Investigations on the pyrolysis behavior of Sapindus mukorossi based on kinetic and thermodynamic parameters

Sapindus mukorossi (SM) is a fast-growing deciduous tree found extensively in tropical and sub-tropical regions of Asia. The conversion of SM seed shell (left over after extracting the pulp and kernal) to value added products through pyrolysis needs in-depth knowledge about its thermal degradation behavior. The present work studies the physicochemical properties, pyrolysis behavior, and kinetics of this less explored biomass feedstock for thermochemical conversion. The elemental composition, gross composition and higher heating value (HHV) of the SM shell is found to determine its energy potential. The kinetics of the pyrolysis reaction influence the breakdown of solid biomass into final products. Thermogravimetric analysis (TGA), wherein the sample is heated at various heating rates (5, 10, 20 degrees C/min) at inert condition reveals the thermal degradation profile of SM seed shell. Three important isoconversional model-free techniques, notably Friedman, Ozawa-Flynn-Wall (OFW), and Kissinger-Akahira-Sunose (KAS) approaches, are employed to obtain the kinetic triplet data, the thermodynamic parameters are also determined. The C, H, N, S and O content of the SM shell was found to be 39.82%, 4.64%, 0.64%, 0.64% and 54.26% respectively. The SM seed shell had a volatile matter, fixed carbon and HHV of 68.5%, 20.9% and 16.6 MJ/kg respectively which revealed its energy potential for thermochemical conversion. From the TGA curve, the maximum thermal degradation was observed between 200 degrees C and 500 degrees C. The values of average activation energy determined using models Friedman, OFW and KAS are 152.28 kJ mol-1, 140.05 kJ mol-1 and 138.14 kJ mol-1, respectively. The frequency factor was found to vary widely between 103 and 1015. The variation in activation energy and frequency factor as the conversion progresses indicated complicated processes during the thermal deterioration of SM. The biomass degradation occurs by diffusion and nucleation mechanisms when the conversion value is between 0.2 and 0.5, and for conversion values in the range of 0.6-0.8, the degradation occurs by diffusion mechanisms. The physicochemical characteristics of SM are found to be comparable with that of the commonly available biomasses. The detailed investigations presented in this paper have clearly demonstrated the viability of SM seed shell as a viable feedstock for the pyrolysis process.