Like GTL, coal-to-liquids (CTL) fuels are produced by isolating the hydrocarbons in existing fossil fuels and converting them to a form of synthetic fuel that can be used in existing vehicles' engines. Manufacturers use two methods to make that conversion. The first, indirect coal liquefaction (ICL), uses the same Fischer-Tropsch process as gas-to-liquids fuels. Of course, processing requires an additional step to convert the solid coal into a gas that can feed the F-T reaction. Solid coal is crushed, and then exposed to high temperature and high pressure, along with steam and oxygen, which react with the coal to produce synthesis gas. This syngas, a mixture of carbon monoxide, hydrogen and other gases, is then used in the Fischer-Tropsch reaction to create liquid fuels. In direct coal liquefaction (DCL), coal is pulverized, and then exposed to hydrogen and high levels of heat and pressure to produce liquid syncrude that can be refined. This second method is not as widely used as ICL.
Coal-to-liquids fuels can be more environmentally friendly, because they burn cleaner than conventional gasoline or diesel. Byproducts of CTL manufacturing, including water, electricity and metals can be sold to offset the costs of CTL processing and make the process more sustainable. But there are serious environmental drawbacks, too. CTL production consumes huge amounts of water before it creates any. It also releases carbon dioxide emissions and large amounts of solid waste called "slag," which is what's left of the coal after all of its usable chemicals have been extracted [source: Van Bibber].