1 Introduction

In the face of global challenges related to greenhouse gases (GHGs) emission and the imperative to transition toward a circular and sustainable economy, microalgae have emerged as a promising solution [1, 2]. Microalgae, evolved about 3 million years ago, have captured huge attention since the early twentieth century due to their widespread distribution and integral role in ecosystems. The limitations of arable land for the growing world population and the persistent food vs fuel dilemma associated with crops underscore the reasons for utilizing microalgae in large-scale commodity products such as biofuels.

Microalgae play a pivotal role in sustainable production, utilizing innovative biological resources, processes, and principles across economic sectors within the bioeconomy. The circular bioeconomy, characterized by the reuse and recycling of materials to maximize value and minimize waste, aligns seamlessly with sustainability goals. Recognized for their versatility in autotrophic, heterotrophic, and mixotrophic growth, microalgae find applications in diverse fields, including biofuels [3], recombinant proteins [4, 5], bioactives [6], pigments [7], antioxidants [8], fine chemicals, biopolymers and nanomaterials, proteins [9], carbohydrates [10], lipids [11, 12], animal and aquaculture feed [13], nutraceuticals (enriched with omega-3-fatty acid) [14], bioremediation [15], biofertilizers [16] and ecosystem services (indicators) [17]. Their unique ability to be cultured in non-arable land, utilizing brackish, marine, or wastewater, along with flue gases as a carbon (carbon dioxide) source, positions microalgae as a compelling alternative to land plants for biofuel production [18]. Microalgae play a crucial role in addressing environmental challenges, contributing to urban and industrial wastewater treatment, and have the potential to transform biomass into biofuels for sustainable energy production [19].

The integration of Hydrothermal Liquefaction (HTL) technology stands as a significant advancement in processing wet microalgae biomass. HTL, a thermochemical conversion method, implicates subjecting wet biomass to high temperature and pressure in the presence of H2O, allowing for the formation of biocrude, the renewable counterpart of oil [20]. This process addresses the energy-intensive drying challenge associated with conventional dry conversion methods. While HTL presents a promising avenue to convert wet biomass into valuable bio-crudes, it introduces challenges that demand careful consideration [21, 22]. The obtained bio-crude may exhibit distinct chemical profiles, including higher nitrogen and oxygen contents compared to crude oil, posing challenges for direct applications as a biofuel [23].

Nevertheless, the scalable and adaptable nature of microalgae cultivation, their rapid growth, and minimal nutrient requirements position them ideally for large-scale production, complementing the HTL process. HTL technology stands out as a pivotal tool in processing microalgae biomass. This innovative thermochemical conversion method allows for the efficient processing of biomass, whether wet or dry, without constraints on lipid content [24]. The byproduct of the HTL process is an energy-dense intermediate known as bio-crude, serving as the renewable counterpart of oil. HTL operates under unique conditions, leveraging the presence of water in hydrothermal settings (250–450 °C and 100–300 bar of pressure) to produce bio-crude from organic matter [25]. The compressed hot water in these circumstances can exist in either a liquid or an extremely dense supercritical form, showcasing the distinctive properties crucial to the success of the reactor systems (Sandquist et al., 2019).

The direct HTL of the wet microalgae biomass has extended significant consideration due to its efficient oil production and low energy usage [26]. This approach utilizes the algae residue left over after lipid extraction, preventing the deterioration of lipids in HTL process. Through this approach, a cogeneration progression can be achieved—high-grade biodiesel from microalgal lipids and low-grade bio-crudes from algal residue (residual carbohydrates and proteins) (Chen et al., 2014). This integrated use of HTL technology not only addresses the challenges associated with wet biomass processing but also opens avenues for producing a spectrum of liquid fuels, contributing to the broader goals of sustainable bioenergy production and circular economy establishment.

In the context of biofuels, which are derived from biomass for use in liquid or gaseous forms for transportation, microalgae represent a distinctive third-generation biofuel. Unlike 1st- and 2nd-generation biofuels, often associated with the usage of human food components, microalgae biofuels offer a encouraging substitute to switch fossil fuels without the disadvantages associated with their predecessors [27]. Leveraging microalgal biomass, various biofuels like biodiesel, biogas, bioethanol, and biohydrogen can be produced, contributing to sustainable energy solutions [28].

Understanding the composition of microalgal biomass is crucial for optimizing biorefinery processes [29]. Differences in lipid, protein, and carbohydrate composition across various microalgal species play a pivotal role in producing valuable compounds [30]. The application of this knowledge in the biorefinery process can lead to the production of useful bioproducts. Wu et al. [31] developed a microalgae biorefinery system integrating HTL and hydrodeoxygenation (HDO) to produce green diesel, naphtha, and heavy oil. The LSBoost model, a machine learning-assisted model, outperformed others in predicting HTL process parameters. The system achieved a green diesel yield of up to 33 wt%, with HTL of Nannochloropsis sp. showing 75% energy recovery [31].

To summarize, the integration of microalgae cultivation and HTL technology offers promising insights into sustainable bioenergy production and the development of platform chemicals. This exploration not only contributes to mitigating GHG emissions but also aligns with broader goals of establishing a circular economy and achieving long-term sustainability in the bioenergy sector. Through the investigation of these synergies, this work aims to deliver a comprehensive review of the advances and future prospects in HTL of wet microalgae biomass, emphasizing its significance for biofuels and platform chemical production from wet microalgal biomass.

2 HTL fundamentals

2.1 Overview of HTL process

The rapid upsurge in the necessity for fossil fuel usage poses a significant environmental concern, contributing to extensive pollution and degradation of natural resources and human health [32]. Consequently, alternative and cleaner sources of energy are sought, and fuels derived from biomass through processes such as transesterification, pyrolysis, and HTL are considered viable options to replace the harmful effects of fossil fuels [33]. The term used to describe the raw material utilized for biofuel production is feedstock and biomass serves as a primary example of this organic matter, rich in carbon and hydrogen. Biomass can be of different forms, including woody biomass, agricultural biomass, and aquatic plants. Particularly, microalgae have garnered significant consideration as a biomass feedstock for biofuels. Notably, the selection of microalgae species as feedstock should prioritize those with high lipid content to ensure optimal biofuel production potential.

HTL represents a thermochemical process where biomass with high water content is converted into bio-crude under relatively moderate temperature and pressure conditions (200 to 400 °C and 100 to 200 bar, respectively; Fig. 1) [25]. According to Jaiswal et al. [34], the optimal range of temperature to maximize the bio-oil and bio-char production is between 300 to 350 °C. At low temperatures (250 °C to 300 °C), HTL process results in solid formation, while moderate temperatures (300 °C to 350 °C) and high temperatures (> 350 °C) favour liquid and gas formation, respectively [35]. Distinctly, HTL feature its aptitude to operate without the need for extensive biomass drying. This allows for the direct implementation of HTL on washed wet biomass. For economic and commercial-scale operations, biomass loading should exceed 10% [26]. During the HTL progression, a sequence of reactions, such as, depolymerization, bond breaking, rearrangement, and decarboxylation, transform biomass into biofuels [22]. Bio-crude produced through HTL exhibits superior stability, miscibility, and a high heating value, making it a promising candidate for sustainable energy solutions [36].

Fig. 1
figure 1

Hydrothermal Liquefaction (HTL) process within a microalgae biorefinery. The different stages include microalgae cultivation, biomass harvest and bioproduct production via HTL conversion. A schematic representation of the HTL phase diagram [37], Hydrothermal Processing unit (Accudyne Systems, Inc., UK) and reaction pathways, including depolymerization, decomposition, and recombination [35].

