A study of reaction kinetics for thermochemical conversion of rice straw

Reaction kinetics for the thermochemical conversion of rice straw of particle sizes 40 mesh and 60 mesh heated at 50 K min{dollar}sp{lcub}-1{rcub}{dollar} and 100 K min{dollar}sp{lcub}-1{rcub}{dollar} in four atmospheres (nitrogen, air, oxygen and carbon dioxide) were studied using a non-isothermal thermogravimetric analysis (TGA) technique. The TGA data were used to develop thermograms and to identify different stages of thermochemical conversion. The TGA parameters, such as the maximum rate of weight loss, temperature at the maximum loss rate, and the residual weight range, were determined for each stage. An analysis of variance (ANOVA) was performed to evaluate the effects of rice straw particle size, heating rate, and atmosphere on these parameters. Two analytical methods involving a first order reaction mechanism, an approximate integral method (AIM) and a direct method (DM), were used to determine the kinetic coefficients (activation energy, E, and frequency factor, {dollar}ksb{lcub}rm o{rcub}{dollar}) for different stages of rice straw conversion. The effects of rice straw particle size, heating rate, atmosphere, and analysis method were also tested on these kinetic coefficients using ANOVA. A multistage model and a multicomponent model were developed and tested to predict the conversion of rice straw under different conditions of particle size, heating rate and atmosphere.; Four different stages of rice straw conversion (Stage I--drying, Stage II--devolatilization, Stage III--ignition and burning, and Stage IV--char oxidation and conversion) were defined. The TGA parameters were mainly influenced by heating rate and atmosphere. The effects of particle size were not significant. The maximum rate of weight loss increased with increase in heating rate, and also it was higher in air and oxygen than in relatively inert atmospheres (nitrogen and carbon dioxide). Simple first-order kinetic models were applied to describe the conversion. The AIM and DM resulted in different kinetic coefficients. The calculated conversion values by using AIM and DM were similar. Heating rate and atmosphere affected the activation energy very significantly. The kinetic coefficients were larger in air and oxygen than those in relatively inert atmospheres (nitrogen and carbon dioxide). Two more stages, after the drying stage, were generally sufficient to estimate the conversion under different conditions. The kinetic coefficients varied widely depending on the temperature and conversion ranges selected. Sample to sample variation was also apparent, most likely a result of poor repeatability in sample temperature measurements, and a result of probable compositional variations for the very small samples ({dollar}sim{dollar}5 mg) used. A multicomponent model, based on a direct first-order reaction mechanism and involving the kinetic coefficients and mass fractions of three major structural components of rice straw, can be used to estimate the conversion of rice straw under different heating rates and atmospheres.