Hydrothermal Liquefaction Process Research Articles

Multiphase Numerical CFD Simulation of the Hydrothermal Liquefaction Process (HTL) of Sewage Sludge in a Tubular Reactor

This article presents multiphase numerical computational fluid dynamics (CFD) for simulating hydrothermal liquefaction of sewage sludge in a continuous plug-flow reactor. The discrete particle method (DPM) was used to analyze the solid particles’ interaction in liquid–solid high shear flows to investigate coupling computational fluid dynamics (CFD). Increasing solid particles’ interactions were observed with the increasing liquid velocity. The study examined the influence of parameters such as flow rate, temperature, and residence time on the efficiency of bio-oil production. An increase in temperature from 500 to 800 K caused an increase in the amount of biocrude oil produced from 12.4 to 32.9% within 60 min. In turn, an increase in the flow rate of the suspension from 10 to 60 mL/min caused a decrease in the amount of biocrude oil produced from 38.9 to 12.9%. This study offers insights into optimizing the flow channel of tubular reactors to enhance the HTL conversion efficiency of sewage sludge into biocrude oil. A parametric study was performed to investigate the effect of the slurry flow rate, temperature, and the external heat transfer coefficient on the biocrude oil production performance. The simulation data will be used in the future to design and scale up a large-scale HTL reactor.

Sustainable biorefining: Hydrothermal liquefaction of diatom biomass for bio-crude and nano-biosilica recovery

Diatom, a unique species of microalgae is a highly promising resource for the production of valuable bio-crude and silica nanomaterial. However, the complex nature of diatom cell structure presents significant downstream processing challenges and leads to inefficient bio-product recovery. To overcome this challenge, this study aimed to develop a one-step approach for the valorization of Denticula sp. dominated diatom consortium using hydrothermal liquefaction (HTL) process with the recovery of three major products. The first output, referred to as biocrude, constituted 22% of the total yield, consisting of 9.5% unbound bio-crude and 12.5% cell wall-bound organic fraction. The bio-crude consisted primarily of oxidized and desirable organic compounds like alcohols (24.4%), long-chain saturated hydrocarbons (23.6%), and esters (17.6%), with only a few undesirable nitrogen-containing compounds (18.9%). The second resultant product comprised nanoporous bio-silica structures, yielding 56%. These silica structures exhibited enhanced quality in comparison to the conventional thermal extraction technique employed for their retrieval and were found to be three-dimensional, highly porous, intact silica structures with low trace elements and a high surface area (53.7 m2 g−1). The third acquired product, the aqueous fraction, was observed to possess elevated levels of nitrate-nitrogen (440 mg L−1) and total dissolved phosphate (975 mg L−1). Overall, the study highlights the potential of HTL as an efficient diatom downstream processing method that allows for the complete valorization of intermediates, resulting in a sustainable biomass conversion and product recovery technology.

Wastewater Nutrient Recovery via Fungal and Nitrifying Bacteria Treatment

In efforts to reduce the consumption of fossil fuels and promote recycling biowaste, there is an interest in the production of post-hydrothermal liquefaction wastewater (HTL-AP) from the hydrothermal liquefaction (HTL) process that converts wet biomass into biocrude oil. This study explores ways of transforming potentially toxic HTL-AP into a fertilizer source for hydroponic cropping systems. This study specifically investigates the integration of the white-rot fungus Trametes versicolor with nitrifying bacteria (Nitrosomonas and Nitrobacter) to convert the organic nitrogen compounds into inorganic nitrogen while also producing the enzyme laccase, which has been shown to remove toxic compounds. This study aims to increase the concentration of nitrate-N to valorize wastewater as a suitable fertilizer by measuring several parameters, including laccase activity, pH, nitrate-N, and ammonia/ammonium-N concentrations, and analyzes interactions to optimize the conversion process. The data support the claim that the simultaneous inoculation of T. versicolor and nitrifying bacteria significantly increases nitrate-N concentrations in HTL-AP, as it increased by 17 times, or an increase of 32.69 mg/L. In addition, HTL-AP treated with T. versicolor and nitrifying bacteria reduced the treatment time by 120 h, highlighting a reduction in personnel time and energy consumption. Therefore, this research accentuates sustainability through fungal and bacterial treatments to develop eco-friendly hydroponic fertilizers. Future research should explore the potential of utilizing the combination of T. versicolor and nitrifying bacteria for the treatment of other industrial wastewaters.