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2.2 HTL mechanisms and reaction pathways

HTL process comprises of 3-major steps i.e. depolymerization, decomposition, and recombination. Biomass, being a heterogeneous mixture of proteins, fats, carbohydrates, and lipids, results in a complex reaction mechanism throughout the entire HTL process (Fig. 1). During HTL, biomass undergoes decomposition and polymer disintegration into smaller compounds, prominent to form bio-crude, gas, and solid compounds [26, 38]. Proper control and optimization of various process parameters (including residence time and temperature) are crucial during the phases of decomposition, condensation, and repolymerization. The depolymerization process entails the gradual breakdown or dissolution of macromolecules based on their physico-chemical characteristics. Temperature and pressure play a crucial role in dissociating long-chain polymers into short-chain hydrocarbons. Depolymerization mimics the natural manner of fossil fuel formation, overcoming the challenging properties of biomass. The presence of cellulose and hemicellulose content enhances the stability of the biofuel, and the water content in HTL facilitates the recycling of organic material energy.

During the decomposition of biomass monomers, different chemical reactions occur, including cleavage, dehydration (elimination of H2O), decarboxylation (elimination of CO2), and deamination (elimination of amino acid group). Simultaneously, hydrolysis breaks down macromolecules into oligomers and monomers (Fig. 1). High temperature and pressure facilitate the hydrolysis of cellulose, forming glucose monomers. Fructose, with higher productivity than glucose, undergoes various reactions, producing by-products such as furfurals, glycolaldehyde, phenols, and organic acid, which are immensely water soluble. The third step involves the recombination and repolymerization of reactive fragments. The absence of hydrogen compounds leads to the reversal of the initial steps. Sufficient hydrogen in the organic matrix during liquefaction results in capping free radicals, forming molecules with stable molecular weights. In cases of hydrogen shortage and excessive free radical concentration, recombination and repolymerization occur, forming high molecular weight compounds, commonly known as coke.

The primary output of the HTL process is liquid bio-crude, accompanied by solid, liquid, and gaseous by-products. HTL is categorized into two main types: lignocellulosic biomass (dry feedstock) and algal biomass (wet feedstock) [26]. HTL of lignocellulose biomass is versatile and can be executed in either batch or continuous reactors. Continuous reactors necessitate a pressurized feeding system, utilizing slurry-based pumps or lock‒hopper systems to manage coarse particles [39]. Pre-treatment and alkaline treatment of lignocellulosic feedstock are critical steps, ensuring particle size reduction and removing contaminants, resulting in a stable slurry ready for pumping. Operating under specific conditions leads to phase separation, yielding a gaseous CO2 stream, bio-crude, and traces of an aqueous phase. The aqueous phase (predominantly water) can be recycled, diminishing water requirements, and enhancing bio-oil productivity. The solid phase (biochar) product obtained can be directly utilized as bio-fertilizer [40].

Processing wet microalgae biomass presents the distinct advantage of nutrient recycling in HTL. The treatment of wet biomass follows a similar approach to lignocellulosic biomass but without the need for pre-treatment. Microalgae, with their small particle size, alleviate challenges associated with pumping of slurry into the reactor chamber [41]. In the HTL process for wet biomass, water and CO2 are recycled for microalgae growth, contributing to a sustainable and closed-loop system [42]. This holistic approach underscores the potential of the HTL process in converting diverse biomass sources into valuable bio-liquids while minimizing environmental impact.

2.3 Key parameters influencing HTL of microalgae biomass

The yield and quality of biocrude achieved from HTL of wet microalgae biomass is impacted by numerous factors, such as, temperature, pressure, residence time, algal species (strain-specific biomass composition), and reaction time, all of which intricately influence the biofuel product [30, 43]. The single-phase HTL process involves maintaining the reaction environment in a single phase, typically water, throughout the entire process. In this method, wet microalgae biomass is subjected to conditions of high temperature and pressure in the presence of water. This allows for the effective conversion of biomass into biocrude oil without the need for additional solvents or phases. Maintaining a single phase simplifies the process and enhances the efficiency of biomass conversion, leading to higher yields of biocrude. Additionally, it facilitates downstream processing and separation of the biocrude from other reaction products. The single-phase HTL process involves maintaining pressure within the range of 4 to 22 MPa, stabilizing the reaction environment for effective hydrolysis and biomass extraction [25]. Optimal residence time, or the duration of the reaction, is crucial for maximizing productivity and biocrude quality. Fast HTL processes, characterized by rapid heating and shorter reaction times, are preferred due to their ability to induce cell structure breakdown efficiently. It's essential to note that reaction time refers to the duration of the HTL process.

The operating temperature (ranging from 360 °C to 530 °C) poses a critical role in microalgae conversion, highlighting the intricate temperature dependence in the HTL process [43]. The dichotomy between batch processing, advantageous for raw material pumping, and the less-explored continuous process poses challenges in maintaining optimal water-biomass ratios. The selection of algae species is paramount due to variations in productivity and yield resulting from unique biochemical compositions and cellular structures [30].

A fast HTL process refers to a method of converting microalgae biomass into biocrude oil through rapid heating and short reaction times under high temperature and pressure. In this, the biomass is subjected to high temperatures typically ranging from 300 °C to 500 °C, and elevated pressures ranging from 10 to 25 MPa for a short duration, often in the order of seconds to minutes. The rapid heating and short reaction times help induce efficient breakdown of biomass constituents (including lipids, proteins, and carbohydrates) into biocrude. Fast HTL processes are favoured for their ability to achieve high biocrude yields while minimizing energy consumption and reactor residence times compared to conventional HTL processes. Fast HTL processes exhibit superior biocrude yields, underscoring the significance of temperature-induced cell structure breakdown [35, 36]. Reaction time, a pivotal factor in HTL, significantly influences both the yield and quality of biocrude. Faeth and Savage [44] assessed the impact of reaction time on biocrude yield and quality in HTL, a fast HTL experiment was conducted with reaction times ranging from 0.25 to 2 min. Under conditions of a sand-bath temperature (~ 600 °C), solids (~ 15 wt.%), and water (~ 11 vol.%), biocrude yields exhibited a rapid increase between 15 and 45 s, attributed to potential cell breakage that released biomolecules, contributing to high yields [44]. However, beyond 0.75 min, biocrude yields declined due to the hydrothermal degradation of cellular components. Systematic reactor loading variations highlight the pivotal role of effective algae concentration in biocrude production [45, 46]. To comprehend the impact of reactor loading on the yield of biocrude, three systematic methods are employed to vary the contents of mini batch reactors in fast HTL. The first method involves fixing water while varying the amount of algae biomass. The second method adjusts the amount of microalgae slurry at a constant solids content (15 wt. %), and the third method maintains a fixed dry algal biomass while modifying water quantity. These variations govern the operative concentration of microalgae solids in hot compressed water at a specified temperature, where effective concentration is defined as the ratio of dry microalgae mass to the combined mass of microalgae and liquid water in the HTL reactor at a specific temperature. The pivotal role of reactor loading variation becomes evident, with low solid content and total mass significantly affecting biocrude yields [45]. However, in reactions with high total mass loadings, such as those involving a high-volume percentage of water, algae solids content exerts less impact on biocrude production. Lower water loading and solids content contribute to increased effective algae concentration, thereby facilitating biocrude production. By controlling reactor loading through the adjustment of solids content and total mass, there is a discernible decrease in yield with the increase in mass. Similarly, the yield of biocrude diminish as water mass increases for a fixed dry algae biomass [47]. This underscores the importance of effective algae concentration during rapid heating for enhanced production of biocrude. Notably, the volume percentage of water has a more significant effect on the yields of biocrude compared to the weight percentage of dry microalgae solids in the slurry [47]. Typically, biocrude yields range from 30 to 70% of the dry algae biomass, with variations influenced by factors such as temperature, pressure, and algae species. Understanding and manipulating these key parameters provides a solid foundation for advancing the efficiency and sustainability of microalgae-derived biofuels through HTL processes [48].

