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

Abstract

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/N₂ 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/N₂ 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.