Group combustion of coal particles

Abstract

Most practical combustor systems such as boilers, gas turbines, Diesel engines, rocket engines, etc. use liquid and/or solid fuels as the energy source. Combustion intensity is controlled by the gasification, mixing and chemical kinetics processes. Coal is the dominant solid fuel source used for energy production. While it finds widespread use in lump form in grate fired boilers and as large particles in fluidized beds, due to the desirable ignition characteristics and high intensity combustion characteristics, most practical combustion devices utilize dense suspensions of finely pulverized particles. Despite this fact, many combustion models treat the particles as though they were isolated, however, interactive effects in suspensions can not be ignored due to the sensitivity of the results to the proper evaluation of the combustion rates of the particles within the suspension. Group Combustion studies have evolved in order to 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. Results are presented for the transient development of the flame in the form of flame radius histories, flame profiles, and total cloud (solid phase) mass histories. A unique method of correcting gas phase and particle kinetics when combustion becomes locally diffusion controlled is presented. This method 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). Carbon dioxide and water vapor disassociation is included through the use of backward reactions. A brief summary of the results is as follows: i.) Two flames axe 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.) an outer diffusion flame which consumes volatiles traveling outward from the cloud...