently available methods of computational fluid dynamics (CFD), the task of simulating full scale circulating fluidized bed combustors is very challenging. In order to simulate the complex fluidization process, the size of calculation cells should be small and the calculation should be transient with small time step size. For full scale systems, these requirements lead to very large meshes and very long calculation times, so that the simulation in practice is difficult. This study investigates the requirements of cell size and the time step size for accurate simulations, and the filtering effects caused by coarser mesh and longer time step. A modeling study of a full scale CFB furnace is presented and the model results are compared with experimental data.

Computational fluid dynamics (CFD) gains popularity in fluidized bed modeling. For model validation, there is a need of detailed measurements under well-defined conditions. In the present study, experiments were carried out in a 40 em wide and 3 m high 2D circulating fluidized bed. Two experiments were simulated by means of the Eulerian multiphase models of the Fluent CFD software. The vertical pressure and solids volume fraction profiles and the solids circulation rate obtained from the simulation were compared to the experimental results. In addition, lateral volume fraction profiles could be compared. The simulated CFB flow patterns and the profiles obtained from simulations were in general in a good agreement with the experimental results.

In this study, mono-disperse flows in squared risers conducted with A and B-type particles were simulated by Eulerian n-fluid 3D unsteady code. Two transport equations developed in the frame of kinetic theory of granular media supplemented by the interstitial fluid effect and the interaction with the turbulence (Balzer et al., 1996) are resolved to model the effect of velocity fluctuations and inter-particle collisions on the dispersed phase hydrodynamic. The studied flow geometries are three-dimensional vertical cold channels excluding cyclone, tampon and returning pipe of a typical circulating fluidized bed. For both type of particles, parametric studies were carried out to determine influences of boundary conditions, physical parameters and turbulence modeling. The grid dependency was analyzed with mesh refinement in horizontal and axial directions. For B-type particles, the results are in good qualitative agreement with the experiments and numerical predictions are slightly improved by the mesh refinement. On the contrary, the simulations with A-type particles show a less satisfactory agreement with available measurements and are highly sensitive to mesh refinement. Further studies are carried out to improve the A-type particles by modeling subgrid-scale effects in the frame of large-eddy simulation approach.

A numerical algorithm is developed for a detailed 3D simulation of the gas/particle flow behavior used for drying of sludge under flue gas atmosphere in the riser section of a circulating fluidized bed. The gas phase is described with standard κ − ε turbulence model, whereas a Lagrangian formulation with a stochastic particle dispersion model is adopted for the particulate phase. Conservation equations of mass and momentum for each phase were solved using the volume numerical technique. Fluid-particle interaction is taken into account to calculate the mass, momentum, and heat transfer between phases. The numerical algorithm is used to predict the circulating fluidized bed performance under various inlet profiles of the flue gas velocity. Gas and particle flow profiles were obtained for velocity and temperature parameters for each phase. The influence of the flue gas inlet velocity and the sludge mass flow rate on drying is discussed.

When a CFB boiler is in automatic control, there are strong interactions between various process variables and inverse response characteristics of bed temperature control target. Conventional Pill control strategy cannot deliver satisfactory control demand. Kalman wave filter technology is used to establish a non-linear combustion model, based on the CFB combustion characteristics of bed fuel inventory, heating values, bed lime inventory and consumption. CFB advanced combustion control utilizes multivariable model predictive control technology to optimize primary and secondary air flow, bed temperature, air flow, fuel flow and heat flux. In addition to providing advanced combustion control to 2×310t/h CFB+1×100MW extraction condensing turbine generator unit, the control also provides load allocation optimization and advanced control for main steam pressure, combustion and temperature. After the successful implementation, under 10% load change, main steam pressure varied less than ±0.07MPa, temperature less than ±1°C, bed temperature less than ±4°C, and air flow (O

2

) less than ±0.4%.

Severe post combustion in the cyclone of CFB boilers could destroy heat absorbing balance among the heating surfaces and cause overheating problem for reheaters and superheaters. However, post combustion in the cyclone is rarely considered in the design phase of a CFB boiler. Based on our previous experiment results, group combustion model is used in this study to estimate the combustion of particles in the cyclone. It is found that the combustion of particles in the cyclone did not contribute as much as we anticipated to the temperature augment in the cyclone because of great oxygen diffusion resistance in near-wall particle layer. Post combustion model in the cyclone is then added into a one-dimensional combustion model of CFB boiler, in which the gas-solid flow, reaction, and heat absorption at different vertical locations in a CFB boiler can be well predicted with the knowledge of operation parameters. The new model was used to estimate the influence of some operation parameters on the post combustion in the cyclone and heat releasing fraction in the cyclone. The prediction results are very good.

The dimension of the hot circulation loop of the supercritical CFB boiler is large, and there are many unknowns and challenges that should be identified and resolved during the development. In order to realize a reasonable and reliable design of the hot circulation loop, numerical simulation of gas-solid flow in a supercritical CFB boiler was conducted by using FLUENT software. The working condition of hot circulation loop flow field, gas-solid flow affected by three unsymmetrical cyclones, air distribution and pressure drop in furnace were analyzed. The simulation results showed that the general arrangement of the 600MWe supercritical CFB boiler is reasonable.

A circulating fluidized bed (CFB) two-stage simulation model was developed. To realize the model results coincident with the design value or real operation value at specified multi-working conditions and with capability of real-time calculation, only the main key processes were taken into account and the dominant factors were further abstracted out of these key processes. The simulation results showed a sound accordance at multi-working conditions, and confirmed the advantage of the two-stage model over the original single-stage simulation model. The combustion-support effect of secondary air was investigated using the two-stage model. This model provides a solid platform for investigating the pant-leg structured CFB furnace, which is now under design for a supercritical power plant.

As industrial Circulating Fluidized bed Combustors (CFBCs) tend to be scaled up, numerous design and operating problems emerge. At the same time uncertainties which concern hydrodynamics, combustion and pollutants formation mechanisms, come in to sight. Along with experience, CFD analysis can play crucial role providing further insight on the complex multiphase combusting flow occurring in CFBCs. This work aims to present a methodology for CFBCs comprehensive modeling, taking into consideration the coupling of hydrodynamics — heat transfer — chemical phenomena that take place in the bed. A combination of acceptable accuracy with high computational efficiency was also an objective. For this purpose, a simple combustion mechanism was integrated in an isothermal model and applied on a 1.2 MWth pilot plant. In this comprehensive model gas, inert-material and fuel are taken into consideration, as three discrete, pure eulerian phases. Solids inventory in the riser as well as temperature of the bed were predicted with satisfactory accuracy. Moreover, major chemical components as O

2

and CO

2

concentrations were predicted along the bed with acceptable accuracy. Concluding, the developed CFD model is capable of efficiently modeling a CFBC. However in order to further increase total accuracy, the need for improved closure equations for the set ofPartial Differential Equations solved was made obvious. Finally, the computational cost for such modeling was found extremely high but not prohibitive for large scale CFBC simulations.

A model for gas phase mixing in fluidized bed boiler furnaces is presented. The model takes its basis in a description of the dynamics of the dense bottom bed which strongly govern the gas mixing up through the furnace. Thus, a time-resolved approach is used to link the modeling to the physics of the underlying processes determining the gas mixing. As output, the model gives the fluctuating flux of gas species, in contrast to the classical modeling approach which is limited to time-averaged gas fluxes. Such a dynamical approach allows assumption of the volatile combustion system as transport-controlled which avoids complete consumption of either oxygen or combustible gases in each modeled cell. Thus, the time-resolved analysis employed enables application of a realistic criterion for the mixing such as that reactants can coincide in both space and time in order to react. While fitting of kinetics is strongly dependent on the system and operational conditions, the present model integrates key system variables such as the bottom bed height and the characteristic pressure-drop constant over the primary air distributor, allowing application of transport-controlled (i.e. infinitely fast kinetics) volatile combustion.

The model of the bottom bed divides the gas flow into two phases, a throughflow and an emulsion gas, and calculates their respective fluctuations in velocity and composition. Having these, in-furnace gas probe measurements can be simulated and compared with in-situ gas suction probe measurements,

i.e.

the probe signal is modeled. Such an approach is crucial in fluidized bed-boilers furnaces since in-situ gas probe measurements in regions with high fluctuations in gas velocity lead to results which are biased towards reducing conditions.

