Carbon Fibre Composites for Ventilation Air Methane Capture

Coal mine methane (CMM) is not only a greenhouse gas but also a wasted energy resource if not utilised. Research and development on mine methane mitigation and utilisation focuses on methane emitted from underground coal mines, in particular ventilation air methane (VAM) capture and utilisation, because (1) it represents most of the methane emissions from coal mines; and (2) it is the most difficult to capture and use as the air volume is large and the methane resource is dilute and variable in concentration and flow rate. Therefore, this project investigated a new process of capturing VAM in high concentration by developing new nano-structured carbon fibre composite adsorbents. VAM can be adsorbed in pores of the adsorbents, and then desorbed by electrical swing or thermal swing into a high methane concentration stream.

This project investigated the feasibility of monolithic carbon fibre composite (MCFC) adsorbent for capturing ventilation air methane (VAM) in order to reduce the fugitive greenhouse gas emission and also to use the captured methane as a clean energy source. During the project, we have successfully fabricated and characterised a series of MCFC adsorbents. The experimental data demonstrated the proof-of-concept of using the MCFC adsorbent to capture VAM. From the study, it was evident that the adsorption capacity of the MCFC adsorbents is double that of the commercial activated carbon. Well above 95% of methane in ventilation air can be captured by MCFC adsorbents. During the study, the methane concentration of 0.56% in the inlet gas stream is reduced to about 0.011% after it passes through the novel carbon fibre composite adsorbent material at ambient temperature and atmospheric pressure. This amounts to a maximum capture efficiency of 98%.

Monolithic carbon fibre composite adsorbents were fabricated at the QCAT laboratory. Various carbon fibres were used as precursor material, and monolithic composite adsorbents were fabricated from each of these carbon fibres. The fabricated monolith adsorbents were tested for maximum CH4 adsorption capacities and adsorption kinetics. The time history of CH4 adsorbed was measured in an adsorption breakthrough test rig. Further, the performance of MCFC adsorbents was compared with commercially available pelletised activated carbon adsorbent.

After a series of trials, optimum conditions for monolith fabrication were identified. The major part of the pore formation and reforming happens during the carbonisation and activation stages. Burn-off (weight loss of the sample during activation), which determines the time of activation was found to be optimal around 20-40%. Activation for a longer time resulting in higher burn-off (more than 40%) was not found to be beneficial. This could possibly be due to closing of the formed pores, which can be seen from the decrease in the measured pore width value. As a result, there was no significant improvement in CH4 uptake beyond this burn-off point and the strength of the sample is significantly reduced. Hence, activation is an important step during the preparation of the carbon fibre monolith adsorbent and the burn-off shows the level of activation. As the burn-off increases, pore characteristics of the material changes and the structural integrity of the sample is reduced.

When VAM capture is captured at atmospheric pressure and ambient temperature, it has a number of advantages including no need for compression or a chilling process etc, which reduces the capital and operational costs significantly for mine site implementation. This needs to be further studied with results from the recommended larger scale test unit trials followed by an economic assessment.