Strategies towards thermochemical valorisation of spent coffee grounds (SCG): Kinetic analysis of the thermal and thermo-oxidative decomposition

Abstract

The thermochemical conversion of spent coffee grounds (SCG) under inert and oxidative conditions was comprehensively evaluated as a feasible valorisation route for this biomass. Dynamic thermogravimetric assays were carried out at different heating rates, where the individual contribution of the pseudo-components (hemicellulose, cellulose, lignin) was assessed by a deconvolution approach with Lorentz curves. The kinetic triplet, constituted by the apparent activation energy (Ea), the pre-exponential factor (A) and the model of reaction (g(a)), was defined for each pseudo-component through the application of iso-conversional methods, Master-Curves and Perez-Maqueda criterion. Under an inert atmosphere, the devolatilization of humidity, decomposition of hemicellulose, cellulose and part of lignin, and finally the completion of lignin were identified, resulting in a remaining percentage of 20% of biochar. For hemicellulose, Ea was 217 kJ mol−1, for cellulose 214 kJ mol−1, and lignin 151 kJ mol−1. Under oxidative conditions, analogous mass-loss stages were found, with the only difference being that biochar was decomposed until only ash (∼2%) was obtained as residue. The advantage of using SCG as biofuels, with a high LHV and HHV (∼20 MJ kg−1), was thus demonstrated. The Ea for the hemicellulose was 194 kJ mol−1, for cellulose 147 kJ mol−1, and for lignin was 173 kJ mol−1. Similar decomposition paths were found regardless of the atmosphere for hemicellulose (F4), hemicellulose (D3), and lignin (F2/4), which suggests that the temperature, the reactant concentration, the particle shape and the diffusion of products control the decomposition of SCG during non-isothermal pyrolytic or combustive thermochemical processes.