Model results, including simulation of the gas suction probe, are analyzed and compared with experimental data from the Chalmers 12 MW

th

CFB boiler and a good agreement is obtained.

The capability of Fluent 6.2.16 to simulate particle mixtures in a laboratory scale 2D circulating fluidized bed (CFB) unit has been tested. In the simulations, the solids were described as one or two particle phases. The loading ratio of small to large particles, particle diameters and the gas inflow velocity were varied. The 40 cm wide and 3 m high 2D CFB was modeled using a grid with 31080 cells. The outflow of particles at the top of the CFB was monitored and emanated particles were fed back to the riser through a return duct. The paper presents the segregation patterns of the particle phases obtained from the simulations. When the fraction of large particles was 50% or larger, large particles segregated, as expected, to the wall regions and to the bottom part of the riser. However, when the fraction of large particles was 10%, an excess of large particles was found in the upper half of the riser. The explanation for this unexpected phenomenon was found in the distribution of the large particles between the slow clusters and the faster moving lean suspension.

Rice is Cultivated in all the main regions of world. The worldwide annual rice production could be 666million tons (www.monstersandcritics.com,2008) for year 2008. The annual production of rice husk is 133.2 million tons considering rice husk being 20% of total paddy production. The average annual energy potential is 1.998 *10

12

MJ of rice husk considering 15MJ/kg of rice husk. India has vast resource of rice husk; a renewable source of fuel, which if used effectively would reduce the rate of depletion of fossil energy resources. As a result a new thrust on research and development in boilers bases on rice husk is given to commercialize the concept. CFD is the analysis of systems involving fluid flow, heat transfer and associated phenomena such as chemical reactions by means of computer-based simulation. High quality Computational Fluid dynamics (CFD) is an effective engineering tool for Power Engineering Industry. It can determine detailed flow distributions, temperatures, and pollutant concentrations with excellent accuracy, and without excessive effort by the software user. In the other words it is the science of predicting fluid flow, heat and mass transfer, chemical reactions and related phenomena; and an innovate strategy to conform to regulations and yet stay ahead in today’s competitive power market. This paper is divided into two parts; in first part review of CFD applied to the various types of boilers based on biomass fuels/alternative fuels is presented. In second part CFD analysis of fluidized bed boilers based on rice husk considering the rice husk based furnace has been discussed. The eulerian multiphase model has used for fluidized bed. Fluidized bed has been modeled using Fluent 6.2 commercial code. The effect of numerical influence of bed superheater tubes has also been discussed.

Hydrodynamic simulation of 2-D gas-solid bubbling fluidized bed containing staggered horizontal tubes was performed. The bubble hydrodynamics, bubble diameter and bubble rise velocity, were investigated and compared with experimental results elsewhere in the literature (

Hull et al., 1999

, Influence of Horizontal tube Banks on the Bubbling and Solids Mixing Behavior of Fluidized Beds. 15th Int. FBC Conference). The Eulerian-Eulerian Two Fluid Model (TFM) implemented in Fluent, version 6.3, was used for the governing equations with closure equations based on the Kinetic Theory of Granular Flow (KTGF). The numerical simulation showed that the horizontal tubes were the main source of bubble break up where bubbles break when they interact with the tubes and grew by coalescence until they reach the next row of tubes. Quantitative investigation of the bubble hydrodynamics also revealed that the predicted average bubble diameter and bubble rise velocity were in good agreement with the experimental results reported in the literature. It was observed that there were small bubbles formed on the lower-half part of the tubes which were usually interacted with an incoming bubble from below and left the tube after coalescence. As a result, the numerical simulation predicted a lower average bubble diameter and bubble rise velocity at the bottom of the tube banks than that reported in the literature.

Two-phase systems where a dense phase of small particles is fluidized with a gas flow appear in many industrial applications, among which the fluidized bed combustors are probably the most important. A homogenization technique allows us to formulate the mathematical model in form of the compressible Navier-Stokes system type with some particularities: 1) the volumetric fraction of the dense phase (analogous to the density in the Navier-Stokes equations) may vanish, 2) the constitutive viscosity law may depend in a nonlinear form on this density, 3) the source term is nonlinear and coupled with state equations involving drag forces and hydrodynamic pressure, and 4) the state equation for the collision pressure of dense phase blows up for finite values of the density. We develop a rigorous theory for a special kind of solutions we call

stationary clouds

. Such solutions exist only under restrictions on the geometry of combustor and on the boundary conditions that usually meet in engineering applications. In return, these solutions have a stationary one-dimensional structure very simple and, from them, it is possible to reconstruct much of the dynamics of the whole system, responding to most of the practical issues of interest. Finally, we study the linear stability for the trivial solutions corresponding to uniform fluidized states injecting plane wave perturbations in our equations. Depending on the parameters of the equations of state describing the collisions between solid particles, hydrodynamic pressure, and the values of blowing boundary condition, we can draw detailed abacus separating stable regions of unstable regions where bubbles appear. Then, we use the dispersion relations of this multidimensional linearized model, combined with the stationary phase theorem, to approach the profiles and the evolution of the bubbles appearing in unstable regimes, and verify that the obtained results adjust to the observations.

A new type of sewage sludge incinerator that combines a pressurized fluidized bed combustor and a turbocharger driven by flue gas was proposed. This plant has four main advantages. (l) The combustion rate can be improved since the oxygen partial pressure in the combustor is increased by the pressurization. The incinerator volume can be substantially smaller than that of an atmospheric incinerator with the same incineration capacity. Thus, the amount of supplementary fuel can be reduced because the heat loss from the incinerator can be decreased. (2) Because the maximum operating pressure is O.3 MPa (absolute pressure), which matches the pressure required for the turbocharger, a pressure vessel is unnecessary. (3) An energy savings of more than 40% can be achieved compared with a conventional plant since the FDF and the IDF are unnecessary. (4) Because steam in the flue gas becomes the working fluid of the turbocharger, the turbocharger can generate surplus air in addition to the combustion air. The surplus air can be used in other processes, such as aeration, in sewage works. We constructed a demonstration plant (4.32 tid scale) at a sewage works in Japan. The operation and combustion characteristics of the plant were clarified, and the design data of a commercial plant was obtained.

As a result, the steady operation exceeded 600 hours in total. Consequently, about 40 % of CO

2

emissions originating from electric power consumption and supplementary fuel is expected to be reduced annually compared with emissions from a conventional plant at an incineration capacity of 100 tons/d. CO, NO

x

and N

2

O emissions in the flue gas were less than half those of a conventional plant due to the effect of the pressurization. Therefore, this proposed incinerator can realize energy recovery and saving as well as a low environmental impact.

Oxy-fuel circulating fluidized bed (CFB) combustion technology is in the stage of initial development for carbon capture and storage (CCS). Numerical simulation is helpful to better understanding the combustion process and will be significant for CFB scale-up. In this paper, a computational fluid dynamics (CFD) model was employed to simulate the hydrodynamics of gas-solid flow in a CFB riser based on the Eulerian-Granular multiphase model. The cold model predicted the main features of the complex gas-solid flow, including the cluster formation of the solid phase along the walls, the flow structure of up-flow in the core and downward flow in the annular region. Furthermore, coal devolatilization, char combustion and heat transfer were considered by coupling semi-empirical sub-models with CFD model to establish a comprehensive model. The gas compositions and temperature profiles were predicted and the outflow gas fractions are validated with the experimental data in air combustion. With the experimentally validated model being applied, the concentration and temperature distributions in O

2

/CO

2

combustion were predicted. The model is useful for the further development of a comprehensive model including more sub-models, such as pollutant emissions, and better understanding the combustion process in furnace.

