An Advanced Numerical Model Of Underground Coal Gasification By The Stream Method, Using Simultaneous Solution
Authors: Pasha M.L. 
Published: December 2007

A comprehensive numerical model of the under-ground coal gasification process via the stream method was developed for a two-dimensional geometry. Seven reactions, involving carbon, carbon dioxide, carbon monoxide, oxygen, water/steam, hydrogen, and methane are accounted for. Nitrogen is the seventh gas component. As a result of gasification, the channel diameter increases with time, and a combustible gas is produced. The model accounts for all of these factors.

Introduction

Considering the fact that vast known coal reserves occur in the United States, much research has been directed toward producing synthetic liquids and gases via above ground gasification and liquefaction of coal which lends itself to large scale operations. The current federal and state pollution control regulations and the enormous cost of mining, transporting and processing make this process commercially unfeasible. Furthermore a low quality coal cannot be used in this process. Some of these problems can be avoided if gasification of coal in situ is utilized.

This process, known as underground gasification of coal, is defined as the controlled burning of coal seam in the presence of gas mixture consisting of mainly nitrogen, oxygen and steam, to produce gasification reactions. A combustible gas mixture containing nitrogen, carbon monoxide, steam, hydrogen, methane and carbon dioxide is formed, which can be used in the generation of electricity on a commercial scale.

This method possesses many advantages over conventional mining and synthetic fuel operations in that it minimizes health hazards to miners, improves process safety, provides ecological benefits in that process safety, provides ecological benefits in that the land surface is left intact, and eliminates much of the need for aboveground plant requirements. Furthermore, it offers a technically simple and feasible method for producing the vast bituminous and low rank coal reserves in the United States (much of which can neither be economically mined nor is acceptable in rank for commercial synthetic fuel operations), and if designed properly can be very economical.

In order to increase the understanding of this process, a series of steady state analytical models process, a series of steady state analytical models by the stream methods were developed by Magnani, by Magnani and Farouq Ali, and by Farouq Ali and Pasha. The steady state assumption of the process enabled the extraction of closed form solutions, which, though complex, were useful for carrying out parameter sensitivity studies. Furthermore, these models were of considerable value for carrying out pseudosteady state simulation of unsteady state underground coal gasification process.

The present model is a numerical extension of the previous models, and as a result is considerably more general. A two dimensional axisymmetrical coal seam is assumed to be ignited at one end and thereafter a mixture of air and steam (or a mixture of oxygen enriched air and steam) is injected. As a result, the combustion zone advances in both axial and radial directions and the channel wall recedes resulting in the production of a combustible gas mixture.

It should be mentioned that while this study has been devoted to the stream method of underground coal gasification, a number of other simulation studies has been reported which consider other gasification processes. These include the works of Kotowski and processes. These include the works of Kotowski and Gunn, Gunn and Whiteman, Winslow, Thorsness and Rosza, and Dinsmoor, Galland and Edgar.

MODEL ASSUMPTIONS

The coal bed consists of 100% carbon. However, the model allows for source terms in all components considered and thus the influx of any of these as a function of distance and time may be simulated. The gasification channel is initially assumed to be of cylindrical geometry, one extremity of which is ignited.