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152 Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 10.1.1 Using the gas to synthesise further products. As noted above the gas produced by gasifiers can be used to manufacture methane or transport fuels. A review of gasifier technologies, with a focus on those suitable for liquid fuel production from syngas, has been recently released by NNFCC. 283 Nitrogen dilution is not acceptable in this type of application and this route is restricted to direct gasifiers using oxygen as an oxidant and indirect gasifiers. Transport fuels produced by Fischer Tropsch synthesis reactions are endothermic using syn-gas (synthetic gas) to produce long chain hydrocarbons that can be distilled to produce transport fuels or intermediates for chemical processing. These reactions commonly require carbon monoxide, carbon dioxide and hydrogen in specific proportions. These proportions are adjusted by use of the shift reaction. The resulting gases are then catalysed at temperatures of 150°C – 300°C, and elevated pressures by an iron or cobalt catalyst. CO + H2O ↔ CO2 + H2 Shift Reaction (2n+1)H2 + nCO ↔ CnH(2n+2) + nH2O Fischer-Tropsch Reaction Methanation is an exothermic process that uses syn gas to produce methane that can be injected into the gas grid. Syn gas is catalytically reacted with steam at temperatures between 340°C – 550°C and up to 60 bar. CO + 3H2 ↔ CH4 + H2O A further variation is the synthesis of methanol which in turn can be converted to a range of organic chemicals and dimethyl ether, potentially a fuel used in diesel engines. Nickel is commonly used as the catalyst. These reactions commonly require carbon monoxide, carbon dioxide and steam in tightly controlled proportions which are obtained by the shift reaction and carbon dioxide absorption. Carbon dioxide removal is done by amine solvent scrubbing or refrigerated methanol scrubbing, both commercial refinery and gasworks processes. The use of catalysts requires a high level of purity from trace contamination, particularly sulphur and metals that must be removed to single ppm levels. This is usually done by passing over beds of zinc and copper oxide. The methanation reaction is highly exothermic and rejects heat at the reaction temperature of over 340°C. This makes it a useful source of process steam and in the past commercialisation of this technology has often been driven by the desire to use this to the full. A process producing 700MW of methane could produce 200MW of steam. Neste Oil, Stora Enso, VTT and Forest Wheeler are exploring ways to transform wood waste in to transport fuels through a Fischer-Tropsch process to give a usable syn-gas at a demonstration plant in Varkaus, Finland. 284 The plant is due to open in Spring of 2009, and will use forest residues such as branches, stumps and trimmings to generate heat and electricity for the attached papermill, and quantities of biowax, the raw material to be refined into transport fuels and its use is to replace food oils. The facility is expected to have a capacity of around 100,000 tonnes p.a. of biowax. 283 Review of Technologies for Gasification of Biomass and Wastes, NNFCC and E4Tech, June 2009. 284 Transforming wood waste into renewable fuel, http://www.nesteoil.com/default.asp?path=1,41,540,10793,10795,11601
Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 Table 79 Scales of operation Direct Downdraft to CHP Local Regional National Updraft to CHP Fluid bed to CHP and power only Updraft to CHP Indirect Fluid bed to CHP Indirectly heated tubes and kilns for CHP and power only Oxygen blown entrained flow to methane and 2 nd Gen Biofuels Fluid bed to methane 153
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