Effect of torrefaction temperature on spent coffee grounds thermal behaviour and kinetics

Abstract

The paper aimed to investigate the influence of the torrefaction temperature (210, 235, 260 °C) on the kinetic and thermodynamic parameters, as well as the reaction mechanism during combustion of spent coffee grounds (SCGs). Non-isothermal thermogravimetric experiments were carried out at four heating rates, namely, 5, 7, 10, and 15 °C/min. Thermogravimetric (TG) and differential thermogravimetric (DTG) results showed torrefaction temperature firstly increases and then decreases the combustion reactivity, reaching the best value for the sample SCGs-235. Starink, Kissinger-Akahira-Sunose (KAS) and Ozawa-Flynn-Wall (OFW) iso-conversional models, master plots method and the compensation effect method were used to calculate the combustion kinetic triplet of the samples. The activation energy of the samples decreased with torrefaction temperature increasing, from 199.76 kJ/mol to 168.91 kJ/mol (with the Starink model). All the combustions were best explained by a reaction order model whose order decreased with the elevated torrefaction temperature and the heating rate of the combustion process. Using kinetic parameters obtained from the Starink method, thermodynamic parameters (enthalpy, Gibbs free energy, and entropy) were also determined, showing that SCGs seemed to be more suitable for the torrefaction at 235–260 °C. The study findings can provide practical information for modelling and designing combustion reactors for clean biofuel production from SCGs.