Enrichment of low concentration methane: an overview of ventilation air methane

Methane (CH4) is the second most important greenhouse gas after carbon dioxide (CO2), but its global warming potential is 21-28 times that of CO2. Coal mining accounts for 9% of global CH4 emissions, among which 60-70% is contributed by ventilation air methane (VAM). Currently the simplest way to reduce CH4 emissions from ventilation air is to thermally oxidize it to CO2; however the low and changeable CH4 concentrations (0.1-1.5% CH4) and the large volume of ventilation air make it a challenge since conventional technologies used for CH4 separation/purification in natural gas (CH4 concentration 55-98%) are not suitable for VAM enrichment. It is therefore highly desirable to concentrate VAM up to levels for further harnessing, as the utilization of VAM can not only reduce CH4 emissions but also provide extra economic benefits to relevant industries. Herein, for the first time, we present a review on both unconventional technologies and materials for VAM enrichment. The feasibility of technologies including vortex tubes, mechanical towers, gas hydrates, membranes and adsorption-based processes has been discussed, with focus on adsorption-based processes. Given that the adsorbents used in adsorption-based processes are one of the key factors for gas enrichment performance, materials including zeolites, porous carbon materials and metal-organic frameworks for methane separation have been critically analyzed and overviewed, covering the summary of the textural properties, CH4 adsorption capacity, CH4/N-2 equilibrium selectivity and CO2/CH4 equilibrium selectivity of these materials under ambient conditions and highlighting some new synthesis strategies to achieve high CH4 adsorption capacity and CH4/N-2 equilibrium selectivity. This review not only provides state-of-the-art technologies and materials for VAM enrichment (also applicable to other low grade CH4), which will inspire further studies to better mitigate and utilize VAM and other low grade CH4, but also supports the upcoming low-carbon economy.