Laboratory Studies Of Coal Drying, Pyrolysis, And Combustion For UCG
Authors: Elliott Guy R.B , Vanderborgh Nicholas E. 
Published: December 2007

Experimental and theoretical analyses show that uncontrolled water invasion during underground coal conversion (UCC) is harmful at all stages of UCC. By contrast, if water invasion is prevented, coal porosity can be created for further processing, pyrolysis can yield uniform hydrocarbon products, gasification can produce a uniform product, coal is fully consumed (not bypassed) during combustion, and environmental problems are minimized.

Introduction

The Four Corners Region of New Mexico has long served as the major source of fossil fuel for the 10% (and growing) of the U.S. population which lives in the American Southwest. Over the long term, assuming continued rejection of the nuclear option, despite a temporary supply of Alaskan oil, and including possible future contributions from the Kaiparowitz possible future contributions from the Kaiparowitz Region of Utah, this fuel source must continue to supply electricity and hydrocarbon fuels for the Southwest. Other regions of the United States, as well, may need hydrocarbons from this Region. However, with the deplation of oil and natural gas supplies, it will be necessary to turn to coal both for electric power and for the hydrocarbon fuels.

Although the coal reserves of the Four Corners and Kaiparowitz Regions are vast, the strippable coal is far more limited—underground mining is already proposed for the Kaiparowitz Region, and the 3 deg. dip of the Four Corners coal seams leaves only relatively small amounts of coal at stripping depth, i.e., less than about 65 m (200 ft). Economic factors thus require that deeper coal be recovered, and safety and environmental factors demand development of new methods to recover that coal. The LASL concept of underground coal conversion (UCC) is one promising new method.

In its complete form (simpler versions are also en visioned, as will be discussed), the LASL advanced concept for subbituminous coals in arid or semi-arid regions involves preliminary physical isolation of the coal from the surrounding aquifers, followed by four chemical steps:The coal is dried at about 120 deg. C to produce greater porosity, to create uniform and reproducible conditions for subsequent processing, to accomplish the drying with low-grade heat, and to recover valuable water.The coal is pyrolyzed at about 300–600 deg. C to recover gaseous and perhaps liquid hydrocarbons. Some hydrocarbons will be sold as fuels or petrochemical feedstocks. Part may be blended with the coal gasification products so that a material of very uniform quality can be supplied.The coal is gasified with O /CO feed to yield an intermediate-Btu fuel.The fuel gas is cleaned and blended. LASL sees the use of this gas at a mine-mouth electricity generation station, but other uses for the fuel can be envisioned.

Because the Navajo Nation is equivocal in its attitude toward recovery of its strippable coal, the longterm fuel supply for present and future Four Corners Region electric generators is uncertain. Other coal supplies must be sought. Conventional underground mining in one expensive possibility. However, chemical recovery of deeper coal by the LASL concept should be reasonably straightforward, if water isolation (as in the previous paragraph) could be accomplished. Once water influx is controlled, successive drying, pyrolysis, then gasification with oxygen/carbon dioxide pyrolysis, then gasification with oxygen/carbon dioxide mixtures would provide a uniform, intermediate-Btu fuel gas with relatively low sulfur content to mix with and augment the powdered coal being burned in the generators. Because of the existing stack-gas cleaning facilities, and because the boilers handle coal pyrolysis gases, such generating stations offer a pyrolysis gases, such generating stations offer a particularly advantageous site for a staged development of particularly advantageous site for a staged development of various concepts ultimately to go into UCC technology for more broad-scale use.

In considering any coal recovery scheme for arid regions, one must recognize the overriding importance of water conservation.

THEORY

It is, perhaps, obvious that coal cannot burn if it is too wet. Control of water influx into coal beds during underground coal gasification (UCG) has seldom been attempted, and underground processing yields have been degraded accordingly. Uncontrolled water influx has implications throughout both the combined steps of simple gasification (UCG) and the individual steps of LASL's UCC.