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use of low-cost subbituminous coal or lignite, especially deposits having high hydrogen-to-carbon ratios;
removal of coal oxygen as carbon dioxide;
complete conversion to liquids with boiling points below 540 °C (1000 °F);
improved selectivity to minimize production of hydrogen, water, and hydrocarbon gases;
coproduction of high-value chemical and other nonfuel products; and
direct use of gas from IGCC systems equipped with hot gas cleanup for F-T synthesis and to produce hydrogen for direct liquefaction.
For direct liquefaction, existing processes require cold gas cleanup, shifting to convert carbon monoxide and water to hydrogen and CO2, and scrubbing to remove CO2. There would be significant energy and capital cost savings if the hot gasifier gas could be used without cooling and further processing. Water/gas shift activity in the catalyst system used would be desirable; however, currently available catalysts are not sufficiently sulfur resistant. The use of hot gasifier product for F-T synthesis would require new catalysts capable of carrying out the reaction in the presence of the sulfur concentrations and traces of heavy metals remaining after hot gas cleanup. More active or selective sulfur-tolerant catalysts could markedly improve both direct liquefaction and the upgrading of coal liquids.
Alternative process chemistries of potential interest include:
coprocessing based on alkylation or transalkylation chemistry rather than hydrogenation,
oxidative depolymerization to oxygenate fuels, and
new depolymerization chemistry followed by fixed-bed catalytic upgrading.
The DOE AR&TD budget for bioprocessing of coal was $1.9 million in FY 1994, and the same funding has been proposed for FY 1995. The main thrusts of the bioprocessing program in recent years have been to explore and apply recent advances in biotechnology to convert coal to liquid fuels and to improve the environmental acceptability of advanced power systems. Activities have included characterization of the metabolic features of bacteria found to remove organic sulfur, mineral matter, and metals from coal and investigation of mechanisms for bioconversion of coal. Most experts in the field now agree that biotechnology is best suited for the manufacture of high-value-added products and is least well suited for the production of very large amounts of low-value-added materials, as in the case of coal processing. Thus, current and proposed future DOE coal program efforts in biotechnology will focus on cleanup of sulfur- and nitrogen-containing compounds in combustion gases, rather than on coal desulfurization and demineralization. The committee notes that, although there are possible opportunities for biological cleanup of flue gas (NOx and SO2 removal), significant