Hydrothermal carbonization of cocoa shell: hydrochar characterization, kinetic triplets, and thermodynamic aspects of the process

This study proposed the use of the kinetic triplet analysis and thermodynamic parameter determination to investigate the hydrothermal carbonization (HTC) of cocoa shell (CS) for energy production. The variations in the Eα were determined by model-free isoconversional methods, the A values were determined using the ASTM E698-18 kinetics approach, and for f ( α ), the master plot methods were used. A progressive variation of Eα values was found, revealing competitive or consecutive reactions during HTC process, as well as multiphasic biomass conversion. From the master plot method, the experimental curve of CS does not perfectly match a single theoretical curve, indicating different reaction mechanisms during the HTC process. However, over the entire conversion range, the n -order reaction model ( n = 1.25) adequately describes the experimental behavior ( R 2 > 0.995). Furthermore, this value of n = 1.25 may imply a relatively low collision probability and can be related to the high extractives and lignin content in CS. Furthermore, for specific values of α , the difference between Eα and Δ H was lower than 3.98 kJ/mol indicating the facility to convert CS to hydrochar. In addition, the Δ S values showed that the process reaches thermodynamic stability during the conversion of CS. Finally, hydrochar characterization showed an energy densification of 40.30% in the case of hydrochar produced at 250 °C, thus demonstrating the feasibility of using this type of biomass for energy purposes. Graphical abstract