Mild Hydrothermal Carbonization Research Articles

Enhanced photo-oxidation of hormones in water using biomass waste-derived hydrochar composites as photocatalysts

This study introduces a green chemistry approach to enhance the catalytic efficiency of metal oxides in photocatalytic reactions using agro-industrial waste. Specifically, rice husks and spent coffee grounds were utilized as carbon sources to produce hydrochars, which served as sustainable supports for ZnO and ZnFe₂O₄ oxides. This method promotes waste minimization and circularity by repurposing agricultural byproducts, reducing the environmental impact of traditional catalyst production. The hydrochars were synthesized through mild hydrothermal carbonization (250 °C for 4 h) and characterized by various physicochemical analyses. Their photocatalytic performance was evaluated using a 20 mg L−1 solution of conjugated estrogens under ultraviolet light (120 W cm−2) for 30 min. Composites with ZnO exhibited a significant increase in oxidation reaction rates due to enhanced porosity and an oxygen-rich structure. Reactive oxygen species (ROS) analysis revealed hydroxyl radicals (OH) as the primary agents, with photocatalytic efficiencies exceeding 83 % after eleven cycles. In toxicity tests, 4 mL of treated solution was applied to 20 Lactuca sativa (lettuce) seeds, kept in the dark at 25 °C for 7 days. Seeds exposed to treated solutions showed improved radicle elongation and germination rates compared to controls, except for those with coffee hydrochar and ZnO. This indicates effective degradation of toxic estrogens without harmful by-products. Overall, this work highlights the advantages of integrating green chemistry principles and circular economy practices, transforming waste into valuable materials to enhance environmental sustainability and catalytic performance.

Insights into pyrolysis process of coconut shell waste hydrochar: In-situ structural evolution and reaction kinetics

Coconut shell (CS) can be efficiently utilized by coupling hydrothermal carbonization (HTC) pretreatment with pyrolysis to meet the goal of global green development. In-situ Raman spectroscopy and thermogravimetric analyzer (TGA) were adopted to investigate the structural evolutions and reaction kinetics characteristics of hydrochar during the pyrolysis process. According to the results, the intensity of peak D and peak G in Raman spectrum decreased with increasing hydrothermal degree. Mild HTC treatment (≤200 °C) led to an increase in ID/IG, indicating the formation of large aromatic rings through small ring condensation. In contrast, severe HTC treatment (≥220 °C) reduced the ID/IG ratio, indicating an increased polycondensation of aromatic compounds and a higher degree of aromatization, resulting in larger molecules sizes. The spectral area below 400 °C reduced with increasing pyrolysis temperature due to the reduction of oxygen-containing functional groups and thermal radiation. Above 400 °C, the total area remains stable as the Raman spectrum scattering ability increases due to char formation and aromatization reactions. HTC-200 exhibited an increase in ID/IG from 0.54 to 0.72 and ID/I(Me + Ke + GL) from 0.58 to 0.68 with increasing pyrolysis temperatures. The activation energy during pyrolysis ranged from 194.50 to 238.34 kJ/mol for CS and 217.51–259.51 kJ/mol for HTC-200. Hydrothermal carbonization had a significant effect on pyrolysis behavior of hydrochar, as it was found that the mild HTC hydrochar followed D-type diffusion models when conversion rate was less than 0.65, and transitioned to higher reaction order in the late-stage of pyrolysis.

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.

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.

Development of a Continuous Hydrothermal Treatment Process for Efficient Dewatering of Industrial Wastewater Sludge

Sludges from the papermaking industry represent a challenging residue stream that is difficult to dewater using conventional processes. The successful development and scale-up of innovative processes from lab- to pilot- to industrial-scale are required to tackle challenges for waste treatment, including paper sludges. Biological paper sludge was treated via a mild hydrothermal carbonization process (TORWASH®) to improve dewaterability of the sludge, including long-duration, continuous testing. Initial lab-scale experiments indicated the optimal treatment temperature for sludge dewatering was 190 °C. Dewaterability improved with increasing temperature, but the obtained solid yield decreased. Scaling-up to a continuous flow pilot plant required a temperature of 200 °C to achieve optimum dewatering. Pilot-scale hydrothermal treatment and dewatering resulted in solid cakes with an average dry matter content of 38% and a solid yield of 39%. This study demonstrates the benefits of hydrothermal carbonization for the dewatering of biological paper sludge without the use of dewatering aids such as fiber sludge or polyelectrolytes. The results also demonstrate the successful adaptation of a lab-scale batch process to a pilot-scale continuous flow process for hydrothermal carbonization of industrial wastewater sludge.

