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Article

Hydrothermal Liquefaction for Biofuel Synthesis: Assessment of VFA (Volatile Fatty Acid) and FAME (Fatty Acid Methyl Ester) Profiles from Spent Coffee Grounds

by
Dimitrios Liakos
1,
Georgia Altiparmaki
1,
Simos Malamis
2 and
Stergios Vakalis
1,3,*
1
Energy Management Laboratory, Department of Environment, University of the Aegean, University Hill, 81100 Mytilene, Greece
2
Laboratory of Sanitary Engineering, School of Civil Engineering, National Technical University of Athens, Zographou Campus, 15780 Athens, Greece
3
School of Chemical Engineering, National Technical University of Athens, 15780 Athens, Greece
*
Author to whom correspondence should be addressed.
Energies 2025, 18(8), 2094; https://doi.org/10.3390/en18082094
Submission received: 24 March 2025 / Revised: 10 April 2025 / Accepted: 15 April 2025 / Published: 18 April 2025
(This article belongs to the Special Issue Advanced Thermochemical and Biochemical Processes for Biomass Transformation to Biofuels and Biochemicals)
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Abstract

:
Spent coffee grounds (SCGs) are an underutilized biomass resource with high potential for renewable energy and bioproduct synthesis. This study applies hydrothermal liquefaction to transform SCGs into high-quality biofuels and value-added biochemicals. Five experiments were conducted over a temperature range of 300 °C to 380 °C, highlighting significant temperature-dependent shifts in product composition. Notably, phenolic compounds peaked at 1180.1 mg/L at 300 °C before declining sharply, while chemical oxygen demand (COD) dropped to a minimum of 13,949.8 mg/L at 350 °C—a temperature that also maximized hydrochar yield (26%) and achieved a high heating value of 32.9 MJ/kg. Plasma chromatographic analyses showed the dynamic behavior of volatile fatty acids (VFAs) and fatty acid methyl esters (FAMEs); maximum levels of acetic (540.7 mg/L), formic (67.8 mg/L), and propionic acids (155.6 mg/L) were recorded at 300 °C, whereas FAMEs such as methyl butyrate, methyl hexanoate, methyl undecanoate, and methyl palmitate increased markedly at higher temperatures due to intensified carboxylation reactions. These results denote the potential of hydrothermal liquefaction to valorize SCGs for the production of biomolecules, expanding the conventional sustainable biofuel production pathways.

1. Introduction

Coffee ranks as the second most valuable commodity in the world, after oil [1]. Spent coffee grounds (SCGs) constitute the most abundant waste produced by the coffee beverage industry and the production of instant coffee, accounting for approximately 45% of coffee waste [2]. Compared to other organic waste, coffee residues contain high concentrations of harmful compounds that are detrimental to the environment, and most coffee waste is disposed of directly in landfills [3]. Organic compounds in SCGs, such as caffeine, tannins, and polyphenols, are particularly problematic for soil when this waste is disposed of [4]. Additionally, disposing of SCGs in landfills poses a significant risk of combustion, which can lead to an excessive production of harmful methane and carbon dioxide, contributing to air pollution and environmental degradation [2]. Besides the environmental issues, disposing of SCGs in landfills is also economically disadvantageous. The high organic content in the waste requires a large amount of oxygen for efficient decomposition [5]. However, SCGs still contain significant amounts of sugars, oils, antioxidants, and other valuable compounds, making them a potential source of energy [3]. The main components of SCGs are cellulose and hemicellulose, which are found in the highest concentrations within the waste. Lignin and protein follow in lesser amounts, along with lipids, mainly fatty acids like linoleic acid and palmitic acid. Additionally, caffeine, phenolic compounds, minerals, tannins, and ash are present in smaller quantities [6]. Coffee residues also have a high moisture content due to their composition, allowing them to retain significant amounts of water, with 5.7 g of water per gram of dry SCGs [1]. Therefore, when combined with their high lignin and protein concentrations, SCGs are a valuable resource for hydrothermal liquefaction [7].
Due to the high moisture content in SCGs, anaerobic digestion has been applied as a waste utilization technology for biogas production. Anaerobic digestion of coffee grounds produces approximately 0.314 L CH4/g VS in biogas. However, adding coffee grounds to other waste for anaerobic co-digestion can have a competitive effect on methane yield [8]. Additionally, the combustion of coffee grounds in a small-scale boiler has been tested. According to Kang et al. [9], this combustion process can generate 6.5 kW of energy, which makes coffee grounds inefficient for combustion in larger-scale boilers. Another widely used process is the extraction of oils from SCGs followed by their transesterification. However, due to the high acidity and water content of the oil, drying and pre-treatment stages, which incur significant energy costs, are necessary before the oil can be used [10]. Given these characteristics, SCGs have been utilized as a raw material for thermal technologies such as pyrolysis and hydrothermal carbonization to manage waste and produce fuel [11]. Regarding the pyrolysis of SCGs, optimal results are achieved after the extraction of waste liquids. This process appears to have significant potential, but the raw material requires costly pre-drying and pre-processing. Low-temperature mild pyrolysis seems to be the most suitable method for converting SCGs, yielding carbon products with significant calorific values ranging from 24 to 26 MJ/kg [12]. Hydrothermal carbonization is a thermochemical process that occurs at temperatures between 180 and 250 °C, producing hydrochar, a solid fuel [13]. According to Hu et al. [14] experiments conducted with SCGs at temperatures ranging from 150 to 230 °C achieved a remarkable maximum energy recovery of 83.93%. However, the calorific value reached a maximum of only 23.54 MJ/kg, which is relatively low compared to other methods.
Based on current energy needs, global energy demand is projected to increase by approximately 28% by 2040. The uncontrolled use of fossil fuels is exacerbating the energy crisis and environmental problems. This situation necessitates a shift towards renewable energy and the creation of a zero-carbon energy system [15]. Hydrothermal liquefaction is a thermochemical process that converts organic waste into liquid fuel products, specifically bio-crude oil and high-value bioproducts. This process occurs at high temperatures ranging from 250 °C to 375 °C, with pressure conditions exceeding the water saturation point at these temperatures. The high temperature and pressure contribute to a short biomass conversion reaction time [16] because they effectively break down the biomass structure to form oily products [17]. During hydrothermal liquefaction, reactions such as depolymerization, bond breaking, rearrangement, and decarboxylation play key roles in breaking down the solid biomass structure into bio-crude oil [18]. Compared to other thermochemical processes like combustion, gasification, and pyrolysis, hydrothermal liquefaction is more efficient because it does not require pre-drying of the biomass. Additionally, it can easily process complex biomass mixtures that contain lignocellulose, protein, and lipids [19]. This technology can not only process and utilize the complex components of biomass but also has the ability to convert them entirely into biofuels [18]. As a result, the abundant availability of organic waste makes it an attractive raw material for biofuel production. SCGs, due to their structural characteristics and the vast consumption of coffee worldwide, constitute a significant amount of organic waste, making them an excellent source for biofuels [19].
The use of hydrothermal liquefaction technology highlights a research gap in the full utilization of the moisture and components of SCGs for the production of high-value biofuels and bioproducts, specifically bio-crude oil and hydrochar with high calorific value. The high temperature and pressure conditions in this process contribute to the high heating value of the solid fuel and the production of long-chain carboxylic acids, which enhance the liquid product of hydrothermal liquefaction. This study focuses on assessing the concentration of fatty acids and the heating value of hydrochar. Additionally, by analyzing the changes in organic load and the concentration of short-chain carboxylic acids (VFAs), the potential for biofuel production through the thermochemical pathways of hydrothermal liquefaction is examined. More specifically, the research focalizes on how thermochemical processes in hydrothermal liquefaction affect the production of VFAs and, by increasing temperature and pressure, the production of longer-chain carboxylic acids.

