Hydrothermal Liquefaction Products Research Articles

Hydrochar fractionation and composition in batch and continuous hydrothermal liquefaction

Understanding differences in hydrochar characteristics in batch and continuous hydrothermal liquefaction (HTL) is crucial in determining suitable valorisation routes for this byproduct, given its high nutrient load and carbon sequestration potential.This study thoroughly characterised hydrochar from batch and continuous HTL at 300, 325 and 350 °C, elucidating their main differences and shedding light on the operational parameters causing them. For this purpose, a bench-scale continuous HTL unit with in-line solids separation was commissioned. It was possible to differentiate primary and secondary char and infer their formation mechanism.The results showed that batch hydrochar yields were higher (24–26%) than continuous (primary and secondary) char yields (15–19%). In both reactions, higher temperatures led to chars with lower carbon and nitrogen contents. The ash content of batch hydrochars was lower than that of continuous primary chars, revealing that the in-line char separator effectively removed inorganic impurities at reaction conditions and produced a cleaner biocrude. The nutrient distribution in the HTL products showed batch biocrudes were more contaminated by Na, K and Fe, while in the continuous biocrudes, only Fe was detected. Moreover, less carbon migrated to the solids from continuous HTL, indicating that removing inorganics may reduce secondary char formation.Batch hydrochars showed a higher presence of oxygenated and nitrogenated compounds, while the continuous primary chars had a higher share of alkanes and alkenes.These differences may imply that batch reactions may not serve as indicators, in terms of hydrochar characteristics, for HTL upscaling to industrial plants.

Separation Technique and Product Evaluation in Hydrothermal Liquefaction (HTL) Thermochemical Conversion Technique

Hydrothermal liquefaction (HTL) is a promising thermochemical conversion technique for the conversion of biomass into biofuels and value-added chemicals. This process involves the treatment of biomass in hot, pressurized water, resulting in the production of a liquid product known as bio-crude. However, the complexity of the bio-crude mixture necessitates effective separation techniques to isolate valuable components. This abstract explores various separation techniques employed in HTL processes, including solvent extraction, fractional distillation, and membrane separation, highlighting their principles and applications. Additionally, the evaluation of HTL products is essential to assess their quality and suitability for different applications. Analytical methods such as gas chromatography-mass spectrometry (GC-MS), high-performance liquid chromatography (HPLC), and Fourier transform infrared (FTIR) spectroscopy are commonly used for product characterization. Understanding the principles and applications of separation techniques and product evaluation methods is crucial for optimizing HTL processes and maximizing the value of bio-crude production.

Exploration of hydrothermal liquefaction of multiple algae to improve bio-crude quality and carbohydrate utilization

With the growing emphasis on biofuels research, enhancing the utilization efficiency of the hydrothermal liquefaction (HTL) process for biomass has emerged as a focal point for researchers. In this study, HTL products from various algae species were analyzed to compare the quality of bio-crude, with a specific focus on the utilization of carbohydrates in biomass. Notably, bio-crude yields from high-lipid algae exhibited less fluctuation with temperature, while the quality showed a significant improvement with increasing temperature, as evidenced by the fatty acids and esters contents in the bio-crude. The results of elemental analysis indicated that bio-crude derived from high-lipid algae had a lower nitrogen content. In the analysis of carbohydrate utilization, it was observed that the aqueous phase products of Chlorella protothecoides demonstrated relatively high yields of furfural and 5-HMF. Similarly, the aqueous phase product of Spirulina exhibited comparatively high yields of levulinic acid. The inclusion of platform compounds in elemental recoveries contributed to an enhancement in the carbon and hydrogen recovery of the product. This study establishes a theoretical foundation for the selection of feedstocks for alternative fuels and highlights the potential of carbohydrates utilization from algal biomass.

