1403 Figure 1. Schematic of an underground coal mine with dedicated drainage system and surface facilities for complex CMM energy recovery ( both: from drainage gas and VAM) (Courtesy of Green Gas International) The gas released from coal seams is a big problem for mining operations. The methane, which is stored in a micro-porous network of coal seams, was formed during a coalification process (Singh, Kumar, 2015; Baris, 2013). The higher rank coal, the more methane it contains (Kholod et al., 2020). Properly adapted ventilation is intended to reduce the concentration of methane per volume unit to a level, that guarantees the safety of workers. VAM is such a mixture of methane and air, where methane concentration is very lean, usually below 1 vol.%. The amount of methane released per 1 ton will be different at different mine locations. For example, in Poland, the amount of VAM emitted into the atmosphere per ton of extracted coal mined is over 9 m3 , in Indian gassy mines the value is more than 10 m3 (Singh, Mallick, 2015). It would be supposed that reducing the number of mines will solve the problem of VAM emissions, or at least reduce it. The problem of methane emission is still present even after decades of the coal mine abandonment (Kholod et al., 2020). Furthermore, it turned out that even a significant reduction of coal extraction and the number of mines does not lead to a reduction of VAM, what has been noticed in Poland. As reported in (Global Green House Gas Emissions Data, accessed on 15th June 2021), there are several factors contributing to this phenomenon as increasing longwall lengths, mining speed, and increasing saturation of coal seam related to deeper exploitation of the raw material (e.g. in some Polish coal mines coal is extracted from 1000 m below the ground level (Dreger, 2019). Characteristic of VAM includes: dust load, particle size, mineral matter of the dust, variation in methane concentration and flow rate, possibility of existence of other gaseous pollutants as H2S or SO2 (Su et al.,2008, Hinde et al., 2016), He, H2, HCN,NH3, NOx, H2S, SO2 (Setiawan et al., 2014) and high amount of water (Fernandez et al., 2015). The fresh air composition supplied to the mine undergoes unfavorable changes, which are the result of mixing gases flowing out of rocks and gases resulting from technological processes and oxidation processes with the pumped air. The temperature of the air rises while it comes into contact with the rock mass and with power devices. Its humidity is also increased. It is worth mentioning, that the availability of detailed data of the VAM characteristics is poor and insufficient. The indicated parameters characterizing ventilation air cannot be generalized on the scale of the mining region or the country. Detailed data is important for selecting the appropriate technology for VAM disposal. A good example is an article (Su et al., 2008) that summarizes the results of the three-month tests of measurement results of compositions of ventilation air emitted to the atmosphere from four gaseous Australian mines. It was shown that the measured values were different between the examined objects but also depend on the currently conducted mining works (see Table 1).
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