Hydrochar Formation Research Articles

Hydrothermal Carbonization of Biomass for Electrochemical Energy Storage: Parameters, Mechanisms, Electrochemical Performance, and the Incorporation of Transition Metal Dichalcogenide Nanoparticles.

Given the pressing climate and sustainability challenges, shifting industrial processes towards environmentally friendly practices is imperative. Among various strategies, the generation of green, flexible materials combined with efficient reutilization of biomass stands out. This review provides a comprehensive analysis of the hydrothermal carbonization (HTC) process as a sustainable approach for developing carbonaceous materials from biomass. Key parameters influencing hydrochar preparation are examined, along with the mechanisms governing hydrochar formation and pore development. Then, this review explores the application of hydrochars in supercapacitors, offering a novel comparative analysis of the electrochemical performance of various biomass-based electrodes, considering parameters such as capacitance, stability, and textural properties. Biomass-based hydrochars emerge as a promising alternative to traditional carbonaceous materials, with potential for further enhancement through the incorporation of extrinsic nanoparticles like graphene, carbon nanotubes, nanodiamonds and metal oxides. Of particular interest is the relatively unexplored use of transition metal dichalcogenides (TMDCs), with preliminary findings demonstrating highly competitive capacitances of up to 360 F/g when combined with hydrochars. This exceptional electrochemical performance, coupled with unique material properties, positions these biomass-based hydrochars interesting candidates to advance the energy industry towards a greener and more sustainable future.

Degradation of toxic components in kitchen waste during hydrothermal carbonization process

Hydrothermal carbonization (HTC) is a promising technique for kitchen waste (KW) disposal. However, KW may contain heavy metals (HMs), aflatoxin, nitrites and sulfites, posing risk for hydrochar utilization. In order to clarify the migration of these toxic components during hydrochar formation process, 7 kinds of kitchen wastes were adopted to carry out HTC experiments at 180–240 °C. Concentration detections results showed that Fe, Mn, Cr, Ni, Cu, and Zn contents were relatively high with a range of 30–2500 mg/kg in KW feedstocks. After HTC treatment, Fe, Cr, Cu were mainly (>80 %) enriched in hydrochar, the enrichment factors of Mn, Ni, Zn was almost above 50 %. From the perspective of species and valence distributions, Mn, Fe, Ni, and Zn in hydrochar were below moderate risk level. No aflatoxin was detected in hydrochar, because AFB1 and AFB2 can be completely degraded into organic acids in the HTC environment at 180 °C. Part benzene rings in aflatoxin were reduced and the C-C bonds were further broken, the C-O bonds were oxidized to produce carboxylic acids. Some benzene rings in aflatoxin remained to form organic acids with aromatic structures. NO2− and SO32- ions with strong ecotoxicity were mostly converted into NO3− and SO42- and flowed into the liquid phase. Some NO2− ions were also converted to and ammonia N and pyridine N; some SO32- ions were converted to sulfonic and sulfinic acid organics, which were successively transformed into sulfonyl/sulfinyl and sulfone/sulfoxide organics. The above findings are of great significance for the transformation mechanism of toxic substances in HTC environment and environment-friendly utilization of hydrochar.

Ionic liquid-catalyzed hydrothermal carbonization of sewage sludge: Effect of residence time and liquid phase circulation on hydrochar characteristic

The ionic liquid (IL, [DMAPA]HSO4) was used to catalyze hydrothermal carbonization (HTC) of sewage sludge (SSL), the study discussed how residence time and liquid-phase circulation influence the characteristics of the hydrochar. The decarboxylation process in IL-catalyzed HTC led to lower hydrochar yield but higher higher heating value (HHV). And the secondary hydrochar formation promoted by IL favored the system for high energy feedback (EF), but iron was involved in the secondary hydrochar formation. At the optimum hydrothermal time (90 min), the EF reached 124 % and the removal efficiencies of Fe, Mg, Mn, and Zn were 66 %, 73 %, 96 %, and 90 %, respectively. Functional groups on organic acids (COO–), amides (CO), and polysaccharides (C–O–C) in the SSL were sequentially removed during the IL-catalyzed HTC process, while cross-linking and graphitization occurred, resulting in higher ignition temperatures and lower metal contents. However, multiple cycles of the liquid phase resulted in a decrease in the catalytic properties of IL, resulting in low EF hydrochar with high metal content and functional group intensity. Catalytic graphitization is the key to obtaining hydrochar with high HHV and low metal content. Therefore, future development of catalysts is needed to facilitate the dehydration, decarboxylation and graphitization process of HTC.

