A simplified method for kinetic modeling of coffee silver skin pyrolysis by coupling pseudo-components peaks deconvolution analysis and model free-isoconversional methods

Biomass is a fundamental renewable resource to develop sustainable energy conversion systems with low environmental impact. The design of biomass pyrolysis, gasification, and combustion facilities requires a particular attention at kinetic aspects. A coffee production residue such as coffee silver skin, which is the main product of the coffee roasting industry, is very suitable for thermochemical conversion, being already dry and containing compounds like the ochratoxin which does not allow its use in the production of biochemicals. In this work, thermal degradation tests of coffee silver skin are made under inert conditions at various heating rates (5, 7, 10, 15 degrees C/min) and a proper kinetic model was considered in order to quantify the activation energy (E-alpha). The variation of E-alpha with the conversion rate (alpha) was higher than 30%, showing that the process is kinetically complex and the kinetics cannot be described by a single-step model, according to the International Confederation on Thermal Analysis and Calorimetry. For this reason, a multi-step model and an analytical approach were used. This involves the deconvolution of the individual peaks of each pseudo-component from the overall differential kinetic curves, followed by the application of a model free-isoconversional method to the separated decomposition steps. The determination of the reaction mechanism f(alpha) of each sub-component (pseudo-cellulose P-CL, pseudo-hemicellulose P-HC, and pseudo-lignin P-LG) was performed by using both the Master-Plots method and the Energy Compensation Effect. The pre-exponential factor, the enthalpy and the Gibbs free energy were also evaluated in order to investigate the pyrolysis characteristics. A second order model mechanism was obtained for P-CL and P-HC, whereas no clear mechanism was found for P-LG when alpha > 0.5.