Underground Gasification for Unrecoverable East Appalachian Coals
Authors: Forrester R.C. , Westmoreland P.R. 
Published: December 2007

Introduction

For more than 100 years, the in-situ extraction of energy in coal in usable form has been recognized as the most potentially active energy production alternative. Fires potentially active energy production alternative. Fires have started spontaneously in coal mines and burned for decades without the help of man, evidence of the technical feasibility of the concept. However, control and economics were problems. From Siemen's proposal of the concept in 1868 through the numerous experiments of many nations, the idea has persisted in spite of limited technical and economic success. Interest in in-situ recovery came to a halt in the 1960's because of the abundance of low-cost oil and natural gas. The changing energy picture of the 1970's has renewed interest in in-situ gasification. Underground coal gasification (UCG) has remained attractive during the years, when compared with mining and surface processing of coal, because the processhas greatly reduced capital requirements,has reduced manpower requirements,can recover otherwise unusable resources, andhas the potential to reduce environmental impact.

Although proposed processes vary greatly as to type of process and operating conditions, all attempt to process and operating conditions, all attempt to introduce a gasifying medium (some combinations of air, oxygen, and steam) into the coal seam to produce combustible gas by partial combustion of coal and to extract the product gas from the coal seam. Several processes are technically feasible, but have been plagued processes are technically feasible, but have been plagued by problems with product quantity and quality control. The primary reason that UCG has not been successful commercially is that the economics has not been favorable. Perhaps the greatest advantage of UCG is that the process offers a way to use a resource that otherwise is unrecoverable by conventional means.

Past UCG Concepts Past UCG Concepts Past processes may be grouped into four general Past processes may be grouped into four general categories:vertical shaft methods,packed-bed methods,long-wall techniques, andblind borehole techniques.

The underground gasification research conducted before 1971 has been reported.' More recent work in this area was presented at UCG symposiums during 1975-77. These techniques generally were developed for coal seams that were below the ground water table and were thick enough to justify the relatively high percentage of drilling through the overburden vs drilling percentage of drilling through the overburden vs drilling in the coal bed. Recently, UCG development has shifted from unrecoverable coal seams to commercially viable coal seams, where UCG is potentially a more economical recovery technique.

Proposed Kentucky UCG Concept Proposed Kentucky UCG Concept In the Appalachian region, numerous coal seams outcrop above the ground water table. These seams are less than 20 in. (51 cm) thick; no underground mining systems exist today for mining these seams. Strip mining can only mine a limited amount of coal in these thin seams because of the excessive overburdens. U.S. reserves of these thin or dipping seams are estimated to range from 150 to 680 billion tons (136 to 617 Pg). The Soviet UCG experiments have shown that applying present UCG techniques to thin seams (30 in. (76 cm)] or less results in a gas of 80 Btu/cu ft (2981 kJ/m3) or less. The reduced heating value of the gas is caused by excessive heat losses to the surrounding strata in the UCG chamber. Reducing the gasification temperature will reduce this heat loss and allow recovery of these seams. We propose a UCG concept that relies on negative pressure in the gasification zone and lower temperatures. pressure in the gasification zone and lower temperatures. With an extension of present technology of auger mining, the process can prepare the seam for UCG. In the initial tests, the gasification ports are auger holes drilled horizontally from the surface; no special fracturing techniques are anticipated since the auger process increases the permeability of the coal layer between successive auger holes. Air and steam enter one auger hole. This gasification medium will pass through the gasification zone and be pumped out of the following auger hole. The gasification zone is slightly subatmospheric. Air can enter through the previous gasified zone and through the entrance port and also can leak through the sides and overburden. The steam/air ratio can be adjusted to compensate for air leakage from other areas. We anticipate that air leakage from outside the seam will be minimal in comparison with the air passing through previously gasified areas. Some gas from the gasification previously gasified areas. Some gas from the gasification process can be used to power the auger process. process can be used to power the auger process. We propose that the augering process can recover 40 to 60% of the coal. UCG should recover an additional 25%. Thus, the proposed recovery will range from 65 to 85%. This is substantially more than other UCG processes. processes. Our process uses much existing technology and has minimal environmental effect because of negative pressures. Developmental areas include research in fire pressures. Developmental areas include research in fire control and auger techniques for extending present augur limits beyond 600 to 2,000 ft (183 to 610 m). Evaluation of the concept began with a computer simulation and laboratory measurements in a quartz tube gasifier with a movable heater for heat-loss compensation.

JPT