The redistribution and migration mechanism of chlorine during hydrothermal carbonization of waste biomass and fuel properties of hydrochars

Abstract

This work investigated the effects of experimental conditions on the chlorine (Cl) behaviour during hydrothermal carbonization (HTC) of three waste biomasses. Temperature was found to be the most critical factor affecting Cl migration. Increasing temperature promoted the conversion of organic Cl to inorganic Cl through nucleophilic substitution and organic C–Cl cleavage reactions. The dechlorination efficiency was approximately 90% at high HTC severity. Specifically, the Cl contents in wheat, straw and eucalyptus hydrochars obtained at 270○C for 120 min drastically declined to 2349 mg/kg, 1943 mg/kg, and 312 mg/kg, respectively. The corresponding dechlorination efficiencies reached 89.93%, 90.81%, and 92.29%, respectively. Based on migration characteristics and theoretical inference, we proposed a Cl migration mechanism in waste biomass during HTC. X-ray photoelectron spectroscopy results revealed abundant oxygen-containing functional groups in the surface of hydrochars obtained at low HTC temperature. In contrast, at severe HTC conditions, the dehydration, decarboxylation, aromatization and polymerization reactions intensely occurred, resulting in lower H/C and O/C atomic ratios and enhanced aromatic C–C and CC. Energy analysis suggested that the natural properties of feedstocks significantly affected the energetic recovery efficiency. This research demonstrated the possibility of converting biomass into low-chlorine and high-energy-grade coal-alternative fuel through the HTC process.