Domestic wastewater sludge valorization: Multi-criteria evaluation of anaerobic digestion vs. hydrothermal liquefaction

The emerging hydrothermal liquefaction (HTL) process is evaluated against the classical anaerobic digestion (AD) processes for stabilizing wastewater sludges and recovering their energy- and nutrient-contents. Although HTL affords faster stabilization, better process stability, and liquid fuel and sterile fertilizer recovery, it suffers from higher energy demand and lower technology readiness level. For a rational comparison of these pathways, a multi-criteria evaluation is conducted considering 21 technical, environmental, economic, and social criteria. Criteria values for the HTL-pathway were derived from laboratory tests while those for the AD-pathway were compiled from literature. Of the 16 process alternatives evaluated, the AD-pathway including nitrogen-recovery by air-stripping and phosphorus recovery by the MEPHREC® process ranked first followed by the HTL-pathway. This multi-criteria study suggests that the HTL-pathway could be engineered as a superior alternative for sludge stabilization and resource recovery if phosphorus recovery and its technology readiness level could be improved.

Continuous wet air oxidation of aqueous phase from hydrothermal liquefaction of sewage sludge

The conversion of organic waste into advanced fuels through hydrothermal liquefaction (HTL) is a promising route. However, for the commercialisation of HTL technology, treatment and valorisation of the generated aqueous by-product are required for its economic viability. This study investigates subcritical non-catalytic continuous wet air oxidation (WAO) for the treatment of the aqueous phase from hydrothermal liquefaction (HTL-AP) of sewage sludge. A custom-made tubular reactor was used at temperatures and residence times similar to the HTL process (300-350 °C; 6-50min), with excess air equivalent to 1.5-2 times the oxygen demand. The WAO process removed up to 95.3% of the chemical oxygen demand and 91.8% of the total organic carbon, generating volatile fatty acids, mainly acetic acid, as intermediary compounds from the oxidation reactions, and CO2 and H2O from the pollutant’s complete degradation. The process effectively eliminated almost all of the organonitrogen compounds, generating NH4+, which reached values of up to 97% of the total nitrogen. Comprehensive molecular characterization performed by FT−Orbitrap MS analysis revealed that the main basic organonitrogen compounds in HTL-AP are distributed in the O1N1, N2, O1N2, N1 classes. For neutral to acidic organonitrogen species, O3N1 is the most representative chemical class. The energy consumption for heating the WAO reactor was up to 40% lower using HTL-AP compared to blanks experiments. Despite good process water clean-up, autothermal operation was not achieved, and improvements in the reactor design are suggested.

Valorization of low heavy metal-accumulating plants through catalytic hydrothermal liquefaction with attapulgite: Product characterization and migration behavior of heavy metals

Harmless treatment of heavy metal-accumulating plants is significant for further development and application of phytoremediation for contaminated soil remediation. In this study, hydrothermal liquefaction (HTL) of low heavy metal-accumulating plants (Jute, Solanum nigrum, and Sorghum) was conducted at different temperatures (280–360 °C) for bio-oil production. Attapulgite and Co/Attapulgite were prepared and tested in HTL process. The maximum bio-oil yield and higher heating value (HHV) was found to be 32.04% and 29.61 MJ/kg from HTL of Solanum nigrum with Co/Attapulgite at 300 °C. The improvement in quality of bio-oil was marked with decreased O/C content (0.26) and newly formed hydrocarbon fraction with the content of 6.24% with Co/Attapulgite catalyst. It was beneficial for immobilization of Mn, Zn and Cr in hydrochar with addition of catalyst. Notably, Mn and Cd were not detected in any bio-oil, and the proportion of Cr in the bio-oil obtained at 360 °C decreased to zero with Co/Attapulgite. XPS and XRD analysis indicated that Mn and Zn existed in the form of Mn3O4, Mn, ZnO and Zn(OH)2 in hydrochar. Finally, potential pathway of heavy metals migration-transformation behaviors during HTL was proposed.

