Kinetic analysis and thermodynamics properties of air/steam gasification of agricultural waste

Abstract

The air/steam gasification of wood sawdust (SD), plum and olive pits (PP, OP) bio-wastes was studied using macro-thermogravimetric analysis at three heating rates (5, 10, 15K/min). Three stages were identified during gasification process: water vaporization; de-volatilization and char gasification. The experimental data were analysed by applying five model-free methods: Flynn-Wall-Ozawa (FWO), Distributed Activation Energy Model (DAEM), Friedman, Starink, and Kissinger-Akahira-Sunose (KAS), to evaluate the gasification kinetic parameters. The FWO method exhibited the best fit to the experimental results. The pre-exponential factor was estimated using the Kissinger's expression. The average apparent activation energy (E) for the char-gasification step was found to be 218.27 (SD), 143.70 (PP) and 87.89kJ mol-1 (OP). The pre-exponential factors were 6.93 1023 (SD), 5.10 1014 (PP), and 3.71 1009 s-1 (OP). A kinetic model to predict the CO release during the bio-waste decomposition was also proposed and validated. The E values for global release of CO were 87.34 (SD), 67.19 (PP), and 133.23kJ mol-1 (OP). In addition, the thermodynamic parameters ΔS, ΔH and ΔG were calculated from the FWO method. The positive values of ΔH evidenced the global endothermicity of the gasification process over the whole range of the conversion degree. The average ΔG values were 130.53 (SD), 148.17 (PP) and 132.91kJ mol-1 (OP). The average ΔS and ΔG values, together with the Arrhenius kinetic coefficient showed that the reactivity for gasification decreased in the following order: SD>OP>PP. The results are in good agreement with previously reported data.