Coal liquefaction is often seen as a way of replacing transportation fuels as oil supplies decline.
The problem with producing liquid fuels from coal (apart from the economics) is this:
Coal contains very little hydrogen; only a few percent.
Liquid fuels, on the other hand, contain from 10-15% hydrogen.
So to get liquid fuels, you have to increase the ratio of hydrogen to carbon.
A certain amount of liquid fuel can be obtained simply by heating coal in the absence of air. This technology was well-developed in earlier times, mainly for making 'coal gas' or 'town gas', before North Sea gas became available.
Large amounts of the vehicle fuel 'benzole' could be manufactured by reacting one of the byproducts, tar, with hydrogen. This is no longer done.
There are two main methods nowadays for making liquid fuels from coal.
1. DIRECT: HYDROCRACKING
Powdered coal is reacted with hydrogen at high temperature in the absence of air. The coal structure breaks down and a host of hydrocarbon molecules, many of them aromatics, are produced. The compounds formed can be varied considerably by altering the reaction conditions and the amount of hydrogen.
2. INDIRECT: GASIFICATION followed by LIQUID FUEL SYNTHESIS
Powdered coal is gasified to produce 'synthesis gas'. Sulphur is removed. Then the hydrogen - methane - carbon monoxide mixture is passed over a catalyst (e.g. the Fischer-Tropsch process) which causes the formation of liquid fuels.
Somewhat inaccurately the reactions can be shown like this:
H2 O + C = CO + H2
H2 O + CO = H2 + CO2
13H2 + 6CO = C6 H14 + 6H2 O
15H2 + 7CO = C7 H16 + 7H2 O
17H2 + 8CO = C8 H18 + 8H2 O , etc
The indirect method is thermally inefficient and for many years was only pursued in South Africa where (1) no oil could be imported, and (2) the low cost of coal offset the inefficiency.
Direct hydrogenation requires high qaulity coal. Indirect hydrogenation works quite well with lower-grade coal.
A by-product of the gasification step is a stream of gas liquor containing ammonia and phenols. The phenols are extraced using a solvent; the ammonia is then recovered by reaction with steam, and converted into useful ammonium compounds.
Coal liquefaction was first developed in Germany in the 1920S's with the development of the Fischer-Tropsch process which remains a core technology in the process.
There are many other ways of liquefying coal. One novel approach, researched by the Coal Research Establishment in the 1980s, involved solvent extraction of coal and then processing the extract.
Given the very large world reserves of coal, there is probably a long-term future for coal liquefaction.
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