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146 Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 Gasification has been successfully used with coal, in the American Great Plains project to generate synthetic natural gas and in South Africa to generate transport fuels. There are also instances internationally where gasification has been used with SRF waste as a feedstock. A gasification for energy process has four steps, the gasifier itself that produces the gas, feedstock preparation to get the fuel in the right form for the gasifier, the clean-up of the gas, and the utilisation of the clean gas for energy. This is shown Figure 19. Step Material state Typical equipment and process Step 1 Pretreatment Step 2 Thermal reactor Step 3 Gas conditioning Solid Waste Raw gas Clean Gas Separation of glass and metals. Screening Shredding Pelleting or briquetting Fluidised bed reactors Externally heated reactors Fixed bed reactors Entrained flow reactors Gas coolers Filters Liquid scrubbers Figure 19 Steps in a gasification process Step 4 Conversion to energy Energy products Steam boilers Engines Gas turbines Chemical synthesis The gasification process The gasification reactions that produce the fuel gas components are endothermic and need a constant supply of heat to sustain production. This heat is supplied by the combustion of part of the fuel. The challenge is for the equipment design to transfer heat from exothermic combustion to endothermic gasification. There are two main types although there are many variants in each category. Direct – This process occurs in a single reactor where oxygen or air and usually superheated steam is supplied to the same vessel so that some combustion occurs in situ providing the required heat to drive the gasification reactions. The direct use of air dilutes the product gas and results in a low calorific value fuel commonly referred to as producer gas. Indirect – Here gasification and combustion process occur in separate fluid bed reactors. Heat is usually transferred by circulating the fluidised bed material between the two beds. This concept is more complex than the direct process but produces a gas with a higher calorific value.
Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 Table 75 Gas compositions of typical gasifier types Compound Typical indirect (Gussing twin fluid bed) % by volume Typical Direct (Downdraft fixed bed) % by volume Direct bubbling fluid bed % by volume Carbon monoxide 22.7 % 21.4 %v 11-15 %v Hydrogen 31.5 %v 15.38 %v 8-13 %v Methane 11.2 %v 2.05 %v 5-8 %v Higher hydrocarbons 4.0 %v 0.5-1.5 %v Carbon dioxide 27.4 %v 11.82 %v 14-16 %v Nitrogen & Oxygen 2.8% 49.35% 35-45 %v Water vapour As dry As dry 14-21 %v LHV, kJ/Nm 3 12,000 - 13,000 5,005 –5,200-5,800 There are a wide variety of commercial processes on the market. The list below gives the basic generic types. 147
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