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Hydrothermal carbonization (HTC) is a promising thermochemical method used to upgrade herbaceous biomass for high-grade fuel. However, the technique yields large amounts of organic process wastewater (PW), which must be properly disposed or reused. In this work, PW was used as water source to prepare HTC feedstock. The evolution of the chemical structure and formation mechanism of hydrochar over the course of four PW recycles were demonstrated by experimental determination of the chemical compositions, equilibrium moisture contents, carbon structures, functional groups, and micro morphologies of the biomass (cornstalk in this study) and resulting hydrochars. The chemical reaction pathways induced during the HTC of biomass was established on the basis of the evolution of the chemical characteristics of the hydrochars and organic components of PW. The results showed significant improvements in the fuel quality of hydrochar as the number of PW recycles increased. The higher heating value, mass yield, and energy recovery efficiency of the hydrochars increased to 23.31 MJ/kg, 62.16%, and 86.24%, respectively, after four PW recycles because PW recycling facilitates the conversion of biomass into a hydrochar matrix with a high carbonization degree and polymerization of intermediates into microspheres. As the number of PW recycles increased, the relative content of furan compounds in the aqueous product decreased and the content of aromatic compounds increased. Further analysis of the aqueous products revealed the polymerization of furfural and 5-hydroxymethylfurfural into aromatic compounds and inhibition of the direct polymerization of aldehydes into aromatic compounds. The results of this work provide a theoretical basis for studying the chemical reaction and evolution mechanism of herbaceous biomass during HTC with recycled PW. Image 1 • PW was recycled to reduce its discharge and the water consumption for HTC. • More microspheres were produced due to polymerization of organic intermediates. • The microspheres have higher reactive oxygen functional groups than the matrix. • About 10% of dissolved organics in PW polymerized again and formed solid particles. • The polymerization of aldehydes into aromatics was inhibited when PW was recycled. [ABSTRACT FROM AUTHOR]

• Co-hydrothermal carbonization (co-HTC) of sludge with lignocellulosic waste. • A FSPG-ORC system was coupled with HTC process to recover waste heat. • Total energy output increased by coupling FSPG-ORC system with HTC process. • The energy ratio of the whole process reached 24.11%. Co-hydrothermal carbonization (co-HTC) of sewage sludge (SS) with corn stalk (CS), as well as process heat recovering method, was proposed by this study for converting the biomass wastes to alternative solid fuel in an economical manner. The mass and energy equilibrium characteristics of the whole process were focused. Results indicated that the higher heating value (HHV) of hydrochar increased by increasing HTC temperature, CS mixing ratio, and reaction time. However, the energy consumption of HTC process would increase with increasing HTC temperature. A flash-organic Rankine Cycle (FSPG-ORC) system was coupled after HTC to recover the heat contained in the HTC slurry product, so as to improve the energy efficiency of HTC process. Slurry product (190 °C–280 °C) was first flashed and 5.62%–20.5% of moisture in slurry product was evaporated to drive a generator, while the remaining slurry was used as a heat source to evaporate organic working fluid which then drive another generator to generate power. After treating 1 ton/batch of SS-CS mixture under a typical HTC condition (HTC at 220 °C for 2 h, and SS:CS = 1:1), 125.1 kg of hydrochar with 15% moisture content was produced, and 114.59 MJ of waste heat contained in HTC slurry product was recovered by FSPG-ORC system. Ultimately, the energy ratio of the whole HTC-FSPG-ORC process reached 24.11%. HTC-FSPG-ORC coupled process converted organic wastes into lignite-like solid fuel in a low-energy manner due to the waste heat was stepwise recovered, and therefore showed a good application prospect. [ABSTRACT FROM AUTHOR]