Bibliometric Analysis of Hydrothermal Co-Processing of Biomass for Energy Generation

Abstract

Waste-to-energy technology plays a crucial role in advancing the circular economy framework, a strategy that contributes to achieving the United Nations Sustainable Development Goals on responsible consumption and production, as well as the provision of affordable and clean energy. Hydrothermal co-liquefaction has emerged as a promising technology for addressing waste material challenges by converting them into valuable biofuels. This review focuses on biomass feedstock classification and provides an overview of hydrothermal co-liquefaction for sustainable waste management and improved energy production. Moreover, the article provides details on integrating other waste treatment methods with hydrothermal liquefaction to promote the circular economy. Research publications from 2015 to 2025 were obtained from Web of Science and Scopus to identify research trends and output across countries and map out future research directions. The retrieved data from Web of Science was analysed for mapping research, keyword occurrence, and network analysis using VOSviewer software. The study highlighted that waste treatment techniques not only mitigate environmental pollution but also provide a sustainable pathway for energy production and contribute to global carbon neutrality. The review shows that biocrude yield varies with blending ratio because of differences in the biochemical composition of feedstocks, which affect reaction pathways and lead to synergistic or antagonistic interactions during co-processing. Therefore, careful selection of biomass feedstock is essential to achieve optimal results.

1. Introduction

Increasing demand for clean and sustainable energy, together with the environmental pollution caused by the use of fossil fuels, has heightened the need for renewable, eco-friendly energy sources [1]. Clean and renewable energy sources include wind, solar, geothermal, hydroelectric, nuclear, and biomass energy [2]. Among these, biomass such as agricultural residues, food waste, and sewage sludge is a promising feedstock due to its abundance, availability, and high organic content [3]. Globally, biomass accounts for approximately 4% of primary energy consumption and is the fourth most consumed energy source after fossil fuels and nuclear energy [4]. It is an effective and efficient energy source that can replace fossil fuels, given the large amounts of agricultural and forestry residues produced globally [5]. Biomass energy offers advantages over fossil fuels because it is renewable, environmentally friendly, produces low levels of heavy metals and sulfur, and is economical [6,7,8].

Hydrothermal liquefaction (HTL) is an effective thermochemical process that converts biomass into energy-rich biocrude without drying the biomass. Waste products from wastewater treatment plants, food processing industries, agricultural production, and processing can be utilised in HTL [9]. The products of HTL include biofuel oil, aqueous phase, and solid content (hydrochar); biofuel may be upgraded into transportation fuels and other chemicals, while hydrochar can be used as fuel, fertiliser, or processed as an adsorbent, and the aqueous phase can be recirculated back into HTL and used as anaerobic digestion feedstock or for growing microalgae [5]. Biofuels are liquid fuels derived from biomass, considered carbon-neutral, with little or no greenhouse gas emissions compared to fossil fuels [10,11]. Biofuels consist of acids, alcohols, phenols, and oligomers, and are categorised into generations based on the feedstock used [12]. First-generation biofuels are those processed from edible crops such as corn, sugarcane, soybean oil, palm oil, sunflower oil, and wheat [13]. The concerns with first-generation biofuels are food security, land availability, and sustainability, as they compete with valuable resources that could otherwise be used for food production. Second-generation biofuels are derived from agricultural waste residues (lignocellulosic biomasses) such as rice straw, sugarcane bagasse, cassava stem, wheat straw, and maize cob [14]. Third-generation biofuel is produced from non-food and photosynthetic biomass, e.g., microalgae [15]. This addresses food security concerns and can be cultivated in wastewater. The biofuels produced in the international market are bioethanol, biobutanol, and biodiesel [10]. The most important factor that influences the properties of biofuel is the composition of biomass [16]. Other factors include operating temperature, reaction time, and solvent. Biofuel derived from plants is biodegradable, renewable, and environmentally friendly compared to conventional fossil fuels [17,18,19]. In contrast, it has higher viscosity and density than petroleum-based fuels, leading to poor atomization and flow characteristics [20]. The higher heating value (HHV) of HTL biofuel is also lower than that of crude oil [5,21]. Recently, hydrothermal co-liquefaction has garnered growing scientific attention as a promising pathway for producing bio-crude from various biomass feedstocks. Over the last decade, the number of publications in this field has increased significantly, reflecting interest in optimising reaction parameters, exploring feedstock synergies, and enhancing both the quality and quantity of biocrude. However, the broad expansion of the literature poses challenges for studies that are fragmented across different biomass feedstocks, process parameters, and analytical methodologies. This study conducted a bibliometric analysis of the literature on hydrothermal co-liquefaction and biocrude production from biomass between 2015 and 2025 to examine publication trends, research output across countries, keyword analysis, and research clusters over this period. The review will help readers gain insight into the hydrothermal liquefaction process and identify the latest advancements in biomass valorisation using HTL and research areas that can be further explored.

2. Bibliometric Approach

This study was conducted to assess recent progress in the hydrothermal co-liquefaction of organic waste. The Web of Science and Scopus databases were consulted for publications between 2015 and 16 November 2025, and only articles and review articles published in English were included. The publication trend obtained from the databases (Web of Science and Scopus) is shown in Figure 1. Upon screening, the cumulative number of published documents obtained from Web of Science and Scopus is 5953 and 3904, respectively, indicating a steady increase in research interest in waste-to-energy technology. Research areas of interest are limited to Engineering, Chemistry, and Energy Fuels. Special focus was given to studies that contribute to the Sustainable Development Goals (SDGs) 6 (Clean Water and Sanitation), 7 (Affordable and Clean Energy), 12 (Responsible Consumption and Production), 13 (Climate Action), and 11 (Sustainable Cities and Communities).

Table 1. Publication outputs of some countries from the Web of Science and Scopus search engines.

Country	Scopus	Web of Science
United States	1104	1117
China	978	2356
Spain	625	1016
Italy	534	884
United Kingdom	466	947
Germany	461	795
Sweden	256	566
Netherlands	159	522
India	152	558
France	128	567
Australia	122	377
Brazil	104	553
Canada	102	405
Denmark	71	271
South Korea	69	320
Poland	68	628
Finland	66	316
Switzerland	57	191
South Africa	52	201
Austria	47	223
Greece	42	168
Pakistan	41	208
Japan	40	291
Norway	37	215
Mexico	36	190
Czech Republic	32	130
Ireland	30	117
Turkey	29	120
Colombia	27	161
Egypt	20	156
Ethiopia	19	50
Nigeria	18	121
Morocco	14	44
Ghana	11	38
Kenya	11	28

presents research output by country of publication, with China and the USA playing dominant roles in advancing research in this area. In contrast, African countries, including South Africa, have comparatively fewer studies in the field in the last decade.