Treatment of swine manure by hydrothermal carbonization: The influential effect and preliminary mechanism of surfactants

Treatment of swine manure by hydrothermal carbonization (HTC) with the aid of different surfactants was first explored in this study. PEG 400 (polyethylene glycol 400) and Tween 80 facilitated the formation of bio-oil. SLS (sodium lignosulfonate) and SDS (sodium dodecyl sulfate) promoted the formation of water-soluble matters/gases. Span 80 enhanced the formation of hydrochar, which resulted in a 50.19 % mass yield, 92.39 % energy yield, and a caloric value of 28.68 MJ/kg. The hydrochar obtained with Span 80 presented a similar combustion performance to raw swine manure and the best pyrolysis performance. The use of Span 80 promoted the transfer of degradation products to hydrochar, especially hydrophobic ester and ketone compounds. Notedly, Span 80 suppressed the synthesis of PAHs during the HTC process, which was reduced to 0.92 mg/kg. Furthermore, the hydrochar produced with Span 80 contained lower contents of heavy metals. On the whole, Span 80 has shown great potential in enhancing the HTC of swine manure. The acting mechanisms of surfactants in the HTC of swine manure included adsorption, dispersion, and electrostatics repulsion.

Insights into PVC-promoted hydrothermal carbonization of manure: Dechlorination, inorganic metals removal, and combustion behaviors

Hydrothermal carbonization (HTC) can serve as a promising route to effectively co-valorize biomass and plastic wastes for solid fuel production. However, manure-derived hydrochar for fuel application remains a significant challenge due to its high ash content and low energy density. Polyvinyl chloride (PVC), as a chlorine-containing plastic, subjected to HTC commonly results in the generation of massive acidic wastewater. Herein, we proposed a PVC-promoted HTC route for high energy–density hydrochar production. By utilizing PVC as an in situ acidic additive, the HTC condition was optimized at 275 °C, 30 min, with a PVC addition of 10 %. The hydrochar obtained under the optimized condition exhibited a higher heating value (HHV) of 25.89 MJ/kg, comparable to bituminous coals, and achieve a high energy recovery rate of nearly 70 %. Moreover, the excellent removal efficiencies of inorganic metals and chlorine both exceeding 95 % (except for Fe at 86 %) were observed, owing to the strong interactions between dechlorination and inorganic metals removal. Importantly, it was evidenced that acidic reaction environment was demonstrated to enhance the carbonization degree and modify the hydrochar formation mechanism. Furthermore, the high comprehensive combustion index S of 3.22 × 10−7 %2/( °C3·min2) and low average Ea of 134.20 kJ/mol were achieved, indicating the favorable combustion behaviors. Overall, this work systematically and comprehensively investigated the PVC-promoted HTC of manure waste for advanced solid fuel production and examined its combustion characteristics, unveiling the hydrochar formation mechanism.

Synthesis of Hydrochars via Hydrothermal Carbonization of Zinc Chloride Activated Cotton Textile Waste

Anthropogenic pollution from fossil fuel combustion due to increasing global populations and emerging industries poses a serious threat to the environment. Therefore, it is vital to explore alternative green energy sources. Hence, this study synthesized renewable solid fuel hydrochars by utilizing biomass of cotton textile waste (CTW) as feedstock. ZnCl2 was selected as a catalyst due to its Lewis acid properties promoting hydrolysis and dehydration of raw materials towards hydrochar formation. The ZnCl2-activated cotton textile waste (ZnCl2-CTW) was prepared via the wet impregnation method at different ZnCl2 loading. The hydrochars were synthesized via hydrothermal carbonization of ZnCl2-CTW in a Parr batch reactor at the reaction temperature of 200 °C for 3 h. The hydrochars and CTW were characterized using a thermogravimetric analyser (TGA) to evaluate the proximate analysis, whereas an elemental analysis and determination of higher heating value (HHV) were conducted employing a CHNS analyser. The proximate analysis indicates the hydrochars volatile matter increases at higher catalyst loading, possibly due to the high content of bio-oil adsorbed on the hydrochars surface. The elemental analysis shows higher carbon and lower oxygen content for hydrochars compared to CTW, mainly due to the dehydration, decarboxylation, and aromatization reactions during HTC. Meanwhile, the HHV of hydrochars increases as catalyst loading increases to 1.5 g, which obtained the highest value of 18.41 MJ/kg, indicating good quality of solid fuel. This demonstrated that the ZnCl2 catalyst was useful in improving the energy density of hydrochars. Overall, ZnCl2 catalyst loading of 1.5 g has exhibited good performance in producing hydrochars via HTC of CTW. Thus, ZnCl2-CTW has the potential to produce hydrochar suitable as solid fuel, contributing to the conversion of waste into energy and the preservation of the environmental.