2. Materials and Methods

The experiment was conducted on the island of Lesvos, using SCGs as the material. The raw material was collected from the cafeteria of the University of the Aegean. As mentioned earlier, coffee consumption trends continue to rise steadily. Coffee-related industries worldwide produce vast quantities of SCGs, which, in most cases, are wasted and not utilized [20].

2.1. Experimental Process

The experimental process began with the collection and analysis of SCGs. The analyses performed on the material included total solids (TS), volatile solids (VS), and moisture determination. Specifically, 4 g of SCGs were placed in an oven at 105 °C for 24 h to remove and calculate the moisture content in the waste, thereby determining the total solids. The material was then further heated at 550 °C for 3 h to calculate the volatile solids.
The hydrothermal liquefaction experiments were conducted in a Parr 4577A (Moline, IL, USA) hydrothermal reactor with a capacity of 1 L. A series of five experiments were carried out at different temperatures, 300 °C, 310 °C, 325 °C, 350 °C, and 380 °C, in order to assess intermediate temperatures, especially close to the perceived transition between HTC and HTL. To regulate the moisture content of the material, which is crucial in hydrothermal treatment, the following quantities were placed in the reactor: for the first four temperatures, 60 g of SCGs and 140 mL of water were added, while at 380 °C, 20 g of SCGs and 40 mL of water were used. The different amounts of water in the experiments were aimed at controlling pressure at higher temperatures and maintaining pressure in the desired range of values, and the scope was not to exceed supercritical conditions. The residence time for the hydrothermal liquefaction experiments was 30 min. Table 1 provides detailed conditions of the HTL experiments. After the process, the reactor was cooled, and the solid and liquid products were collected for further analysis.

2.2. Methods of Analysis

After the completion of hydrothermal liquefaction, several analyses were conducted on both the liquid and solid products. The liquid product was analyzed for pH, total phenols, and chemical oxygen demand (COD), as well as the concentrations of volatile fatty acids (VFAs) and fatty acid methyl esters (FAMEs). The solid product, on the other hand, was separated from the liquid, filtered, and then assessed for hydrochar mass yield and high heating value.
The analyses began with the determination of pH, using a De Bruyne Instruments Consort C932 pH-meter, (Wichelen, Belgium) which was calibrated at pH 4 and 7 for greater accuracy [21]. To calculate total phenols, the samples were initially mixed with water. The SCGs300 and SCGs350 samples were mixed 1/10, the sample SCGs310 was mixed 1/100, and the SCGs325 and SCGs380 samples were mixed 1/200. The phenol analysis involved preparing a mixture containing 6 mL of water, 1 mL of diluted sample, 0.5 mL of phenol reagent, 1.5 mL of sodium carbonate, and 1 mL of water. This procedure was followed for all samples. The mixtures were then placed in a shaded area for two hours before measuring the phenol concentration using a Hach DR/2400 spectrophotometer [21]. For COD measurement, the liquid phase of the HTL samples was also mixed with water. In the SCGs300, SCGs350, and SCGs380 samples, a mixture of 1/25 was performed, while in the SCGs310 and SCGs325 samples, a mixture of 1/30 was carried out. To measure COD concentration, a mixture was prepared containing 2.8 mL of silver sulfate, 1.2 mL of potassium dichromate, and 2 mL of the mixed sample. This procedure was repeated for all samples. The mixtures were then digested in a Hach 45600 COD reactor (Loveland, CO, USA) at 150 °C for 2 h. After digestion, the COD concentration was measured using the Hach DR/2400 spectrophotometer (Loveland, CO, USA), following the APHA [22] methodology. It is worth noting that the mixtures in phenols and COD were different so that their concentrations would be within the spectrum of measurement of the curve in the spectrophotometer.
In the next stage of the analyses, the concentration of VFAs was measured. The process began by centrifuging the liquid for 10 min, followed by filtration. Equal amounts of isopropanol and the filtered sample were then mixed and placed in a sonic bath at 40 °C for 15 min to extract the VFAs using the organic solvent. The mixture was then centrifuged again for 10 min, after which the supernatant liquid, containing isopropanol and VFAs, was collected. This solution was introduced into a distillation column to evaporate the isopropanol, leaving only the VFAs. The resulting liquid was filtered using 0.45 µm filters. The efficiency of the extraction method was assessed using the CRM46975 Supelco (Darmstadt, Germany) Volatile Free Acid Mix as a standard, with the recovery efficiency estimated at 59.6 ± 0.2%. The concentration of VFAs was then measured using a gas chromatograph, specifically the Shimadzu (Kyoto, Japan) Nexis 2030 GC-BID gas chromatograph with an Agilent (Santa Clara, CA, USA) J&W HP-FFAP column. The parameters of the method included a 1 μL sample injection volume, an injection temperature of 160 °C, an oven temperature program ranging from 80 °C to 230 °C, a flow rate of 59 mL/min, and a BID detector temperature of 280 °C. All the calibration curves and the raw GC analysis data are provided as Supplementary Materials of this study.
The next phase of the analyses involved calculating the concentration of FAMEs. Transesterification was required to determine the FAMEs, and the process consisted of the following steps. Initially, 200 µL of the sample was mixed with 4 mL of a 0.5 M Methanolic-KOH solution. This mixture was heated to 80 °C for 15 min with two stirring stages. Then, 1.6 mL of an HCl (4:1) solution was added and the mixture was heated to 80 °C for 25 min. After heating, the mixture was cooled, and 8 mL of deionized water was added. Next, a total of 15 mL of hexane was introduced into the mixture to extract the FAMEs. The addition of hexane caused phase separation, and the supernatant, consisting of hexane and FAMEs, was collected. The liquid was then filtered using a 0.45 µm filter, with 2 g of anhydrous sodium sulfate at the end of the syringe to absorb moisture. FAMEs were measured using the same gas chromatograph as VFAs. For the measurement method, the MEGA 10 column was used. Additional method parameters included a 1 μL sample injection volume, an injection temperature of 240 °C, an oven temperature program ranging from 40 °C to 230 °C, a flow rate of 66.5 mL/min, and a BID detector temperature of 240 °C. The final stage of the analyses involved measuring the high heating value of both the hydrochar produced at each temperature and the original material (SCGs). The measurements were conducted using a Parr 6400 calorimeter (Moline, IL, USA). Before beginning the process, the calorimeter was calibrated with benzoic acid tablets. The high heating value for each temperature was then measured using a 0.3 g sample. All the data that are presented in the form of figures are statistically analyzed and include error bars, except the pH analysis due to the identical measurements.

