Kinetic study of the thermal and thermo-oxidative decomposition of spent coffee grounds under inert and oxidative atmospheres

The growing request of energy by the global population, related to the improvement of life quality, has generated an increase of environmental pollution due to the massive use of fossil fuels. In this context, the development of new technologies may implement the use of biomass as a renewable source of energy and to generate value-added products. Particular interest has been dedicated to the lignocellulosic biomass. Coffee is one of the most popular beverages in the world and the second commodity traded after petroleum. One of his most abundant by-products is the spent coffee grounds (SCG), which contain a huge number of organic compounds that could be used for energy production and added-value product generation. In this study, SCGs were examined in terms of proximate analysis, ultimate analysis and calorific values and subsequently subjected to thermal and thermo-oxidative decomposition by means of thermogravimetric analysis (TGA). Thermogravimetric thermograms were obtained at different heating rates. Results were assessed through a deconvolution approach in order to ascertain the contribution of each component of the SCGs (hemicellulose, cellulose, lignin) to the global decomposition process. Several mathematics models were tested, and the Lorentz one was selected as it was the one better describing the experimental data. Finally, the kinetic triplet, constituted by the activation energy (Ea), the pre-exponential factor (A) and the model of reaction (f(a)), was defined through the application of iso-conversional methods, Master-Curves and Perez-Maqueda criterion. The obtained thermal and kinetic parameters related to SCG may help in the optimization of its thermo-chemical conversion, conducted through pyrolysis or combustion, in order to speed up its application in industrial processes.