Technical and Economic Assessment of Mine Methane Mitigation and Utilisation Technologies

Coal mine methane (CMM) is not only a greenhouse gas but also a wasted energy resource if not utilised. This project addressed technical and economic merits/issues of the ventilation air methane (VAM) mitigation and utilisation technologies when implemented at mine sites, with a specific consideration of gas cleaning technology required for the VAM mitigation and utilisation plants. The technical difficulties associated with mitigation and utilisation of dilute mixtures of methane and air are explored using basic thermodynamic analyses of various mitigation and utilisation processes. The results of the analyses show that the thermodynamic availability (the theoretical maximum amount of work that can be done) of methane is fairly constant down to very low concentrations (less than 0.1%). However, the mixing of methane and air is an irreversible process, and the effect of process irreversibilities (inefficiencies) increases with decreasing methane concentration. This is what makes mitigating and utilising VAM more difficult as the methane concentration decreases.

Gas cleaning is one of major focuses of this project, i.e. determination of gas cleaning technologies for mine ventilation air before it enters the VAM mitigation and utilisation units. Based on the study of gas cleaning requirement, cleaning performance of each cleaning technology, and discussion with manufacturers, two types of barrier filters were recommended for the mine ventilation air cleaning process: self-cleaning and static. In addition, there is no need for the removal of gaseous contaminants (H2S, SO2) because their concentrations in ventilation air of case study mines A and D are well below the cleaning requirements by the VAM mitigation and utilisation units. 

Four types of plant systems were configured to identify technical merits or issues for the application of various VAM technologies at the two mines. The thermal flow reverse reactor (TFFR), catalytic flow reverse reactor (CFFR) or catalytic monolithic reactor (CMR) plants can mitigate 100% of VAM. The 1% methane turbine performs better than 1.6% methane turbine in terms of utilising VAM. Its combination with gas engines is the best option, and promises to use much more VAM, and offers substantially higher power then the other systems. In addition, drainage gas supply continuity is very important for continuously operating the turbines and gas engines without efficiency penalties and significant load reductions.

Major economic indicators including capital cost, operating costs, internal rate of return, and break-even cost have been examined to explore the broader issue of CH4 destruction versus energy recovery, and the effect of gas cleaning on the economics. The CMR VAM mitigation plant has the lowest capital cost of the three VAM mitigation plants, and has a lower net cost of operation. Gas cleaning certainly affects the economics of VAM mitigation plants, but not critically at the two mines. The lean burn turbine plants have higher capital and operating costs than the mitigation plants, but the sale of electricity provides a financial benefit that results in no need of values of carbon credit for a financially viable operation at Mine A. The use of gas engines only is the most financially viable case for both mines. The 1% methane turbines, with or without gas engines, is the most consistent performer with very little variation in performance between Mine A and Mine D, and promises to use much more ventilation air methane than others assessed in this report. Available drainage gas supply is also an important factor affecting both the power plant performance and economics.