Hydrothermal Carbonization Process Research Articles

Research Needs and Pathways to Advance Hydrothermal Carbonization Technology

Hydrothermal carbonization (HTC) is a proven cost-effective and energy-efficient method for waste management and value-added product recovery. There are, however, several issues that require further improvement or research. Identifying the strengths and weaknesses of HTC in comparison to traditional pyrolysis is crucial for scientists to choose between them or use both (complementary) to achieve specific product properties. Additionally, sharing information on diverse modeling approaches and scales is crucial to enhance the robustness and universality of HTC process models. In addition, the study on the applicability of hydrochars on target applications such as soil amendment is crucial to give back nutrients to soils and face the dependence on finite specific feedstocks in this field. Also, proper management of the process by-products, especially process water, must be addressed to improve the carbon and hydric footprint of the process. Reviewing the suitability of HTC to treat specific challenging wastes, whose strength is not related to their calorific value but to their nutrient composition (i.e., manures), is also an appealing topic for HTC research. This paper aims to tackle the above-mentioned issues through an updated review and discussion of research gaps that require further investigation.

Bi-directional prediction of hydrothermal carbonization characteristics of agroforestry and livestock wastes with variable components: Graph learning model-aided waste recycle

Agricultural resource management is essential in present-day society. The hydrothermal carbonization (HTC) is a promising technology for recovering valuable resources from wastes. However, there is a paucity of efficient approaches to predict and comprehend the agricultural waste HTC process. Machine learning modeling approaches have emerged as an important tool for simulating thermochemical reaction processes such as HTC. However, previous studies all limit to predicting products properties of single-component HTC, which are contrary to the complex composition of raw materials, and demonstrate poor generalization performance. Herein, the homogeneity graph data is employed to represent the HTC data with variable components. Then, based on graph encoder-decoder transformer method, a bi-directional prediction model is established to model the HTC process of agroforestry and livestock wastes. In the case of the forward prediction, the model can predict the composition and yield of hydrochar according to the given HTC conditions. The comparisons with the experimental data other than the training and validation datasets show that the prediction accuracy (R2) of ash, carbon and nitrogen contents of the hydrochar are 0.895, 0.908 and 0.840, respectively, which proves the good generalizability of the model. Moreover, owing to the embedding vector, which connects the reactant data with the product data in HTC, the model can also achieve backward prediction of HTC reaction conditions such as raw material composition and reaction temperature. Furthermore, by dimensionality reduction analysis of the embedding vector in the model, the similarity of any two HTC reactions can be quantified. The established HTC-graph neural network (GNN) model shows both high accuracy and good generalization ability in bi-directional prediction of HTC process. Based on the effectiveness of the proposed model and the given evaluation indexes, the corresponding HTC parameters can be determined, which greatly benefits to the resources saving and energy consumption reduction.

Recycling waste chaff to prepare biochar with co-decorated CoCO3/CoNiO2 for anti-bacterial microwave dissipation

In order to address the recession of absorption properties and service life in electromagnetic absorbers caused by the bacterial propagation, the design of microwave absorbing composite with excellent antimicrobial character is an urgent requirement. Herein, the grain husk-derived carbon is co-decorated with CoNiO2 and CoCO3 nanoparticles through a facile hydrothermal-carbonization process. The defect structure and graphitization in biochar, complex interfaces between multi-components, and partial magnetic resonances can be regulated with the calcination temperature. With an appropriate impedance matching from balanceable electromagnetic parameters, the C4 composite treated by 650 °C realizes an optimal electromagnetic absorption of -41.87 dB at 2.5 mm thickness with its efficient absorption bandwidth of 4.45 GHz at 1.49 mm, which is mainly owing to the powerful dielectric dissipation caused by polarization and conduction losses together. Additionally, the composite exhibits a higher antimicrobial rate of 97.75 % against ubiquitous E. coli, indicating this work provides a reference for designing antimicrobial microwave absorption materials with better practicality.