Carbon-rich and low-ash hydrochar formation from sewage sludge by alkali-thermal hydrolysis coupled with acid-assisted hydrothermal carbonization

The production of carbon-rich and low-ash hydrochar from sewage sludge is attracting interest due to its great application prospect in high value-added carbon materials fields, but which is impossible through direct hydrothermal carbonization. In this study, alkali-thermal hydrolysis followed by acid-assisted hydrothermal carbonization was thus proposed. Thermal hydrolysis at strong alkaline environment was more effective than acid one to promote the dissolution of organic matters and restrain the release of inorganic matters from sludge, which created a favorable condition for hydrochar formation in a carbon-rich and low-ash way. Alkali-thermal hydrolysis began to show a positive effect on the dissolution of organics in sludge when temperature exceeded the threshold of 90 °C, and an increase of 9.77 % was found at 150 °C when compared to 30 °C. Acid-assisted hydrothermal carbonization of alkali-thermal hydrolysate (ATH) at pH 1.0 strongly promoted condensation polymerization of dissolved organics to form hydrochar and meanwhile inhibited introduction of dissolved inorganics. The nanosized microparticulate hydrochar derived from ATH-30 had a carbon and ash content of 50.98–61.31 % and 10.76–12.09 %, while the micro-sized microspheric hydrochar with multiple deposition layers formed from ATH-150 showed a better performance in carbon-rich and low-ash aspect where a carbon and ash content of 58.24–70.07 % and 0.40–3.24 % was realized, both of which were obviously superior to the direct hydrochar (carbon 34.86 % and ash 46.11 %). The condensation of dissolved organics during alkali-thermal hydrolysis stage is important to the carbonization degree of hydrochar. This study provides a new perspective in sludge disposal and production of advanced carbon materials.

Co-hydrothermal carbonization with process water recirculation as a valuable strategy to enhance hydrochar recovery with high energy efficiency

This study aims at valorizing the residual aqueous phase from hydrothermal carbonization (HTC) of Sicilian agro-wastes in order to enhance the hydrochar recovery, positively affecting the process energy balance. Process waters (PW) obtained from HTC and co-HTC using orange peel waste and fennel plant residues were used as recycled solvent in experiments carried out at the temperatures of 180 and 230 °C. The results showed that an additional hydrochar formation was promoted during recirculation of solvent, leading to average increments of solid mass yield of 10.5 wt% for tests conducted at 180 °C and 3.9 wt% for 230 °C. After five consecutive recirculation phases in co-HTC runs, the hydrochar yield increased up to 18.2 wt%. The low H/C and O/C atomic ratios values, found after recirculation, indicate that organic acids, accumulated in the PW, may catalyze the process and promote the biomass deoxygenation by boosting dehydration and decarboxylation. The recovered PWs from conversion steps with deionized water were also carbonized in absence of the solid feedstock in order to quantify their contribution in hydrochar formation during recirculation and thus the synergistic interactions. After recirculation, energy recovery averagely augmented by more than threefold, showing that the proposed strategy could significantly improve the sustainability of HTC.

