Diffusional effects in CO2 gasification experiments with single biomass char particles.: 2.: Theoretical predictions

We studied the diffusional effects that occur during the gasification of single char particles in a thermogravimetric analysis (TGA). In the first part of this study, which has been described in a companion paper, the diffusional effects in char particles were experimentally investigated at various particle sizes, CO2 partial pressures, and temperatures. In this second part, a kinetic particle model that takes into account the diffusion-reaction processes within a finite-size particle was formulated, based on a recently developed modeling approach. It was further extended to allow for intraparticle heat effects and heat- and mass-transfer phenomena that occurs in the external gas layer. The model satisfactorily explains the experiments reported in part I and highlights the importance of the diffusional effects at high temperatures and large particle sizes-i.e., a large Thiele modulus. This theoretical treatment sheds light on the different physical aspects involved in a typical char gasification test. Intraparticle mass limitation was identified as the main factor responsible for the strong resistance found at large particle size and high temperature. External heat and mass transfer were also determined to have a relevant role. Although the situation in full-scale gasifiers is more complex than that in a TGA using a single oxidant, the methodology developed here is capable of capturing the major physicochemical processes with minor computational difficulties. Thus, it is a first step toward rigorously estimating the gasification rate of a single char particle with minor calculations.