Combustion Characteristics of Carbon: Influence of the Zone I−Zone II Transition on Burn-Out in Pulverized Coal Flames

Abstract

Transition of reaction from Zone II to Zone I during char combustion in a pulverized coal (pc) flame is shown to result in significant and discontinuous drop, jointly, in temperature and in reaction rate, with a correspondingly significant increase in burn-out time. In the particular case studied, as example, the estimated increase in burn-out time is by a factor of at least 3. The reduction in reaction rate is partly due to a (300 °C) drop in temperature, but it is jointly due to the doubling of the (operational) activation energy associated with the Zone transition. The probability of such Zone transition on particles in pc flames is controlled by the thermal balance that, at the end of combustion, is shown to depend critically on the “seen” wall temperature. In the case of a boiler, particles seeing the relatively cold walls may “extinguish”, and this is a possible or probably cause of increased loss-on-ignition (LOI) that is increasingly being reported for pc combustion in boilers.