High-pressure CH4 and CO2 sorption isotherms as a function of coal maturity and the influence of moisture

• All coals studied have CO2 vs. CH4 excess sorption capacity ratios > 2.
• Moisture-induced reduction of gas sorption capacity is lesser for high rank coals.
• Gas sorption properties of low-rank coals are controlled by functional groups.
• Microporosity and polyaromatic rings control sorption properties of high-rank coals.

Methane (CH4) and carbon dioxide (CO2) sorption isotherms have been measured on an Australian subbituminous, a German high-volatile bituminous and a German anthracite coal in the dry and moisture-equilibrated state. The purpose was to study the variation of CH4 and CO2 sorption capacities of the dry coals as a function of rank and the influence of water on the sorption properties. Methane sorption isotherms were measured at 303, 308, 318 and 334 K (30, 35, 45 and 61 °C), and CO2 isotherms at 318, 334 and 349 K (45, 61 and 76 °C).The excess sorption capacity of coals is always higher for CO2 than for CH4. The CO2 and CH4 sorption capacity of dry coals as a function of rank follows a parabolic trend reported in earlier studies, with a minimum at ~ 1% vitrinite reflectance. This trend is more pronounced for CO2 than for CH4. For moisturised coals a linear increase in CO2 and CH4 sorption capacity with coal rank was observed. Moisture reduces the gas sorption capacity of coals significantly. Moisture content therefore is a first-order control for the gas sorption capacity of low rank coals up to bituminous rank, with much higher impact than temperature or maturity. The moisture-induced reduction in CO2 and CH4 sorption capacity decreases with increasing coal rank. It correlates linearly with the oxygen content, which in turn correlates qualitatively with the amount of hydrophilic and carboxylic functional groups as evidenced by FTIR analysis.The influence of sorbed water on the sorption capacity is highest at low pressures (low surface coverage θ < 0.3). The dry/moist sorption capacity ratios converge towards 1 with increasing pressure (high surface coverage θ ≈ 0.7).