Kinetic and thermodynamic evidences of the Diels-Alder cycloaddition and Pechmann condensation as key mechanisms of hydrochar formation during hydrothermal conversion of Lignin-Cellulose

We investigated the kinetics and thermodynamics of the Diels-Alder cycloaddition and Pechmann condensation to evaluate the significance of both mechanisms toward hydrochar formation during hydrothermal carbonization and liquefaction of biomass. We performed well-designed experiments of semi-continuous hydrothermal conversion of pure compounds representative of lignin-cellulose fractions (i.e., D-(+)-cellobiose and 1-(3,4-dimethoxyphenyl)-2-(4-hydroxy-2-methoxyphenoxy)propane-1,3-diol) and corresponding hydrochar precursors at 220–370 °C for 60 min. We developed thermodynamically-feasible multi-phase reaction pathways and specified the kinetic and thermodynamic parameters of each pathway by fitting the reversible power-law kinetic model, Arrhenius equation, and correlation between conventional definition of Gibbs free energy and the Gibbs free energy isotherm equation into experimental data of time evolution of chemical species concentration measured using GC–MS, XRD, and μGC. Reaction pathways of mixtures of pure compounds were a combination of those for individual pure compounds. We discovered that endothermic Diels-Alder cycloaddition involving dienes and dienophile was prominent in D-(+)-cellobiose conversion pathways and functioned as an intermediate reaction producing precursors for various endothermic and exothermic precipitation mechanisms. The temperature dependence of solid organics formation via this indirect route depended on the net ΔH of a set of reaction pathways after Diels-Alder cycloaddition. In contrast, we found that the Pechmann condensation involving phenols and β-ketoester/β-ketoacid was significant in the conversion pathway of 1-(3,4-dimethoxyphenyl)-2-(4-hydroxy-2-methoxyphenoxy)propane-1,3-diol and facilitated direct precipitation of organics including 4,10-dimethyl-2,7-dioxo-2,7-dihydropyrano[2,3-g]chromene-5-carbaldehyde and 4,5-dimethoxy-2,7-dioxo-6,7-dihydro-2H-benzo[g]chromene-10-carboxylic acid. The endothermic nature of Pechmann condensation preferred higher temperatures for a higher equilibrium conversion toward solid product formation. Results in this study contributed to the improved understanding of fundamentals of hydrochar formation.

Hydrothermal carbonization of coking sludge: Formation mechanism and fuel characteristic of hydrochar

In this study, the chemical structures, fuel characteristic, and formation mechanism of hydrochar during hydrothermal carbonization (HTC) at 150–270 °C for 0–120 min were investigated using coking sludge (CS) as the feedstock. The results showed that the yield decreased from 96.86 to 60.98%, whereas the carbonization rate increased from 6.74 to 93.41% at 270 °C. More stable structures with aromatic and N-heterocycles rings were formed through hydrolysis and polymerization. The H/C and O/C ratio decreased from 1.75 to 0.60 to 1.04 and 0.09, and the combustion stability index (Hf) decreased from 0.86 to 0.60 °C.103, and the flammability index (S) increased from 24.16 to 26.42 %/(min2 °C3) 10−8, indicating an improvement of fuel performance. A kinetic model to describe the conversion of organic components of CS was developed to elucidate the formation mechanism of hydrochar combined with the change of water-soluble intermediates (SM). The solid-solid conversion reaction of protein and humus components was the predominant hydrochar formation pathway, with an activation energy (Ea) of 26.06 kJ/mol. The polymerization of aromatic compounds slightly participated in the hydrochar formation, with an Ea of 86.12 kJ/mol. The water-soluble intermediates mostly transformed into inorganic substances (IS) through decarboxylation, deamination, or decomposition reaction, with an Ea of 5.73 kJ/mol. This study provided insights for understanding the formation of hydrochar from CS through HTC, which is vital for controlling the polymerization of intermediates and solid-solid conversion to enhance the carbonization efficiency.

Formation and evolution of PVC waste-derived hydrochar

Hydrothermal carbonization (HTC) is an effective method for converting solid waste PVC into multifunctional chlorine-free hydrochar. However, the formation and evolution of hydrochar derived from PVC are not clear. In this study, a combination of ultimate analysis, SEM, BET, FTIR, XPS, and 13C NMR analytical techniques was employed for a comprehensive characterization of the surface morphology and molecular structure of rigid PVC(RPVC)-based hydrochars. The influence of additives on the dechlorination of RPVC and structural evolution was investigated, leading to the establishment of the formation and evolution pathway of hydrochars. The results demonstrate that the CaCO3 filler facilitates dechlorination of RPVC by reacting with HCl and providing an additional channel for HCl diffusion. The chlorine removal rate reaches 96.08% at 280 °C and the residual chlorine in hydrochar is detected primarily on its surface. Prior to dechlorination, RPVC underwent a significant isomerization reaction, resulting in the transformation of hydrochar from long straight-chain alkanes into a polymerized aromatic structure. This transformation led to the formation of hydrochar with a relatively stable core structure and a highly active shell structure. These shell layers primarily contain C-O, CO, and O-CO. This study provides vital information for the design of large-scale processes for handling polyvinyl chloride (PVC)-containing waste and upgrading PVC waste into high-value multifunctional materials.