Continuous Hydrothermal Carbonization of Olive Pomace and Orange Peels for the Production of Pellets as an Intermediate Energy Carrier

The ever-increasing volumes of food waste generated and the associated environmental issues require the development of new processing methods for these difficult waste streams. One of the technologies that can treat these waste streams directly is hydrothermal carbonization. In this work, olive pomace and orange peels were treated via a mild hydrothermal carbonization process (TORWASH®) in a continuous-flow pilot plant. For olive pomace, a solid yield of 46 wt% and a dry matter content of 58% for the solid press cakes were obtained during continuous operation for 18 days. For orange peels, the values were lower with 31 wt% solid yield and a 42% dry matter content during 28 days of continuous operation. These values corresponded fully with initial laboratory-scale batch experiments, showing the successful transformation from batch to continuous processing. The obtained hydrochar from both feedstocks showed an increase in higher heating value (HHV) and a significant reduction in ash content. Pellets produced from the solids met the requirements for industrial use, demonstrating a large increase in the deformation temperature and a significant reduction in the potassium and chlorine content compared to the original feedstock. These results indicate the excellent potential of these pellets for combustion applications.

Hydrothermal Carbonization of Waste Biomass: A Review of Hydrochar Preparation and Environmental Application

The concept of a bio-based economy has been adopted by many advanced countries around the world, and thermochemical conversion of waste biomass is recognized as the most effective approach to achieve this objective. Recent studies indicate that hydrothermal carbonization (HTC) is a promising method for the conversion of waste biomass towards novel carbonaceous materials known as hydrochars. This cost-effective and eco-friendly process operates at moderate temperatures (180–280 °C) and uses water as a reaction medium. HTC has been successfully applied to a wide range of waste materials, including lignocellulose biomass, sewage sludge, algae, and municipal solid waste, generating desirable carbonaceous products. This review provides an overview of the key HTC process parameters, as well as the physical and chemical properties of the obtained hydrochar. It also explores potential applications of produced materials and highlights the modification and functionalization techniques that can transform these materials into game-changing solutions for a sustainable future.

Insights about the formation of secondary char during HTC processes

This investigation aims to get insight about the extent of formation of secondary char (SC) during the hydrothermal carbonization (HTC) of lignocellulosic biomass (walnut shell) by introducing a new role on the system; a porous solid able to partially adsorb chemicals from the process water.In this way, the potential migration of the feedstock degradation products from process water was followed along different time conditions at constant temperature, by adding a porous activated carbon (AC) into the HTC reaction media; two types of adsorbents were tested, differing on their pore size distribution. It was found that the addition of AC decreased the adsorption of decomposition products on the primary char, what had a significant effect on its solid yield, decreasing it from the beginning of the process (a decrease of about 30% of final HC mass was observed) and affecting the kinetics and the capture of chemicals on the HC surface.At the same time, AC gained in mass, up to a value neat 40%, which was associated to its saturation, as confirmed by the pore volume characterization of the adsorbent, and also because SC microspheres were found on its surface with increasing number and size for runs performed for longer periods.The physicochemical properties of both HC and ACs, were influenced by the change introduced in the system; these modifications were followed by elemental and calorific value analyses, thermal degradation profiles, surface chemistry analyses by FT-IR spectroscopy, and SEM-EDX imaging.

Fate of antibiotics and hormones during hydrothermal carbonization of poultry litter: degradation kinetics and toxicity assessment of filtrates and hydrochars

This study investigated degradation kinetics of five selected organic micropollutants (OMPs) present in poultry litter (namely: sulfadiazine, tetracycline, and doxycycline hyclate (antibiotics); estrone and 17-β-estradiol (hormones)) during hydrothermal carbonization (HTC) treatment as the temperature stepwise increased to 250 °C. All five pure OMPs were completely degraded before 250 °C was reached during the HTC process. Nevertheless, presence of poultry litter slowed down the degradation of OMPs. Through elemental mass balance calculation, it is noted that after 15 min (temperature less than 137 °C), 69–82% of organic carbon and 50–66% of organic nitrogen initially consisting part of the target antibiotics were fully mineralized. Both HTC filtrates and hydrochars obtained from poultry litter inhibited Escherichia coli and Bacillus subtilis growth. A combination of high salinity, high nutrients, dissolved organic carbon, and other ions in the filtrate as well as the adsorption of OMPs on hydrochars were probably the reason for the high toxicity.

