Monitoring light hydrocarbons in Brazilian coal mines and in confined coal samples

Monitoring light hydrocarbons (LHCs) in coal mines, particularly methane, is important not only because of their implications for global climate change but also for economic and safety reasons. Furthermore, the identification and quantification of LHCs in coal mine air samples and desorbed from confined coal may contribute to a better understanding of coal seam characteristics. The paucity of information about the levels of methane in Brazilian underground coal mines can be attributed to their difficult access and a lack of adequate procedures for correct gas sampling. The aim of this study is to optimize and apply standard gas chromatography procedures to determine LHC levels in the air of coal mines and in confined coal from five mines under operation, three underground (A, B, C) and two surface (D, E) mines, in southern Brazil. The results indicate methane (C1) levels varying from 3 ppm to 27% in the atmosphere of the underground mines. Mine A presented high levels of all LHCs analyzed (C1 to C5), while only C1 and C2 were detected in mine B, and hydrocarbons ranging from C1 to C4 were found in mine C. On the other hand, surface mines presented narrow concentration range for C1 (3 ppm to 470 ppm) and C2-C3, with higher levels observed for puncture explosive points. Among LHCs, methane is desorbed in higher concentrations from confined coals and the presence of C2 was detected in all samples while C3-C5 were only observed in coals from underground mines. These data are consistent with those obtained from the air gas samples collected in the mines under study. Geological events such as faulting and intrusions can accelerate the release of gas or the trapping of large amounts of previously released methane. The LHC emissions from coal mines were found to be highly variable, indicating the need for a comprehensive survey of Brazilian coal mine emissions. (author)