World tendencies in environmental protection points out necessity of reduction of CO

2

emission to atmosphere. The one of the main sources of CO

2

emission is placed in energy sector where electric energy and heat are produced based on fossil fuels combustion. Therefore, it seems to be necessary to perform research on CO

2

emission reduction in this sector. The main aim of work presented in this paper was focused on the analysis and assessment of CO

2

separation from flue gases on the total efficiency of conventional power station. The paper shows the numerical calculations performed with IPSEpro simulation software by SimTech.For the CO

2

separation the PTSA (pressure-Temperature Swing Adsorption) process was chosen and the numerical as well as simulation model of such process was formulated. The calculations were made for few different adsorbents taking into account varying values of such thermodynamic parameters of separation process like temperature or pressure. Results obtained from calculations point out that mixed PTSA technology is not very energy consuming process. Owing to utilisation of waste heat for sorbent regeneration, it does not decrease the total efficiency for more than 0.6%. However, that is caused by separation only, while after that CO

2

must be compressed for further treatment.

The performance of a fluidized bed catalytic reactor was investigated for NO

x

reduction using a Fe/ZSM-5 catalyst and propylene as the reducing agent. The effects of inlet NO

x

concentration, propylene to NO

x

molar ratio, flue gas oxygen concentration and fluidizing gas velocity on NO

x

conversion were studied using simulated flue gases. The results showed that the NO

x

conversion decreased with decreasing the inlet NO

x

concentration, and propylene to NO

x

molar ratio. The increase in flue gas oxygen concentration also imposed a significant negative impact. As the flue gas oxygen concentration increased, NO

x

reduction decreased while propylene conversion increased at a given propylene to NO

x

molar ratio, due to the oxidation of propylene in the presence of high oxygen concentration. The increase in fluidization gas velocity reduced the contact time between reactant and the catalyst particles due to the increased bubble size and bubble rise velocity. As a result, the NO

x

reduction decreased as the gas velocity increased.

Municipal sewage sludge has proven to eliminate alkali metals and chlorine related problems during combustion of straw and refuse derived fuels (RDF). However, the mechanisms involved have not been clarified. The aim of this work was to gain more knowledge about the behaviour of sewage sludge and detergent zeolites in combustion and about their effects on alkali metal chemistry.

Co-combustion tests with combinations of municipal sewage sludge, wood and straw were carried out in a 12 MW fluidised bed (FB) boiler. In addition, a detergent zeolite, Doucil A24, was used as additive during co-combustion of wood and straw. The chemical characteristics of fuels and fly ashes were studied using several methods, such as chemical fractionation and scanning electron microscopy with element analysis by energy dispersive fluorescence detection (SEM-EDX) and X-ray diffraction (XRD).

In the co-combustion tests involving sewage sludge no KCl was found in the flue gas prior to the convection pass. The zeolite addition was less effective but the KCl concentration was reduced to some degree in favour of HCl compared to the reference case. Both SEM-EDX and XRD confirmed the presence of potassium-aluminium-silicates in the fly ash fraction in all cases. In addition, the laboratory study showed that Doucil A24 had the ability to capture potassium and KCl at temperatures in the range of 700–900°C.

The co-firing of biomass and fossil fuel in the same power plant is one of the most important issues when promoting the utilization of renewable energy in the world. Recently, the co-firing of coal together with biomass fuel, such as “densified refuse derived fuel” (d-RDF or RDF-5) or RPF (refuse paper & plastic fuel) from waste, has been considered as an environmentally sound and economical approach to both waste remediation and energy production in the world. Because of itscomplex characteristics when compared to fossil fuel, potential problems, such as combustion system stability, the corrosion of heat transfer tubes, the qualities of the ash, and the emissionof pollutants, are major concerns when co-firing the biomass fuel with fossil fuel in a traditional boiler. In this study, co-firing of coal with RDF-5 was conducted in a 22t/h bubbling fluidized bed (BFB) steam boiler to investigate the feasibility of utilizing RDF-5 as a sustainable fuels in a commercial coal-fired steam BFB boiler. The properties of the fly ash, bottom ash, and the emission of pollutants are analyzed and discussed in this study.

An experimental study on co-combustion of sludge and coal were conducted in a circulating fluidized bed incinerator with the dense bed cross section area of 0.23m×0.23m and the height of 7m. The mercury mass balance was measured and the distribution of mercury speciation in flue gas was discussed. Effects of major operational parameters such as Ca/S molar ratio, desulfurization sorbents, excess air coefficient, co-combustion temperature, and SO

2

and NO

x

concentrations on the distribution of mercury speciation in flue gas, mercury in fly ash and slag were investigated during the co-combustion process. The results show that majority of mercury goesinto the fluegas in which the elemental mercury is the major speciation. Ca-based sorbent can remove Hg

2+

in flue gas effectively, in which CaO has better mercury removal effect than CaCO

3

. The content of Hg

2+

in flue gas increases with increasing of the concentration of SO

2

and NO

x

in flue gas. It can also be concluded that the excessair coefficient exerts dominant influences on mercury speciation among the flue gas, fly ash and bottom ash.

It has been shown that addition of either sulfur and/or aluminosilicates such as kaolinite may reduce alkali induced deposit formation when firing biomass fuels. Sewage sludge is a fuel containing substantial amounts of sulfur and aluminosilicates, such as zeolites. In this work different amounts of sewage sludge (0, 2, 4, 6 and 8%

en

) were co-fired with bark in a bench-scale BFB. SO

2

and HCl emissions were measured and deposits were sampled during 3 hrs with an air-cooled probe with a surface temperature of 500°C at two different locations with flue gas temperatures of 850°C and 650°C, respectively. The test results showed that an increase of the share of sewage sludge to the fuel mixture increased theformation of HCl and simultaneously decreased the Cl-content in the deposits. Usually this is considered to be a sign of sulfation of alkali chlorides. However, the increase of HCl canalso be caused by AI-silicates capturing alkali, thus releasing Cl as HCl to the gas phase. AIthough, sulfur increased in the fuel input with an increased share of sewage sludge, this was not reflected in the gaseous emissions as may be expected. Up to 4%

en

sewage sludge was fired together with bark without increasing the sulfur content in theemissions. At higher shares of sewage sludge the sulfur emissions increased linearly with an increase of sewage sludge. The amount of water soluble potassium fed into the boiler remained relatively constant in the different tests. This potassium is usually released as volatile salts. Nevertheless, the amount found in deposits decreased with an increase in sludge feeding. In this paper it was shown that interaction of potassium with AI-silicates in the bed is a probable cause for the decrease of potassium in the deposits, while both the sulfation of potassium chlorides and possibly also, the alkali capture by AI-silicates can weaken the deposition of Cl.

Gasification of wastes may come out as an alternative technology to produce a gas with many potential applications, from direct burning in a boiler or motor to the production of synthetic chemicals and hydrogen. High tar production and high operational costs are preventing gasification wider dissemination. Besides these problems, the presence of NH

3

in the syngas may have a negative impact as it can be converted into nitrogen oxides if the gas is further burnt. To reduce NH

3

formation it is required a full understanding of how operational parameters contribute to the formation/reduction of this pollutant. A full studyon the effect of fuel composition, temperature and equivalence ratio on the formation of NH

3

is given. Experimental results are compared to theoretical ones obtained with FactSage software. It is also analyzed the effect of feedstock mineral matterin NH

3

release during gasification. Toaccomplish a significant decrease in the release of NH

3

, different catalysts and sorbents were tested with the aim of achieving high energy conversions and low environmental impact.

Anthropogenic CO

2

production is caused primarily by fossil fuel combustion. In consequence, it is increasingly necessary to find ways to reduce these emissions when fossil fuel is used. CO

2

capture and storage (CCS) appears to be among the most promising. All of the CCS technologies involve producing a pure stream of CO

2

either by concentrating it from the flue gases, or by using pure oxygen as the combustion gas. The latter option, oxy-fuel combustion, has now been well studied for pulverized coal combustion, but hasreceived relatively little attention to date in the case of oxy-fuel circulating fluidized bed combustion. Recently, oxy-fuel CFBC hasbeen examined ina 100 kW pilot plant operating with flue gas recycle at CanmetEnergy. The results strongly support the view that this technology offers all of the advantages of air-fired FBC, with one possible exception. Emissions such as CO or NO

x

are lower or comparable to air firing. It is possible to switch from air-firing to oxy-firing mode easily, with oxygen concentrations as high as 60–70%, and flue gas recycle levels of 50–60%. Only sulphur capture is poorer. However, this result is not in good agreement with other studies, and the reasons for this discrepancy need further exploration. Here, longer tests have confirmed previous findings from CanmetEnergy with two coals and a petroleum coke. It also appears that changing from direct to indirect sulphation with the petroleum coke improves sulphur capture efficiency, although a similar effect could not be confirmed with coal from these results.

