Conventional Hydrothermal Carbonization Research Articles

Study of two approaches for the process water management from hydrothermal carbonization of swine manure: Anaerobic treatment and nutrient recovery

Hydrothermal carbonization (HTC) is a promising alternative to transform biomass waste into a solid carbonaceous material (hydrochar) and a process water with potential for material and energy recovery. In this study, two alternatives for process water treatment by conventional and acid-assisted HTC of swine manure are discussed. Process water from conventional HTC at 180 °C showed high biodegradability (55% COD removal) and methane production (∼290 mL STP CH4 g−1 CODadded) and the treatment in an upflow anaerobic sludge blanket reactor allowed obtaining a high methane production yield (1.3 L CH4 L−1 d−1) and COD removal (∼70%). The analysis of the microbiota showed a high concentration of Synergistota and Firmicutes phyla, with high degradation of organic nitrogen-containing organic compounds. Acid-assisted HTC proved to be a viable option for nutrient recovery (migration of 83% of the P to the process water), which allowed obtaining a solid salt by chemical precipitation with Mg(OH)2 (NPK of 4/4/0.4) and MgCl2 (NPK 8/17/0.5), with a negligible content of heavy metals. The characteristics of the precipitated solid complied with the requirements of European Regulation (2019)/1009 for fertilizers and amendments in agricultural soils, being a suitable alternative for the recycling of nutrients from wastes.

Upgrading of Corn Stalk Residue and Tannery Waste into Sustainable Solid Biofuel via Conventional Hydrothermal Carbonization and Co-Hydrothermal Carbonization

Upgrading of Corn Stalk Residue and Tannery Waste into Sustainable Solid Biofuel via Conventional Hydrothermal Carbonization and Co-Hydrothermal Carbonization

Advancing sustainable development goals with machine learning and optimization for wet waste biomass to renewable energy conversion

With the rapid surge in global population, urbanization, and improved living standards, there has been a significant increase in untreated wet waste. This escalation poses a critical challenge for waste management and raises considerable environmental concerns. One pivotal solution is the conversion of this wet waste into valuable energy resources and carbon-neutral mediums, a strategy aligning with the United Nations Sustainable Development Goal 7 (SDG7) - Affordable and Clean Energy, and Sustainable Development Goal 15 (SDG15) - Life on Land. Hydrothermal carbonization (HTC) and pyrolysis have emerged as effective technologies due to their ability to process large volumes of wet waste and convert it into energy-rich and carbon-stable products with high energy efficiency. This study introduces a novel framework incorporating advanced machine learning techniques (including deep neural networks, random forest, and eXtreme gradient boosting) with dual-objective optimization. This approach enables a comparative analysis of the solid products from HTC and pyrolysis, focusing on their Carbon Stability Index (CSI) and Return on Energy Investment (REI) index. This assessment enables the tailoring of char production towards specific applications, yielding optimal conditions for high energy efficiency and stable carbon sequestration. In a case study involving wet food waste, the analysis indicated a significant enhancement from 4.83 to 14.43 in REI and an increase from 47.4 to 57.98 in CSI compared to conventional HTC solutions. Moreover, the uncertainty analysis demonstrated that our machine learning-based model provides robust and reliable predictions, with most of the R-squared (R2) values exceeding 0.90, meeting or exceeding the R2 values reported in similar ML models of biomass conversion in the literature, reinforcing its potential for practical applications. Our study also plots the solid products in a Van Krevelen diagram, showing composition changes during conversion. These findings exemplify the potential of these processes in bolstering the primary objectives of SDG7 and SDG15, thereby contributing to a more sustainable future.

