An Appraisal of Underground Coal Gasification
Authors: Watson William I. 
Published: December 2007

This paper was prepared for the 48th Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in Las Vegas, Nev., Sept. 30-Oct. 3, 1973. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgement of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made.
Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Introduction
It is quite clear that coal has supplied and will continue to supply a substantial amount of the energy needs in the U.S. A recent study indicated that coal supplied about 19 percent of U.S. energy needs in 1971 and will supply about 17 percent in 1985. Because of the projected future increase in total energy requirements and short supplies of alternate fuels such as natural gas, the actual tons of coal needed in the U.S. is expected to approximately double between now and 1985.
The conventional way to utilize the energy value of coal is to mine it either by surface or underground methods and then burn or process the coal in some way to make use of its energy value.
Another potential way is to recover the energy values by underground gasification, which we define as the partial combustion of coal in situ to produce combustible gases. This can be done through boreholes and provides a potential way to use our coal resources with greater safety and lower environmental cost. It could also increase the utilization of coal resources by giving a higher recovery of the fuel values in a coal seam and by the utilization of coal seams not now minable by conventional methods.
Underground gasification also has the very important potential to reduce or eliminate many of the undesirable side effects associated with conventional surface and underground coal mining such as acid mine drainage, refuse dumps, spoil banks, stream siltation, subsidence, uncontrolled fires, and the health and safety hazards in underground mines where the payments for "black lung" disability alone represent over $1 billion per year.
It is important to understand that the process of recovering energy from coal ends not process of recovering energy from coal ends not with the solid or gaseous product from the mine, but when these products are converted to electricity, to a high heating value gas, or used as a raw material for chemical manufacture. Thus, the energy form in which the coal deposit is recovered must be coupled to the demand centers. Fig. 1 illustrates a coupling that could be used to connect underground coal gasification product gases, which will have a low heating value, to U.S. energy demand market centers.
Fig. 1 shows the alternative of a supercharged-boiler/gas-turbine cycle that could be adopted to supply air, steam and oxygen for the gasifying agent. There are other alternatives, of course, with the simplest being that of using the mine product gas as a fuel in a conventional mine mouth power plant. plant.