Hydrothermal Carbonization Temperature Research Articles

From scraps to purification: innovative use of food waste-derived hydrochar in eradicating pharmaceutical pollutants.

Chemical products (CPs) such as carbamazepine and naproxen, present in aquatic environments, pose significant risks to both aquatic life and human health. This study investigated the use of hydrothermally carbonized food waste-derived hydrochar (AC-HTC) at three distinct temperatures (200, 250, and 300°C) as an adsorbent to remove these CPs from water. Our research focused on the impact of hydrothermal carbonization temperature on hydrochar properties and the effects of chemical activation with phosphoric acid on adsorption capacity. Hydrothermal carbonization increased the hydrochar's surface area from 1.47 to 7.52 m2/g, which was further enhanced to 32.81 m2/g after activation with phosphoric acid. Batch adsorption experiments revealed that hydrochar produced at 250°C (AC-HTC-250) demonstrated high adsorption capacities of 49.10mg/g for carbamazepine and 14.35mg/g for naproxen, outperforming several conventional adsorbents. Optimal adsorption occurred at pH 4, aligning well with the Langmuir and pseudo-first-order models. The hydrochar showed potential for regeneration and multiple uses, suggesting its applicability in sustainable wastewater treatment. Future research will explore scalability and effectiveness against a broader range of pollutants.

Hydrothermal carbonization of invasive plant biomass as a tool for its safe utilization and production of artificial humic substances

Abstract Invasive plants may cause significant environmental risks by affecting biological diversity, ecosystem services, agriculture, and forestry. Safe utilization of invasive plant biomass by obtaining new outcome products useful for bioeconomics is a challenging but promising solution. Hydrothermal carbonization (HTC) presents a sustainable and cost-effective approach to transforming invasive plant biomass into new products while simultaneously supporting carbon capture aims. This study utilized invasive plants such as lupine [Lupinus polyphyllus Lindl.], Sosnowsky’s hogweed [Heracleum sosnowskyi Manden.], and Japanese knotweed [Polygonum cuspidatum Siebold & Zucc.] as biomass sources for HTC to produce artificial humic substances (AHS). The greatest impact on the yield of AHS (maximum gained yield 62%) has the duration of the HTC process (up to 6 h) and temperature of the treatment (from 160 up to 250 °C), as well as the catalyst used (an alkaline medium is preferable). During the HTC treatment, significant changes in the invasive plant biomass composition occur as indicated by the removal of labile components of organic matter. Obtained AHS are essentially similar to natural humic matter and have biostimulatory properties; thus, they can find application in agriculture and other areas of bioeconomics.

Food waste recycling to Yarrowia biomass due to combined hydrothermal carbonization and biological treatment

Food waste (FW) to value-added application represents a demanding opportunity in the circular economy. As one of the approaches, FW hydrothermal carbonization (HTC) delivers an aqueous phase (HTC-AP), which would be suitable when applied to microbial growth and lipid production. In this context, this current work explored HTC-AP from FW obtained through HTC (temperatures 200 to 260 °C) for microbial growth of 14 different Yarrowia species. Among these strains, Y. lipolytica, Y. keelungensis, Y. porcina, and Y. galii showed the ability to utilize HTC-AP. In HTC from 200 °C high optical density (OD600) reaching above 1.2 for the 4 species was observed. An increase in the temperature by 20 °C declined the growth of Y. lipolytica, Y. porcina, and Y. galii by 17%. However, Y. keelungensis showed good growth regardless of the HTC temperature. Further, the lipid produced better at higher biomass 240 °C compared to 260 °C during the HTC. Further, Y. yakushimensis showed the highest growth rate among all the analyzed media (0.12 – for medium 2 (0.45 NH4+ and 200 °C). Although the composition of HTC-AP is very diverse and often toxic, some analyzed yeast species can use the contained compounds as a carbon source for biomass and lipid biosynthesis. The presented possibilities are a very good starting point for developing processes on a larger scale.

