Adsorption-based Enrichment and Stabilization of Methane in Mine Ventilation Air (MVA)

This report briefly examines the application of adsorption technology to the enrichment of methane in mine ventilation air (MVA). The aim of the study was to determine if adsorption technology is capable of providing enrichment of methane to the required level (assumed to be 1%v/v) to feed a VOCSIDIZER unit and if so, what the energy and CO2 emissions penalty was. Both pressure and temperature regeneration modes were to be studied and an initial assessment of infrastructure and engineering issues to be made. In addition, the role of adsorbent development was assessed and recommendations were made for future work in this area.

The study demonstrated that vacuum swing adsorption (VSA) and temperature swing adsorption (TSA) are both capable of enriching methane from low levels (down to 0.01%v/v) up to 1% v/v. In the case of VSA, provision of vacuum levels down to 0.1 kPa may be necessary to provide the enrichment using an activated carbon adsorbent. In the case of TSA, a hot purge gas stream at 180°C is needed to remove the adsorbed impurity - in this case carbon adsorbents are not appropriate due to safety concerns.

In both regeneration modes (TSA and VSA), the energy consumed by the separation process impacted negatively on the overall energy output of the VOCSIDIZER process and the CO2 emissions reduction attainable. In the case of vacuum swing adsorption, for inlet CH4 concentrations of less than 0.1% v/v, no net CO2 reduction is achieved (assuming no power is generated by the VOCSIDIZER). In the case of temperature swing adsorption, no net CO2 reduction is achieved for inlet CH4 concentrations of less than approximately 0.03% v/v. In both cases, the power consumed for adsorbent regeneration was significantly more (~ 80 times) than the minimum energy needed from a second law analysis. This suggests there is considerable scope for improved systems/process development.

The physical equipment items needed for application of VSA technology are well beyond that currently commercially available and suggest that TSA is more appropriate. To avoid the long cycle times and large bed sizes usually associated with TSA, it is recommended that honeycomb rotary adsorbers be investigated for their potential application to MVA methane enrichment.