Pyrolysis of household coffee vis-à-vis tea waste: A detailed insight into physicochemical properties, kinetics, and thermodynamics study

Abstract

The present study compares the kinetics and thermodynamics of the pyrolysis process of coffee and tea waste with respect to their physicochemical properties to analyze their potential as an energy and carbon source. Coffee and tea waste exhibit a promising source as a biofuel that has gross calorific values of 22.7 MJ kg⁻¹ and 20.2 MJ kg⁻¹, with significant differences in the overall volatile conversion of 76 % and 65 %, and as final carbon with a yield of 22 % and 31 %, respectively. Kinetic analyses using isoconversional methods show a trend of activation energy and frequency factor with conversion for both samples, with a significant difference at a conversion beyond 0.6 due to the higher lignin content in coffee waste. The predicted master plots indicate complex pyrolysis kinetics for both samples. Furthermore, the reaction kinetics determined by the multivariate regression approach, assuming parallel independent reactions of the nth order applicable to all heating rates, provide the individual mass change and carbon yield of each reaction process that can be controlled using experimental conditions. Finally, the thermodynamic parameters indicate that the pyrolysis process of both coffee and tea waste is nonspontaneous and endothermic, and its reactivity increases with conversion.