Optimization of coalbed methane liquefaction process based on parallel nitrogen reverse Brayton cycle under varying methane contents and liquefaction ratios

The recovery of coalbed methane (CBM) is of significance in preventing coal mining accidents, environmental protection, and energy conservation. Liquefying natural gas purified from CBM, as an efficient storage and transportation technology, provides a convenient way to get CBM products. A CBM liquefaction process based on parallel N2-RBC is investigated to explore the technical feasibility of purifying liquefied natural gas (LNG) from CBM solely through a liquefication process. Sensitivity analysis was conducted to explore the impacts of design parameters on system performance, followed by a global optimization using a genetic algorithm under varying methane contents and liquefaction ratios. Analysis of the optimized results revealed that the specific energy consumption (SEC) and refrigerant flow rate decrease as the discharge pressure of the refrigerant compressor increases, and the decline rate slows down when it exceeds 3000 kPa. SEC decreases with a rise in methane content in CBM, and the decline rate accelerates when methane content in CBM exceeds 60%. SEC and methane recovery rate increase with an increment in liquefaction ratio, while methane concentration in the liquid product decreases. LNG can be feasibly extracted from CBM containing methane content exceeding 70% solely through a liquefaction process while maintaining an acceptable methane recovery rate and SEC.