Distribution characteristics and migration pathways of metals during hydrothermal liquefaction of municipal sewage sludge in the presence of various catalysts

A series of hydrothermal liquefaction (HTL) experiments with two different samples of municipal sewage sludge (MSS) were conducted at 350 °C for 30 min residence time in a high pressure batch reactor. The main aim of the study was to explore the distribution and migration pathways of a broad range of metals and metalloids in the HTL products (bio-oil, char and aqueous phase) obtained in the presence of various homogeneous and heterogeneous catalysts (Na2CO3, Li2CO3, K2CO3, Ba(OH)2, Fe2O3, CeO2, NiMo/MoO3, MoS2, Ni/NiO, SnO2, FeS). The elements under study included 16 environmentally significant metals and metalloids (As, B, Ba, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, Sb, Se, Sn, Zn and Hg). The study showed that the quantitative migration of the tested metals and metalloids to the particular HTL products, relative to their initial content in the raw sludge, is different for the individual elements. Most metals exhibited a particularly strong affinity to the solid fraction (biochar). In the obtained HTL bio-oils, all tested elements were identified, except of Cd. It was also found that B and As have high affinity to the aqueous phase. A direct effect of catalysts on the contents of some elements in the products was also proved by the study, e.g. increased concentration of Cr in the biochar when Fe2O3 was used as a process catalyst. Due to the wide scope of the tested elements and broad range of catalyst used, the results obtained represent a unique and comprehensive set of environmental data compared to similar HTL studies previously conducted for MSS.

Hydrothermal Liquefaction: How the Holistic Approach by Nature Will Help Solve the Environmental Conundrum.

Hydrothermal liquefaction (HTL) represents a beacon of scientific innovation, which unlocks nature's alchemical wonders while reshaping the waste-to-energy platform. This transformative technology offers sustainable solutions for converting a variety of waste materials to valuable energy products and chemicals-thus addressing environmental concerns, inefficiencies, and high costs associated with conventional waste-management practices. By operating under high temperature and pressure conditions, HTL efficiently reduces waste volume, mitigates harmful pollutant release, and extracts valuable energy from organic waste materials. This comprehensive review delves into the intricacies of the HTL process and explores its applications. Key process parameters, diverse feedstocks, various reactor designs, and recent advancements in HTL technology are thoroughly discussed. Diverse applications of HTL products are examined, and their economic viability toward integration in the market is assessed. Knowledge gaps and opportunities for further exploration are accordingly identified, with a focus on optimizing and scaling up the HTL process for commercial applications. In conclusion, HTL holds great promise as a sustainable technology for waste management, chemical synthesis, and energy production, thus making a significant contribution to a more sustainable future. Its potential to foster a circular economy and its versatility in producing valuable products underscore its transformative role in shaping a more sustainable world.

Unraveling the effects of sodium carbonate on hydrothermal liquefaction through individual biomass model component and machine learning-enabled prediction

Despite sodium carbonate (Na2CO3) being commonly utilized as a catalyst in actual biomass hydrothermal liquefaction (HTL), its impact on individual biomass components hasn't been well-examined. This study thus delves into the role of Na2CO3 in HTL of biomass model components (carbohydrate, lignin, protein, and lipid) at varying conditions. Na2CO3 at 5 wt% amplified carbohydrate degradation into biocrude and aqueous-gaseous products (AG), resonating with previous work on carbohydrate-rich feedstocks. While Na2CO3 has a marginal effect on lignin HTL, it negatively influences lipid HTL. Here, 5 wt% and 13.5 wt% Na2CO3 decrease the biocrude yield from 95.6% to less than 10%, simultaneously increasing the AG yield to approximately 90%. This is presumably due to the interaction of lipid decomposition intermediates (fatty acids) with sodium cations, resulting in water-soluble soap. For protein HTL, a low Na2CO3 concentration (5 wt%) has no significant impact on product formation, but excessive Na2CO3 (27 wt%) converts a considerable portion of biocrude into AG. In addition to these explorations that provided insights for actual biomass HTL, we also developed a machine learning model to adequately predict HTL biocrude yield by taking Na2CO3 effect into consideration. The Adaboost machine learning model displayed the most satisfactory prediction performance (training and testing R2: 0.96, 0.8) among the three investigated machine learning models. The feature importance analysis reveals lipid content and Na2CO3 concentration as pivotal factors over HTL process conditions and other biochemical components. This research provides fundamental insights into the Na2CO3 role in HTL product formation pathway and offers an intelligent and precise prediction model for HTL biocrude yield, thereby advancing the development of more efficient and sustainable bioenergy production processes.

