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The effect of hydrothermal carbonization on the properties of cellulose present in lignocellulosic biomass was investigated for converting it into a renewable energy resource with high energy recovery efficiency. The biochar obtained from cellulose subjected to hydrothermal carbonization showed a significant increase in its carbon content and a calorific value. 13C NMR spectroscopy showed that when raw cellulose was subjected to hydrothermal carbonization above 220 °C, the resulting biochar had more aromatic and aliphatic fractions than those in raw cellulose. The resulting composition of the biochars was comparable to that of solid fuels and was between that of lignite and sub-bituminous coal. Therefore, cellulose, the main component of lignocellulosic biomass, was used to investigate the effects of varying the reaction temperature during hydrothermal carbonization. The energy recovery efficiency calculations showed that the optimum reaction temperature for the transformation of a mixture of cellulose was approximately 220 °C.

Hydrothermal carbonization (HTC) reactor systems used to convert wet organic wastes into value-added hydrochar are generally classified in the literature as liquid water-based (HTC) or vapor-based (VTC). However, the distinction between the two is often ambiguous. In this paper, we present a methodological approach to analyze process conditions for hydrothermal systems. First, we theoretically developed models for predicting reactor pressure, volume fraction of liquid water and water distribution between phases as a function of temperature. The reactor pressure model predicted the measured pressure reasonably well. We also demonstrated the importance of predicting the condition at which the reactor system enters the subcooled compression liquid region to avoid the danger of explosion. To help understand water–feedstock interactions, we defined a new solid content parameter %S(T) based on the liquid water in physical contact with feedstock, which changes with temperature due to changes in the water distribution. Using these models, we then compared the process conditions of seven different HTC/VTC cases reported in the literature. This study illustrates that a large range of conditions need to be considered before applying the label VTC or HTC. These tools can help in designing experiments to compare systems and understand results in future HTC research.

Hydrothermal treatment (HTT) offers the potential to upgrade low-value biomass such as digestate (DG) or forest residue (FR) by producing solids and liquids for material use or energetic utilization. In this study, microwave-assisted HTT experiments with DG and FR as feedstocks were executed at different temperatures (130, 150, 170 °C) and with different holding times (30, 60, 90 min) to determine the influences on product properties (ash and elemental concentrations, calorific values and chemical compounds). In general, DG and FR reacted differently to HTT. For the DG solids, for instance, the ash concentration was reduced to 8.68%DM at 130 °C (initially 27.67%DM), and the higher heating value increased from 16.55 MJ/kgDM to 20.82 MJ/kgDM at 170 °C, while the FR solids were affected only marginally. Elements with importance for emissions in combustion were leached out in both HTT solids. The DG and FR liquids contained different chemical compounds, and the temperature or holding time affected their formation. Depending on the designated application of HTT, less severe conditions can deliver better results. It was demonstrated that different low-temperature HTT conditions already induce strong changes in the product qualities of DG and FR. Optimized interactions between process parameters (temperature, holding time and feedstock) might lead to better cost–benefit effects in HTT.

This work evaluates the effect of hydrothermal carbonization (HTC) as a pretreatment and post-treatment technique to anaerobic digestion (AD) of dairy sludge. HTC's effect on AD was evaluated based on energy recovery, nutrient transformation, and hydrochar utilization. The first approach was executed by performing HTC under a range of temperatures before mesophilic AD. HTC optimal pretreatment temperature was 210 °C for 30 min residence time. HTC pretreatment significantly increased the methane yield potential by 192%, the chemical oxygen demand removal by 18%, and the sludge biodegradability during AD by 30%. On the other hand, the application of HTC after AD (post-treatment) increased the total energy production, i.e., in addition to methane, a hydrochar with a caloric value of 10.2 MJ/kg was also obtained. Moreover, HTC post-treatment improved the steam gasification performance of the AD digestate. From the fertilizer quality point of view, HTC implementation generally boosted the concentrations of macro, micro, and secondary nutrients, suggesting its suitability for use as a liquid fertilizer. Overall, the findings of the present study indicate that if bioenergy production were the main target, HTC post-treatment following AD would lead to the most promising outcomes.

