Coal and Biomass Gasification

Gasification and pyrolysis are promising thermochemical processing technologies for the conversion of complex feedstocks like coal, lignocellulosic biomass and refuse-derived fuels (RDF) into energy and fuels. The quality of the products such as syngas and liquid oil and the process efficiencies depend greatly on the operating parameters of the process, which in turn depend on feedstock characteristics. Hence, it is imperative to map the salient properties of the feedstock to the process characteristics. This review highlights the techniques adopted for characterizing different varieties of coal, biomass and RDF. The various physicochemical and thermal properties discussed in this chapter include density, porosity, specific surface area, thermal conductivity, specific heat, calorific value, thermal stability, pyrolysate composition, proximate and elemental composition, and ash composition. A compendium of proximate analysis (moisture, volatile matter, fixed carbon, ash), ultimate analysis (elemental C, H, N, S, O) and higher heating value data for a large number of solid fuels is provided. The implications of these on the process and product characteristics are addressed. As ash is known to act as a catalyst in the pyrolysis process and cause issues like corrosion and deposition in gasifier systems, the effect of its composition on relevant process parameters is discussed. Finally, the existing challenges and requirements in fuel characterization are discussed.

This chapter deals with the basic thermodynamics and chemical kinetics pertaining to the various physicochemical phenomena that are collectively termed as the phenomenon of gasification. Although the phenomena associated with the gasification of various feedstocks differ from each other in detail, the underlying thermodynamics is more or less common and is attempted to be captured here. The technology of gasification also has a wide variety, and this results in different phenomena having varying grades of importance in each. Thermodynamics of a phenomenon is described in terms of conservation equation for mass and the first law, often discussed under the headings of stoichiometry and energetics of a phenomenon, and in terms of the second law, which determines the equilibrium state at the end of the phenomenon, and thus defines the product compositions in gasification when the reactor is maintained at a given pressure and temperature. The variety of phenomena involved in gasification, namely drying of feedstock, its pyrolysis, homogeneous and heterogeneous reactions which form part of the gasification in the form of oxidation and reduction reactions, proceed at different rates in a given system, and also vary widely between different types of gasification systems. Hence, it is important to study the kinetics of these phenomena, in addition to the study of thermodynamic equilibrium states pertaining to these phenomena. Owing to the fact that each of these phenomena is extremely complex, in mathematical modelling of these phenomena, often apparent mechanism and their thermodynamics and kinetics are studied. This leads to a variety of models and thermodynamic and kinetic data in the literature, often in apparent conflict with each other. This chapter also attempts to identify some of these conflicts through the experience of the authors in modelling gasification phenomena.

Carbonaceous solid materials are converted into gaseous fuel through the gasification process. A limited supply of steam, air, oxygen, or a combination of these serves as gasifying agent. Depending upon the gasifying agent used, the fuel gas will contain mainly hydrogen, carbon monoxide, carbon dioxide, methane, higher hydrocarbons, and nitrogen (if air is used). In gasification, different technologies are used depending upon the requirement. Technologies used for gasification can broadly be classified into four groups; fixed bed or moving bed gasification, fluidized bed gasification, entrained bed gasification, and plasma gasification. In the present chapter, a detail discussion on the design, working principle, merits and demerits of different types of gasifiers are presented. Some of the important commercial gasifiers installed worldwide are also discussed.

Fluidized bed reactors are used in different industries to carry out multiphase chemical reactions. In these reactors, the fluid is passed through the reactor bed having granular solid materials. The velocity of the fluid is kept high enough to suspend these materials resulting to behave them like fluids. Such reactors are classified as bubbling bed, fast circulating bed or dual bed systems combining two beds depending upon the fluid velocities and constructions of the reactors. For combustion and gasification processes, circulating or dual fluidized bed systems are often preferred because they are more efficient having high throughput. However, the hydrodynamics of such fluidized beds, using normally low-grade feedstocks, is very complex and plays a critical role for successful operation of the plant. Lots of experimental and theoretical investigations are done in this area; however, the available information on the hydrodynamics is limited. In this chapter, the hydrodynamics of circulating fluidized bed systems has been discussed.

