A Two-Dimensional Mathematical Model of the Underground Coal Gasification Process

Abstract

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Abstract A two-dimensional mathematical model of the underground coal gasification process is developed, and is used to study the effect of the process variables on the total heat recovery by the gasification process. The model simulates the stream method of underground coal gasification, using the producer gas stoichiometry. Specifically, producer gas stoichiometry. Specifically, reaction kinetics and heat and mass transfer are considered in a horizontal circular channel in a coal seam. Air is injected at one end, where the coal seam is ignited. The carbon dioxide formed is reduced to carbon monoxide farther downstream. The mathematical equations describe the conduction and generation of the heat at the surface of the coal, convective heat transport by the gas stream, and gas phase mass transfer of the chemcial species. The equations were solved analytically, obtaining a closed form solution, which was employed for studying the process, for a variety of operating options. The model is unique in that it considers heterogeneous, intermediate combustion, i.e. the reaction zone temperature is not high enough so that diffusion controls the process, nor is it low enough that reaction kinetics dominate. It was found that the use of oxygen or enriched air is superior to air as the gasification agent, i.e. the total heat content of the produced gas increases with an increase in the oxygen concentration. An increase in the mean axial velocity leads to an increase in the total heat content of the produced gas up to a certain point, beyond which a decrease is observed. The gasification process is more effective in small diameter channels. Results obtained show that the production of carbon monoxide is critically dependent on the gasification agent, the reduction zone temperature, the average gas velocity, the contact time for the reduction reaction, and the channel diameter for a given reactivity of coal. Introduction Underground gasification of coal has received a great deal of attention in the past, as the numerous publications and field tests indicate. Currently, there is renewed interest in the process, first on account of its environmental advantages, and secondly, because the economics of the process are likely to be more favorable now than was the case in the earlier field tests. The process has not been analyzed and modeled mathematically to any great extent. In a previous publication, the authors presented the development of a mathematical model of the process, assuming producer gas stoichiometry, homogeneous reaction producer gas stoichiometry, homogeneous reaction kinetics, and one-dimensional heat and mass transport. This paper extends the previous model to consider two-dimensional mass transport, and heterogeneous reaction kinetics.