Single char particle combustion at moderate temperature: effects of ash

Abstract

The effects of ash content, ash layer heat and mass transfer on a single granulated char particle (10–18 mm in diameter) combustion in an air stream (12 cm s −1 in cold base) are studied. The transient temperature of various ash content char particle burning in the surrounding gas temperatures of 900 to 1200 K is simulated. Agreement between simulated and experiment results is obtained by adjusting the ash layer diffusion coefficient and heat conductivity, surface emissivity and the reaction rate constant. The reaction rate constant plays an important role in modeling the initial stage of char particle combustion even when the overall rate is ash layer diffusion controlled. It determines the particle heating rate in the initial stage of combustion, and then indirectly influences the peak temperature. The ash layer diffusion resistance affects the rate controlling processes and the pattern of the time-temperature profile. The higher ash content char particle burns with a lower peak temperature and earlier temperature decrease due to the lower ash layer porosity and lower ash layer diffusion coefficient. It is concluded that the high ash particle combustion is controlled by ash layer diffusion except in the initial stage of combustion. As for the lower ash content char particle, it is controlled mainly by reaction at lower ambient temperature and by film diffusion at a higher temperature in the earlier stage. However, in the last stage, it is controlled by ash layer diffusion. The transition occurs when the ash layer is formed and the diffusion resistance is significant, and it is at that time that the particle reaches its peak temperature.