Flame Propagation Velocity for Co-combustion of Pulverized Coals and Gas Fuels

Abstract

A model was developed to evaluate the flame propagation velocity for co-combustion of pulverized coals and gas fuels such as ammonia, methane, hydrogen, and other fuels. For coal combustion, the maximum flame propagation velocity increased when the particle diameter decreased or the volatile content increased. The limit value of the maximum flame propagation velocity was extrapolated to zero particle diameter and 100% volatile matter (VM) content. The value became equivalent to the maximum flame propagation velocity of hydrocarbon fuels such as methane. However, the flame propagation velocity of coal was usually lower than that of gas fuels due to the delay of pyrolysis reactions. Also, residual solid particles (ash, char, and VM that have not been pyrolyzed yet) might slow down the gas combustion reactions. A model was developed to evaluate the flame propagation velocity for co-combustion. Previously, a model had been developed for solid fuels with diameter distribution. A weighting factor had been introduced to represent the contribution of each particle diamter. The factor had been the ratio of the number of particles. For gas fuels, instead of the number of particles, the weighting factor was expressed as a function of the equivalence ratio and the burning velocity of gas fuels. The flame propagation velocities were analyzed for co-combustion of coal–methane and coal–ammonia. The calculated results could reproduce the characteristics of the experimental results. Very recently, experimental results for coal–ammonia have been reported by Hadi et al. The flame propagation velocity for co-combustion was larger than the maximum value when ammonia or coal burned alone. This phenomenon was not observed for co-combustion of coal–methane. This phenomenon could be reproduced by the present calculation. The present model is useful for the design and development of burners for solid–gas co-combustion.