Behaviors and interactions during hydrothermal carbonization of protein, cellulose and lignin

Proteins participation will inevitably reconstruct the conversion route of lignocellulosic components and the formation of N-containing hydrochar during hydrothermal carbonization process. Herein, the behaviors and interaction mechanisms of protein, and typical biomass model chemicals (i.e. cellulose and lignin) were in-depth investigated. Results showed that the interaction of the binary mixture led to a reduction of hydrochar yield, but increased nitrogen concentration in hydrochar. Nitrogenous heterocyclic structure of hydrochar increased due to the interaction of protein with cellulose/lignin via forming Schiff bases and melanoids, wherein the content graphitic and pyridine nitrogen in protein-lignin hydrochar substantially increased from 5.4 % and 2.3 % to 9.7 % and 25.8 %, respectively. The increased ID/IG value from 0.81 to 0.91 in cellulose-protein hydrochar indicated the concentration of fused aromatic rings (≥6) based on Raman analysis. Carbon defects and disordered structure in hydrochar dramatically grew with the occurrence of the Maillard reaction and aldol-condensation of the intermediates. The nitrogenous substance in the aqueous phase increased by 30 %, resulted from protein decomposition accelerated by the hydrolysis of cellulose. This study provides a full understanding towards the transformation process of hydrochar from protein-containing biomass.

A Polystyrene supported Scandium (III) microencapsulated Lewis acid catalyst for hydrothermal carbonization of glucose

Hydrothermal carbonization of glucose in the absence and presence of recoverable solid polystyrene-supported microencapsulated Lewis acid catalyst has been carried out at 180 and 200 °C for 6, 12, and 24 h using either 0.25 g or 0.5 g of catalyst. The effects of temperature, residence time, and catalyst loading on the resulting hydrochars were investigated. At the lowest temperature (180 °C) and the shortest residence time (6 h) without the presence of a catalyst, the glucose conversion was the lowest as expected. No hydrochar was formed at 180 °C for 6 h without using any catalyst. In catalytic runs, the formation of hydrochar was observed under identical conditions although the yields of hydrochar were low. The use of the catalyst increased the yield of acid compounds (acetic, glycolic, lactic and levulinic) at the expense of 5-hydroxymethylfurfural under identical conditions. Except for 180 °C for 6 and 12 h, the use of the catalyst decreased the yield of hydrochar. The use of the catalyst led to increasing the diameter of carbon sphere particles under identical conditions. The presence of the catalyst resulted in the production of a notable amount, approximately 20 wt% of levulinic acid as a by-product.

Low nitrogen and high value hydrochar preparation through co-hydrothermal carbonization of sludge and saw dust with acid/alcohol assistance

High hydrothermal temperature and solid-N enrichment were the key issues during co-hydrothermal carbonization (co-HTC) of sludge and lignocellulose biomass. The technology of acid-alcohol assisted co-HTC was proposed and the relevant reaction mechanisms were investigated in this study. The results showed that acetic acid and ethanol promoted the Maillard and Mannich reaction respectively at 453 K through catalytic degradation of different organics. Above 483 K, acetic acid contributed to deamination and reduced solid-N by 9.18%, but there was no decrease in yield because acetic acid participated in hydrochar formation by repolymerization. Ethanol preferred to increase production by 5.82% without extra solid-N enrichment, promoting NH4+-N transformation into quaternary-N with more organic groups substitution. Solvent mixture had best performance on dehydration, carbonization and denitrification reaction to upgrade hydrochar fuel quality. The hydrochar obtained was more approach to lignite with high fixed carbon and heating value as well as low nitrogen content. Thus the fuel property and substance composition could be selected by adjusting the combination of hydrothermal temperature and organic solvent.

