Vented Hybrid Explosions of Methane and Coal Dust

The interaction of a vented methane explosion flame front (and induced flow) with a coal dust layer just prior to the vent exit, was experimentally investigated in a small cylindrical explosion vessel with two different open vent areas at low level, using different methane/air mixtures and different amounts of coal in the layer. The gas-only explosion tests were characterized by two pressure peaks, one associated with the onset of venting of burnt gas and one resulting from the combustion of the flammable gas cloud vent formed by the expelled mixture prior to the flame arrival. The pressure development was compared to methane/air explosions without the coal powder. It was estimated that the velocities induced by the gas explosions were more than sufficient to entrain the powder. However, with the exception of the 10% explosions all other tests showed a small reduction of maximum overpressure and a slowdown in the explosion development, attributed to the energy removal by the powder dispersion and powder action as a heat sink. For the 10% mixtures the main effects were the increase in the severity of the external explosion pressure peak, by a factor of 2.5. This was due to the highest exit velocity for entrainment of coal dust plus the entrainment of external air to oxidize the entrained coal in the external vented flame and hence increase the back pressure from the external explosion.