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Hydrothermal carbonization (HTC) is a useful method to convert wet biomass to value-added products. Fruit waste generated in juice industries is a huge source of moist feedstock for such conversion to produce hydrochar. This paper deals with four types of fruit wastes as feedstocks for HTC; namely, rotten apple (RA), apple chip pomace (ACP), apple juice pomace (AJP), and grape pomace (GP). The operating conditions for HTC processing were 190 °C, 225 °C, and 260 °C for 15 min. For all samples, higher heating value and fixed carbon increased, while volatile matter and oxygen content decreased after HTC. Except for ACP, the ash content of all samples increased after 225 °C. For RA, AJP, and GP, the possible explanation for increased ash content above 225 °C is that the hydrochar increases in porosity after 230 °C. It was observed that an increase in HTC temperature resulted in an increase in the mass yield for RA and GP, which is in contrast with increasing HTC temperature for lignocellulose biomass. Other characterization tests like thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) also showed that the HTC process can be successfully used to convert fruit wastes into valuable products.

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Hydrothermal carbonization (HTC) is a promising thermochemical process for the conversion of biomass with high moisture content, and therefore, interest in the development of continuous technology is growing. However, the development of continuous HTC systems requires the in-depth understanding of reaction kinetics, heat transfer mechanisms, exothermic or endothermic nature of the reaction, effect of operational parameters, recyclability of process water, and variability in biomass feedstock. The current paper discusses the effects of recyclability of process water and particle size on the product yield. To investigate the water recycling effect, HTC experiments were performed on sawdust using recycled water in a batch reactor. Moreover, to study the particle size effect, 3 small reactors containing specific particle sizes of biomass were placed in a larger reactor simultaneously to ensure consistency in the process condition. The results showed that the process water recycling increases the mass yield and higher heating value (HHV) of the solid product by 12% and 2%, respectively, after the first recycle. Physicochemical properties of hydrochar were studied by carrying out the ultimate and proximate, HHV, thermogravimetric coupled with Fourier transform infrared spectroscopy, and Brunauer−Emmett−Teller analysis, and results are discussed. Regarding the particle size study, higher mass yield, a decrease in heating value, and an increase in total organic carbon in the process water were observed by increasing the particle size.