Experimentally determined overall burning rates of pulverized-coal chars in specified O 2 and CO 2 environments

Abstract

Measurements of the size, temperature and velocity of individual pulverized-coal char particles are used to determine the overall burning rates of particles flowing in gaseous environments of specified O2 and CO2 contents. The overall burning rates of the char particles are essentially unaffected by the carbon dioxide content of the ambient gas. When the single film model of a burning carbon particle with CO as the sole heterogeneous reaction product is employed, the chemical reaction rate coefficients determined in gaseous environments of fixed CO2 levels but varying O2 levels exhibit the same temperature dependence. For the Missouri bituminous coal studied, a reaction rate coefficient of 28.4 exp (−2000/RT) g/cm2-s is determined for the C−O2 heterogeneous reaction. The apparent activation energy is in the range expected for burning limited by the combined effects of pore diffusion and the intrinsic chemical reactivity of the particle material. Based on the overall particle burning rates determined, the Thiele modulus estimated for each particle size is in the range which suggests little penetration of oxygen into the particle pores. The results support the single film model of a burning particle with CO as the sole reaction product for coal chars burning under typical pulverized-coal combustion conditions. The results indicate that the double film model of a burning carbon particle is not applicable for particles less than about 130 μm burning under such conditions. A reaction rate constant for the C−CO2 heterogeneous reaction a thousand times greater than the rate constants reported in the literature is required in order to get agreement between measured and calculated temperatures for a given particle size when the double film model is employed.