Investigating adsorption properties of CO2 and CH4 in subbituminous coals from Mamu and Nsukka formations: a molecular simulation approach

Abstract

Abstract The study aimed to investigate the adsorption properties of methane (CH 4 ) and carbon dioxide (CO 2 ) in subbituminous coals from the Mamu and Nsukka formations, focusing on the CO 2 -Enhanced Coalbed Methane (ECBM) method. Proximate, ultimate, and FT-IR analyses determined the quality, age, and functional categories of these coals, confirming their subbituminous nature. Using molecular dynamics (MD) simulations, a unique amorphous subbituminous coal model was developed to study adsorption phenomena. Isosteric heat and adsorption isotherms for pure CO 2 and CH 4 were analyzed, alongside Grand Canonical Monte Carlo (GCMC) simulations to assess CO 2 adsorption selectivity in a binary CO 2 and CH 4 mixture. Results showed that CO 2 required more isosteric heat than CH 4 in single-component scenarios and demonstrated stronger electrostatic interactions with heteroatom groups in the coal model, explaining its higher adsorption preference. In binary adsorption experiments, CO 2 exhibited a higher affinity under specific conditions, particularly influenced by pressure variations. At lower pressures, CO 2 selectivity decreased rapidly with increasing temperature, while at higher pressures, the influence of temperature diminished. These findings have established a theoretical and practical basis for optimizing CO 2 -ECBM extraction in Nigeria, highlighting the preferential adsorption of CO 2 over CH 4 in subbituminous coals from the Mamu and Nsukka formations under varying pressure and temperature conditions. Implementing CO 2 -ECBM extraction and storage in Nigeria could boost economic viability and help achieve net-zero goals, using insights from this study to guide policy development. Graphical Abstract