First Line Ash Transformations of Coal and Biomass Fuels during PF Combustion

Abstract

During a combustion process, fuel undergoes various physical and chemical transformations. First-line physical transformations such as char oxidation, devolatilization, and fragmentation are the most important aspects of the ash formation process in the radiant zone. These transformations will be the same for all fuels but the extent to which they occur will be different due to varying mineral matter contents in the carbon matrix and size distribution. At typical pulverized fuel firing conditions, characterized by the high initial heating rates (105 °C/s) and temperature (1450−1600 °C) the overall, reaction rate is initially controlled by the rate of volatiles diffusion through the pores of a char particle along with chemical reactions. A transition to a kinetically controlled regime is generally observed in the later stages of the combustion process. The extent of the fragmentation will be different in both regimes. In this paper an attempt is made to shed light on the first-line ash transformations of different coal and biomass fuels, by carrying out a thorough experimental study in an advanced lab-scale facility under typical PF-firing conditions, including air-staging. Ash release, conversion, size reduction, and distribution, alongside with the change in inorganic chemical compositions, are derived at different char burnout levels in the reactor at 20, 90, 210, and 1300 ms residence times. Results indicate that at typical PF firing conditions, char combustion will mainly be in a kinetic-diffusion controlled regime, even with extended residence times. Char oxidation and devolatilization are found to depend to a large extent on the fuel mineral matter and its association inside the char matrix under these conditions. However, fragmentation of the fuel particles depends on char devolatilization and oxidation. During the initial heating and devolatilization, smaller particles of coal and biomass were found to be converted faster than larger particles. After a certain conversion, larger-sized particles fragment more than smaller particles, likely due to the high temperature gradient. Attrition, breakage, and percolative fragmentation were observed throughout under the kinetic-diffusion controlled regime. On the basis of the results at hand, a qualitative predictive tool is also suggested to gauge the extent of first-line physical transformations.