The Modeling of Cavity Formation During Underground Coal Gasification

This paper identifies key operating variables in underground coal gasification and determines the underlying reasons for the unsuccessful field tests. A mathematical model of a gasification channel process is used to interpret the complicated physicochemical phenomena involved. Important factors in the growth of a cavity and its ultimate size also are identified. Introduction The object of underground coal gasification (UCG) is the recovery of the energetic and chemical content of coal without mining. A gaseous mixture composed of nitrogen, oxygen, steam, and carbon dioxide in variable proportions is introduced in a coal seam prepared for proportions is introduced in a coal seam prepared for gasification. Combustion and gasification reactions occur in situ. Carbon monoxide, carbon dioxide, hydrogen, water vapor, methane, nitrogen, and other hydrocarbon products are obtained in an easily used form for the production of electric power or the manufacture of chemicals. The idea of underground coal gasification is not new but dates back 100 years. Historical reviews of pre-1965 technology for UCG can be found in the Little report, Wang, and Elder. However, the review of worldwide literature in UCG found in these reports was incomplete because of a lack of information about Soviet contributions to the technology. Only recently has a complete picture of the Soviet UCG experience been obtained with picture of the Soviet UCG experience been obtained with large-scale translation of Russian technical literature as reported by Gregg et al. The most significant efforts besides those in the U.S.S.R. were in Great Britain and in the U.S. Experimental work in England began in 1949 and field test results have been well documented by Gibb. The studied coal seam was horizontal, but only 3-ft thick; the average heating value obtained during a 4-month period was only 3.1 x 10 to the 6th J/m3 (75 Btu/scf). Both blindborehole and borehole-producer methods were tested. The project experienced many problems, including excessive water leakage into the system, oxygen bypassing, and recycling of gases in the cavity, which caused oxidation of CO to CO2 and excessive gas outlet temperatures. Steam injection reduced high temperatures, but this was not considered a productive operating strategy. Coal recoveries of as much as 84 percent were obtained, but the average heating value of the gas still did not exceed 3.1 x 10 to the 6th J/m3. U.S. tests were conducted first in Alabama by the USBM from 1948 to 1962. Test results have been summarized by Elder. The gasified coal was highly volatile A bituminous in a horizontal seam. Leakage of gases to the surface was excessive (as much as 30 percent) because of the shallow depths, high operating pressure, and extensive fracturing program. Typical heating values with different injected species included air (1.8 x 10 to the 6th J/m3); 34-percent O2, enriched air (2.0 x 10 to the 6th J/m3); 34-percent O2/43-percent N2/25-percent steam (4.4 x 10 to the 6th J/m3); 65-percent O2/30-percent steam (5.3 x 10 to the 6th J/m3); and steam only (6.0 x 10 to the 6th J/m3). Various gasification and seam-preparation methods were attempted, including electrolinking, hydraulic fracturing, and reverse combustion, but none gave satisfactory results. More recently, the Laramie Energy Research Center has been conducting field tests since 1972 on the gasification of thick seams of sub-bituminous coal near Hanna, Wyo. Three separate tests have been performed, with successful results. JPT P. 695