3 Current state of HTL research on microalgae

In response to the escalating global demand for sustainable energy solutions, researchers have increasingly encouraged on biofuels production as a key avenue for mitigating environmental impacts [49]. One promising approach involves the liquefaction process, a versatile method capable of deriving biocrude from various renewable biomass sources. This section provides an insight into the current landscape of HTL, placing specific emphasis on the utilization of wet microalgae biomass within the process, recognizing its substantial relevance and contribution to the ongoing advancements in sustainable energy production.

3.1 Process optimization

In-depth exploration of process conditions underscores their crucial role in optimizing biocrude performance, with factors like temperature, pressure, catalyst types, solvent usage, microalgae/solvent ratio, and residence time influencing product properties significantly [50, 51]. The optimization of HTL for wet microalgal biomass is a key undertaking, demanding meticulous consideration of temperature, pressure, residence time, and feedstock composition. Temperature evaluation depends on biomass type, with domestic wastewater sludge requiring temperatures from 250 to 400 °C for biofuel production [52]. Pressure, operating between 4 to 22 MPa, influences yield by aiding water density, facilitating hydrolysis, and biomass extraction [52]. Residence time affects biocrude production and quality, with increased yield and gas formation as temperature rises. The growth rate and lipid productivity of microalgae have become a focal point for next-generation biofuels. HTL emerges as a promising method, obviating energy-intensive drying processes for wet biomass. Challenges include high heteroatom content in microalgal bio-crude, affecting hydrogen consumption and catalyst deactivation [53]. Mitigation strategies involve pretreatment, catalyst use, and vacuum distillation. Bio-crude separation commonly uses dichloromethane (DCM), impacting yield and properties [54]. Batch reactor modifications and flash heating in HTL experiments aim to optimize the process and enhance bio-crude properties. Commercializing microalgal biofuels faces challenges of feedstock and conversion process costs, making HTL biofuel with by-products potentially promising [55].

Optimizing the HTL process for microalgae wet biomass involves various techniques to enhance biofuel production and improve the properties of the obtained bio-crude [30]. Some key process optimization techniques include,

  • Temperature and Pressure Control: In the pursuit of optimizing HTL for microalgae wet biomass, careful evaluation and adjustment of HTL temperature are essential based on the specific type of microalgae biomass [43]. A common temperature window for microalgae falls between 360 to 530 °C, ensuring efficient biofuel production. Simultaneously, maintaining optimal pressure conditions within the range of 4 to 22 MPa is crucial. This not only aids in keeping the process in a single phase but also contributes to water density, facilitating hydrolysis and biomass extraction [25].

  • Residence Time Optimization: To enhance yield while managing the potential formation of gases and heavy components that could impact quality, researchers experiment with adjustments to residence time based on reaction temperature [56]. Understanding the impact of residence time on the transfer of heteroatoms, such as N-containing compounds, from the aqueous phase to the oil phase is a key consideration in achieving optimal results.

  • Feedstock Considerations: Selecting microalgae species with higher growth rates and lipid productivity (e.g. Nannochloropsis sp.) is a critical facet of optimizing the HTL process. Additionally, different studies in the past decade have explored the impact of various microalgal biomass compositions on overall efficiency of the hydrothermal liquefaction process [24, 57, 58].

  • Pretreatment Strategies: Exploration of pretreatment methods emerges as a strategy to reduce heteroatom content in bio-crude. Specific pretreatment conditions applied to microalgae have shown promise in diminishing N- and O-contents in the resulting bio-crude.

  • Catalyst Utilization: Researchers experiment with a variety of catalysts to alter the bio-crude profile [43]. Catalysts, including Ni/TiO2, demonstrate the potential to decrease nitrogen content, although this may come with an impact on oxygen content [59]. The selection of homogeneous and heterogeneous catalysts is contingent upon the particular biomass feedstock, aiming to augment both yield and quality of bio-crude.

  • Vacuum Distillation: Implementing vacuum distillation stands out as a technique to enhance bio-crude quality [60]. This process involves reducing oxygen and metallic content while adjusting boiling point ranges. However, researchers must grapple with trade-offs, including a potential decrease in bio-crude yield and limited reduction in the nitrogen-to-carbon ratio.

  • Bio-crude Separation Techniques: Exploring alternative methods for separating bio-crude, solid residuals, and aqueous phases is imperative. Filtration is identified as one such method that effectively reduces heteroatom content in bio-crude compared to traditional solvent extraction methods employing dichloromethane (DCM) [61, 62]. Investigating the mechanism and effects of different solvents on bio-crude properties is an ongoing area of study.

  • Batch Reactor Modifications: Optimizing batch reactor designs becomes instrumental in shortening the heating process and minimizing interference with HTL outcomes [63]. The incorporation of pressure tanks and flash heating are explored to improve efficiency and shorten processing times, aiming for more streamlined and effective hydrothermal liquefaction [64].

  • Thermochemical Characterization: To address challenges in achieving accurate thermochemical characterizations, researchers employ thermodynamic models to estimate performances [65, 66]. This involves a comparative analysis across different models to enhance the understanding of HTL outcomes and contribute to the optimization of the process for microalgae wet biomass [30].

These techniques, ranging from pretreatment to advanced separation methods, help optimize biocrude performance, address challenges and pave the way for the commercial viability of microalgal biofuels, contributing to sustainable energy solutions.

3.2 Catalyst development

The efficacy of the HTL significantly relies on the development and optimization of catalysts, which play a pivotal role in influencing reaction kinetics, product distribution, and overall efficiency. More specifically, the HTL of microalgal wet biomass, catalysts are crucial to enhance the yield and quality of biocrude, with their effectiveness influenced by different control parameters (such as temperature, residence time) and reactor system. Temperature is identified as the most influential operating parameter, with an optimal range of 270 to 400 °C for HTL [67]. Catalysts are often employed in the subcritical temperature range (270–350 °C; Fig. 1), with notable exceptions exploring supercritical conditions (> 373.94 °C; Fig. 1). The two main classifications of catalysts are homogeneous and heterogeneous. Homogeneous catalysts comprise alkaline salts and organic acids, whereas heterogeneous catalysts encompass transition metals, metal oxides, and activated carbon [68]. Alkali catalysts, such as potassium carbonate (K2CO3), have shown promise in enhancing biocrude yield, particularly for lignocellulosic biomass [68]. However, their efficiency may vary based on the feedstock composition. For high-protein-containing microalgae, alkali catalysts may negatively impact conversion efficiency. Acid catalysts (e.g. formic acid and acetic acid) have been explored, but their effectiveness is generally considered less promising, especially for lignocellulosic biomass [69].

