The initial step in the production of liquid fuels, such as diesel fuel, gasoline, or methanol from coal is the gasification of coal to produce syngas. Coal gasification is commercially deployed today. Several different types of gasifier may be selected and used, but complete gasification is required for downstream synthesis operations. An entrained-flow gasifier (GE/Texaco) was used for the coal-gasification evaluations here, as in coal to electricity (Chap. 2). For biomass gasification, a steam/oxygen-blown fluidized-bed gasifier was used. Biomass gasification is technically ready for aggressive commercial demonstration but is not commercially robust enough to assure efficient, effective commercial deployment today and will evolve with further development.
In gasification sufficient carbon is burned using oxygen to increase the temperature to above 1,200 - 1,400°C. At this temperature the char remaining reacts with water (steam) to produce an equilibrium mixture of CO, H2, and CO2, which is referred to as syngas. The overall reaction is:
The syngas produced is about 55% CO, 34% H2, and 10% CO2 on a volume or molar basis, along with low concentrations of impurities. The syngas composition will depend on the gasifier and the feedstock, and the impurities must be removed.
After gasification, the hydrogen to carbon monoxide ratio is adjusted to that required for the desired synthesis reaction by the water-gas shift reaction:
The CO2 is then removed from the gas stream, and the remaining synthesis gas can be catalytically converted to synthetic liquid transportation fuel. The CO2 that is removed can either be vented to the atmosphere as a stream of pure CO2, or can be further compressed and transported to a geologic storage site and sequestered. In either case, this CO2 removal is a required step for the synthesis process.
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