Technological Assessment of a Recycle Reactor for VAM Abatement

Underground coal mining emits high volumes of methane, diluted in ventilation air to between 0.4 and 1%. As this is below the lower flammability limit of methane, alternative mitigation techniques need to be examined. This project examined the potential of catalytic combustion, where methane is converted to carbon dioxide over palladium-based catalysts at temperatures below 500oC. A VAM mitigation unit was developed consisting of an in-operando catalyst regeneration system. This system has been examined with alumina supported palladium catalysts, with the deactivation mechanisms investigated via spectroscopic techniques to identify potential reactivation conditions. It has been suggested that the presence of water vapour in the feed stream decreases catalytic activity, particularly at lower temperatures, due to the formation of inactive hydroxyl groups on the catalyst surface, blocking access to the PdO sites for methane dissociation. The role of the support is also shown to be important, with results indicating supports influence low palladium dispersion and TPD suggesting that increased oxygen mobility would decrease the rate of catalyst deactivation caused by the presence of water. It was found that high methane conversion could be achieved over Pd/Al2O3 at 480oC for extended time on stream, with deactivation able to be reversed by exposure to dry air, oxidising carbon deposits. Significant quantities of carbon deposits were observed, and it was estimated that approximately 94kg of carbon are deposited (in a typical 700 m3s-1 VAM stream) over a 450 hour time period. This can be regenerated over a time period of 10 minutes. The ability for a system to reactivate catalysts in operando allows for the potential of a continuous VAM mitigation system, which we have demonstrated experimentally.

A heat recovery system was modelled and a recovery system was designed to enable the VAM mitigation system to operate with minimal additional fuel, with process viability with respect to energy consumption and costings simulated with Aspen Plus software.