Co-hydrothermal carbonization of pine residual sawdust and non-dewatered sewage sludge – effect of reaction conditions on hydrochar characteristics

Waste valorization is mandatory to develop and consolidate a circular bioeconomy. It is necessary to search for appropriate processes to add value to different wastes by utilizing them as feedstocks to provide energy, chemicals, and materials. For instance, hydrothermal carbonization (HTC) is an alternative thermochemical process that has been suggested for waste valorization aiming at hydrochar production. Thus, this study proposed the Co-HTC of pine residual sawdust (PRS) with non-dewatered sewage sludge (SS) – two wastes largely produced in sawmills and wastewater treatment plants, respectively – without adding extra water. The influence of temperature (180, 215, and 250 °C), reaction time (1, 2, and 3 h), and PRS/SS mass ratio (1/30, 1/20, and 1/10) on the yield and characteristics of the hydrochar were evaluated. The hydrochars obtained at 250 °C had the best coalification degree, showing the highest fuel ratio, high heating value (HHV), surface area, and N, P, and K retention, although presenting the lowest yields. Conversely, hydrochar functional groups were generally reduced by increasing Co-HTC temperatures. Regarding the Co-HTC effluent, it presented acidic pH (3.66–4.39) and high COD values (6.2–17.3 g·L−1). In general, this new approach could be a promising alternative to conventional HTC, in which a high amount of extra water is required. Besides, the Co-HTC process can be an option for managing lignocellulosic wastes and sewage sludges while producing hydrochar. This carbonaceous material has the potential for several applications, and its production is a step towards a circular bioeconomy.

Carbon dots prepared by different bottom-up methods: a study on optical properties and the application as nanoprobes for metal ions detection

Carbon dots (CDs) have emerged in the last few years as new carbon nanostructures due to their excellent optical and physicochemical properties and thus promising applications in different areas. In this work, we describe the preparation of different CDs obtained from distinct bottom-up methods and also using different low molar mass precursors in order to study advantages and disadvantages of each experimental method used. Different techniques were performed to characterize the nanoparticles, such as Transmission electron microscopy, Potentiometric titration, Fourier transform infrared, Fluorescence and Ultraviolet-visible spectroscopy. The CDs obtained using conventional hydrothermal carbonization showed the highest values of quantum yield (QY), whereas the nanoparticles obtained by the heating reflux reaction method exhibited the lowest QY values mainly due to incomplete polymerization and/or carbonization reactions of the material. In addition, a study to evaluate the use of the CDs as nanoprobes for metallic ions showed that one of the samples exhibited high selective and sensitive response toward Fe3+, with a detection limit of 0.89 µM. Furthermore, the nanoparticles used as sensors were obtained with an environmentally friendly strategy, and the CD-based photoluminescence method is simple, has a short response time and present low-cost.

Strategies to improve swine manure hydrochar: HCl-assisted hydrothermal carbonization versus hydrochar washing

The work focuses on the study of hydrochar upgrading from hydrothermal carbonization (HTC) of swine manure by HCl-assisted HTC or washing with HCl or acetone, as a post-treatment to conventional HTC. Conventional HTC of swine manure yields a low-quality hydrochar (C content ~ 38 wt.%, higher heating value (HHV) ~ 15 MJ kg−1, and ash content up to 32 wt.%). HCl-assisted HTC (0.5 M HCl at 230 ℃) substantially reduced the ash content up to ~10 wt.% in the hydrochar and increased the C content to 58 wt.%, reaching a HHV of 23 MJ kg−1. However, the N and S contents remained at values similar to those of the swine manure. Washing post-treatment of conventional hydrochars with HCl or acetone significantly improved the C content and the HHV in the range 47–58 wt.% and 19–25 MJ kg−1, respectively, as well as the ash removal with values 7-11 wt.%. Washing the hydrochar with acetone significantly reduced the N and S contents, obtaining a carbonaceous material with properties suitable for solid biofuel according to ISO/TS 17225–8, (N < 3 wt.%; S < 0.15 wt.%; HHV > 17 MJ kg−1; and ash < 10 wt.%). Hydrochars obtained by HCl-assisted HTC and HCl/acetone washing post-treatment yielded higher thermal stability, as well as better reactivity and low ash agglomeration indexes than compared to conventional hydrochars. Washing post-treatment with acetone proved to be the best strategy to obtain improved hydrochars from swine manure for industrial use as a solid biofuel.

