ASSESSMENT OF CARBON DIOXIDE SEQUESTRATION IN FORT UNION COAL BEDS IN THE NORTHERN ROCKY MOUNTAINS-GREAT PLAINS COAL BASINS

Carbon dioxide (CO₂) sequestration in coal beds, which are too deep to mine at present economic and technological constraints, is an environmentally attractive technique, and if successful, might reduce the amount of greenhouse gas emissions released to the atmosphere. A common estimate of CO₂ adsorption by coal beds is assumed to be about twice that of methane (CH₄) for bituminous coal. However, new CO₂ and CH₄ adsorption isotherms of subbituminous and lignite coal beds, as much as 200-ft thick, in the Paleocene Fort Union Formation from the Powder River Basin (PRB), Williston Basin (WB), and Green River Basin (GRB) indicate that these low-rank coals might be capable of adsorbing 5 to 9 times more CO₂ than CH₄.

The CO₂ storage capacity was calculated from the 1999 U.S. Geological Survey assessment of coal resources in the PRB, WB, and GRB, and from CO₂ and recently analyzed adsorption isotherms of selected coal beds. There are 326 billion short tons (295 metric tons) of coal in the Wyodak-Anderson coal zone below a depth of 500 ft (152 m), which is presently considered too deep to be surface mined in the PRB. This coal zone could sequester as much as 390 TCF (11 TM³) of carbon dioxide from coal-fired power plants presently in operation in the PRB. In the WB, the Harmon-Hansen coal zone contains an estimated 23 billion short tons (21 metric tons) of coal below 500 ft (152 m) of overburden, which could sequester as much as 25 TCF (0.71 TM³) of carbon dioxide from coal-fired power plants presently in operation in the WB. In the GRB, the Deadman coal zone contains an estimated 3.7 billion short tons (3.4 metric tons) of coal below 500 ft (152 m), which could sequester as much as 3.7 TCF (0.11 TM³) of carbon dioxide from coal-fired power plants presently in operation in the GRB.

As of 2001 13 mine-mouth power plants emitted more than 81,160,700 short tons of carbon dioxide (1.315 TCF or 0.03724 TM³) in the PRB, WB, and GRB. At this rate of emission of CO₂, the presently unminable coal beds in these basins theoretically could sequester CO₂ for the next 300 years from these power plants. The close proximity (0-100 mi distance) of these coal beds to the power plants makes them ideal geological storage sinks.