(123a) Solvent Extraction of Low Grade Coals for Clean Liquid Fuels

Solvent extraction of bituminous coals has been used as a means of coproducing clean liquid transportation fuels as well as solid fuels for gasification. Coal solvents are created by hydrogenating coal tar distillate fractions to the level of a fraction of a percent, thus enabling bituminous coal to enter the liquid phase under conditions of high temperature (above 400 oC). The pressure is controlled by the vapor pressure of the solvent and the cracked coal. Once liquefied, mineral matter can be removed via centrifugation, and the resultant superheavy crude can be processed to make pitches, cokes as well as lighter products.

Low emission liquefaction processes are particularly important in a scenario in which greenhouse gas mitigation is essential. Likely such scenarios will emphasize the use of technologies such as wind and nuclear power for central station power, while hydrocarbons will be increasingly reserved for liquid transportation fuel applications.

Lower rank coals such as sub-bituminous coal and lignite are desirable feedstocks for this process due to their low cost, high hydrogen to carbon ratio and high aliphaticity compared to bituminous coals, which can result in superior transportation fuels. However, these advantages are partially offset by the high moisture content and high ash content which typically accompany lignite and sub-bituminous coals. In particular, ash content of approximately 20% is problematic because centrifugation might not succeed in increasing the ash content of the tails. Hence, much of the liquid product would be contained in the nominal tails rather than in the separated liquid centrate, if conventional centrifugation techniques were utilized.

In order to overcome this inherent difficulty, a more complete liquid separation can be accomplished by vacuum distillation. Mineral matter is further heat treated to produce a value-added slag product. Solids separation can be over 90% effective using this technique depending upon the degree to which coal molecules are broken down during the solvent extraction process. The result is correspondingly higher yield of lighter products such as transportation fuels, with lower yield of heavy hydrocarbon products such as pitches and coke precursors.