Experimental and numerical verifications of biochar gasification kinetics using TGA

Abstract

The Thermogravimetric Analyzer (TGA) is a standard state-of-the-art instrument used for investigating char gasification kinetics. However, one major flaw of the TGA is the build-up of a stagnant gas region between the empty space of the crucible mouth and the char sample layer, which leads to poor gas-solid contacting. Diffusion is the predominant mechanism for the transport of reactive gas from the crucible mouth to the porous char layer. Therefore, this work aims to present a numerical model for evaluating the kinetic data using a TGA, encapsulating the three diffusion phenomena affecting the reactive gas inside the TGA crucible (i) diffusion of gas between the char layer and crucible mouth, (ii) diffusion of gas within the inter-particle voids of the char layer and (iii) diffusion of the gas within the intra-particle pores of the char particle. The model was first formulated and validated for the non-porous char particles and then extended to the porous particles. The simulation results predicted by the model using the re-evaluated parameters are in good agreement with the experimental results. It was shown that the diffusion of the gas within the inter-particle voids of the char layer in TGA has a significant impact on gasification processes inside TGA sample. In particular, the consideration of diffusional effects within the crucible resulted in an 8.5% increase in the activation energy. A sensitivity analysis showed that the model was highly sensitive to the change in the specific surface area of the char.