Elemental migration and transformation during hydrothermal liquefaction of biomass

Towards Negative Emissions: Hydrothermal Carbonization of Biomass for Sustainable Carbon Materials

The contemporary production of carbon materials heavily relies on fossil fuels, contributing significantly to the greenhouse effect. Biomass is a carbon-neutral resource whose organic carbon is formed from atmospheric CO2. Employing biomass as a precursor for synthetic carbon materials can fix atmospheric CO2 into solid materials, achieving negative carbon emissions. Hydrothermal carbonization (HTC) presents an attractive method for converting biomass into carbon materials, by which biomass can be transformed into materials with favorable properties in a distinct hydrothermal environment, and these carbon materials have made extensive progress in many fields. However, the HTC of biomass is a complex and interdisciplinary problem, involving simultaneously the physical properties of the underlying biomass and sub/supercritical water, the chemical mechanisms of hydrothermal synthesis, diverse applications of resulting carbon materials, and the sustainability of the entire technological routes. This review starts with the analysis of biomass composition and distinctive characteristics of the hydrothermal environment. Then, the factors influencing the HTC of biomass, the reaction mechanism, and the properties of resulting carbon materials are discussed in depth, especially the different formation mechanisms of primary and secondary hydrochars. Furthermore, the application and sustainability of biomass-derived carbon materials are summarized, and some insights into future directions are provided.

Effects of Fe3+ on Hydrothermal Humification of Agricultural Biomass

Hydrothermal humification technology for the preparation of artificial humic matters provides a new strategy, greatly promoting the natural maturation process. Iron, as a common metal, is widely used in the conversion of waste biomass; however, the influence of Fe3+ on hydrothermal humification remains unknown. In this study, FeCl3 is used to catalyze the hydrothermal humification of corn straw, and the influence of Fe3+ on the hydrothermal humification is explored by a series of characterization techniques. Results show that Fe3+ as the catalyst can promote the decomposition of corn straw, shorten the reaction time from 24 h to 6 h, and increase the yield from 6.77 % to 14.08 %. However, artificial humic acid (A-HA) obtained from Fe3+ -catalysis hydrothermal humification contains more unstable carbon and low amount of aromatics, resulting in a significantly decreased stability of the artificial humic acid. These results provide theoretical guidance for regulating the structure and properties of artificial humic acid to meet various maintenance needs.

Comparative exploration of biological treatment of hydrothermal liquefaction wastewater from sewage sludge: Effects of culture, fermentation conditions, and ammonia stripping

Hydrothermal liquefaction wastewater from sewage sludge (sludge HTLWW) is an emerging waste stream that requires treatment before being discharged into the environment. Biological treatment of sludge HTLWW is an attractive option due to the low cost and operational flexibility. In this study, we investigated and compared the performance of three bacterial strains and four fungal strains for biodegradation of sludge HTLWW. Our screening experiments established pH and mineral supplementation (iron, magnesium, calcium, and phosphorus) conditions that greatly improved COD removal and chemical compound degradation by the microbes. An ammonia stripping pretreatment improved COD removal efficiency of Rhodococci jostii RHA1 by 44%. All tested bacterial strains can tolerate 10× dilution of HTLWW and remove 35-44% of COD in 2-15 days, while all tested fungal strains were able to tolerate 20× dilution and were better at degrading phenolic compounds than bacteria. HTLWW treatment with biomass pellets of fungus Aspergillus niger NRRL 2001 achieved the best COD removal efficiency of 47% in 12 days without the need of nutrient supplementation. Comparisons on chemical compound degradation by the tested microbes suggested that organic acids in HTLWW were highly degradable, followed by phenolic compounds. N-heterocyclic compounds were resistant to biodegradation and were removed by 38%. This study demonstrated pure culture biological treatment of sludge HTLWW with diverse types of microorganisms, which would guide the culture development and bioprocess parameter optimization for treating HTLWW of different compositions.

