Ventilation Air Methane has a very low methane concentration. Thus it is very difficult to use or to utilize. VAM contains the biggest amount of methane which is released by coal mines, but up to now there are no big scale technologies for its destruction or utilisation. However, currently a lot of research activities have been executed regarding thermal oxidation as a new technology to utilise VAM. Some demonstration projects are running or will be started in the future.
Especially in the frame of JI projects it is a very interesting project activity for generation of emission reduction units by restructure of VAM. Moreover, the methane is released in the vicinity of the ventilation shaft. This location is expected to have a long life time. So installed plants do not have to be moved, if the digging area changes in the underground.
VAM utilisation by thermal oxidation
Even if methane concentration of VAM is quite low (typically 0,3 to 0,7 percent), the volume of air that ventilation systems move is so big, that they actually form the largest source of methane emissions from underground coal mines. Basing on the experience of exhaust air treatment (e.g. air with solvent damp) a technology for VAM utilisation was developed: flow-reversal oxidation.
Flow-reversal oxidation - operating principals
If the inflow of ambient-temperature ventilation exhaust air is maintained in only one direction, the heated area of the bed would migrate across the bed in the direction of flow until the heat essentially is blown out of the bed. If that happens, bed temperatures would no longer be adequate to sustain auto-thermal operation, and the system would cease oxidizing subsequent VAM inflows. To preclude cooling of the bed, dampers and valves redirect the flow of incoming ventilation exhaust air from one side of the bed to the other, typically on a timeframe of every two or three minutes. This flow-reversal process, which is managed by a programmable logic controller, maintains the hot area of the bed in the middle of the oxidizer, where it is available to support oxidation of a constant stream of VAM over time.
However, there are a few drawbacks that show the inefficiency of the oxidizing process. In the first place it is the decrease in pressure during the VAM flow through the oxidizing chamber. There is also a lot of dirt in the flow of the VAM that causes contamination of the filling material and the valves. Another problem is the fluctuating heat dispersion. The dispersion effuses in a parabolic profile. So, in the middle of the chamber it is warmer than in the peripherals. The VAM from the ventilation shaft has to be compressed to compensate the pressure losses in the air – methane – mixture filtration and oxidation area. This additional pressure can’t be produced from the ventilation fan. All operating VAM projects to utilise CMM are in a research or a demonstration phase. At the moment two companies are notably active in developing VAM technologies. Moreover a third company is developing a VAM utilisation, but has not demonstrated its technology yet.
For more information see the description of these technologies from companies' papers:1. Vocsidizer