In the catalytic thermochemical conversion of biomass, heterogeneous catalysts play a vital role, providing benefits like elevated catalytic activity, selectivity, recyclability, and reusability. Their adaptable structure, shape selectivity, and enduring characteristics render them highly suitable for biomass conversion processes on an industrial scale processes [70]. The creation of heterogeneous catalysts with substantial catalytic active sites is crucial for attaining enhanced reaction rate, particularly in cascade biomass conversion process. There are three main categories of heterogeneous catalysts: redox metals or acidic metal oxides (e.g. ZrO2, CeO2, Cu/CuFe2O4 and Fe/CuFe2O4), noble metal based catalysts (e.g. Rh/Ce0.9Pr0.1O2, Pd/Al2O3), and non-noble metal based catalysts (e.g. LaFeAl11O19, Co3O4/CeO2), which are widely utilized in catalytic HTL of various biomass [70, 71]. Notably, redox metals, particularly Fe-based catalysts, exhibit facile regeneration owing to rapid oxidation–reduction kinetics [72]. These catalysts are recognized for improving the formation and hydrogenation of light organic compounds within the water-soluble fraction. Despite their high cost, noble metal-based catalysts demonstrate remarkable catalytic activity in reducing various oxygen-containing compounds to hydrocarbons in biomass. An example is the effective use of Pd–Ir–ReOx/SiO2 catalysts in the hydrogenation of furfural to 1,5-pentanediol [70, 73]. On the other hand, non-noble metal catalysts, including Co, Mo and Ni demonstrate effectiveness in facilitating faster denitrogenation and deoxygenation reactions [74]. Notably, Co/CNT catalysts significantly boost biocrude yield in the HTL of Spirulina sp. biomass [70]. Research on redox catalysts, such as Fe-based catalysts, indicates their capability to generate H2 and facilitate the conversion of reactive chemical species through hydrogenation during HTL [75]. Duan and Savage et al. [76] reported the initial use of traditional hydrocarbon processing catalysts in the hydrothermal liquefaction of microalgae in aqueous environments, generating biocrude oil from the biomass of Nannochloropsis sp. (HTL performed at 350 °C). Across six different heterogeneous catalysts (Pd/C, Pt/C, Ru/C, Ni/SiO2–Al2O3, CoMo/γ-Al2O3, and zeolite) tested, the absence of added H2 resulted in higher crude bio-oil yields with consistent properties (heating value ~ 38 MJ/kg), showcasing unique desulfurization activity in the supported Ni catalyst [76].

The overall goal of catalysis in HTL is to maximize biocrude yield while minimizing the generation of undesired byproducts [77]. Achieving a balance between increased yield and improved biocrude quality, as measured by higher heating value (HHV), is a complex task. HHV (MJ/kg) can be calculated using the Dulong-Berthelot formula (Eq. 1) [78, 79], where C, H, N and O are in wt%.

$${\text{HHV}}= 0.3414{\text{C}}+1.4445{\text{H}}- \frac{N+O-1}{8}$$
(1)

The Energy Recovery (ER) is introduced as a metric to evaluate the efficiency of the catalytic process, considering both yield and HHV [80]. ER can be mathematically expressed as Eq. 2,

$$ER=\frac{{Y}_{biocrude} \times { HHV}_{biocrude}}{{{\text{HHV}}}_{feed}}$$
(2)

where Ybiocrude represents the biocrude yield (wt %) obtained from the HTL process, HHVbiocrude and HHVfeed denote the HHV values (MJ/kg) of resultant biocrude and the biomass feed, respectively. Table 1 presents biocrude yields (wt %) obtained from HTL of different microalgal strains with diverse homogeneous and heterogeneous catalysts.

Table 1 Biocrude yields from catalytic HTL of different microalgae strains
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Recently, Zhang et al. [95] examined the catalytic HTL of different microalgae strains at varying temperatures using ZSM-5 and MCM-41 catalysts. The highest yield of bio-oil (39.7 wt %) was achieved with a 5 wt % ZSM-5 catalyst during ozone-air flotation harvested microalgae. Combining ozone-flotation with HZSM-5 catalyst produced a bio-crude enriched in aliphatic, cyclic, and aromatic compounds suitable for fuel production [95]. Ozone-flotation improved the oxidative stability of biodiesel by reducing unsaturated bonds, minimizing the formation of ketonic and carboxylic compounds during storage. Deamination and deoxygenation reactions were witnessed at higher temperatures, alleviated by the 5 wt % catalyst, resulting in reduced nitrogen and oxygen content. Bio-crude from ozone-floated microalgae displayed higher HHV and ER [95].

The unique challenges posed by wet microalgal biomass include high moisture content, complex biomass composition, and temperature and pressure sensitivity. Microalgal biomass typically contains a high moisture content, posing challenges in terms of maintaining effective heat transfer and achieving optimal reaction conditions [96]. Catalysts need to address the specific characteristics of wet biomass, promoting efficient liquefaction despite the presence of water [97]. Furthermore, microalgal biomass is chemically diverse, containing proteins, lipids, carbohydrates, and other organic compounds [98]. Catalysts must be tailored to selectively target specific components to produce desired biofuels and platform chemicals, thereby minimizing unwanted by-products [99, 100]. Given that HTL operates at elevated temperatures and pressures, catalysts are required to remain active and stable under harsh conditions. Catalyst deactivation or decomposition can hinder the overall effectiveness of the process.

To address the intricate challenges posed by HTL of wet microalgal biomass, different studies have employed diverse catalyst development strategies aimed at enhancing reaction rates, selectivity, and overall process efficiency [67, 101, 102]. These strategies include the use of bifunctional catalysts, supported metal catalysts, tailored nanocatalysts, and catalyst stability and regeneration.

Bifunctional catalysts, possessing both acidic and basic sites, have shown promise in hydrothermal liquefaction [103]. They can facilitate hydrolysis, dehydration, and decarboxylation reactions, optimizing the conversion of microalgal biomass into bio-oils and platform chemicals. Similarly, supported metal catalysts (e.g. nickel and ruthenium) on various supports, have demonstrated catalytic activity in promoting hydrogenation and deoxygenation reaction, thereby contributing to the upgrading of bio-oils and enhancing their quality as potential biofuels. Nanocatalysts, with controlled particle sizes and surface properties, offer opportunities for improved catalytic performance [104]. Tailoring catalysts at the nanoscale can enhance surface area, reactivity, and selectivity, enabling more precise control over HTL reactions. Additionally, addressing catalyst stability and developing regeneration strategies are critical aspects of catalyst design [105]. Catalytic materials that resist deactivation and can be regenerated for prolonged use contribute to the economic viability of HTL [106].

Ongoing innovations in catalyst development hold significant promise for improving the efficacy and sustainability of HTL applied to wet microalgal biomass. The critical role of catalyst selection and optimization in overcoming challenges associated with HTL of microalgal wet biomass is evident. Factors such as the type of catalyst, its concentration, and specific HTL process conditions profoundly impact biocrude production efficiency. Many research groups around the world are actively exploring diverse strategies for catalyst development to enhance the overall performance of hydrothermal liquefaction [107,108,109].

3.3 Exploration of novel microalgal strains

The exploration of novel microalgal strains for HTL represents a cutting-edge avenue in the quest for sustainable biofuels and platform chemicals [110]. As mentioned, HTL, a thermochemical conversion process, involves the use of heat and pressure to wet biomass, leading to the breakdown of complex organic molecules into valuable liquid products which include bio-crude oil (a versatile feedstock capable of refinement into transportation fuels like diesel and jet fuel), bio-oils (rich in organic compounds, serving as precursors for myriad platform chemicals essential in chemical manufacturing processes), aqueous fractions containing organic acids and soluble compounds (fostering the production of fertilizers and additional chemical intermediates). Microalgae have emerged as encouraging feedstocks due to their rapid growth rates, high lipid content, and ability to thrive in diverse environments [29, 55]. To diversify feedstock resources, this subsection investigates ongoing efforts to identify and characterize novel microalgal strains for HTL. Beyond the fundamental exploration of strain-specific lipid content and adaptability, it explores the potential of these strains for efficient hydrothermal conversion. This analysis contributes to the broader understanding of how different microalgal strains influence the HTL process. Figure 2 shows the commonly explored microalgae strains in HTL research for biofuel and platform chemical production and the biomass composition (% dry weight) of different microalgae strains.

