Detailed numerical simulations of flame propagation in coal-dust clouds

Abstract

A detailed numerical study was conducted to understand the transient flame propagation process in coal-dust clouds. The model includes detailed chemistry for the gas-phase combustion; devolatilisation kinetics; full coupling between the gas and solid phases; and radiative heat transfer. Furthermore, it solves the gas- and particle-phase momentum equations for the two-phase dynamics. The results show that the flame-speed oscillation phenomenon, which in a previous study was observed for carbon-dust clouds, was not observed for high-volatile coal dust. This is because for high-volatile dusts, such as coal, surface reactions have little impact on flame propagation, which is in fact dominated by volatile combustion. The flame speed largely depends on the devolatilisation rate. For the same reason, neither radiative emission nor absorption is important in high-volatile dust flames because of the much shorter timescale of volatile combustion. The flame structure can be divided into five zones: unburned, preheat, devolatilisation, reaction (gas phase), and post-reaction. Lastly, flame speeds increase when particle size decreases, mainly because the heat released from volatile combustion can be more effectively transported to smaller particles through conduction and convection. This will raise their temperatures more quickly and more profoundly and result in a faster devolatilisation rate and thus faster flame speed.