3. Results

The analyses carried out on the SCGs revealed that they are a material with a high moisture content. According to Osorio-Arias et al. [23], the moisture content of SCGs typically fluctuates around 60%. In this study, the moisture content was found to be 62.7%, with total solids at 33.2% and volatile solids at 0.6%. The remaining percentage to reach 100% involved the percentage of solid carbon. On the other hand, pH measurements of the liquid products from hydrothermal liquefaction indicated that the pH values were acidic across all temperatures. This is likely because the material has a low ash content, approximately 1.3% [24], and a low nitrogen content of 2.3%, including nitrogenous compounds [25], which are factors that can contribute to an alkaline pH. The pH analysis results are presented in Figure 1. The lowest pH value was observed in the liquid product from 300 °C (SCGs300), which was 4.67. Almost the same pH value was derived from hydrothermal liquefaction SCGs at 300 °C and was 4.8 according to the experiments of Yang et al. [4]. As the temperature increased, the sample values remained acidic. In the liquid product from 380 °C (SCGs380), there was a slight decrease in pH. The highest pH value was found in the SCGs350 sample, reaching 5.92. Also, according to Muller et al. [26], in their SCG hydrothermal liquefaction experiments, there was a range in pH from 4.4 to 6.7.
Regarding the total phenol concentration, the highest value was observed at 300 °C, measuring 1180.2 mg/L. A sharp drop in concentration occurred at 310 °C, where the lowest phenol concentration of 365.5 mg/L was recorded, as shown in Figure 2. In general, phenol concentrations in bio-crude oil produced from SCGs are low due to the raw material’s low lignin content [4]. The lignin content in SCGs is approximately 23.3% [24]. After the sharp drop at 310 °C, an increase in phenol concentration was observed at 325 °C, reaching 870.4 mg/L, followed by a gradual decrease in concentration up to 380 °C. This increase in the concentration of phenols is due to the breaking down of the hydrochar; additionally, organic components present in its pores were transferred to the liquid phase [27]. This is because the phenols form light cyclic hydrocarbons that remain in the bio-crude, and heavier polycyclic hydrocarbons that form the biochar are trapped in it [28]. On the other hand, this decline at higher temperatures occurs because high temperatures contribute to the destruction of phenols [27]. From the hydrothermal liquefaction of non-lignocellulosic biomass and, more specifically, sewage sludge at 350 °C and a residence time of 1 h, the concentration of phenols was reduced to 1365 mg/L, according to Liu et al. [29]. Also, following the hydrothermal liquefaction of wastewater in a similar temperature range from 290 °C to 350 °C for 30 min of residence time, the concentration of phenols ranged from 1500 mg/L to 2300 mg/L and there was a decrease in temperature increase according to Basar et al. [28]. On the other hand, following the hydrothermal liquefaction of microalgae Chlorella Kessleri at temperatures of 270 °C, 300 °C, 330 °C, 345 °C, from 270 °C to 300 °C, there was a decrease in the concentration of phenols from 90 mg/L to 63 mg/L; at 330 °C, there was an increase in the value reaching 126 mg/L; and at 345 °C, it increased again to 208 mg/L, according to Alimoradi et al. [30]. The reduction in phenols can be explained by a combination of thermal cracking and polymerization, as shown by the study of Li et al. [31].
The calculation of the high heating value (HHV) revealed that its initial value in SCGs increased through hydrothermal liquefaction, with the results presented in Figure 3. The highest HHV was observed at 350 °C, reaching 32.9 MJ/kg, while the lowest HHV among the hydrothermal liquefaction samples was at 310 °C, measuring 29.7 MJ/kg. According to Yang et al. [4], the HHV of SCGs at 350 °C is 31.0 MJ/kg, noting that the product has a relatively low heating value due to its lower lignocellulose content compared to other biomass types. Initially, the HHV of SCGs was estimated at 21.7 MJ/kg, which is slightly higher than the 19.0 MJ/kg measured by Caetano et al. [32]. As for other biomass species in a similar temperature range, the HHV of hydrochar from corn cob at 310 °C is 26.6 MJ/kg, while at 370 °C, it is 27.6 MJ/kg for a residence time of 1.5 h [33]. On the other hand, hydrochar from olive tree at processed temperatures of 300 °C and 350 °C with a residence time of 1 h amounts to 26.0 MJ/kg and 26.5 MJ/kg, respectively [34]. Similar values to those of hydrochar from SCGs are observed in food waste with a moisture content of more than 70%. More specifically, hydrochar from food waste at 300 °C and a residence time of 1 h presents a calorific value of 31.0 MJ/kg, similar to that of hydrochar from the SCGs300 sample [35]. The fluctuation in the HHV of the solid and liquid HtL products has been discussed by Liakos et al. [36] and has been attributed to the carboxylation reactions that take place in the range of 300–350 °C.
The mass yield of hydrochar as a percentage is presented in Figure 4. It was observed to be 20% or lower at temperatures of 300 °C (20%), 310 °C (8%), and 325 °C (11%). At 350 °C, the hydrochar yield increased to 26%, followed by a slight decrease at 380 °C, where it reached 24%. Also, Saengsuriwong et al. [37] showed similar rates to the hydrothermal liquefaction food waste at temperatures of 280, 310, 340, and the mass yields of the solid product ranged from 10% to 20% w/w. Still, following the hydrothermal liquefaction of sewage sludge by Shah et al. [38] at temperatures of 350 °C and 400 °C, the mass yield of the solid ranged from 10–12%.
Measurements of the COD are presented in Figure 5, showing an increase up to 310 °C. At this temperature, the highest COD concentration among all the samples was observed, reaching 28,012.2 mg/L. At the next temperature (325 °C), there was a slight decrease, with a concentration of 23,816.4 mg/L. According to Chen et al. [39], bio-crude oil from rice straw at 320 °C has a COD of 29,020 mg/L. At high temperatures, specifically of 300 °C and above, the COD is elevated because thermochemical processes, such as repolymerization reactions, occur, increasing the concentrations of organic compounds (e.g., ketones, amines, hydrocarbons). This enhances the yield of bio-crude oil and the organic load in the liquid phase [40]. Subsequently, a sharp decrease in COD concentration (13,949.4 mg/L) was observed at 350 °C, likely due to an increase in hydrochar yield. This temperature also corresponded to the lowest COD concentration. Following this, there was a slight rise in concentration at 380 °C, reaching 17,382 mg/L. In comparison with non-lignocellulosic biomass, the COD of sludge from hydrothermal liquefaction, specifically at 330 °C, showed a concentration ranging from 5458 mg/L to 7265 mg/L [41]. Also, concentrations of other biomass species such as bio-crude from the hydrothermal liquefaction of sorghum bagasse at 300 °C and at 350 °C COD ranged from 20,000 mg/L to 26,000 mg/L [42].
The measurements of volatile fatty acids (VFAs) are presented in Figure 6. The VFAs produced included acetic acid, formic acid, propionic acid, isobutyric acid, butyric acid, and isovaleric acid. The highest concentrations of VFAs were observed at 300 °C, with acetic acid at 541 mg/L, formic acid at 68 mg/L, propionic acid at 156 mg/L, and isovaleric acid at 41 mg/L standing out at this temperature. Subsequent temperatures showed a rapid decrease in VFA concentrations, except at 350 °C, where there was a slight increase in acetic acid and propionic acid, reaching 126 mg/L and 75 mg/L, respectively. Also, according to Basar et al. [28], in the hydrothermal liquefaction of wastewater at temperatures from 290 °C to 350 °C, a decrease in the concentration of VFAs to 325 °C was observed; there was a considerable increase in valeric acid. This increase may have occurred from the possible decomposition of a longer-chain volatile fatty acid at this temperature point [28]. In this case, as well as in the case of hydrochar breakdown, organic products are released in the liquid phase and help in the secondary production of acetic acid and propionic acid, which embody the highest concentrations at all temperatures compared to other volatile fatty acids. The above two cases can explain the significant increase in the two VFAs. The reduced production of VFAs at higher temperatures is likely due to the formation of bio-crude oil from SCGs, which consist predominantly of long-chain carboxylic acids as the temperature increases [4].
The determination of fatty acid methyl ester (FAME) concentrations, carried out via gas chromatography, revealed high concentrations, as presented in Figure 7. A wide range of FAME production was observed, with concentrations increasing as the temperature rose. In particular, the highest concentrations were found for methyl butyrate, methyl hexanoate, methyl undecanoate, methyl palmitate, methyl stearate, methyl oleate (cis-9), and methyl linoleate. The highest concentrations were observed at temperatures of 325 °C, 350 °C, and 380 °C, with the last temperature standing out significantly. This indicates that as the temperature increases, the production of long-chain carboxylic acids also increases, consistent with findings from Yang et al. [4], where bio-crude oil from SCGs at 275 °C primarily contained lighter fatty acids such as n-Hexadecanoic acid, octadecanoic acid, 9–12 octadecadienoic acid, and octanoic acid. According to Caetano et al. [32], the lipid content, which is directly related to fatty acids, is higher when the coffee grounds are dry. Also, according to Tang et al. [43], hydrothermal liquefaction experiments of microalgae Scenedesmus obliquus at temperatures of 290 °C, 305 °C, 320 °C, 335 °C and 350 °C showed that in the production of fatty acids, the largest percentage comprised the fatty acids of C16–C18, more specifically, hexadenoic acid, palmitic acid, octadecadienoic acid, octadecenoic acid, stearic acid, eicosanoic acid. These fatty acids accounted for 80% of the total production of fatty acids. It was also observed that fatty acids formed at lower temperatures were retained in bio-crude oil at higher temperatures [43].