Luminescent carbon dots obtained from different precursors and methods and their applications as sensors for metal ions

Carbon dots (CDs) represent a rising class of nanomaterials with numerous optical and physicochemical properties that make them useful for a variety of applications. In this work, three different methods, conventional hydrothermal carbonization (CHC), microwave-assisted irradiation and heating reflux reaction technique, were used as different bottom-up approaches to obtain different CDs. The carbon nanoparticles were prepared using three different carbon precursors (maleic, fumaric and adipic acids) and one nitrogen source (ethylenediamine). The CDs were characterized using different techniques including Fourier transform infrared spectroscopy, Transmission electron microscopy, Ultraviolet-visible, Fluorescence and Zeta potential measurements. Both, the experimental method used to obtain the CDs and the different precursors, presented a considerable influence on the quantum yield values (QY), and also on the fluorescence emission curves. The molar proportion between the carbon and nitrogen source was varied to improve the QY, and one of the samples prepared by CHC method reached QY = 25.4%. The carbon nanoparticles obtained by CHC were used as nanoprobes for metal ions and exhibited great selectivity and sensitivity for Fe3+ ions, reaching a limit of detection of 35 nM, which is lower than most reported values for CD-based PL method used as probes for Fe3+ ions.

Characteristics of Hydrochar and Liquid Products Obtained by Hydrothermal Carbonization and Wet Torrefaction of Poultry Litter in Mixture with Wood Sawdust

Poultry farms with floor-standing poultry generate large amounts of poultry litter waste. The direct application of this waste as an organic fertilizer does not ensure sustainable and cost-efficient utilization of all waste fractions, and can also be linked to environmental hazards. Therefore, the development of new technologies is required for processing poultry litter into a safe product with higher added value. In this work, the characteristics of activated carbon derived from hydrochar, along with the liquid products obtained from hydrothermal carbonization (HTC) and the wet torrefaction (WT) of poultry litter, were investigated. Poultry litter (PL) was applied in a mixture with sawdust (SD) in the following ratios: 1:0 (PL/SD 1:0), 1:1 (PL/SD 1:1), 1:2 (PL/SD 1:2), and 2:1 (PL/SD 2:1). WT processing took place in an innovative fluidized bed system in a superheated steam medium with low overpressure (less than 0.07 MPa) at 300 °C and 350 °C for 30–45 min. Conventional HTC processing was performed in a water medium at 220 °C for 1–4 h. The hydrochar produced in the experiments was activated with steam for 1 h at 450–750 °C. The porosity characteristics of activated hydrochar were measured, including pore size, pore volume, and specific surface area, in view of potential industrial applications as an adsorbent. Additionally, the contents of 5-hydroxymethylfurfural (HMF), as high-value product, were determined in the liquid products obtained from HTC processing, as well as in the condensate obtained after WT processing. Specific surface areas of the activated hydrochars may still be too low for application as adsorbent material. Hence, its use as a biofertilizer and soil improver should be preferred. Interestingly, the liquid fraction obtained from the innovative WT process displayed a significantly higher 5-HMF content compared to the conventional HTC process.

Valorization of tree leaves waste using microwave‐assisted hydrothermal carbonization process

AbstractFallen leaves of landscape trees, as an emerging biomass waste, were valorized using conventional hydrothermal carbonization (HC) and microwave‐assisted hydrothermal carbonization (MHC) pretreatments, and were comparatively characterized for physicochemical properties and thermal degradation kinetics. The results show that MHC is superior to conventional HC operation, because at 200℃, the MHC process not only gives a higher hydrochar yield (45.09 vs. 39.47 wt%) with significantly reduced energy consumption (0.63 vs. 2.74 MJ g−1), but also is more effective in removing K and Si. For isoconversional kinetic analysis, the FWO method provides better results than the KAS method as the latter failed to fit the tree leaves sample (R2 &lt; 0.9). The thermal degradation kinetics at high temperatures (&gt;400℃) showed that the hydrochar obtained from the MHC process has a lower average activation energy of ~190 MJ kg−1 than the conventional HC process (~260 MJ kg−1). This study reveals the potential for valorization of the landscape tree wastes via the MHC process.