Fig. 2
figure 2

Microalgae strains in HTL research. Microscopy images of commonly explored microalgae strains in HTL research for biofuel and platform chemical production, accompanied by a bar plot depicting the biochemical compositions (% dry weight)

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Generally, microalgae biomass productivity ranges from 0.04 g/L/day to 0.37 g/L/d under photoautotrophic conditions. Similarly, lipid productivity varies widely, ranging from 0.0174 g/L/day (for Thalassiosira pseudonana) to 0.061 g/L/d (for Nannochloropsis sp.). This broad range underscores the diverse lipid production potential among different microalgal strains, highlighting the importance of strain selection for optimizing biofuel production. Based on different strains analysed for lipid production, Nannochloropsis sp. had been reported to exhibit the highest lipid productivity of 0.061 g/L/d (with corresponding biomass productivity at 0.21 g/L/d) followed by Scenedesmus sp. with 0.0539 g/L/d (biomass productivity at 0.26 g/L/d) and Chlorella sp. with 0.0537 g/L/d (biomass productivity at 0.28 g/L/d).

The key factors in strain selection for HTL include lipid content, biomass productivity, stress tolerance, nutrient utilization efficiency and genetic diversity. Microalgae with high lipid content are particularly desirable for HTL as lipids are a major precursor for biofuel production [111]. The exploration of novel strains with inherently high lipid accumulation or those amenable to genetic modification for enhanced lipid synthesis is a key focus. Rapid biomass growth is essential to ensure a sustainable and economically viable feedstock for hydrothermal liquefaction. Strains that exhibit high biomass productivity under diverse environmental conditions, including varying nutrient availability and temperature ranges, are prioritized. Given the varying environmental conditions in which microalgae may be cultivated, stress tolerance becomes a critical trait for ensuring consistent and robust performance [112, 113]. Strains that demonstrate resilience to stressors such as temperature fluctuations, salinity changes, and nutrient variations contribute to the overall stability of the HTL process [112]. Microalgae strains capable of efficient nutrient utilization contribute to the sustainability of the HTL process. Strains that can thrive in nutrient-deficient conditions or efficiently assimilate nutrients from wastewater streams align with the aim of making a circular and resource-efficient system [114, 115]. Exploring a diverse range of microalgal strains allows for the identification of unique characteristics and traits that may enhance the HTL process [116]. Genetic diversity can also contribute to the resilience of the overall biofuel production system against environmental fluctuations and stressors [117, 118]. Notable strains include Dunaliella tertiolecta, Chlorella vulgaris, Nannochloropsis gaditana, Tetraselmis suecica, Porphyridium purpureum, Phaeodactylum tricornutum, Scenedesmus obliquus and Scenedesmus almeriensis [30]. These strains contribute to high lipid content, biomass productivity, and adaptability to wet conditions, nutrient utilization efficiency, and genetic diversity, respectively. The comprehensive analysis enables better understanding of how different microalgal strains influence the HTL process, providing a foundation for targeted research efforts and advancing the sustainable production of biofuels and platform chemicals.

3.4 Recent advances in HTL technology

Recent advancements in HTL technology have propelled it to the forefront of wet algal biomass processing, showcasing advantages over alternative conversion methods. While conventional HTL processes have demonstrated efficacy in yielding bio-oil from various algae species, ongoing developments are imperative to optimize energy efficiency, reduce processing costs, and improve GHGs reduction via innovative approaches [119, 120]. The inherent challenges lie in the compositional disparities among algal species, the intricate nature of algal biomass, and the diverse optimal conditions required for HTL processing. The reliability of cost estimates and predicted environmental impacts is constrained by limited commercial-scale data [121]. Addressing these challenges necessitates comprehensive research encompassing various aspects, such as understanding the interactions between physical and chemical systems, refining predictive models for diverse microalgal biomass and operating conditions, designing robust continuous operation HTL reactor systems, and developing cost-effective, long-lasting catalysts [77]. Furthermore, overcoming engineering issues like phase separation and product purification is crucial [77]. The Sequential Hydrothermal Liquefaction (SEQHTL) process, in particular, stands out for its green technology attributes, leveraging water as a solvent and offering lower operational and capital costs due to mild operating conditions [122]. The two-stage approach enables fractionation of biomass for simultaneous high-value co-product recovery and biofuel production, creating potential revenue streams. While preliminary Techno-Economic Analysis (TEA) suggests the economic viability of HTL for algal biofuel production on a large scale with low biomass production costs, further research, especially on large-scale systems, is imperative for the commercialization of HTL and SEQHTL in biofuels and bioproduct applications [122].

In the realm of HTL advancements for microalgal biomass conversion, Watson et al. (2021) showcased an integrated dark fermentation–HTL (DF-HTL)—with notable environmental and energetic benefits compared to traditional HTL methods. Watson et al. [123] explored the potential of integrated DF-HTL for enhancing biocrude oil production from algal bloom microalgae. Compared to conventional HTL, DF-HTL significantly increased biocrude oil yield, carbon and energy content, and energy conversion ratios. It also reduced aqueous byproduct yield, carbon, nitrogen, and ammonia content, lowering environmental impact. However, DF-HTL improves N-content in biocrude oil. The study demonstrates the efficacy of DF-HTL as a promising method, offering environmental and energetic advantages over traditional HTL, particularly for high carbohydrate and protein containing feedstocks [123].

3.4.1 Sequential hydrothermal liquefaction process

While conventional HTL proves effective in converting wet microalgal biomass into biocrude, it encounters limitations arising from the incidence of proteins and carbohydrates in biomass. These components significantly impact the quality of the final product by introducing excessive amounts of O and N, leading to undesirable qualities such as oil acidity, polymerization, high viscosity, and a high-boiling distribution [124]. These challenges necessitate intricate upgrading processes for the derived bio-oil. Moreover, the presence of carbohydrates can impede phase separation efficiency in gravity methods, compelling the use of organic solvent extraction. Furthermore, proteins and carbohydrates may transform into inhibitory chemicals, causing a net loss of nutrients from the culture system, making the removal and recovery of these compounds imperative [38].

Several studies have reported that SEQHTL process addresses these challenges by employing a two-stage approach [122, 125,126,127]. In the initial stage, operations are conducted at lower temperatures (140–180 °C), focusing on breaking down the algal cell wall without inducing thermal degradation of co-products. This low-temperature hydrothermal treatment facilitates the extraction of various compounds, including polysaccharides, proteins, amino acids, pigments, and inorganic nutrients. Subsequently, in the second stage, the treated algae undergo liquefaction at moderate temperatures (240–250 °C) and reduced pressure (3.5 MPa) resulting in the generation of bio-oils, water extractives, biochar, and gaseous products. SEQHTL offers several advantages over conventional HTL. In the second stage, the diminished biomass content contributes to a reduction in water and energy requirements. Improved mass transfer between subcritical water and lipid molecules during this stage enhances oil extraction efficiency. Comparative studies indicate that SEQHTL results in a 50% reduction in biochar yield, an increase in bio-oil yields, and a 15% decrease in energy consumption (due to mild operating conditions) per unit of bio-oil obtained compared to DHTL [122, 126]. Studies indicate that the highest polysaccharide recovery occurs at 160 °C in the first stage, with the second stage operating at 240 °C [128, 129]. The bio-oil quality is influenced by temperature variations in the second stage, with lower temperatures yielding higher concentrations of fatty acids, particularly unsaturated acids.