Application of corn stover derived pyrolyzed hydrochars for efficient phosphorus removal from water: Influence of pyrolysis temperature

Excessive phosphorus load to surface water is undesirable since it can cause eutrophication. In this study, three different pyrolyzed hydrochars from corn stover were synthesized by applying hydrothermal carbonization (HTC) at 260 °C and then eventual pyrolysis at 400°, 600°, and 800 °C, respectively, and finally were used for phosphorus removal from aqueous solution. By linking the physicochemical properties of these pyrolyzed hydrochars investigated here, we tried to comprehend the effect of HTC and pyrolysis temperature on the hydrochar structure and phosphorus adsorption. Results show that high pyrolysis temperatures (600 °C and 800 °C) enhanced the hydrochar's phosphorus adsorption compared to low pyrolysis temperature (400 °C). HTC260P800 type hydrochar showed a fast phosphorus adsorption kinetics, while the HTC260P600 showed an overall high adsorption capacity, with the maximum phosphate adsorption capacity of 5090 mg/kg calculated using isotherm model. Three different adsorption isotherm models (Langmuir, Fruendlich, and Redlich-Peterson) were used to fit the isotherm data; among them, the Redlich-Peterson isotherm model fit the data best for all three hydrochars. pH was found to be a critical factor in terms of phosphorus removal, and the optimum pH was found to be 5, probably due to the enhanced electrostatic interaction between positively charged hydrochar and negatively charged phosphate ions. The desorption experiment showed that a small fraction of the adsorbed phosphate (15.6 % to 24.4 %) was released from the spent hydrochar samples, which might be due to the strong attachment of phosphate ions to the sorption sites. Reusing the spent pyrolyzed hydrochar showed lower phosphate adsorption capacity than the fresh ones, ranging from 423 mg/kg to 903 mg/kg.

Hydrothermal carbonization of Chinese medicine residues: Formation of humic acids and combustion performance of extracted hydrochar

Aiming at the sustainable management of high-moisture Chinese medicine residues (CMR), an alternative way integrating hydrothermal carbonization (HTC), humic acids (HAs) extraction and combustion of remained hydrochar has been proposed in this study. Effect of HTC temperature, HTC duration, and feedwater pH on the mass yield and properties of HAs was examined. The associated formation mechanism of HAs during HTC was proposed. The combustion performance of remained hydrochar after HAs extraction was evaluated. Results show that the positive correlation between hydrochar yield and HAs yield is observed. According to three-dimensional excitation emission matrix (3D EEM) fluorescence intensity, the best quality of HAs is achieved with a yield of 8.17 % at feedwater pH of 13 and HTC temperature of 200 °C. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses show abundant aromatic and aliphatic structure as well as oxygenated functional groups in HAs, which is like commercial HAs (HA-C). Besides, in terms of comprehensive combustion index (CCI), HTC can improve the combustion performance of CMR, while it becomes a bit worse after HAs extraction. Higher weighted mean apparent activation energy (Em) of hydrochar indicating its highly thermal stability. HAs extraction reduces Em and CCI of remained hydrochar. However, it can be regarded a potential renewable energy. This work confirms a more sustainable alternative way for CMR comprehensive utilization in near future.