Production and characterization of biocrude from Persian Gulf Sargassum angustifolium using hydrothermal liquefaction: Process optimization by response surface methodology

In this study, the Sargassum angustifolium macroalgae was used as the feedstock of the hydrothermal liquefaction (HTL) process. The effects of operating conditions of temperature, residence time, and feedstock concentration on the yield of the HTL products were modeled using the response surface methodology of the Box-Benken design. Based on the proposed model, the maximum biocrude yield was predicted at 350 °C, 35 min residence time, and 8.6 wt% of the feedstock concentration, confirmed by the experiment. The biocrude has a higher heating value of 33.91 MJ/kg, with the main components including ketones and alkanes. Sensitivity analysis of operating conditions revealed that the HTL process temperature has the most effect on the yield of products, and increasing temperature causes an increase in the biocrude and gas phase yields while decreasing the aqueous and solid phase yields. Among the cross-effect of parameters, the temperature-residence time interaction has the highest impact on the biocrude and gas phase yields. A comparison of the structural analysis of the solid phase and the algae showed the solid residue of the HTL process is highly porous and potentially a good candidate as an adsorbent. The physicochemical properties of the aqueous phase revealed its potential application to algae cultivation. Finally, three pretreatment methods of ultrasonication, microwave irradiation, and size variation of the feedstock were used to increase the biocrude yield at both maximum and minimum yield conditions. The results showed that microwave-assisted HTL is the most effective process, increasing the biocrude yield from 26.15 wt% to 34.77 wt%.

Hydrothermal liquefaction potential of wastewater treatment sludges: Effect of wastewater treatment plant and sludge nature on products distribution

The rapid growth of urban populations necessitates the development of more efficient and environmentally sustainable methods for waste treatment and resource recovery. Hydrothermal liquefaction (HTL) has emerged as a promising technology for converting organic waste into bio-crude oil. In this study, the HTL products of various waste fractions (Fat, oil, and grease (FOG), food waste, primary and secondary sludge, and digestate) produced or handled by four wastewater treatment plants in Denmark were analysed. Additionally, the effect of mixing the produced waste was investigated. The HTL experiments were conducted at temperatures 300, 325, and 350 °C, with a residence time of 20 min and dry matter content of 16.7%. The highest HTL bio-crude yields were obtained at a temperature of 325 °C for all feedstocks except FOG (300 °C). The mixing of wastes resulted in a positive synergistic effect, presenting an energy recovery between 62 and 76% and bio-crude yields higher than expected due to interactions between the feedstock's biochemical composition. Based on the biochemical composition and on the feedstocks fraction, two models are proposed for the prediction of bio-crude yields, with an error between 4 and 5% in absolute terms. In addition, important process water parameters, as well as the nutrient recovery and heavy metal load of the solid residues are analysed and discussed, giving a comprehensive overview of the potential of different wastewater treatment plant sludges.

Catalytic hydrothermal liquefaction of orange peels into biocrude: An optimization approach by central composite design

Global instability, persistent increase in pump-price, inflation and depletion of fossil fuel resources amidst the continuous discharge of greenhouse gases from fossil fuels combustion call for urgent attention. The application of novel catalyst towards an improved biocrude production and enhanced biomass conversion are required for effective performance of hydrothermal liquefaction of biomass feedstocks. In the present study, iron supported carbon nanospheres (Fe/CNSs) catalyst has been developed using wet impregnation approach and explored for its catalytic potency in improving yield of biocrude using an optimization approach; central composite design. Hydrothermal liquefaction (HTL) was adopted as a process route where the catalyst dosage (3–6 wt%) and the reaction temperature (330–430 °C) were optimized on the constant weight of orange feedstock (10 g), reaction time of (15 mins) and solvents’ ratio of 3:1 (acetone to ethanol). The performance of the catalyst was found to be aided with the even dispersion of the Fe on the surfaces of the CNSs support and improved surface area. The effects of reaction temperature were observed to be more progressive on biocrude yield formation over the catalyst loading. The optimum biocrude yield, solid residue, biomass conversion and gas yield were obtained to be 71.09 wt%, 28.18 wt%, 71.82 wt% and 40.14 mL/g at 430 °C and 3 wt% catalyst loading. The obtained analysis of variance suggests a good correlation between the experimental and predicted data in all responses. The selectivity of Fe/CNSs catalyst to high yield of phenolics and aromatic compounds in the biocrude suggest that, the biocrude can be further be upgraded into transportation fuel through hydrodeoxygenation process. The findings have further suggested the viability of the developed Fe/CNSs as an effective catalyst for improved HTL products in a batch reactor.