Boron and nitrogen co-doped porous carbon for supercapacitors: A comparison between a microwave-assisted and a conventional hydrothermal process

Abstract An ultrafast synthesis method is proposed for a boron and nitrogen co-doped porous carbon using biomass material as a precursor through a combination of a microwave-assisted hydrothermal method (MHT) followed by a pyrolysis process. Compared with a conventional hydrothermal carbonization method (HTC), the MHT method exhibits unparalleled advantages, such as a high energy efficiency and a uniform and ultrafast reaction. The pore structure, elemental composition and electrochemical performance of the materials from the two methods were studied and compared. The results showed that when water was used as the solvent, a porous texture with a surface area of 823 m2 g−1 was obtained by the HTC after 24 h; in contrast, a similar surface area of 813 m2 g−1 could be easily obtained in only 20 min with MHT. Furthermore, when boric acid was used as the solvent, MHT-20min-10% presented similar heteroatom (B and N) contents as the HTC sample but with a significantly improved surface area (994 vs. 666 m2 g−1) and specific capacitance (120 vs. 106 F g−1) compared to those of HTC-24h-10%. Therefore, compared with the HTC, MHT is an ultrafast synthesis method and also is the best option to obtain high surface area porous carbon materials doped with B and N heteroatoms with suitable pore size distribution, which are beneficial in electrode materials for supercapacitors.

Hydrothermal Polymerization Catalytic Process Effect of Various Organic Wastes on Reaction Time, Yield, and Temperature

The purpose of this study is to optimize the processing conditions (temperature, pressure, process time, yield rate) for the conversion of biomass to a high-energy density biofuel. The hydrothermal polymerization (HTP) catalytic process has been developed for production of biofuel via hydrothermal processing using an acid-based catalyst. This study has shown that the HTP catalytic process for a reference feedstock lowered the temperature by 10 to 40 °C, reduced the pressure requirement by 1 to 2 MPa, increased the rate of yield by 22%, and shortened the total processing time by up to 3 h when compared to the conventional hydrothermal carbonization (HTC) process. FTIR spectrum analysis of the HTP catalytic biofuel has shown that lignin in the biomass is preserved, while the pure HTC process destroyed the lignin in the biomass. GC/MS analysis of the process liquid determined the changes of the intermediate soluble components as a function of time. By measuring the 2,5-hydroxymethyl furfuralde concentration in solution, an endpoint determination could be made. This study also determined the approximate analysis of the HTP biofuel from various organic wastes such as cotton, cow manure, wood waste, paper waste, sugarcane bagasse waste, and food waste.

Mild hydrothermally treated brewer's spent grain for efficient removal of uranyl and rare earth metal ions.

The increasing concerns on uranium and rare earth metal ion pollution in the environment require sustainable strategies to remove them from wastewater. The present study reports an eco-friendly approach to convert a kind of protein-rich biomass, brewer's spent grain (BSG), into effective biosorbents for uranyl and rare earth metal ions. The employed method reduces the energy consumption by performing the hydrothermal treatment at a significantly lower temperature (150 °C) than conventional hydrothermal carbonization. In addition, with the aid of the Maillard reaction between carbohydrates and proteins forming melanoidins, further activation processes are not required. Treatment at 150 °C for 16 h results in an altered biosorbent (ABSG) with increased content of carboxyl groups (1.46 mmol g−1) and a maximum adsorption capacity for La3+, Eu3+, Yb3+ (pH = 5.7) and UO22+ (pH = 4.7) of 38, 68, 46 and 221 mg g−1, respectively. Various characterization methods such as FT-IR, 13C CP/MAS NMR, SEM-EDX and STA-GC-MS analysis were performed to characterize the obtained material and to disclose the adsorption mechanisms. Aside from oxygen-containing functional groups, nitrogen-containing functional groups also contribute to the adsorption. These results strongly indicate that mild hydrothermal treatment of BSG could be applied as a greener, low-cost method to produce effective adsorbents for uranyl and rare earth metal ion removal.