The experiments of the fly ash recycle combustion using Guizhou anthracite were carried out in a bench scale circulating fluidized bed (CFB) combustor. Effects of some key operating parameters such as recycle ash to coal mass ratio (Ca to S molar ratio), temperature, reactivation modeof fly ash, circulation rateand fluidization velocity on the desulfurization efficiency were intensively investigated. It is shown that thelimestone utilization efficiency could be improved about 30% with the following operating conditions: the mass ratio of fly ash (reactivated by water and dried at 90°C) to coal was 0.45, the furnace temperature was 880°C, the water to ash mass ratio was 4.5% (the water-to-calcium molar ratio was 0.55) and circulation rate was 18.

The de-NO

x

potential of coal and of dried and pelletized sewage sludge, a waste-derived fuel candidate for cofiring with coal, is assessed. The experimental procedure is based on operation of a bench scale fluidized bed reactor where NO-doped nitrogen is contacted with batches of the fuel. A second type of experiment has been purposely designed to assess the loss of reactivity of chars toward gasification by NO

x

as char is heat-treated for pre-set times at temperatures typical of fluidized bed combustion. A simple phenomenological model is developed to shed light on the basic features of the interaction between heterogeneous char-NO

x

reaction and thermal annealing of the char.

Since under fluidized bed conditions N

2

O is produced as a by product of the De-NO

x

process, the thermal decomposition of N

2

O was investigated under conditions relevant to those in FBC installations. Laboratory experiments were made in a current of nitrogen using a fixed bed of pure quartz sand or sand with 10% (wt.) of the solids tested, CaO and Fe

2

O

3

. With a sand bed the decomposition was slightly faster than in the empty reactor and the reaction was first order with respect to [N

2

O]. Both fresh CaO and Fe

2

O

3

strongly catalysed N

2

O decomposition. Their effectiveness diminished after they were heated to temperatures typical for FBC, but they still retained appreciable activity. This activity went down with increasing particle size. The flue gas components investigated were O

2

, water vapour and CO

2

. Their presence appeared to interfere with N

2

O decomposition and increased with the concentration of the additive. The observations indicated that this could only be due to heterogeneous effects. Thus the effects of the bed solids and of the gas phase components are opposed. The effects associated with N

2

O decomposition have proved to be surprisingly complex and instead of supplying simple answers, this work uncovered more problems.

SO

2

emission from coal combustion is the important problem in many countries. This paper aims to evaluate the possibility of simultaneous reduction of SO

x

and fine ash particles during combustion of coal added with inorganic Ca-containing sorbent and organic Mg-containing sorbent in the fluidized bed conditions. Compared to addition of limestone particles to coal, the use of these sorbents produces the ultra-fine active oxides in the coal/char at higher temperature. The formed ultra-fine active oxides provide larger reaction surface area for the S and chemical sorbents, and, therefore, the high desulfurization efficiency will be expected in the fluidized bed coal combustion. In addition, the addition of chemical additives can affect the mineral transformation process during combustion. The results indicate that at certain temperature, higher sulfur removal efficiency can be obtained for selected Ca- and Mg-rich sorbents than those of natural limestone under fluidized bed combustion conditions. It is mainly due to the fine dispersion of Ca and Mg in impregnated coal so that a good is obtained between calcium and sulfur-containing coal particles. The addition of additives has a visible impact on the particle size distribution and chemical composition of the PM, wherein, it improves the degree of coalescence of sub-micron and fine mineral particles, which reduces PM

2.5

emissions. For the selected coal, the effect on the reduction of PM

2.5

emissions strongly depends on the addition and the type of sorbent being used.

Attrition and fragmentation of limestone during FB combustion of sulphur-bearing fuels have a profound influence on sorbent inventory and particle size distribution establishing at steady state in the bed. A population balance model is presented aiming at the prediction of the inventory and of the particle size distribution of sorbent particles establishing at steady state in the bed of an air-blown CFBC. The core of the model is represented by a population balance equation on sorbent particles which embodies terms expressing the extent/rate of each attrition/fragmentation process. The effect of the progress of sulphation on attrition/fragmentation is also taken into account. Constitutive equations needed to quantify attrition/fragmentation are developed on the basis of published data. Model results are presented and discussed with the aim of clarifying the influence of particle attrition/fragmentation on sorbent inventory and particle size distribution in a CFBC and on the closely related variables. Research needs and priorities within the specific field of investigation are also discussed.

The extent and mechanism of the sulphation and carbonation of a limestone, dolomite and chalk, have been compared with a novel, synthetic sorbent (85 wt% CaO and 15 wt% Ca

12

Al

14

O

33

), from experiments in a small, electrically-heated fluidised bed. The sorbent particles were either (i) untreated, but then sieved into two particle sizes and reacted with SO

2

of two different concentrations, or (ii) cycled 20 times between (a) carbonation in 14 vol.% CO

2

in N

2

, and (b) calcination, in pure N

2

, at 750°C. The uptake of SO

2

by untreated limestone and dolomite was generally low (<0.2 g

SO

2

/g

sorbent

) and dependent on particle size, confirming previous results. In comparison with limestone and dolomite, the untreated chalk and the synthetic sorbent were found to be substantially more reactive with SO

2

; their final uptake was significantly higher (> 0.5 g

SO

2

/g

sorbent

) and essentially independent of the particle size. Hg-intrusion porosimetry, performed on calcined sorbents, revealed that the volume inside the pores of limestone and dolomite was entirely in small pores (<200 nm dia.), confirmed by EDAX analysis. The small pores were easily plugged, hindering the diffusion of SO

2

through the particle. On the other hand, calcined chalk and fresh synthetic sorbent possessed large volumes in wide pores (> 200 nm dia.); these bigger pores were not blocked by newly formed CaSO

4

. This allowed sulphation to proceed uniformly throughout the particle. It was also found that the uptake of SO

2

by limestone, dolomite and chalk was substantially lower when the particles had been subjected to cycles of calcination and carbonation in CO

2

prior to sulphation; this was attributed to a loss of volume inside the small pores during carbonation and calcination, confirmed by Hg-intrusion porosimetry. The uptake of SO

2

by the synthetic sorbent, on the other hand, was much closer to that achieved when it was used untreated, because large pores remained accessible after cycling.

Recent studies show that deposit formation and agglomeration in fluidized bed boilers may be aggravated by a high phosphorus content besides alkali metals, chlorine and sulphur in a fuel. This paper presents the fate of phosphorus during co-combustion of wood chips and wood pellets with rapeseed cake pellets, a high phosphorus fuel in a 12MW CFB boiler. 12 hour tests with 12% and 18% (energy basis) of rapeseed cake with wood were performed with and without limestone addition. All fuels were characterised by means of standard fuel analyses combined with chemical fractionation. Retrieved ash samples were analysed using wet chemical analysis complemented with SEMlEDXA. Gaseous alkali metal chlorides as well as HCI and SO

2

were measured upstream of the convective pass at a flue gas temperature of 800°C where also the deposit samples were collected with a deposit probe. The composition of deposits was studied with SEMlEDXA. Analyses of bed material particle cross-sections showed phosphorus compounds present within a K-silicates matrix between the agglomerated sand particles, indicating direct attack of gaseous potassium compounds on the bed surface followed by adhesion of rich in phosphorus ash particles. Build-up of the deposits took place mainly on the windward side of the probe; where up to 9 wt-% of phosphorus was present. SEMlEDXA shows that rapeseed cake addition caused an increase of K, Na besides P indicating presence of low melting phosphate salts in the deposits. During limestone addition in the deposit samples the increase of CI could be noticed however no significant change in P content was observed. This paper shows that agglomeration and fouling when co-firing rapeseed cake may be linked to its high content of organically bonded phosphorus — phytic acid salts; together with high content of water soluble fraction of alkali metals chlorides and sulphates in the fuel mixture.