Evaluation of sugar cane bagasse hydrothermal liquefaction products for co-gasification with coal as green coal pellet production

This study investigates the production and properties of green coal pellets formed by combining hydrochar and coal in different ratios (3,1, 1:1, and 1:3). It was found that higher ratios of hydrochar to coal resulted in increased mechanical strength of the green coal pellets, highlighting the binding properties inherent in hydrochar. Furthermore, all the green coal and hydrochar pellets exhibited remarkable hydrophobic qualities. Among the different ratios tested, the green coal pellets composed of equal amounts of hydrochar and coal demonstrated the most favourable pyrolysis behaviour based on these indices. The gasification reactivities (R50%) at 1000 °C were measured as 107.9 min−1 for hydrochar pellets, 28.2 min−1 for sugarcane bagasse pellets, and 17.7 min−1 for coal. By blending coal with hydrochar, the green coal pellets exhibited improved pyrolysis and gasification properties compared to coal alone. Notably, the synergistic effect observed in the green coal samples increased with higher ratios of hydrochar.

An integrated eco-friendly and efficient approach towards heavy metal removal from industrial wastewater and co-generating bio-oil using wheat straw via hydrothermal liquefaction

Industrialization and population growth have adversely affected our environment. Heavy metal-contaminated industrial wastewater and the extensive use of fossils fuel are the prime reason for the degradation of the environment. In this research work, removal of heavy metals (Ni, Fe, and Cr) was achieved from synthetically prepared wastewater in a two-step process, i.e., metal-impregnation and hydrothermal liquefaction (HTL). In the first stage, metal removal was investigated via batch impregnation process using wheat straw by varying parameters such as pH (2 − 12), biomass dosages (0.5 – 1.5 g), and contact time (up to 48 h). The maximum metal removal achieved at this stage was 55 – 65 % (Ni – 58.78 ± 1.5 %, Fe – 62.55 ± 1.8 %, and Cr – 63.51 ± 1.7 %). In the second stage, almost complete removal of heavy metals (99.9 %) was accomplished during the hydrothermal liquefaction of metal-loaded wheat straw and filtered wastewater (used as solvent) collected after the biosorption process. The HTL experiment of metal-impregnated wheat straw was performed by varying temperatures, biomass–to–wastewater ratio, and residence time. Nano-metals (oxides/hydroxides) were formed in the 10–50 nm range during the impregnation process, enhancing the HTL product's yield and selectivity. MP-AES analyzed the heavy metals concentration, and XRD confirmed the formation/reduction of nano-metals. TEM and FE-SEM analysis illustrated the particle size and morphology. CHNS, FTIR, and GC-MS analysis identified the heavy and light bio-oil compositions. In the present study, we have addressed the issues of heavy metal removal and fulfilling the energy demand gap via a greener and eco-friendly approach with the concept of waste–treat–waste and waste–to–energy. The present method provides an efficient outcome and can be a sustainable solution for the two prominent issues.

