Thermogravimetric, kinetic and thermodynamic behaviour of raw and hydrothermally pretreated oil cakes during pyrolysis and TG-FTIR analysis of the gaseous products

Abstract

The pyrolysis of raw and hydrothermally (HTC) pretreated pumpkin (PC) and hemp (HC) oilseed cakes was investigated for the first time using thermogravimetric, kinetic and thermodynamic analyses. The influence of the HTC pretreatment and the type of reaction liquid (whey or water) on the pyrolysis was investigated and the pyrolysis gases were analysed.

The HTC pretreatment increases the biochar yield with values of up to 44 wt% compared to raw oil cakes (∼27 wt%). The HTC pretreatment with whey resulted in a higher energy and biochar yield and better biochar properties than the pretreatment with water. The tested oil cakes provided comparable energy yields, although HC provided higher biochar yields, while PC biochar showed higher hydrophobicity. The kinetic modelling shows that the activation energies ($E_{\alpha }$) for the pyrolysis of the raw oil cakes varied between 93.6 and 529.9 kJ/mol for PC and between 71.3 and 669.9 kJ/mol for the HC sample. HTC pretreatment in water media increased the $E_{\alpha }$ values, while the use of whey led to a decrease in the $E_{\alpha }$ values. TG-FTIR analysis of the emitted gases showed that the HTC treatment affected the release of CO₂ and hydrocarbons as well as the pyrolysis mechanism and reaction pathways.