4. Discussion

Within the scope of the meta-analysis of the returned yields from the hydrothermal treatment of SCGs, studies reveal that although the main constituents of spent coffee grounds (SCGs) are relatively consistent—typically around 50 wt% combined cellulose and hemicellulose, and roughly 20 wt% each of lignin and protein—the minor constituents such as oil (ranging from approximately 7–15 wt%) can vary with factors like coffee type and geographical origin. In a recycling study by Leow et al., coffee oil was extracted via reflux using different solvents with yields of about 10%, and the use of acetone-extracted SCGs as fillers in epoxy composites led to enhanced mechanical properties, with a tensile strength of 23.4 MPa, a flexural modulus of 3.02 GPa, and a flexural strength of 42.9 MPa, suggesting that even small variations in SCG composition can have measurable impacts on process outcomes [3,44].
Taking into account recent studies, hydrochar has emerged as a versatile and sustainable material with a wide range of applications. It can serve as a solid fuel and enhance the energy value of biomass, while its tailored surface chemistry makes it an effective adsorbent for the removal of heavy metals, organic contaminants, and toxins from wastewater. Moreover, hydrochar finds utility as a soil amendment that improves soil fertility and water retention, and it is increasingly being explored as a catalyst support in chemical processes, as well as for applications in energy storage and carbon sequestration. These multifunctional uses not only contribute to waste valorization but also address critical environmental and energy challenges by converting waste into value-added products. Although hydrochar shows promise as a sustainable material, its widespread adoption is hindered by reactor design challenges, issues in scale-up, and secondary waste management—including the treatment of contaminated process water—which complicate industrial implementation [45,46]. Furthermore, considerable variability in feedstock compositions and experimental conditions leads to inconsistent product characteristics and a lack of standardized methodologies, which limits its predictable performance in applications such as adsorption and catalysis [47,48].
The hydrothermal liquefaction of spent coffee grounds (SCGs) has proven to be an effective method for producing high-quality bio-crude oil that contains significant concentrations of fatty acid methyl esters (FAMEs). This bio-crude oil has the potential to serve as a valuable feedstock for the production of renewable fuels, contributing to the global transition toward sustainable energy sources. In addition to the liquid fraction, the process also generates the solid by-product hydrochar. This material exhibits a remarkable heating value, making it a promising candidate for use as a solid biofuel or as a precursor for advanced carbon-based materials. Due to their inherent oxygen content, FAMEs support cleaner combustion by reducing particulate matter emissions when used in high-speed diesel engines while offering a renewable alternative to fossil fuels. Their established production process and compatibility with existing diesel engines underscore FAMEs’ potential for widespread application in transportation and power generation, as presented by Kim et al. [49]. The authors also mention the application of solvent extraction for the recovery of FAMEs. Nonetheless, the efficiency of extraction solvents can be further examined with the development of relevant models based on COSMO or COSMO-RS [50].
The combined production of bio-crude oil and hydrochar highlights the potential of hydrothermal liquefaction as a versatile and efficient process for valorizing SCGs, reducing waste while simultaneously generating multiple forms of renewable energy. Finally, volatile fatty acids (VFAs) serve as critical intermediates in the conversion of biomass to bio-crude, being rapidly produced during anaerobic digestion and subsequently transformed into fuels and high-value chemicals. Their ability to be derived from diverse waste biomass—without competing with food resources—presents a sustainable, cost-effective platform for biofuel production and various chemical syntheses [51].
To optimize the utilization of bio-crude oil, future research should focus on the development and implementation of a biorefinery capable of upgrading bio-crude into high-purity biodiesel. This step is crucial for the commercial exploitation of hydrothermal liquefaction products and their integration into the renewable fuels market. Another promising research area involves the activation of hydrochar and its application as a catalyst in the recovery of more qualitative and volatile fractions of bio-crude oil. The proper modification of hydrochar could significantly enhance process efficiency, leading to the production of high-value fuels with improved properties. Furthermore, it is essential to investigate the gaseous products generated during hydrothermal liquefaction, with particular emphasis on carbon dioxide (CO2). Understanding the impact of CO2 on both liquid and solid fuels could reveal new mechanisms influencing the thermochemical conversion of biomass. By studying these processes, it will be possible to optimize production conditions to yield high-value fuels with a lower environmental footprint.

5. Conclusions

Experiments on hydrothermal liquefaction were conducted using a Parr 4577A hydrothermal reactor at temperatures of 300 °C, 310 °C, 325 °C, 350 °C, and 380 °C. The material used was SCGs, which have a high moisture content (63%). After the experiments, analyses were performed on the liquid and solid products of hydrothermal liquefaction. Parameters such as pH, total phenols, COD, volatile fatty acids (VFAs), FAMEs, mass yield of hydrochar, and the high heating value (HHV) of the solid product were measured. VFAs and FAMEs were analyzed using a Shimadzu Nexis 2030 plasma gas chromatograph with Agilent HP-FFAP and MEGA 10 columns, respectively. The pH of the samples was acidic, likely due to the high production of carboxylic acids. Phenols were present in low concentrations across all samples, with the highest value occurring at 300 °C (1180.1 mg/L) and the lowest at 310 °C (365.6 mg/L). Phenol concentrations generally decreased as the temperature increased. COD showed an increase at the first three temperatures, followed by a decrease at 350 °C, likely due to the increased hydrochar yield at this temperature. The HHV peaked at 350 °C, reaching 32.9 MJ/kg. Additionally, VFAs were found in very low concentrations at all temperatures, with the highest concentrations observed at 300 °C. The VFAs produced included acetic acid, formic acid, propionic acid, isobutyric acid, butyric acid, and isovaleric acid. The reduced production of VFAs coincided with the formation of long-chain carboxylic acids. It was observed that higher temperatures led to greater concentrations of FAMEs in the bio-crude oil, with methyl butyrate, methyl hexanoate, methyl undecanoate, methyl palmitate, methyl stearate, methyl oleate (cis-9), and methyl linolenate having the highest concentrations. More specifically, by observing the production of VFAs and FAMEs, it was realized that the increase in temperature and pressure conditions triggered more intense decarboxylation and carboxylation reactions, causing a transition from short-chain carboxylic acids to fatty acids.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/en18082094/s1.

Author Contributions

Conceptualization: S.V.; Data curation: D.L. and S.V.; Formal Analysis: D.L. and S.V.; Investigation: D.L., G.A. and S.V.; Methodology: G.A., S.M. and S.V.; Project administration: G.A., S.M. and S.V.; Resources: S.M. and S.V.; Supervision: S.M. and S.V.; Validation: G.A. and S.V.; Writing—original draft: D.L. Writing—review and editing: S.V. All authors have read and agreed to the published version of the manuscript.

Funding

The research was funded by the project TRINEFLEX, which has received funding from the European Union’s Horizon Europe research and innovation program under Grant Agreement No 101058174 “TrineFlex”.

