Construction of reaction rate equation of biomass based on integral method improvement

Aiming at the problem that the temperature integral term cannot be integrated in the classical Arrhenius integral equation, a method that assumes the biomass conversion rate in the pyrolysis process as a function of time t and temperature T was suggested, making the temperature integral term can be integrated and effectively avoiding the computational errors caused by the simplification of the temperature integral term in the isoconversional integration method. The idea of combining the isoconversional method and the model fitting method to analyze the biomass pyrolysis kinetics was proposed: Using the isoconversional linear integral method to solve the reaction activation energy E based on class II thermodynamic equation, the activation energy E obtained by the model fitting method was compared with that obtained by the isoconversional method to select the optimal reaction mechanism function, then the selected mechanism function was re-substituted into the class II thermodynamic equation to obtain the value of the pre-factor A. The above analysis method was used to analyze the pyrolysis kinetics of five kinds biomass (corn cob, corn straw, sorghum straw, peanut shell and beech wood), verifying the feasibility of this analysis method. Non-isothermal thermogravimetric experiments with 7 linear heating programs were performed for each biomass, the obtained experimental data were linearly fitted based on class I and class II thermodynamic equations, respectively. The fitting correlation coefficients of the two classes of thermodynamic equations were all more than 0.95. The results showed that the activation energy E solved by class II thermodynamic equation was more sensitive to temperature, and the difference between the experimental data and the calculated value obtained by the kinetic parameters, which solved by the class II thermodynamic equation, was lower than that of the class I thermodynamic equation. Hence, the TG value calculated by the kinetic parameters of the class II thermodynamic equation was more close to the experimental data than that of the class I thermodynamic equation, and the accuracy of the kinetic parameters calculated by the class II thermodynamic equation was higher. According to the characteristics of the pyrolysis reaction that the activation energy E had a high linear fit with the logarithm of A and the activation energy with a conversion rate of 0.05 to 0.9 has a little fluctuation, the pyrolysis kinetic parameters solved by the isoconversional method can be simplified to a set of kinetic parameters corresponding to a specific conversion rate. Taking the conversion rate corresponding to the maximum pyrolysis rate as the dividing point, the TG values obtained from three sets of representative pyrolysis kinetic parameters (less than, equal to and higher than this point) were compared with the experimental data. The results showed that the TG values solved by the pyrolysis kinetic parameters of the maximum pyrolysis rate were closest to the experimental data. Using the kinetic parameters of the maximum pyrolysis rate as the kinetic parameters of the whole pyrolysis process can reduce the number of kinetic parameters while ensuring the calculation accuracy, thereby can reduce the computational cost of numerical simulation and provide a basis for numerical simulation of pyrolysis reactions in engineering.