A review of hydrothermal carbonization of municipal sludge: Process conditions, physicochemical properties, methods coupling, energy balances and life cycle analyses

Hydrothermal carbonization (HTC) is an effective way to harmlessly dispose municipal sludge (MS). This work comprehensively reviewed critical hydrothermal parameters of HTC, reaction mechanisms, physicochemical characteristics, applications of target products, technology coupling, energy balance, economic evaluation and life cycle analyses. The formation of hydrochar is predominantly attributed to the polymerization of remarkably reactive intermediates generated through the degradation of biopolymers in MS, as well as the solid-solid conversion of their insoluble components. Physicochemical properties of target products are closely related to process parameters and chemical compositions of MS, which provide a promising opportunity to acquire desired products by optimizing reaction conditions. The combination of other wastes with MS, and the coupling of different processes (e.g., pyrolysis) are potential optimization methods of HTC. The synergy of technological coupling enables the augmentation of value within the product. Typical energy balances of HTC indicate that net energy recovery exists even when the moisture content of municipal sludge is up to 91%. The combination of hydrochar as a substitute for fossil fuels and anaerobic digestion or recycling of process water is very favorable for the life cycle assessment of HTC of MS. Ultimately, some prospects for HTC of MS are highlighted.