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

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, 15 K/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 87.89 kJ mol(-1) (OP). The pre-exponential factors were 6.93 10(23) (SD), 5.10 10(14) (PP), and 3.71 10(09) 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 133.23 kJ mol(-1) (OP). In addition, the thermodynamic parameters Delta S, Delta H and Delta G were calculated from the FWO method. The positive values of Delta H evidenced the global endothermicity of the gasification process over the whole range of the conversion degree. The average Delta G values were 130.53 (SD), 148.17 (PP) and 132.91 kJ mol(-1) (OP). The average Delta S and Delta G 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.