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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