Activation of hydrochar derived from food waste via hydrothermal carbonization for Cu(II) adsorption

ABSTRACT Currently, heavy metal pollution is a challenge that needs to be solved urgently, and there is a broad prospect for food waste(FW) utilization. In this paper, activated hydrochar (AHC) made from FW according to actual proportion was produced by hydrothermal carbonization (HTC) and activated with K2CO3. The effects of the AHC on Cu(II) adsorption were investigated using static adsorption experiments. The results indicated that the sample fabricated at 160°C (FW-160) for 6 h had the best adsorption performance with the maximum adsorption amount of Cu(II) (36.77 mg/g). The oxygenated functional group on the surface of FW-160 was observed. The pseudo-second-order kinetic model fitted the experimental results better, which proved that chemisorption was the main adsorption mechanism, and the surface complexations, including oxygenated functional groups, were the predominant factors in hydrochar(HC) sorption for Cu(II). The adsorption amount of the sample simulating rotten FW with Aspergillus niger (FW-160A) was 24.82 mg/g. Through the HTC process, the rotten FW can be made into heavy metal absorbents which can be stored for long term, and finally realize the reduction of volume and cost of storage.

Machine learning, conventional and statistical physics modeling of 2,4-Dichlorophenoxyacetic acid (2,4-D) herbicide removal using biochar prepared from Vateria indica fruit biomass

The adsorption properties of 2,4-Dichlorophenoxyacetic acid (2,4-D) onto biochar, obtained through HCl-assisted hydrothermal carbonization process of Vateria indica fruits (VI-BC), were extensively studied using traditional and statistical physics approaches. The traditional adsorption investigations encompassed kinetics, equilibrium, and thermodynamics studies. Subsequently, the Hill statistical physics model was employed to interpret the mechanism. Also, artificial neural network (ANN) and adaptive neuro-fuzzy inference system (ANFIS) machine learning tools were successfully employed to model the adsorption data wherein both models had high prediction potential (R2 > 0.99). The outcomes demonstrated that the produced VI-BC exhibited remarkable adsorptive traits, having a considerable specific surface area (111.54 m2/g), pore size (5.89 nm), a variety of functional groups, and appropriate attributes for efficiently adsorbing 2,4-D. For 10 mg/L 2,4-D, at pH 2.0 and with 0.3 g/L dose, an impressive 91.67% adsorption efficiency was achieved within a 120-min. Pseudo-second-order model aptly depicted the kinetic behavior of 2,4-D adsorption, while the Freundlich model provided a more accurate representation of the isotherms. 2,4-D maximum adsorption capacity stood at 131.39 mg/g at 303 K. The Hill statistical physics model elucidated that the adsorption primarily occurred via physisorption mechanisms, involving electrostatic attractions, π-π conjugation, and pore filling. This conclusion was further substantiated by post-adsorption characterization of the VI-BC. Thermodynamic analysis indicated that the interactions between VI-BC and 2,4-D were favorable, spontaneous, and exothermic. The calculated low energy of adsorption (1.255 kJ/mol) and ΔH° value (−20.49 kJ/mol) further supported physisorption as the dominant mechanism. In summary, this study underscores the significant potential of the newly developed biochar as a promising alternative material for efficiently removing the 2,4-D herbicide from polluted environments.