Keyword Analysis and Research Themes

To evaluate research trends in hydrothermal co-liquefaction within the academic community, co-occurrence analysis of keywords was performed using VOSviewer (v1.6.20). In total, 4582 keywords were obtained from the 5953 published articles that emerged from Web of Science. The minimum number of occurrences of the authors’ keywords was set at five, resulting in 290 keywords linked to others. Figure 2 shows an overlay visual of the keyword distribution across the average publication year from 2021 to 2023. Biomass was the most frequently used keyword, with 586 occurrences and 2098 links to other keywords. Pyrolysis was the second most linked keyword, with 108 occurrences and 522 links to other keywords. Others included circular economy (120 occurrences, 522 links), conversion (85 occurrences, 396 links), energy (78 occurrences, 386 links), lignocellulosic biomass (77 occurrences, 380 links), bio-oil (64 occurrences, 367 links), gasification (55 occurrences, 330 links), thermochemical conversion (49 occurrences, 298 links), hydrothermal liquefaction (35 occurrences, 236 links) and others. Keywords with fewer occurrences (10) with link strength (<30) included biogas, biomass waste, catalytic conversion, blend, etc.

The obtained keywords were further processed to identify themes related to biocrude energy production via hydrothermal liquefaction. Figure 3 shows the five main clusters of keywords in networks, with distinct colour codes. Cluster 1 (Red) shows the prevalence of topics that discuss the circular economy. This cluster comprises 164 keywords related to research on energy system integration, the circular bioeconomy model, and sustainability assessment, indicating that research efforts are now increasingly focused on resource reuse and circular economy principles. Cluster 2 (Green), with 155 items linked to lignocellulosic biomass, highlights the most prominent pretreatment methods and biomass conversion routes. It reveals research studies related to biorefinery technologies, the utilisation of agricultural waste, and various pathways for biofuel production, highlighting optimal utilisation of biomass resources for fuel, material recovery, and chemical production. Cluster 3 (Blue), with 86 keywords, focuses on the utilisation of waste and environmental management. The major linked keywords are bioenergy, biogas, biodiesel, biomethane, green hydrogen as an alternative energy source, and anaerobic digestion as the most studied conversion technology for their production. Cluster 4 (Yellow) consists of 138 keywords linked to thermochemical conversion technologies (gasification, combustion, and pyrolysis). This indicates a shift towards modelling, optimisation, and co-processing of waste to achieve higher yields. The cluster clearly overlaps with hydrothermal liquefaction, indicating it has been a research hotspot over the last decade. Cluster 5 (Purple), with 271 keywords linked to biomass, focuses on the characterisation of materials and reaction mechanisms. The keywords are linked to thermogravimetric analysis, biochar adsorption properties, and thermal decomposition. This means that researchers must place more emphasis on understanding reaction pathways and reactor design to optimise biofuel production. From the clusters, there is a lack of studies on the reaction mechanism, relating the breakdown of feedstock constituents to biocrude quality. There is also a research gap in integrating pathways from feedstock selection through conversion and upgrading, as well as in conversion processes, life-cycle assessment, and technoeconomic analysis.

3. Biomass and Interaction

Biomass is organic waste that contains chemical energy in the form of carbon and hydrogen, which can be utilised as an energy source [22]. Biomass has been widely investigated as a potential feedstock for biocrude production through thermochemical routes. Valorisation of biomass as an alternative energy source is attracting interest not only due to the finite nature of fossil fuels, but also the rise in global warming associated with the usage of fossil fuels [21]. Biomass-based fuels are a renewable and sustainable alternative to conventional ones [23]. There are several types of biomass, categorised as agricultural waste and residues, forestry residues, food processing wastes, livestock waste, algae, sewage, municipal solid waste, and plastic waste (Figure 4).

3.1. Classification and Characteristics of Biomass

3.1.1. Agricultural Biomass

Agricultural biomass is derived from organic materials, such as plants, crop residues, and urban waste. These materials are readily available, renewable, and can be obtained from agricultural activities. They include corn stalks, corn cobs, rice husks or stalks, wheat straw, sugarcane bagasse, and straw. Agricultural biomass can be used as a source of energy in many ways, depending on its physical and chemical properties and availability. The lignin, cellulose, and hemicellulose content of agricultural biomass makes it a viable feedstock for biocrude production [12,25,26]. Cellulose is a linear polymer, a polysaccharide, composed of glucose and connected by β-glycosidic bonds. These connections help form a crystalline structure in cellulose that can be broken down by thermal treatment at 250–350 °C [23]. The complex structure of cellulose is first hydrolysed into glucose monomers and, at high temperatures, further broken down into aldehydes, furan derivatives, and small molecular acids [27,28]. However, hemicellulose decomposition occurs at lower temperatures, usually between 180 and 350 °C [22]. During hydrothermal liquefaction, hemicellulose undergoes the following reaction pathways: dehydration, oxidation, self-lactonisation, deoxygenation, esterification, and condensation, yielding macromolecular fragments [29]. The decomposition of lignin occurs at higher temperatures within the range of 200 °C to 800 °C due to strong inter- and intramolecular hydrogen bonds induced by hydroxyl and other polar groups [22,23]. Compared with cellulose and hemicellulose, lignin has a more complex structure and a broader molecular-weight distribution [30]. During hydrothermal liquefaction, the lignin structure undergoes hydrolysis, carbon–carbon bond breakdown, benzene ring degradation to form phenolic compounds, and repolymerization leading to coke formation [31]. Condensation must be prevented to ensure successful lignin conversion during hydrothermal liquefaction.

Table 2. Hydrothermal liquefaction of agricultural biomass and the resultant biocrude yield and quality.

