Coalbed gas desorption in canisters: Consumption of trapped atmospheric oxygen and implications for measured gas quality

Desorption canisters are routinely employed to quantify coalbed gas contents in coals. If purging with inert gas or water flooding is not used, entrapment of air with ∝ 78.08 vol.% nitrogen (N2) in canisters during the loading of coal results in contamination by air and subsequent overestimates of N2 in desorbed coalbed gas. Pure coalbed gas does not contain any elemental oxygen (O2), whereas air contamination originally includes ∝ 20.95 vol.% O2 and has a N2/O2 volume ratio of ∝ 3.73. A correction for atmospheric N2 is often attempted by quantifying O2 in headspace gas and then proportionally subtracting atmospheric N2. However, this study shows that O2 is not a conservative proxy for air contamination in desorption canisters. Time-series of gas chromatographic (GC) compositional data from several desorption experiments using high volatile bituminous coals from the Illinois Basin and a New Zealand subbituminous coal document that atmospheric O2 was rapidly consumed, especially during the first 24 h. After about 2 weeks of desorption, the concentration of O2 declined to near or below GC detection limits. Irreversible loss of O2 in desorption canisters is caused by biological, chemical, and physical mechanisms. The use of O2 as a proxy for air contamination is justified only immediately after loading of desorption canisters, but such rapid measurements preclude meaningful assessment of coalbed gas concentrations. With increasing time and progressive loss of O2, the use of O2 content as a proxy for atmospheric N2 results in overestimates of N2 in desorbed coalbed gas. The indicated errors for nitrogen often range in hundreds of %. Such large analytical errors have a profound influence on market choices for CBM gas. An erroneously calculated N2 content in CBM would not meet specifications for most pipeline-quality gas. (author)