Zinc Chloride as a Catalyst in Hydrothermal Carbonization of Cocoa Pods Husk

In this study, we have successfully synthesized a high surface area of activated carbons (ACs) from cocoa pod husk (CPH) by using a combination of hydrothermal carbonization (HTC) and chemical activation in the presence of ZnCl2 as both of catalyst as well as activating agent. During HTC, the effect of HTC temperature (200 and 225 °C) and ZnCl2 to biomass mass ratio (1:1, 2:1, 3:1) as a catalyst to the characteristics of the obtained hydrochar (HC) and ACs were investigated. The obtained hydrochar (HCs) was then processed by using ZnCl2 chemical activation with a fixed HC to ZnCl2 mass ratio of 1:4 and the mixture was pyrolyzed at 600 °C for 1 h under an inert atmosphere. The results showed that the addition of catalyst during HTC presented different AC morphology, where the carbon samples were decorated with carbon microspheres. An increase of surface area was observed, where the ACs synthesized from catalyzed HC at biomass to ZnCl2 ratio of 1:3 at temperature 200 °C gave a higher surface area of 1954 m2/g compared to that of without catalyst (1165 m2/g). The catalysis effect was more profound at the HTC temperature of 200 °C compared to 225 °C, as reflected in the significant increase of AC surface area. The addition of a catalyst creates ACs with narrow pore distribution compared to that of synthesized in the absence of a catalyst. It was also observed that the ACs from catalyzed hydrochar possessed higher oxygenated functional groups (OFG) than those without catalysts.

Hydrothermal carbonization of corncob for hydrochar production and its combustion reactivity in a blast furnace.

A key factor restricting the application of biochar in the steel industry is its high-quality upgrading. This paper evaluated the characteristics of hydrochar produced by HTC (hydrothermal carbonization) process of corncob to be used as a solid fuel. HTC temperatures (240-300°C) and HTC water-reused times (1-3 times) were examined for their effects on hydrochar yield, physicochemical characteristics, and combustion properties. The results showed hydrochar yields, O/C, and H/C parameters decreased as HTC temperature and water-reused times increased, while its high heating value increased. Due to dehydration and decarboxylation, hydrochar showed similar characteristics to those in bituminous coal. The removal efficiency of alkali metal K reached 99% after HTC treatment. Carbonaceous hydrochar had become more compact, orderly, and stable with increasing amounts of aromatic functional groups, C = C, and C = O. Hydrochar, as a biofuel, has higher ignition energy and is more stable than corncob due to its high carbonaceous order degree. To calculate combustion kinetic parameters, the Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) methods were applied. The results revealed that Eα (average activation energy) was quite similar between the two models. HC-300 had an Eα of 262kJ/mol. HTC could be an efficient way to reutilize corncob biomass into clean biofuels with high calorific value.

Analysis of Carbon Dots Synthesized at Different Hydrothermal Carbonization Temperatures and Their Ferric Ion Detection Performances

In this article, carbon dots (CDs) are fabricated via hydrothermal carbonization (HTC) at different temperatures ranging from 120 to 210 °C. Regarding the temperature variations, they demonstrate a crucial role in specific identification in the ability to differentiate the chemical structures and optical properties of the resulting CD features. Both heteroatoms‐doped core structure, and the functionalized surface state display a changed that correlates with the temperature variations mentioned. The CDs which are synthesized at 180 °C contain the appropriate balance of doped heteroatoms and surface functional groups which further display the most promising performance of the quenching effect of up to 17% toward ferric ions. This work contributes to the growing understanding of CDs and provides valuable insights for the future development of applications that may favor certain interest in the creation or designing process in various applications.

Effects of Fe-modified digestate hydrochar at different hydrothermal temperatures on anaerobic digestion of swine manure

Hydrothermal carbonization temperature is a key factor in controlling the physico-chemical properties of hydrochar and affecting its function. In this study, effects of hydrochar and Fe-modified hydrochar (Fe-HC) prepared at 180 °C (180C-Fe), 220 °C (220C-Fe) and 260 °C (260C-Fe) on anaerobic digestion (AD) performance of swine manure was investigated. Among the three Fe-HCs, 220C-Fe had the highest amount of Fe and Fe2+ on the surface. The relative methane production of control reached 174 %-189 % in the 180C-Fe and 220C-Fe treatments between days 11 and 12. The degradation efficiency of swine manure was highest in the 220C-Fe treatment (61.3 %), which was 14.8 % higher than in the control. Fe-HC could act as an electron shuttle, stimulate the coenzyme F420 formation, increase the relative abundance of Methanosarcina and promote electron transport for acetotrophic methanogenesis in the AD. These findings are helpful for designing an efficient process for treating swine manure and utilizing digestate.

