积分方法改进的生物质热解反应速率模型构建

针对经典Arrhenius方程中温度积分项不可积的问题,通过设定热解过程中生物质转化率同时为时间(和温度r的函数,使温度积分项可积分,有效避免积分法动力学分析中因简化而导致的计算误差。基于此热动力学方程(Ⅱ类热动力学方程)采用等转化率线性积分法求解反应活化能E,并结合模型拟合法选取最优反应机理函数,将选取的机理函数重新代入Ⅱ类热动力学方程积分式解得指前因子A的值。基于等转化率线性积分法分别采用Ⅰ类及Ⅱ类热动力学方程对玉米芯等5种生物质热解过程进行动力学分析,结果显示2类热动力学方程求得生物质活化能E的决定系数均高于0.95。Ⅱ类动力学方程求解的动力学参数计算的动力学分析值与试验值的吻合度高于Ⅰ类动力学方程。根据热解反应的活化能E与lnA具有高度线性拟合性,且转化率0.05～0.85间活化能波动不大这一特点,采用最大热解速率处转化率对应的热解动力学参数简化热解过程的动力学参数,可减小数值模拟的计算成本,为工程上热解反应的数值模拟提供一定的基础。

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.