MANUAL Published: 08 November 2019
MNL3720160019

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

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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.

Author Information

Davis, Burtron
University of Kentucky, Center for Applied Energy Research, Lexington, KY, US
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Developed by Committee: D02
Pages: 115–143
DOI: 10.1520/MNL3720160019
ISBN-EB: 978-0-8031-7090-2
ISBN-13: 978-0-8031-7089-6