CaO-based looping cycles for CO

2

capture at high temperatures are based on cyclical carbonation of CaO and regeneration of CaCO

3

. The main limitation of natural sorbents is the loss of carrying capacity with increasing numbers of reaction cycles, resulting in spent sorbent ballast. Use of spent sorbent from CO

2

looping cycles for SO

2

capture is a possible solution investigated in this study. Three limestones were investigated: Kelly Rock (Canada), La Blanca (Spain) and Katowice (Poland). Carbonation/calcination cycles were performed in a tube furnace with original limestones and samples thermally pretreated for different times (i.e., sintered). The spent sorbent samples were sulphated in a thermogravimetric analyzer. Changes in the resulting pore structure were then investigated using mercury porosimetry. Final conversions of both spent and pretreated sorbents after longer sulphation times were comparable or higher than those observed for the original sorbents. Maximum sulphation levels strongly depend on sorbent porosity and pore surface area. The shrinkage of sorbent particles during calcination/cycling resulted in a loss of sorbent porosity (≤48%), which corresponds to maximum sulphation levels ∼55% for spent Kelly Rock and Katowice. However, this is ∼10% higher than for the original samples. By contrast, La Blanca limestone had more pronounced particle shrinkage during pretreatment and cycling, leading to lower porosity, <35%, resulting in sulphation conversion of spent samples <30%, significantly lower than for the original sample (45%). These results showed that spent sorbent samples from CO

2

looping cycles can be used as sorbents for SO

2

retention if significant porosity loss does not occur during CO

2

reaction cycles. For spent Kelly Rock and Katowice samples final conversions are determined by the total pore volume available for the bulky CaSO

4

product.

In this work, a simplified description of combustion and nitrogen oxides chemistry was implemented in a 1.5D model framework with the aim to compare the results with ones earlier obtained with a detailed reaction scheme. The simplified chemistry was written using 12 chemical components. Heterogeneous chemistry is given by the same models as in the earlier work but the homogeneous and catalytic reactions have been altered. The models have been taken from the literature. The paper describes the numerical model with emphasis on the chemistry submodels. A simulation of combustion of bituminous coal in the Chalmers 12 MW boiler is conducted and the results are compared with the results obtained earlier with the detailed chemistry description. The results are also compared with measured O

2

, CO, NO and N

2

O profiles. The simplified reaction scheme produces equally good results as earlier obtained with the more elaborate chemistry description.

All the experiments were conducted in a pilot scale vortexing fluidized bed combustor (VFBC), an integration of circular freeboard and rectangular combustion chamber. The dimension of the freeboard is 0.75 m LD. and 4.6 m in height. The cross section of the combustion chamber is 0.8×0.4 m

2

and the height of the combustion chamber is 1.35 m. The secondary air injected nozzles were installed tangentially at the bottom of the freeboard. Coal was used as the fuel. Silica sand was employed as the bed material. Acetic acid was used as the reductant to reduce NO emissions. The operating conditions, such as the stoichiometric oxygen in the combustion chamber, the bed temperature and the injecting location of acetic acid, were determined by means of response surface methodology (RSM), which enables the examination of parameters with a moderate number of experiments. In RSM, the NO emission concentration at the exit of the VFBC is used as the objection function. A mathematical model for the NO emission as a function of the operating conditions was empirically proposed. The results show that NO emission increases with the stoichiometric oxygen in the combustion chamber and the bed temperature. NO emission can be decreased by injecting the acetic acid into the combustion chamber. The NO emission decreases with the height of the acetic acid injecting location above the distributor. Meanwhile, the bed temperature has more important effect on the NO emission than the other two factors.

A new approach to evaluate NO formation and reduction processes separately in fluidized bed coal combustion has been investigated. Sixteen different coals were burned in a lab-scale bubbling fluidized bed combustor with three different char loadings by changing the inlet oxygen concentration to 20 %, 15 % and 10 %. When O

2

consumption rate in the bed becomes zero, N conversion ratio to NO (

X

NO

) indicates a special value without contribution of NO reduction by char particles in the bed. We call it “ultimate NO conversion ratio (

X

NO

*

)”. This imaginary NO conversion ratio,

X

NO

*

, expresses an NO conversion of a coal particle which burns singly in a fluidized bed combustor. Since there are no other char particles in the bed,

X

NO

*

should express a coal’s original NO emission.

X

NO

*

can be determined by an extrapolation of a line, which is determined by a least-squares method, showing the relationship between

X

NO

and O

2

consumption rates for each tested coal. By this technique, the contribution of char particles to NO reduction and the original NO emission of the tested coals can be separated from each other.

X

NO

*

depended on the N content and OIN molar ratio. Effect of the bed temperature on the sensitivity of

X

NO

on char loading, which expresses the effect of NO reduction by char particles, was very different among the different coals. On the other hand, the contribution of char particles on N

2

O emission was affected little and experiments showed that a burning char particle cannot reduce N

2

O. This result suggests that the reduction by the char particles involves the competitive reaction of NO and N

2

O, and the effect of the N

2

O reduction by burning char particles was less than NO reduction. Consequently, experiments showed that the effect of NO reduction by char particles was very important in the fluidized bed combustion when final NO

x

emission is evaluated deeply.

The experimental study was carried out on the emission characteristics of nitrogen oxide in a 0.3MW circulating fluidized bed as well as analysis of the formation and destruction mechanism of nitrogen oxide. Several variables associated with the combustion system were investigated in the experiment, which include bed temperature, excess air, primary-to-secondary air ratio, and coal species. It is found that the NO

x

emissions decrease along the height of the furnace, while the N

2

O emissions increase continuously. More NO

x

emissions and less N

2

O emissions are generated as the bed temperature increases. It is also illustrated that an increase of air stoichiometry leads to a significant NO

x

increase and an obvious N

2

O increase. And air staging is proved to be an effective way to control the nitrogen oxide emissions in the fluid bed. Besides, it is also shown in the experiment that the concentration of NO

x

and N

2

O is higher during the combustion of anthracite than that during the combustion of bituminous.

Pyridine has been chosen as the nitrogenous model fuel to research the effect of metal oxide (CaO, MgO and Fe

2

O

3

) on the emission of nitrogen oxides in the range of fluidized bed combustion temperature. The experiment was presented in a fixed bed reactor with metal oxide layer, and Ff-IR gas analyzer was used to monitor the emission of N

2

O and NO. When there is no metal oxide loaded, results show that the maximum peak of N

2

O is at 725°C. Under the rich oxygen condition, NO increases monotonically with temperature increasing, however, under the lean oxygen condition there is a maximum peak of NO at 675°C, NO decreases to a minimum value at 725°C, and then increases slowly. With the addition of metal oxide, the loaded experimental results show that the addition will not change the general trend of nitrogen oxides, but it affects the emission in a certain extent and the effect has relation to temperature, oxygen atmosphere and metal oxide added. Under the lean oxygen condition, both CaO and Fe

2

O

3

inhibit the formation of N

2

O and NO in all temperature range; the effect of Fe

2

O

3

on NO is negligible above 900°C; MgO inhibits N

2

O at temperature higher than 725°C, and inhibits NO only from 625°C to 700°C. Under the rich oxygen condition, CaO and MgO decrease the N

2

O emission efficiently, but Fe

2

O

3

can only act at the temperature higher than 700°C. For the NO emission, CaO and MgO inhibit NO just above 775°C, however, Fe

2

O

3

promotes NO greatly especially below 700°C, and it even promotes NO

2

to 122.65ppm at 625°C. The results and discussion are helpful to demonstrate the effect of metal elements on the emission of nitrogen oxides in fluidized bed combustion.

Infraserv Hochst, located in the Industrial Park Hochst, has longstanding experience with the operation and development of large scale FBC plants.

In previous conferences we provided information about the “Reduction of NO

x

-Emissions by taking primary measures” and the “Debottlenecking of FBC furnaces with incineration by oxygen”.