Nitrogen self-doped hierarchical porous carbon via penicillin fermentation residue (PR) hydrothermal carbonization (HTC) and activation for supercapacitance

The safe disposal and resource utilization of biomass waste are of great significance. For the first time, we have prepared penicillin fermentation residue (PR) N self-doped porous carbon through hydrothermal carbonization (HTC) combined with activation. Mild HTC method keeps stable carbon skeleton, rich nitrogen and oxygen functional groups. The hydrochar is activated as a precursor material to realize high specific surface area and naturally doped nitrogen. At 600 ℃, the hierarchical porous carbons obtained with KOH and ZnCl 2 as activators are BC@KOH600 and BC@ZnCl 2 600, with specific surface areas of 615.68 m 2 /g and 792.58 m 2 /g, respectively. The porous carbon prepared with ZnCl 2 as an activator has higher N content than KOH. BC@ZnCl 2 600 has 4.75% N content, among which Pyridine-N is up to 36%. The BC@KOH600 and BC@ZnCl 2 600 supercapacitors have specific capacitances of 160.3 F/g and 209.2 F/g at 1.0 A/g in the three-electrode system, respectively, which display excellent rate performance and cycle stability. The BC@KOH600 and BC@ZnCl 2 600 symmetric supercapacitor, constructed with a two-electrode configuration, shows an energy density of 8.55 Wh kg −1 and 8.79 Wh kg −1 respectively and outstanding capacitance retention of 91.01% and 87.98% after 10,000 charge-discharge cycles at 5.0 A/g in an aqueous 6 M KOH electrolyte system. These results demonstrate a very effective method for the preparation of porous carbon using antibiotic residue for supercapacitors, which also provides a solution for the treatment of similar hazardous biomass wastes. • Porous carbon was prepared by two-step method, which was activated by PR hydrochar. • The sample BC@KOH600 and BC@ZnCl 2 600 had a specific surface area of 615.68 m 2 /g and 792.58 m 2 /g. • BC@ZnCl 2 600 has 4.75% N content, among which Pyridine-N is up to 36%. • The porous carbons had good capacitance characteristics and cycle stability.

Improving Kraft Pulp Mill Energy Efficiency through Low-Temperature Hydrothermal Carbonization of Biological Sludge

Of the various waste and side streams created in a kraft pulp mill, the biological sludges from the wastewater treatment plant are some of the most problematic to handle. Incineration is becoming a common solution as landfilling is no longer permitted by legislation in many countries, but this is also problematic due to the high moisture content, poor drying characteristics, and high ash content in the solids. This study evaluates the technical potential of mild hydrothermal carbonization (HTC) at 160 °C for 3 h to improve the energy efficiency of on-site incineration as a biosludge handling method. HTC treatment transforms wet organic substrates into a hydrophobic carbonaceous material (hydrochar). The heating value and elemental composition of both the sludge and the hydrochar product were analyzed. Based on this, a hydrothermal carbonization model developed earlier was adjusted for the feedstock, and process integration modelling performed to evaluate the performance impact on the power and heat generation at the mill. The results indicate that if the alternative is combustion in the power boiler, HTC pre-treatment could allow a significant increase in power generation. If the sludge is combusted in the recovery boiler, a practice often avoided in order to not introduce non-process elements to the chemical recovery cycle but sometimes necessary due to, e.g., absence of a power boiler, a much smaller increase is obtained. The increase is smallest if the freed evaporator plant capacity cannot be utilized for increasing the firing liquor dry solids content.

Process Optimization for Preparation of Hydrochar with Abundant Surface Functional Groups and Promising Adsorption Capacity

The study attempted to optimize the synthesis process of hydrochar from bamboo utilizing hydrothermal carbonization (HTC). The process conditions were optimized to maximize the amount of Acid Oxygen Functional Groups (AOFG) for better adsorption capacity, and the yield of hydrochar for better economic feasibility. The hydrochar obtained was subjected to Methylene Blue (MB) adsorption capacity, to assess its potential as lowcost adsorbent for wastewater treatment. Among the process parameters, temperature was found to significantly influence the AOFG amount and the yield, the optimum conditions were found to be a temperature of 180 °C, duration of 24 h and pH of 5, with AOFG amount of 1108 μmol/g and carbon yield of 52.45%. Although with low surface area, the hydrochar had a maximum MB adsorption capacity of 91.74 mg/g. The adsorption kinetic followed the pseudo second-order kinetic model, while the adsorption isotherm resembled Langmuir isotherm model. The experimental results established quantitative relationship between the process conditions and the AOFG amount and yield of bamboo based hydrochar, which also suggested a pragmatic way to enhance the adsorption capacity of hydrochar as low-cost adsorbent by maximizing the surface functional groups under mild hydrothermal carbonization conditions.

