Development of New Generation Carbon Composites for VAM Capture

Capturing ventilation air methane (VAM) as a way of fugitive methane mitigation, and using it as a clean energy source has been a challenging issue because of its large quantity and low methane level (0.3-1%). It is important to capture the fugitive methane in a cost-effective way. For VAM capture and enrichment technologies with solid adsorbents, there are mainly two areas to improve the performance and to reduce the total capital investment cost: adsorbent capacity and capture processes. This project aimed to develop new generation carbon composite adsorbents derived from low-cost and readily available biomass (macadamia nut shells (MNSs)), carbon nanotubes (CNTs) and phenolic resins, focusing on methane capture capacity, so that the VAM capture capacity can be enhanced more than 30 %, compared to that of the carbon fibre composites used in our previous project (C19054). The principal objectives were successfully accomplished.

To evaluate the dynamic VAM capture performance of the carbon composites, breakthrough tests were conducted with a simulated ventilation air (VA). Regardless of VA flow rate, the methane breakpoint times (BTs) were significantly increased.  For a practical VAM capture system design, denser adsorbents would save more capital costs as the composite volume is related to column sizes. When normalised over the composite volume, the BTs of all new composites (C2, C3 and C4) were improved significantly (mostly more than 70 %), compared to that of C1. In addition, as real VA contains high humidity, the effect of moisture on the VAM capture performance was investigated and found to be insignificant for all newly developed carbon composites (C2, C3 and C4) as well as C1.

As both the methane adsorption capacities and the bulk densities of composites C2, C3 and C4 were enhanced more than 30 %, compared to those of carbon fibre composites (C1), the amount of composites required can be substantially reduced, leading to reduction in the column space by more than 45 %.

Considering the costs of composites and column fabrication were estimated at about 44% and 15 %, respectively, of the total capital investment (TCI) costs for the VAM capture unit system, it is expected that this project would lead to a significant reduction in capital costs.

Considerable effort was made to develop new carbon composites from macadamia nut shells, carbon nanotube and phenolic resin to enhance VAM capture performance throughout the project. The following are recommended for further research. Firstly, as new carbon composites with greater methane capacities, the captured methane quality needs to be confirmed with regeneration processes which include steps of initial vacuum swing, temperature swing, final vacuum swing, air purging and cooling. It is expected that the captured methane quality would be higher than that with carbon fibre composites. The composites developed in this project need to be enlarged to compare with the results from our previous project (C19054). In this way, real costs associated with composite fabrication can be determined. Secondly, site trials are recommended to scale up CSIRO's VAM capture prototype unit to deal with 1 m3/s of VA. This will lead to development of large-scale composite fabrication and mass production processes. More realistic techno-economic analysis can be obtained throughout site trials as it is expected that there would be more safety requirements such as instrument selection, gas sensors, test unit fabrication and electrical connections. Thirdly, newly developed carbon composites from this project can enhance the CSIRO's VAM capture technology which can be applied for upgrading coal mine methane with lower costs than capturing VAM. The regeneration processes developed in our previous project (C19054) can be revisited and simplified to reduce the capital costs as well as the operating cost. Lastly, the upgraded coal mine methane can be stored in our newly developed composites as a buffer tank between coal mine methane streams and gas engines. As methane is mostly in the adsorbed phase, it is safer to transport the buffer tank than compressed gas.