Influence of the Oily Fraction on the Thermal Behavior of Coffee Grounds During Pyrolysis: A Kinetic Study Based on Thermogravimetric Data

Abstract

This study investigates the impact of the oily fraction on the thermal behavior during the pyrolysis of spent coffee grounds (SCG), a biomass abundantly found in waste. Thermogravimetric analysis (TGA-DTG) was conducted at heating rates ranging from 5 to 50°C/min, with a maximum temperature of 800°C, to evaluate the thermal behavior and kinetic parameters of the thermochemical decomposition of SCG and oil-extracted spent coffee grounds (SCGE), along with their biopolymer components. The comparison of the oily fraction in SCG and SCGE was carried out using thermogravimetric analysis and Fourier transform infrared (FTIR) spectroscopy. Results indicated that for both biomasses, the decomposition temperature increased with higher heating rates, although the overall decomposition patterns remained unaffected. The FTIR results revealed a progressive transformation of organic compounds into carbonaceous materials, with significant changes around 400°C. During pyrolysis, the reactivity of biopolymers followed the order: hemicellulose > cellulose > lignin. The kinetic study of pyrolysis by applying the Kissinger method provided an E_a value of 122.149 kJ/mol for SCG and 115.814 kJ/mol for SCGE, corresponding to the decomposition of hemicellulose. KAS and FWO methods showed that the activation energy (E_a) of hemicellulose, cellulose, and lignin increased with the conversion level. Compared to SCGE, the increase in activation energy (E_a) observed for SCG can be attributed to the presence of free fatty acids in the oil, which slow down the reaction transition. After oil extraction (SCGE), the transition becomes more gradual, indicating that the removal of oil fractions promotes a decrease in activation energy. These results highlight the influence of the oil fraction on the thermal behavior of the material.