Study of interaction of entrained coal dust particles in lean methane-air premixed flames

This study investigates the interaction of micron-sized coal particles entrained into lean methane-air premixed flames. In a typical axisymmetric burner, coal particles are made to naturally entrain into a stream of the premixed reactants using an orifice plate and a conical feeder setup. Pittsburgh seam coal dust, with particle sizes in the ranges of 0-25 μm, 53-63 μm, and 75-90 μm, is used. The effects of different coal dust concentrations (10-300 g/m3) entrained into the mixture of methane-air at three lean equivalence ratios, ϕ, of 0.75, 0.80 and 0.85, on the laminar burning velocity are studied experimentally. The laminar burning velocity of the coal dust-methane-air mixture is determined by taking high quality shadowgraph images of the resulting flames and processing them using the cone-angle method. The results show that the laminar burning velocity reduces with the addition of coal dust having particle sizes in the ranges of 53-63 μm and 75-90 μm, irrespective of the equivalence ratio values. However, burning velocity promotion is observed for one case with particle size in the range of 0-25 μm at an equivalence ratio of 0.75. Two competing effects are considered to explain these trends. The first effect is due to volatile release, which increases the overall equivalence ratio and thus, the flame temperature and burning velocity. The second is the heat sink effect that the coal particles take up to release the volatiles. This process reduces the flame temperature and accordingly the burning velocity also. A mathematical model is developed considering these effects and it is seen to successfully predict the change of laminar burning velocity for various cases with different dust concentrations and equivalence ratios of the gas mixture. Furthermore, the implication of this study to coal mine safety is discussed.