Geotimes
Highlights
Coal Science
Leslie Ruppert

According to figures from the Energy Information Agency, global coal consumption in 2002 was 5,263 million short tons (mst) — up from 5,116 mst in 2001 (consumption data). Coal is used to produce most of the world's electricity, and research is currently focused on assessment of resources, coal quality parameters, coal utilization, combustion, emission technologies, coal waste management and by-product recovery. More recently, however, three issues have come to the research forefront: coal bed methane (CBM), carbon-dioxide sequestration and mercury pollution.

Interest in coal bed methane is widespread and exploratory drilling is occurring in countries around the world, from Australia to Ukraine. The United States is the leading producer of CBM, where it accounts for almost 10 percent of domestic natural gas production. The San Juan and Warrior basins continue to be the largest producers in this country, but exploration and production from low-rank coal in the Powder River Basin has increased rapidly and will continue to grow. New exploratory CBM drilling has been proceeding in the Paleocene/Eocene Wilcox Group coals along the Gulf Coast, and production has started in the Upper Cretaceous Olmas coals of South Texas. In addition, Pennsylvanian coals from the Illinois and Appalachian Basins and coals in Oklahoma and Arizona continue to be targeted for new CBM exploration.

Increased anthropogenic atmospheric carbon dioxide from fossil fuel combustion is considered by some to be one of the primary causes of global warming. This year, new pilot studies designed to evaluate the commercial viability of injecting and permanently sequestering carbon dioxide from coal-fired power plants into unmined coal beds are being funded by the U.S. Department of Energy. Coal beds are unique in their sequestration characteristics because they have much higher gas storage capacities than other potential geologic media. Once injected, carbon dioxide competes with methane for pore space within the coal, usually resulting in enhanced methane production by displacing the methane in the coal, where it is preferentially adsorbed onto the coal surfaces.

Heightened interest in CBM and carbon-dioxide sequestration has resulted in an impetus to document the microstructure of coal and its components so that the processes affecting adsorption and desorption of gases in coal beds may be defined and modeled. Two new studies last year reported significant progress on this front. In one report, scientists at the U.S. Geological Survey showed that, contrary to previous work on bituminous coals, adsorption capacities for carbon dioxide can be 6 to 18 times higher than the adsorption capacities of methane in some subbituminous coals and lignites. This research implies that deep, uneconomic low-rank coals may be potential reservoirs for carbon-dioxide sequestration, even when the methane content is low.

In another study, Karancan and Mitchell (International Journal of Coal Geology, v. 53, issue 4, p. 201-218) examined the behavior of different coal components, or microlithotypes, in whole coal samples exposed to carbon dioxide with X-ray computerized tomography. They found that different microlithotypes display varying degrees of gas adsorption and swelling, with clay and inertinite-rich components showing the highest adsorption and swelling while vitrite and liptite components showed the least change. Knowing the adsorption properties of coal lithotypes is critical to predicting gas injection rates for carbon-dioxide sequestration and production rates of CBM recovery.

Mercury emissions from coal fired power plants in the United States are estimated to be 43 tons annually. Globally, concerns are widespread over air quality issues, and concentrations of mercury in fish are leading governments to investigate or regulate mercury emissions. In the United States, the proposed Clear Skies Act, calling for caps on mercury, nitrogen oxide and sulfur dioxide emissions from electric power plants, was a dominant environmental news story last year. This past February, a bill was submitted to the U.S. House of Representatives and Senate as H.R. 999 and S. 485, respectively. It calls for mercury reductions of 46 percent and sulfur dioxide reductions of 60 percent of current emissions by 2010, with more cutbacks scheduled for the future (EPA's Clear Skies). The regulations would have a direct impact on the composition of coal by-products and waste products. High-sulfur coal producers may actually benefit from such regulations, however, as methods used to clean sulfur from coals are likely to remove the mercury from the coal as well.

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Ruppert is a coal geologist and inorganic petrologist with the U.S. Geological Survey's Eastern Energy Resources Team. E-mail: lruppert@usgs.gov.

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