Synergistic Characteristics and Capabilities of Co-hydrothermal Carbonization of Sewage Sludge/Lignite Mixtures

Co-hydrothermal carbonization (co-HTC) has been reported as a potential technology for enhancing fuel qualities and subsequent combustion behaviors of high-moisture raw biowastes or coals. For the purpose of real energy utilization, two low-fuel-quality materials, sewage sludge (SS, a biowaste) and lignite (LC, a coal), were employed to explore the characteristics and capabilities of target hydrochars produced from co-HTC of their mixture (1:1 mass ratio) in a temperature range from 120 to 300 degrees C. The results demonstrated that the hydrochars exhibited excellent synergistic characteristics, such as more fixed carbon, less volatile matter, an enhanced higher heating value, and an improved coalification degree (lower O/C and H/C ratios), which was probably explained by the involved dehydration, decarboxylation, and aromatization reactions enhanced by the mutual interactions between SS and LC during the co-HTC process. Furthermore, the combustion of hydrochars was observed to have maximal synergistic capabilities at approximately 240 degrees C, with a corresponding deviation value of -21.96% and a comprehensive combustion index of 10.28 x 10(-8) min(-2) degrees C-3. These findings, which are related to the synergistic effects of the co-HTC technique, could indicate a feasible approach for the effective energy utilization of low-fuel-quality raw biowastes or coals.