Correlations between the physicochemical properties of hydrochar and specific components of waste lettuce: Influence of moisture, carbohydrates, proteins and lipids

Synergistic production of nitrogen-rich hydrochar and solid biofuels via co-hydrothermal carbonization of microalgae and macroalgae: When nitrogen circularity matters

This work explores the synergies between N-rich (Chlorella pyrenoidosa) microalgae and N-deficient (Undaria pinnatifida) macroalgae for the production of N-containing hydrochar and solid biofuels via co-hydrothermal carbonization (co-HTC). The impact of the feedstock (each alga alone and all possible binary mixtures) was comprehensively assessed under different temperatures (180-260 °C) and times (60-240 min). The synergies between micro and macroalgae governed product distribution, nitrogen transformation pathways, and hydrochar quality, with these effects varying by processing conditions. Biomass synergies enhanced hydrochar quality at lower temperatures through deoxygenation reactions and/or liquid-phase repolymerization. In contrast, at higher temperatures, interactions between carbohydrates and proteins via solid-phase Maillard and Mannich reactions decreased hydrochar fuel quality but enriched nitrogen functionalities, such as pyridine-N. Optimization revealed that high N retention and hydrochar yield (up to 23%) were achieved by mixing up to 50 wt% macroalgae with microalgae at 223 °C for 174 min, maintaining functional N content (6 wt% N, 16% pyridine-N). Additionally, an energy-dense hydrochar (34% yield and 26 MJ/kg HHV) was synergistically produced by co-treating 70 wt% microalgae and 30 wt% macroalgae at 180 °C for 60 min. This synergistic algal approach highlights the potential of synergistic algal co-HTC to enhance nitrogen circularity, improve feedstock flexibility, and support sustainable biofuel and material production from marine resources.

Xylan-based near-infrared fluorescent probes for monitoring viscosity abnormalities in living cells and zebrafish

Viscosity is a crucial indicator of the flow state of proteins, lipids, and polysaccharides in the cell microenvironment and plays a vital role in maintaining normal cellular activities. Abnormal viscosity in any part of the cell constituents can lead to various diseases in the organism. For instance, abnormal mitochondrial viscosity can lead to diseases, such as diabetes and Parkinson's disease. Therefore, real-time monitoring of changes in mitochondrial viscosity in both pathological and physiological environments is relevant. This study describes a water-soluble xylan-based near-infrared fluorescence probe that can detect changes in cellular viscosity. The designed mitochondria-targeting near-infrared fluorophores were introduced into modified xylan to form a viscosity-sensing fluorescent probe (NI-XylV). The fluorescence intensity of NI-XylV at 590 and 670 nm gradually increases with an increase in viscosity caused by environmental changes, enabling the sensitive detection of viscosity changes in mitochondria within living cells. NI-XylV exhibits good photostability, biocompatibility, excellent mitochondrial targeting, and broad application prospects as a bio-based fluorescence probe.

Carbon-rich and low-ash hydrochar formation from sewage sludge by alkali-thermal hydrolysis coupled with acid-assisted hydrothermal carbonization

The production of carbon-rich and low-ash hydrochar from sewage sludge is attracting interest due to its great application prospect in high value-added carbon materials fields, but which is impossible through direct hydrothermal carbonization. In this study, alkali-thermal hydrolysis followed by acid-assisted hydrothermal carbonization was thus proposed. Thermal hydrolysis at strong alkaline environment was more effective than acid one to promote the dissolution of organic matters and restrain the release of inorganic matters from sludge, which created a favorable condition for hydrochar formation in a carbon-rich and low-ash way. Alkali-thermal hydrolysis began to show a positive effect on the dissolution of organics in sludge when temperature exceeded the threshold of 90 °C, and an increase of 9.77 % was found at 150 °C when compared to 30 °C. Acid-assisted hydrothermal carbonization of alkali-thermal hydrolysate (ATH) at pH 1.0 strongly promoted condensation polymerization of dissolved organics to form hydrochar and meanwhile inhibited introduction of dissolved inorganics. The nanosized microparticulate hydrochar derived from ATH-30 had a carbon and ash content of 50.98-61.31 % and 10.76-12.09 %, while the micro-sized microspheric hydrochar with multiple deposition layers formed from ATH-150 showed a better performance in carbon-rich and low-ash aspect where a carbon and ash content of 58.24-70.07 % and 0.40-3.24 % was realized, both of which were obviously superior to the direct hydrochar (carbon 34.86 % and ash 46.11 %). The condensation of dissolved organics during alkali-thermal hydrolysis stage is important to the carbonization degree of hydrochar. This study provides a new perspective in sludge disposal and production of advanced carbon materials.