To produce hydrochar with less volatile matter (VM) and more fixed carbon (FC) to increase its stability, this study compared the hydrothermal carbonization (HTC) of hen (HM) and swine (SM) manures at typical HTC sub-critical temperature of 210 °C and slightly super-critical temperature of 400 °C. Physico-chemical properties such as proximate analysis; ultimate analysis; Brunauer–Emmett–Teller (BET) surface area; higher heating value (HHV); chemical oxygen demand (COD); and inorganic nutrients of hydrochar, gaseous, and liquid products were determined. As expected, both VM and yield decreased with temperature. The heats of HTC reactions were estimated to be exothermic, ranging from −5.7 to −8.6 MJ/kg. The FC approximately doubled, while VM significantly decreased with a yield of 42.7%, suggesting the high potential of producing more stable hydrochar via near-critical HTC (NCHTC) treatment of SM. Additional work is needed before recommendations on carbonization temperatures can be made. Specifically, there is a need to experimentally investigate how the chars produced from each carbonization condition influence plant growth and soil emissions.

The effect of FeCl3 catalyst on the hydrothermal carbonization of dragon fruit stem (DFS) was investigated in reaction temperature range of 160 to 220 °C, in reaction time varying from 1 to 7 h. The obtained results showed that all properties of hydrochar were remarkably enhanced, in which the carbon content of non-catalyzed and FeCl3-catalyzed hydrochar increased about 12.65 and 29.37 wt.% compared to the raw material DFS. The higher heating value (HHV) and the mass yield of produced hydrochars were between 15.72–25.88 MJ kg−1 and 46.1–58.7%. The conditions for producing hydrochar as solid fuel were found optimized at 220 °C and retention time of 5 h with FeCl3 catalyst. The obtained results suggested that the presence of FeCl3 catalyst might help reduce reaction time and energy input. Besides, based on the characteristic results by FTIR, SEM, and TG/DTG analyses, the reaction temperature is the parameter having the most significant influence on the structural morphology and thermal stability of the final hydrochars. The HTC process of DFS with FeCl3 catalyst opens up a potential application direction in enhancing the value of agricultural by-products into a carbon-rich solid fuel source.

The production of low cost and environmentally friendly high performing carbon materials is crucial for a sustainable future. Sustainable Carbon Materials from Hydrothermal Processes describes a sustainable and alternative technique to produce carbon from biomass in water at low temperatures, a process known as Hydrothermal Carbonization (HTC). Sustainable Carbon Materials from Hydrothermal Processes presents an overview of this new and rapidly developing field, discussing various synthetic approaches, characterization of the final products, and modern fields of application for of sustainable carbon materials. Topics covered include: • Green carbon materials • Porous hydrothermal carbons • HTC for the production of valuable carbon hybrid materials • Functionalization  of hydrothermal carbon materials • Characterization of HTC materials • Applications of HTC in modern nanotechnology: Energy storage, electrocatalysis in fuel cells, photocatalysis, gas storage, water purification, sensors, bioapplications • Environmental applications of HTC technology: Biochar production, carbon sequestration, and waste conversion • Scale-up in HTC Sustainable Carbon Materials from Hydrothermal Processes will serve as a comprehensive guide for students and newcomers in the field, as well as providing a valuable source of information for researchers and investors looking for alternative technologies to convert biomass into useful products.

