As the air volumes that must be processed for abating VAM emissions are very large, the modularity of the processing system is important. VOCSIDIZERs are arranged in groups of four units on two levels in a so-called 'VAM cube', each capable of processing 250,000m3 h-1 of ventilation air. Large installations consist of multiple VAM cubes, each with a footprint of approximately 500m2 (Figure 12.3).
As the system is an air-processing installation, the CH4 reduction capacity relates directly to the VAM concentration, where a 0.3 per cent CH4 concentration 'generates' annual emission reductions per 'VAM cube' of around 80,000 tonnes of CO2-eq, 0.6 per cent CH4 around 160,000 tonnes CO2-eq and 0.9 per cent CH4 around 240,000 tonnes CO2-eq. An installation processing 1 million m3 h-1 of coal mine ventilation air (equivalent to the capacity of four VAM cubes) can therefore achieve annual emission reductions of the order of 1 million tonnes of CO2-eq.
Despite being in an extremely dilute form, the amount of energy contained in the mine ventilation air can be very high. From a VAM cube processing approximately 250,000m3 h-1 of ventilation air, the thermal energy recoverable from 0.6 per cent VAM is around 10MW of heat energy. Increasing or decreasing the VAM concentration by 0.3 per cent means increasing or decreasing the recoverable energy content by around 7MW.
As previously mentioned, the main limitation to energy generation using VAM VOCSIDIZER technology is not the extremely dilute nature of the 'fuel', but rather the requirement for the system to be located in close vicinity to the mine ventilation shaft.
The carrier of the thermal energy produced from the VAM can be hot water, hot oil or steam, depending on the intended use of the recovered energy.
This can include space heating, cooling (by driving absorption chillers) or for the generation of electricity. Normally, the thermal energy requirement (for example space heating in buildings) near the mine ventilation shaft is very limited. Therefore, in most cases, it is sufficient to apply secondary heat exchange to the exhaust ducts that leave the VOCSIDIZER equipment, to provide for the relatively small heat energy demands on-site (for example hot water for miners' showers and heating of buildings) - a single VAM cube processing ventilation air with an average VAM concentration of 0.6 per cent can generate 8MW of 70°C water.
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