Agricultural Biomass	Operating Conditions	Biocrude Yield (wt.%db)	HHV (MJ/kg)	References
Giant juncao grass	T = 250–350 °C, t = 30 min	50	27.7–30.8	[32]
Mustard flour	T  =  280 °C, time  =  30 mi	38	37–39	[33]
Sugarcane bagasse	T = 300–350 °C, time = 0–30 min	36	-	[34]
Tobacco waste	T = 280–340 °C, time = 15–45 min	52	28	[35]
Barley straw	T = 285 °C, time = 45 min	42.70	30.70	[21]
Tomato plant residues (TPR)	T = 220–280 °C, time = 15–60 min	45.1	18.5	[36]
Wheat straw	350–400	32.34	35	[37]
Lactuca scariola plants	T = 220–300 °C, time = 0–30 min.	10.49–12.5	24–30	[38]
Olive oil residues	250–350	38.9–47.8	31.42–35	[39]
Pine wood	T = 280 °C, time = 15 min	24–34	-	[40]
Corn stover	250–375	29.25	35.13	[41]
Sawdust	T = 280 °C, time = 15 min	34.9	31.77	[42]
Sawdust	250	52.3		[43]
Soybean straw	320	15.8	32.98	[44]

provides a summary of biocrude yield and higher heating value from hydrothermal liquefaction of selected agricultural biomass. The results indicate that the composition of agricultural biomass, including lipids, carbohydrates, and proteins, influences reaction pathways, product distribution, and the energy content of biocrude derived from hydrothermal liquefaction, explaining the trade-off between biocrude yield and energy density. Feedstocks such as sugarcane bagasse, wheat straw, corn stover, Lactuca scariola plants, and soybean straw are rich in carbohydrates, mainly cellulose and hemicellulose, whereas feedstocks like pinewood and sawdust have a high lignin content. Generally, feedstocks with high carbohydrate and lignin levels tend to have lower conversion efficiencies, with carbohydrates leading to aqueous-phase products and lignin promoting char formation. Conversely, feedstocks such as mustard flour have high protein levels, which support moderate biocrude yields. It can be concluded that the variation in reported yields is attributable to differences in reaction conditions and feedstock pretreatment methods used in the studies.

3.1.2. Forest Residues

Forest residues are the leftover materials from harvested trees, including leaves, bark, branches, stumps, sawdust, wood chips, off-cuts, and woody debris. Approximately 40% of the wood harvested can be left as forest residues, representing a significant amount of unused wood that can be utilised as an alternative fuel [45]. Forest residues contain a significant amount of lignin, which makes their degradation require high temperatures [46]. Due to their high lignin content, forest residues produce more solid residue than they do oil [47]. Numerous researchers have utilised forest residues as HTL feedstocks for bio-crude production, including wood waste [48], construction wood waste [49], sawdust [50], pine sawdust [51], barks [52], cypress [53], tobacco stem [54], beech wood [55], and white poplar dust [56].

3.1.3. Livestock Waste

Livestock wastes are byproducts and residues generated during livestock production and processing, such as feed residues, manure, bedding waste, and slurry. Manure is a commonly used livestock waste utilised in hydrothermal liquefaction for energy generation. Manure has a high ash content, which has been demonstrated to be detrimental to biocrude yield [57]. Examples of livestock waste that have been studied for hydrothermal operations include livestock manure [58], swine and dairy manure [59], and cattle manure [60].

3.1.4. Algae Biomass

Algae is a third-generation biomass used as a feedstock to generate biofuels through biological, biochemical, and thermochemical processes [61]. Its shorter development cycles and its ability to adapt to varied climatic conditions, contaminated land, and water bodies make it advantageous over lignocellulosic biomass [62]. Depending on the species, algae are rich in lipids, carbohydrates, and proteins, which makes them a potential feedstock for different conversion routes [63]. Hydrolysis is the primary degradation reaction in the hydrothermal liquefaction of algae, releasing fatty acids and glycerol from triglycerides and phosphate chains of phospholipids. The products of glycerol and fatty acids include acrolein, acetaldehyde, formaldehyde, methanol, propionaldehyde, allyl alcohol, aldehydes, ketones, alkenes, alkanes, and polyaromatic compounds [64,65]. Carbohydrates are hydrolysed into mono or disaccharides and then converted into cyclic oxygenated molecules or small acids [66]. Examples of algae species that have been studied for hydrothermal liquefaction include Gracilaria gracilis and Cladophora glomerata [67], Spirulina sp. and Tetraselmis sp. [68], Nannochloropsis, Pavlova, and Isochrysis [69].

3.1.5. Sewage and Municipal Solid Waste

Sewage sludge is a byproduct of wastewater treatment plants that contains a high water content, carbohydrates, proteins, lipids, and other toxic and non-toxic inorganic elements [70]. Sewage sludge has been treated conventionally through anaerobic digestion, landfilling, incineration, and land application. However, each of these conventional treatment methods has its challenges, including the uptake of heavy metals by plants for land application, shrinking landfill space, high energy demand in incineration, and associated CO₂ emissions [71]. The high moisture content and organic content of sludge make it a suitable feedstock for hydrothermal processing [72]. Its utilisation as a source of energy helps reduce the disposal burden and, at the same time, serves as an alternative to fossil fuels [73]. Sewage sludge contains high volatile matter, making it a bioresource for biocrude production and material recovery [74]. The potential of sewage sludge for energy generation has been extensively studied by several researchers [75,76,77].

Municipal solid waste is waste generated from households, institutions, hospitals, and commercial establishments and contains inorganic materials such as plastic, paper, and metal, along with organic waste such as kitchen and green waste [22]. Okoligwe, et al. [78] reported that the use of municipal solid waste via hydrothermal liquefaction yields significant mass and energy efficiency. Mostly for HTL studies, solid waste co-liquefied with other waste materials improved biofuel yield and other materials recovery. Some of the studies include the conversion of solid waste into solid fuel through hydrothermal carbonisation [79], co-hydrothermal carbonisation of food waste digestate and yard waste [80], and the influence of process parameters on food waste valorisation [81].

3.1.6. Plastic Waste

Lifestyle changes driven by consumerism and rapid technological advancements have created a “throwaway society” characterised by increased consumption of single-use and disposable products, particularly packaging materials, resulting in a surge in waste. Specifically, plastics have found wide application across many key sectors of human life, such as construction, healthcare, engineering, and food packaging, thereby increasing demand and use and, in turn, generating waste. The plastics in solid waste include polypropylene, polyvinyl chloride, polyethylene, polystyrene (PS), and high-density polyethylene (HDPE), which account for approximately 10% of landfill trash by mass [46]. Dumping of plastic wastes in waterways, oceans, and other water bodies is a hazard of concern to environmentalists, creating a nuisance to both flora and fauna [82]. Recently, plastic waste has been thermochemically converted to bio-gas, bio-oil, and heat in an environmentally safe manner due to its high calorific value, similar to petroleum fuel [83]. Studies on hydrothermal liquefaction of plastic wastes for biocrude production have been reported [84,85,86,87]. However, pure plastics require catalysts to facilitate their breakdown under HTL conditions [88].

