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Environ Eng Res > Volume 28(3); 2023 > Article
Venkatachalam, Sekar, Ravichandran, Sengottian, Sukumar, Chenniappan, and Ramachandran: A review on bio-crude production from algal biomass using catalytic hydrothermal liquefaction process


The rising demand for cleaner energy among the nations has turned the focus from fossil fuel-based energy sources to renewables like biomass, solar, wind, etc. Biomass conversion to fuel has increased research in recent times due to its ease and availability throughout the year. Hydrothermal liquefaction is the process where biomass converts to a liquid product via complex reaction mechanisms. This review aims to summarize the hydrothermal liquefaction of algal biomass and the improvements in the bio-crude yield using heterogeneous and homogenous catalysts. Many references have been reviewed to provide the sources for the process and have been critically well structured. This review also provided information regarding the reaction pathway for algal biomass and the effects of process parameters like temperature, residence time, pH, etc. The focus of the review is on the effects of various catalysts based on their dosage whose results collected from various sources have been tabulated. The review briefly discusses the applications of products formed during hydrothermal liquefaction after post-processing.

1. Introduction

Fossil fuel plays a major role in our global energy needs. In recent times the world faces a difficult situation wherein the usage of fossil fuel is getting costlier day by day due to its non-renewable nature. In this respect, biofuel is considered one of the best alternative sources of renewable energy, to reduce the usage of fossil fuels [1] and also minimalize CO2 emissions and harmful gases [24]. Biomass is renewable organic material that comes from food crops (1st generation), agricultural and forest wastes (2nd generation), algae (3rd generation), and municipal wastes (4th generation). From the above, the first generation of biomass is not suitable for conversion as it would directly affect the food source, and biofuels made using 2nd and 4th generation biomass are less productive than 3rd generation biomass. There are multiple advantages to using algae as a feedstock for biofuel due to its ease of production [57]. Multiple studies have been performed to analyze the efficiency of algae as a rich source of biofuel, where different types of algae (micro and macro) were considered along with various homogenous and heterogeneous catalysts. Ali A.Jazie et al studied the effects of Hβ zeolite as a functional catalyst to improve the yield of F.vesiculosus algae which produced and yield of 38.47% [8]. Nguyen Thúy Lan Chi et al proposed a technique where the bio-char produced through thermochemical conversion can be utilized as a catalyst for the biofuel production from algal biomass, showing that the selection of catalyst in hydrothermal processing is important as the quality and property of the biofuel varies accordingly [9].
The ways through which the algal feedstock is converted into biofuel are classified into Thermal, Chemical, Thermochemical, and Biochemical [7, 10]. The thermochemical methods are further classified into the dry and wet processes. In Dry thermochemical conversion, the feedstock should be free from moisture or with a minimal amount of moisture. Whereas in wet thermochemical conversion, the feedstock can contain moisture or liquid [11].

1.1. Dry Thermochemical Conversion

It consists of several methods like pyrolysis, torrefaction, carbonization, liquefaction, and gasification, mostly the feedstock needs to be moisture free [11]. Carbonization is a slow process where the biomass is heated in the inert atmosphere to produce a carbon product, pyrolysis is a process where the biomass is burnt in an inert atmosphere mostly in nitrogen at around 300°C – 900°C [12], liquefaction is divided into direct and indirect, where the biomass is liquefied at around 160 to 280°C in the presence of glycerol [13], gasification is the process where the biomass is heated above 1000K under inert atmosphere to produce gaseous fuels [14].

1.1.1. Pyrolysis

The conversion technique in which moisture-free biomass is heated in the presence of nitrogen, are classified into flash/fast pyrolysis, slow pyrolysis based on the heating rate, microwave-assisted pyrolysis based on the heating medium, and hydropyrolysis which takes place in the high-pressure hydrogen environment. Selection of the right kind of conversion method is critical to obtaining desired results [15]. Slow pyrolysis typically happens at a very low heating rate in the range of 5–10°C / min, this technique produces an estimate of 43% by volume of oil as investigated by Scott Grierson et al [16]. Fast pyrolysis in the contrast has a higher heating rate in the range of 100 – 200°C/s, and very little residence time during the reaction, higher heating rates upwards of 500°C were also studied by Xiaoling Miao et al to produce higher grade bio-oil, the product characterization showed that the bio-oil produced through fast pyrolysis contains lesser oxygen content compared with slow pyrolysis [17].