Co-hydrothermal carbonization of microalgae and digested sewage sludge: Assessing the impact of mixing ratios on the composition of primary and secondary char

The role of microalgae cultivation in wastewater treatment and reclamation has been studied extensively, as has the potential utility of the resulting algal biomass. Most methods for processing such biomass generate solid residues that must be properly managed to comply with current sustainable resource utilization requirements. Hydrothermal carbonization (HTC) can be used to process both individual wet feedstocks and mixed feedstocks (i.e., co-HTC). Here, we investigate co-HTC using microalgae and digested sewage sludge as feedstocks. The objectives were to (i) study the material's partitioning into solid and liquid products, and (ii) characterize the products' physicochemical properties. Co-HTC experiments were conducted at 180-250°C using mixed microalgae/sewage sludge feedstocks with the proportion of sewage sludge ranging from 0 to 100 %. Analyses of the hydrochar composition and the formation and composition of secondary char revealed that the content of carbonized material in the product decreased as the proportion of sewage sludge in the feedstock increased under fixed carbonization conditions. The properties of the hydrochars and the partitioning of material between the liquid phase and the hydrochar correlated linearly with the proportion of microalgae in mixed feedstocks, indicating that adding sewage sludge to microalgae had weak or non-existent synergistic effects on co-HTC outcomes. However, the proportion of sewage sludge in the feedstock did affect the secondary char. For example, adding sewage sludge reduced the abundance of carboxylic acids and ketones as well as the concentrations of higher molecular weight cholesterols. Such changes may alter the viable applications of the hydrochar.

The Role of the Mannich Reaction in Nitrogen Migration during the Co-Hydrothermal Carbonization of Bovine Serum Albumin and Lignin with Various Forms of Acid-Alcohol Assistance

Co-hydrothermal carbonization (co-HTC) of N-rich and lignocellulosic biomass is a potential way to produce hydrochar with high yield and quality, but the nitrogen will also enrich in a solid product. In this study, a novel co-HTC with acid-alcohol assistance is proposed, and the model compounds bovine serum albumin (BSA) and lignin were used to investigate the role of the acid-alcohol-enhanced Mannich reaction in nitrogen migration. The results showed that the acid-alcohol mixture could inhibit nitrogen enrichment in solids and the order of the denitrification rate was acetic acid > oxalic acid > citric acid. Acetic acid promoted solid-N hydrolysis to NH₄⁺ while oxalic acid preferred to convert it to oil-N. More tertiary amines and phenols were generated with oxalic acid-ethanol addition and then formed quaternary-N and N-containing aromatic compounds through the Mannich reaction. In the citric acid-ethanol-water solution, NH₄⁺ and amino acids were captured to form diazoxide derivatives in oil and pyrroles in solids through both nucleophilic substitution and the Mannich reaction. The results are able to guide biomass hydrochar production with the targeted regulation of nitrogen content and species.

Evolution of elemental nitrogen involved in the carbonization mechanism and product features from wet biowaste

Hydrothermal carbonization (HTC) represents elegant thermochemical conversion technology suitable for energy and resource recovery from wet biowaste, while the elemental nitrogen is bound to affect the HTC process and the properties of the products. In this review, the nitrogen fate during HTC of typical N-containing-biowaste were presented. The relationship between critical factors involved in HTC like N/O, N/C, N/H, solid ratio, initial N in feedstock, hydrothermal temperature and residence time and N content in hydrochar were systematic analyzed. The distribution and conversion of N species along with hydrothermal severity in hydrochar and liquid phase was discussed. Additionally, the chemical forms of nitrogen in hydrochar were elaborated coupled with the role of N element during hydrochar formation mechanism and the morphology features. Finally, the future challenges of nitrogen in biowaste involved in HTC about the formation and regulation mechanism of hydrochar were given, and perspectives of more accurate regulation of the physicochemical characteristics of hydrochar from biowaste based on the N evolution is expected.

