Non-isothermal kinetics of cellulose, hemicellulose, and lignin degradation during cocoa bean shell pyrolysis

Abstract

The waste generated by the cocoa industry consists of lignocellulosic material that can be used as a source of energy. To investigate this potential, the pyrolysis of cacao bean shell (CBS) was analyzed through thermogravimetric analysis (TGA) using nitrogen as the carrier gas. The process was performed at four different heating rates (β1 = 20 °C min−1, β2 = 30 °C min−1, β3 = 40 °C min−1, and β4 = 50 °C min−1) from 40 to 600 °C. The CBS degradation process was found to be similar to lignocellulosic biowaste, and the Friedman, Osawa-Flynn-Wall (OFW), and Kissinger-Akahira-Sunose (KAS) isoconversion methods were used to obtain kinetic parameters. The Friedman model had the best fit for the experimental data, and the activation energy (Ea) of the overall process increased with the conversion ratio due to changes from the broken bond reaction in the early stages to the secondary cracking response in the later stages. Due to the complexity of the thermal degradation of CBS, a detailed analysis was made by Fourier deconvolution to analyze the process in each stage of its pseudocomponents. The drying Ea (26.8–58.6 kJ/mol) was determined, along with the Ea of P-extractives (38.2–81.9 kJ/mol), P-hemicellulose (59.5–128.6 kJ/mol), P-cellulose (74.2–159.4 kJ/mol), P-lignin (79.8–173.4 kJ/mol), and carbonaceous degradation (102.2–186.2 kJ/mol). FTIR analysis showed that compounds such as CO2, esters, hydrocarbons, and phenols are released in the gas fraction during CBS pyrolysis. These results are useful for developing phenomenological models of the process and designing reactors to exploit the energetic potential of CBS through pyrolysis.