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148 Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 Table 76 Generic gasifier types Type Description Direct/ indirect Fixed bed Fuel, air and the produced gas flow co currently direct downdraft downwards through a reactor with a converging base section. Fixed bed updraft Fuel and air flow counter currently through a cylindrical bed. direct Fluidised bed The waste is gasified in a hot bed of sand or direct direct limestone that is agitated by air and the gas product. This is known as a fluidised bed. Char burns in situ to provide heat. The fluidised bed can be retained within the reactor vessel as a coherent bed with only gas and dust leaving or the velocity of the gas can be increased to the point that bed material is transported from the top of the vessel necessitating its recirculation back to the reactor. These two patterns are respectively referred to as bubbling and circulating fluidised beds. Fluidised bed The waste is gasified in a turbulent bed of sand or indirect indirect limestone. Char from the reaction is transferred with sand to a separate vessel where it is burned in air. Heat is transferred by sand circulation. As with the direct type the fluidised beds can be bubbling or circulating. Heated kilns The waste is tumbled in a rotating drum heated by the combustion of some of the product gas indirect Small heated tubes The waste is progressed by a screw or ram through a tube heated by the combustion of some of the product gas. indirect Large heated tubes The waste is progressed by a screw or ram through a tube heated by the combustion of some of the product gas. Indirect Large entrained Finely divided or liquid fuel is gasified as a flame direct flow with oxygen. Plasma gasification High temperature plasma from an electric arc dissociates the waste into its component molecules Direct Feedstocks The feedstock quality requirements depend on the size and the design of the gasification process. While most thermal gasification processes work best with dry regularly sized feedstocks there are some limited examples of processes that will accept unsorted MSW. In general, the smaller the gasification unit, the more stringent the quality requirements will be for the feedstock being used. The requirements for each of the types identified above are summarised below.
Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 Table 77 Gasifier feedstock requirements Gasifier Type Fixed bed downdraft Typical Size MW fuel 1 MW modules Fixed bed 10 MW updraft modules Fluidised 25MW bed direct or upwards indirect Heated kilns 20MW modules Small heated tubes Large heated tubes Large entrained flow 10 MW modules Fuel Quality Regularly sized, dry and clean wood chips with low ash. Possibly pellets Wood chip SRF Waste Wood Clean Wood most qualities Most dry shredded materials Most shredded materials not necessarily dry Most dry shredded materials not necessarily dry 50MW Minimally sorted or prepared 100 MW upwards Plasma 2MW upwards T C Fuels – Thermochemical Fuels materials Capable of high pressure pneumatic transport or slurry component Most shredded materials not necessarily dry Domestic Pellet Industrial Pellet Unsorted Waste Wet Wastes Agricultural Residues T C Fuels With the exception of the direct fluidised beds none of the gasification processes above can be considered to be widely available commercially. The allocation of feedstock is therefore an estimate based on AEA knowledge. Primary Gas Cleaning The gas from the gasifier contains a range of contaminants. The most important are dust and residual organic compounds or tars that can cause difficulties for the following conversion process. The extent of cleaning necessary varies with the end use of the gas, so a fluidised bed gasifier supplying hot gas to a coal fired utility boiler will have minimal dust removal whereas a unit supplying gas to a reciprocating engine will need reliable dust, tar and acid gas removal. Synthesis processes require a very high level of purity to avoid poisoning of the catalysts used in the reactions - these are more properly regarded as part 149
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