Theoretical study on the effect of particle spacing on ignition and combustion behavior of pulverized coal particles

In order to explore coal combustion behaviors under complex combustion organization conditions, the influence of particle spacing on ignition and combustion behavior of typical bituminous coal is investigated. By using Computational Fluid Dynamics (CFD), the interaction of pulverized coal particles is simulated and analyzed. The evolution of ignition delay time, ignition mode, as well as the key flow/species field with the variation of particle spacing were intensively studied. The results show that, when the particle spacing is large, the combustion behavior of particle group is similar to that of single particle; for a 70 μm bituminous coal particle, homogeneous ignition occurs later than heterogeneous ignition in the ambience of 1 500 K-0.2O2, and this phenomenon is not sensitive to the adoption of gas-phase reaction mechanism. However, when the particle spacing is small (≤8d), the gas phase combustion behavior of pulverized coal particles is quite different from that of single coal particles. Due to the interactions between particles, the volatiles accumulate around the boundary of downstream particle. Therefore, the gas-phase ignition occurs at the boundary of downstream particle, and then gradually develops to the upstream particle. This promotes the formation of gas-phase continuous flame. The change of the flame pattern leads to the decrease of homogeneous ignition time for both the upstream and downstream particles. It will further lead to the shift of ignition mode towards homogeneous ignition, which is much prominent for the downstream particle. Moreover, when the flame envelope is formed in multi-particle combustion, the fuel-rich region in the particle vicinity extends. It will bring significant impact on the subsequent formation of nitrogen oxides during combustion.