1.1.2. Torrefaction

The process of heating the biomass at lower temperatures compared with pyrolysis in the range of 200–300°C [18] in the atmospheric pressure in the presence of nitrogen to produce solid char as the main product, liquid, and gas as by-products. The conversion method is classified into two categories wet and dry torrefaction based on the temperature and residence time, both types have distinct advantages and disadvantages when used in the conversion of microalgae to solid fuel. Since dry torrefaction (200°C–300°C) is simply mild pyrolysis, there is a requirement for dry torrefaction that the moisture of the biomass is reduced to 10% to get high performance, in contrast, wet torrefaction (180°C–260°C) requires a complex reactor system to maintain high pressure which ultimately increases the cost of the overall process [19]. The primary advantage of both processes is the superior characteristics of the bio-char produced, and the bio-char produced can be filtered, dried, and can be used directly as fuel without further enhancement [20].

1.2. Wet Thermochemical Conversion

Wet thermochemical conversion is classified into three methods viz. Hydrothermal carbonization, Hydrothermal liquefaction and Hydrothermal gasification. Hydrothermal carbonization mainly focuses on the production of solid products (bio-char) and this process is carried out at low to medium temperatures (180°C–275°C) and at lower pressure below 2MPa [21]. Hydrothermal liquefaction emphasizes the production of a liquid product termed bio-crude which is further converted into bio-oil after treatment, this process takes place at medium to high temperatures (280°C–370°C) and pressure ranging between 10–25MPa [22]. Hydrothermal gasification is also identified as supercritical water gasification (SCWG), in this process the primary product is combustible high calorific value gases. This gasification process is carried out at temperatures around 400°C–700°C and pressure 25MPa-30MPa [10].

1.2.1. Hydrothermal liquefaction

Hydrothermal liquefaction is a process where biomass either in the presence of a catalyst or without a catalyst is converted into bio-crude. In hydrothermal liquefaction the temperature is around 280°C – 370°C and the pressure around 10MPa – 25MPa [23]. The yield and efficiency of the process depend upon the processing conditions like temperature, pressure, biomass loading and residence time, the conversion of biomass to product also depends on the algal strain used, based on the information multiple attempts are made with various processes conditions to produce a liquid product. Hydrothermal liquefaction is where the depolymerization of algal feedstock takes place, which helps in the production of bio-crude [24]. The produced bio-crude has higher energy content than produced in pyrolysis along with lower energy consumption [10, 25]. The higher heating value is shown in Fig. S1, obtained through hydrothermal liquefaction is an important factor if the produced bio-crude is to be used as fuel. The proximate and ultimate analysis of algae species are given in Table 1 and Table 2, this data helps in the identification of suitable micro/macroalgae species for further processing.

2. Chemistry of the Process

Very little information is known about the chemistry behind the hydrothermal liquefaction process, the basic process that happens is that during the process water will be at a subcritical condition and hence will involve in the decomposition of the biomass into bio-crude and other valuable products [40]. The main composition of microalgal species is carbohydrates, protein, and lipids [41]. These components undergo degradation in subcritical water to give a basic reaction mechanism of biomass decomposition, depolymerization (dehydration, decarboxylation, and deamination), and the recombination process. Even though hydrothermal liquefaction conversion is difficult to understand, only very few authors have identified the conversion pathways [10, 42]. The conversion pathway for hydrothermal liquefaction is shown in Fig. S2.

2.1. Mechanism of Carbohydrates in Hydrothermal Liquefaction

Carbohydrates are complex sugars like cellulose, hemicellulose, polysaccharides and starch [43]. When these components undergo hydrothermal processing it rapidly forms glucose and a few other saccharides via hydrolysis, which further undergo degradation. In this regard, the hydrolysis rate depends upon the nature of polysaccharides like starch and hemicellulose break down quicker compared to cellulose [10].

2.1.1. Cellulose

Cellulose on the whole contains glucose units linked using beta-linked D-Glucose units, which help in the formation of stronger inter and intramolecular hydrogen bonds. Due to the nature of cellulose being linear chained and crystalline it does not undergo any significant deformation at low temperatures like starch [44]. Cellulose fiber is one of the most abundant organic polymers in the world, also a hydrophilic compound that does not dissolve in any organic solvents or water at low temperatures. They are complex carbohydrates that are essential in bio-oil formation and oil yield. These during the reaction process get hydrolyzed into simple sugars, based on the type of algae (high lipid and low lipid) used about 10–20% of the overall yield is obtained from carbohydrates. Based on the studies found that cellulose depolymerizes at temperatures below 400°C to form benzene, propionic acid, and 4-hydroxyphenyl alcohol [27] and alcohols and phenols [45]

2.1.2. Hemicellulose

As a heteropolymer consisting of various sugars like glucose, galactose, mannose and xylose. Due to the presence of a side group a less uniform structure of the hemicellulose. Around 30% of the biomass comprises hemicellulose and they vary according to the biomass type taken because the structure of the hemicellulose is branched they are weaker than cellulose and are easily degradable at low temperatures above 180°C and also hydrolysis in both acid and base catalyst[43]. The main products obtained through hydrothermal liquefaction of hemicellulose are furfurals, carboxylic acids, and aldehydes at higher temperatures around 300°C [46].

