Torrefaction and hydrothermal carbonization pretreatments of rice impact nature of biochar from subsequent pyrolysis

Abstract

Torrefaction and hydrothermal carbonization (HTC) as important pretreatment methods for processing starch-rich food might affect pyrolysis behaviors of the food waste. This was investigated herein by torrefaction and HTC of rice at 200 ºC and subsequent pyrolysis of the pretreated rice at 600 ºC for probing the influence on evolution of the products. The results indicated that the torrefaction could not remarkably affect yield of the pyrolytic products, but made the torrefied rice more hydrophobic via removing some polar functionalities. The HTC at 200 ºC significantly enhanced overall yield of bio-oil from rice. The HTC also enhanced aromatization degree, making the hydrochar more difficult to crack, reducing the tar yield significantly, increasing the activation energy of 162.3 kJ/mol in pyrolysis of rice to 301.6 kJ/mol in pyrolysis of the hydrochar, and improving combustion performance of the resulting biochar. The HTC could form dominantly microsphere morphology in the hydrochar and biochar. The in-situ IR characterization of the pyrolysis process showed that the torrefaction could induce some deoxygenation reactions to form some aromatics while the HTC pretreatment removed a significant portion of aliphatic structures. Life cycle assessment indicated that HTC pretreatment of rice lowered environmental pollution and fossil fuel depletion during pyrolysis.