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In-situ coal conversion processes change coal into liquids and gases underground by chemical reactions which include oxidation, hydrogenation, and pyrolysis. The reactants include oxygen, hydrogen-donor solvents, and hydrogen under extreme reaction conditions. The conditions for these reactions may involve elevated temperatures (250 degrees-600 degrees C), high pressures (to 1000 psi), the presence of coal-derived solvents, water flood or solid fill, and extended duration (10 minutes to 4 hours). Products are solid coal particles, volatile matter Products are solid coal particles, volatile matter from the coal, and possibly pollutant gases exemplified by carbon dioxide, carbon monoxide, hydrogen sulfide, methane, ethane, nitrogen, and traces of sulfur or nitrogen oxides.
Potential hazards for the environment are many. The entry which is drilled or mined may admit impurities to aquifers or may drain and deplete aquifers. Fortunately, coal mining or oil and gas well drilling technology is available to ameliorate these difficulties. The reaction at high temperatures provides heat to the lithographic column and adds to the heat flow. Additional heat flow for conversion near the surface would be catastrophic for plant or animal life. The reaction would therefore be best practiced in deep coal seams. The elevated pressures could possibly fracture confining pressures could possibly fracture confining strata, particularly those which react and are weakened by the reactants, or products, to release liquids, volatile matter, and gases into adjacent strata or the surface. The seams selected for in-situ conversion should be naturally competent, possess low gas or liquid permeability and have adequate strength to resist permeability and have adequate strength to resist fracture. Chemical reactivity should be low for the confining strata exposed to the conversion reaction conditions.
After conversion the voided seam would lead to rock falls, ground and surface subsidence, and seismic effects. While these might be permissible in remote areas without undue ecological permissible in remote areas without undue ecological damage, in populated regions the careful control of underground conversion would be required to provide pillars or radial rows of supporting provide pillars or radial rows of supporting coal substance.
The ecological effects of noise, buildings, and the presence of man are reduced with this technology compared to conventional mining and conversion technology.