2.1.3. Starch

Starch is made up of polysaccharides which consist of monomers like glucose connected with α-(1→6), β-(1→4) bonds. α-(1→4) is glucopyranoside which is a polymer and is soluble in cold water. β-(1→6) is a glucoside branch that is insoluble in cold water [47]. In the hydrothermal liquefaction process, It is usually referred to as amylum, as it contains two different molecules like amylose (helical), amylopectin (branched), and a liner structure. Though starch and cellulose have similar properties, starch is insoluble in cold water and is the most common type of carbohydrate available [48, 49]. During the processed starch first gets converted into small fragments of light molecular compounds through decarboxylation and hydrolysis, after which through cyclization and condensation the water-soluble organics get converted into new compounds and bio-char [48, 50].

2.2. Mechanism of Protein

Protein is the prime constituent of microbial biomass. Generally, protein consists of one or many peptide chains, which give rise to the chain of amino acids. The proteins are made up of amino acids that are grouped into 21 groups based on their side chain properties. All are linked through a peptide bond, which undergoes decarboxylation and deamination reaction. As the result of these reactions, the produced products are acids, amines and aldehydes [51].
A peptide bond is the one that links the amino acids primarily an amide bond between amino and carboxyl groups [52]. Nitrogen is the important molecule present in the protein due to its presence in amino groups. Thus it is the main contributor in terms of nitrogen content in both the aqueous phase and organic phase, the nitrogen molecules are hydrolyzed and present in the aqueous phase as amine compounds and those compounds are not in the form of pyrrole or pyrazine in the bio-crude [53]. Since the peptide bond in the protein is far more stable than the glucoside bonds present in starch and cellulose due to slow hydrolysis occurs. The yield of amino acids during hydrothermal liquefaction is very minimal compared to the conventional process due to the rapid hydrolysis of amino acids in the hydrothermal liquefaction (HTL) condition [10, 35].

2.3. Mechanism of Lipids

Lipids are fats present in organisms and they have similar chemical structures to the hydrocarbons used as fuels. And it has an aliphatic character, which is referred to chemically as triacylglycerides (TAG), a compound of fatty acids and glycerol [54]. Fats and oils are insoluble in ambient conditions, but triacylglycerides are easily hydrolyzed under subcritical conditions with or without the use of catalysts. As the result of hydrolysis, the triacylglycerides are converted into glycerol and fatty acids [55].

2.3.1. Glycerol and fatty acids

As one of the main products formed during hydrolysis, it is an important compound used in bio-crude production. It can synthesize specialty chemicals that can be used for many propose. On hydrolysis, triacylglycerides degrade to produce acrolein, acetaldehyde, propionaldehyde, methanol, allyl alcohol, formaldehyde, ethanol and gas products, mainly CO2, CO, and H2. Fatty acids have higher thermal stability but they are converted into long-chain hydrocarbons, which have good properties for energy or fuel [10, 56]. The conversion pathway for proteins, lipids and carbohydrates is shown in Fig. 1.

2.3.2. Mechanism of Algaenans

Algaenan’s conversion process is important in the conversion of microalgae in hydrothermal liquefaction. But not many studies are conducted on the conversion of algaenans. This is a macropolymer of hydrocarbon, which has the characteristic of resistance to chemical treatment than other microalgae fractions. Its resistance property makes it difficult to study and analyze. There is not much data available on the hydrothermal liquefaction of Algaenans [10].

2.5. H/C, O/C, N/C Ratio

To identify the effectiveness of produced bio-crude as a potential fuel, the molar ratios of hydrogen, oxygen and carbon of bio-crude are plotted against the values of known liquid fuels in the Van-Krevelen plot. When compared with existing fossil fuel data, the obtained bio-crude is found to have a higher percentage of oxygen and hydrogen, which shows the need for a further purification process. The H/C vs O/C ratios of a few algal bio-crude is given in Fig. 2.

3. Non-Catalytic Hydrothermal Liquefaction

In non-catalytic hydrothermal liquefaction, water plays a major role in both catalysts as well as solvents. When the Catalyst is not used for the hydrothermal liquefaction process water is used as a catalyst [57, 58]. The properties of water under subcritical conditions are very important to understanding the degradation pathway along with hydrothermal processing of biomass, at subcritical and critical conditions of water, a gas such as CO2, and O2 are miscible. Organic compounds are dissolved in water in critical and subcritical conditions. Whereas the inorganic compounds which use solvent will decrease in critical conditions [5963]. The bio-crude yield concerning various temperatures is given in Table 3.

