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Hydrothermal carbonization (HTC) and Fenton pretreatment (FP) are effective methods currently being used to treat lignin. However, there are few reports of combining the two techniques and applying them in the treatment of lignin. Additionally, the synergistic effect of the two techniques on the physicochemical properties of lignin has not been widely investigated. Therefore, this study innovatively combined Fenton pretreatment with hydrothermal carbonization to investigate the synergistic effect on two types of structurally different lignans, and to reveal the changes in physicochemical properties of lignin under the synergistic effect. In this work, 13C NMR, XPS, FTIR, XRD, Raman spectroscopy, and automated calorimetry were used to determine the physicochemical properties and characterize the chemical structure of lignin. The results showed that the synergistic effect of FP–HTC increased the carbon fixation rate of herbal lignin (HL) and de-alkalized lignin (DAL) by 46.07 and 59.27%, respectively, and heating value of HL and DAL by 45.53 and 46.07%, respectively. The synergistic action of FP–HTC broke the stubborn and complex chemical barrier of lignin, and effectively enhanced the degree of aromatization and demethoxylation of lignin. It is notable that the synergistic effect of FP–HTC induced the formation of more stable 4-O-5 bonds and C–C bonds in the lignin structure, resulting in a moderate enhancement of the lignin chemical structure and material properties. In this study, the synergistic effect of Fenton pretreatment and hydrothermal carbonization is utilized to convert lignin into sustainable carbon materials, and the effect of the synergistic effect on the physicochemical properties of lignin is analyzed in detail, which provides valuable references for the subsequent research on the conversion of waste lignin into carbon materials. [ABSTRACT FROM AUTHOR]

Waste straw contains a large amount of lignin, and its resource utilization is not only in line with the national double carbon development strategy, but also to alleviate environmental pollution. Hydrothermal carbonization is a new thermochemical conversion technology, which has attracted much attention because it can directly transform carbon containing waste raw materials with high moisture content and low energy density. To investigate the physicochemical properties and aromatization changes of lignin hydrochar, hydrothermal carbonization experiments were carried out at 290 °C and a solid–liquid ratio of 1:20 for 0.00, 0.25, 0.50, 1.00, 1.50, 2.00, 4.00, 8.00 h, respectively. The experimental results shows that hydrothermal carbonization can increase the combustion quality of lignin. Physical and chemical properties analysis shows that with the increase of hydrothermal carbonization time from 0 to 2 h, the hydrochar content increased from 21.21% to 26.02% and the HHV of hydrochar increased from 20.01 MJ/Kg to 26.32 MJ/Kg. When the holding time exceeded 2 h, the carbon content and calorific value of hydrothermal tended to be stable. With the increase of holding time, FTIR analysis and XRD analysis show that the free hydroxyl groups in water-soluble lignin were easily combined with intramolecular and intermolecular hydrogen bonds, thus forming an ordered crystal arrangement. Subsequently, the crystal structure formed a well-arranged long chain through a strong hydrogen bond network, forming a ring structure in the process of aromatization. Aromatic ring structure accumulated, aromatization wave peak increased with holding time and aromatization intensified. Hydrochar crystal particles became larger and arranged in order. At the same time, the surface functional group detection and degree of crystallization were almost unchanged when holding time exceeded 2 h. The surface morphology of hydrochar was observed by SEM as follows: when the hydrothermal carbonization reaction of lignin entered the insulation stage, the microsphere structure began to aggregate and then became larger. When the holding time reached 2 h, the growth rate of carbon microspheres noticeably slowed. Therefore, the optimal hydrothermal carbonization time of lignin is 2 h, and hydrochar fuel has the best performance and aromatization. [ABSTRACT FROM AUTHOR]

Two new lignin feedstocks (HLF and DALF) were obtained by Fenton pretreatment of herbal lignin (HL) and de-alkalized lignin (DAL), and hydrothermal carbonization of each lignin sample was carried out at 290 °C with different holding times to investigate the impact of Fenton pretreatment on the physicochemical properties of the hydrothermal materials. The analyses based on 13C solid-state NMR, XPS, FTIR, SEM and fully automated calorimetry showed that the optimal holding time for hydrothermal carbonization of lignin at 290 °C was 30 min. The two types of lignin were treated with Fenton pretreatment to remove some methoxy groups and make the aromatic structural units more compact. Hydrothermal carbonization removed a small amount of α O 4 and β O 4 bonds from the structure, increased the guaiacyl units [G], decreased the syringyl units [S], and formed more stable 4-O-5 and C C bonds, which in turn polymerized into 10–300 µm irregular spherical carbon particles or porous honeycomb carbon blocks. The calorific values of HLF290-30 and DALF290-30 were increased to 31.04 MJ/kg and 32.17 MJ/kg, respectively, by the combination of Fenton pretreatment and hydrothermal carbonization. Overall, the Fenton pretreatment of lignin enhanced the degree of aromatization and removal of methoxylation during hydrothermal carbonization of lignin. This study provides a sustainable and green strategy for the preparation of high calorific value hydrochar as well as all-round utilization of lignin. [Display omitted] • Hydrothermal carbonization holding time is determined as 30 min. • The calorific values of HLF290-30 and DALF290-30 are 31.04 MJ/kg and 32.17 MJ/kg. • Fenton pretreatment is an effective way to produce high calorific value hydrochar. [ABSTRACT FROM AUTHOR]

