A new approach to studying pore diffusivity during char combustion at FBC conditions

Abstract

A new technique of studying pore structure evolution during fluidized bed combustion (FBC) of char has been developed. Evolution of nitrogen oxides during char combustion has been found to be very sensitive toward changes in pore diffusivities. For a nonporous particle, nitric oxide is formed largely at the surface of the particle and diffuses out of the pores before it is destroyed. On the other hand, nitric oxide is formed and destroyed throughout a porous particle, resulting in lower conversions of char nitrogen to NO. The instantaneous conversion of char nitrogen to NO is very sensitive toward changes in pore structure. N 2 O, which is formed by the reaction of NO with char nitrogen, can also be used as an indicator for studying changes in pore diffusivity during combustion. Since ambient NO levels and pore diffusivity determine the diffusion of NO into or out of the particle, NO concentration within the particle affects the trends in the N 2 O formation with changing carbon conversion. This effect is compounded by the changes in pore diffusivity and the pore diffusion length as the char particle burns. By fitting the model to the experimental data, mesopores were found to control the pore diffusivity of the reacting species and, hence, the location of heterogeneous reactions occurring inside the pores.