Hydrothermal carbonization of agro-industrial pomaces: Evaluating operational factors for realistic applications

This study aims to develop a strategy for valorizing apple pomace (AP) and grape pomace (GP) through hydrothermal carbonization (HTC). We studied the production of hydrochar, with a focus on recycling the liquor and exploring its anaerobic digestion (AD), in line with circular economy principles. Initially, we performed HTC on AP and GP, with and without a drying pre-treatment, using two different batch reactors (50 mL and 2 L) at 180, 200, and 220 °C. The effects of reactor size, feedstock, and its possible pre-treatment, as well as reaction temperature, on product properties and carbon distribution were assessed. In the subsequent phase, HTC experiments were conducted with process water (PW) recirculation to favor hydrochar production. We selected a target HTC temperature of 200 °C. After five recirculation cycles, hydrochar yield increased for both AP and GP compared to HTC at 200 °C with fresh tap water. Although hydrochar composition remained largely unchanged, PW exhibited progressively higher total organic carbon (TOC) levels, indicating that recirculation concentrates soluble organics in the liquid phase. These organics could react and polymerize into secondary char, thereby increasing hydrochar yield. As an alternative option to valorize PW, we performed its AD, evaluating the effect of PW recycling on AD. The process was not inhibited by the stronger acidity and the increased concentration of organic compounds in recycled PW: all tested PWs showed biogas production in line with typical organic residues.