In-Situ Coal Gasification: Model Calculations and Laboratory Experiments

One concept for in-situ coal gasification involves fracturing thick, deep, coal seams using chemical explosives. The resultant high-permeability zone then would be ignited and reacted with a steam/ oxygen mixture to produce medium-Btu gas suitable for upgrading to pipeline quality in a surface plant. This paper discusses the calculational modeling and supporting laboratory experiments relating to the gasification process. The primary aim of this preliminary work is to predict and correlate reaction preliminary work is to predict and correlate reaction and thermal-front propagation rates and product gas composition as a function of bed properties and process operating conditions. process operating conditions. Our initial efforts are restricted to onedimensional, transient Darcy flow in a permeable packed bed. The numerical calculations include a packed bed. The numerical calculations include a detailed description of the reacting system chemistry (13 species) with appropriate reaction rates and over-all heat and mass transport in the system. Comparison of calculated results with experimental data from a packed-bed combustion tube shows good agreement for reaction-zone propagation rates and produced-gas compositions. propagation rates and produced-gas compositions. However, the sensitivity of the calculations to other reaction-rate and transport-coefficient models should be investigated. Introduction In-situ coal gasification has received renewed interest recently. It offers four potential advantages over conventional mining and subsequent surface processing of coal: (1) the product gas may be processing of coal:the product gas may be cheaper because of lower capital investment requirements;environmental damage is likely to be lower;hazards to miners are avoided; andit may make possible the exploitation of coal resources too deeply buried for economical recovery by conventional strip or deep mining operations. The Lawrence Livermore Laboratory (LLL) packed-bed concept for coal gasification was packed-bed concept for coal gasification was originated in 1972. Major program funding by the U.S. ERDA began in 1974. The LLL concept is designed to recover medium-Btu gas from the thick, deeply buried, subbituminous coal deposits prevalent in the western U.S. After upgrading in a prevalent in the western U.S. After upgrading in a surface facility the product gas would have sufficiently high energy density to make pipeline distribution attractive economically. The packed-bed concept calls for creating a permeable zone of coal by detonating chemical permeable zone of coal by detonating chemical explosives in an array of drilled boreholes. The top of the resulting permeable zone is supplied and a steam/oxygen reactant mixture is supplied. The oxidation reactions produce a high-temperature zone that propagates through the bed as a slowmoving thermal wave. The thermal wave first dries the coal downstream from the reaction zone and then pyrolyzes (devolatilizes) it, forming a char. The char undergoes further reactions with the steam present. The major products of the over-all process include H2, CO, CH4, and CO2 as gases, process include H2, CO, CH4, and CO2 as gases, and water and tar as liquids. Mathematical modeling and laboratory experimentation have been carried out to increase understanding of the important parameters of the in-situ gasification process. The purpose of this paper is to present a mathematical description of paper is to present a mathematical description of the gasification process, together with results obtained from calculations and laboratory-scale gasification reactor experiments. The long-range goal of our modeling effort is to acquire the ability to predict resource recovery for a variety of different field geometries and operating conditions. This is a multidimensional, multiphase flow problem. The preliminary model described here is a transient, one-dimensional model of the gasification process in a packed bed. The primary reason for its development is to provide a framework in which to test the importance of accurate specification of the large number of physical and chemical processes involved in gasification. This will be accomplished primarily through comparisons with carefully controlled experiments performed in the 1.6-m reactor. SPEJ P. 105