We are now able to provide information on the preparatory processes we are involved in as operators of our newest IFBC plant with an incineration capacity of 700,000 tJa RDF (Refuse Derived Fuel) and 270 MW combustion heat output.

Limestone attrition is a major cause of loss of limestone during pressurized fluidized bed combustion. In the authors’ previous works, the analysis of published results of solid attrition and desulfurization was conducted to determine the attrition rate expression. The specific attrition rate (rate of decrease in diameter) was estimated to be second order with respect to particle diameter in the previous work. This rate expression implies that reduction of feed size of limestone is effective for suppression of loss of limestone by attrition. However, too much grinding of raw limestone will increase the content of fine particles that are readily elutriated by gas stream and do not contribute to the sulfur capture. In this work, modeling works are conducted for particle attrition and desulfurization in order to predict the effect of feed size of limestone on total consumption of limestone and desulfurization is discussed. Optimum particle size to suppress limestone consumption was approximately 0.7 mm (as

D

p50

). However, the control of solid drain rate from the bottom was found to have more influence on total limestone consumption rate. Emissions of SO

2

from low sulfur coal (S=0.33%) could be sufficiently low irrespective of limestone feed size but SO

2

emissions from coals with higher sulfur content than 0.5% were anticipated to increase drastically. Such drastic change in SO

2

emissions with the change in sulfur content is attributable to non-linear nature of reaction rate for attrition-enhanced desulfurization by limestone.

The present paper deals with the strategic field of production of clean fuels with very low to zero emissions. A two stage fluidized bed process for catalytic decomposition of methane has been investigated. Firstly, the fluidized bed has been operated for the thermo-catalytic decomposition (TCD) of methane to produce hydrogen and solid carbon, which deposited on the catalyst. Secondly, the carbon oxy-combustion has been carried out to regenerate the catalyst producing a separated CO

2

stream candidate to be directly fed to a sequestration unit. Experiments have been carried out in a laboratory scale bubbling fluidized bed reactor (26mm ill) using a home-made copper dispersed on γ-alumina as catalyst operated at 800°C. The carbon oxy-combustion regeneration strategy have been compared to the carbon combustion one on the basis of the efficiency of carbon removal and the performance ofregenerated catalyst with respect to the TCD process. The effect of multiple cycles of decomposition and regeneration steps has been also quantified. A reasonable cyclic process has been simulated switching between two different feeds, the first containing CH

4

and the second containing the regeneration stream. Experimental activity confirmed the possibility ofproducing a CO

2

stream that can be finalized to a sequestration unit but also indicated some drawbacks related to the oxy-combustion regeneration strategy which affect the production of CO

x

species during the methane decomposition stage.

Characterizations of ash deposits from co-firing/co-combusting of a woody biomass (i.e., white pine) and lignite coal were investigated in a fluidized-bed combustor using a custom designed air-cooled probe installed in the freeboard region of the reactor. Ash deposition behaviors on a heat transfer surface were comprehensively investigated and discussed under different conditions including fuel type, fuel blending ratios (20–80% biomass on a thermal basis), and moisture contents. For the combustion of 100% lignite, the compositions of the deposited ash were very similar to those of the fuel ash, while in the combustion of 100% white pine pellets or sawdust the deposited ash contained a much lower contents of CaO, SO

3

, K

2

O and P

2

O

5

compared with the fuel ash, but the deposited ash was enriched with SiO

2

, Al

2

O

3

and MgO. A small addition of white pine (20% on a heat input basis) to the coal led to the highest ash deposition rates likely due to the strong interaction of the CaO and MgO (from the biomass ash) with the alumina and silica (from the lignite ash) during the co-combustion process, evidenced by the detection of high concentrations of calcium/magnesium sulfates, aluminates and silicates in the ash deposits. Interestingly, co-firing of white pine pellets and lignite at a 50% blending ratio led to the lowest ash deposition rates. Ash deposition rates in combustion of fuels as received with a higher moisture content was found to be much lower than those of oven-dried fuels.

The polycyclic aromatic hydrocarbons (PAHs) and heavy metals (Hg, Pb, Ni, Cr, Cu, Zn) emission characteristics in flue gas from co-combustion of petrochemical sludge and coal were investigated in a pilot-scale circulating fluidized bed (CFB) incinerator with a thermal input of 0.2 MW. Experimental results show that when mass ratio of petrochemical sludge/coal increases from 10% to 40%, PAHs, Hg and Zn emissions increase, while Pb and Ni emissions decrease. Mass mixing ratio has no obvious effect on the emissions of Cr and Cu. As combustion temperature increases, PAHs emission decreases at first and then increases, there is an optimum combustion temperature inhibiting PAHs formation, while emissions of Hg, Pb, Cu, Zn increase relatively rapidly. With Ca/S molar ratio increasing, PAHs, Hg, Pb, Ni, and Cu emissions decrease due to adsorption by calcium sorbents. Limestone is effective in adsorbing Pb and Ni, while lime is suitable to adsorb Hg and Cu. However, Ca/S molar ratio doesn’t have much influence on the emissions of Cr and Zn. Some combustion parameters are recommended based on stable combustion and low pollutant emission.

Over 90% of electricity produced in Estonia is made by power plants firing local oil shale and 25% of the boilers are of the circulating fluidised bed (CFB) variety. In 2007 approximately 6.5 million tons of ash was acquired as a byproduct of using oil shale for energy production. Approximately 1.5 million tons of that was ash from CFB boilers. Such ash is deposited in ash fields by means ofhydro ash removal.

Ash field material properties have undergone changes as a result of CFB ash deposition — the ash field surface is not stabilising, the ash is not becoming petrified and is unsuitable for building dams needed for the hydro technology application.

The analytical research was dedicated to determining the reasons for this different ash field behaviour of ash from CFB and pulverised firing (PF) boilers based on changes of ash properties. Comparative CFB and PF ash studies were conducted: chemical analysis, surveys with X-ray difIractometers (XRD) and scanning electron microscopes (SEM), study of binding properties. Bottom and electrostatic precipitator ash was scrutinised, with its typical rougher and finer particles, accordingly. Such ash types comprise the predominant current ash deposit content in ash fields. The ash samples were taken from boilers of the Balti Power Plant.

The Estonian oil shale mineral part consists mainly of the following minerals: calcite 44.0%, dolomite 19.5%, quartz 8.7%, orthoclase 10.5%, hydromuscovite 8.6%. These minerals comprise 91.3% of the mineral part and it should be noted that 63.5% of that are minerals in the carbonaceous part and 27.8% — in the terrigeneous part. When oil shale is fired, thermal decomposition of these minerals occur into simpler compounds, coupled with volatilisation of some compounds and formation of novel minerals, and there are changes in the mineral phase state as well. The ash binding properties and behaviour in ash fields depend on these changes, the extent ofwhich is determined by the firing temperature in the boiler furnace.

The comparative studies indicate that the introduction of the CFB method of firing has caused several important changes in the mineralogical composition and properties of oil shale ash. The reason behind this is that the CFB boiler furnace firing temperature is approximately 600°C lower than in the PF counterpart, resulting in weaker fuel mineral decomposition and lower novel mineral formation intensity during combustion. The ash solidification tendency is in this instance substantially less than that observed in PF ash. The compressive strength of the electrostatic precipitator ash test samples from CFB boilers was 4.4 N/mm

2

; from PF boilers — 15.3 N/mm

2

, the corresponding specific surface area: 4 533–9 806 cm

2

/g and 707–3 966 cm

2

/g.

The obtained data confirms the need to alter the ash storage technology.

Aerosol and fly ash sampling was carried out at a 80MWth bubbling fluidised bed (BFB) boiler plant co-firing solid recovered fuel (SRF), spruce bark and paper mill wastewater sludge in two experimental conditions. The SRF-Bark ratio in the fuel mix was kept constant at 50%–50% on dry mass basis in both experiments but two sludge proportions were used: 15% and 4% on dry mass basis. Aerosol samples were collected from the superheater region of the boiler furnace and fly ash from the electrostatic precipitator (ESP). Na, K, Cl and S were found to be in mainly water soluble compounds in the aerosols sampled by means of a Dekati type Low Pressure Impactor (DLPI). Bromine was found in several weight percentages in aerosols and it was amongst the main elements in some of the samples collected. Bromine is assumed to mainly originate from flame retarded plastics and textiles in the SRF. According to the measurements, the fate of Br seems to be analogous to the other main halogen, Cl, and its conversion from fuel to aerosols was high, indicating a strong tendency to form bromine salts.