Coupling influences of organic components and temperature on nitrogen transformation and hydrochar characterization during hydrothermal carbonization of sewage sludge

Nitrogen (N) in sewage sludge (SS) should be reduced if it is to be used to produce clean solid fuels. However, the N transformation during hydrothermal carbonization (HTC) of SS is not yet fully understood. Since the composition of SS is complex, it is wise to study a model compound, which should have typical functional groups of organic components. Hence, in this study, six model components (protein, lipid, cellulose, hemicellulose, humic acid, and lignin) representing the main organic components in SS were mixed with SS and treated at 150-270 °C for 1 h. The influence of the organic component and reaction temperature on hydrochar yield, hydrochar characterization, and N distribution in the products was investigated. Except for proteins and lipids, all the other components were found to contribute to the N content and aromatization of the hydrochar. Humus shows the best comprehensive performance in terms of both reducing the N content and increasing the aromaticity. The strongest effects of hemicellulose and cellulose on N retention in hydrochar are found to occur at 210 °C and 240 °C, respectively. The N retention caused by lignin is correlated with the Mannich reaction at 240 °C, while humus significantly promotes N transformation at 240 °C. For carbohydrates, lignin, and humus, the temperatures required for increasing the N content and aromaticity maintain a high degree of consistency. Although protein pulls down the energy recovery (ER) and yield of the hydrochar, observations indicate that it favors the carbonization process. This finding can be used for estimating the N content and quality of hydrochar and provides references for future research targeting the upgrading of hydrochar.

Machine learning for hydrothermal treatment of biomass: A review

Hydrothermal treatment (HTT) (i.e., hydrothermal carbonization, liquefaction, and gasification) is a promising technology for biomass valorization. However, diverse variables, including biomass compositions and hydrothermal processes parameters, have impeded in-depth mechanistic understanding on the reaction and engineering in HTT. Recently, machine learning (ML) has been widely employed to predict and optimize the production of biofuels, chemicals, and materials from HTT by feeding experimental data. This review comprehensively analyzed the application of ML for HTT of biomass and systematically illustrated basic ML procedure and descriptors for inputs and outputs of ML models (e.g., biomass compositions, operation conditions, yield and physicochemical properties of derived products) that could be applied in HTT. Moreover, this review summarized ML-aided HTT prediction of yield, compositions, and physicochemical properties of HTT hydrochar or biochar, bio-oil, syngas, and aqueous phase. Ultimately, future prospects were proposed to enhance predictive performance, mechanistic interpretation, process optimization, data sharing, and model application during ML-aided HTT.

Pyrolysis and Co-Combustion of Semi-Dry Sewage Sludge and Bituminous Coal: Kinetics and Combustion Characteristics

To reduce the energy consumption and cost of the drying of sewage sludge (SS) and to ensure stability during combustion, the pyrolysis and co-combustion characteristics of semi-dry SS after the dehydration of flocculant and bituminous coal (BC) were studied in this work. The results show that the decrease in moisture content accelerates the release of volatile substances, and the increase in heating rate can also enhance the release of water and volatile matters. Furthermore, in the co-combustion of semi-dry SS and BC, the increase in mixing ratio (from 0% to 60%) of semi-dry SS caused the ignition and burnout temperature to decrease from 481 °C to 214 °C and from 702 °C to 627 °C, respectively. During co-combustion, the infrared spectra showed that the temperature range of 300–700 °C was the main gas precipitation area, and the main gaseous products were CO2, NOx, SO2, and volatile organic pollutants (VOCs).

Effects of lignocellulosic biomass type on nutrient recovery and heavy metal removal from digested sludge by hydrothermal treatment

Sludge is a nutrient-rich organic waste generated from wastewater treatment plants. However, the application of sludge as a nutrient source is limited by its high contents of water and pollutants. In this study, the effects of biomass type on nutrient recovery and heavy metal removal from digested sludge by hydrothermal treatment (HTT) were investigated. Blending biomass with digested sludge for HTT at 180-240 °C increased the recovery of nitrogen in the treated solids. At the HTT temperature of 240 °C, HTT with hardwood sawdust led to the highest nitrogen recovery of 70.6%, compared to the lowest nitrogen recovery of 36.5% without biomass. Blending biomass slightly decreased the recovery of phosphorus compared to those without biomass. Nevertheless, the lowest phosphorus recovery of 91.3% with the use of hardwood sawdust at the HTT temperature of 240 °C was only ∼7.0% less than that without biomass. Blending biomass reduced the contents of macro-metals such as Ca, Fe, Mg and Al in treated solids but the metal contents varied with different biomasses. Regarding the heavy metals, the use of rice husk did not decrease the contents of Ni and Co while blending bagasse did not decrease the content of Cr at HTT temperatures of 210 °C and 240 °C compared to the use of other biomasses. The different effects of biomass type on nutrient recovery and heavy metals were likely related to the types and abundances of organic acids such as acetic acid, oxygen-containing functional groups such as C-OH and COOH, oxide minerals such as silica from biomasses and the overall effects of these factors. This study provides very useful information in selection of lignocellulosic biomass for HTT of sludge for nutrient recovery and heavy metal removal.