Over the years, gasification technology has been established as one of the efficient thermochemical conversion processes catering to a wide variety of applications like thermal, power generation and liquid fuel production through Fischer–Tropsch route. However, there are issues with the conversion devices when the biomass feed material changes and hence understanding of product gas behaviour and its variability is important in order to utilize the biomass gasification technology effectively in the long run. The current chapter addresses these issues relating to biomass characterization, product gas estimation and utilization, and advances in this technology. Furthermore, an example of an equilibrium model formulation for prediction of product gas generated from rice husk has been presented, and a brief about reaction kinetics has been discussed. A comparison of model result and experimental data has also been briefly presented. The recent trends in biomass gasification research show a promising future for this technology. Moreover, techno-economic evaluations prove that biomass gasification is not only technically viable but also a sound economic option. It is expected that biomass gasification will contribute more to the global energy requirements and thus to the economy in the coming future.

Producer gas, derived from biomass gasification, contains a wide variety of compounds organic as well as inorganic, apart from the gas species and particulate matters. The hydrocarbon compounds present in the raw gas, which have comparatively lower molecular weights, act as fuel in gas turbine or gas engines. Hydrocarbons with higher molecular masses are collectively called tars. Relatively simpler tars often polymerize into more complex structures. These heavier species quickly condense, some even solidify, and choke the particulate filters and other restrictions and valves in the gas paths, causing serious obstruction to continuous operation of the application components. Some other impurities, like sulphides and halides, too cause damages to the materials of downstream equipment. It is, therefore, essential to remove the tars and impurities in the product gas to the extent possible. Tars also pollute the environment if discharged untreated. If, however, tars could be cracked and converted to permanent gas species, the producer gas calorific value could be improved substantially. Tars can be eliminated or effectively converted or their production rates can be reduced by certain measures. They include installing separation devices, modifying the conditions and parameters of gasification, modifying the gasifier design, using additives and catalysts. This chapter discusses these measures or processes that are aimed at tackling the tars.

It has been realized in recent decades that a proper investigation of gasification reactor requires the detailed information over the entire flow field, as well as time, at multiple scales. Such detailed information needs the use of sophisticated measuring techniques with capability to provide the required information over the entire flow field, as well as time, at multiple scales. Aside from the mean velocities and volume fractions, information about the flow fluctuations or dynamics (quantified in terms of cross-correlations and auto-correlations) is also desirable. In addition, it is preferable if such techniques are amenable to automation to reduce extensive human involvement in the data collection process. While such data are “stand-alone” sets of information, which can be used for design and scale-up strategies, it also provides information that is crucial to establish the validity of conventional models like phenomenological flow models describing residence time distribution (RTD), as well as more recent and sophisticated models like those based on computational fluid dynamics (CFD). In fact, it almost seems imprudent to validate CFD predictions on overall holdup and flow rates, because these spatial integrals of point properties are simply averages of a complete flow field that a CFD code is designed to and claims to compute. Thus, fair validation must involve validation at multiple scales, for which one needs experimental information also at multiple scales (and not just spatial and temporal averages). Several experimental techniques have been reported in past to quantify the flow field in gas–solid gasification reactors, with each technique having its own advantages and disadvantages. In this chapter, details of pressure, solid velocity, solid fraction, and RTD measurement techniques will be presented. Techniques will be divided majorly in two types, invasive and non-invasive. The postprocessing methods for each technique, advantages, and limitations will be discussed. Finally, some of the recent findings on gas–solids circulating fluidized bed using radioactive particle tracking (RPT) technique will be discussed in detail to explain the use of the experimental techniques for design and scale-up of these reactors.

Underground coal gasification is an in situ coal utilization technique that has immense potential as a future clean coal technology. UCG possesses a number of advantages including the ability to use deep and unmineable coals. The most important component of UCG is the underground “cavity”—which serves as a chemical reactor with rich interplay of kinetics and transport. Field and laboratory-scale experiments have revealed several interesting features of the UCG cavity. Modeling studies on the UCG cavity involve fundamental models and CFD simulations. In this chapter, we will discuss various experiments and models of UCG cavities, with a focus on the effects of reaction chemistry and thermomechanical spalling on cavity evolution.

