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Coal-to-liquids has two major drawbacks: it is expensive and it causes carbon pollution. Logic dictates that it should first become more economical and environmentally friendly, since sustained high oil prices won’t be enough to make coal liquefaction economically viable without large-scale public investment. Turning dirty coal into a clean-burning liquid fuel remains something of a challenge for the energy industry. The long-standing association with air pollution has deterred many from taking it seriously. Coal liquefaction has traditionally been a route taken by countries such as Germany in the 1930s and SA, which during the apartheaid era had no other options. The technique was developed further by Sasol, which was attempting to meet the country’s energy needs when economic embargoes could prevent oil imports. The US and other nations also explored the technology during the oil crisis in the mid-1970s, but global research soon came to a virtual standstill.
Yet with oil prices showing no signs of dropping, governments and energy companies are displaying a renewed interest in coal, which is encouraging scientists and engineers to revisit the technologies for retrieving petroleum-like liquid fuels. The predominant coal liquefaction process, developed by Franz Fischer and Hans Tropsch at the Kaiser Wilhelm Institute for Coal Research in 1923, uses high temperatures and pressures to break down the long hydrocarbon chains in coal, forming a mixture of carbon monoxide and hydrogen. This is passed over an iron- or cobalt-based catalyst to produce a liquid that contains hydrocarbons of various lengths. The liquid can then be refined in the same way as crude oil. This process is indirect, producing a gas mixture that is then recombined into liquid hydrocarbons. Direct approaches have also been proven, but on a smaller scale. However Elliot Kennel, a chemical engineer at West Virginia University, says that direct liquefaction is less economical than the Fischer-Tropsch process because it consumes large amounts of hydrogen gas, which is expensive to produce. So Kennel is working to reduce the amount of hydrogen needed. This will also cut the energy to drive the process, but will yield less petrol in the refinery and more thick, tar-like residues. The by-products are low in sulphur and heavy metals, making them easy to sell. Industrial companies have joined forces to build a $600-million pilot plant in Pennsylvania. The plant will use the Fisher-Tropsch process to convert about 4,300 tonnes of coal a day into transportation fuels, heat, electricity and various chemical by-products. The project is being supported by the US National Energy Technology Laboratory and involves Sasol, Shell, Californian energy consultancy Nexant, and Uhde, a German chemical-engineering company. In China several commercial-scale coal-liquefaction projects are under way, including one direct liquefaction project in which Sasol and Shell are involved, which, when it opens in 2008 will convert 6,400 tonnes a day of coal into oil products in its first year aloneMaking petroleum cheaper, more efficient and more environmentally friendly is key to capturing market share and justifying further resaearch. This is really the biggest drawback for coal liquefaction. Because processing a tonne of coal produces much more CO2 than crude oil, the environmental case rests on the hazy prospect that the CO2 can be ‘captured’ or sequestered. None of the pilots under construction so far will achieve that.
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