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

CODChemical Oxygen Demand
°CDegrees of Celsius
FAMEsFatty Acid Methyl Esters
HHVHigh Heating Value
HtLHydrothermal Liquefaction
MJ/kgMegajoule per kilogram
mg/LMilligram per liter
%Percentage
SCGsSpent Coffee Grounds
TSTotal Solids
VFAsVolatile Fatty Acids
VSVolatile Solids

References

  1. Kovalcik, A.; Obruca, S.; Marova, I. Valorization of spent coffee grounds: A review. Food Bioprod. Process. 2018, 110, 104–119. [Google Scholar] [CrossRef]
  2. Forcina, A.; Petrillo, A.; Travaglioni, M.; di Chiara, S.; De Felice, F. A comparative life cycle assessment of different spent coffee ground reuse strategies and a sensitivity analysis for verifying the environmental convenience based on the location of sites. J. Clean. Prod. 2023, 385, 135727. [Google Scholar] [CrossRef]
  3. McNutt, J.; He, Q.S. Spent Coffee Grounds: A Review on Current Utilization. J. Ind. Eng. Chem. 2019, 71, 78–88. [Google Scholar] [CrossRef]
  4. Yang, L.; Nazari, L.; Yuan, Z.; Corscadden, K.; Xu, C.C. Hydrothermal liquefaction of spent coffee grounds in water medium for bio-oil production. Biomass Bioenergy 2016, 86, 191–198. [Google Scholar] [CrossRef]
  5. Saberian, M.; Li, J.; Donnoli, A.; Bonderenko, E.; Oliva, P.; Gill, B.; Lockrey, S.; Siddique, R. Recycling of spent coffee grounds in construction materials: A review. J. Clean. Prod. 2021, 289, 125837. [Google Scholar] [CrossRef]
  6. Battista, F.; Barampouti, E.M.; Mai, S.; Bolzonella, D.; Malamis, D.; Moustakas, K.; Loizidou, M. Added-value molecules recovery and biofuels production from spent coffee grounds. Renew. Sustain. Energy Rev. 2020, 131, 110007. [Google Scholar] [CrossRef]
  7. Zhang, S.; Yang, J.; Wang, S.; Rupasinghe, H.V. Experimental exploration of processes for deriving multiple products from spent coffee grounds. Food Bioprod. Process. 2021, 128, 21–29. [Google Scholar] [CrossRef]
  8. Kim, J.; Kim, H.; Baek, G.; Lee, C. Anaerobic co-digestion of spent coffee grounds with different waste feedstocks for biogas production. Waste Manag. 2017, 60, 322–328. [Google Scholar] [CrossRef]
  9. Kang, S.B.; Oh, H.Y.; Kim, J.J.; Choi, K.S. Characteristics of spent coffee ground as a fuel and combustion test in a small boiler (6.5 kW). Renew. Energy 2017, 113, 1208–1214. [Google Scholar] [CrossRef]
  10. Marx, S.; Venter, R.; Karmee, S.K.; Louw, J.; Truter, C. Biofuels from spent coffee grounds: Comparison of processing routes. Biofuels 2022, 13, 537–543. [Google Scholar] [CrossRef]
  11. Kim, H.J.; Oh, S.C. Hydrothermal carbonization of spent coffee grounds. Appl. Sci. 2021, 11, 6542. [Google Scholar] [CrossRef]
  12. Vakalis, S.; Moustakas, K.; Benedetti, V.; Cordioli, E.; Patuzzi, F.; Loizidou, M.; Baratieri, M. The “COFFEE BIN” concept: Centralized collection and torrefaction of spent coffee grounds. Environ. Sci. Pollut. Res. 2019, 26, 35473–35481. [Google Scholar] [CrossRef]
  13. Afolabi, O.O.; Sohail, M.; Cheng, Y.L. Optimisation and characterization of hydrochar production from spent coffee grounds by hydrothermal carbonisation. Renew. Energy 2020, 147, 1380–1391. [Google Scholar] [CrossRef]
  14. Hu, Y.; Gallant, R.; Salaudeen, S.; Farooque, A.A.; He, S. Hydrothermal carbonization of spent coffee grounds for producing solid fuel. Sustainability 2022, 14, 8818. [Google Scholar] [CrossRef]
  15. Bodur, F.G.; Güngören-Madenoğlu, T.; Özdemir, G.; Ballice, L.; Kabay, N. Biochemical and biohythane production from anaerobically digested water plant by hydrothermal liquefaction/gasification. Int. J. Hydrogen Energy 2024, 52, 183–201. [Google Scholar] [CrossRef]
  16. Thomsen, L.B.S.; Anastasakis, K.; Biller, P. Hydrothermal liquefaction potential of wastewater treatment sludges: Effect of wastewater treatment plant and sludge nature on products distribution. Fuel 2024, 355, 129525. [Google Scholar] [CrossRef]
  17. Wei, C.; Xu, Y.; Li, Y.; Wei, W.; Feng, Y.; Li, Z.; Xu, L. Life-cycle assessment of microalgae liquid biofuel production in biofilm cultivation system via conversion technologies of transesterification, hydrothermal liquefaction and pyrolysis. J. Clean. Prod. 2024, 436, 140559. [Google Scholar] [CrossRef]
  18. Ni, J.; Qian, L.; Wang, Y.; Zhang, B.; Gu, H.; Hu, Y.; Wang, Q. A review on fast hydrothermal liquefaction of biomass. Fuel 2022, 327, 125135. [Google Scholar] [CrossRef]
  19. Liu, G.; Chen, F.; Cao, K.; Jin, B.; Rao, L.; Jin, X.; He, F.; Huang, Q. Fe-catalyzed low-temperature hydrothermal liquefaction of coffee shells: Influence of operating parameters on bio-oil yield, distribution of bio-oil products. J. Energy Inst. 2024, 113, 101525. [Google Scholar] [CrossRef]
  20. Atabani, A.E.; Ali, I.; Naqvi, S.R.; Badruddin, I.A.; Aslam, M.; Mahmoud, E.; Almomani, F.; Juchelková, D.; Atelge, M.R.; Khan, T.M.Y. A state-of-the-art review on spent coffee ground (SCG) pyrolysis for future biorefinery. Chemosphere 2022, 286, 131730. [Google Scholar] [CrossRef]
  21. Sklavos, S.; Gatidou, G.; Stasinakis, A.S.; Haralambopoulos, D. Use of solar distillation for olive mill wastewater drying and recovery of polyphenolic compounds. J. Environ. Manag. 2015, 162, 46–52. [Google Scholar] [CrossRef] [PubMed]