The content of residual carbon in fly ash of CFB boilers is a litter high especially when low-grade coal, such as lean coal, anthracite coal, gangue, etc. is in service, which greatly influences the efficiency of boilers and fly ash further disposal. Reburn of fly ash through collection, recirculation in CFB furnace or external combustor is a possibly effective strategy to decrease the carbon content, mainly depending on the residual carbon reactivity. In this work, the combustion properties of residual carbon in fly ash and corresponding original coal from large commercial CFB boilers (Kaifeng (440t/h), and Fenyi (410t/h), all in china) are comparably investigated through experiments. The residual carbon involved was firstly extracted and enriched from fly ash by means of floating elutriation to mitigate the influence of ash and minerals on the combustion behavior of residual carbon. Then, the combustion characteristic of two residual carbons and the original coal particles was analyzed with thermogravimetric analyzer (TGA, STA409C from Nestch, Germany). It was observed that the ignition temperature of the residual carbon is much higher than that of original coal sample, and the combustion reactivity of residual carbon is not only dependent on the original coal property, but also the operating conditions. The influence of oxygen content and heating rate was also studied in TGA. The O

2

concentration is set as 20%, 30%, 40% and 70% respectively in O

2

/N

2

gas mixture with the flow rate of 100ml/min. It was found that higher oxygen content is favor for decreasing ignition temperature, accelerating the combustion rate of residual carbon. And about 40% of oxygen concentration is experimentally suggested as an optimal value when oxygen-enriched combustion is put into practice for decreasing residual carbon content of fly ash in CFB boilers.

Calcium sulphoaluminate cements, mainly composed by 4CaO·3Al

2

O

3

·SO

3

and 2CaO·SiO

2

, are special hydraulic binders which require limestone, bauxite and gypsum as natural raw materials for their manufacture. In order to save bauxite and natural gypsum, it has been explored the possibility of using, among the raw mix components, FBC waste together with pulverised coal fly ash or anodization mud and, when necessary, flue gas desulphurization gypsum. Mixtures containing limestone (29–39%), FBC waste (30–44%), pulverised coal fly ash (0–13%) or anodization mud (0–32%), bauxite (0–18%) and flue gas desulphurization gypsum (0–8%) were heated for 2 hours in a laboratory electric oven at temperatures ranging from 1150° to 1300°C. The X-ray diffraction patterns on the burnt products generally showed a good conversion of the reactants and a high selectivity degree towards 4CaO·3Al

2

O3·SO

3

, particularly at 1250°C.

CFB fly ash from separators was mixed with water or the mixture of water and additives under the temperature of 363K by use of a blender. Then, this compound of fly ash and water or additives was pumped into a CFB combustion chamber by a sludge pump. Because the temperature of flue gas was high in CFB, the fly ash was hydrated fast and agglomerated in the same time. Through this process, the size of agglomerating fly ash is larger than the original particle and the relative residence time of agglomerated fly ash in CFB becomes longer. Therefore, the rate of utility of calcium in fly ash improves and the content of carbon in fly ash decreases. This results in a low Ca/S and low operational cost for CFB boiler. The additive is one key factor, which affects the rate of desulfurization of agglomerated fly ash. Effect of different additives on rate of desulfurization is not same. Cement and limestone are beneficiated to sulfur removal of agglomerated fly ash, but sodium silicate does not devote to the rate of sulfur removal of agglomerated fly ash.

By using Low Pressure Impactor, fly ash is sampled at the ESP inlet and outlet of a 300MW utility boiler. The composition, mass and element size distribution of fly ash was measured by X-ray fluorescence and 0.001mg precision microbalance, respectively. Mass and element size distribution of Si, Al, Fe, Ca, Mg, S, Cu, Pb, Zn and Mn is of bimodal. Fly ash diameters of the fine and the coarse are about O.Ium and 2.36–3.95 μm, respectively. Efficiency of ESP for submicron ash is lower than that for normal variables ranging 62∼83%, while for ash bigger than 10μ it is almost 100%. Mg, S, Cu, Zn and Pb are obviously enriched in fine ash, especially Cu, Zn and Pb. They are found in PM

2.5

accounting for 50∼60%. The relative enrichment coefficient of Cu, Zn and Pb is about 30∼40. The relative enrichment coefficient of Si, Al and Ca decreases with the ash size decreasing. No direct connection is found between the change ofrelative enrichment coefficient ofFe, Mn and their ash size.

In order to study atmosphere chemical component, a model to depict the characteristic of the absorption process of gaseous pollutant by a liquid aerosol with internal circulation and chemical reaction was established. Based on the finite volume method, SIMPLE algorithm was employed to numerically study the experimental works of Walcek, which indicates that the present models and method are feasible. Then, the transient momentum and mass transfer characteristics of SO

2

into a droplet which radius is 5μm were evaluated numerically, and the results indicate that the chemical reaction increases the rate of mass transfer and the quasi-saturated time of aerosols, which provided theory basis for the heterogeneous reaction of liquid aerosols.

Characterizations of combustion and emission of four kinds of herbaceous biomass pellets were investigated in a 0.15 MWt circulating fluidized bed. Corn stalk, wheat stalk, cotton stalk and king grass, which are typical herbaceous biomass in China, were chosen for this study. Temperature profile, emission in flue gas and agglomeration were studied by changing the combustion temperature between 750°C and 880°C. The combustion efficiencies are in the range from 97.4% to 99.4%, which are relatively high due to the homogeneous temperature profiles and good circulating fluidization of bed material. Suitable combustion temperatures for the different herbaceous biomass are mainly depended on the emission and bed agglomeration. SO

2

and HCl concentrations in flue gas are in direct proportion to the sulfur and chlorine contents of the herbaceous biomass. Agglomeration at the cyclone leg and the loop seal is the main reason for defluidization in the CFB combustor.

Bed material coatings and the consequent agglomeration of bed material are main ash-related problems in FB-boilers. The bed agglomeration is a particular problem when combusting biofuels and waste materials. Whereas SEM-EDS together with automated image processing has proven to be a convenient method to study compositional distribution in coating layers and agglomerates, it is a relatively expensive technique and is not necessarily widely available. In this contribution, we explore the suitability of LA-ICP-MS to provide analogous information of the bed.

Agricultural wastes (straw, sunflower or millet husk, etc.) are difficult to use as fuel because of low bulk density and relatively big ash content with a low melting point. It is possible to produce agropellets of agricultural wastes which are suggested to combust in a fluidized bed of pellets alone, their char particles and ash. The characteristics of the process of fluidization of agropellets are investigated at room temperature. The experiments on agropellet combustion in a fluidized bed are carried out in an experimental set-up. The results of the experiments have shown that in such a bed the pellets produced of straw and millet husk combust with the same rate as those of wood though the latter contain 8.76 – 19.4 times less ash. The duration of combustion of the same portion of straw pellets in a fluidized bed is 3.74 – 7.01 times less than the duration of combustion of cut straw in a fixed bed. Besides, the movement of agropellets prevents agglomeration and slagging of a boiler furnace.

Combustible waste is very heterogeneous and the variation in chemical composition is of great significance for the performance of the combustors in terms of boiler availability and power efficiency. For example, the content of alkali, Chlorine and sulfur affect agglomeration, fouling and corrosion mechanisms, which often limits the steam data and requires counteracts such as soot blowing and outages. An increased knowledge on favorable levels and ratios of fuel components are therefore highly important when developing waste combustors, both existing and future. However, to be able to make good predictions of reactions, reliable fuel analyses are a necessity and they are difficult to perform because of the heterogeneity of waste. As a consequence, it is also difficult to complete pro-active measure to reduce unwanted reactions.

