Thermokinetics analysis of biomass based on model-free different heating rate method

Abstract: Thermokinetics analysis can test the relationship between physical and chemical properties of material and temperature through controlling heating rate. Through thermokinetics analysis, we can study the combustion, pyrolysis and gasification reaction kinetics of biomass, decide the reaction kinetics model and calculate the reaction kinetics parameters, such as activation energy and pre-exponential factor. In the article, we chose 6 kinds of biomass raw materials, including corn straw, wheat straw, cotton stalk, pine sawdust, peanut shell, and residue of sweet sorghum. The thermal gravity analysis (TG) experiments were carried out, and 8 loss curves were obtained under non-isothermal conditions at linear heating rate of 5, 10, 20 and 30 ℃/min. The 99.99% nitrogen continuously passed and the temperature rose from room temperature to 600℃. The initial sample weight was always within the range of 3-4 mg. The method of different heating rates was applied to non-isothermal data. The Friedman method and the Flynn-Wall-Ozawa method were used for the estimation of the activation energy, and the Malek method was used for the decision of the reaction kinetics model, which were defined as the sample of the pre-exponential factor and the conversion function, respectively. The results showed that the pyrolysis process of biomass included 3 main stages: drying and preheating stage, volatile matter evaporation stage and carbonization stage. The higher the total moisture in biomass, the greater the mass loss rate for the sample at the first stage. Volatile matter evaporation stage was the most important stage in the pyrolysis process, in which the mass loss rate of the sample increased rapidly with the increase of the temperature. The carbonization stage was mainly the continued pyrolysis of lignin, and carbon and ash were the final products. In the whole range of conversion rate, the activation energy of biomass was not a fixed value, and it would increase gradually with the increase of conversion rate. Due to the influence of the particle size, the buoyancy and the non homogeneous phase, in the range of conversion rate <0.2, and >0.8, the TG curve was difficult to meet the requirements of the temperature at different heating rates under the same conversion rate. In the volatile matter evaporation stage, the activation energies obtained by Friedman method and Flynn-Wall-Ozawa method were almost the same and hardly changed with the conversion rate. The pyrolysis activation energy of the biomass ranged from 144.61 to 167.34 kJ/mol, and the correlation coefficient was almost between 0.99 and 1.00. This shows that the calculation method of the activation energy is reliable in this paper. Among biomass raw materials, corn straw and wheat straw belonged to gramineous crops, whose activation energy was high, 167.34 and 167.20 kJ/mol respectively; lignification degree of cotton stalk, pine sawdust and peanut shell was higher, whose activation energy was lower, 154.06, 147.29 and 146.91 kJ/mol respectively; residue of sweet sorghum was processed by biochemical process, whose activation energy was the lowest, 144.61 kJ/mol. The reaction kinetics models of the biomass conformed the Avrami-Erofeev function. This shows that because the composition and structure of different biomass materials are basically the same, the reaction kinetics models are basically the same. But, there were some differences in the reaction orders. The reaction order of corn stalk and peanut shell was 3; the reaction order of wheat straw, cotton stalk and residue of sweet sorghum was 2; and the reaction order of pine sawdust was 1.5. The pre-exponential factor of the biomass ranged from 26.66 to 33.97 s-1. Our results show that biomass pyrolysis is an extremely complex multi-step process, which has different activation energy and reaction kinetics model in different temperature range. This is important theoretical basis for the optimization of process conditions and engineering amplification of biomass pyrolysis process.