4. Catalytic Hydrothermal Liquefaction

It is widely demonstrated that the presence of a catalyst would promote the conversion of biomass into bio-crude which leads to higher yield and quality product. In hydrothermal liquefaction, different types of catalysts are used [48, 59, 71]. There are two types of catalysts that are being used in hydrothermal liquefaction, homogeneous catalysts and heterogeneous catalysts. The most widely used catalyst in hydrothermal liquefaction such as alkaline solutions are K2CO3, Na2CO3, LiOH, NaOH and KOH. The key role of catalysts is to decompose the macromolecules(cellulose and hemicellulose) present into small fragmented molecules that are unstable, reactive, and repolymerize into oil-based compounds [72]. For different species of microalgae, the bio-crude yield varies. It is due to the different lipid content present in the algae. For spirulina, the yield value is significantly increased for the catalyst NaOH. For the other catalyst spirulina bio-crude yield is lower and similar to each other. In the case of nanochloropsis, the bio-crude yield is maximum for the catalyst of KOH and the bio-crude yield for other catalysts is lower [7376]. The bio-crude yield of four algal biomass in presence of an organic and inorganic catalyst is given in Fig. S3.

4.1. Homogenous Catalyst

Usually, hydrothermal liquefaction uses catalysts that are water-soluble at room temperature (e.g., Na2CO3, KOH, CH3COOH and HCOOH). As a result, they often encourage algae to liquefy by improving the hydrothermal process. Normally Na2CO3 is the most used homogenous catalyst in the hydrothermal liquefaction process [77, 78]. Because it will increase the bio-crude and solid residue yield among the homogenous catalyst. A carbonate, hydroxide and carboxylic acid homogenous catalyst does not appear to be very common in hydrothermal reactions. Due to their low susceptibility to catalyze the decarboxylation of fatty acids, aromatization and isomerization coupled with their non-recyclable nature [79]. The homogenous catalyst is used during the reaction to some degree, and because its recovery cost is high, it is difficult to select it for cyclic use [8082]. Some of the commercially available homogenous catalyst ad their effectiveness on the bio-crude yield is summarized in Table 4.

4.2. Heterogeneous Catalyst

The heterogeneous catalyst has the advantage in the separation of liquefied products from solid residue and it is reused after proper treatment [86]. The heterogeneous catalysts are widely used due to their higher catalytic efficiency, low corrosion and higher bio-crude recovery. One of the most widespread disadvantages of homogenous catalysts is the damages that occur due to chemical reaction, which is not present in heterogeneous catalysts. Heterogeneous catalysts have been extensively studied for hydrothermal liquefaction, such as supported metal catalysts (Pt, Ni, Pd, Ru, and others) [79, 87]. In algae hydrothermal liquefaction, metal catalysts have a complex effect on bio-crude yields, but all catalysts do not increase yields significantly. Examples of materials with good catalytic desulfurization activity include Pd/C, Pt/C, Ni/SiO2-Al2O3, and CoMo/*-Al2O3 [38, 77, 88]. Table 5. provides the summary of heterogeneous catalyst and their yield.

5. Effects of Process Parameters on the Bio-Crude Yield

5.1. Temperature

The temperature of the hydrothermal process is crucial in the bio-crude formation and their yield, the addition of catalysts not only increases yield but also the characteristics of the bio-crude obtained [65, 88]. An increase in temperature usually leads to quicker depolymerization thus producing higher bio-crude yield along with gaseous products, but this also depends on the other process conditions like catalyst dosage, residence time, etc., thus bio-crude yield may vary at the same temperature due to variation in other process conditions or type of catalyst [94, 95]. One of the major reasons that hydrothermal liquefaction yield of algal biomass is higher compared to other sources is the presence of peptide bonds, during lower temperatures the peptide bonds are stable thus resulting in lower bio-crude yield. But at higher temperatures in the range of 300°C – 400°C the peptide bonds are less stable and rapidly hydrolyses to produce higher bio-crude yield [63, 96]. The yield differences due to temperature are shown in Fig. 3. which clearly shows that yield of bio-crude is increased with an increase in temperature.

5.2. Pressure

Hydrothermal liquefaction is also influenced by pressure when it comes to decomposition and hydrolysis. An increase in pressure is used during the hydrothermal liquefaction process where higher enthalpies are not required in phase change [97, 98]. To identify the effect of pressure in the process during the experiments all the other parameters are kept constant while the pressure is varied, this showed that there is a significant increase in bio-oil as the pressure increases [99]. Pressures above the critical pressure can be maintained to control the rate of hydrolysis and biomass dissolution, which can increase the productivity of bio-crude through improved thermodynamics [100]. Catalytic run for the same conditions shows a decrease in the amount of oil produced, this is due to the blocking of catalyst active site by high-density solvent when the pressure is increased [101]. This emphasizes that selection of catalyst is crucial during high-pressure liquefaction.

