Laboratory Studies of Gasification Kinetics for Western Coals Under Conditions Expected During In-Situ Combustion

Kinetics for the reaction of steam with western U.S. subbituminous chars are described for operating conditions characteristic of those expected during underground coal gasification (UCG). The mineral matter present in these coals has been found to have significant catalytic activity for the water/gas shift reaction. Also, the inorganic constituents of brackish waters occurring naturally in western aquifers have been found to have little effect on the gasification rates. Introduction The in-situ gasification of coal offers significant potential as a means of increasing U.S. utilization of underground hydrocarbons for fuel conversion or producing petrochemical feedstocks. The primary advantage of in-situ gasification is utilization of coal reserves that cannot be recovered economically by conventional techniques. Additional advantages lie in the potential reduction of capital and operating costs, pollution control costs, feedwater quality requirements, and health and safety problems associated with conventional coal-processing technologies. Early efforts to develop in-situ gasification technology have been described in reviews by Capp et al.1 and Elder.2 Since the 1930's, there have been extensive developmental efforts in the USSR, and since 1972 there has been a resurgence of experimental studies - both in the laboratory and in the field - in the U.S. and Canada. Both Soviet tests and recent tests carried out in the Western Hemisphere are discussed in detail in Ref. 3. Depending on the geological conditions of the coal seam and the properties of the coal, the configuration and operating procedures of an underground gasifier can vary significantly. Regardless of the configuration of the underground gasifier and the preparation techniques used for coal seam gasification, the gasifier can be envisioned as consisting of several distinct reaction zones very similar to those occurring in moving-bed gas producers, such as a Lurgi gasifier. The zone nearest the product recovery well is the drying and pyrolysis zone, in which water is driven from the coal and the pyrolysis reactions occur. Tars produced in this zone are driven forward continually into the cooler regions of the seam, with a portion being cracked to lighter hydrocarbons. Cracking proceeds until the tars are light enough to be carried with the product gas stream out of the coal seam. The reducing zone or gasification zone is immediately behind the pyrolysis zone. In this area, the primary reactions areEquations 1 through 5 The water necessary for Reaction 1 is supplied either by injection of steam with the air or oxygen or by natural intrusion of water into the reaction zone if the coal seam is a natural aquifer (as is the case for many western coal seams). Behind the gasification zone is the combustion zone that supplies the process heat. Heat is transferred from the combustion zone to the gasification zones primarily by convection of the product gases rather than by conduction through the solid char and coal.