Investigation into the roles of surface oxide complexes and their distributions in the carbon-oxygen heterogeneous reaction mechanism

The combustion and gasification of chars are complex processes involving various subprocesses. These include mass and heat transfer to and from char particles, mass diffusion through the pores of the char, surface area evolution with char conversion, and heterogeneous reaction of gas molecules with sites and chemisorbed species on the internal surfaces of the char particle.; An extensive study was undertaken to characterize the intrinsic reactivity of char to oxygen. A variety of carbonaceous materials was employed in the study. The chars were subjected to a variety of heat treatment procedures, after which, oxy-reactivity tests were executed under kinetics-limited conditions using a pressurized thermogravimetric analyzer (PTGA). The PTGA system was also employed in performing BET-CO2 gas adsorption tests, which were used to determine the specific surface areas of chars during conversion.; A comprehensive char oxidation model was developed based on the results of the oxy-reactivity and surface evolution tests. The formulation includes an 11-step carbon-oxygen heterogeneous reaction mechanism and a surface area evolution model. The kinetic and pore structure parameters were carefully determined in order to reproduce the results of the oxy-reactivity and CO 2-adsorption experiments. The model was used to investigate the effects of surface oxide complexes and their distributions on char reactivity as well as to investigate the impact of surface area evolution on the char conversion process. A qualitative assessment of the impacts of inorganic matter and heat treatment was also performed.; The effect of surface area evolution was investigated by comparing a quasi-steady surface model with a dynamic surface model. It was demonstrated that the quasi-steady approximation resulted in an under-prediction of surface species, especially with increasing conversion. It was also demonstrated that activation energy distributions can characterize the reaction energetics of a char, and that approximating these distributions by average or effective activation energies can give rise to misleading results.; An increase in the inorganic composition of a char generally resulted in an increase in char reactivity. While some results suggested a decrease in char reactivity with heat treatment, other results suggest increase in reactivity due to heat treatment, an unexpected result that merits further investigation.