Is nitrification inhibition the bottleneck of integrating hydrothermal liquefaction in wastewater treatment plants?

Sewage sludge management poses challenges due to its environmental impact, varying composition, and stringent regulatory requirements. In this scenario, hydrothermal liquefaction (HTL) is a promising technology for producing biofuel and extracting phosphorus from sewage sludge. However, the toxic nature of the resulting process water (HTL-PW) raises concerns about integrating HTL into conventional wastewater treatment processes. This study investigated the inhibitory effects of HTL-PW on the activity of the main microbial functions in conventional activated sludge. Upon recirculation of the HTL-PW from the excess sludge into the wastewater treatment plant, the level of COD in the influent is expected to increase by 157 mgO2⋅L-1, resulting in 44% nitrification inhibition (IC50 of 197 mg⋅L-1). However, sorption of inhibitory compounds on particles can reduce nitrification inhibition to 27% (IC50 of 253 mg⋅L-1). HTL-PW is a viable carbon source for denitrification, showing nearly as high denitrification rates as acetate and only 17% inhibition at 157 mgO2⋅L-1 COD. Under aerobic conditions, heterotrophic organic nitrogen and organic matter conversion remains unaffected up to 223 mgO2⋅L-1 COD, with COD removal higher than 94%. This study is the first to explore the full integration of HTL in wastewater treatment plants for biofuel production from the excess activated sludge. Potential nitrification inhibition is concerning, and further long-term studies are needed to fully investigate the impacts.

Hydrothermal liquefaction of lignocellulosic biomass with potassium phosphate and iron and their binary mixture: A comprehensive investigation on the yields and compositions of biocrude and solid residue

Hydrothermal liquefaction of corn, soybean, rice and wheat straws with K3PO4, Fe and Fe + K3PO4 at 320 °C for 30 min was examined. The addition of K3PO4 led to the highest biocrude yields from hydrothermal liquefaction of rice straws (39.20 wt%). Particularly, the biocrude yields from K3PO4-catalyzed hydrothermal liquefaction of corn and rice straws were ∼ 10 wt% higher than those from non-catalytic run (19.4 and 27.8 wt%). Catalytic hydrothermal liquefaction with K3PO4 had minimal impact on the elemental compositions of biocrudes and solid residue. Furthermore, K3PO4 promoted the enrichment of low-boiling components in biocrudes by 2.02 wt%. for hydrothermal liquefaction of wheat straw. Moreover, the incorporation of K3PO4 induces the occurrence of dense porous structure on the surface of solid residue, making it highly suitable as an adsorbent or catalyst carrier. Finally, potential reaction network and mechanisms of catalytic hydrothermal liquefaction of straw have been proposed and discussed detailly.

Biofuel production by hydro-thermal liquefaction of municipal solid waste: Process characterization and optimization

The significant growth of the global population, as well as the increase in energy demand and the limitations of energy generation from fossil fuels, have become a serious challenge over the world. To address these challenges, renewable energies like biofuels are recently found as a proper alternative to conventional fuels. Although biofuel production by using various techniques such as hydrothermal liquefaction (HTL) is considered one of the most promising methods to provide energy, the challenges correlated to its progression and development are still striking. In this investigation, the HTL method was employed to produce biofuel from municipal solid waste (MSW). In this regard, the effect of various parameters such as temperature, reaction time and waste-to-water ratio on mass and energy yield were assessed. It should be stressed that the optimization of biofuel production has been accomplished by the Box-Behnken method using Design Expert 8 software. Based on the results, biofuel production has an upward trend by increasing temperature to 364.57 °C and reaction time to 88.23 min Whereas, there is an inverse relationship between the biofuel waste-to-waterater ratio, in both the context of mass and energy yield.

