Molecular hydrogen formed via low temperature oxidation of bituminous coals as the source of explosions in underground coal mines

Explosions in deep mines (or other confined spaces containing bituminous coals) are usually attributed to methane gas concentrations above the LEL or accumulation of fine coal dust which can undergo fast radical reactions. In order to avoid occurrence of such conditions in deep coal mines, methane detectors are installed and good ventilation is required. Coal piles stored for long periods at ambient temperatures might undergo autocatalytic heating if the rate of heat dissipation from the pile is lower than the heat release due to the chemisorption/oxidation of atmospheric oxygen inside the pores of the coal. Some (5-10%) of the reacting oxygen produces carbon dioxide but some carbon monoxide, low molecular weight organic molecules (C1-5) and water are also released. It has been established that bituminous coal exposed to mild oxidation conditions (temperature range 40-120°C in air atmosphere) emits also molecular hydrogen as a side product. These reactions have been observed for a variety of bituminous coals worldwide. Surprisingly, the amount of hydrogen released (which is a reduction product) are linear to the amount of oxygen consumed. The hydrogen is produced via oxidative decomposition of formaldehyde groups with surface hydroperoxides and is catalyzed by the coal surface. Thus if there is an accumulation of hydrogen in a crack in the coal seam in a underground mine and the concentration increases to 4.1% (LEL in air) initiation of explosion by electrostatic charge or a spark is feasible. Indeed in the last decade several unexplained explosions have been reported in coal mines worldwide despite appropriate ventilation and no detection of methane had been observed. We have checked if accumulation of hydrogen in a deep coal mine might reach the LEL of 4.1%. The results show that in extreme cases this is indeed the case and furthermore that during transportation of bituminous coal by sea hydrogen concentrations might reach levels of up to 10% in ship holds (which are also a confined space). These observations may indicate that molecular hydrogen accumulation in confined spaces might increase the risk to explosions and that appropriate measure should be taken in order to check this possibility.