  22. APHA. Standard Methods for the Examination of Water and Wastewater, 20th ed.; American Public Health Association, Port City Press: Baltimore, MD, USA, 1998. [Google Scholar]
  23. Osorio-Arias, J.; Delgado-Arias, S.; Cano, L.; Zapata, S.; Quintero, M.; Nuñez, H.; Ramírez, C.; Simpson, R.; Vega-Castro, O. Sustainable management and valorization of spent coffee grounds through the optimization of thin layer hot air-drying process. Waste Biomass Valorization 2020, 11, 5015–5026. [Google Scholar] [CrossRef]
  24. Yang, J.; Niu, H.; Corscadden, K.; He, Q.; Zhou, N. MW-assisted hydrothermal liquefaction of spent coffee grounds. Can. J. Chem. Eng. 2022, 100, 1729–1738. [Google Scholar] [CrossRef]
  25. Yang, L.; Havard, P.; Corscadden, K.; Xu, C.C.; Wang, X. Co-liquefaction of spent coffee grounds and lignocellulosic feedstocks. Bioresour. Technol. 2017, 237, 108–121. [Google Scholar] [CrossRef]
  26. Muller, L.C.; Pearson, J.; Louw, J.; Marx, S.; Kumar Karmee, S. Hydrothermal liquefaction of spent coffee grounds followed by biocatalytic upgradation to produce biofuel: A circular economy approach. Biofuels 2022, 13, 779–788. [Google Scholar] [CrossRef]
  27. Vakalis, S.; Georgiou, A.; Moustakas, K.; Fountoulakis, M. Assessing the effect of hydrothermal treatment on the volatile solids content and the biomethane potential of common reed (Phragmites australis). Bioresour. Technol. Rep. 2022, 17, 100923. [Google Scholar] [CrossRef]
  28. Basar, I.A.; Liu, H.; Eskicioglu, C. Incorporating hydrothermal liquefaction into wastewater treatment–Part III: Aqueous phase characterization and evaluation of on-site treatment. Chem. Eng. J. 2023, 467, 143422. [Google Scholar] [CrossRef]
  29. Liu, H.; Basar, I.A.; Eskicioglu, C. Hydrothermal liquefaction for sludge-to-energy conversion: An evaluation of biocrude production and management of waste streams. Energy 2023, 281, 128268. [Google Scholar] [CrossRef]
  30. Alimoradi, S.; Stohr, H.; Stagg-Williams, S.; Sturm, B. Effect of temperature on toxicity and biodegradability of dissolved organic nitrogen formed during hydrothermal liquefaction of biomass. Chemosphere 2020, 238, 124573. [Google Scholar] [CrossRef]
  31. Li, X.; Liang, J.; Zhang, L.; Zhang, S.; Xu, L.; Wang, Y.; Hu, X. Coke Formation in the Co-production of Hydrogen and Phenols from Pyrolysis-Reforming of Lignin. Int. J. Hydrogen Energy 2022, 47, 32835–32848. [Google Scholar] [CrossRef]
  32. Caetano, N.S.; Silva, V.F.; Melo, A.C.; Martins, A.A.; Mata, T.M. Spent coffee grounds for biodiesel production and other applications. Clean Technol. Environ. Policy 2014, 16, 1423–1430. [Google Scholar] [CrossRef]
  33. Zhang, L.; Wang, Q.; Wang, B.; Yang, G.; Lucia, L.A.; Chen, J. Hydrothermal carbonization of corncob residues for hydrochar production. Energy Fuels 2015, 29, 872–876. [Google Scholar] [CrossRef]
  34. Duman, G.; Balmuk, G.; Cay, H.; Kantarli, I.C.; Yanik, J. Comparative evaluation of torrefaction and hydrothermal carbonization: Effect on fuel properties and combustion behavior of agricultural wastes. Energy Fuels 2020, 34, 11175–11185. [Google Scholar] [CrossRef]
  35. Saqib, N.U.; Baroutian, S.; Sarmah, A.K. Physicochemical, structural and combustion characterization of food waste hydrochar obtained by hydrothermal carbonization. Bioresour. Technol. 2018, 266, 357–363. [Google Scholar] [CrossRef] [PubMed]
  36. Liakos, D.; Altiparmaki, G.; Moustakas, K.; Malamis, S.; Stergios Vakalis, S. The fate of Volatile Fatty Acids (VFAs) during the thermodynamic transition from hydrothermal carbonization to hydrothermal liquefaction: HtC-to-HtL. Sustain. Chem. Pharm. 2024, 41, 101683. [Google Scholar] [CrossRef]
  37. Saengsuriwong, R.; Onsree, T.; Phromphithak, S.; Tippayawong, N. Biocrude oil production via hydrothermal liquefaction of food waste in a simplified high-throughput reactor. Bioresour. Technol. 2021, 341, 125750. [Google Scholar] [CrossRef]
  38. Shah, A.A.; Toor, S.S.; Conti, F.; Nielsen, A.H.; Rosendahl, L.A. Hydrothermal liquefaction of high ash containing sewage sludge at sub and supercritical conditions. Biomass Bioenergy 2020, 135, 105504. [Google Scholar] [CrossRef]
  39. Chen, H.; Zhang, C.; Rao, Y.; Jing, Y.; Luo, G.; Zhang, S. Methane potentials of wastewater generated from hydrothermal liquefaction of rice straw: Focusing on the wastewater characteristics and microbial community compositions. Biotechnol. Biofuels 2017, 10, 140. [Google Scholar] [CrossRef]
  40. Lachos-Perez, D.; Torres-Mayanga, P.C.; Abaide, E.R.; Zabot, G.L.; De Castilhos, F. Hydrothermal carbonization and liquefaction: Differences, progress, challenges, and opportunities. Bioresour. Technol. 2022, 343, 126084. [Google Scholar] [CrossRef]
  41. Song, H.; Yang, T.; Li, B.; Tong, Y.; Li, R. Hydrothermal liquefaction of sewage sludge into biocrude: Effect of aqueous phase recycling on energy recovery and pollution mitigation. Water Res. 2022, 226, 119278. [Google Scholar] [CrossRef]
