Sorption of carbon dioxide, methane and nitrogen in dry coals at high pressure and moderate temperature

In view of the growing interest in enhanced coal bed methane recovery, a technique under investigation as a possible approach to the geological storage of carbon dioxide in a CO2CO2 capture and storage system, we have studied experimentally the interaction of different gases in contact with coal at pressure and temperature levels typical of coal seams. In particular, samples of coals of different geological origin and different characteristics have been considered. A gravimetric method using a magnetic suspension balance has been applied to measure the excess sorption isotherms of CO2CO2, CH4CH4 and N2N2 between 33 and 60 °C and up to 200 bar. The obtained sorption isotherms on two coal samples from Australia are consistent with the data measured in a laboratory there. Maximum sorption capacities of the coals investigated lie within the range of 5–14% weight per unit mass of coal for CO2CO2 and of 1–3% for CH4CH4 and N2N2. Moreover, for all gases sorption capacity has been found to decrease with increasing vitrinite reflectance.A Langmuir-like model has been applied to the sorption data, by fitting the isotherm parameters to the experimental values. The results confirm that this equation is a valuable option to describe gas sorption on coal. Moreover, it is shown that the two mechanisms involved in the gas uptake process, namely adsorption and absorption, can be separated only if independent experimental data about coal swelling are available. The proposed model is compared to another isotherm equation presented in the literature, namely a modified Dubinin–Radushkevich adsorption isotherm, which uses a linear law to describe absorption.