Chapter 5 | Coal-to-Liquid Conversion Processes: A Review

Four processes for the conversion of coal to liquid products are described. Pyrolysis is the simplest of these processes, but usually less than 50 % of the carbon can be converted to liquid product with the remainder being a carbon char. Direct coal liquefaction resembles pyrolysis except it is conducted in an atmosphere of high hydrogen pressure. Modern direct coal liquefaction processes involves a catalyst that is active for hydrogenation. In one approach, the catalyst is present in the reactor in cases in which the conversion of coal occurs and a highly hydrogenated solvent is generated in a separate catalytic hydrogenation reactor. Today, direct coal liquefaction is being practiced commercially in China, and these processes utilize an iron catalyst. A significant fraction of the heteroatoms remain in the initial liquid products of these two processes. The Fischer-Tropsch synthesis (FTS) and methanol-to-gasoline (MTG) conversion are indirect coal liquefaction processes. In the first step, the coal is converted to a synthesis gas (mixture of hydrogen and carbon monoxide), which then is cleaned of catalyst poisons. The hydrogen–carbon monoxide ration is adjusted to about 2:1, which is needed for the next step. For FTS, both low- and high-temperature processes are utilized commercially. To operate the high-temperature process only, dry gases and liquid fuels are produced. With low-temperature FTS, one-half or more of the product is a wax that must be hydrocracked to produce gasoline and diesel range fuels. Commercially, only the Sasol operation in Secunda and the recent Chinese plants use iron catalysts; all other plants use cobalt catalysts. The other indirect process first converts the synthesis gas to methanol, which is converted to a high-octane gasoline using a ZSM-5 or similar zeolite catalyst. A commercial-scale plant was operated in New Zealand. The products from the indirect processes are essentially free of heteroatoms and are environmentally friendly.