Effect of the reactor heating rate on bio-crude yield and quality from hydrothermal liquefaction of different sludge

This study scrutinizes the effect of the heating rate obtained in two reactors of different volume (10 mL and 500 mL) on the production of bio-crude during the hydrothermal liquefaction (HTL) process applied to sludges of different nature: a municipal, a paper mill and an agricultural sludge. The effect of the reactor scale on the chemical composition of the bio-crude and the solid residue (co-product of HTL process) is also evaluated by elemental, GC–MS and FT-IR analysis. Results suggest that different heating rates along HTL exert an almost negligible influence on the bio-crude yield and limited effect on the chemical composition when the reaction time under the isothermal conditions is kept at 10 min. In fact, regardless of the heating rate, the bio-crude yields on dry basis from municipal, paper mill and agricultural sludges are equal to 21 %, 21 % and 12 %, respectively. These results could be explained with the different reactive pathway and kinetics of the sludges macro-components.

Integration of hydrothermal liquefied sludge as wastewater for anaerobic treatment and energy recovery: Aqueous phase characterization, anaerobic digestion performance and energy balance analysis

For the hydrothermal liquefaction (HTL) process as the final method for waste activated sludge, the liquid by-product of the process, i.e., the HTL aqueous solution, poses the greatest bottleneck due to its large volume and contaminant levels. In this study, the effects of hydrothermal liquefaction pretreatment (HTL) on the performance of anaerobic digestion and energy balance of the waste-activated sludge (WAS) aqueous phase were investigated. The results showed that HTL could greatly increase the hydrolysis of WAS and enhance the protein, polysaccharide and volatile fatty acid contents in the aqueous phase. The anaerobic digestion showed that the optimal gas and methane production efficiency was achieved at 160 °C. Positive net energy gain was obtained in each process (ΔE) and the optimal HTL heat recovery rate was obtained at 160 °C. HTL was found to improve the hydrolyzing bacterial community in anaerobic digestion but had little effect on the archaeal community.

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.

Aqueous phase recycling: impact on microalgal lipid accumulation and biomass quality.

The potential success of microalgal biofuels greatly depends on the sustainability of the chosen pathway to produce them. Hydrothermal liquefaction (HTL) is a promising route to convert wet algal biomass into biocrude. Recycling the resulting HTL aqueous phase (AP) aims not only to recover nutrients from this effluent but also to use it as a substrate to close the photosynthetic loop and produce algal biomass again and process this biomass again into new biocrude. With that purpose, the response to AP recycling of five Chlorellaceae strains was monitored over five cultivation cycles. After four successive cycles of dynamic growth under nutrient-replete conditions, the microalgae were cultivated for a prolonged fifth cycle of 18 days in order to assess the impact of the AP on lipid and biomass accumulation under nutrient-limited conditions. Using AP as a substrate reduced the demand for external sources of N, S, and P while producing a significant amount of biomass (2.95-4.27 g/L) among the strains, with a lipid content ranging from 16 to 36%. However, the presence of the AP resulted in biomass with suboptimal properties, as it slowed down the accumulation of lipids and thus reduced the overall energy content of the biomass in all strains. Although Chlorella vulgaris NIES 227 did not have the best growth on AP, it did maintain the best lipid productivity of all the tested strains. Understanding the impact of AP on microalgal cultivation is essential for further optimizing biofuel production via the HTL process.