3.4.2 Biocrude yield prediction using machine learning

In the realm of HTL technology, the precise estimate of biocrude yield is essential for the competent design and optimization of the processes of chemical conversions. Traditionally, empirical and mechanistic models have been employed for this purpose, but recent advancements have introduced machine learning (ML) algorithms as a promising alternative. Different studies have explored the application of machine learning models to predict the yield of HTL products, incorporating an extensive array of input variables (such as biomass proximate and ultimate profiles, biochemical compositions, and reaction conditions) [130,131,132]. Cheng et al. [133] pioneered the construction of machine learning models for simultaneous prediction of the yield of biocrude, aqueous phase, hydrochar, and gases across diverse varieties of biomass. They found that the random forest (RF) model outperformed regression tree (RT) and multiple linear regression (MLR) models. Many later studies also utilized machine learning for multi-objective analysis and optimization of biocrude yield, nitrogen content, and energy recovery, demonstrating the effectiveness of models such as Gradient Boosting Regression (GBR) and Particle Swarm Optimization (PSO) [132]. Shafizadeh et al. [134] created four ML models, including Support Vector Regression (SVR), Generalized Additive Model (GAM), Neural Network Regression (NNR), and Gaussian Process Regression (GPR), to predict biocrude yield, elemental composition, and HHV based on biomass and processing conditions. The GPR model outperformed others (R2 = 0.97), highlighting the influence of feedstock characteristics over processing conditions. This study also created a user-friendly software tool incorporating the GPR model for rapid estimation of HTL product yields, biocrude composition, and HHV [134]. Comparative analyses have shown that machine learning-based predictions outperform traditional mathematical equations, indicating the potential for improved accuracy in predicting biocrude yield in HTL processes. These advancements underscore the transformative role of ML in enhancing the predictability and efficiency of HTL technology.

3.4.3 Kinetic modelling for HTL

Kinetic modelling plays a pivotal role in understanding the HTL of microalgae, shedding light on the intricate processes involved in the transformation of biomass into valuable products. Activation energy, the minimum energy required for a chemical reaction, is a key parameter in these models, guiding researchers in resolving reaction kinetics. Different studies from have reported the distinct activation energy requirements for different conversion pathways involved in the conversion of solid microalgae biomass into biocrude, which are summarised in Fig. 3 [135,136,137,138,139,140]. In the macroscale kinetic modelling, studies have focused on various biomass feedstocks, emphasizing the importance of understanding activation energies for different components [141]. For example, the transformation of Spirulina sp. wet biomass into light and heavy biocrude revealed distinct activation energies, while comprehensive models considered protein-carbohydrate interactions and identified preferential pathways [142]. Mesoscale kinetic modelling utilized model components like soy protein concentrate, demonstrating the preferential production of gases over biocrude from aqueous phase. Additionally, studies explored the impact of ethanol on HTL kinetics, revealing reduced activation energies and supporting ethanol's role as a co-solvent. Microscale kinetic modelling, although less explored, employed monomers to represent compounds, providing insights into aqueous phase and biocrude yields. Sheehan et al. [140] developed a kinetic model for microalgae HTL, including novel interactions among algal proteins, carbohydrates, and lipids. While these interactions improved data fitting, the model without interactions performed better in predicting biocrude yields. The model accurately predicted 70 biocrude yields within ± 5wt% and 42 yields within ± 10wt%, showcasing its superior performance in capturing the influence of HTL conditions and algal composition on biocrude yields [140]. Overall, kinetic modelling in HTL of microalgae offers a comprehensive approach to optimize process parameters and design efficient systems for biomass conversion.

Fig. 3
figure 3

Activation energies required for converting solid microalgae biomass into biocrude. A bar plot of activation energy (kJ/mol) vs. conversion pathway. The variations in different pathways, including solid to biocrude, solid to light biocrude, solid to heavy biocrude, carbohydrate to biocrude, carbohydrate to light biocrude, protein to biocrude, protein to light biocrude, lipid to biocrude, and lipids to light biocrude, are shown

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4 Biofuel and platform chemicals from HTL of microalgae

Biofuel and platform chemicals derived from HTL of microalgae hold significant promise for advancing sustainable bioenergy production. Particularly, lipid-rich microalgae strains are gaining attention due to their efficient photosynthetic capabilities and minimal land requirements, making them ideal candidates for HTL applications. The diverse range of algae types, including microalgae, macroalgae, and mixed-culture algae, has been under scrutiny for HTL, with a recent focus on exploring low-lipid, high-protein algae such as Chlorella sp. and Spirulina sp. In the pursuit of optimizing HTL processes, critical parameters like reaction time and temperature play a pivotal role in influencing both the quantity and quality of the resulting biocrude.

While much emphasis has been placed on high-lipid microalgae, recent studies have revealed a growing interest in the utilization of low-lipid, high-protein algae. Notably, mixed-culture algae sourced from wastewater environments have exhibited potential by providing a sustainable approach, concurrently reducing nutrient loads. Moreover, municipal sludge has been recognized as a viable feedstock for HTL, offering versatility for conversion processes like SlurryCarb [71]. Nevertheless, to advance the application of HTL, addressing existing gaps through systematic studies is imperative, especially concerning various sewage sludge types and varied process variables.

Recycling the aqueous phase obtained from HTL in the reaction medium has emerged as a noteworthy strategy, significantly enhancing bio-oil yields from microalgae due to several key factors [143]. Firstly, the aqueous phase contains low molecular weight compounds such as organic acids, aldehydes, and alcohols, which are by-products of biomass degradation during HTL (Fig. 1). These compounds play a crucial role in enhancing biomass degradation by acting as catalysts or co-reactants in the hydrothermal conversion process [144]. Additionally, recycling the aqueous phase helps to maintain the balance of reactants and products within the HTL reactor, ensuring optimal conditions for bio-oil production. Moreover, the aqueous phase may contain water-soluble nutrients and minerals that can be beneficial for microalgae growth, thus promoting the sustainability of the HTL process. Hu et al. [145] investigated the recycling of aqueous phase from HTL of Chlorella vulgaris biomass to boost bio-crude oil yield and reported that bio-crude oil yield increased by 32.6 wt% (Na2CO3-HTL) and 16.1 wt% (HCOOH-HTL) when recycling aqueous phase from catalytic HTL experiments. Chen and Quinn [146] comprehensive study delves into the techno-economic and life cycle aspects of algae biofuel production through HTL. Their findings reveal baseline fuel prices, cost reductions achieved through temperature adjustments, and detailed environmental metrics. The study provides valuable insights into the economic viability and environmental impact of HTL-based biofuel production.

Another noteworthy exploration by Guo et al. 2022 involves the co-hydrothermal liquefaction (co-HTL) process of microalgae, specifically Chlorella vulgaris and Nannochloropsis gaditana, in conjunction with various plastics. This study uncovered positive synergistic effects, leading to greater plastic decomposition and increased HTL crude oil yields. The optimization of co-processing conditions using response surface methodology further refined the process, demonstrating a wide range of applications for the resultant crude oils. Notably, the study highlighted the potential of different crude oils for specific applications, for example, N. gaditana combined with polycarbonate crude oil identified as more suitable for aromatic chemicals production and N. gaditana combined with polystyrene crude oil favourable for biofuel applications [147].

Figure 4 represents the results of examining the elemental composition of biocrude obtained from HTL of wet microalgae biomass under different catalysts (H2O, Na2CO3, HCOOH), provides insights into the Higher Heating Value (HHV), energy recovery, change in heat content of feedstock (△Hc Feedstock), change in heat content of biocrude (△Hc Biocrude), and Energy Conversion Ratio (ECR) [148]. These parameters contribute to a comprehensive analysis of the HTL process's efficiency and potential applications. The aqueous phase obtained from HTL of wet microalgae biomass offers a significant potential for extracting valuable organic compounds. Recent studies indicate the production of monomers like Vinyl acetate and glycolic acid, with high concentrations of the latter having applications in cosmetics [149, 150]. Although microalgae-derived aqueous phase has a complex organic distribution, its potential for chemical production remains comparatively underexplored. However, production costs, accounting for 70% of chemical prices, demand advanced and cost-efficient technologies for sustainable microalgae-based chemical extraction [144].

