A KINETIC-MODEL OF CARBON BURNOUT IN PULVERIZED COAL COMBUSTION

The degree of carbon burnout is an important operating characteristic of full-scale suspension-fired coal combustion systems. Prediction of carbon loss requires special char combustion kinetics valid through th e very high conversions targeted in industry (typically >99.5%), and v alid for a wide range of particle temperature histories occurring in f ull-scale furnaces. The present paper presents high-temperature kineti c data for five coal chars in the form of time-resolved burning profil es that include the late stages of combustion. The paper then describe s the development and validation of the Carbon Burnout Kinetic Model ( CBK), a coal-general kinetics package that is specifically designed to predict the total extent of carbon burnout and ultimate fly ash carbo n content for prescribed temperature/oxygen histories typical of pulve rized coal combustion systems. The model combines the single-film trea tment of char oxidation with quantitative descriptions of thermal anne aling, statistical kinetics, statistical densities, and ash inhibition in the late stages of combustion. In agreement with experimental obse rvations, the CBK model predicts (1) low reactivities for unburned car bon residues extracted from commercial ash samples, (2) reactivity los s in the late stages of laboratory combustion, (3) the observed sensit ivity of char reactivity to high-temperature heat treatment on second and subsecond time scales, and (4) the global reaction inhibition by m ineral matter in the late stages of combustion observed in single-part icle imaging studies. The model ascribes these various char deactivati on phenomena to the combined effects of thermal annealing, ash inhibit ion, and the preferential consumption of more reactive particles (stat istical kinetics), the relative contributions of which vary greatly wi th combustion conditions. (C) 1998 by The Combustion Institute.