5.3. Residence Time

Residence time is termed as the time required for the hydrothermal reaction to occur after the desired temperature or pressure is reached, this excludes heating and cooling time [102, 103]. This residence time is very crucial in obtaining higher quality bio-crude and other useful products. Lack of residence time may result in incomplete conversion, while excessive residence time can result in the decomposition of desirable products [39, 48, 104]. Small residence times are preferred over longer residence times during the hydrothermal liquefaction at higher temperatures as the decomposition and hydrolysis of components are rapid. The addition of a catalyst at the time would improve the conversion as it provides space for the reaction to occur much quicker than that process without it under the same process conditions [105, 106].

5.4. pH

The reaction medium pH is very important during the hydrothermal process, as the reaction pathway depends on pH, researchers have identified that hydrothermal liquefaction of biomass produces a higher yield in alkaline and acidic conditions depending upon the type of catalysts and solvents [107, 108]. The compounds like levulinic acids and 5-(Hydroxymethyl) furfural (5-HMF) do not decompose in acidic conditions, The biomass produces carboxylic acids function groups such as acetic acids and lactic acids, due to the self-degradation of H2O to H+ and OH at very high temperatures where the pH does not affect the conversion [65, 109]. During the process, if the reaction condition is weakly alkaline as the reaction progresses the pH changes to acidic [109112]. The products obtained during hydrothermal liquefaction are primarily liquid (bio-crude & aqueous), and the pH condition of the reaction medium is very important [113116]. When the bio-crude yields are compared with the pH of the solution it is found that to ensure higher yield at a lower temperature the reaction medium is to be acidic but at the same time to produce higher yields at medium to higher temperatures the reaction medium is to be alkaline as given in Fig. S4. But it is very important to consider the fact that there will be a difference in yield at same process condition but with different catalysts as they tend to affect the reaction medium, so selection of catalyst is very important during hydrothermal process.

5.5. Biomass Loading

Biomass loading is crucial in determining the process and the yield to be obtained, during the hydrothermal liquefaction process the ratio of biomass to water will be lower compared with carbonization to facilitate hydrolysis to produce higher amounts of liquid products [117]. Maintaining the optimum loading is very important and it is reported that when biomass loading is increased the bio-crude yield will also increase but beyond a certain point, the yield will reduce due to degradation of biomass through cyclic formation, polymerization, and condensation reactions [118, 119]. Bita Motavaf and Phillip E. Savage (2021) reported that with biomass loading of 10–20% the yield of the bio-oil increased and reached the maximum of 30% at the critical temperature [120]. Gargi Goswami et al (2022) investigated the catalytic conversion of lipid-enriched microalgae and found out that the maximum yield of 28% was obtained at the biomass loading of 8.86% and corresponding catalyst dosage [121]. Christopher Jazrawi et al (2013) studied the continuous hydrothermal liquefaction of algae and found that at 350°C and loading of 10% produced a yield of 41.7 wt% [122]. Peter J.Valdez et al (2012) discovered that when the biomass loading for the hydrothermal liquefaction of Nannochloropsis sp. from 5–35 wt% showed an increased bio-crude yield of 36% to 46% [123]. Thus, multiple studies have reported that when the biomass loading is increased the bio-crude yield also increases but that depends on the biomass and the process conditions.

5.6. Catalyst Dosage

When considering catalytic hydrothermal liquefaction, the dosage of the catalyst is very important. The yield will not be optimum if there is too little or too much catalyst in the system [124]. Wenjia Wang et al investigated the catalytic hydrothermal liquefaction of Nannochloropsis sp. using metal-based TiO2 catalyst, the results showed that the yield of bio-crude increased from 30% to 42 % when Ni-based TiO2 is used, the results also showed a decrease in the yield of 29.06% if Fe was used, all the other process variable sare constant [89]. These two catalysts Na2CO3 and HCOOH are homogenous catalysts, which produces lower yield when compared with heterogeneous catalyst. For heterogeneous catalysts, Ni/Al2O3 and Co/Al2O3 are taken for the hydrothermal liquefaction process for chlorella vulgaris. When Ni/Al2O3 is used as the catalyst for the reaction the bio-oil yield is around 30% at a temperature of 350°C and for Co/Al2O3 bio-oil yield is around 38.7% at a temperature of 350°C [125].