Process evaluation of mild hydrothermal carbonization to convert wet biomass residue streams into intermediate bioenergy carriers

Hydrothermal carbonization (HTC) is a promising process for the upgrading of wet biomass residues. Models of HTC processes, in particular at continuous pilot-scale, are needed to move HTC from lab-scale to industrial scale. This study presents a process model for mild HTC, dewatering and conversion to intermediate energy carriers (bio-pellets and biogas for power and/or heat production) of three wet biomass residue streams: paper sludge, olive pomace and orange peels, based on lab- and pilot-scale experiments. In addition, the process energy efficiency and feedstock utilization of the HTC process is calculated and compared with conventional treatment options for the chosen residues, i.e., direct anaerobic digestion (olive pomace, orange peels) or combustion after conventional dewatering (paper sludge). The process model indicates that the HTC pilot-scale process is much more efficient in terms of feedstock utilization to produce heat and/or power than the reference scenarios. The process energy efficiency of the HTC process (pilot-scale) was calculated to be 26 %, 63 % and 40 % for paper sludge, olive pomace and orange peel feedstocks, respectively. For all feedstocks, both the solid and liquid-generated products are equally important for improving the overall process energy efficiency. This study demonstrates the potential benefits of HTC processes for upgrading wet biomass waste streams based on continuous pilot-scale data.

Optimization of hydrochar production from almond shells using response surface methodology, artificial neural network, support vector machine and XGBoost

Herein, we investigate how to improve the hydrochar yield via hydrothermal carbonization process, focusing on the use of almond shells as a carbon resource. We have employed various modeling techniques, including response surface methodology, artificial neural networks, support vector machines and XGBoost, to determine the optimal conditions for maximizing hydrochar yield. The results highlight the excellent performance of the ANN and XGBoost models, with high R² values and lower RMSE values, while the SVM model offers insights into the interactions between the factors. The choice of modeling method depends on the specific nature of the problem and accuracy requirements. In summary, this study contributes to a better understanding of HTC processes and opens the way to further optimization of hydrochar performance, with significant practical implications in the energy and environmental fields. It also highlights the importance of further research to explore more modeling techniques.

Fe/N co-doped magnetic porous hydrochar for chromium(VI) removal in water: Adsorption performance and mechanism investigation

Four kinds of Fe/N co-doped porous hydrochar were prepared by one/two-step N-doping schemes using microwave/traditional pyrolysis methods for removing Cr(VI) from aqueous phase. Heterocyclic-N was introduced through CO(NH2)2-based hydrothermal carbonization process, which could adjust the electronic structure of the hydrochar framework. Furthermore, Fe0 and Fe3O4 were embedded into hydrochar via carbothermal reduction reaction using FeCl3 as the precursor, which improved the reducibility and magnetism of the material. The modified hydrochar exhibited pH-dependency and rapid kinetic equilibrium, and the maximal adsorption amount of magnetic porous hydrochar obtained by microwave-assisted one-step N-doping (MP1HCMW) reached 274.34 mg/g. Meanwhile, the modified hydrochar had a high tolerance to multiple co-existing ions and the removal efficiency maintained above 73.91 % during five regeneration cycles. Additionally, MP1HCMW efficiently removed Cr(VI) via pore filling, electrostatic attraction, ion exchange, reduction, complexation, and precipitation. Summarily, Fe/N co-doped porous hydrochar was a feasible adsorbent with outstanding remediation potential for Cr(VI)-contaminated water.

Enhancing the Fuel Properties of Spent Coffee Grounds through Hydrothermal Carbonization: Output Prediction and Post-Treatment Approaches

The reuse potential for the large annual production of spent coffee grounds (SCGs) is underexploited in most world regions. Hydrochars from SCGs produced via hydrothermal carbonization (HTC) have been recognized as a promising solid fuel alternative. To increase demand, optimization of the HTC and two post-treatment processes, washing and agglomeration, were studied to improve hydrochar in terms of energetic properties, minimizing unwanted substances, and better handling. HTC experiments at three scales (1–18.75 L) and varying process conditions (temperature T (160–250 °C), reaction time t (1–5 h), and solid content %So (6–20%) showed that the higher heating value (HHV) can be improved by up to 46%, and most potential emissions of trace elements from combustion reduced (up to 90%). The HTC outputs (solid yield—SY, HHV, energy yield—EY) were modeled and compared to published genetic programming (GP) models. Both model types predicted the three outputs with low error (<15%) and can be used for process optimization. The efficiency of water washing depended on the HTC process temperature and type of aromatics produced. The furanic compounds were removed (69–100%; 160 °C), while only 34% of the phenolic compounds (240 °C) were washed out. Agglomeration of both wet SCG and its hydrochar is feasible; however, the finer particles of washed hydrochar (240 °C) resulted in larger-sized spherical pellets (85% > 2000–4000 µm) compared to SCGs (only 4%).