Biomass gasifiers with capacities exceeding 1 MW have large biomass consumption, and mixture of biomasses need to be used as feedstock in these gasifiers. In this chapter, we have presented a review of our studies in gasification of biomass blends using approaches of non-stoichiometric equilibrium, semi-equilibrium, and kinetic models. Initially, gasification of biomass mixtures has been assessed using thermodynamic equilibrium and semi-equilibrium (with limited carbon conversion) model employing Gibbs energy minimization. Influence of operating parameters such as equivalence ratio, temperature of gasification, and composition of the biomass mixture has been evaluated using two criteria, viz net yield and LHV of the producer gas. Interestingly, optimum operating conditions for all biomass mixtures have been established as equivalence ratio ~0.3 and gasification temperature ~800 °C. The kinetic model analysis of gasification of biomass based on a circulating fluidized bed gasifier. A series of chemical reactions was considered for obtaining complete mass balance. Although the profiles of molar composition, net yield and LHV of the producer gas predicted by kinetic model matched with equilibrium models qualitatively, significant quantitative difference was evident. The processes of char gasification and tar oxidation have slow kinetics that adversely affects the carbon conversion in the riser of the circulating fluidized bed gasifier.

An overview of the computational fluid dynamics (CFD) modelling techniques used to study multiphase reacting flow in fluidized bed reactors is presented in this chapter. Research in fluidized bed gasifiers has gained momentum in recent years due to their various industrial applications. Experimental investigation of such intricate process requires sophisticated and expensive measuring techniques. Moreover, it is very difficult to capture essential process details of these systems by available experimental methods. On the other hand, numerical simulation offers a viable approach to experimental investigations. The numerical simulations not only offer a better insight into the complex gas–solid flow dynamics, but it also carries paramount importance in the design and optimization of fluidized bed systems. The present chapter primarily focuses on the CFD modelling fundamentals and their application pertaining to fluidized bed reactors. Detailed description of gas–solid flow modelling and chemical reaction kinetics is given separately.

Entrained flow gasification is perhaps the most adopted gasification technology around the world. Most of the IGCC deployments around the world have chosen entrained flow gasification as the coal conversion technology. It offers high temperatures, high mass throughput, low amount of tars and oils in the flue gas stream, and high carbon conversion efficiencies. The drawbacks of this technology include frequent maintenance of critical equipment such as wall refractory and injectors. This chapter focuses on both the commercial aspects, in terms of worldwide deployment and operational experience, and the technical aspects of entrained flow gasification. The technical discussion is centered on the computational fluid dynamics modeling, owing to the deep complexity inherent in the turbulent fluid mechanics of these systems. There is a dedicated discussion on the char consumption model, including the heterogeneous kinetics, as well since it plays a key role in determining the sizing and overall design of the gasifier.

The chapter gives an overview of new techniques developed and used in coal, biomass and waste materials gasification. All of the above-mentioned materials have similar properties for hydrocarbon content. As a consequence, most of them are used for power and heat generation through gasification technology. The chapter discusses advanced kinetic as well as computational fluid dynamics (CFD) modelling schemes valid for a wide range of coal and biomass materials using a downdraft gasifier. The models show validated results with experimental data. Underground coal gasification (UCG) is also discussed, modelled and verified to some extent. Applications leading to the combined heat and power (CHP) generation from syngas produced through the gasification of such feedstocks are presented.

Circulating fluidized bed (CFB) in chemical looping combustion (CLC) is a novel carbon capture technology which offers great advantage for high efficiency and low cost. To obtain a thorough understanding of the hydrodynamics behavior inside the reactors as well as CLC process, numerical simulations are conducted. Computational fluid dynamics (CFD) simulations are performed with dense discrete phase model (DDPM) to simulate the gas–solid interactions. CFD commercial software ANSYS Fluent is applied for the simulations. Two bed materials of different particle density and diameter, namely the molochite and Fe100, are used in studying the hydrodynamics and particle behavior in a fuel reactor corresponding to the experimental setup of Haider et al. at Cranfield University in U.K. Both the simulations reach satisfactory agreement with the experimental data concerning both the static pressure and volume fraction at various heights above the gas inlet inside the reactor. It is found that an appropriate drag law should be used in the simulation depending on the particle size and flow conditions to obtain accurate results. The simulations demonstrate the ability of CFD/DDPM to accurately capture the physics of CFB-based CLC process at pilot scale which can be extended to industrial-scale applications.

