Matching gasification technologies to coal properties

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Abstract

The gasification of coal to produce hydrogen for use either in power generation or/and for synthesis applications and transport is attracting considerable interest worldwide. Three types of generic gasifiers (entrained flow, fluidised bed and fixed bed gasifiers) presently in use in commercial gasification plants or under development worldwide are described. Their suitability for processing all types of coals is discussed. This includes an assessment of the impact of some of the major properties of coal on the design, performance and maintenance of gasification processes.

Introduction

Global electricity demand is increasing at about three times the rate of total energy while the industry is expected to reduce CO2 emissions because of global warming. As a consequence, there is pressure to improve the efficiency of energy use through changes in technology and to produce energy vectors such as H2 with near zero emissions of greenhouse gases. Oxygen-blown gasification may be the most attractive route for the production of H2 from coal with CO2 capture and sequestration as CO2 can be removed from the pressurised syngas (pre-combustion) rather than the exhaust gas (post-combustion). Removing CO2 from the exhaust gas in conventional combustion processes is feasible, but extremely expensive as this is carried out at atmospheric pressure and implies the treatment of a much larger volume of gas (10 times the volume of syngas). Another attraction of gasification technologies and, in particular, of Integrated Coal Gasification Combined Cycle (IGCC) is the possibility of cogeneration of electricity, H2 and chemicals. This contributes to the improvement of power generation efficiency compared with conventional pulverised coal fired plants as well as the reduction of emissions of greenhouse gases and particulates to the atmosphere (Clayton et al., 2002, Collot, 2003, Collot, 2004).

H2 is currently produced from coal for use as an intermediate for the synthesis of chemicals such as methanol, ammonia/urea, Fischer–Tropsch products and substitute natural gas (SNG) all over the world. However IGCC technology for commercial-scale plants is relatively recent. A summary of the main existing gasification processes is given in the following.

There are presently sixty five Chevron Texaco owned or licensed gasification facilities worldwide that produce power, chemicals and H2 from coal (6 plants), oil derivatives and natural gas. Three of the coal gasification facilities produce ammonia, one produces town gas and electricity, one is an IGCC plant and one is producing methanol and chemicals. There are also other gasification projects in development or engineering for the production of diesel, H2/steam, syngas or electricity from either coal, natural gas or oil derivatives (Preston, 2003).

Sasol, which was established in 1950 with the prime objective to convert low grade coal into petroleum products and chemical feedstocks currently operates three major coal to liquid (CTL) complexes based on the former Lurgi gasification process (now known as Sasol–Lurgi dry bottom gasifier) in South Africa for the gasification of coal into Fischer–Tropsch (FT) products (Van Dyk et al., 2001). The Great Plains Synfuels plant (Dakota Gasification) located in North Dakota (USA) has been producing substitute natural gas (SNG) from lignite using the same technology since 1984 (Lukes and Wallach, 2003).

There are presently five gasification plants using the Shell gasification technology. Only one of them, the Nuon Power Buggenum IGCC plant in the Netherlands (formerly named Demkolec) which was started up in 1994, is fed with coal for the production of electricity. All the other gasification plants are fed with petroleum wastes to produce chemicals and/or H2 (Postuma et al., 2002). Eight other coal gasification plants using the Shell Coal Gasification Process (SCGP) for the production of chemicals are planned to be built in China and one in the USA (the Waste Management and Processors Inc project). The plants will all produce syngas for ammonia/urea, Fischer–Tropsch liquids production or H2 for other chemical plants (methanol, oxo), replacing naphtha reformers, oil gasifiers or outdated coal gasifiers (Ploeg, 2001, Zuideveld, 2003). It is expected that the same technologies as the ones developed at the Nuon Power Buggenum IGGC facility, based in the Netherlands, will be used for the construction of future SCGP plants with power/H2.

New projects based on demonstrated or commercial technologies for the production of hydrogen from coal for power generation are presently under development all over the world.

The EAGLE project (coal Energy Application for Gas, Liquid and Electricity) is one of two Clean Coal Technologies (CCT) projects sponsored by the Japanese New Energy and Industrial Technology Development Organisation (NEDO) and the Ministry of Economy, Trade and Industry (METI) as part of a new strategy called the ‘Deployment of Coal Utilization Technology Development Strategy for the 21st century’. The objective of the EAGLE project is the development of an Integrated coal Gasification Fuel Cell combined cycle (IGFC) (Wasaka and Kubota, 2003).

In Italy, a partnership of Sotacarbo, Ansaldo Ricerche, Enea and the University of Cagliari is presently developing a pilot scale gasifier for the production of H2 and power from coal/biomass and waste mixtures. The process, based on a 5 MW (thermal) gasifier combined with an internal combustion engine, will generate 0.2 MW power (Pratola et al., 2002).

The New Zealand government through its science funding agency, the Foundation for Research, Science and Technology, has approved funding to develop a ‘technology platform’ for hydrogen energy. CRL Energy Ltd is presently constructing a small scale atmospheric air blown fluidised bed gasifier pilot plant fed with local lignites for the production of an equivalent of 50 kW hydrogen energy (Pearce, 2003; S. Pearce, pers. comm., 2003).

The FutureGen project in the USA is a 10 year, US$ 1 billion, demonstration project which was launched by the US government in February 2003 for the production of H2 from coal. The 275 MW prototype plant known as FutureGen will serve as a large-scale engineering laboratory for testing new clean power, carbon capture and coal to hydrogen technologies. Every aspect of the prototype plant will be based on cutting edge technologies (US DOE, 2003).

