Biomass-derived carbon composites for enrichment of dilute methane from underground coal mines

Ventilation air methane (VAM), which is the main source of greenhouse gas emissions from coal mines, has been a great challenge to deal with due to its huge flow rates and dilute methane levels (typically 0.3-1.0 vol%) with almost 100% humidity. As part of our continuous endeavor to further improve the methane adsorption capacity of carbon composites, this paper presents new carbon composites derived from macadamia nut shells (MNSs) and incorporated with carbon nanotubes (CNTs). These new carbon composites were fabricated in a honeycomb monolithic structure to tolerate dusty environment and to minimize pressure drop. This paper demonstrates the importance of biomass particle size distributions when formed in a composite and methane adsorption capacities at low pressures relevant to VAM levels. The selectivity of methane over nitrogen was about 10.4 at each relevant partial pressure, which was much greater than that (6.5) obtained conventionally (at very low pressures), suggesting that capturing methane in the presence of pre-adsorbed nitrogen would be a practical option. The equilibrium and dynamic performance of biomass-derived carbon composites were enhanced by 30 and 84%, respectively, compared to those of our previous carbon fiber composites. In addition, the presence of moisture in ventilation air resulted in a negligible effect on the dynamic VAM capture performance of the carbon composites, suggesting that our carbon composites have a great potential for site applications at coal mines because the cost and performance of solid adsorbents are critical factors to consider.