A comprehensive insight into devolatilization thermo-kinetics for an agricultural residue: Towards a cleaner and sustainable energy

Stubble burning is a widely implemented agricultural malpractice that poses a serious environmental threat along with air pollution and global warming as the major consequences. This study aims to investigate an alternate conversion of the rice stubble feedstock to the thermochemical route by detailed kinetic and thermodynamic analysis. Initial assessment about the feedstock was made through physical and chemical characterization which indicated its energy potential with the high carbon content 47.61% along with 14.43 MJ kg-1 heating value. Subsequently, devolatilization kinetics was studied through a series of non-isothermal thermogravimetric analysis which showed the dependence on the plant cell wall component with overall conversion in active pyrolysis zone was 50.30%. Reaction pathways suggested a chain initiation and propagation mechanism associated with each constituent macromolecule. Friedman and simplified distributed activation energy methods were employed and the average apparent activation energy was estimated as 261.47 and 251.08 kJ mol-1 with corresponding pre-exponential factor as 7.85 x 1036 and 1.74 x 1021 s- 1 respectively. Kinetic compensation effect was also observed within these parameters with its constants as 0.070 mol kJ-1 s- 1 and 22.24 s-1. Master plots indicated that nuclei growth starts in the initial part of the reaction which exponentially increases to random nucleation and finally diffusion effects are dominant at the end of devolatilization. The empirical model mathematically showed an excellent correlation with the experimental data with a fitting percentage of 69.34%. Finally, thermodynamic parameters indicated an endothermic, non-spontaneous and high degree of randomness in the reaction. Altogether, the thermo-kinetic study suggested the energy feasibility of the overall process.