New insights into the hydrothermal carbonization process of sewage sludge: A reactive molecular dynamics study

In this work, reactive molecular dynamics simulations using the ReaxFF force field were performed to investigate the hydrothermal carbonization (HTC) process of sewage sludge. The focus was on the reaction mechanism, the effect of water content and temperature on the formation of different products. The results showed that temperature was the most important factor controlling the HTC process, with higher temperatures leading to more rapid carbonization reactions and lower H/C and O/C ratios in the hydrochar product. The estimated activation energy of the HTC process was in the range of 126–169 kJ/mol, depending on the water content. The H/C and O/C ratios of the hydochar product obtained from the simulation were in excellent agreement with experimental data. Overall, this work provides valuable new insights into the reaction mechanism and key factors influencing the HTC process of sewage sludge.

Hydrothermal carbonization of Calotropis procera leaves as a biomass: Preparation and characterization

Hydrothermal carbonization (HTC) is a biowaste treatment process and a promising technology for many applications. The use of Calotropis procera (CP), found in the Egyptian desert, as a biosource feedstock for the HTC process. In the present work, different process conditions were applied during the HTC process. The influence of temperature, time, and particle size was studied to produce hydrochar using CP leaves as biomass. The hydrochar samples were characterized by many analytical techniques. The temperature and time controlled the severity of the reaction; they caused a significant decrease in the yield (wt.%) and caused an increase in the carbon content of the hydrochar product, with a value of 57.03 % at 240 °C and a time of 180 min. Although time in this case has a more significant effect than temperature, a long reaction time influences the hydrochar yield. FTIR showed the formation on hydrochar with high content of lignin at high temperature and longtime. The O/C decrease with increase the temperature and time as evidence of decarboxylation reaction occurs and the reaction move toward carbonization and increase the HHV of hydrochar than raw biomass that recorded 40.2 MJ/Kg at 240 °C, time 180 min and biomass particle size 150 μm. The hydrochars produced under all condition have negative surface charge in wide range of pH and all have negative charge over pH 4–5 and most suitable pH value is between 6 and 7. The best conditions recorded were temperature 200–240 °C and reaction time 120–180 min at this conditions hydrochar has a stable negative charge in slightly acidic and basic media which let it able to use in applications of removing the cationic pollutants from wastewater, and the high heating value give opportunity to use in energy production applications.

Lab-scale engineered hydrochar production and techno-economic scaling-up analysis

Despite the extensive use of engineered hydrochar (EHC) for contaminants adsorption in water, little is known about the scaling-up of EHC production which has kept the technology at a low readiness level (TRL). Full-scale EHC production was simulated to help bridge this knowledge gap. A systematic analysis was performed where EHC was produced from rice straw using hydrothermal carbonization (HTC) at 200 °C with iron addition. A techno-economic evaluation model was employed to simulate the production process and to estimate energy requirements, configuration, and cost scenarios for the HTC process. The minimum selling price (MSP) analysis of the engineered hydrochar was found to be almost half compared to the market price for other similar sorbents ($ 76/t vs. $136/t) suggesting that EHC production is feasible for scaling up. Finally, as a trial, the resulting material was tested for its efficacy in the adsorption of an anionic organic contaminant (e.g., Congo Red, C32H22N6Na2O6S2) in water to identify its potential for water treatment. Experimental results showed that EHC adsorbed > 95% CR suggesting significant adsorption capability and feasibility for production scale-up.

