Heat and mass transfer within thermogravimetric analyser: From simulation to improved estimation of kinetic data for char gasification

Abstract

Thermogravimetric analysis is a standard experimental method to determine kinetic data for heterogeneous coal char reactions. However, it is necessary to implement a sample holder to weigh the char sample, which may influence the mass and heat transfer towards the crucible. Thus, in this work the heat and mass transfer phenomena in a TGA were investigated. Experiments were carried out with hearth furnace char at temperatures between 1073 and 1273K and a pressure of 1bar. Based on the experiments, computational fluid dynamics was utilized to study the macroscopic transport mechanism employing a heterogeneous reaction on the external surface at a temperature of 1100K. For the species transport it is shown that convection plays a minor role in the crucible and diffusion is predominant. The aim of the work is to present a methodology for evaluating the resultant kinetic data taking the resistances in heat and mass transfer into account. For this purpose, the true species concentration and temperature at the sample were calculated. Therefore, the relevant heat and mass transport phenomena were modeled using a zero-dimensional approach considering spatially uniform concentrations and temperatures in the transient process. The model was validated by detailed CFD simulations of the reacting packed bed of char particles in the range of 900–1250K. It was found out that the concentration of the gasification agent was about 14% lower than expected as the maximal deviation. Thus, the consideration of diffusional effects within the crucible resulted in higher activation energies and different reaction orders when the kinetic parameters were determined.