The review shows that each biomass category has unique constituents that influence its suitability for biocrude production. Agricultural biomass and forest residues are rich in lignocellulosic materials, making them highly suitable for HTL. Agricultural biomass often produces relatively high biocrude due to carbohydrate depolymerisation and lower energy content, resulting from its high oxygen content. Forest residues are rich in lignin, which promotes the formation of aromatic-rich biocrude, enhancing its energy density. However, its high lignin content can resist complete depolymerisation, resulting in low biocrude yield. Algae are rich in lipids and proteins, which favour higher biocrude yields and higher heating values than those of agricultural biomass and forest residues. Sludge offers a balanced composition (proteins, lipids, and inorganic ash) that influences product distribution but requires upgrading due to high heteroatom content. Plastic waste, while challenging to degrade, can be converted into biocrude oil with appropriate catalysts. The mixed composition of solid waste presents both opportunities and challenges for efficient biocrude production. In general, biomass such as algae, forest residues, and lignin- and lipid-rich plastics can produce biocrude with higher heating values, while agricultural biomass and sludge rich in carbohydrates show higher yields but lower heating values. This underscores the need for co-liquefaction and process optimisation to improve biocrude yield and quality. Additionally, the HTL product distribution from biomass feedstocks is dependent on reaction conditions and biomass composition.

3.2. Synergistic Interaction Among Biomass Feedstocks

The biochemical content of biomass feedstocks determines the liquefaction mechanism and product distributions, as well as the interactions when co-processed. Agricultural waste (corn cobs, barley straw, corn stover, wheat straw), forest residues (wood chips, sawdust, barks), and some fractions of municipal solid waste are dominated by lignocellulosic structures. In contrast, algae and food processing waste are rich in protein and lipids. Generally, during HTL, biomass rich in lipids produces a high biocrude yield with higher carbon and hydrogen content, while lignocellulosic biomass promotes the formation of phenolics, oxygenated compounds, and a solid product due to lignin repolymerization [89]. A synergistic reaction occurs when lignocellulosic biomass is co-liquefied with high-lipid content biomass, as a result of interactions between hydrogen intermediates from lipids and stabilising free radicals released during carbohydrate and lignin depolymerisation, thereby suppressing the formation of solid residues and increasing biocrude yield and quality [88,90]. Biocrude quality improves when nitrogenous biomass, such as algae and sewage sludge, is co-liquefied with carbon-rich materials such as lignocellulosic biomass due to dilution and redistribution of nitrogenous compounds. Co-liquefaction of Cyanobacteria with bagasse reduced the nitrogen content from 7 wt% to 4.2 wt% and sulfur content from 0.7 wt% to 0.4 wt% [91]. Similarly, co-hydrothermal liquefaction of rice straw and sludge reduced nitrogen and sulfur content in bio-oil [92].

Synergistic effects are observed when mineral-rich feedstocks, such as sewage sludge and livestock waste, are blended with organic-rich, low-ash biomass, such as food waste, algae, and forest residues. The mineral constituents promote hydrolysis and depolymerisation reactions, while the organic constituents donate energy-dense intermediates to compensate for the yield losses [93,94]. Plastic waste has a high hydrogen content, and its co-liquefaction with biomass rich in oxygen yields strong synergistic effects. It donates its hydrogen component to enhance deoxygenation reactions and reduce oxygenated compounds through polymerisation, leading to a higher energy content and improved fuel quality [95,96]. The combination of biomass and plastic often yields synergistic effects, with the overall bio-oil yield and quality exceeding the sum of the individual contributions. This is due to the interaction between the degradation products of biomass and plastics, which can enhance the formation of hydrocarbons and reduce the formation of undesirable compounds [97].

4. Overview of Hydrothermal Liquefaction (HTL)

Hydrothermal liquefaction is a thermochemical conversion process for producing biofuel by converting wet biomass into high-value energy products as a sustainable alternative to fossil fuels [34]. HTL effectively overcomes the high moisture content of agricultural residues, sludge, food waste, algae biomass, and organic matter without the need for energy-intensive drying, thereby reducing the associated energy costs [88,98]. The liquefaction process is carried out at temperatures between 220 and 350 °C and a pressure of 5−20 MPa [21,22]. During HTL, various reactions break down the molecules of organic biomass to produce biocrude, gas, biochar, and aqueous products [99]. The suitability of diverse biomass materials for biocrude production via hydrothermal liquefaction has been investigated, considering their availability, renewability, and cost-effectiveness. Recently, Liu, et al. [100] liquefied mixed sewage sludge for biocrude production and reported a high biocrude yield of 42.6 wt% and energy recovery of 64%. Saengsuriwong, et al. [101] performed hydrothermal liquefaction of food waste at 340 °C and observed a high biocrude yield of 40% w/w and energy recovery of approximately 70%. Evcil, et al. [102] investigated the effect of metal chlorides on product yields and compositions from HTL of olive oil residues at 250–330 °C for 5–60 min. It was found that the use of catalysts reduces the yield of biocrude and solid residue. Other studies on HTL of biomass include wheat straw [103], crop straws [44], barley straws [104], soybean straw [105], rice straw [106], algae biomass [107], and paddy straw [108]. Nevertheless, liquefaction of a single feedstock presents practical challenges due to its low fuel quality, seasonal variations that limit consistent supply, and high oxygen, nitrogen, and impurities in the resulting bio-oil, which reduce quality and conversion efficiency due to factors like low energy density, high ash content, or slow reaction kinetics [109]. This shortcoming has increased interest in co-liquefaction, in which two or more feedstocks are combined to leverage their complementary properties and achieve synergies. Figure 5 shows a schematic diagram of hydrothermal liquefaction technology.

4.1. Hydrothermal Co-Liquefaction

Co-hydrothermal liquefaction (co-HTL) combines feedstocks to increase bio-oil yield and improve its properties by adjusting the feedstock blend [111]. As shown in Figure 6, it utilises all kinds of waste, reduces logistics costs associated with collection and transportation, and enhances the processability of slurry feedstock for continuous processes [46]. The co-liquefaction effect on biocrude yield is the difference between the observed yield from mixed feedstocks and the weight-average yield calculated from the individual feedstocks. The effect is said to be synergistic if positive, antagonistic if negative, or additive if there is no effect.

$$\mathrm{CE}=Y_{actual}-\sum Y_i\times x_i$$

(1)

From Equation (1), Y_actual is the yield from co-liquefaction of mixed feedstock, Y_i indicates the yield from liquefaction of individual feedstock, x_i is the mass fraction of individual feedstock in the mixture, and CE is the co-liquefaction effect. The extent of synergistic or antagonistic effect depends on the biomass mixing ratio and hydrothermal liquefaction reaction conditions, such as temperature and residence time [112].