Chlorine migration during hydrothermal carbonization of recycled paper wastes and fuel performance of hydrochar

Wastes from recycled paper industry (WRP) was hydrothermally carbonized to remove impurities to produce alternative biofuel. The chlorine behaviors during hydrothermal carbonization (HTC) and the fuel properties of the generated hydrochar were evaluated. The degradation of lignocellulosic components improved free -OH bond and small free cellulosic fragments facilitating dechlorination. Meanwhile, dechlorination process stimulated degradation of biomass. The carbon content and density of WRP were improved by HTC resulting in high heating value increasing from 5044.40 MJ/m3 to 17,004.88 MJ/m3. Pollutants emission from hydrochar combustion would be effectively controlled because of the removal of sulfur and chlorine. Hydrochar enjoyed higher ignition temperature and average combustion rate while lower burnout temperature, indicating that the combustion range has been narrowed, thus improving the combustion efficiency. The increase of HTC temperature facilitated hydrochars combustion and reduce energy consumption during hydrochar combustion. Mild HTC operating conditions (220 ℃ for 90 min and 240 ℃ for 60 min) were supposed to effectively improve the combustion intensity with stability meanwhile reached the highest dechlorination efficiency. These results evidence the feasibility of converting WRP to clean biofuel via HTC.

Innovative particleboard material from the organic fraction of municipal solid waste

This study presents a challenging approach that addresses the efficient management of the organic fraction of municipal solid waste (OFMSW) by hydrothermal carbonization (HTC) for the development of novel sustainable low-CO2 building materials. Mild HTC treatment at 180 °C for 2 h transformed low-grade OFMSW into a renewable carbonaceous solid (hydrochar), which displays promising properties for application in particleboards. Taking advantage of the presence of extractives acting as natural binders, the hydrochar particles with sizes of <0.3 mm, 0.3–1 mm, and 1–2 mm agglomerate successfully by simple pressing at 3 MPa for 7 min at room temperature (∼ 25 °C). The resulting binderless monolithic probes display a density of 838 and 883 kg/m3 for the finest and coarsest grain sizes, respectively, and approximately 30% porosity. The mechanical resistance is enhanced by the use of larger particle sizes, and values of modulus of rupture and tensile strength of 21.64 MPa and 18.99 MPa are reached, respectively. The thermal conductivity of the probes in the range of 0.091–0.132 W/(m∙K) suggests the potential of OFMSW-derived hydrochar for thermal insulation panels.

Conversion of Slaughterhouse Wastes to Solid Fuel Using Hydrothermal Carbonization

In this study, cattle and pig slaughterhouse wastes (SHWs) were hydrothermally carbonized at 150–300 °C, and the properties of SHW-derived hydrochar were evaluated for its use as a solid fuel. The results demonstrated that increasing the hydrothermal carbonization (HTC) treatment temperature improved the energy-related properties (i.e., fuel ratio, higher heating value, and coalification degree) of both the cattle and pig SHW-derived hydrochars. However, the improvements of cattle SHW-derived hydrochars were not as dramatic as that of pig SHW-derived hydrochars, due to the lipid-rich components that do not participate in the HTC reaction. In this regard, there was no merit of using HTC treatment on cattle SHW for the production of hydrochar or using the hydrochar as a solid fuel in terms of energy retention efficiency. On the other hand, a mild HTC treatment at approximately 200 °C was deemed suitable for converting pig SHW to value-added solid fuel. The findings of this study suggest that the conversion of SHWs to hydrochar using HTC can provide an environmentally benign method for waste treatment and energy recovery from abandoned biomass. However, the efficiency of energy recovery varies depending on the chemical composition of the raw feedstock.

Modeling of &lt;i&gt;Agave Salmiana&lt;/i&gt; bagasse conversion by hydrothermal carbonization (HTC) for solid fuel combustion using surface response methodology