5.7. Bio-Crude Yield

The yields of bio-crude and other end products produced by hydrothermal liquefaction of microalgae with and without catalyst are shown in Tables 3, 4 & 5. Peigao Duan and Phillip E. Savage while investigating the hydrothermal liquefaction of Nannochloropsis. sp using heterogenous catalyst found that bio-crude yield increased from 35% (No Catalyst) to around 65% (with Pd/C) [7]. With the detailed investigation of different kinds of literature, it was found that in the aspect of bio-crude yield is not only based on triglycerides but also includes carbohydrates, proteins and fibers [126]. Because hydrothermal liquefaction has such a high potential for conversion of different types of biomacromolecules into bio-crude, it applies to a wide range of microalgae and wet biomass. Though catalysts are different based on their chemical nature, structure, etc., it was found that few of them produced the same yields which are interesting but still in-depth discussion must be done to verfy the same [127, 128]. In few instances the bio-crude yield is low compared with non-catalytic conditions as the presence of catalyst at certain conditions will enable hydrocracking which reduces the molecular size and produce more volatile compounds rather than heavy liquid compounds [129, 130].

6. Application

The algae biomass is converted into bio-crude, aqueous phase, and bio-char. Every end product has its own and different applications. The aqueous phase can be recycled for algal growth since it contains more amount of micro and macronutrient required for algal growth. And it is also used for agricultural and irrigation propose[131, 132]. The bio-char application is identified using the physicochemical characterization of hydrothermal liquefaction bio-char [133, 134]. This bio-char contains high nitrogen and carbon content along with inorganic metals like Mg, Ca, P, and K allowing the bio-char to be utilized in the soil in agricultural practices[135]. It will improve soil respiration, soil enzyme activity, seed germination, and soil respiration. The bio-char consists of surface-active moieties which help the bio-char act as a good absorbent for removing heavy metals and pollutants from wastewater. The refining and upgrading required for the possible ratio for blending with crude fuel are found using the physicochemical characterization of bio-oil. De-polymerization of the biomass of microalgae gives oxygen content which gives numerous oxygenated compounds such as furans, alcohols, phenols ketones, aldehydes, and other oxygenated compounds. High content of oxygen in the bio-crude will have various problems such as poor miscibility with fossil fuels, rendering side reactions of bio-crude low thermal stability, and highly reactive nature [119].
Low ratio H/C has a disadvantage in the fuel properties not allowing the direct application of bio-crude as a transport fuel for the vehicle. While these properties are not suitable for biofuels, it is clever to produce biofuels with the same properties as conventional fuels and also confirm the minimal emission of engines. It is important to carry out a standard crude oil refining process of the bio-crude before being utilized for blending commercial uses [136, 137].

7. Conclusion

Hydrothermal liquefaction of microalgae seems to be an emerging technology for bio-oil production. Compared with pyrolysis this method has quite a few advantages like no pre-removal of moisture is needed for the biomass, much less energy required and at lower temperatures, the process is not risky, and the yield obtained though a little less is still a great advantage over pyrolysis, the disadvantage is the amount of hydrogen and oxygen in the final product that reduces the calorific value. This can be seen in the elemental analysis of the produced bio-crude, to rectify this drawback the use of external agents like catalysts are used. At earlier stages, homogenous catalysts (inorganic & organic) were used but the recent development of heterogeneous catalysts was found to be more attractive in some cases but they are still in the developmental stage and cost more than homogenous catalysts. The presence of catalysts not only improves the bio-crude yield but also reduces the by-product formation and parallel reactions that would produce undesirable products. The end products can be specifically produced by changing the catalysts.
In algae, hydrothermal liquefaction, Ni/TiO2, Pt/Al2O3, ZrO2/SO4 & Ru/C produced higher yields compared with other heterogeneous catalysts, on the other hand, sodium-based homogeneous catalysts produced higher yields. Based on a comprehensive understanding of the catalyst mechanism, the development of catalysts with higher activity and stability, longer lifespan, and lower cost remains important. In addition, collecting and purifying water-insoluble and water-soluble bio-crudes separately seems to be a better approach than treating them comprehensively.

Supplementary Information


The authors express their acknowledgments towards Kongu Engineering College for their facility support for this research. The authors would like to thank all the researchers who performed the experiments that provide the base of this review.


Future Prospective

Though the hydrothermal liquefaction process produces higher-grade bio-crude at a much lesser cost than other conventional processes there is a long way to be more effective and economical. The hydrothermal process is still at large a lab-scale process with very little output that will not be sufficient if it is converted into industrial scale, so a lot of research and development is needed in this area to streamline the process and convert it into a large scale process that can compensate the demand in the future [40, 48].

Author Contributions

S.S. (Ph.D.) & S.R.R (Assistant professor) wrote the manuscript and devised the methodology. C.D.V. (Professor) conceptualized and supervised the writing of manuscript. M.S. (Assistant professor) conducted the formal anaslysis and review of the manuscript. K.C.S. (Bachelor’s), D.C. (Bachelor’s) & G.A.R. (Bachelor’s) equally contributed to the data collection and curation of the manuscript.