Two new IGCC projects, the Kentucky Pioneer Energy project (Kentucky) and the Lima Energy project (Ohio) and one existing IGCC power plant, the Wabash River IGCC (Indiana), are being developed by Global Energy Inc in the USA. The objective of the Kentucky Pioneer Energy project co-sponsored by the US DOE, is to demonstrate the reliability, availability and maintainability of a utility-scale IGCC system using a high sulphur bituminous coal, coal fines and pelletized refuse-derived fuel (RDF) blend in a BGL (British Gas/Lurgi) gasifier (Bailey, 2001). The Lima Energy 580 MW gasification plant project is based on the use of the E-GAS™ technology, for the co-generation of H2 and electricity from petcoke. The Wabash River IGCC power plant also designed with an E-GAS™ entrained flow gasifier has been operating with a range of local coals since 1995. A molten carbonate fuel cell is currently being installed at the Wabash River IGCC plant instead of as originally planned, at the Kentucky Pioneer Energy plant. It is expected that operation of the integrated IGCC-fuel cell will start in spring 2004 first with natural gas followed soon after with coal syngas. The Wabash River IGCC power plant and the Lima Energy projects are owned by Global Energy although ConoPhillips recently acquired the patents and intellectual property associated with the E-GAS™ Technology for Gasification (P. Amick, pers. comm., 2004).

There are also three projects of cogeneration plants (power and chemicals) projects sponsored by the US DOE as Early Entrance Coproduction Plants (EECP) (Amick et al., 2003, Rich et al., 2003, Shah and Schrader, 2003, Strickland and Tsang, 2003) and one project for the production of 100 t/d of dimethyl ether (DME) from coal in Japan (Ohno and Omiya, 2003). More details on these projects can be found in Collot (2004).

New concepts based on the gasification of coal for the production of hydrogen are presently under development. Some of the concepts are based on the combination of three steps which include the gasification of coal (either steam gasification or hydrogasification), the shift reaction and carbon dioxide removal. Examples of this type of concept are the Absorption Enhanced Reforming (AER) process developed in Germany (Weimer et al., 2002), the Advanced Gasification-Combustion (AGC) project (Rizeq et al., 2002) and the Zero Emission Coal Alliance (ZECA) process (Ziock et al., 2002, Ziock et al., 2003) developed in the USA, and the Hydrogen Production Reaction Integrated Novel Gasification (HyPr-RING) process developed in Japan (Lin et al., 2002, Lin et al., 2003). Other concepts under development include membrane reactors (Sammells and Barton, 2003) and molten bath processes (HydroMax®, HyMelt®) adapted from metal smelting processes existing in the iron making industry (Alchemix Corporation, 2003, Trowbridge et al., 2002). More details on these new concepts can be found in Collot (2003).

Section snippets

Coal gasification and its applications

Gasification is defined as the reaction of solid fuels with air, oxygen, steam, carbon dioxide, or a mixture of these gases at a temperature exceeding 700 °C, to yield a gaseous product suitable for use either as a source of energy or as a raw material for the synthesis of chemicals, liquid fuels or other gaseous fuels. More details concerning the mechanisms of these reactions and their kinetics can be found in Kristiansen (1996).

Common gasifying agents used in industrial gasifiers include a

Entrained flow gasifiers

In entrained flow gasifiers, coal particles concurrently react at high speed with steam and oxygen or air in a suspension mode called entrained fluid flow. Short gas residence times (seconds) give them a high load capacity but also require coal to be pulverised. Coal can either be fed dry (commonly using nitrogen as a transport gas) or wet (carried in a slurry water) into the gasifier. They usually operate at high temperatures of 1200–1600 °C and pressures in the range of 2–8 MPa. Entrained

Fluidised bed gasifiers

Fluidised bed gasifiers, with the exception of the Transport Reactor Gasifier which is midway between a fluidised bed and an entrained flow gasifier, can only operate with solid crushed fuels (coal: 0.5–5 mm, less than 50 μm for the Transport Reactor Gasifier) that are introduced into an upward flow of gas (either air or oxygen/steam) that fluidises the bed of fuel while the reaction is taking place. The bed is either formed of sand/coke/char/sorbent or ash. Residence time of the feed in the

Moving bed gasifiers

Moving bed gasifiers are only suitable for solid fuels with a particle size in the range of 5–80 mm. A mixture of steam and oxygen is introduced at the bottom of the reactor and runs counter-flow to the coal. Coal residence times in moving bed gasifiers are of the order of 15 to 60 min for high pressure steam/oxygen gasifiers and can be several hours for atmospheric steam/air gasifiers. The pressure in the bed is typically of the order of 3 MPa for commercial gasifiers with tests realised at up

Conclusions

Hydrogen is currently and mainly used as an intermediate for the synthesis of chemicals and clean fuels. However with the move towards the ‘hydrogen economy’ there is an incentive to use Hydrogen itself as an energy carrier itself. New programmes and research projects, which are particularly dedicated to the production of Hydrogen from coal, are presently underway worldwide. In this paper some coal properties that have an influence on gasifier design, operation and performance have been

Acknowledgements

The author would like to thank Dr Geoffrey Morrison, head of publication at the IEA Clean Coal Centre for his help in preparing the manuscript.

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