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Hydrothermal carbonization (HTC) is a promising and effective technology for upgrading the fuel quality of high‐moisture organic wastes. However, a large amount of energy is required to heat the feedstock to high temperature for HTC. Therefore, the maximum waste heat should be recovered to increase the energy efficiency of the HTC process. In this work, a flash‐organic Rankine cycle (FSPG‐ORC) system was coupled after HTC to recover the heat contained in the HTC slurry product. Various operating factors including the flashing temperature, the HTC temperature, the solid‐water ratio (S/W) of the feedstock, and the organic working fluids in the ORC section were studied to obtain their influences on thermodynamic performance of the coupled system. Results indicated that the net power (Wnet) of the FSPG‐ORC system increased from 185.64 to 774.30 kW through the increase of the HTC temperature from 160 to 280°C. In addition, the Wnet and exergy efficiency of the FSPG‐ORC system also increased with increasing the flashing temperature of HTC slurry product. The coupled FSPG‐ORC system converted waste heat to electric energy and increased the energy efficiency of the HTC process. [ABSTRACT FROM AUTHOR]

The use of seawater as a substitute for pure water as supplemental moisture raises questions about its effect on the physicochemical properties of hydrochar. Therefore, this study aimed to investigate the feasibility of using seawater as supplemental moisture by comparing the physicochemical properties of products obtained through Co-hydrothermal carbonization of chicken manure and cornstalk under seawater and deionized water conditions. By varying the HTC temperature and blending ratios of CM and CS to investigate comprehensively the effect of seawater. Results indicated that the hydrochar yield experienced a variation from 54.54% to 57.40%, while the IC value changed from 7.69% to 8.46% as the ratio of CM:CS shifted from 3:1 to 1:3 under seawater conditions. The higher heating value of the hydrochars obtained under seawater conditions was lower than those obtained under deionized water conditions. This suggests that seawater conditions promote the hydrolysis reaction of organic solid waste. Furthermore, it was observed that when no lignin hydrolysis reaction occurred, seawater conditions had no discernible effect on the fuel quality of the hydrochar. However, at an HTC temperature of 250 °C, the fuel quality of the hydrochar obtained under seawater conditions was notably inferior to that of the hydrochar obtained under deionized water. Thus, an HTC temperature lower than 250 °C is necessary for the hydrothermal carbonization of organic solid waste under seawater conditions. Moreover, the relative content of surface -C-(C, H)/C C of the hydrochar obtained under seawater conditions was lower than that obtained under deionized water conditions, indicating that the hydrochar had a low degree of aromatization. Additionally, there was a significant increase in the immobilized Mg atoms in the hydrochar under seawater conditions, which affected the hydrochar yield and higher heating value of the hydrochar. This research presents a theoretical foundation for preparing solid fuels and materials using hydrothermal carbonization of saltwater as supplemental moisture. • Lignin hydrolysis occurred under seawater conditions impacted the fuel quality of the hydrochar. • The higher heating value of hydrochar reduced under seawater conditions. • The HTC temperature should control below 250 °C under seawater conditions. • Mg atoms immobilized in the hydrochar under seawater conditions was considerable. • Lignin hydrolysis occurred under seawater con ditions impacted the fuel quality of the hydrochar. [ABSTRACT FROM AUTHOR]

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]