The interaction of intermediates produced from the breakdown of various components of different biomass mixed during co-liquefaction gives rise to variation in biocrude yield and quality, and chemical components [113]. Many researchers have reported hydrothermal co-liquefaction using different types of biomass.

Figure 6. Co-liquefaction of biomass for high-yield biocrude production (modified with an OpenAl Copilot tool), copyright from Li, et al. [113].

Figure 6. Co-liquefaction of biomass for high-yield biocrude production (modified with an OpenAl Copilot tool), copyright from Li, et al. [113].

4.1.1. Co-Liquefaction of Sewage Sludge with Various Feedstocks

Many researchers have co-liquefied sewage sludge with algae biomass. Mishra and Mohanty [114] investigated the co-liquefaction of sewage sludge and microalgae. A synergistic effect was observed, improving both the yield and quality of the biocrude through interactions among mineral elements in sewage sludge. Xu, et al. [115] co-liquefied sewage sludge with microalgae (Chlorella) under subcritical conditions. A synergistic effect on biocrude yield was observed, attributed to the hydrolysis of SS ash, thereby improving biocrude yield. However, the SE had no impact on biocrude quality due to the inhibition of deoxygenation reactions and the loss of hydrogen and carbon in the feedstocks as gaseous products during co-HTL. An antagonistic effect was observed during fast hydrothermal co-liquefaction of high-ash sludge and Chlorella microalgae. The sludge contained a high ash content, which was responsible for the repolymerization of carbohydrate intermediates into solid products and incomplete hydrolysis due to short contact time, resulting in a low biocrude yield [116]. Sewage sludge has also been co-processed with other feedstocks. Zhang, et al. [117] examined the hydrothermal co-liquefaction of sewage sludge and agricultural biomass to produce biocrude. The synergistic effect of the sewage sludge-wheat straw mixture on biocrude yield was 20 wt%. The effect was attributed to the presence of alkaline metals and nitrogen in sewage sludge, which served as catalysts during co-hydrothermal liquefaction (co-HTL) treatment, enhancing biocrude yield. A similar result was observed from co-HTL of binary sewage sludge and cow manure [118]. Wang, et al. [119] investigated the co-liquefaction of sewage sludge with soy oil, soy protein, and starch. The highest synergistic effect was observed with the blend of sewage sludge and starch, followed by the blend of soy oil and sewage sludge, although it produced the highest biocrude yield. The interaction between starch and sewage sludge results in a synergistic effect; however, the inherent properties of starch and sludge do not yield a high biocrude yield. The slight synergistic effect observed in the co-liquefaction of sewage sludge and soy oil was due to the chemical reaction of amino acids and reducing sugars. Leng, et al. [120] investigated the co-liquefaction of sewage sludge and rice straw at 300 °C and reported a synergistic effect on both biocrude yield and quality. The synergy was attributed to the hydrolysis of the alkaline component of sewage sludge into intermediates, which facilitate biomass degradation and promote biocrude production. The synergistic effect on biocrude quality was attributed to the interaction between lignin and protein-derived fragments, which influence the fuel chemical constituents. Studies have shown that co-processing of sewage sludge and agricultural biomass produces biocrude with higher energy content and improved quality [121,122]. Overall, the effects of co-liquefaction depend on several operating parameters, including biomass feedstock composition, temperature, and catalysts. Depending on the feedstocks used, co-liquefaction can increase biocrude yield and energy recovery. It is also noted that antagonistic effects can occur during co-HTL, underscoring the importance of carefully selecting biomass feedstocks.

4.1.2. Hydrothermal Co-Liquefaction of Algae with Other Biomass

Several researchers have investigated the co-liquefaction of algae with other biomass to improve product distribution. Hossain, et al. [123] reported an increase in biocrude yield resulting from the synergistic effect of co-liquefaction of microalgae with peat at 300 °C for 60 min. Feng, et al. [111] co-liquefied microalgae (Spirulina) and grass (Spartina alterniflora) using ethanol and water as co-solvents for bio-oil production at 360 °C. A similar result was obtained from the liquefaction of microalgae (Spirulina platensis, SP) and macroalgae (Enteromorpha prolifera, EP) by Jin, et al. [124] and Yuan, et al. [125]. An antagonistic effect on biocrude yield was observed when lignocellulosic biomass was blended with microalgae [126]. Furthermore, Zhang, et al. [127] co-liquefied Laminaria japonica and sweet potato vine for biocrude production. The synergistic effect was attributed to the chemical compositions of the feedstocks, resulting in a low nitrogen content and a high phenolic content in the biocrude oil. Similarly, Wang, et al. [128] performed co-liquefaction of Chlorella pyrenoidosa and sweet potato residue and reported that the synergistic effect improved the heating value and energy recovery of biocrude due to decarboxylation and dehydration reactions between the feedstocks. The Maillard reaction between protein and starch components increased the hydrocarbon content of the biocrude.

Overall, the high protein and lipid contents of algae make it a highly valuable co-liquefaction feedstock that facilitates synergistic interaction, enhances fuel quality through denitrogenation and deoxygenation, and increases calorific value. However, the nitrogen content and the N-containing compounds in the biocrude derived from algae co-liquefaction are generally high. Additionally, the success of co-liquefaction depends on the careful selection of complementary biomass, as lignin-rich residue may reduce biocrude yield despite improving energy recovery.

4.1.3. Hydrothermal Co-Liquefaction of Lignocellulosic Biomass and Other Types of Feedstocks

Agricultural wastes, such as corn stalks, rice stalks, sugarcane bagasse, wheat straw, vegetable waste, wood chips, bark, and aspen wood, are readily available and have been co-liquefied with other biomass for biocrude production. Liu, et al. [129] co-liquefied corn stover (CS) and cow manure (CM) at temperatures of 375–450 °C for 12–30 min at mass ratios of 1:2, 1:1, and 2:1. The synergistic effect on biocrude yield (24.66 wt%) was observed with a mass ratio of 1:1. Sharma, et al. [130] co-liquefied wheat straw, eucalyptus, and pinewood biomass at an equal ratio under supercritical conditions. The antagonistic effect on biocrude yield was attributed to the similarity in biomass composition, which led to repolymerization and cracking reactions and converted organic matter into solid and gaseous products. It could be deduced that co-liquefaction of biomass feedstock with the same constituents is not favourable for enhancing bio-crude yield. Tirumareddy, et al. [33] co-liquefied lipid-rich mustard meal (MM) and canola meal (CM) to produce biocrude. A synergistic effect was observed at a CM: MM mass ratio of 1:3 on both biocrude yield and quality. The synergistic effect on biocrude quality was attributed to complex reactions among the different feedstock components, resulting in the formation of oxygenated products that might not form when processing individual feedstocks. Xia, et al. [131] carried out co-liquefaction of rice straw with microalgae (Nannochloropsis) in a glycerol–water mixture at 350 °C for 10 min in the presence of K₂CO₃. The synergistic effect on biocrude yield was 8.86 wt% at a 1:1 mass ratio. Increasing the algae percentage in the blend decreased the solid residue yield. A synergistic effect was observed to improve biocrude quality, attributed to protein–carbohydrate interactions. Some other studies have also reported a synergistic effect on both biocrude yield and quality, attributed to the easy hydrolysis of hemicellulose and cellulose components of lignocellulosic biomass into intermediates at lower temperatures [132,133,134,135]. Generally, most studies reviewed reported a synergistic effect on biocrude yield from agricultural residues when combined with other biomass, and further research is needed on the reaction mechanism.