Hydrothermal carbonization (HTC) of Agave Salmiana bagasse was investigated to assess the potential of hydrochar as an energy resource. The effect of operating conditions on the mass yield and the energy quality of hydrochar was examined by varying the temperature and the holding time over the ranges of 180–250 ℃ (T) and 0–60 min (t), respectively. These parameters were screened using the response surface methodology through a Doehlert design. Performances of HTC were assessed on the basis of the hydrochar mass yield higher heating value ultimate and proximate analyses. In the studied domain, the modeling results indicated that the hydrochar mass yield varies between 45% and 86% and higher heating valuesfrom18 to 23 MJ.kg-1, which is similar to that of peat. In addition, the volatile matter and fixed carbon fractions of hydrochars ranged from 54 to 80% and between 12 and 36%, respectively. Energy yield modeling indicated that the mass yield is the most influent parameter. The maximal energy yield value was obtained at 180 ℃ with time equal to 0 min. Globally, the evolution of H/C ratio is amplified for temperatures greater than 215 ℃ by increasing the holding time from 30 to 60 min. For O/C ratio the maximal variation is below these conditions. It is concluded that increased hydrothermal carbonization conditions favor dehydration reactions, while decarboxylation reactions are favored in mild HTC conditions. The combustion characteristics such as ignition peak and burnout temperatures were significantly modified for hydrochars. The model results indicated that lowest ignition and peak temperature values are obtained at both low-temperature and low-holding time, and high-temperature and high-holding time zones. An increase in the burnout temperature was correlated with the increase of the fixed carbon content in hydrochars.

Platanus orientalis leaves based hierarchical porous carbon microspheres as high efficiency adsorbents for organic dyes removal

Water contamination caused by hazardous organic dyes has drawn considerable attention, among all of the techniques released, adsorption has been widely used, which however to a large degree is dependent on the development of high efficiency adsorbents. Waste biomass based porous carbon is becoming the new star class of adsorbents, and thus contribute more to the sustainable development of the society. In this work, for the first time to the best of our knowledge, abundant waste fallen Platanus orientalis leaves are employed as the raw material for hierarchical activated porous carbon (APC) microspheres via a mild hydrothermal carbonization (210 °C, 12.0 h) followed by one-step calcination (750 °C, 1.0 h). The APC microspheres exhibit a specific surface area of 1355.53 m2·g−1 and abundant functional groups such as OH and CO. Furthermore, the APC microspheres are used as the adsorbents for removal of RhB and MO, with the maximum adsorption capabilities of 557.06 mg·g−1 and 327.49 mg·g−1, respectively, higher than those of the most porous carbon originated from biomass. The adsorption rates rapidly approach to 98.2% (RhB) and 95.4% (MO) within 10 min. The adsorption data can be well fitted by Langmuir isotherm model and the pseudo-second-order kinetic model, meanwhile the intra-particle diffusion and Boyd models simultaneously indicate that the diffusion within the pores is the main rate-limiting step. Besides, the APC microspheres also demonstrate good recyclability, and may also be applied to other areas such as heterogeneous catalysis and energy storage.

One-step preparation of phosphate-rich carbonaceous spheres via a hydrothermal approach for phosphopeptide analysis

Ti4+-IMAC carbonaceous spheres prepared via a one-step mild hydrothermal carbonization (HTC) process exhibited satisfactory enrichment ability for phosphopeptides in biosamples.

Catalytic performance of sulfonated carbon-based solid acid catalyst on esterification of waste cooking oil for biodiesel production

Carbonaceous material containing sulfonate groups as solid acid catalyst was generated by one-step mild hydrothermal carbonization of glucose. The process took place in the presence of hydroxyethylsulfonic acid at 180°C for 4h. The obtained sulfonated carbon (C-SO3H) was microsphere with diameter size of 50–100 micrometer. The sulfonate groups were further attached on the surface of the carbonaceous materials. The catalytic esterification of waste cooking oil (WCO) with methanol could be easily achieved by using C-SO3H. Free fatty acids (FFAs) in WCO could be decreased up to 93.4% at 60°C for 3h. The biodiesel yield was achieved about 87% within 1h at 60°C by transesterification using 1% NaOH (w/w) as catalyst. The C-SO3H was demonstrated to have good stability with only 7% decrease in FFAs conversion after 5 repeat uses. The sulfonated carbon-based solid acid catalyst was thus designed to be an active, stable and reusable solid acid as an environmentally benign replacement for homogeneous catalyst.

Improving the Energy-Related Aspects of Biowaste Treatment in an Experimental Hydrothermal Carbonization Reactor

Hydrothermal carbonization (HTC) is a thermochemical conversion process with the potential to treat the prevalent wet urban biowaste in low- and middle-income countries. The generated hydrochar solids are a hygienic, homogenized, carbon rich and energy dense product with economic value that can be used as an alternative to wood-based charcoal or fossil fuel. Obtaining a satisfactory energy efficiency of the process is, however, one of the prerequisites for the possible breakthrough of this technology. In an experimental HTC reactor, a model kitchen/market waste feedstock (17.8 MJ/kgdb) was hydrothermally carbonized with varying loading rates (TS 20 and 25 %) under mild operational conditions with peak temperatures of 160–190 °C and process times of 2–10 h above 160 °C. The aim was to evaluate the energy ratio of the process under these conditions while examining the impact on the hydrochar quality. Results show that the chemical properties of the produced hydrochar and its heating value were of moderate quality (21.1–24.4 MJ/kgdb), showing similar characteristics like torrefied products. HTC of a 25 % TS-load during 2 h at 180 °C and maximum pressure of 18.3 bar resulted in a char chemical output energy that is twice as high as the electrical energy consumed in the process. If considering the theoretical methane potential of the process water, the energy ratio could be increased to 2.6; while reactor insulation could further enhance this ratio to 3. This article reveals the merits of mild HTC and provides relevant knowledge for attaining an optimized, energy efficient HTC system.