Role of Accumulated Alkali and Alkaline-Earth Chloride Salts in Hydrochar Formation Produced from Lignocellulosic Biomass

Hydrothermal carbonization (HTC) is an effective method for recovering useful carbon-rich solids from biomass and wet organic waste. This study investigated the effects of the type and amount of alkali and alkaline-earth chloride salts (AAECS) in HTC media on the properties of cornstalk hydrochar by analyzing the yield, elemental composition, surface functionality, and morphology, as well as fuel characteristics. The results suggest that the abundance of AAECS in HTC media may increase the aromaticity and energy recovery of hydrochar by facilitating the dehydration and decarboxylation of biomass. Both anions and cations in AAECS are responsible for these variations, especially for Ca2+, which can selectively affect the dehydration reaction of biomass by promoting the formation of anhydride and inhibiting the reaction between carboxyl and hydroxy to form a lactone. In addition, the high concentration of AAECS in HTC media is beneficial for improving the electron-donating capacity of hydrochar. This work could provide important insights into AAECS’ effects on hydrochar preparation, which arose from the recirculation of HTC aqueous solution.

Hydrothermal Carbonization of Corn Stover: Structural Evolution of Hydro-Char and Degradation Kinetics

Hydrothermal processing of biomass may be able to overcome a series of problems associated with the thermochemical conversion of lignocellulosic material into energy and fuels. Investigating the process parameters and an adequate process description is one of the first steps to being able to design and optimize a certain treatment concept. In the present article, we studied process evolution with respect to reaction time in order to evaluate structure changes and kinetics of corn stover decomposition in a hydrothermal reactor. The effect of the biomass-to-H2O ratio was also investigated. A pilot-scale reactor of 18.75 L was used to conduct hydrothermal processing runs at 250 °C at different reaction times (60, 120 and 240 min) and biomass-to-H2O ratios (1:10, 1:15 and 1:20). Solid phase products were characterized by thermogravimetry (TG), scanning electron microscopy (SEM), elemental composition (EDX), crystalline phases by X-ray diffraction (XRD) and surface area (BET). For the experiments with a constant reaction time, the yields of hydro-char, aqueous and gaseous phases varied between 31.08 and 35.82% (wt.), 54.59 and 60.83% (wt.) and 8.08 and 9.58% (wt.), respectively. The yields of hydro-char and gases tend to increase with higher biomass-to-H2O ratios, while aqueous phase yields are lower when using lower ratios. As expected, the yields of liquid and gases are higher when using higher reaction times, but there is a reduction in hydro-char yields. TG showed that 60 min was not enough to completely degrade the corn stover, while 120 and 240 min presented similar results, indicating an optimized time of reaction between 120 and 240 min. SEM images, elemental composition and XRD of hydro-char showed that higher biomass-to-H2O ratios increase the carbonization of corn stover. The surface area analysis of hydro-char obtained at 250 °C, 2.0 °C/min, a biomass-to-H2O ratio of 1:10 and 240 min showed a surface area of 4.35 m2/g, a pore volume of 18.6 mm3/g and an average pore width of 17.08 μm. The kinetic of corn stover degradation or bio-char formation was correlated with a pseudo-first-order exponential model, exhibiting a root-mean-square error (r2) of 1.000, demonstrating that degradation kinetics of corn stover with hot-compressed H2O, expressed as hydro-char formation, is well described by an exponential decay kinetics.

Hydrothermal Carbonization of Sewage Sludge into Solid Biofuel: Influences of Process Conditions on the Energetic Properties of Hydrochar

Hydrothermal carbonization (HTC) is an attractive, green technology for the management of sewage sludge. In this study, low-value secondary sewage sludge was subjected to an HTC treatment in a 1 L batch hydrothermal reactor and transformed into a high-energy-density hydrochar under varying HTC conditions (temperature of 150–300 °C, carbonization time of 30–150 min and a solid loading of 10–30%). The resulting hydrochar fuel characteristics were analyzed for ultimate and proximate analyses, functional group composition and energetic parameters. It was found that the hydrochar yield decreased with the increasing HTC temperature and reaction time, primarily due to the loss of organic volatile matter and functional groups. Under the optimum conditions of 150 °C, 30 min of carbonization time and 30% solid loading, 80.56% of the hydrochar was recovered, providing a maximum energy yield of 90.32% and a high heating value of 18.49 MJ/kg. Compared to the raw sewage sludge (H/C ratio of 2.67 and O/C ratio of 0.51), the hydrochar also had lower H/C and O/C atomic ratios of 1.42 and 0.18, respectively. The results suggest that significant dehydration and decarboxylation during the HTC treatment of sewage sludge have resulted in the formation of carbonaceous hydrochar with energetic properties close to the sub-bituminous coals.

