Kinetic studies of hydrothermal carbonization of avocado stone and analysis of the polycyclic aromatic hydrocarbon contents in the hydrochars produced

Abstract

Model-free and model-fitting kinetic approaches were used to investigate the hydrothermal carbonization (HTC) of avocado stone. The total solid mass yield was used to estimate the kinetic parameters and reaction mechanism. The results obtained show that the HTC process for avocado stones could be divided into four temperature zones. Up to 160 °C, the initial induction period where the reaction rate is relatively slow. The second zone (160 to 200 °C) corresponds to the maximum decomposition rate. The third zone between 200 and 220 °C corresponds to the stabilization zone; in this zone, the total solids yield is practically constant, and finally, the polymerization zone, between 220 and 250 °C, where an increase in total solid yield was observed. The kinetic parameters and reaction mechanism were determined in two temperature zones, from 150 to 210 °C and 210 to 250 °C. In the first zone, the decomposition of avocado stone during the HTC process followed a random nucleation reaction mechanism (Avrami-Erofeev-1) with the activation energy of 87.84 ± 3.28 kJ/mol. In contrast, in the second zone, it followed a first-order reaction model, and the activation energy was 230.96 ± 28.84 kJ/mol. Analysis of PAHs with gas chromatography-mass spectrometry showed that the number of PAHs in the hydrochar increases with an increase in temperature from 190 to 250 °C. The 3–4 rings PAHs were dominant in the hydrochars prepared at temperatures between 230 and 250 °C, while two rings were largely prevalent in the hydrochar obtained at low temperatures.