Any technology needs to be environmentally sustainable to be successful. The biomass gasification technology is often perceived to be carbon neutral. However, these perceptions need to be confirmed using a rigorous life-cycle assessment (LCA). This chapter presents a sustainability assessment of the biomass gasification technology for the production of ammonia. Conventional ammonia production that is based on hydrocarbon feedstock is known to be energy-intensive and tends to make a substantial contribution to the global greenhouse gas emissions. Therefore, an environmentally benign feedstock in the form of biomass is proposed as an alternative. Biomass, when used as a feedstock for ammonia production, is expected to yield a considerable reduction in environmental impacts. This chapter undertakes a cradle-to-gate life-cycle assessment (LCA) for ammonia production from biomass through the gasification route. Three different biomass feedstocks, namely wood, straw, and bagasse, are compared for their environmental sustainability by using different environmental indicators. Furthermore, these feedstocks are modeled for cultivation in three different geographical regions. The results suggest that different biomass feedstocks and geographical regions have their own niche environmental advantages. The global warming potential (GWP) for the straw-based ammonia production was found to be close to natural gas-based ammonia production. Contrariwise, 78% reduction in GWP compared to natural gas-based ammonia production is noticed when bagasse is used as a feedstock for ammonia production.

This review focuses on the fundamentals, recent technology development, environmental and economic analyses, and commercialization of power generation by gasification of municipal solid wastes (MSW) and biomass wastes for distributed power application. Design and operational factors affecting the performance and emission characteristics of power generation systems using syngas are reviewed. The performance characteristics include maximum power output, engine efficiency, and specific fuel consumption of various technologies. Emissions characteristics include levels of carbon dioxide (CO₂), carbon monoxide (CO), nitrogen oxides (NO_x), unburned hydrocarbon (HC), sulfur dioxide (SO₂), and polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/PCDF). Large-scale system (>1 MW) is typically selected for power generation via MSW gasification, which is generally accomplished using plasma-based gasification followed by the use of internal combustion (IC) engines or gas turbines to achieve high efficiency. Plasma is preferred for treating MSW due to its unique capability to ionize materials, minimize tars, and improve syngas quality. Besides, co-gasification of MSW and biomass is also an alternative for power generation. Finally, techno-economic and life cycle analyses of power generation from plasma gasification system are summarized.

Indoor air quality is one of the prime concerns as it relates directly to the health of occupants. Detrimental pollutants from burning of solid fuels range from CO to NO, NO₂ and suspended particles containing polynuclear aromatic hydrocarbons pose serious threats to human lives. Incorporation of alternative means of heating and near-complete combustion of biomass feedstock would be a better solution to this problem. Gasification based natural draft and forced draft cook stoves are helpful in improving the wood utilization efficiency and reducing the harmful pollutants. The combustion unit of gasification based cook stove is equipped with the provision of primary and secondary air, which facilitates the combustion and makes near-complete utilization of feedstock practical. Thermal draft in the chimney, a control unit of overall process generates pressure difference which facilitates the incoming of primary as well as secondary air and sidewise keeps the flue gases moving. Utilization of solar heating for preheating the feedstock is another step for making the total process efficient. In this chapter, we will discuss the advantages and challenges of different cook stove designs and feasibility of incorporating the solar heating in them.

In dual fluidized bed gasification technology, the gasification/pyrolysis and combustion reactions are decoupled and conducted in two separate fluidized bed reactors connected by circulating inert or catalytic bed material. Hence, a nitrogen-free high-quality syngas is produced. The configuration obviates the need of a capital-intensive air separation unit. It is a complex reactor system, and the challenge lies in selecting appropriate bed material/catalyst, understanding flow patterns and heat transfer characteristics, and designing and operating such a system. This chapter reviews the basic concept, critical components, hydrodynamics, and process characteristics of this technology presenting the current state of the art.