Hydrochar-based facile and greenwood bio-adhesives production from distilled spent grain

The use of urea-, melamine-, and phenol–formaldehyde resin adhesives is endangering human health and damaging the environment. Developing formaldehyde-free plywood bio-adhesive is becoming a hot spot, and this study, for the first time, reports the preparation of facile and green wood bio-adhesives using hydrochar derived from distilled spent grain (DSG) by hydrothermal carbonization (HTC), which is promising for treating and valorizing high-moisture bio-waste such as DSG. HTC contributed to the dewatering, screening, and activation of active components in DSG, benefiting their interactions with cross-linkers and wood panels. The results show that the HTC temperature can significantly affect the reaction activity of hydrochar, thus influencing bonding strength. BD-180 bio-adhesive (mixture of cross-linkers glycerol triglycidyl ether/polyvinyl alcohol and hydrochar from HTC of DSG at 180 °C) had an excellent wet shear strength (1.24 MP), meeting the Chinese national standard GB/T 9846–2015 (≥0.7 MPa). Maillard reaction was a key reaction during HTC to destroy the secondary structure of proteins, resulting in the release of abundant –OH and –NH2. These functional groups in hydrochar were the major active sites available for the formation of covalent bonds (e.g., C–N and C–O–C) between bio-adhesive and wood panels, promoting the improvement in wet shear strength of bio-adhesives. This work provides a new method for the valorization of DSG and contributes to the development of green wood bio-adhesives without formaldehyde problems.

Elaboration of cost-effective adsorbents derived from sewage sludge through hydrothermal carbonization: Influence of process parameters and feedstock components on the adsorption performance

Sewage sludge (SS)-derived hydrochar by hydrothermal carbonization (HTC) have demonstrated its potential as an adsorbent. This study investigated the effect of HTC parameters and feedstock components on the adsorption performance of hydrochar, with the aim of enhancing its adsorption performance without modification, thereby expanding applications of waste-derived adsorbent under cost constraints. Water, ash and five model compounds were used to adjust the typical components of SS, then hydrochar was prepared under various HTC parameters. The adsorption quantity of the model pollutants (tetracycline and methylene blue) on hydrochar decreased with an increase in HTC temperature, and there was a minor decrease with prolonged reaction times. Changes in the HTC parameters did not affect the adsorption mechanism of the hydrochar. Notably, humus and lignin emerged as key components affecting the adsorption performance of hydrochars. Compared with the blank control, the adsorption capacities of tetracycline and methylene blue by hydrochar after the addition of 30 wt% humic acid increased by 16.1% and 101.0%, respectively. Moreover, the adsorption capacity of methylene blue increased by 58.5% after adding 30 wt% alkali lignin. The underlying mechanism was elucidated based on hydrochar characterization. Changes in HTC parameters affected the properties of hydrochar and its adsorption performance. The effect of component adjustment on the adsorption performance of hydrochar was attributed to abundance of oxygen-containing functional groups formed by the model compounds during hydrolysis. Component adjustment facilitated the binding of groups to the hydrochar, thereby offering more adsorption sites. This study provides a valuable reference for utilizing SS in the preparation of efficient and low-cost adsorbents.

Effect of Acid-Assisted Hydrothermal Carbonization Temperatures on the Hydrochar Properties for Supercapacitor Application

