Coal Conversion in the United States with Emphasis of Studies at The University of Utah

Abstract

Recent international events have demonstrated the need for additional energy sources to supplement or replace petroleum and natural gas in the United States. Petroleum and natural gas provide over 75‰ of the energy. Supplies of these fuels have not kept pace with demand and in recent years the gap between supply and demand has been widening, resulting in a search for alternate fuels and in the importation of petroleum.Vast reserves of coal make it an attractive fuel, but problems of environmental pollution and inconvenience in handling have restricted its use. Processes for conversion of coal are receiving much attention from Government and Industry. Coal conversion processes under study include liquefaction by hydrogenation, by pyrolysis and by solution in solvents ; gasification and methanation to produce high B. t. u. gas; and gasification to produce low B. t. u. gas. Processes for production of liquids or semisolid products and for the production of high B. t. u. gas are being tested at the pilot plant level and are being scaled-up as rapidly as possible. Low B. t. u. gasification processes will receive increased support during fiscal 1975.The University of Utah has been involved in the study of coal properties and coal conversion processes for over ten years. A liquefaction process under study involves the direct hydrogenation of dry coal, impregnated with catalyst, at moderately high pressures and at short residence times. Hydrogenation occurs in small diameter tube reactors. Both free-fall and entrained-flow designs have been investigated. In the entrained flow reactor coal is fed into the reaction zone by a hydrogen stream at a flow rate in the turbulent range of flow.Metal halides such as ZnCl2 and SnCl2·2H2O have been found to be effective catalysts when impregnated on the coal from aqueous solution. A temperature range of 450°C to 650°C has been found to be effective for hydrogenation. Conversions to liquid and gaseous products of up to 85‰ are attainable, while 60-65‰ conversion isattainable at less severe conditions. Over 60‰ conversion is achieved in a 5mm diameter by 35m reactor tube at 510°C and 135kg/cm2 hydrogen pressure at feed rates of 10kg/hour. Residence times are estimated to be 4-6 seconds, giving a relatively high space utilization rate.Studies on the mechanism of coal hydrogenation by impregnated metal halide catalysts indicate that these catalysts react with coal to form a site which is active for hydrogenation. Pyrolysis of matal halide impregnated coals results in dehydrogenation of hydroaromatic structures in the coal and a subsequent decrease in the evolution of tars and gases. The Lewis acid properties of the metal halide are lost on heating the impregnated coal samples to 400°C. An increase in the surface areas and the develop-ment of a capacity to chemisorb hydrogen indicate that the micropore structure of the coal in enlarged upon heating the impregnated coals . Unimpregnated coals show a different behavior.Although zinc halides and stannous chloride are very effective catalysts, they present problems in terms of expense, corrosion of the system by the catalyst and the recovery of the catalyst. Up to 93.8‰ of the zinc can be recovered by extraction from the liquid products and char.