Application of life cycle assessment and machine learning for the production and environmental sustainability assessment of hydrothermal bio-oil

The hydrothermal bio-oil (HBO) production from biomass conversion can achieve sustainable and low-carbon development. It is always time-consuming and labor-intensive to quantitative relationship between influential variables and bio-oil yield and environmental sustainability impact in the hydrothermal conditions. Machine learning was used to predict bio-oil yield. Life cycle assessment (LCA) is further conducted to assess its environmental sustainability effect. The results demonstrated that gradient boosting decision tree regression (GBDT) have the most optimal prediction performance for the HBO yield (Training R² = 0.97, Testing R² = 0.92, RMSE = 0.05, MAE = 0.03). Lipid content is the most significant influential factor for HBO yield. LCA result further suggested that 1 kg of bio-oil production can cause 0.02 kg ep of SO₂, 2.05 kg ep of CO₂, and 0.01 kg ep of NO_x emission, and environmental sustainability assessment of HBO is exhibited. This study provides meaningful insights to ML model prediction performance improvement and carbon footprint of HBO.

Influence of hydrothermal carbonization conditions on the porosity, functionality, and sorption properties of microalgae hydrochars

Green microalgae is a possible feedstock for the production of biofuels, chemicals, food/feed, and medical products. Large-scale microalgae production requires large quantities of water and nutrients, directing the attention to wastewater as a cultivation medium. Wastewater-cultivated microalgae could via wet thermochemical conversion be valorised into products for e.g., water treatment. In this study, hydrothermal carbonization was used to process microalgae polycultures grown in municipal wastewater. The objective was to perform a systematic examination of how carbonization temperature, residence time, and initial pH affected solid yield, composition, and properties. Carbonization temperature, time and initial pH all had statistically significant effects on hydrochar properties, with temperature having the most pronounced effect; the surface area increased from 8.5 to 43.6 m2 g-1 as temperature was increased from 180 to 260 °C. However, hydrochars produced at low temperature and initially neutral pH generally had the highest capacity for methylene blue adsorption. DRIFTS analysis of the hydrochar revealed that the pH conditions changed the functional group composition, implying that adsorption was electrostatic interactions driven. This study concludes that un-activated hydrochars from wastewater grown microalgae produced at relatively low hydrothermal carbonization temperatures adsorb methylene blue, despite having low surface area.

A review on the emerging conversion technology of cellulose, starch, lignin, protein and other organics from vegetable-fruit-based waste

A large amount of vegetable-fruit-based waste (VFBW) belonging to agricultural waste is produced around the world every year, imposing a huge burden on the environment and sustainable development. VFBW contains a lot of water and useful organic compounds (e.g., cellulose, minerals, starch, proteins, organic acids, lipids, and soluble sugars). Taking into account the composition characteristics and circular economy of VFBW, many new emerging conversion technologies for the treatment of VFBW (such as hydrothermal gasification, ultrasound-assisted extraction, and synthesis of bioplastics) have been developed. This review summarizes the current literature discussing the technical parameters, process, mechanism, and characteristics of various emerging conversion methods, as well as analyzing the application, environmental impact, and bio-economy of by-products from the conversion process, to facilitate solutions to the key problems of engineering cases using these methods. The shortcomings of the current study and the direction of future research are also highlighted in the review.

Response surface optimization of product yields and biofuel quality during fast hydrothermal liquefaction of a highly CO2-tolerant microalgae

The effects of biochemical components and processing variables (e.g., temperatures, solid-liquid ratio, ethanol concentration, and time) during fast hydrothermal liquefaction of a highly CO₂-tolerant microalgae (Micractinium sp.) on the product yields and biofuel quality were explored using response surface methodology coupled with central composite design. Results showed that the maximum bio-oil yield (51.4 %) was obtained at 321 °C for 49 min at ethanol concentration of 75 % and solid-liquid ratio of 15.3 %. Among different studied parameters, ethanol concentration showed the highest significant impact on the bio-oil yield due to the low P-value and high F-value in ANOVA analysis. Furthermore, the chemical compositions of bio-oils were determined, which showed that the increase of ethanol concentration in the solvent not only increased the bio-oil yield but also promoted the bio-oil quality by reduction of carboxylic acids and nitrogen-containing compounds with simultaneous enhancement of esters in the bio-oil. The present results show that fast hydrothermal liquefaction is a promising approach to convert the microalgae into high quality biofuels rich in esters.