Preparation of solid organic fertilizer by co-hydrothermal carbonization of peanut residue and corn cob: A study on nutrient conversion

With continuous recognition of green, organic and pollution-free products, the organic fertilizer plays an increasingly important role in agricultural production. Hydrothermal carbonization (HTC) is an efficient and environmentally friendly biomass treatment technology that can achieve value-added utilization of solid wastes. This study evaluated the potential of two typical agricultural and forestry wastes (corn cob and peanut residue) in preparing as solid organic fertilizers through HTC. The effects of reaction temperature, residence time, and the raw material composition on hydrochar yield, total nutrient content (TNC), nitrogen recovery, and nutrient elements transformation in HTC were investigated. Corn cob was proven to be not an ideal raw material for the preparation of organic fertilizers because of the low TNC and the high C/N ratio of its hydrochar. On the contrary, peanut residue was suitable for preparing organic fertilizers due to its high TNC and appropriate C/N ratio. The co-HTC of corn cob and peanut residue could further improve the N recovery rate from 8.52% (for peanut residue only) to 19.51% due to the synergistic effect between them. Under the optimal hydrothermal conditions of 240 °C, 120 min, and mixing ratio of 1:1, the hydrochar yield was as high as 27.86%, and the C/N value (11.98) and TNC (6.331%) were both appropriate as fertilizer. Furthermore, the potential migration and transformation paths of nutrients including N, P, K and metal elements in the co-HTC were analyzed. The thermodynamic conditions and raw materials composition significantly affect the migration and transformation of N, P and K between solid and liquid. N dissolved into process water (mainly ammonia) would migrate into hydrochar and bio-oil with increasing of reaction temperature. P was fixed in hydrochar through precipitation and adsorption reaction with metal ions. Further, adjusting pH or adding metal salts can promote the fixation of N and P in solid.

Nitrogen migration in products during the microwave-assisted hydrothermal carbonization of spirulina platensis

Nitrogen has a vital influence on the properties of the microwave-assisted hydrothermal carbonization (MHTC) products of Spirulina platensis (SP). The effects of hydrothermal temperature (140-220 °C) and time (1-4 h) on the product distribution and nitrogen migration of SP in MHTC were studied. Increasing temperature led to an increase in the carbon content, and a decrease in the nitrogen content in hydrochar. Protein-N was the major nitrogen-containing species in hydrochar. The total nitrogen in liquid phase increased significantly with increasing temperature. Carbon dots were found to be one of the valuable products in the liquid phase. Higher temperatures improved the amine-N level and reduced the quaternary-N content in carbon dots. A close correspondence was found between the N-containing species and the luminescence centers of carbon dots. A possible nitrogen migration mechanism was proposed to provide guidance for the potential application of the products.

Effect of solvent and feedstock selection on primary and secondary chars produced via hydrothermal carbonization of food wastes

Hydrothermal carbonization is a thermochemical process that converts wet waste biomass into hydrochar, a renewable solid fuel that comprises a coal-like primary phase and an oily secondary phase. The varying oxidation rates of these phases may result in an inefficient energy recovery when combusting the hydrochar, as secondary char is more reactive. Brewer's spent grain, dairy cheese whey and food waste were hydrothermally carbonized at 250 °C. The hydrochars were extracted using six solvents to evaluate the hydrochar partitioning between primary and secondary char phases. Feedstock nature and solvent selection impact the amount and composition of these phases detected. For lipid-rich feedstocks, ethanol extracts up to 50 wt% secondary char enriched in liquid fuel precursors from a solid primary char with enhanced coal-like characteristics. For substrates rich in carbohydrates, proteins, and lignocellulose, less secondary char is produced. Acetone and dichloromethane remove the oily secondary char and maximize primary char yield.