Comparative assessment of liquid product from hydrothermal treatment of lignosulfonate in batch and nozzle reactors for aromatic compounds recovery

Lignosulfonate is one of the main by-products of the pulping process, with enormous potential for application as an alternative source of aromatic and bioactive compounds, especially in a closed-loop Circular Economy Concept. The lignin-derived bioactive compounds have attracted enormous interest in the scientific community due to their positive effects on human health. In the present paper, hydrothermal treatment technologies of the 10 wt% and 20 wt% lignosulfonate solutions i.e., slow HTL in batch reactor and fast HTL in continuous-flow (CF) reactor, are compared, and discussed, to assess their potential for obtaining bioactive and aromatic compounds. The hydrothermal liquefaction (HTL) in the two reactors was carried out at 275 °C at a pressure of 21.4 MPa, a 1:3 feed-to-water ratio. GC-MS and LC-MS-QTOF analyses were used to determine the chemical composition of the liquid products. According to the chromatogram integrated peak areas (PA), the fast HTL of the lignosulfonate produced the most isoeugenol (35.8–49.2%), eugenol (10.4–16.9%), and homovanillyl alcohol (13.8–18.4%). Slow HTL product, on the other hand, was rich in guiacol (62%) and apocynin (19.7%). LC-MS analysis of the product showed a similar composition to the liquid product in terms of heaviest compounds. Observed m/z values in the chromatograms indicated a presence of the compounds with masses between 121.159 and 934.021 u, possible configurations of the polycyclic phenolic compounds are presented.

The effect of chlorella hydrothermal products on the heavy oil upgrading process

In-situ upgrading can improve the scale of heavy oil development, but the efficiency of in-situ hydrothermal hydrogen supply is limited. In this paper, the combination of algae hydrothermal liquefaction and heavy oil upgrading and viscosity reduction process is proposed. The liquefaction products of hydrothermal liquefaction are used to supply hydrogen to the heavy oil upgrading and viscosity reduction process, thereby enriching the supply path of active hydrogen. The effects of hydrothermal liquefaction temperature of Chlorella and the addition of 5% Fe/HZSM-5 catalyst on the liquefaction rate and product distribution of Chlorella were investigated, and the upgrading and viscosity reduction process of heavy oil with the participation of hydrothermal liquefaction products was discussed. The results show that when the temperature of upgraded heavy oil is 280 °C and 5 wt% of hydrothermal liquefaction products are added and reacted for 24 h can make the viscosity reduction rate of heavy oil reach 84.32%, and 31.2% of resin in heavy oil was converted into aromatic hydrocarbon. Elemental analysis showed that the upgraded heavy oil H/C ratio increased, and GC–MS analysis showed that the distribution of algae hydrothermal liquefaction products could be adjusted by the catalyst, and more aromatics were produced after the addition of the catalyst, which confirmed that the aromatics in hydrothermal liquefaction products had a significant effect on the quality improvement and viscosity reduction of heavy oil. Its molecular weight was also found to be reduced from 853 to 326 in the average molecular weight test (GPC). The efficient carbon sequestration of algae is combined with the demand for carbon dioxide emission reduction in oilfields to realize the recycling of carbon resources.

Incorporating hydrothermal liquefaction into wastewater treatment – Part I: Process optimization for energy recovery and evaluation of product distribution

• Hydrothermal liquefaction (HTL) conditions were optimized for max energy recovery. • Energy recovery was linearly and positively correlated to biocrude yield. • HTL byproducts (aqueous and hydrochar) were minimized at the optimal conditions. • High HTL reaction severities improved biocrude quality (more C and less heteroatoms). • Elemental distribution (including metals) among HTL products was evaluated. The treatment of significant amounts of municipal sewage sludge requires novel and efficient technologies. This study evaluated hydrothermal liquefaction as a means to sustainably convert sludge waste into a renewable energy source – biocrude, which can mitigate both environmental and energy-related challenges. Response surface methodology was employed to investigate the effects of reaction temperature (290–360 °C) and residence time (0–30 min) on product yield and biocrude quality. Both the highest and the lowest reaction temperature or residence time had negative effects on biocrude yield and energy recovery (ER), while high reaction severities improved biocrude quality. Under optimized conditions (332 °C for 16.9 min), biocrude yield (48.9%, dry ash-free) and ER (70.8%) were maximized. Biocrude composition followed the order of N-heterocycles > O-heterocycles > hydrocarbons, while nitrogenous compounds reduced, and hydrocarbons increased with reaction temperature. More distillable fractions in biocrude were also produced at higher reaction severities. The possible reaction pathways of biocrude formation were discussed and updated to include catalytic effects on inherent metals and Brønsted (acidic and basic) sites. The high content of O (7.8–13.1%), N (4.4–4.9%), and TAN (48.6–63.6 mg KOH/g) suggested the necessity of biocrude upgrading. Separating and recycling trace metals (e.g., 497–656 mg/kg Fe) from biocrude are necessary to relieve upgrading challenges. C, N, and P were mostly distributed into HTL biocrude, aqueous, and hydrochar, respectively, allowing their recovery. Most metals were concentrated in hydrochar. The results contribute to the advancement of the state of the art in biorefinery, which will guide the design of full-scale HTL sludge treatment systems combining resource recovery.

