DEVOLATILIZATION AND COMBUSTION CHARACTERISTICS OF COAL PARTICLES

For the proper understanding of fixed and fluidized-bed coal combustors and gasifiers, it is necessary to have the detailed knowledge of the devolatilization and combustion characteristics of coal particles. Both of these properties are involved functions of the nature of coal, particle size, temperature, pressure, heating rates, environmental conditions (inert, oxidizing or reducing) etc. In an actual system devolatilization and combustion phenomena may interact with each other and thereby may further complicate the capability with which predictions for such systems can be made. In this review, an attempt has been made to synthesize together the available information on both of these fronts to facilitate understanding of those coal conversion technologies where either or both of these phenomena may play an important role. Various qualitative features of devolatilization of different types of coals are described as well as various mechanisms, phenomenological and chemical models. Single and multireaction models are discussed in relation to the kinetic expressions for the pyrolysis reactions. Various physical factors which influence the pyrolysis, volatile yield and product composition are enumerated and discussed. Various properties of plastic and nonplastic coals which influence heat and mass transport are described and their consequences analyzed. These are particularly significant for large particles, several mm in diameter, which are commonly used in coal conversion processes of combustion and gasification. In addition, to transport considerations secondary reactions are also very important for such systems and these are discussed at length in this review for both plastic and nonplastic coals. Areas where our present knowledge is deficient and more research work is essential are outlined. In a similar fashion the combustion behavior of small and large particles is analyzed and the physical factors which influence such characteristics are discussed. Both qualitative features and quantitative mathematical models which shed light on the combustion properties of single coal particles are discussed and their consequences analyzed in terms of coal particle size, structural properties, operating parameters etc. Particular aspects which need to be further researched are identified. © 1990.