Selective Absorption of Methane by Ionic Liquids

The connection of a ventilation air methane (VAM) abatement plant directly to a ventilation shaft raises significant safety issues for underground coal mines. Eliminating the risk of a fire or explosion caused by an abatement plant is considered a step-change in abatement technology. To eliminate this risk, the plant must operate below the autoignition temperature of methane. A concept developed by The University of Newcastle using ionic liquids can selectively absorb methane from ventilation air below 100°C. The process operates approximately 500°C below the autoignition temperature of methane, completely eliminating the risk of a mine fire caused by an abatement system. The ionic liquid looping process would produce a concentrated methane stream that could be utilised for power or heat. Hence VAM would be converted to carbon dioxide, reducing greenhouse gas emissions from the mining activities. The captured methane would be sufficient to provide the energy required for the process and therefore there would be no increase in the consumption of electricity for the process.

The principal vision of this project was to carry out fundamental investigations into the chemistry of ionic liquids and low-concentration methane mixtures and develop a proof-of-concept process for the absorption and desorption of methane using ionic liquids. To fulfil the above vision, the following objectives were defined:

• Determine the effect of temperature on methane solubility and selectivity;
• Determine the effect of pressure on methane solubility and selectivity;
• Determine the optimal ionic liquid properties for the absorption of methane from low-concentration methane-air mixtures;
• Determine the effect of temperature and pressure on the desorption rate of methane from ionic liquids;
• Evaluate the performance of ionic liquids and undertake a preliminary feasibility assessment;
• Report the key findings and provide recommendations for advancing the next phases of the program of study.

The key findings for the project were:

• Methane absorption decreased with increasing ionic liquid molecular mass;
• Gas absorption decreases with increasing temperature;
• Gas absorption occurs at low pressures and increases with increasing pressure;
• The absorption process is reversible when desorption occurs at moderate temperatures;
• Methane is selectively absorbed from air at low concentrations, at selectivities greater than predicted ideal selectivities;
• Overall assessment: from a technical standpoint the absorption of VAM using ionic liquids is quite feasible.

Translating the findings of this project to a practical unit for VAM abatement requires further examination of the process at laboratory- and pilot-scales under a continuous mode of operation. Such examinations are a key step in the transition from the fundamental chemistry investigations already completed in this project to the demonstration of the process in an appropriate absorption technology.

This project identified that the absorption process is not solely governed by the ionic liquid chemical properties and the ionic liquid physical properties and process conditions influence the quantity of methane absorbed. Given the scale dependency of process conditions, the investigations necessary to elucidate the mass transfer mechanisms and the key process variables must be conducted at laboratory- and pilot-scale. Packed beds were identified as the most suitable technology for gas absorption in ionic liquids. Specifically, rotating packed beds have shown to increase mass transfer rates by an order of magnitude compared with conventional packed beds. Given the high gas flow rates of ventilation air from underground coal mines, a high mass transfer rate is likely to be necessary for the absorption of VAM.