THE POTENTIAL FOR CARBON DIOXIDE SEQUESTRATION IN COAL BEDS: NEW EVIDENCE FROM METHANE AND CARBON DIOXIDE ADSORPTION ANALYSES OF COALS FROM LIGNITE TO ANTHRACITE

Published data on methane adsorption in coals are much more common than adsorption data on carbon dioxide and most of those measurements were made on bituminous coals. Initial published studies of the two gases reported that the ratio of carbon dioxide adsorption to methane adsorption in bituminous coals was approximately 2 to 1. In contrast, more recently determined adsorption isotherms from samples of low-rank coals (lignite to subbituminous) from North Dakota, Texas, and the Powder River Basin, Wyoming, document much higher values of the carbon dioxide to methane ratio - ranging from 6 to 18.

Based upon data from our low-rank coals from the Northern Great Plains and Texas, and from published analyses of bituminous coals, the carbon dioxide/methane adsorption ratio correlates with coal rank. The lower rank coals (lignites) have the higher adsorption ratios. This correlation appears to be controlled by the fact that methane adsorption values correlate significantly with rank parameters, such as calorific value, moisture, and fixed carbon; whereas carbon dioxide adsorption values are scattered randomly through coal ranks. This lack of correlation between carbon dioxide adsorption and coal rank is unexpected and may be related to the compositional differences of the coals. Within our sample sets, which include coals from lignite to anthracite, the anthracite samples adsorb the greater amounts of methane.

Deep, unminable low-rank coal beds should not be overlooked as sites in which to sequester carbon dioxide. Although the methane adsorption volumes of the low-rank coals are relatively low, they may adsorb as much, or more, carbon dioxide as coals of higher rank. The potential for recovery of methane from the coals as a by-product of the sequestration of carbon dioxide exists for coals of all ranks.