The role of char surface structure development in pulverized fuel combustion

The present work is concerned with the effect of different bituminous coal chars pore surface structure on their combustion behavior. The chars were sampled in a semi-industrial coal jet flame of 2.5 MW thermal input. The solid samples from the jet flame were compared with samples tested in an isothermal plug flow reactor. For surface characterization, Nz-adsorption and scanning electronmicroscopy were applied. Differences in the BET-surface area up to one order of magnitude were observed for char samples collected in both combustion facilities. It was concluded that the larger surface area of the plug flow reactor char samples was due to a micropore structure, which was developed during devolatilization. The higher the initial particle heating rate was, the larger was the micropore structure and thus larger pore surface area resulted. Thus chars were expected to show different intrinsic reactivities. Nevertheless, since the control of internal structure on char consumption decreases as the temperature grows, an attempt was made to model char burnout in the jet flame making use of the kinetic parameters derived from the plug flow reactor experiments. Theoretical burnout curves fit remarkably well the experimental data, revealing that the porous structure may play a minor role in pulverized fuel combustion processes. Diffusive transport and reaction rates were of the same order of magnitude. Thus bulk diffusion may play a significant role, governing the global rate so that the internal porous structure is not significantly involved in the combustion process. (C) 2000 Elsevier Science Ltd. All rights reserved.