Modelling and Simulation Studies on the Characteristic Behaviour of Macroalgae Derived Bio-Oil

Abstract With the expanding need for renewable and sustainable energy resources, the demand for biomass derived bio-oil particularly microalgae or macroalgae based bio-oil is growing. In this regard, understanding the characteristics of bio-oil as a function of several influencing operational parameters is essential. In this study modelling and simulation technique based on Aspen HYSYS was applied to investigate the characteristics of macroalgae Ulva prolifera derived bio-oil from Hydrothermal Liquefaction (HTL) process as a function of the influencing parameters like temperature, kinematic viscosity, and weight percent of the bio-oil assay, cut yield behaviour, and standard liquid density. Modelling and simulation help to optimise the process parameters and design the process layout for large-scale production. According to the simulation results the cut yield of off gases, light naphtha, heavy naphtha, light distillate, heavy distillate, gas oil and residues are shown at specific final boiling point (FBP) temperatures of 70 °C, 70 °C - 110 °C, 110 °C - 221.1 °C, 221.1 °C - 304.4 °C, 304.4 °C - 371.1 °C, 371.1 °C - 537.8 °C and 537.7 °C respectively. Whereas, above a temperature of 300 °C, the weight percentage of aromatic components increased steadily. The increase in percentage composition of the aromatic components is due to the reduction of the paraffinic components. The density of the liquid bio-oil was steadily increasing until a temperature of 200 °C.

Aqueous-phase product treatment and monetization options of wet waste hydrothermal liquefaction: Comprehensive techno-economic and life-cycle GHG emission assessment unveiling research opportunities

While wet waste hydrothermal liquefaction technology has a high biofuel yield, a significant amount of the carbon and nitrogen in the feedstock reports to the aqueous-phase product. Pretreatment of this stream before sending to a conventional wastewater plant is essential or at the very least, advisable. In this work, techno-economic and life-cycle assessments were conducted for the state-of-technology baseline and four aqueous-phase product treatment and monetization options based on experimental data. These options can cut minimum fuel selling prices by up to 13 % and life-cycle greenhouse gas emissions by up to 39 % compared to the baseline. These findings highlight the substantial influence of aqueous produce treatment strategies on the entire wet waste hydrothermal liquefaction process, demonstrating the potential for optimizing economic viability and environmental impact through further research and development of milder treatment methods and diversified by-product valorization pathways.

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.

Experimental investigation and optimization of dual fuel combustion using diesel/gasoline and Bio-oil extracted from Co-thermal liquefaction of paint/biomass wastes: An approach towards waste to energy

A higher-grade transport fuel from both non-biodegradable paint waste and biomass plant Prosopis Juliflora is obtained in the most economical and eco-friendly way by hydrothermal liquefaction process. The yielded bio-oil is experimented on a diesel engine in dual fuel combustion (DFC) strategy and compared with the conventional diesel combustion (CDC). In DFC, the secondary fuels (gasoline, bio-oil, and premixed gasoline-bio-oil blends) are port injected at different premixed ratios (10, 20 and 30%), while the primary diesel fuel is directly injected into the cylinder. DFC of diesel - bio-oil (DB) exhibited higher brake thermal efficiency than premixed gasoline at all power outputs and performed closer to neat diesel. Lower smoke emission is observed in DB than neat diesel and the DFC of diesel-gasoline (DG) at all conditions. At 30% premixed ratio, the smoke opacity of DFC of diesel and premixed bio-oil - gasoline (DGB) is 65.7% and 68% respectively compared to that of CDC (76.4%). At all loads, average NOx emission of DGB (20% gasoline) is abated by 23.2, 7.8 and 12.9% than CDC and 20% of DB and DG respectively but almost equal to neat diesel at full load. The experimental data are used to train an artificial network (ANN) and the trained ANN is used to generate sufficient data for optimizing the fuel concentration using TOPSIS algorithm to realize an overall improved performance and emissions. The optimized combustion modes arrived from this study are: CDC for 25% load, DFC of DGB (D90G2B8) for 50% load and DFC of DB for 75% (D89B11) and 100% (D86B14) loads.