  42. Bi, Z.; Zhang, J.; Peterson, E.; Zhu, Z.; Xia, C.; Liang, Y.; Wiltowski, T. Biocrude from pretreated sorghum bagasse through catalytic hydrothermal liquefaction. Fuel 2017, 188, 112–120. [Google Scholar] [CrossRef]
  43. Tang, S.; Shi, Z.; Tang, X.; Yang, X. Hydrotreatment of biocrudes derived from hydrothermal liquefaction and lipid extraction of the high-lipid Scenedesmus. Green Chem. 2019, 21, 3413–3423. [Google Scholar] [CrossRef]
  44. Leow, Y.; Yew, P.Y.M.; Chee, P.L.; Loh, X.J.; Kai, D. Recycling of Spent Coffee Grounds for Useful Extracts and Green Composites. RSC Adv. 2021, 11, 2682–2692. [Google Scholar] [CrossRef]
  45. Islam, M.T.; Sultana, A.I.; Chambers, C.; Saha, S.; Saha, N.; Kirtania, K.; Reza, M.T. Recent Progress on Emerging Applications of Hydrochar. Energies 2022, 15, 9340. [Google Scholar] [CrossRef]
  46. Ighalo, J.O.; Akaeme, F.C.; Georgin, J.; de Oliveira, J.S.; Franco, D.S.P. Biomass Hydrochar: A Critical Review of Process Chemistry, Synthesis Methodology, and Applications. Sustainability 2025, 17, 1660. [Google Scholar] [CrossRef]
  47. Masoumi, S.; Borugadda, V.B.; Nanda, S.; Dalai, A.K. Hydrochar: A Review on Its Production Technologies and Applications. Catalysts 2021, 11, 939. [Google Scholar] [CrossRef]
  48. Fang, J.; Zhan, L.; Ok, Y.S.; Gao, B. Minireview of Potential Applications of Hydrochar Derived from Hydrothermal Carbonization of Biomass. J. Ind. Eng. Chem. 2018, 57, 15–21. [Google Scholar] [CrossRef]
  49. Kim, N.-J.; Lim, S.-J.; Chang, H.N. Volatile Fatty Acid Platform: Concept and Application. In Emerging Areas in Bioengineering, 1st ed.; Chang, H.N., Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2018; pp. 172–191. [Google Scholar]
  50. Rezaei Motlagh, S.; Harun, R.; Radiah Awang Biak, D.; Hussain, S.A.; A. Elgharbawy, A.; Khezri, R.; Wilfred, C.D. Prediction of Potential Ionic Liquids (ILs) for the Solid–Liquid Extraction of Docosahexaenoic Acid (DHA) from Microalgae Using COSMO-RS Screening Model. Biomolecules 2020, 10, 1149. [Google Scholar] [CrossRef]
  51. Islam, M.A.; Rainey, T.J.; Ristovski, Z.; Brown, R.J. Performance and Exhaust Emissions of Diesel Engines Using Microalgae FAME and the Prospects for Microalgae HTL Biocrude. Renew. Sustain. Energy Rev. 2018, 82, 4269–4278. [Google Scholar] [CrossRef]
Energies 18 02094 g001
Figure 1. pH values for the HTL liquid fraction for temperatures of hydrothermal liquefaction between 300 and 380 °C.
Figure 1. pH values for the HTL liquid fraction for temperatures of hydrothermal liquefaction between 300 and 380 °C.
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Energies 18 02094 g002
Figure 2. Total phenol concentration for the HtL liquid fraction for temperatures of hydrothermal liquefaction between 300 and 380 °C.
Figure 2. Total phenol concentration for the HtL liquid fraction for temperatures of hydrothermal liquefaction between 300 and 380 °C.
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Energies 18 02094 g003
Figure 3. High heating value of the HtL solid fraction (hydrochar) for temperatures of hydrothermal liquefaction between 300 and 380 °C.
Figure 3. High heating value of the HtL solid fraction (hydrochar) for temperatures of hydrothermal liquefaction between 300 and 380 °C.
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Energies 18 02094 g004
Figure 4. Mass yield of hydrochar (solid fraction) for different conditions of HtL treatment for temperatures of hydrothermal liquefaction between 300 and 380 °C.
Figure 4. Mass yield of hydrochar (solid fraction) for different conditions of HtL treatment for temperatures of hydrothermal liquefaction between 300 and 380 °C.
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Energies 18 02094 g005
Figure 5. Concentrations of chemical oxygen demand (COD) for the HtL liquid fraction for temperatures of hydrothermal liquefaction between 300 and 380 °C.
Figure 5. Concentrations of chemical oxygen demand (COD) for the HtL liquid fraction for temperatures of hydrothermal liquefaction between 300 and 380 °C.
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Energies 18 02094 g006
Figure 6. Volatile fatty acid concentrations for the HtL liquid fraction for temperatures of hydrothermal liquefaction between 300 and 380 °C.
Figure 6. Volatile fatty acid concentrations for the HtL liquid fraction for temperatures of hydrothermal liquefaction between 300 and 380 °C.
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Energies 18 02094 g007
Figure 7. Concentrations of fatty acid methyl esters for the HtL liquid fraction for temperatures of hydrothermal liquefaction between 300 and 380 °C.
Figure 7. Concentrations of fatty acid methyl esters for the HtL liquid fraction for temperatures of hydrothermal liquefaction between 300 and 380 °C.
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Table 1. Conditions for conducting the HTL experiments.
Table 1. Conditions for conducting the HTL experiments.
SamplesTemperature (°C)Pressure (bar)Resident Time (min)
SCGs30030084.330
SCGs31031097.430
SCGs325325118.830
SCGs350350163.530
SCGs38038089.830
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MDPI and ACS Style

