Hydrothermal co-carbonization of sewage sludge and high concentration phenolic wastewater for production of solid biofuel with increased calorific value

Hydrothermal carbonization has proven to be an effective method to convert sewage sludge into solid carbon-rich hydrochar, of which the high ash content is generally regarded as limiting factor for energy-related potential application. Hydrothermal co-carbonization of sewage sludge and high concentration phenolic wastewater was firstly investigated in this study as a scenario for production of clean solid biofuel with increased calorific value and decreased ash content, by means of polycondensation reactions between phenols and highly reactive intermediates of biopolymers in sewage sludge. Results showed that addition of phenolic wastewater into sewage sludge led to a substantial increase of hydrochar yield (1.83–31.11%) and higher heating value (1.01–10.01%) and a remarkable decrease of ash content (1.39–25.68%). The reaction temperature and residence time had a complicated impact on the contribution of phenol to hydrochar production. An increase in phenol mixing ratio was beneficial to the improvement of mass yield and higher heating value but showed a less influence on the reduction of ash content. The synergistic effects between phenols and sewage sludge on the production of solid biofuel were closely related to the phenolic species, which followed the order in the aspects of hydrochar yield increase and ash content decrease: phloroglucinol > p-nitrophenol > m-cresol > phenol. The changes of phenols content in liquid phase and variations of functional groups in hydrochars implied that phenols interacted with sewage sludge and incorporated into hydrochars during the hydrothermal co-carbonization. TG-DSC confirmed the improved combustion reactivity of the phenols-sludge-derived hydrochars.