Catalytic hydrothermal liquefaction of Camelina sativa residues for renewable biogasoline production

ABSTRACT Camelina sativa plant residue was used as feedstock for the production of biogasoline (a mixture of paraffinic and olefinic hydrocarbons) by catalytic hydrothermal liquefaction (cHTL) process. The experiments were conducted at temperature range 200–375°C and pressure range 10–19 MPa. Molecular hydrogen was added for the hydro-deoxygenation processes. Various experiments were performed to understand the contributions of process parameters such as temperature, pressure, hydrogen, and retention time on reaction conversion and products distribution. This was followed by screening of different catalyst supports – γ-Al2O3, HZSM-5, SiO2-Al2O3, SBA-15, and Al2O3-HZSM-5 to understand their impacts as well as catalytic activities. The Al2O3/HZSM-5 exhibited the best properties and was impregnated with a varying range of cobalt metal for the HTL of Camelina sativa. The 5%Co/γ-Al2O3-HZSM-5 catalyst exhibited highest liquefaction conversion (79%) and biogasoline yield (43%) at optimized reaction conditions due to the presence of strong brønsted acidity on the catalyst were that enhanced both the hydrocarbon cracking and hydrogenation leading to the production of low boiling point and low molecular weight hydrocarbon within the boiling point range of fossil gasoline. Across all the data point used in the work, the average deviation was within ± 3%, these are mostly indicated by the error bars.

Unravelling the Role of Biochemical Compounds within the Hydrothermal Liquefaction Process of Real Sludge Mixtures

This paper aims to provide a contribution to understanding the role of sludge macro-components (lipids, proteins and carbohydrates) on the yield/quality of bio-crude obtained via hydrothermal liquefaction (HTL). This was pursued by analysing the HTL process of real sludges and the mixtures thereof at different compositions, a topic that has been explored in a very limited way in the pertinent literature. The HTL experiments were run with municipal sludge, tannery sludge and a mixture of them in 25:75, 50:50 and 75:25 weight ratios in a batch reactor at 350 °C and for different residence times. The outcomes for a single sludge showed a greater bio-crude yield for the municipal one (42.5% at 10 min), which is linked to its significant carbohydrate content. The results obtained from the sludge mixtures suggested that a carbohydrate-to-protein mass ratio of 2:1 would maximise the bio-crude yield (average value of about 38%). Moreover, LC-MS and NMR analyses highlighted that the mixed sludges contributed to the formation of a higher number of compounds after the HTL treatment, with respect to the pure municipal or tannery sludge, with an increase in amine, alcohols and aromatic compounds.

Integrated biorefinery approach for seaweed Sargassum fusiforme: A step towards green circular economy

This study aims to maximize the utilization of Sargassum fusiforme (SF), embedding the concept of a green circular economy. We developed a green extraction process using water and ultrasound assistance to extract high-value SF polysaccharides (SFPS). A 2k factorial design coupled with center points evaluated the curvature effect in the extraction process. Optimal extraction conditions were identified to be a mass-liquid ratio of 1:59 (w/v), an extraction temperature of 70 °C and time of 88 min, yielding 15.30 wt% SFPS, validated through experiments. SFPS demonstrated significant antioxidant capacity, indicated by their ferric ion reduction ability and superoxide anion scavenging ability. Post-extraction, the waste ethanol solution and SF residue were employed as feedstock for hydrothermal liquefaction (HTL), aiming to produce biocrude oil. We explored various parameters affecting the yield and quality of the resultant biocrude. The optimal HTL conditions for SF residue included a temperature of 280 °C and a reaction time of 45 min. The biocrude obtained using the waste solution exhibited higher carbon content (69.83%) and higher heating value (33.42 MJ/kg) compared to freshwater, indicating enhanced energy recovery (87.65% vs. 15.39%). Also, the presence of ethanol in waste solution facilitated a decrease in fatty acid content from 48.50% to 24.50%, while simultaneously increasing ester content from 6.37% to 35.54%, presumably due to the trans-esterification of fatty acids into esters. Utilizing the waste ethanol solution as HTL processing medium also led to a much lower greenhouse gas emissions (1714.0 kg CO2-eq) compared to that of freshwater (7735.4 kg CO2-eq) as evidenced by the life cycle assessment. This study's innovative approach not only valorizes the waste streams from SFPS extraction but also adds value by generating high-quality biocrude oil, offering practical solutions for waste reduction and resource recovery, fostering sustainable development and circular economy initiatives.