The investigation of biomass-based, cost-effective, efficient, and environmentally materials with high power density and fast ion/electron transfer is intensively carried out for the development of renewable energy storage devices. Pyrolysis and hydrothermal carbonization (HTC) are two common methods of thermochemical conversion to synthesize biomass-derived based carbon. Compared to the pyrolysis method, HTC is a more promising strategy because it can be carried out without a pre-drying process, has a high yield, low ash content, and requires a relatively low temperature (180-250 °C). The carbon produced from the HTC process is known as hydrochar. This study reports the acid-assisted hydrothermal carbonization temperature on the hydrochar properties and its application for supercapacitor electrodes. Hydrochar was synthesized from extracted avocado seed waste with potassium permanganate and sulfuric acid catalyst solution at 200 °C for 12 h. The effect of one- and two-stage HTC temperature on the hydrochar properties were compared. The hydrochar characterization includes yield, SEM, XRD, FTIR, and cyclic voltammetry analysis. According to the characterization and analysis results, hydrochar produced has the 3D porous network morphology and the highest specific capacitance of 73.54 F/g. In conclusion, hydrochar derived from avocado seed through the acid-assisted HTC can be a potential way for supercapacitor electrodes.

A first-of-its-kind study on founding concatenated trio fuels production platform: Biodiesel from lipids, biomethane by anaerobic digestion of defatted biomass, and hydrochar by hydrothermal carbonization of Oscillatoria willei biogas slurry (OWBS)

Sustainable production of multiple biofuels from algae/spent algal biomass through a concatenated route imparts a new energy and environmental sustainability platform. Thus, this is a first-of-its-kind study on establishing a novel and integrated trio fuels production platform using a cyanobacterium Oscillatoria willei (O. willei); - (i) production of biodiesel from O. willei (ii) Anaerobic digestion of defatted biomass for biomethane production (iii) Hydrothermal carbonization (HTC) of O. willei biogas slurry (OWBS) for hydrochar production. Initially, a maximum dry weight of 1.63 g/L was obtained from O. willei on the 18th day, and further, it contains 12.86 % lipids, 41.37 % proteins, and 34.21 % carbohydrates. Biodiesel (Bioenergy- I) produced from the test strain was 67.87 % and it contains preferable fatty acids such as C16:0, C16:1, C18:1, C18:2, and C18:3. The defatted biomass was used for anaerobic digestion (AD) for biomethane (Bioenergy- II) production for up to 45 days. The OWBS from the AD process was used as a feedstock in HTC for the production of hydrochar (Bioenergy- III) under different HTC temperatures (200, 220, 240, 260, and 280 °C) and time (60, 90, 120, 150 and 180 min). The highest hydrochar yield from OWBS was estimated to be 41.65 % at 200 °C and 60 min. Further, elemental analysis of hydrochar revealed 37.6, 4.8, 1.7, 0.1, and 28.3 % of C, H, N, S, and O, respectively with 27.5 % ash content. Based on the results, it is concluded that this novel sequence of strategy delivering trio biofuels along with biogas slurry nutrient removal has not been investigated till now to the best of our knowledge, which will lay a road map for the production of multiple bioenergies along with waste management.

Sustainable manufacture and application of biochar to improve soil properties and remediate soil contaminated with organic impurities: a systematic review

Biochar production and application have become increasingly popular in the past 15 years. Biochar, derived from diverse biomass types, offers a rich carbon source created through thermal combustion. Biochar production primarily depends on pyrolysis conditions and feedstock type. This review focuses on the multifaceted aspects of biochar, encompassing hydrothermal carbonization, gasification, and pyrolysis temperatures in biochar production and its role in bioeconomy and soil remediation. Biochar has yielded valuable insights, notably in decreasing nutrient leaching, curbing greenhouse gas (GHG) emissions, reducing the bioavailability of environmental pollutants, sequestering carbon (C) in soils, and enhancing agricultural productivity. Consequently, it has emerged as a valuable commodity for the bioeconomy, which involves harnessing bioresources through bioengineering to create economically valuable products. As a marketable output, biochar finds application in energy, diverse biochar-based product manufacturing, and the agricultural sector. Thus, biochar production not only enhances soil quality but also unlocks additional revenue streams. This review underscores the critical role of feedstock selection and pyrolysis conditions in optimizing biochar production. Furthermore, it highlights biochar as a sustainable and effective tool for improving various soil types and remediating soil contamination caused by organic impurities, including persistent organic compounds and antibiotics.