Hydrochars produced from hydrothermal carbonisation of faecal simulant (FS) at 180 °C for 30 min and sewage sludge treated via CAMBI process at 165 °C for 30 min were used for adsorption of organic pollutants in effluent from an anaerobic digester (AD). The adsorption potential of the hydrochars investigated was compared with that of a commercial powdered activated carbon (PAC). It was found that the CAMBI and FS hydrochars were effective only after chemical activation. KOH activation increased chemical oxygen demand (COD) removal from 33.0 to 59.6% and 75.2% for FS and CAMBI, respectively. Extra activation with HCl improved the adsorption efficiency of FS, increasing COD removal to 79.3%, but not for the CAMBI hydrochar even though its surface area was increased. Acidic pH aided organics removal for both hydrochars. PAC adsorption capacity was the highest (> 90%), and this was not affected by pH. All the adsorbents could successfully remove the organics in a very short time as 30 min. The optimum dosage of the hydrochars to reach a high uptake of organics was 30.0 g/L. The adsorption reaction followed a pseudo-second-order kinetic model for all the hydrochars. The adsorption onto the hydrochars correlated well with Temkin isotherm model and followed a type II and IV isotherm type, except PAC which was described by the Langmuir isotherm with a high adsorption capacity of 400.0 mg/g. The study demonstrated that hydrochars of FS and CAMBI activated with KOH are efficient and sustainable adsorbents for the removal of organics from AD effluent.

Pig manure currently results in sizeable greenhouse gas emissions, during storage and spreading to land. Anaerobic digestion and hydrothermal carbonisation could provide significant greenhouse gas mitigation, as well as generate renewable heat and power (with anaerobic digestion), or a peat-like soil amendment product (with hydrothermal carbonisation). The greenhouse gas mitigation potential associated with avoidance of pig manure storage and spreading in the UK, EU, and China, as well as the potential to provide heat and power by anaerobic digestion and soil amendment products by hydrothermal carbonisation was herein determined. In each case, the mono-conversion of pig manure is compared to co-conversion with a 50:50 mixture of pig manure with grass. Anaerobic digestion displayed a greater greenhouse gas mitigation potential than hydrothermal carbonisation in all cases, and co-processing with grass greatly enhances greenhouse gas mitigation potential. China has the largest greenhouse gas mitigation potential (129 MT CO2 eq), and greatest mitigation per kg of pig manure (1.8 kgCO2/kg pig manure volatile solids). The energy grid carbon intensity has a significant impact on the greenhouse gas mitigation potential of the different approaches in the different regions. Pig manure is generated in large amounts in China, and the energy generated from biogas offsets a higher carbon intensity grid. Greenhouse gas savings from the anaerobic digestion of pig manure and grass have been calculated to provide a significant potential for reducing total greenhouse gas emissions representation in China (1.05%), the EU (0.92%), and the UK (0.19%). Overall, the utilisation of pig manure could bring about substantial greenhouse savings, especially through co-digestion of pig manure with grass in countries with large pig farming industries and carbon intense energy mixes.

Carbon quantum dots (CQDs) are nanomaterials with a particle size range of 2 to 10 nm. CQDs have a wide range of applications such as medical diagnostics, bio-imaging, biosensors, coatings, solar cells, and photocatalysis. Although the effect of various experimental parameters, such as the synthesis method, reaction time, etc., have been investigated, the effect of different feedstocks on CQDs has not been studied yet. In this study, CQDs were synthesized from hydroxymethylfurfural, furfural, and microcrystalline cellulose via hydrothermal carbonization at 220 °C for 30 min of residence time. The produced CQDs showed green luminescence behavior under the short-wavelength UV light. Furthermore, the optical properties of CQDs were investigated using ultraviolet-visible spectroscopy and emission spectrophotometer, while the morphology and chemical bonds of CQDs were investigated using transmission electron microscopy and Fourier-transform infrared spectroscopy, respectively. Results showed that all CQDs produced from various precursors have absorption and emission properties but these optical properties are highly dependent on the type of precursor. For instance, the mean particle sizes were 6.36 ± 0.54, 5.35 ± 0.56, and 3.94 ± 0.60 nm for the synthesized CQDs from microcrystalline cellulose, hydroxymethylfurfural, and furfural, respectively, which appeared to have similar trends in emission intensities. In addition, the synthesized CQDs experienced different functionality (e.g., C=O, O-H, C-O) resulting in different absorption behavior.