Innovative mathematical approach for hydrothermal carbonization process using an inverse method: Experimental analysis, rheology behavior, and numerical comparative investigation

Hydrothermal carbonization (HTC) shows promise for converting biomass into carbon solids, but simulating it at high biomass-to-water ratios is difficult due to limited models. Existing numerical methods have not been sufficiently tested and validated under such conditions. An experimental approach and a novel non-Newtonian mixture model (NNM) were developed to improve simulation accuracy. This model incorporated rheological parameters using COMSOL Multiphysics software and an inverse method, especially the Levenberg-Marquardt algorithm. The results show that the proposed design achieved desired HTC conditions within 55–70 min, depending on the ratio. Besides, the innovative NNM model proved more effective for predicting thermal behavior and exhibited significantly reduced computation time than other CFD models in the literature, with a maximum 5 % relative deviation, indicating lower computational costs. Infrared camera measurements validated the heat behavior predicted by the NNM. Moreover, during HTC, the mixture behaves as a Newtonian fluid at a 1/6 ratio, and as a shear-thinning fluid at 1/3 and 1/2 ratios, with viscosity decreasing with increasing shear rate and temperature. This study presents innovative findings in the field of HTC, offering a reliable NNM model for simulating HTC under challenging conditions, addressing the mixture's rheology, and proposing a novel design.

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.

Generating colloidal Fe/C composites via hydrothermal carbonization – A critical study

The increasing need for water remediation due to water scarcity and the accumulation of persistent pollutants in water cycles requires innovative materials and their more sustainable synthesis processes. The aim of this work was therefore to develop an approach for the bottom-up synthesis of colloidal Fe/C composites. It was tested whether the composites fulfill the main criteria for use as injectable adsorbents and reducing agents for the removal of chlorinated pollutants in in-situ groundwater remediation. After screening different Fe and C precursors, the bottom-up synthesis of a particulate and reactive Fe/C composite was developed via hydrothermal carbonization (HTC) of mixtures of ferrous and sodium gluconate in a one-pot HTC and subsequent carbothermal reduction at 800 °C. The produced particles show good dispersibility with particle diameters in the range of d10 ≥ 2 µm, d50 ≈ 11 µm and d90 ≤ 40 µm in aqueous suspensions. By varying the molar ratio of Fe to gluconate in the HTC process, the final content of zero-valent iron equivalents (ZVI) in the composite could be adjusted between (24 ± 12) and (49 ± 15) wt.-%. The chemical reactivity of the composite was tested by reductive dechlorination of chloroform (CF) as a test reaction. The characterization of the composites before and after reductive dechlorination with X-ray diffraction (XRD) shows that not only pristine Fe0 but also Fe3C is active in the reductive dechlorination reaction. Palladization of the composite material shifts the product selectivity from dichloromethane as the main product to the non-chlorinated products methane (up to 77 mol-%) and ethane (up to 9 mol-%) without any external H2 feed. The dispersiblity and intrinsic reactivity of the synthesized composites meet the requirements for application in water remediation where contaminants need to be retained and degraded, e.g. for groundwater remediation or protection by the in-situ generation of permeable barriers using modern injection technologies.

Improved energy recovery from yard waste by water-starved hydrothermal treatment: Effects of process water and pressure

This study investigated the feasibility of a high-loading process with less water consumption for the valorization of wet biomass waste through hydrothermal carbonization (HTC) with and without N2 pressurization from the views of water saving, carbon utilization, and energy recovery. The results revealed that reducing the liquid-to-solid ratio from 10 to 2.5 significantly improved carbon storage in hydrochar due to preferential carbon sequestration as the solid phase (59.9%) instead of being lost in the liquid phase (∼10%). The pressurized HTC process resulted in a higher stability hydrochar through the devolatilization of secondary char that was less stable, yet resulted in ∼10% 15% more carbon transformation to the gas phase. A cost-benefit analysis further demonstrated the potential of the high-loading HTC process for enhancing energy recovery while minimizing energy consumption during hydrochar production from high-moisture yard waste.