Efficient production of high calorific value solid fuel from palm oil empty fruit bunch by pressurized hydrothermal carbonization

The efficient production of high calorific value solid fuel from palm oil empty fruit bunch (EFB) was investigated by experimenting hydrothermal carbonization (HTC) with initial absolute pressures of 0.1 MPa, 1.3 MPa, and 3.1 MPa in N2 atmosphere and in CO2 atmosphere. The pressure increase in HTC is expected to promote the improvement of calorific value in efficient way, while the alteration of reaction atmosphere from N2 to CO2 is aimed at obtaining higher extent of carbonization. Increasing the initial pressure of N2 improved calorific value of the derived fuel from 19.3 MJ/kg to 22.2 MJ/kg, while creating only trivial effect on energy yield. Increasing the initial pressure of CO2 further enhanced the carbonization and resulted in fuel with maximum calorific value of 23.6 MJ/kg. However, it also resulted in up to 6% reduction on energy yield compared with the cases with N2. The advantages of pressurized HTC in N2 atmosphere and in CO2 atmosphere over conventional HTC were evident when used for the production of fuels with comparable heating value. Process efficiencies of the pressurized HTC in both N2 and CO2 atmospheres were shown to be 4–8% higher than those of conventional HTC with normal initial reaction pressure.

Optimization of the conventional hydrothermal carbonization to produce hydrochar from fish waste

Fish waste disposal is a major cause for concern for the seafood processing industries. Fish processing generates enormous quantities of waste as almost 45% of the live weight of fish is regarded as waste. Current ways of managing fish waste involves dumping in oceans, landfills, or treating them with already established strategies. Dumping these wastes without any form of treatment is far from being environmental friendly. Current utilization strategies suffer from disadvantages such as incomplete utilization of solid and liquid wastes or generation of new waste effluents that needs further processing. Therefore, there is a need to find an alternate/supplemental method of seafood utilization. Previously, we have reported the use of microwave hydrothermal carbonization (MHTC) to carbonize fish waste to hydrochar. Here, a conventional heating method such as a custom autoclave reactor is reported that could also be used to carbonize fish waste to hydrochar. Upon response surface design optimization, it was found that a maximal yield of hydrochar (~ 35%) can be achieved at a holding temperature of 180 °C and at a holding time of 120 min. We have also characterized the elemental, proximate, energy, and surface properties of hydrochar produced by conventional hydrothermal carbonization (CHTC). It was found that the quality of the hydrochar produced by MHTC is largely comparable to CHTC. This further proves that HTC could be employed to generate energy from non-lignocellulosic wastes such as fish waste while getting rid of the waste in an eco-friendly manner.

Process characteristics for microwave assisted hydrothermal carbonization of cellulose

The process characteristics of microwave assisted hydrothermal carbonization of cellulose was investigated and a first order kinetics model based on carbon concentration was developed. Chemical properties analysis showed that comparing to conventional hydrothermal carbonization, hydrochar with comparable energy properties can be obtained with 5–10 times decrease in reaction time with assistance of microwave heating. Results from kinetics study was in great agreement with experimental analysis, that they both illustrated the predominant mechanism of the reaction depend on variations in the reaction rates of two co-existent pathways. Particularly, the pyrolysis-like intramolecular dehydration reaction was proved to be the predominant mechanism for hydrochar generation under high temperatures. Finally, the enhancement effects of microwave heating were reflected under both soluble and solid pathways in this research, suggesting microwave-assisted hydrothermal carbonization as a more attracting method for carbon-enriched hydrochar recovery.

