Theoretical investigation of CO2/N2-enhanced coalbed methane recovery in coal-derived asphaltenes

Abstract

The global energy supply relies heavily on fossil fuels, contributing significantly to greenhouse gas emissions and climate change. Despite efforts to reduce CO${}_2$ emissions through renewable energy, substantial amounts continue to be released. Coal bed methane recovery (CBM) presents opportunities in this regard, necessitating a thorough understanding to minimize associated risks. To gain better insight into N${}_2$-ECBM and CO${}_2$-ECBM processes, an asphaltene model was selected, heteroatoms (CH${}_2$, NH, CO, S) were incorporated into it, and the adsorption of CO${}_2$, CH${}_4$, and N${}_2$ was investigated. We utilized grand canonical Monte Carlo (GCMC), molecular dynamics (MD), and density functional theory (DFT) for this purpose. The results show that CO${}_2$ has the highest adsorption energy, with values of up to −12 kJ/mol on asphaltene models, compared to −8 kJ/mol for CH${}_4$ and −7 kJ/mol for N${}_2$. Temperature was found to reduce adsorption capacity, while increased pressure positively affected gas uptake, agreeing with experiments. The isosteric heats of adsorption for all gases were below 10 kJ/mol, indicating a physisorption process. Additionally, the self-diffusion coefficient increases with temperature in accordance with experiments, with CO${}_2$ displaying the lowest value among the studied gases, suggesting a stronger affinity to the asphaltene surface. DFT analysis showed that the adsorption of gases on asphaltene molecules was a physical process (van der Waals interaction). These findings highlight the suitability of asphaltene structures for CO${}_2$-ECBM, where CO${}_2$ can be used to displace methane in coal seams, aiding both methane recovery and greenhouse gas mitigation. The strong interaction between CO${}_2$ and asphaltene suggests that asphaltene-based adsorbents could be developed for more efficient carbon capture and storage technologies.