Group combustion of coal particles

Group Combustion studies predict the effect of interactions on ignition and/or combustion of droplet/particle sprays/suspensions. The model presented in the current work deals with the interactive combustion of a spherical cloud of uniformly spaced monosized coal particles. A method of correcting gas phase and particle kinetics when combustion becomes locally diffusion controlled allows for the use of rather large time steps when the temperature is high (fast kinetics) and when the species concentration is low (diffusion flame).; A brief summary of the results is as follows: (i) Two flames are formed during the early stages of combustion; (1) a "pseudo-premixed" flame which propagates inside the cloud consuming oxygen and providing the particles within the cloud the enthalpy required for pyrolysis, and (2) and outer diffusion flame which consumes volatiles traveling outward from the cloud. (ii) Initially, the cloud mass decreases slowly until ignition. Once ignited, the cloud mass decreases rapidly as the inner flame propagates inward and the particles pyrolyze. Once pyrolysis is complete, the cloud temperature is low and no oxygen exists within the cloud, as such, the cloud mass remains constant. Once the cloud can no longer supply volatiles to the outer flame at a rate faster than the outer flame can consume them, the outer flame collapses on the cloud and oxygen is available for the combustion of the residual char particles. Thus, the cloud mass begins to decrease again. (iii) The inclusion of backward reactions results in a finite value of oxygen concentration throughout the cloud. The oxygen reacts heterogeneously resulting in a high carbon monoxide mass fraction within the cloud during char oxidation. This carbon monoxide diffuses outward and is consumed in the gas phase outside the cloud resulting in a thick reaction zone around the cloud and enhanced combustion rates. (iv) Decreased particle diameters, for the same particle mass within the cloud, result in increased volatile yields due to the high heating rates provided by the inwardly moving flame. The increased yields result decreased burning time, since the flame radius, and hence the reaction zone, is greater which serves to enhance the combustion rate. (Abstract shortened with permission of author.)...