This chapter addresses studies conducted on a new approach to clean combustion via gasification process progressing on the earlier work on packed bed reverse downdraft (REDS) combustion. The additional element is the development of continuous combustion device. The studies are aimed at the use of prepared (in terms of size and dryness) biomass in a broad range of densities (100 to 1000 kg/m\(^3\)) in a newly conceived scalable combustion scheme. The range of power levels includes domestic demands (\(\sim \) 1 kg/h), semi-industrial needs (3 to 20 kg/h), and larger industrial requirements (50 kg/h and more). System can deliver hot gases at a flame temperature from 1150 to 1200 \(^\circ \)C. In domestic stoves, CO emissions are within the permissible limits (CO:CO\(_2\) ratio of 0.006 ± 0.002) and PM2.5 emissions showed incremental steady values of a maximum of 30 \(\upmu \)g/m\(^3\). An important aspect addressed here concerns the mode of assessment of efficiency and emissions from these stoves. It is suggested that recent expectations of domestic stove emissions need revision in favor of known concepts from other combustion devices. The second part is concerned with the use of coal of permitted ash content (of 21%, but up to 34%) sized to 2–8 mm for thermal applications and clean cold combustible gas applications. Studies on the flame propagation behavior in packed beds in REDS with air show rates about half of that with biomass. With air–steam mixtures, carbon conversion beyond 99% and avoidance of ash fusion are achieved. Operation of the bed with heated coal (\(\sim \) 120 \(^\circ \)C) and air up to 160 \(^\circ \)C are considered beneficial to reduce the flaming time and char conversion times. The fixed bed studies provide inputs for evolution to mildly fluidizing strategy for complete conversion of coal without ash fusion.

Electricity generation through coal–thermal route is one of the highest contributors to environment pollution through greenhouse gas emission, which has given rise to issue of climate change risk. Among different alternatives of renewable energy, an important source is biomass-based energy. Utilization of biomass for energy production in coal-fired power plants is essentially in terms of partial substitution of coal feed with biomass. Major challenge in this route is fluctuating supply and varying compositions of biomass. It can be overcome by adopting co-gasification technology (using mixed feed of biomass and low-grade coal) for power generation. In this chapter, we have presented a critical review and analysis of the literature in the area of co-gasification of biomass and coal. Analysis in this paper touches upon several facets of co-gasification process such as effect of biomass/coal ratio, the composition (proximate/ultimate analyses of biomass/coal), gasification media, temperature and heating rates on the gasification kinetics, producer gas composition, and yield. The synergistic effects between gasification of coal and biomass have been reviewed. The alkali/alkaline earth metal content in the ash of biomass catalyzes the kinetics of the gasification of coal char. However, if coal has high silica content, adverse reaction between silica and potassium oxides can deactivate the catalytic effect. Actual chemical mechanisms related to this synergy have also been described and discussed. Finally, a brief review of the literature on gasification of coal/biomass blends in bubbling/circulating fluidized bed gasifiers has also been presented.

Gasification of solid fuels exhibits a vital role in power plants for electricity generation using gas turbines, fuel cells and for various chemical productions such as hydrogen, methanol, liquid hydrocarbons, etc. Chemical looping combustion (CLC) and plasma gasification are the recent emerging advanced technologies for the production of clean energy from solid fuels. CLC operation eliminates the energy penalty of air separation unit (ASU) in an oxy-fuel combustion unit and paves a way for carbon capture and storage. Another promising technology for the conversion of solid fuel into syngas is plasma gasification. Tar-free syngas with high-calorific value can be obtained in this technology. Further, co-gasification of coal and biomass is another option to utilize renewable energy, which reduces a considerable amount of greenhouse gas emission. In this chapter, the conversion efficiencies of solid fuels such as coal and biomass in the CLC technology are compared with liquid and gaseous fuels. The complexity of solid fuel-based CLC operation and future research scope of the CLC technology are discussed. Also, the percentage conversion of solid fuels and CO₂ yield in pilot plant-scale CLC experiments is reviewed. The syngas composition and carbon conversion efficiency of plasma gasification are compared for biomass and coal. The feasibility of co-utilization of coal and biomass in these gasification technologies is also explained.