To enhance the hydrothermal carbonization (HTC) process on biomass waste and improve the quality of biomass solid fuel. Corn straw was pretreated with acid washing and subsequently hydrothermally carbonized at 180–270 °C. The solid product obtained (hydrochars) was compared with the solid product produced from untreated hydrothermally carbonized straw. The results show that the acid pretreatment removed 7.9% of the ash from the straw. ICP and XRD analysis show that most of the alkali and alkaline earth metals have been removed. This addresses the defect of high ash content as the HTC temperature increases. The HHV of hydrochars produced by HTC after acid washing can reach 27.7 MJ/kg, which is nearly 10% higher than that of hydrochars prepared without acid washing pretreatment, and nearly 70% higher than that of straw raw materials. Elemental analysis and FTIR analysis show that the acid washing pretreatment changed the content and structure of the biomass components in the straw, resulting in a more complete HTC reaction and higher carbon sequestration. The decrease of H/C and O/C deepened the degree of coal-like transformation of hydrochars, with the lowest approaching the bituminous coal zone. The combustion characteristics of the hydrochars prepared after acid washing were significantly upgraded, the comprehensive combustion index and thermal stability of hydrochars both increased. Therefore, HTC after acid washing pretreatment is beneficial to further improve the high heating value and combustion characteristics of hydrochar. [ABSTRACT FROM AUTHOR]

The production of coal as an alternative to solid fuel through co-hydrothermal carbonization (co-HTC) of polyvinyl chloride (PVC) and lignocellulose biomass can play a key role in improving the environment and energy supply. However, how to improve the synergistic effect of the reaction is still an urgent problem. In this study, the co-HTC method of acid-base pretreated straw and PVC was proposed to improve the synergistic effect of the reaction. Through the comparison and analysis of the products, the change of reaction mechanism and the reason for the increase of synergistic effect were speculated. The results showed that these pretreatment methods could effectively enhance the synergistic effect of the co-HTC process. Alkali pretreatment destroyed the connection between hemicellulose and lignin, increased the relative content of cellulose, accelerated the hydrothermal reaction, and increased the energy density of the product. The HHV of hydrochar increased from 26.89MJ/kg to 30.83MJ/kg and the energy recovery efficiency reached 68.85% by using base modified straw powder (BSP). Acid pretreatment destroyed the connection between cellulose and hemicellulose in straw, degraded part of lignin into phenols, which strengthened the substitution reaction of Cl in the co-HTC process and improved the dechlorination efficiency of the product. The chlorine content of hydrochar decreased from 7.84% to 6.44% and the carbon retention reached 84.49% by using acid-modified straw powder (ASP). The ICP-OES analysis showed that more insoluble metal salts in hydrochar were dissolved in solution, which reduced the risk of slagging and corrosion when hydrochar as a substitute for coal combustion. The co-hydrothermal carbonization method of acid-base pretreatment of straw and polyvinyl chloride to improve the synergistic effect of the reaction. [Display omitted] • The synergistic effect of co-HTC of PVC and acid-base pretreatment straw was studied. • Base pretreatment can accelerate the co-HTC and increase the product energy density. • Base pretreatment can increase HHV of the product from 26.89MJ/kg to 30.83MJ/kg. • Acid pretreatment can increase the dechlorination efficiency of the product from 91.95% to 92.22%. • Alkali pretreatment is better for fuel heat release, acid pretreatment is better for fuel clean use. [ABSTRACT FROM AUTHOR]

[Display omitted] Mechanism of migration transformation of chlorine in PVC during co-hydrothermal carbonization. • The Cl migration and transformation of SP and PVC during the co-HTC were investigated. • 5-HMF and phenol promoted PVC dechlorination and HCl from PVC accelerated SP hydrolysis. • PVC hydrochar accumulated on blender while that of SP was mainly present in solution. • Cl in solid products decreased from 19.55% to 4.84% prolonging the reaction time. • Soluble lignin reacts with incompletely dechlorinated PVC to form soluble organic-Cl. Co-hydrothermal carbonization (co-HTC) of lignocellulosic biomass and polyvinyl chloride (PVC) is an effective method to remove organic-Cl in PVC. However, the pattern of redistribution and transformation behavior of Cl is not very clear during the co-HTC reaction. Therefore, the co-HTC of typical lignocellulosic biomass straw powder (SP) and PVC was conducted at 240 ℃ with increasing reaction times (0, 30, 60, 90, and 120 min) and a mixing ratio of 1:1 to speculate the mechanism of Cl migration. As a result, the solid-phase products were recovered in the solution (PS-1) and on the blender (PS-2), which both contained Cl but showed distinguished properties, so the PS-1 and PS-2 were analyzed separately. The XPS, 13C NMR, and FT-IR analysis showed that the product characteristic of PS-1 was closer to that of SP hydrothermal carbonization, while that of PS-2 was closer to that of PVC hydrothermal carbonization, and the Cl in the solid-phase product was converted to HCl as the reaction proceeded. In addition, the GC–MS analysis revealed the existence of organic-Cl in the liquid-phase product, presumably formed from the reaction of incompletely dechlorinated PVC with soluble lignin, and the organic-Cl then decomposed into inorganic-Cl in the form of HCl as the reaction proceeded. This work provides a comprehensive mechanical analysis for the co-HTC of lignocellulosic biomass with PVC. [ABSTRACT FROM AUTHOR]