Kinetic evaluation of hydrate-based coalbed methane recovery process promoted by structure II thermodynamic promoters and amino acids

Coalbed methane recovery is crucial to coal mine safety, greenhouse gas emission reduction and economic benefits. Gas hydrate technology can be employed to separate CH4 from the N2-rich coal mine gas. To enhance hydrate formation kinetics, separation performance and CH4 recovery, we examined the synergistic effect of structure II (sII) hydrate promoters and amino acids on hydrate formation of 30 mol% CH4/70 mol% N2 mixture, involving three thermodynamic promoters (tetrahydrofuran (THF), cyclopentane (CP) and propane) and four amino acids (l-cysteine, l-methionine, l-tryptophan and l-lysine). Amino acids exhibited extraordinary promotion effects on the propane system with over 10 times improvement in kinetics. CP-amino acid systems excelled in separation performance by enriching CH4 content from 30 mol% to 70 mol%. THF-amino acid systems showed moderate performance on kinetics and separation but achieved the highest CH4 recovery up to 50.27%. The impacts of amino acids were system-dependent: they did not alter the hydrate growth pattern of the THF system but completely changed the hydrate morphology of CP and propane systems. Two possible mechanisms were discussed including interfacial tension alteration and hydrophobicity of amino acids. These insights would provide a basis for further optimizing the hydrate process promoted by sII promoters and amino acids for applications including coalbed methane recovery and beyond.