Fig. 4
figure 4

Biocrude analysis of HTL of wet microalgae biomass. The bar plot illustrates the elemental composition of biocrude obtained from HTL (350 °C and ∼200 bar) of wet microalgae biomass under different catalysts (H2O, Na2CO3, HCOOH; 1 M each). The Higher Heating Value (HHV), energy recovery, change in heat content of feedstock (△Hc Feedstock), change in heat content of biocrude (△Hc Biocrude), and Energy Conversion Ratio (ECR) are also depicted for comprehensive analysis [148]

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The aqueous phase contains essential inorganic compounds, Nitrogen, Phosphorus, and Potassium, valuable as commercial fertilizers. While ammonia recovery, especially through air stripping, is effective for nitrogen, challenges persist in potassium extraction. The varying composition of the aqueous phase demands continuous efforts to recover both organic and inorganic compounds. Furthermore, as non-renewable fuel sources deplete, aqueous phase, rich in N, P, and K, emerges as a promising medium for algae cultivation. Different studies highlight the potential of aqueous phase to support algal growth, with a focus on parameters like growth period, aeration, and temperature [144, 151, 152]. Algae efficiently assimilate nitrogen and phosphorus from the aqueous phase, making it a viable growth medium, emphasizing the need for continued research and optimization.

In conclusion, the combination of microalgae and diverse waste materials, when subjected to HTL processes, opens up promising avenues for sustainable bioenergy production. Recycling strategies, in particular, enhance the overall efficiency of the HTL process for microalgae, paving the way for potential commercialization at a larger scale. More specifically, recycling the aqueous phase significantly improves the economics and footprint of continuous biocrude oil production [144]. This technique enhances feed characteristics, energy balance, and oil quality, contributing to a reduced carbon footprint. This also minimizes operating costs associated with water treatment technology. However, successful recirculation requires isolating and removing recalcitrant compounds. Aqueous phase recycling holds great promise for enhancing biocrude oil properties and achieving sustainable energy efficiency at a large scale. The exploration of different microalgae species, optimization of co-processing with plastics, and detailed elemental analysis contribute to a comprehensive understanding of the HTL process's intricacies and potential applications. As research progresses, addressing challenges in both organic and inorganic compound recovery and refining growth parameters for algal cultivation will further solidify the role of microalgae in advancing sustainable bioenergy solutions.

5 Advantages, challenges and limitations of HTL

HTL technology capitalizes on the unique properties of water under subcritical conditions, providing numerous advantages (Fig. 5). It facilitates the conversion of wet microalgae biomass, eliminating the need for energy-intensive drying. The use of a solvent minimizes unwanted side reactions by diluting product concentrations at lower temperatures, surpassing the energy efficiency of fast pyrolysis [153]. Additionally, HTL enables total biomass processing, yielding an aqueous phase rich in essential nutrients for microalgae growth. Although bio-oil from microalgae HTL may exhibit challenging properties for direct use, such as high acidity and a low H/C ratio, upgrading techniques like hydrotreatment and hydrodeoxygenation can enhance stability and fuel properties [124, 154]. Operating at high temperatures and pressures, HTL induces complex chemical reactions, resulting in diverse products, including hydrochar, gas phase, and an aqueous phase [155]. Valorization of these by-products aligns with circular bioeconomy principles, offering opportunities for gas utilization in microalgae growth, hydrochar incorporation into various applications, and recycling of water-soluble products for further HTL or as substrates in anaerobic digestion [68].

Fig. 5
figure 5

Venn diagram—HTL of Wet Microalgal Biomass. The Venn diagram highlights the overlapping areas of advantages (efficiency, versatility, high yield), challenges (energy input, catalyst dependence, product variability), and limitations (lack of standardization, economic viability concerns, integration challenges) of HTL applied to wet microalgal biomass

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The HTL of wet microalgal biomass stands at the forefront of innovative biofuel and platform chemical production. However, challenges persist, demanding further research (Fig. 5). The inherent high moisture content poses a substantial challenge, impacting overall energy efficiency and necessitating elevated inputs to overcome the heat of vaporization [156, 157]. Beyond moisture, the complex composition of microalgal biomass introduces nuances, leading to varied product distributions and optimizing conditions for extracting specific products [158]. Thermal stability becomes a crucial consideration, particularly for thermally unstable microalgal species, potentially causing undesired reactions and product degradation. Catalysts, vital for efficiency enhancement, present challenges in terms of selection and potential deactivation, compounded by the cost and availability of suitable catalysts [70]. Addressing corrosion and material compatibility issues becomes paramount due to the harsh reactor conditions—high temperatures and pressures. Scaling up from laboratory to industrial operations introduces complexities, including challenges in process optimization, scalability, and the absence of standardized protocols. The delicate balance between energy input and output is fundamental, with the energy required for the process potentially nearing or surpassing the energy content of the produced biofuels. Economic viability is a pervasive challenge, encompassing the overall cost of hydrothermal liquefaction, from biomass cultivation to processing and catalysts, necessitating competitiveness with other biofuel production methods and conventional fuels [1]. These considerations collectively underscore the multifaceted challenges inherent in HTL of wet microalgal biomass. Ongoing research endeavours and technological innovations aim to overcome these hurdles, unlocking the true potential of this process for sustainable biofuel and platform chemical production.

5.1 Reactor design and engineering

The challenges and limitations in HTL reactor design and engineering underscore the transition from lab-scale batch reactors to scaled-up continuous systems. Wagner et al. [159] investigated microalgae HTL using a cost-effective continuous flow system designed for wastewater-treated microalgae. The system, operated at temperatures of 300 °C–340 °C and flow rates of 3–7 mL/min, utilized high-pressure N2 for feed delivery. Employing high heating rates and extended reaction times, the continuous system yielded significantly enhanced bio-crude (bio-crude yield of 21.9 wt%, high algal ash content of 29.9 wt% and low lipid fraction of 7.9 wt%) compared to a batch reactor under equivalent conditions, emphasizing the importance of cost-effective continuous processing in lab-scale experiments for informed scale-up decisions [159].

Continuous reactors offer precise control over reaction parameters, energy recovery, and economic feasibility, as demonstrated by Reliance's journey from batch to demo-scale continuous systems [108]. Operational challenges, such as biomass slurry heating issues and agitator seal leakage, have been addressed through innovative modifications. Product separation from the mixed stream remains a critical challenge, with high solids and ash content posing risks of blockages [108].

5.2 Scalability challenges

Scaling up the HTL of wet microalgal biomass for biofuels and platform chemicals poses several challenges in transitioning from lab-scale experiments to large-scale industrial operations. Some key scalability challenges include,

  • Reactor Design and Engineering: Lab-scale HTL reactions are often conducted in batch reactors, which allow for precise control of reaction conditions [160]. Scaling up to continuous and larger reactors introduces challenges in maintaining efficient heat transfer, optimizing residence times, and ensuring uniformity in the reaction environment. Designing reactors that can handle the volume and flow rates required for industrial-scale production while maintaining optimal conditions is a significant challenge [159].

  • Heat Transfer and Temperature Control: Achieving and controlling the desired temperature profile in large-scale reactors is challenging. Ensuring efficient heat transfer throughout the reactor while preventing temperature variations and fluctuations is crucial for maintaining the desired reaction kinetics. This becomes more complex as reactor dimensions increase [43, 46].