Role of sea salt in modulating biomass-to-biocrude conversion via hydrothermal liquefaction

Hydrothermal liquefaction (HTL) employs subcritical water as a reaction medium for the conversion of biomass into biocrude, however, most research relies on freshwater as the reaction medium, posing a critical resource challenge. Although preliminary investigations have demonstrated the viability of seawater as an alternative medium for HTL, the mechanistic role of sea salt in HTL of biomass remains poorly understood. In the present study, individual biomass components—carbohydrate, lignin, protein, and lipid—were subjected to HTL under varying sea salt concentrations and temperatures. Sea salt exerted an inhibitory effect on biocrude yield during carbohydrate HTL process, but no inhibitory effect was observed in HTL of lignin, protein, and lipid. As for the impact of sea salt relative to that of sodium chloride (NaCl), it was found that at a moderate sea salt concentration (13.5 wt% relative to feedstock mass) and temperature (270 °C), the minor constituents in sea salt were favorable for HTL of protein and lipid in terms of biocrude yield, energy recovery, and carbon recovery. Our findings offer valuable insights into the benefits and challenges of employing seawater as a reaction medium and illuminate the complex roles that sea salt and its minor constituents play in HTL processes.

Waste Sludge: Entirely Waste or a Sustainable Source of Biocrude? A Review.

Biomass-derived biocrude is gaining greater recognition from people in general as an alternative fuel source to traditional fossil fuels. Worldwide, a great deal of research is being done to develop fuels made from sustainable biomass in order to replace the current conventional energy sources. Waste sludge has been thought of as a viable raw biomass source because of its accessibility, affordability, high lignin content, and higher heating value. Additionally, considering sludge contains a high proportion of moisture and water acts as a catalyst during the hydrothermal liquefaction (HTL) process, it is the best choice for thermochemical conversion. From the ultimate component value ranges obtained from elemental analysis, it can be demonstrated that the C, H, and higher heating value (HHV) of petrocrude are approximately 8.78%, 23.5%, and 10.66% higher than those of biofuel. According to the overall analysis, co-liquefaction of waste vegetable oil and swine manure can result in 87.97% bio-oil at 340°C. The temperature, retention period, inclusion of catalysts, and use of solvents, however, can all affect this proportion. To support this illustration, it has been assessed from the study that municipal wet sewage sludge can produce an HHV of 28.52MJ/kg when water is used as the solvent. However, 34.14MJ/kg, or 16.5% more than the previous one, can be produced for the same amount of biomass, when the mixture of water and methanol serves as the solvents. This review article highlights an array of waste sludge categories, their chemical properties, and their conversion through the HTL process. It also features a Van Krevlen diagram with a graphical representation of essential operating parameters. This review research illustrates one of the best strategies for producing biofuel in which waste sludge can be used as raw material through the HTL conversion process, considering the prospective mass commercial production of biocrude oil.

Lignin extraction from kraft black liquor and its conversion to phenol-rich oil by hydrothermal liquefaction process

This study exploited lignin from black liquor as feedstock in the hydrothermal liquefaction process. The operation conditions such as temperature, operation time, and solvent ratio (by adding ethanol) were explored, with temperature identified as the most significant parameter. The optimum condition generating the highest oil yield (58.9 %) was found at an operating temperature, time, and solvent ratio of 275 °C, 30 min, and 1:1 water-to-ethanol ratio, respectively. The extracted oil was further characterized, revealing phenolic-rich oil containing about 83.4 % phenol, which was dominated by guaiacol. The lignin-to-oil conversion was initiated by thermochemical degradation comprising a series of multiple chemical reactions. The overall process reduced sulfur, nitrogen, and oxygen content linked to reaction instability, favoring solid char yield. The resulting higher heating value of the extracted phenol-rich oil was about 27.8 kJ/g, almost twice that of the raw black liquor only having 14.1 kJ/g.