Thermochemical decomposition of coffee ground residues by TG-MS: A kinetic study

Abstract

Dynamic pyrolysis tests of coffee grounds residues (CGR) at heating rates in the range from 5 to 100 °C/min and at maximum temperature of 500 °C were carried out using a thermogravimetric analyser coupled to a mass spectrometer (TG-MS), for online evolved gas analysis, to determine kinetic parameters of thermochemical decomposition of CGR and its biopolymer constituents. During the pyrolysis, the maximum decomposition rate of each biomass component increased linearly with the heating rate. The slope increased with the biopolymer reactivity in the following sequence: hemicellulose > celluose > lignin.Main gases produced during the pyrolysis of CGR were oxygen containing species derived from parent biopolymers and primary and secondary vapours (250–425 °C), primarily H2O, followed by CO and CO2. The use of the Beta zeolite had only negligible effect on deoxygenation reactions, however it significantly promoted cracking reactions of pyrolysis vapours increasing the light hydrocarbons (C1–C2) formation with the subsequent improvement in the heating value of the pyrolysis gas.Kinetic parameters for any of the individual biopolymers in CGR were estimated using the model-free isoconversional dynamic methods Kissinger–Akahira–Sunose (KAS) and Flynn–Wall–Ozawa (FWO) models. The average value for the apparent activation energy of the individual biopolymers (hemicellulose, cellulose and lignin) in CGR calculated by KAS and FWO methods were estimated as 214, 241 and 266 kJ/mol, respectively; whilst for the CGR as a whole it was 242 kJ/mol. The two model-free isoconversional dynamic methods have been shown to be useful tools for assessment of biomass pyrolysis kinetic parameters, as they can provide Ea values for use in reactor design models.