Conventional Hydrothermal Carbonization of Shrimp Waste

Shrimp is among the most consumed seafood across the world. Shrimp-processing operations generate enormous quantities of waste. Current strategies of shrimp waste management and the current utilization strategies suffer from several disadvantages, especially from being not environmentally friendly. This warrants alternate methods to completely utilize shrimp waste. Previously, we have shown that microwave hydrothermal carbonization (MHTC) can be used to treat shrimp waste to produce hydrochar. In this study, conventional hydrothermal carbonization (CHTC) using a customized autoclave reactor was performed to treat shrimp waste. Upon using response surface design, it was found that at a holding temperature of 186 °C and a time of 120 min, a maximal hydrochar yield of ∼29% was achieved. Furthermore, characterization results of elemental, proximate, energy, and surface properties of CHTC shrimp waste hydrochar were found to be comparable to that of the MHTC hydrochar from shrimp waste. In addition, the hydroc...

Comparative evaluation of conventional and microwave hydrothermal carbonization of human biowaste for value recovery.

This paper compares conventional and microwave hydrothermal carbonization (HTC) of human biowaste (HBW) at 160 °C, 180 °C and 200 °C as a potential technology to recover valuable carbonaceous solid fuel char and organic-rich liquor. Also discussed are the influence of HTC heating methods and temperature on HBW processing conversion into solid fuel char, i.e. yield and post-HTC management, dewaterability rates, particle size distribution and the carbon and energy properties of solid fuel char. While HTC temperatures influenced all parameters investigated, especially yield and properties of end products recovered, heating source effects were noticeable on dewatering rates, char particle sizes and HBW processing/end product recovery rate and, by extension, energy consumed. The microwave process was found to be more efficient for dewatering processed HBW and for char recovery, consuming half the energy used by the conventional HTC method despite the similarity in yields, carbon and energy properties of the recovered char. However, both processes reliably overcame the heterogeneity of HBW, converting them into non-foul end products, which were easily dewatered at <3 seconds/g total solids (TS) (c.f. 50.3 seconds/g TS for a raw sample) to recover energy-densified chars of ≈17 MJ/kg calorific value and up to 1.4 g/l of ammonia concentration in recovered liquor.

Preparation of high performance supercapacitor materials by fast pyrolysis of corn gluten meal waste

Fast pyrolysis of biomass wastes which is usually finished in a few seconds for the preparation of porous carbonaceous materials (PCMs) is much faster and more energy-efficient than the conventional hydrothermal carbonization (HTC) method and does not require the use of solvents.

Microwave-assisted and conventional hydrothermal carbonization of lignocellulosic waste material: Comparison of the chemical and structural properties of the hydrochars

This study compares the chemical and structural properties of the hydrochars prepared from microwave-assisted and conventional hydrothermal carbonizations of Prosopis africana shell, a waste plant material. The preparation involved heating the raw material in de-ionized water at 200°C for 5⿿20min, and 120⿿240min in the microwave and conventional oven respectively. The prepared hydrochars were characterized using the scanning electron microscope, nitrogen sorption measurement, Fourier transform infrared spectroscopy, CHN analyzer, thermogravimetric analysis, and nuclear magnetic resonance. The results showed that the microwave-assisted hydrothermal carbonization process is fast in the carbonization of the Prosopis africana shell as shown by the level of conversion attained within the short time. This study presents new data on the comparison of the hydrochars from microwave-assisted and conventional hydrothermal carbonization processes of the same lignocellulosic material in terms of their properties.

Effects of hydrolysis and carbonization reactions on hydrochar production

Hydrothermal carbonization (HTC) is a thermal conversion process which converts wet biomass into hydrochar. In this study, a low-energy HTC process named “Two-stage HTC” comprising of hydrolysis and carbonization stages using faecal sludge as feedstock was developed and optimized. The experimental results indicated the optimum conditions of the two-stage HTC to be; hydrolysis temperature of 170°C, hydrolysis reaction time of 155min, carbonization temperature of 215°C, and carbonization reaction time of 100min. The hydrolysis reaction time and carbonization temperature had a statistically significant effect on energy content of the produced hydrochar. Energy input of the two-stage HTC was about 25% less than conventional HTC. Energy efficiency of the two-stage HTC for treating faecal sludge was higher than that of conventional HTC and other thermal conversion processes such as pyrolysis and gasification. The two-stage HTC could be considered as a potential technology for treating FS and producing hydrochar.