An analysis of the transient combustion and burnout time of carbon particles

The various coupled and transient processes controlling the gasification mechanism and burnout time of carbon particles were analyzed, with emphasis on the influence of the initial particle size for the size range that is relevant to the firing of pulverized solid fuels. The formulation recognizes the suppression of the envelop gas-phase CO flame because of the small particle size, and allows for the three surface reactions of C + O2, C + CO2, and C + H2O, as well as radiation heat transfer because of the potential high temperature attainable by the carbon particle. Results show that while the particle temperature continuously increases during the combustion of sufficiently large particles, the gasification actually consists of three phases: namely an initial particle heating period, an activation period for the surface reactions, and a diffusion-controlled, d2-law gasification period characterized by perpetually maximized surface reaction rates in spite of the continuously decreasing particle size. Radiation heat transfer is shown to have the same magnitude as those of reaction heat release and conduction, and actively affects the particle gasification response. For smaller particles, activation of the surface reactions is either substantially delayed subsequent to the initial heating period, or is completely suppressed, which respectively leads to either long burnout times or incomplete particle gasification. Influences due to the ambient oxygen concentration and the presence of CO2 and H2O as the oxidizer were also studied. Comparisons with literature experimental data show adequate agreement.