This work has investigated the composition of the fuel during one year in a 40 MW commercial BFB waste plant. Twelve samples have been performed in order to estimate the variation of key components. The fuel samples were analyzed chemically for alkali and several other components of interest, such as Cl and S. Moreover, thermo-chemical multi-phase equilibrium calculations were performed for prediction of the chemical composition of the furnace in the temperature range 400–1000 °C. In this temperature range NaCl and PbCl

x

are formed as well as gas phase HCl and solid silicates. Furthermore, the calculations show that the chemistry is very sensitive to the input chemical composition, suggesting that already a minor shift in fuel mineral matter may change the behavior of the fuel radically in terms ofits fouling and corrosion tendency.

Power production in Estonia is predominantly based on combustion of a local low-grade fossil fuel Estonian oil shale. Due to the high content of carbonaceous mineral matter in oil shale, its combustion is related to formation of lime-containing ashes (content of free CaO 10–30%) which could be utilized as sorbents for CO

2

. In the present research CO

2

uptake by circulating fluidized bed and pulverized firing ashes from different technological devices (furnace, cyclones etc) of an operating power plant was studied and the effect of pre-treatment (grinding, calcination at different temperatures) of these ashes on their capture capacity was estimated using thermogravimetric, SEM, X-Ray and EDX analysis methods. It was found that capture capacities were determined mainly by free CaO content in the ashes, thereby, fluidized bed ashes showed higher CaO conversion levels (19.2–74.2%) as compared to pulverized firing ones (8.7–51.8%). Pre-treatment conditions influenced noticeably CO

2

uptake. Grinding decreased CO

2

capture capacity of fluidized bed ashes, calcination at higher temperatures decreased capture capacity of both types of ashes. Clarification of this phenomenon was given. Kinetic analysis of the process has been carried out, mechanism of the reactions and respective kinetic constants have been estimated.

The mixture of horse manure and bedding materials (peat and sawdust) appear to be a potential biofuel. The chemical compositions of horse manure, bedding materials, and the mixture of these have been characterized by SEM-EDS and ICP-OES. In addition, the compositional distribution of ash of these materials has been determined and this facilitates the estimation of their propensity for ash-related problems in boilers. NO

x

and SO

x

emissions from the co-combustion are also discussed on the basis of the chemical composition of fuel mixtures. It seems that co-combustion of horse manure and bedding materials in FB boilers is suitable for small-scale energy production.

FTIR, thermo-gravimetric analysis techniques and molecular modelling were employed to study the effect of CO

2

on fuel-nitrogen evolution under oxy-combustion conditions. The main objective is to compare NO

x

emissions at several molar fractions of O

2

using Ar or CO

2

as balance gas in a fluidized bed reactor. A char with about 16% N content was prepared by pyrolysis of polyacrylonitrile. This sample facilitated NOx evolution experiments due to the abundance of nitrogen complexes, and aided the identification and quantification of several N species by means ofFTIR. Results indicate that the presence of CO

2

enhances NO

2

formation. A complementary study was carried out by molecular modelling of the experimental reactions using the Gaussian 03 package. Different heterogeneous and homogeneous interactions between CO

2

and char N-species were simulated. The results thus obtained show that the presence of CO

2

during combustion can facilitate NCO formation which is a very reactive intermediate species that can be readily oxidized in the gaseous phase.

In order to understand the combustion phenomena of woody material in a vortexing fluidized bed combustor (VFBC), a series sawdust combustion tests were carried out in a pilot scale VFBC. The sawdust is from the residuals of timber industry. The pilot VFBC consists of a 0.4m×0.8m×0.75m (W×L×H) of rectangular combustion chamber and a cylindrical freeboard with 0.75m LD. The secondary air is injected at the position of 1.75m above distributor. The effects of operating parameters on the combustion efficiency, temperature distribution, and combustion proportion are investigated in this present work.

The results of this study indicate that as primary air increasing, more heat released in the splashing zone; however, more unburned carbon particles reduce combustion efficiency. Higher secondary air flow prolonged the residence time of the unburned carbon particles to make the heating release ratio rise on the second air zone and freeboard zone. The combustion efficiency and heating release ratio increase with excess air, because of more oxygen reacted with combustibles.

The features of the performance of sawdust combustion in VFBC reflect the specific fuel properties of high volatile biomass. The results of this study will help us to design a VFBC which can extract the biomass energy of woody biomass.

From the view of the reliability and the techno-economy, the rolling ash cooler is feasible for the large-scale CFB boilers. However, existing studies on heat transfer in rolling ash cooler mainly focused on heat balance calculation and cold, hot test on the ash cooler outputs. In the heat balance calculation, the value of the overall heat transfer coefficient (a) is usually estimated by the experience, lacking of the support of experimental data.

In this paper, a dynamic experimental system was built to measure the overall a in the rolling ash cooler and the influencing factors, including ash size, ash amount and rotational speed, using the orthogonal design. The results showed that the ash size has the greatest effect on a, and rotating speed has the least. The heat transfer in rolling ash cooler could be enhanced by increasing the rotating speed and decreasing the ash size. The a increased with ash amount when the ash amount is small, but keep nearly constant when ash amount exceeds a certain value.

This paper investigates Eulerian simulation approach of uranium hexafluoride production in fluidized bed pilot. Mass transfer is modeled by using the shrinking particle model. The model successfully predicts expected amount of uranium hexafluoride. As heat transfers with wall are neglected, temperature increases within the reactor and there is no gradient in wall-normal direction of reactor. External diffusion model of reactive gas around particle is developed. Effect of fluorine diffusion within nitrogen is found to be negligible under the simulation conditions. Moreover, inter-particle radiative heat transfer between particles of dense phase in the bed is investigated in the frame of Rosseland approximation.

A successful design of circulating fluidized bed (CFB) boiler should have the highest combustion efficiency, economic operation, and optimum availability. There is a coupled phenomenon of an oxygen lean zone existing in the CFB boiler furnace which depresses combustion efficiency and particle (group) falling down faster and faster when it falls along the waterwall, abrading the tube metal effectively. A new secondary air design for the oxygen lean zone and erosion protection is conceived by using staged and large velocity differential secondary air. For example, a part of concentrate supplied secondary air has been divided into two parts: a low velocity part and a high velocity part. The low velocity part is used for rigid gas layer to reduce the particle falling velocity, and the high velocity part is used for oxygen supply. It is believed that 40∼6Om/s projecting air velocity could send new oxygen to at least half furnace depth in a short projecting lift as shown in calculation. In another view point, operational superficial gas velocity has an obvious effect on waterwall metal erosion, with a lower operation velocity having lower erosion.

Three-dimensional mathematical model has been developed as a tool for co-combustion of paper sludge and coal in a 130 tJh Circulating Fluidized Bed (CFB) boiler. Mathematical methods had been used based on a commercial software FLUENT for combustion. The predicted results of CFB furnace show that the co-combustion of paper sludge/coal is initially intensively at the bottom of bed; the temperature reaches its maximum in the dense-phase zone, around l400K. It indicates that paper sludge spout into furnace from the recycle inlet can increase the furnace maximum temperature (l396.3K), area-weighted average temperature (l109.6K) and the furnace gas outlet area-weighted average temperature(996.8K).The mathematical modeling also predicts that 15 mass% paper sludge co-combustion is the highest temperature at the flue gas outlet, it is 1000.8K. Moreover, it is proved that mathematical models can serve as a tool for detailed analysis of co-combustion of paper sludge and coal processes in a circulating fluidized bed furnace when in view of its convenience. The results gained from numerical simulation show that paper sludge enter into furnace from the recycle inlet excelled than mixing with coal and at the underside of phase interface.

Applying an underfeed system, the underfeed circulating spouted bed was designed as a desulfurization reactor. The main objective of the technology is to improve the mixing effect and distribution uniformity of solid particles, and therefore to advance the desulfurization efficiency and calcium utility. In this article, a series of experimental studies were conducted to investigate the fluidization behavior of the solid-gas two-phase flow in the riser. The results show that the technology can distinctly improve the distribution of gas velocity and particle flux on sections compared with the facefeed style. Analysis of pressure fluctuation signals indicates that the operation parameters have significant influence on the flow field in the reaction bed. The existence of injecting flow near the underfeed nozzle has an evident effect on strengthening the particle mixing.