Methane Recovery from Low and Variable Concentrations in Mine Ventilation Air: A Review of Adsorption and Membrane Separation

Coal mine ventilation air typically contains around 0.5% methane, although this is highly variable. With the large air flows involved in a typical mine, methane is being vented at around 1kg/s. Methane is approximately twenty-one times worse as a greenhouse gas than carbon dioxide, so the removal of the methane from mine ventilation air is attractive environmentally as well as providing a potential energy source. This review focuses on capturing the methane and concentrating it before subsequent combustion as a fuel. Two methods of concentration are reviewed: adsorption and membrane separation. Methods of implementing these technologies are also considered.

In general, separation of methane and air is not easy. The molecules are similar in size and in their ability to be polarised, making it difficult to separate them based on these parameters using membrane or adsorption respectively. Despite this, there are slight differences which may be exploited, however the process is not likely to be simple or cheap given the difficulty of the separation.

Membrane systems for gas separations are not yet widely available commercially. The most promising membranes for removing methane from air are silica, DDR and Sr-ETS-4. The latter two are zeolite membranes. Further work will be required before these membranes are available. A combination of membrane technology with other separation technology (scrubber-stripper) has also been proposed for removal of volatile organics from high volume air streams. The system has not been assessed for air/methane separation and would require careful selection of an appropriate membrane. It might be relevant for the current application.

Pressure Swing Adsorption and Temperature Swing Adsorption are much more preferred. Both are considered less expensive than membrane separation and preliminary work suggests they are able to remove low levels of methane from air using the correct adsorbent. Temperature swing adsorption does not require compression of the large air flow and only slight heating to desorb the methane, making it particularly attractive. This system has been considered previously for mine ventilation air in Germany using activated carbon.

From a range of adsorbents considered, activated carbon, zeolite 13X and ion-exchanged clinoptilolite (a natural zeolite) are the most promising. Adsorption characteristics can be altered by incorporating other species or by controlling preparation conditions. Activated carbon can be modified by deposition of bromine which blocks nitrogen adsorption, or impregnated with MoO2.

K- and Ba-exchanged clinoptilolite enhance the separation in favour of methane compared to natural clinoptilolite. Synthetic zeolite 13X performs similarly to K-clinoptilolite. Preparation conditions such as the activation temperature and time also affect the gas adsorption and separation performance of these adsorbents. Other adsorbents considered in this study but are not prospective for the current application include silica gel, gamma alumina, clays, oil shale and coal.

Relatively little work has been published involving the removal of low levels of methane from air. It would be advantageous to investigate whether activated carbon or identified zeolites are able to remove very low levels of methane from air by adsorption. A preliminary study is currently underway in this laboratory. If the systems show promise, a more exhaustive study would be needed. The combination of scrubber-stripper-membrane for low level gas separation also shows promise and could be interesting to test for the current application.

Based on the literature available, in the short term, the most prospective process would be adsorption, probably using activated carbon, to concentrate the methane in the air stream. This is the focus of the preliminary laboratory work. In the longer term, membrane separation may become feasible based on recent advances of zeolitic membranes for separating gases of similar size, and on combination with other types of processes.