Research progress, trends, and future prospects on hydrothermal liquefaction of algae for biocrude production: a bibliometric analysis

The rising demand to settle a sustainable energy source is guiding researchers in the production of biofuels. The liquefaction process is an alternative to obtaining biocrude from different types of renewable biomass and can mitigate environmental impacts. All papers published since 2000, which are related to the hydrothermal liquefaction process that aims to obtain biocrude are analyzed in the present study using the bibliometric approach to provide the selected database. Furthermore, the use of algae biomass in the liquefaction was also a discussed topic considering its high relevance in the process. The focus of the present study was to evaluate the evolution of the current state of the art in these topics and also to indicate trends and courses that it might be taken in the future. The database used in the bibliometric analysis was taken from the Web of Science (WoS) and the papers were selected by two different search equations. With the selected data, the use of BibExcel, VOSviewer, and PowerBi software was useful to guide the discussion and to create graphics and visual networks. As shown in the results, it was noticeable the influence of China and the USA on the field, considering the high number of publications from these countries. Moreover, the main authors were indicated considering their citation numbers, publications, and local h-index factor. Based on the author's keywords, the most significant and recent topics on liquefaction were listed. Among them, technical-economic analysis, nutrient, and energy recovery, response surface methodology, and kinetic model are highlighted. This may indicate a new direction being taken by researchers besides the operational parameters' studies.

Graphical Abstract

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.

Rethinking quantified methods for arsenic speciation and risk in a biowaste hydrothermal liquefaction system

Controversy exists to quantify the fate and speciation of Arsenic (As). We investigated its characteristics by As-containing algae in various pH hydrothermal liquefaction (HTL) system, specifically via two classical methods, i.e. the European Community Bureau of Reference (BCR) and Wenzel's method. Solid residue immobilized 11.23-16.55% of As, and 88.07-82.44% was in aqueous by the pH regulators (e.g., CH₃COOH, HCl, and KOH). ICP-MS and XRD analysis revealed that As (V) was converted into As (III) and As (0) in the solid residue, while the As (V) was mainly converted into As (III) in the aqueous phase during HTL. When the classified forms of As in solid residue are compared, Wenzel's method was more appropriate for dividing the bio-availability forms of As, whereas BCR was better for estimating the toxic-potential forms of As. Subsequently, pH regulators raised the risk of As in solid residue associated with the increasing of unstable forms. The amide was hydrolyzed to carboxylic acid with acidic additives, which weakened the reducing environment in the HTL process. In contrast, the amide was hydrolyzed to ammonia with the alkaline additives, which enhanced the reducing environment and increased the risk of As in products. This work provided a new insight in systematically evaluating the risk and speciation of As in HTL.

Comparative study of machine learning methods integrated with genetic algorithm and particle swarm optimization for bio-char yield prediction

In this study, Machine learning (ML) models integrated with genetic algorithm (GA) and particle swarm optimization (PSO) have been developed to predict, evaluate, and analyze biochar yield using biomass properties and process operating conditions. Comparative study of different ML algorithms integrated with GA and PSO were performed to improve the ML models architecture and parameters selection. The results proposed that Ensembled Learning Tree (ELT-PSO) model outperformed all other models and is favored for biochar yield prediction (R² = 0.99, RMSE = 2.33). The partial dependence plots (PDPs) analysis shows the potential effects of each influencing parameter impact on the biochar yield and as well as shows that how these factors will interact during the pyrolysis process. A user-friendly software was developed based on the ELT-PSO model to avoid extensive and expensive experimentations without requiring considerable ML understanding. Difference recorded by GUI was less than 2% with experimental yield.