Interpretable ensemble prediction for anaerobic digestion performance of hydrothermal carbonization wastewater

Hydrothermal carbonization (HTC) is a method to improve fuel quality that can directly treat wet solid waste, but the treatment produces large amounts of wastewater. Hydrothermal carbonation wastewater treatment for methane production by anaerobic digestion can lead to waste utilization and energy saving. However, anaerobic digestion performance prediction of HTC wastewater is challenging due to the complexity of influencing factors. This study applies interpretable machine learning combined with ensemble learning to construct ensemble prediction models for the biogas yield and CH4 concentration. The machine learning ensemble model can integrate the advantages of single models and effectively improve the prediction accuracy of the anaerobic digestion performance of HTC wastewater, with the best R2 reaching 0.836 and 0.820, respectively, which is better than 0.780 and 0.802 of the best single models. The SHapley Additive exPlanations theory is combined with the ensemble models to show that anaerobic digestion reacted time with HTC temperature, pH, and COD has a coupling effect on daily biogas yield and CH4 concentration.

Characteristics and phytotoxicity of hydrochar-derived dissolved organic matter: Effects of feedstock type and hydrothermal temperature

The dissolved organic matter (DOM) with high mobility and reactivity plays a crucial role in soil. In this study, the characteristics and phytotoxicity of DOM released from the hydrochars prepared from different feedstocks (cow manure, corn stalk and Myriophyllum aquaticum) under three hydrothermal carbonization (HTC) temperatures (180, 200 and 220°C) were evaluated. The results showed that the hydrochars had high dissolved organic carbon content (20.15 to 37.65 mg/g) and its content showed a gradual reduction as HTC temperature increased. Three fluorescent components including mixed substance of fulvic acid-like and humic acid-like substances (C1, 30.92%-58.32%), UVA humic acid-like substance (C2, 25.27%-29.94%) and protein-like substance (C3, 11.74%-41.92%) were identified in hydrochar DOM by excitation emission matrix spectra coupled with parallel factor analysis. High HTC temperature increased the relative proportion of aromatic substances (C1+C2) and humification degree of hydrochar DOM from cow manure, while it presented adverse effects on the hydrochar DOM from corn stalk and Myriophyllum. aquaticum. The principal component analysis suggested that feedstock type and HTC temperature posed significant effects on the characteristics of hydrochar DOM. Additionally, seed germination test of all hydrochar DOM demonstrated that the root length was reduced by 8.88%-26.43% in contrast with control, and the germination index values were 73.57%-91.12%. These findings provided new insights into the potential environmental effects for hydrochar application in soil.

Hydrothermal carbonization of oil palm trunk: Hydrochar properties and combustion behaviors

Oil palm trunk (OPT) is a solid residue discharged from palm-oil upstream industry, with high potential as a feedstock for solid fuel production. However, its high moisture content and low energy density causes the limitation of utilization. In this research, hydrothermal carbonization (HTC) was applied to upgrade OPT. Effect of different reaction temperature and residence time on thermal characteristics and combustion behaviors of hydrochar obtained from HTC were investigated. OPT gradually converted into hydrochar with higher carbon content and heating values. The proximate and ultimate analyses indicated that fuel properties of hydrochars enhanced by increasing HTC temperature and residence time. The high heating values (HHV) increased by 7.18% at 180 °C, 30 min and reached up to 48% at 240 °C, 60 min. Meanwhile, combustion behaviors of hydrochars based on thermogravimetric analysis (TGA) were examined. The results showed that hydrochars had a higher ignition temperature than that of the raw OPT, leading to a more stable combustion. Furthermore, slagging and fouling problems were greatly minimized owing to the removal of a significant portion of alkaline remained in OPT, especially potassium (66% removal) and calcium (48% removal). The results revealed that OPT is of great potential as a solid fuel employing HTC process.