Insights on the Effect of Heavy Metals on Subcritical Hydrothermal Recycling of Heavy Metal-Contaminated Biomass and Its Derived Porous Carbon Properties Using Cu as a Case

Subcritical hydrothermal liquefaction (HTL) technique has shown great potential in recycling heavy-metal-contaminated biomass derived from phytoremediation and biosorption. However, the effect of heavy metals on the physicochemical characteristics of HTL products and their derivatives has not been elaborated so far. Herein, various Cu content-contaminated derived biomass was recycled by HTL. We found that over 99% of Cu was enriched in hydrochar at 300 °C, enabling us to obtain clean chemicals (e.g., levulinic acid and acetic acid). Furthermore, a large amount of Cu(0) was formed, especially at high temperatures, because HTL products acted as electron donors. Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry analysis showed that Cu catalyzed the depolymerization and deoxygenation of macromolecular lignins/carboxylic-rich alicyclic molecule-like compounds, resulting in the production of more levulinic acid. Two-dimensional FTIR correlation spectroscopy analysis indicated that the intensity of C═C increased first, and the intensity of C–O–C decreased later as the Cu content increased. Notably, the low-risk hydrochar could be upgraded into porous carbon (PC) with excellent CO2 capture performance (up to 6.64 mmol/g), whereas Cu enhanced the CO2 capture capacity of PC by increasing its porosity. Undoubtedly, such findings have important implications for recycling heavy-metal-contaminated biomass into high value-added products.

Investigation of the molecular characterization and antioxidant activity of bio-oil produced from high moisture oil palm residues employing hydrothermal liquefaction

Oil palm residues are a promising feedstock for green chemical production; however, they are high in moisture and difficult to use. This study focuses on the conversion of high-moisture biomass, namely empty fruit bunch (EFB), oil palm trunk (OPT), and oil palm frond (OPF), into phenolic-rich bio-oil employing non-catalytic hydrothermal liquefaction (HTL) at 250, 275, and 300 °C for a 30-min holding time. Ethyl acetate was used to extract the HTL product. The results showed that the degree of depolymerization was 83–97%. The weight-average molecular weights (Mw) of the bio-oil produced from EFB at 300 °C (EFB-300 °C), OPT-300 °C, and OPF-275 °C were 130, 599, and 100 g mol−1, respectively. When the HTL temperature was increased, the Mw of the bio-oil products decreased due to depolymerization. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry showed that the molecular weights of the bio-oil products from EFB, OPT, and OPF were m/z 197–271, 208–334, and 203–343, respectively. The ratios of syringyl (S) and guaiacyl (G) units of lignin were in the range of 0.352–0.731. The main lignin components were p-hydroxyphenyl (H) and G units, which favor phenolic resin substitution. The antioxidant activity of the extracted bio-oil from HTL at 300 °C was very strong (IC50: 17.1–39.6 ppm), indicating its potential for future research and applications.

FT-ICR MS characterization of bio-binders for road pavement from HTL of microalgae residues

Hydrothermal liquefaction (HTL) of microalgae-derived feedstocks into a bio-binder was investigated in the absence or presence of a catalyst. The resulting products of HTL of Scenedesmus species (sp.) residues present rheological properties close to those of a petroleum-derived bitumen. The chemical composition of the biobinder was analysed by two-dimensional gas chromatography (GCxGC-MS), gel permeation chromatography (GPC), and negative and positive-ion laser desorption/ionization FT-ICR MS to provide a complementary analysis of the biobinder and understanding of the role of catalysts. A detailed description of the heavy nitrogen compounds contained in the biobitumen is given.

