Influence of organic acids with varying chain lengths on the physicochemical properties and combustion performance of hydrochar from sewage sludge

Organic acids are commonly used additives in the hydrothermal carbonization (HTC) of sewage sludge (SS). Previous studies have demonstrated that organic acids can effectively increase the carbon content of hydrochar, reduce its nitrogen content, and improve its combustion performance. Due to significant differences in molecular structure and chemical reactivity among organic acids with varying carbon chain lengths, they may exert distinct influences on hydrochar yield, carbon content, and combustion behavior. However, there remains a lack of systematic research on the effects of organic acids with varying carbon chain lengths during the HTC process. In this study, an orthogonal experimental design was employed to investigate the influence of C1 to C9 organic acids on the carbon content of hydrochar under various temperatures (170–230 °C) and concentrations (0.1–0.5 mol/L). The results showed that as the carbon chain length increased, organic acids were more likely to incorporate into the hydrochar structure through chemical reactions, resulting in a gradual increase in both carbon content and calorific value. Furthermore, the addition of long-chain organic acids led to a reduction in nitrogen content and enhancement of the combustion characteristics of the hydrochar. Meanwhile, long-chain organic acids were also beneficial for the recovery of nutrients from the aqueous phase. These findings provide a systematic reference for improving hydrochar quality and promoting aqueous phase resource recovery in the HTC of SS using organic acids.