Effect of particle distance on combustion behaviors through 1-D model with Neumann boundary condition

Abstract Moderate or intense low oxygen dilution (MILD) combustion is a promising technology for coal utilization due to the uniform heat flux and low NOx emission. In such coal cloud combustion, it is essential to investigate the effect of surrounding particles on the single coal combustion behaviors. In this work, the effect of particle distance on ignition, volatile flame, soot volume fraction and combustion modes are studied through a 1-D model with Neumann boundary condition. For pulverized coal combustion, heterogeneous ignition is dominating rather than homogeneous ignition. When particle number density is small, the ignition time of coal jet is dominated by that of large particles; while when particle number density is large, the ignition time of coal jet is dominated by that of small particles. According to the transient oxygen distribution at different particle distances, when the particle distance is large, the combustion is diffusion controlled; while when the particle distance is small, the combustion is kinetics controlled. At low oxygen mole fraction, soot emission increases with increasing particle distance because the yields-promoting effect dominates. At high oxygen mole fraction, the consumption-strengthening effect competes with the yields-promoting effect, thus the soot emission increases first and then decreases as particle distance increases. The numerical method in this work is able to predict the combustion modes at different particle number density. At least 90 times of the particle diameter is required for the size of the calculating domain when the Dirichlet boundary condition is used for the modeling of a single coal particle combustion.