Competing Interests

The authors express no competing interest in the current study


This review did not receive any specific grant from funding agencies in the public, commercial or non-profit sectors.


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Fig. 1
Conversion pathway of algae during hydrothermal liquefaction.
Fig. 2
Van Krevelan plot for algae species
Fig. 3
The bio-crude yield of algae species at different temperatures
Table 1
Proximity Analysis of Microalgae Species
S.no Species Moisture (wt%) Volatile (wt%) Ash (wt%) Fixed carbon (wt%) Ref
1. C. sorokiniana 2.81 ± 0.04 64.62 ± 0.50 10.08 ± 0.40 22.48 ± 0.19 [26]
2. G. sulphuraria 2.41 ± 0.34 63.41 ± 0.66 13.27 ± 0.55 20.90 ± 0.94 [26]
3. C. vulgaris 1.56 ± 0.22 63.07 ± 0.48 8.36 ± 0.54 27.01 ± 0.32 [26]
4. Nannochloropsis salina 4.95 86.76 2.48 5.81 [27]
5. Salicornia sp. - 43.96 46.27 9.76 [28]
6. C. merolae - 80.29 1.76 17.95 [28]
7. Sargassum tenerrimum 5.7 61.5 26.5 6.3 [29]
8. D. tertiolecta 4.98 54.48 4.98 27.0 [5]
9. Chlorella pyrenoidosa 6.3 94.4 5.6 - [2]
10. Arthrospira platensis - 73.58 9.35 17.07 [30]
11. Tetraselmis sp. 7.31 ± 0.14 77.69 ± 1.05 15.99 ± 0.87 6.32 ± 0.34 [31]
12. D. tertiolecta. 5.48 57.13 11.76 25.63 [32]
13. C. pyrenoidosa 6.3 94 5.6 - [33]
14. Microcystis - 70.1 6.1 14.1 [34]
15. Spirulina.sp 6.8 90.4 9.6 - [35]
Table 2
Ultimate Analysis of Microalgae Species
S.no Species C (wt%) H (wt%) N (wt%) S (wt%) O (wt%) HHV (MJ/kg) Ref
1. Nannochloropsis salina 55.16 6.87 2.73 1.27 33.97 - [27]
2. Chlorella sorokiniana 58.02 ± 0.530 7.66 ± 0.023 10.52 ± 0.415 10.52 ± 0.415 23.07 26.59 [26]
3. G. sulphuraria 57.32 ± 0.260 7.84 ± 0.028 12.15 ± 0.078 12.15 ± 0.078 21.25 27.01 [26]
4. Chlorella vulgaris 55.93 ± 0.528 7.67 ± 0.324 10.43 ± 0.136 0.9 ± 0.009 25.07 25.5 [26]
5. A. protothecoides 57.8 9.8 2.2 0.6 29.6 28.63 [36]
6. S. almeriensis 48 7.6 6.9 0.5 37 22.08 [36]
7. Porphyridium 51.3 7.6 8.0 - 33.11 14.7 [37]
8. S. almeriensis 41.9 6.7 5.9 0.8 44.73 37.2 [38]
9. Phaeodactylum tricornutum 37.5 6.47 7.25 0.83 27.3 37 [38]
10. Salicornia sp. 19.99 ± 0.62 3.13 ± 0.32 2.70 ± 0.20 0.99 ± 0.04 73.19 ± 0.98 5.43 [28]
11. C. merolae 48.93 ± 0.43 7.31 ± 0.03 9.38 ± 0.29 1.24 ± 0.09 33.14 ± 0.11 16.52 [28]
12. Tetraselmis sp 71 9.5 5 0.6 14.0 35.0 [39]
13. Desmodesmus sp. 75.8 8.6 6.3 - 9.1 35.4 [39]
14. Spirulina maxima 49.6 6.9 11.2 - 25.7 - [38]
15. N. gaditana 47.6 7.5 6.9 0.5 25.1 37 [38]
Table 3
Ultimate Analysis and Bio-Oil Yield without Catalyst
S.no Species Temperature (°C) Loading (wt%) C (wt%) H (wt%) N (wt%) S (wt%) O (wt%) HHV (MJ/kg) Oil yield (%) Ref
1. N. oceanica 250 °C 25 75.5 10.8 6.11 0.0282 8.99 38.5 - [64]
300 °C 77.4 11.0 6.95 0.257 7.44 39.6 -
