Char particle burning behavior: Experimental investigation of char structure evolution during pulverized fuel conversion

The present work provides experimental results on structural changes of burning char particles derived in an entrained flow reactor. Changes in the char particle diameter, apparent and true density, specific surface area, and porosity are studied at different residence times using a bituminous coal with a moderate ash content of 11.8 wt%. Results show the significance of mineral matter on structural properties of samples extracted along the combustion. The apparent density of a sample composed of mineral and organic matter increases progressively with proceeding char conversion, whereas the isolated char density decreases. The mean particle diameter decreases during conversion, indicating regime II conditions, with pore diffusion being the rate-controlling step. The decrease in char density and diameter follow well-known power law expressions, where the exponents are shown to be dependent on the effectiveness factor, and, thus the reaction regime. However, an ideal burning behavior in regime III, where the diffusion of oxidant through the boundary layer is rate-controlling, is not observed. Instead, large openings in the char structure enable the penetration of oxidant into the particle interior, in particular for highly porous cenospherical char particles. Hence, it is recommended to consider large openings and voids in the char structure and their changes, as well as mineral matter effects in advanced char combustion models.