4.1.4. Hydrothermal Co-Liquefaction of Organic Waste and Other Biomass

The moisture-laden nature of organic waste makes its handling and disposal challenging, especially through incineration and landfilling. Therefore, a sustainable approach, such as the valorisation of these wastes for resource recovery, is necessary. Hu, et al. [136] co-liquefied cattle manure and peanut residue at 270 °C for 30 min to improve bio-oil production. An increase in the mass fraction of peanut residue in co-liquefaction was found to increase biocrude yield, attributed to the hydrolysis of proteins and lipids into smaller molecules at higher temperatures. A synergistic effect was also observed in improving biochar yield, resulting from the Maillard and Mannich reactions that occur during co-liquefaction. A synergistic effect was also reported for the co-liquefaction of faecal sludge and peat biomass, attributed to interactions among the lipid, protein, and carbohydrate constituents of the biomass, which significantly influence biocrude production through the Maillard reaction [123]. Likewise, Opu, et al. [137] observed that the co-HTL of faecal sludge with water hyacinth at a 1:1 ratio showed a synergistic effect on biocrude production due to the catalytic effect of biomass composition. Hydrothermal co-liquefaction of cow manure and wheat straw was carried out by dos Passos, et al. [138]. Positive synergy was observed between manure and straw in biocrude yield. Chen, et al. [139] co-liquefied swine manure and algae at 300 °C for 1 hr. The optimum biocrude yield was reported at a mass ratio of 3:1 compared to other blending ratios. The same result was obtained for the co-liquefaction of faecal sludge and organic solid waste by Kabir and Khalekuzzaman [140]. The review highlights that synergistic effects occur between organic wastes and other biomass, improving both the yield and chemical composition of the biocrude oil compared to single feedstocks. The synergy is attributed to the interaction between the biochemical composition and catalytic mineral elements of the feedstocks.

4.1.5. Hydrothermal Co-Liquefaction of Plastic Waste and Other Biomass

The consistent production of plastic waste poses significant health risks and environmental challenges, and a substantial portion of this waste ends up in landfills, natural landscapes, and water bodies. Therefore, environmentally friendly techniques such as plastic valorisation have been explored to convert plastic waste into valuable products. A low biocrude yield antagonistic effect was observed from the co-liquefaction of waste plastics and food waste at different temperatures (290–370 °C) and times (30–60 min) [141]. The AE was attributed to the stability of the fatty acids, which inhibits interactions with plastics and allows mass transfer to other product phases. Yuan, et al. [142] studied the effects of temperature and time on the co-liquefaction of low-density polyethylene plastics and rice straw. The reaction temperature and time were observed to improve product distribution and properties to a certain level. Hongthong, et al. [143] co-liquefied plastics with pistachio hulls at 350 °C for 15−60 min at different blending ratios. It was observed that the biocrude yield decreased as the percentage of plastics in the blend increased, except for PET and nylon, for which there were no significant differences in biocrude yield with the blending ratio. Baloch, et al. [144] performed catalytic co-liquefaction of polyethylene and sugarcane bagasse at supercritical conditions to produce biocrude. Catalysts were observed to improve the conversion rate and yield of biocrude. In general, co-liquefaction of plastics with biomass is an efficient route in valorisation. However, its efficiency is strongly dependent on the type of plastic, the blending ratio, the operating conditions, and the catalyst. Optimising operating parameters such as temperature, residence time, and catalysts is essential to maximise biocrude yield and minimise unwanted byproducts.

5. Waste-to-Energy and Circular Economy

5.1. Circular Economy and Sustainable Development Goals

Industrialisation and reliance on non-renewable energy sources have led to increased waste production and climate change, necessitating strategies for the circular economy to cut carbon emissions by 45% and achieve carbon neutrality by 2030 and 2050, respectively, as outlined in the Paris Agreement [145]. The circular economy is an effective approach to reducing environmental pollution and greenhouse gas emissions, as well as to developing policies for sustainability [146]. It is closely linked to other economic sustainability strategies, such as industrial ecology and industrial symbiosis, aiming at transforming the linear value chain into a circular system [147]. Traditional waste treatment methods often harm the environment through pollution and resource depletion. Therefore, the circular economy provides a framework to replace the linear model through waste reduction, reuse, recycling, and recovery, supporting sustainable development while enhancing environmental quality, economic growth, and social equity [148]. Figure 7 describes the concept of a low-carbon economy. Developing a waste-to-energy supply chain is essential for achieving circular economy goals by promoting sustainable energy planning and operation and enabling the recovery of materials for bioenergy production [149]. The energy transition seeks to incorporate renewable energy sources into the energy mix, replacing fossil fuels with clean options such as solar, wind, and biomass [150]. Valorisation of biomass such as food waste, agricultural waste, and sewage sludge supports bio-circular economy principles by producing biofuels and other valuable products [151]. Waste conversion technologies such as hydrothermal liquefaction are environmentally friendly and support a zero-waste approach, as their byproducts, such as hydrochar and aqueous phase water, can be reused as adsorbents for water and wastewater treatment and organic fertilisers for soil remediation, irrigation, and biogas production via anaerobic digestion [152]. This approach is valuable for maintaining the energy–environment nexus and protecting the environment by reducing carbon footprints [153].

While many countries, driven by the advantages of the waste-to-energy approach, have adopted this fast-evolving technology, emphasis is being placed on the Sustainable Development Goals, and many national corporations are focusing on reducing energy consumption through the circular economy [145,154]. Waste-to-energy technologies can offset the greenhouse gas emissions associated with over-reliance on fossil fuels, as embedded in SDG 7 (Affordable and Clean Energy). It is noted that every 1000 kg of waste utilised to generate electricity replaces one-quarter of a barrel of oil, which can reduce greenhouse gas emissions by approximately 26 million tons of CO₂ in the US alone [155].

