Mine Gas Quality Control: Methane Purification

Mine methane has several differing natures, being simultaneously a safety hazard, a Greenhouse gas and a potentially saleable product. It is therefore to the advantage of mine operators to evaluate if the waste methane can be processed to economic advantage, while also reducing the Greenhouse gas impact of the mine and utilising energy that would otherwise be wasted. Previously, the major research work in this area in Australia has examined options for producing electricity from the waste gas, however the low local electricity prices have made this difficult to achieve economically. Natural gas prices are comparatively high and, therefore, it appears more likely to be viable to purify the methane to a saleable quality. A number of processes suitable for methane purification were examined to determine the economic and technical feasibility of their use at Australian mines. Process simulations were prepared for the processes that appeared feasible and the results used to develop a cost model, which is available in Microsoft Excel spreadsheet form. Two processes appeared to be economically feasible if the feed gas could be maintained over 70% methane. Pressure swing adsorption (PSA) plant can produce pipeline quality methane economically, if a local user is available or a pipeline operator is prepared to purchase it at the mine site. Cryogenic plant has a higher capital cost but can produce a higher value liquefied natural gas (LNG) product economically. This product has an advantage of the gas product, as it can be stored and transported more readily. The economic analysis is sensitive to the feed composition, so increasing the concentration of methane or reducing the variability of flow and composition can significantly improve the economics. Both plants are robust, can be supplied in modular form, are relocatable, are fully automated to minimise operator requirements and can be self-powered using waste methane streams. Greenhouse gas emissions in the form of methane are minimal if the feed gas is maintained at methane content higher than 70%. At lower feed concentrations, a waste stream of dilute methane is likely to be produced.