  • Pressure Management: Scaling up HTL processes requires managing high pressures efficiently. Maintaining the necessary pressure levels while ensuring safety, reliability, and cost-effectiveness in large-scale systems poses challenges. Pressure reduction systems and safety measures become more critical as the scale increases [161].

  • Solid–Liquid Separation: Separating the solid and liquid phases after HTL reactions is challenging at an industrial scale. Dealing with high solids content in the reaction products and preventing blockages in downstream equipment are significant issues. Developing effective and continuous separation techniques is essential for uninterrupted operation [77, 162].

  • Feedstock Handling and Processing: Handling and processing large quantities of wet microalgal biomass present logistical challenges. Efficient pre-treatment methods that can handle varying compositions of microalgae, as well as issues related to impurities and contaminants, become more pronounced at scale [163].

  • Energy Efficiency: Ensuring energy efficiency in the HTL process at an industrial scale is crucial for economic viability. Balancing the energy input for heating and maintaining the process conditions against the energy output from the produced biofuels and platform chemicals is a challenge in large-scale systems [164].

  • Economic Viability: Scaling up introduces economic challenges related to capital costs, operational expenses, and overall process economics. Achieving cost-effective production of biofuels and platform chemicals becomes a critical consideration for the commercial success of large-scale HTL operations [77, 109, 165].

Addressing these scalability challenges requires interdisciplinary efforts involving engineering, chemistry, and process optimization. Continuous research and development are essential to overcome these challenges and make HTL of wet microalgal biomass a viable and sustainable technology for biofuel and platform chemical production on an industrial scale.

6 Future prospects and research directions

Future prospects and research directions for HTL of wet microalgal biomass for biofuels and platform chemicals encompass key areas aimed at enhancing efficiency, sustainability, and commercial viability. Advancements in HTL technology should concentrate on optimizing reaction conditions, exploring novel catalysts, and refining reactor designs for scalability. The identification and development of microalgal strains with tailored compositions, high growth rates, and resilience to environmental conditions are crucial for feedstock exploration. This involves investigating genetic modifications and cultivation strategies to boost biomass productivity and quality. Research should also target hybrid approaches that integrate HTL with other biomass conversion technologies to provide comprehensive solutions for biofuel production and platform chemical synthesis.

Efforts should focus on understanding and mitigating challenges related to reactor design, energy recovery, and product separation. Innovative separation techniques are needed to efficiently handle the complex product streams generated during HTL, ensuring a seamless and economically viable process. Exploring the utilization of by-products and waste streams from HTL processes can contribute to a circular economy approach, minimizing waste and maximizing resource efficiency. Additionally, assessing the environmental and economic sustainability of HTL-derived products will be crucial for broader market acceptance.

In the aqueous phase obtained from HTL, versatility is highlighted, showcasing its capability to process various biomass types for the recovery of valuable products, making it a crucial candidate for producing chemicals, fuels, and electric gases. However, the review emphasizes the need for further investigation into the advantages and limitations of different conversion approaches for the aqueous phase. The section discusses the potential of algae cultivation in the aqueous phase, emphasizing the challenges related to inhibitory compounds and the necessity to process these compounds for enhanced biomass yield. Anaerobic digestion is recognized as a valuable technique, but challenges such as poor conversion of organics with nitrogen and the need for additional valorization techniques are acknowledged. Recycling of the aqueous phase in the HTL process is proposed to enhance bio-oil yield, yet economic evaluations and considerations for decreased yield after multiple recycles are highlighted. The extraction of value-added chemicals from the aqueous phase is suggested, with an emphasis on economic evaluation and applicability to complex compositions. The summary underscores the need for further investigation, economic evaluations, and the resolution of sustainability and economic barriers to enable the large-scale success of aqueous phase techniques. Additionally, challenges and advantages of specific valorization techniques, including algae cultivation, methanation, hydrothermal gasification, recycling, and extraction of value-added chemicals, are outlined in the provided table.

To enhance the microalgae conversion process, a shift towards bio-based catalysts is recommended, considering the high cost associated with synthetic catalysts. The recycling of waste energy can contribute to reduced operational costs. However, challenges such as the high cost of equipment for process scale-up pose limitations to research progress. Promising microalgae conversion processes, including the use of bio-based solvents and supercritical fluids for biodiesel production, hydrothermal liquefaction for biogas production, and ultrasound/microwave enhanced extraction for bio-oil production, hold potential for high yields and economic viability. Future research is expected to focus on these techniques, attracting increased interest from researchers. Additionally, addressing knowledge gaps and research questions related to heat sinks for reducing electricity-generated heat, catalytic technologies for converting microalgae biomass into fungible fuels, and the effects of different blend ratios on biomass biodiesel properties will contribute to advancing the field.

As part of future research directions, considerations for heat sinks to mitigate greenhouse gas emissions, the development of oxygen carrier catalysts for enhanced carbon conversion efficiency, mechanisms to improve cellulose conversion in biomass, measures to mitigate the effects of cogeneration of electricity, alternatives to toxic solvents for scaling up yeast lipid fermentation conversion, and the effects of blend ratios on biomass biodiesel properties will be crucial for further advancements in microalgae conversion technology. Addressing these research questions will contribute to the sustainable and efficient utilization of microalgae biomass for biofuels and platform chemicals.

Contemplating future prospects, it is crucial to acknowledge the advancements and challenges specific to HTL processes for wet algal biomass. While HTL offers advantages over other conversion methods, including its ability to process wet algal biomass, the optimization of conventional HTL processes is imperative for increased net energy return, cost reduction, and improved greenhouse gas (GHG) reduction. Challenges arise from the compositional differences among algal species, the complex nature of algal biomass, and the need for specific optimal conditions, making general predictions and operations challenging. The focus of previous research primarily centered on testing HTL performance with various algae species and optimizing bio-oil yield. To perfect HTL for commercial application, there is a need for in-depth investigations in several areas. These include improving the understanding of interactions within physical and chemical systems, especially regarding cell structural changes and reaction pathways in different algal biomass constituents. Enhancing performance predictability for different algal biomass and operating conditions is crucial, along with developing robust continuous HTL reactor systems, fostering flexibility in harvesting high-value co-products. Other priorities involve developing cost-effective and durable catalysts, addressing engineering challenges like phase separation and product purification, and conducting comprehensive economic and environmental analyses based on reliable data from operating systems at the demonstration scale. Incorporating these advancements will be vital for the successful future implementation of HTL processes in the algal biomass sector.

7 Conclusion

The review comprehensively explores the advances and future prospects of HTL applied to wet microalgal biomass for biofuels and platform chemicals. The intrinsic ability of HTL to efficiently process various biomass types, particularly wet microalgae, underscores its versatility and potential in sustainable biofuel and platform chemical production. Notably, the sequential hydrothermal liquefaction (SEQHTL) process emerges as a promising extension, offering enhanced economic feasibility through lower operational costs and simultaneous recovery of high-value co-products. The techno-economic analysis suggests the potential for cost-competitive algal biofuel production, contingent on large-scale operations and optimized biomass production costs. However, commercialization hurdles persist, mandating further research focus on scaling up processes, demonstrating viability with large-scale systems, and addressing knowledge gaps. The future prospects of HTL hinge on holistic approaches, encompassing advancements in reactor design, catalyst development, feedstock exploration, and innovative integration with biorefinery processes. As the bioenergy landscape evolves, continued interdisciplinary research efforts will pave the way for HTL to emerge as a pivotal technology in the sustainable production of biofuels and platform chemicals from wet microalgal biomass.