By converting waste into valuable products and recycling nutrients, these technologies promote SDG 12 (Responsible Consumption and Production). Additionally, reducing greenhouse gas emissions through renewable energy production contributes to climate change mitigation (SDG 13). The adoption of a waste-to-energy strategy in low- and middle-income countries such as South Africa can reduce or eliminate the combustion of woodfuels for energy generation and subsequently reduce emissions from burning and forest degradation [156]. Waste-to-energy (WTE) technologies have the potential to transform waste into sustainable energy, reduce the environmental impacts of fossil fuels, thereby mitigating climate change, and promote the circular economy. The adoption of circular economy principles is still in its infancy in Africa; however, if fully adopted, it will support the advancement of the UN Sustainable Development Goals [157]. Thus, to realise the full potential of waste-to-energy technologies, researchers, industry, and government must collaborate to achieve a sustainable circular economy.

5.2. Techno-Economic and Life-Cycle Considerations of Co-HTL

Co-HTL leverages the synergistic effects of different feedstocks to improve biocrude yield and quality. For example, co-liquefaction of sewage sludge with lignocellulose biomass resulted in higher bio-crude yields and improved energy content [158]. Similarly, co-HTL of cattle manure and corn cob showed improved biocrude yields and higher heating values [159]. However, the economic feasibility of HTL is influenced by the choice of feedstocks and operating conditions. The use of acetone as a co-solvent increased biocrude yield but increased the operating costs due to challenges in solvent recovery [160]. A study on wood–glycerol co-liquefaction reported an MSP of $1.14 per litre of gasoline equivalent (LGE), which reduced to $0.82/LGE when only wood was used as feedstock [161]. Another study found the breakeven selling price of co-HTC hydrochar to be $117 per ton, with potential reductions to $106 per ton for higher-capacity plants [162]. The capital and operating costs of co-HTL are also significant factors in determining the system’s viability. Elhassan, et al. [163] reported that ethanol production from Shorea sawdust required a capital cost of USD 7.59 million with a payback period of 16.2 years. Co-processing of algae with food waste could boost production scale by 10–60%, and scalability could reduce the minimum selling price to <$5.00/GGE [164]. Sensitivity analyses have identified biocrude yield and feedstock prices as key factors affecting the minimum fuel selling price of biocrude [161]. Comprehensive uncertainty analyses are essential to quantifying the economic and environmental measures of the co-HTL process. For instance, a U.S.-based analysis estimated a biocrude yield range of 42.2% to 52.4% and a minimum fuel selling price range of $2.28/GGE to $3.45/GGE [165].

Life-cycle assessment of co-HTL evaluates its environmental impacts across various stages, from feedstock preparation to bio-oil production and utilisation. Co-HTL processes generally reduce global warming potential (GWP) compared to conventional methods. For instance, co-liquefaction of sludge and microalgae resulted in a GWP of 25.49 kg CO₂-eq, which is significantly lower than that of traditional methods [166]. The co-HTL of plastic waste with algae has been reported to improve bio-oil quality and energy density and to reduce greenhouse gas emissions, supporting circular economy principles [167]. Zhang, et al. [166] observed that pretreatment, such as HTC, can significantly reduce environmental burden. Integration of HTL with anaerobic digestion for wastewater treatment and biogas generation showed a favourable environmental outcome [168]. Similarly, integrating photovoltaic technology with HTL can achieve negative carbon emissions by using renewable energy and biochar for soil carbon sequestration [169]. Compared with other thermochemical conversion technologies, such as fast pyrolysis, co-HTL processes perform better environmentally [170].

The technoeconomic assessment of co-HTL of biomass reveals that while the process can improve bio-crude quality and operational efficiency, economic feasibility is highly dependent on feedstock costs, process parameters, and capital investments. Sensitivity and uncertainty analyses are crucial for optimising the process and making it competitive with conventional fuels. Co-hydrothermal liquefaction has substantial potential to reduce carbon emissions throughout its life cycle by optimising process conditions, utilising renewable energy sources, and effectively managing by-products such as biochar. However, further research and development are needed to address uncertainties and enhance the technology’s overall efficiency and sustainability.

6. Integration and Hybrid Approach

An integrated thermochemical–biochemical technology could be a viable option to overcome the shortcomings of standalone conversion processes. The barriers affecting the full implementation of some of the standalone technologies include a long retention time, incomplete conversion rates, and the presence of toxic compounds that inhibit microbial activities in the case of anaerobic digestion, feedstock variability, high energy demand, and production of unwanted byproducts in hydrothermal liquefaction [171,172]. The schematic view of an integrated system for waste valorisation is presented in Figure 8. The integrated systems complement one another and offer advantages, including improved waste valorisation, reduced operational costs, and optimised energy recovery [173]. For example, hydrochar derived from the hydrothermal carbonisation of biomass can be used as a carbon-rich feedstock in gasification to improve energy recovery and minimise waste [174]. Also, the aqueous product from the hydrothermal liquefaction process can be used as a substrate to produce biogas via anaerobic digestion for nutrient reuse and residual energy recovery [175]. Similarly, biochar from thermochemical technologies can serve as a microbial carrier matrix in biochemical processes and adsorb inhibitory compounds [176,177].

6.1. Integration of Anaerobic Digestion and Hydrothermal Liquefaction/Carbonisation

High organic content in anaerobic digestion (AD) digestate poses a significant environmental threat if not properly managed [172]. AD digestate can be hydrothermally processed to produce value-added products such as biocrude, thereby reducing pollution associated with its disposal. Studies have reported improved energy recovery from integrated anaerobic digestion and hydrothermal liquefaction/carbonisation. Digestate from anaerobic digestion of sewage sludge was hydrothermally treated at a temperature of −220 °C and produced an improved hydrochar yield of 56–73% [178]. Similarly, a low biocrude yield of 7 wt% db, with a higher heating value of 36.7 MJ/kg, a high biochar yield of 46 wt% db, and a lower heating value of 4.58 MJ/kg, was reported from hydrothermal liquefaction of AD digestate [179]. The higher heating value suggests that the biocrude can be upgraded into a dense energy fuel. Biochar can be utilised as a soil amendment due to its low heating value. Tito, et al. [180] valorised the digestate from the organic fraction of municipal solid waste via HTL at temperatures of 300–360 °C and residence times of 10–60 min. An increase in temperature was reported to improve biocrude yield but alter its chemical compositions. Klüpfel, et al. [181] studied the HTL behaviour of digested urban and agricultural wastes and reported energy recovery of 35–43%. Other studies have also investigated the HTL of digestate with improved yield and energy recovery [182,183,184]. Integrating hydrothermal liquefaction (HTL) and anaerobic digestion (AD) provides a promising approach for efficient waste management and improved energy recovery.