Characterization of hydrochar obtained from hydrothermal carbonization of wheat straw digestate

Hydrothermal carbonization (HTC) of wheat straw digestate was performed at 180–260 °C for 2–8 h. The resulted hydrochars were analyzed by ultimate analyzer. Elemental carbon and oxygen concentration of hydrochars were fitted with power law correlation. Moreover, chemical structures of feedstocks and hydrochars were investigated by 13C cross-polarization magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR) spectroscopy. In particular, a procedure including CP dynamics analysis was applied to obtain semi-quantitative information on the composition of the analyzed materials from 13C CP/MAS spectra. Up to a process temperature of 220 °C, digestate-derived hydrochar contained primarily crystalline cellulose and lignin. At 260 °C, crystalline cellulose was degraded and more aliphatic carbon and lignin-rich hydrochars were produced. Ester bands corresponding to hemicellulose disappeared at mild HTC conditions at 180 °C and 2 h.

Optical texture of hydrochar from maize silage and maize silage digestate

Hydrochar is a promising solid fuel produced from hydrothermal carbonization (HTC) of wet biomass feedstocks. Raw maize silage and corresponding hydrochars from 230 and 250°C, and maize silage digestate and corresponding hydrochar from 230°C were analyzed in this study. Optical microscopy has been used to follow the transformations occurring in the hydrochars. The terms biocharospheres and biocharoplast have been proposed to describe the spherical carbonaceous material formed from the cell-walls and the molten homogeneous shapeless material, respectively, generated from biomass during hydrothermal carbonization. Hydrochar produced from digestate under mild HTC condition had a granular structure indicating an acceleration of the nucleation with the formation of submicrometer size spheres. The reflectance of the hydrochar increased with increasing carbon content and allowed the description of sample heterogeneity, which would be useful to follow the transformations under treatments of different severities.

Chemical and morphological changes in hydrochars derived from microcrystalline cellulose and investigated by chromatographic, spectroscopic and adsorption techniques

Hydrothermal carbonization (HTC) can be used for converting the biomass into a carbon-rich material, whose application as a fuel requires higher heating value, whereas soil amendment needs stable carbon. This work was focused on the characterization of hydrochars derived from microcrystalline cellulose. The chars were investigated using elemental analysis, Brunauer-Emmett-Teller technique, nuclear magnetic resonance spectroscopy, Raman, Fourier transform infrared, and electron spin resonance spectroscopy. Severity in temperature between 230 and 270°C with reaction times between 2 and 10 h only affect the carbon content moderately. The results show that aromatization of HTC chars correlates well with temperature, which was further supported by the increase of organic radicals with decreasing g values at higher temperatures. Based on these results, the energetic use of chars favors mild HTC (T<230°C and t≤6 h), while the soil amendement favors serve conditions (T≥230°C, and t>6 h).

Novel carbonaceous nanocomposite pellicle based on bacterial cellulose

A novel carbonaceous nanocomposite pellicle was prepared by one-step mild hydrothermal carbonization of bacterial cellulose pellicle soaked in glucose solution. The nano-fibrous structure of bacterial cellulose remains intact while carbonaceous spheres with nanometre sizes were generated from glucose during hydrothermal carbonization at 180 °C for 3 h. The carbonaceous spheres formed in situ and anchored on the nanofibers’ surface expanded the nanostructure of bacterial cellulose and the water-holding capacity decreased slightly from 98% to 93%. Approximately 80% w/w of the dried carbonaceous pellicle was found to be nanometre sized carbonaceous spheres. The size of carbonaceous spheres decreased significantly but had a 60% increase of surface carboxyl groups when the carbonaceous pellicle was prepared in the presence of acrylic acid. The carbonaceous spheres in the never-dried pellicle demonstrate a high adsorption capacity towards heavy metal ions. Approximately, 200 mg of Pb(II) and 1134 mg of Fe(III) can be adsorbed per gram of carbonaceous pellicle, respectively.