Clean solid fuel produced from cotton textiles waste through hydrothermal carbonization with FeCl3: Upgrading the fuel quality and combustion characteristics

Highlights: • Clean solid fuel prepared by hydrothermal carbonization with FeCl₃ was proposed. • FeCl₃ promoted the side reaction of hydrothermal transformation to pseudo lignin. • The pseudo lignin deposited on the hydrochars surface as a film structure. • Pseudo lignin-attached hydrochars emerged better fuel characteristics. Cotton textiles waste (CTW) was employed by hydrothermal carbonization (HTC) process catalyzed with FeCl₃ to prepare hydrochars. FeCl₃ could preliminarily lower the initial hydrothermal temperature, catalyze dehydration and decarboxylation to facilitate the formation of furfural derivatives. Subsequently, the synergistic effect of FeCl₃ and HTC regulated the side reaction pathway to convert more furfural derivatives into pseudo lignin structure, which further updating the thermal characteristics and combustion properties of hydrochars. The results demonstrated that the fuel ratio increased from 0.07 to 0.87 and the higher heating values (HHVs) of hydrochars became 1.05–1.67 times of CTW (18.63 MJ/kg). Moreover, FeCl₃ also enhanced the efficient conversion from volatile matters to fixed carbon and elevated the ignition temperature of hydrochars from 292.7 to 431 °C as compared to the CTW. Furthermore, the potential formation mechanism and development trend of the pseudo lignin in the process of HTC with FeCl₃ were also proposed. The study proposed a way to promote the formation of high value-added by-products in the HTC process, and also provided a novel approach to obtain a better combustion performance of clean solid fuel.