6.2. Integration of Hydrothermal Liquefaction and Anaerobic Digestion

The hydrothermal liquefaction process produces a large volume of aqueous products rich in organic compounds, making it a good energy source for biological conversion [172]. It contains complex organic compounds that can inhibit biological degradation, thus requiring pretreatment before disposal. Several studies have reported pretreatment methods for removing refractory contaminants from aqueous wastewater, including oxidation, adsorption, solvent extraction, and others [185,186]. Anaerobic digestion can be integrated with HTL to enhance resource recovery from aqueous products. Algae HTL wastewater was co-digested with manure by Fernandez, et al. [187]. The results showed that HTL wastewater at concentrations up to 30% (v/v) influenced biodegradation and methane generation between 263.4 and 327.2 mL/gVS_in. However, above 40% (v/v), chemical compounds inhibited biodegradation. Azarmina and Eskicioglu [188] co-digested the HTL aqueous phase with municipal sludge and found that thermophilic conditions at low loading rates favoured COD removal, whereas higher loading rates led to process inhibition due to increased nitrogen and salt concentrations, but partial microbial acclimatisation was observed over time. A study by Egerland Bueno, et al. [189] reported that biostimulation of sludge aids the anaerobic digestion of microalgae HTL aqueous products by improving methane yields and decreasing inhibition at higher organic loadings. This was attributed to the augmentation of Mesotoga and Methanomethylovorans microorganisms, which facilitate the degradation of recalcitrant compounds. Cox and Eskicioglu [190] pretreated HTL aqueous waste to remove toxic ammonia and phenolics and evaluated the influence of the pretreatment on the performance of anaerobic co-digestion of treated aqueous waste with municipal sludge under both thermophilic and mesophilic conditions. Ammonia stripping effectively reduced inhibitory compounds in the aqueous product, achieving approximately 80% ammonia and 32% phenol removal under optimal conditions. A high organic carbon aqueous byproduct from co-HTL of brewery waste and sewage sludge was anaerobically digested, yielding significantly higher methane yields and 80.64% energy recovery, demonstrating that co-liquefaction enhances bioenergy recovery efficiency [191].

7. Challenges Facing HTL Technology

HTL is a promising technology for energy generation, but several key challenges hinder its large-scale application and commercialisation. These challenges include:

• Economic and technical feasibility—High equipment costs, energy demand, and feedstock logistics are significant barriers to commercialisation. Techno-economic assessments indicate that reducing these costs is essential for making HTL competitive with conventional fuels [192].
• Feedstock variability—The inconsistency in biomass chemical composition and physical properties poses a challenge for continuous operation and reliable biofuel production [193,194]. Co-liquefaction of feedstock can mitigate some variability issues, but understanding the interactions between different feedstocks is necessary to optimise processing conditions [122].
• Complexity of bio-oil composition—HTL-derived biocrude contains a complex mixture of organic compounds, making refining challenging and affecting the stability and quality of the bio-oil, necessitating advanced purification techniques [195].
• Reactor material constraints and formation of unwanted products—The HTL reactor requires high-temperature and -pressure conditions, which can be challenging and costly. Inefficient heat and mass transfer within the reactor can affect conversion efficiency and selectivity, leading to lower yields and higher costs for product separation and purification [196].
• Catalyst deactivation—Heterogeneous catalysts can suffer from deactivation due to fouling, sintering, and poisoning, which reduces their effectiveness over time. They can be regenerated and reused, though this can be complex and costly [197].

Addressing these challenges through innovative solutions and process optimisation is crucial for the successful commercialisation and widespread adoption of HTL technology.

8. Research Outlook and Conclusions

8.1. Research Outlook

For an efficient liquefaction process, future research should focus on the co-liquefaction of feedstocks in a continuous reactor to optimise the process. Advancements in hydrothermal co-liquefaction should focus on optimising reaction conditions and synthesising low-cost catalysts to improve efficiency. Future studies should explore a hybrid approach that integrates hydrothermal liquefaction with hydrothermal carbonisation to enhance overall system efficiency and maximise energy recovery. Utilisation of various byproducts of hydrothermal liquefaction will curtail waste and contribute to the circular economy through materials recovery.

For scalability, hydrothermal liquefaction must address challenges related to feedstock variability, catalyst durability, and reactor scalability. Efforts should focus on applying artificial intelligence optimisation methods to manage the reactor, as well as on developing a model that integrates hydrothermal liquefaction with other conversion processes, such as gasification, anaerobic digestion, and hydrothermal carbonisation. Addressing these challenges effectively will evolve this waste-to-energy technology from the bench scale to commercial scale, thereby contributing to the advancement of green energy and the circular economy.

8.2. Conclusions

Hydrothermal liquefaction is a conversion technology for the valorisation of organic wastes into high-value energy products. HTL biocrude can be upgraded into transportation fuels, including gasoline, aviation fuels, and marine diesel. Hydrothermal co-processing of biomass feedstocks is a promising technology that enhances biocrude yield and quality through synergistic interactions among feedstock constituents. Feedstocks such as agricultural waste, forest residues, food processing waste, algae, and plastics have been evaluated for co-liquefaction. Co-liquefaction technology has proven to be an efficient waste-to-energy process and offers a unique blend of selectivity and integration potential to advance the circular economy framework. The synergistic effects and potential outcomes of co-liquefaction are premised on several operating conditions, including interactions among biomass feedstocks, temperature, time, and catalysts. Therefore, careful selection of biomass feedstocks and operating parameters is essential, as variations in biomass composition can significantly influence the quality and yield of biocrude. The review demonstrates that utilising waste in co-liquefaction reduces the carbon footprint of fossil fuels, mitigates greenhouse gas emissions, and promotes sustainable energy production. The review also shows that hybridisation of hydrothermal liquefaction with other thermochemical or biochemical technologies improves energy recovery and sustainability.

Finally, waste-to-energy technology promotes resource efficiency and reduces greenhouse gas emissions, aligning with circular economy principles to advance climate action and directly contributing to the UN Sustainable Development Goals and the Paris Agreement on climate change, while supporting the African Union Agenda 2063 by fostering sustainable growth and energy security across the continent.