Hydrothermal carbonization of sludge: Effect of steam release on products properties and wall sticking phenomenon

In order to understand hydrothermal carbonization (HTC) process more effectively, the effect of steam-release process at different temperatures (160, 180, 200, and 220 °C) on product properties and wall sticking phenomenon were studied. Experimental results indicated that the approximate equilibrium moisture content (AEMC) of hydrochar obtained with (SRHC) and without (HC) steam-release process were both negatively correlated with zeta potential (R = −0.96326 for HC and R = −0.95825 for SRHC). More importantly, the absolute value of the zeta potential of SRHC was lower than HC indicating steam-release process significantly improve dewatering performance of sludge after HTC treatment. The results of FE-SEM and N2 adsorption–desorption experiments indicated steam-release process was favorable for the formation of porous hydrochar. The variation of surface functional groups showed that the intensity of all peaks in SRHC was significantly lower than HC, indicating steam-release process which significantly affects surface chemical properties of samples. As for liquid, TOC, TN and COD value of the condensate were lower than filtrate, indicating steam-release process which produces “cleaner” condensed liquid with less organic matter content. The wall sticking phenomenon in the steam-release process became more severe with increasing temperature and the amount of sticky wall sample (SWS) increased from 0.392 g at 160 °C to 2.131 g at 220 °C, which indicated that temperature was key factor affecting the sticking phenomenon on the inside wall of the reactor. The contact angle of SWS and SRHC decreased from 60.083° and 77.283° at 160 °C to 51.317° and 67.967° at 220 °C, and the polar free energy of SWS (12.52–14.00 mN/m) was significantly higher than that of SRHC (1.85–4.53 mN/m), indicating increasing hydrophobicity and reducing polar free energy of sample were key factors in improving sticking phenomenon.

Growth mechanism of glucose-based hydrochar under the effects of acid and temperature regulation.

Improving the tailorability of hydrochar synthesis is an effective way to enhance its performance and utilization efficiency. In this study, the growth rate, morphology, and molecular structure of hydrochar were controlled by regulating the pH and temperature of the hydrothermal carbonization process. Growth process analysis indicates that hydrochar has three growth periods: induction, rapid, and stable growth periods. It is mainly controlled by 5-hydroxymethylfurfural (HMF), which is formed by converting glucose and its transformation products. The regulation of acid can significantly shorten the induction period of hydrochar, even under low-temperature conditions (<180℃), and increase the growth rate of hydrochar. However, the degree of hydrochar adhesion varies: the lower the temperature, the greater the degree of its adhesion. Molecular structural analysis demonstrates that hydrochar mainly consists of furan structural domains and aromatic clusters, and its surface is rich in oxygen-containing functional groups. The degree at which hydrochar was aromatized was improved by increasing the reaction temperature (160-220℃); whereas the regulation of acid reduced it and increased the content of oxygen-containing functional groups on the hydrochar surface. Based on these results, it is proposed that hydrochar formation has five stages and three growth periods with or without acid regulation.

Hydrothermal carbonization of alfalfa: role of processing variables on hydrochar properties.

Hydrothermal carbonization of alfalfa is a potential way to reuse agricultural waste. However, the effects of hydrothermal conditions on the properties of alfalfa-derived hydrochar are not clear. Herein, this study investigated the impact of different synthesis conditions (e.g., heating temperature, heating time, and solid to liquid ratio) on the formation and properties of hydrochar. Characterization and thermogravimetric analysis results revealed that with the increase of hydrothermal temperature and the extension of time, cellulose in alfalfa broken down more completely, and the number of carbon spheres and the aromatization degree increased, while the functional groups decreased. Furthermore, there was a surge in the carbon content, fixed carbon yield, high heating value, reduced oxygen, and volatile content. Additionally, the enhancement solid-liquid ratio could effectively improve the energy and mass yields. In all, by adjusting the process parameters of hydrochar, cleaner and higher productivity products could be obtained. This study provides theory basis for the production of target hydrochar that is used to soil amendments, adsorbents, and energy sources in the future.