Green solvents to enhance hydrochar quality and clarify effects of secondary char

Several limitations hinder the industrial-scale implementation of hydrothermal carbonization (HTC) of biomass, especially the quality of as-carbonized hydrochar. This work investigates solvent extraction of hydrochars to enhance their potential applications. Hydrochars were produced at several HTC temperatures (190, 220, 250 °C) from cellulose and extracted using combinations of green polar solvents (ethyl acetate, acetone, and methanol). Results show that the composition of the extractable fraction resembles that of the HTC liquor, rich in carboxylic acids and furan derivatives, while the non-extractable solid phase shows improved thermal profiles devoid of highly volatile compounds. Carbon microspheres (non-dissolvable secondary char) are unaffected by extraction. The organics adsorbed on the hydrochar surface comprise highly volatile species and solvent washing effectively removes them.

Hydrothermal carbonization (HTC) of dairy waste: effect of temperature and initial acidity on the composition and quality of solid and liquid products

Background: Hydrothermal carbonization (HTC) of dairy processing waste was performed to investigate the effect of temperature and initial pH on the yield and composition of the solid (hydrochar) and liquor produced. All hydrochars met the EU requirements of organo-mineral solid fertilizers defined in the Fertilizing Products Regulation in terms of phosphorus (P) and mineral content. Methods: Laboratory scale HTC was performed using pressurized reactors, and the products (solid and liquid) were collected, stored and analyzed for elemental composition and nutrient content using Inductively coupled plasma optical emission spectroscopy (ICP-OES), ultraviolet-visible spectrophotometry (UV-Vis) and other analytic techniques. Results: Maximum hydrochar yield (60.67%) was observed at T=180℃ and pH=2.25, whereas the maximum P-recovery was 80.38% at T=220℃ and pH=4.6. The heavy metal content of the hydrochars was mostly compliant with EU limitations, except for Ni at T=220℃ and pH=8.32. Meanwhile, further study of Chromium (Cr) species is essential to assess the fertilizer quality of the hydrochars. For the liquid product, the increase in temperature beyond 200℃, coupled with an increase in initial acidity (pH=2.25) drove P into the liquor. Simultaneously, increasing HTC temperature and acidity increased the concentration of NO 3 - and NH 4 + in the liquid products to a maximum of 278 and 148 mg/L, respectively, at T=180℃ and pH=4.6. Furthermore, no direct relation between final pH of liquor and NH 4 + concentration was observed. Conclusions: HTC allows for the production of hydrochar as a potential fertilizer material that requires further processing. Adjusting HTC conditions enhanced P-recovery in the hydrochar, while retrieving higher nitrate concentrations in the liquid product. Optimizing HTC for the production of qualified hydrochars requires further treatment of Cr content, studying the availability of P in the products and enhancing the hydrochar yield for economic feasibility.

Hydrothermal carbonization mechanism of agricultural waste under different conditions: An experimental and ReaxFF molecular dynamics study

Different hydrothermal carbonization (HTC) conditions will affect the yield, physicochemical properties and application of agricultural waste hydrochar. The hydrochar prepared under different HTC conditions was systematically studied through various experimental methods, and the experimental results were further verified and explained by ReaxFF molecular dynamics method. The results show that the higher HTC temperature, higher liquid-solid ratio and longer reaction time can reduce the yield of solid product. The volatile content of rice straw (RS) hydrochar decreased with the progresses of HTC, but the fixed carbon content increased. The HTC process is accompanied by the breakage of C-H bonds, C-O bonds, C-C bonds and O-H bonds, with C-O bond breaks comprising the most. The HTC process is also accompanied by the C-C bonds regeneration, indicating that decomposition and polymerization reactions coexist in the HTC process. At higher HTC temperature and longer reaction time, RS hydrochar showed lower specific surface area and porosity. The effect of the HTC temperature is the largest, while the effect of holding time is the smallest. Therefore, to expand the potential use of RS hydrochar, it is important to thoroughly investigate the influence of HTC conditions on the physicochemical properties of RS.