Liakos, D.; Altiparmaki, G.; Malamis, S.; Vakalis, S. Hydrothermal Liquefaction for Biofuel Synthesis: Assessment of VFA (Volatile Fatty Acid) and FAME (Fatty Acid Methyl Ester) Profiles from Spent Coffee Grounds. Energies 2025, 18, 2094. https://doi.org/10.3390/en18082094

AMA Style

Liakos D, Altiparmaki G, Malamis S, Vakalis S. Hydrothermal Liquefaction for Biofuel Synthesis: Assessment of VFA (Volatile Fatty Acid) and FAME (Fatty Acid Methyl Ester) Profiles from Spent Coffee Grounds. Energies. 2025; 18(8):2094. https://doi.org/10.3390/en18082094

Chicago/Turabian Style

Liakos, Dimitrios, Georgia Altiparmaki, Simos Malamis, and Stergios Vakalis. 2025. "Hydrothermal Liquefaction for Biofuel Synthesis: Assessment of VFA (Volatile Fatty Acid) and FAME (Fatty Acid Methyl Ester) Profiles from Spent Coffee Grounds" Energies 18, no. 8: 2094. https://doi.org/10.3390/en18082094

APA Style

Liakos, D., Altiparmaki, G., Malamis, S., & Vakalis, S. (2025). Hydrothermal Liquefaction for Biofuel Synthesis: Assessment of VFA (Volatile Fatty Acid) and FAME (Fatty Acid Methyl Ester) Profiles from Spent Coffee Grounds. Energies, 18(8), 2094. https://doi.org/10.3390/en18082094

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