Characterization of municipal solid waste residues for hydrothermal liquefaction into liquid transportation fuels

This paper presents and evaluates a new method for characterising municipal solid waste residues for assessing the performance of thermochemical conversion technologies to produce fuels. The method combines information from three complementary analytical techniques to estimate the quantity of key organic waste fractions and was demonstrated using two commercial waste residues: ‘BRDF’ and ‘Floc’ produced from the mechanical processing of domestic waste. Cellulose content (mostly paper and textiles) is estimated using acid hydrolysis, while thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) are combined to determine the plastics (LDPE and PET) and non-volatile fractions such as lignin of the wastes. High mass balance closures were achieved for both residues, although the nature of the non-volatile fraction was difficult to verify. Hydrothermal liquefaction (HTL) of cellulose rich BRDF (34.0% cellulose) produced much higher biooil yields than Floc (26.8% and 12.2%, respectively), with a cellulose content of only 22.4%. In both cases, most of the plastic and non-volatile waste fractions partitioned into the solid HTL product, representing a potential method for separating the plastic fractions from other waste components. Importantly, this combined waste characterization method can be used for characterization of any municipal waste residue using acid hydrolysis, TGA and FTIR data, providing accurate information about feedstock composition. It enables comparison between different waste valorisation studies of complex waste residues.

The Role of Catalysts in Biomass Hydrothermal Liquefaction and Biocrude Upgrading

Hydrothermal liquefaction (HTL) of biomass is establishing itself as one of the leading technologies for the conversion of virtually any type of biomass feedstock into drop-in biofuels and renewable materials. Several catalysis strategies have been proposed for this process to increase the yields of the product (biocrude) and/or to obtain a product with better properties in light of the final use. A number of different studies are available in the literature nowadays, where different catalysts are utilized within HTL including both homogeneous and heterogeneous approaches. Additionally, catalysis plays a major role in the upgrading of HTL biocrude into final products, in which field significant developments have been observed in recent times. This review has the ambition to summarize the different available information to draw an updated overall picture of catalysis applied to HTL. The different catalysis strategies are reviewed, highlighting the specific effect of each kind of catalyst on the yields and properties of the HTL products, by comparing them with the non-catalyzed case. This allows for drawing quantitative conclusions on the actual effectiveness of each catalyst, in relation to the different biomass processed. Additionally, the pros and cons of each different catalysis approach are discussed critically, identifying new challenges and future directions of research.

Hydrothermal liquefaction of sewage sludge – effect of four reagents on relevant parameters related to biocrude and PFAS

Catalytic hydrothermal liquefaction (HTL) can convert wet biomass into crude-like oil and has potential to degrade pollutants during the thermal depolymerization process. To avoid negative environmental impacts from per- and polyfluoroalkyl substances (PFAS) in sewage sludge after its final disposal, we comprehensively evaluated catalytic destruction of PFAS in sludge through HTL. The results showed that adding red mud to HTL was beneficial for the biocrude yield and significantly increased the carbon conversion efficiency and energy recovery of the hydrothermal process. HTL at 300 °C for 2 h degraded > 96% of spiked perfluorooctanoic acid (PFOA). The mass of spiked perfluorobutanesulfonic acid (PFBS) and perfluorooctanesulfonic acid (PFOS) and pre-existing perfluorohexanoic acid (PFHxA) and perfluoroheptanoic acid (PFHpA), however, increased in the HTL product streams. The remaining PFAS after HTL mainly stayed in the biocrude phase. Our findings indicated that HTL under the studied condition was not able to degrade all PFAS existed in sewage sludge. Although adding red mud could improve the HTL performance with respect to biocrude, this reagent did not improve the removal of perfluoroalkyl acids (PFAAs). As a matter of fact, the hydrothermal process amended with the selected reagents resulted in increased mass of PFBS, PFHxA, and PFHpA in the HTL products due to degradation of PFAA precursors. Thus, other treatment approaches for PFAS removal from sewage sludge need to be identified to prevent possible contamination of environments receiving PFAS-containing sludge.