Kinetics and Thermodynamic Properties of Torrefied Rice Straw
In order to fully understand the pyrolysis characteristics of rice straw(RS)and its torrefied samples,a vertical tubular furnace was used to torrefy RS for 30 min at 200,240 and 280℃.Then the effects of three heating rates of 5,10 and 15℃/min on the pyrolysis characteristics of RS and its torrefied samples were studied using a thermogravimetric analyzer.The Flynn-Wall-Ozawa(FWO)method,Kissing-Akahira-Sunose(KAS)method and distributed activation energy model(DAEM)method were used to calculate the pyrolysis kinetic parameters and thermodynamic parameters of RS and its torrefied samples,and the Criado method was used to determine reaction mechanism.The results showed that the thermal decomposition process of RS can be divided into three stages:drying,devolatilization and coking,and the greatest mass loss occurred in the devolatilization stage.During the process of thermal decomposition of RS and torrefied samples,TG curves and DTG curves moved towards high temperature with the increase of heating rate.With the increase of torrefaction temperature,the activation energies(E)and pre-exponential factor(A)of the torrefied samples calculated by the three models decreased,and the pyrolysis reaction rate of the torrefied samples tended to be more stable.The average Gibbs free energy(ΔG)calculated by the three models increased from 67.05 kJ/mol of RS to 91.96 kJ/mol of torrefied samples at 280 ℃.The average enthalpy change(ΔH)of torrefied samples at 280℃ was positive,and the average entropy change(ΔS)was negative,indicating that the endothermic reaction dominated the pyrolysis process of RS.And it became closer to the thermodynamic equilibrium state as the torrefaction temperature rose.The calculation results of Criado method showed that the pyrolysis mechanism of torrefied samples conformed to the three-dimensional diffusion model D3 when the conversion rate was less than 0.3,and it shifted to the phase boundary reaction model R2 when the conversion rate was greater than 0.3.
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