Characterizing thermogenic coalbed gas from Polish coals of different ranks by hydrous pyrolysis

Abstract

To provide a better characterization of origin and volume of thermogenic gas generation from coals, hydrous pyrolysis experiments were conducted at 360 °C for 72 h on Polish coals ranging in rank from lignite (0.3% R r) to semi-anthracite (2.0% R r). Under these conditions, the lignites attained a medium-volatile bituminous rank (1.5% R r), high-volatile bituminous coals attained a low-volatile bituminous rank (1.7% R r), and the semi-anthracite obtained an anthracite rank (4.0% R r). Hydrous pyrolysis of a coal, irrespective of rank, provides a diagnostic δ 13C value for its thermogenic hydrocarbon gases. This value can be used quantitatively to interpret mixing of indigenous thermogenic gas with microbial methane or exogenous thermogenic gas from other sources. Thermogenic methane quantities range from 20 dm 3/kg of lignite (0.3% R r) to 0.35 dm 3/kg of semi-anthracite (2.0% R r). At a vitrinite reflectance of 1.7% R r, approximately 75% of the maximum potential for a coal to generate thermogenic methane has been expended. At a vitrinite reflectance of 1.7% R r, more than 90% of the maximum potential for a coal to generate CO 2 has been expended. Assuming that these quantities of generated CO 2 remain associated with a sourcing coal bed as uplift or erosion provide conditions conducive for microbial methanogenesis, the resulting quantities of microbial methane generated by complete CO 2 reduction can exceed the quantities of thermogenic methane generated from the same coal bed by a factor of 2–5.