Co-hydrothermal carbonization of sewage sludge and model compounds of food waste: Influence of mutual interaction on nitrogen transformation

This study reports the transformation behavior of nitrogen during the co-hydrothermal carbonization of sewage sludge and model compounds (microcrystalline cellulose, starch, lignin, and xylan) of food waste at 220 °C, with a focus on the reaction routes between starch/xylan and NH₄⁺. Most of the nitrogen in the raw sludge was transformed into organic-N (44.6%) and NH₄⁺ (23.3%) in the aqueous product, and only 20.3% of nitrogen was retained in the hydrochar. The added model compounds could react with organic-N (i.e., amino acids and amines) and NH₄⁺ in aqueous products through Maillard and Mannich reactions, generating heterocyclic-N (especially pyrrole-N) which further polymerizes to form nitrogen-containing polyaromatic hydrochar. This leads to an increase in the retention rate of nitrogen to 36.8-50.9%, especially upon the addition of starch and xylan. During the hydrothermal carbonization of starch/xylan in the NH₄⁺ solution, the polymers are first hydrolyzed into monomers, followed by their further reaction with NH₄⁺ to generate pyrrole-N and pyridine-N in aqueous products (especially xylan), and the pyrrole-N can then polymerize with aromatic clusters to form hydrochar-N. The results show that the model compounds of food waste substantially affect the nitrogen transformation pathways during hydrothermal carbonization, mainly because of the structures of their monomers. These findings can guide the production of sludge-based hydrochar with the targeted regulation of nitrogen content and species.

Co-hydrothermal carbonization of swine and chicken manure: Influence of cross-interaction on hydrochar and liquid characteristics

Swine and chicken manures are abundant solid wastes that can be converted into carbonaceous materials through hydrothermal carbonization (HTC). Owing to their unique biochemical compositions, co-HTC of these two types of manures may have significant implications for the generated products. We investigated the co-HTC of swine manure and chicken manure to understand the influence of the interaction between contrasting manures on the properties of the derived products. The results indicated that co-HTC treatment enhanced the formation of solid product and improved the C and N contents, heating value, and energy yield of the resulting hydrochar. Regarding the ignition temperature and comprehensive combustion index, the combustion properties of the hydrochar were enhanced owing to the mutual effect of the HTC intermediates. Additionally, the interaction of the intermediates significantly impacted the transfer of nitrogenous species and generation of organic acids and organic polymers with fused-ring structures. Therefore, co-HTC processing of animal manures could potentially provide a sustainable pathway for the conversion of animal waste into solid products with improved characteristics compared to those produced by treating the two feedstocks separately.

A review on nitrogen transformation in hydrochar during hydrothermal carbonization of biomass containing nitrogen

Biomass is a type of renewable and sustainable resource that can be used to produce various fuels, chemicals, and materials. Nitrogen (N) in biomass such as microalgae should be reduced if it is used to produce fuels, while the retention of N is favorable if the biomass is processed to yield chemicals or materials with N-containing functional groups. The engineering of the removal and retention of N in hydrochar during hydrothermal carbonization (HTC) of biomass rich in protein is a research hot spot in the past decade. However, the N transformation during HTC has not yet been fully understood. In order to mediate the migration and transformation of N in hydrochar, the present review overviewed i) the characteristics of hydrochar and the original feedstock, ii) the possible N transformation behavior and mechanisms, and iii) the effect of factors such as feedstock and pyrolysis parameters such as temperature on hydrochar N. The high temperature and high protein content promote the dehydration, decarboxylation, and deamination of biomass to produce hydrochar solid fuel with reduced N content, while the Millard and Mannich reactions for lignocellulosic biomass rich in carbohydrate (cellulose, hemicellulose, and lignin) at medium temperatures (e.g., 180-240 °C) significantly promote the enrichment of N in hydrochar. The prediction models can be built based on properties of biomass and the processing parameters for the estimation of the yield and the content of N in hydrochar.

Effects of process water recirculation on solid and liquid products from hydrothermal carbonization of Laminaria

Hydrothermal carbonization (HTC) is a promising thermo-chemical technology to treat wet biomasses for production of hydrochars but produces excessive process water. In this study, recirculation of process water from HTC of macroalgae Laminaria was investigated for 12 rounds. Recycling process water increased the hydrochar yield, carbon recovery rate and high heating value from 13.3% to 17.1%, from 22.9% to 32.6%, and from 18.4 MJ/kg to 20.5 MJ/kg after 12 rounds, respectively. The process water recirculation could partly alleviate the toxicity of process water through seed germination test. Volatile fatty acids (VFAs) predominantly accumulate with process water recirculation. The increased proportion of VFAs on chemical oxygen demand could promote methane production of diluted process waters, a 12.3% increase was observed in the round 10, compared with initial process water. These results showed that recycling the process water could reduce water consumption significantly and enhance energy recovery efficiency.