Research on Coalbed Gas Separation and Recovery in Coal Medium-Mixed Promoter Systems via Hydrate Technology

Efficient separation and recovery of low-concentration coalbed methane (LCCBM) are vital for energy utilization and greenhouse gas mitigation. However, hydrate-based separation technologies still struggle to balance kinetic performance and separation efficiency. Given coal’s intrinsic affinity for methane, this study explores its synergistic effect with mixed promoters for LCCBM separation. Experiments evaluated the separation of a 25%CH 4 /67%N 2 /8%O 2 mixture at 283.2 K (3 and 7 MPa) using two promoter systems: 5.56% (molar concentration) tetrahydrofuran (THF) and 0.5% (mass concentration) L -tryptophan (L-Trp), 5.56% (molar concentration) cyclopentane (CP) and 0.5% (mass concentration) L-Trp. We investigated four coal particle sizes (M1: 0.90–2.00, M2: 0.43–0.90, M3: 0.25–0.43, M4: 0.18–0.25 mm), using coal-free and 0.08 wt % expanded graphite (EG) systems as controls. The results demonstrate that, compared to the coal-free system, the comprehensive separation performance in both systems was improved. Coal particles altered the gas–liquid–solid interface and hydrate growth pattern, causing nonmonotonic changes in kinetics and separation. In the THF system, coal significantly promoted hydrate growth, the initial rate was approximately 2.84 times higher than that of the coal-free system without causing a decrease in separation performance. In the CP system, CP adsorption by coal slightly decreased the kinetics but significantly enhanced CH 4 selectivity, the methane mole fraction in the hydrate phase increased from 25% to 51.26%. M3 proved optimal, balancing high gas uptake and separation efficiency. While 0.08 wt % EG showed similar trends, coal provided superior enhancements.