350 °C 77.4 10.7 6.98 0.227 7.32 39.3 -
2. Golenkinia sp 250 °C 25 73.7 9.06 6.29 0.346 8.76 35.9 - [64]
300 °C 76.0 9.02 6.56 0.274 6.11 37.0 -
350 °C 76.6 8.97 6.59 0.160 5.86 37.1 -
3. Isochrysis 250 °C 14 75.23 12.3 3.92 0.47 7.77 33.14 47.1 [65]
300 °C 81.96 12.23 4.42 0.37 0.75 34.66 51.0
4. Chlorella sp. 350 °C 10 79.6 10.8 4.7 0.117 5.0 41.5 94.8 [66]
405 °C 83.5 11.3 3.6 0.0721 1.5 44.7 68.9
5. Spirulina sp. 300 °C 16 70.5 10.1 5.4 1.1 12.9 35.3 - [67]
350 °C 74.5 9.7 6.9 1.0 7.9 36.7 -
6. Tetraselmis sp. 300 °C 16 68.5 8.4 4.7 1.1 17.3 32.1 - [67]
350 °C 71.4 9.5 5.7 0.9 12.5 34.9 -
7. Dunaliella tertiolecta 250 °C 1:5–1:10 71.3 9.1 5.3 0.4 12.2 34.6 44.8 [68]
375 °C 72.0 8.8 6.2 0.3 9.9 34.9 55.3
8. Nannochlo-ropsis gaditana 250 °C 1:5–1:10 71.5 9.7 3.7 0.2 11.5 35.4 34.4 [68]
375 °C 74.7 9.9 5.2 0.4 8.5 37.2 54.3
9. Porphyridi-um purpureum 250 °C 1:5–1:10 69.1 8.4 5.0 0.5 15.2 32.7 24.7 [68]
375 °C 73.9 8.2 6.8 0.7 8.7 35.0 47.1
10. Phaeodact-ylum tricornutum 250 °C 1:5–1:10 62.9 8.0 4.7 0.3 12.0 30.3 40.8 [68]
375 °C 73.4 9.1 5.8 1.0 7.8 35.9 54.3
11. Scenedesm-us almeriensis 250 °C 1:5–1:10 72.6 9.4 4.1 0.3 12.5 35.3 35.7 [68]
375 °C 74.3 9.1 6.1 0.4 8.4 36.2 58.1
12. Pavlova 250 °C 14 79.75 12.42 4.01 - 1.09 33.90 53.0 [65]
300 °C 80.02 13.21 5.02 0.46 6.37 34.16 56.6
13. Scenedesm-us obliquus 250 °C 5–7 69.3 9.1 5.1 0.2 12.9 33.8 17.6 [68]
375 °C 73.2 8.9 6.3 0.3 8.1 35.6 50.6
14. Chlorella pyrenoidos-a 200 °C - 67.56 8.26 7.18 16.07 0.93 31.8 59.8 [69]
350 °C 71.85 8.71 6.69 0.86 11.89 34.7 59.9
15. L. Saccharin-a 250 °C 5–20 76.6 7.6 5.2 - 10.3 34.6 - [70]
275 °C 77.2 7.3 5.9 - 9.5 34.5 -
Table 4
Ultimate Analysis and Bio-Oil Yield using Homogenous Catalysts
S.no Loading (wt%) Temperature (°C) Time (min) Catalyst C (wt%) H (wt%) N (wt%) S (wt%) O (wt%) Oil yield (%) Ref
1. 1:5 to 1:10 250 - NaOH 70.6 9.2 5.5 0.4 12.3 33.0 [83]
2. 7.5 203 120 C2H2O4 51.6 7.9 9.8 0.6 - - [84]
3. 1:5 to 1:10 300 30 Na2CO3 72.7 8.8 6.3 0.6 11.5 36 [83]
4. 1:10 350 15 KOH 35.2 4.57 1.35 0.64 37.39 17 [38]
5. 1:3 230 20 H2SO4 67.5 6.4 3.5 - 22.7 28.43 [82]
6. - 360 50 Na2CO3 72.6 8.28 4.46 - 33.67 25.8 [85]
7. 1.10 300 30 KOH 45.07 7.64 3.88 - 35.52 - [38]
8. 14 250 - Na2CO3 80.17 12.40 4.54 0.70 1.08 - [65]
9. - 300 20 Ce 15 52 17 - 10 47 [79]
10. 17 350 60 Na2CO3 45.73 5.96 1.27 - 47.04 51.6 [40]
Table 5
Ultimate Analysis and Bio-Oil Yield using Heterogeneous Catalyst
S.no Species Loading (wt%) Temp (°C) Time (min) Catalyst C (wt%) H (wt%) N (wt%) S (wt%) O (wt%) Oil yield (%) Ref
1. Nannochloropsis sp. - 390 10 Ni/TiO2 69.42 9.72 6.54 0.33 13.99 69.70 [89]
2. Chlorella 20 350 30 Pt/C 63.57 7.34 12.19 2.84 14.06 - [90]
3. Scenedesmus almeriensis 40 400 60 Pt/Al2O3 82.7 11.0 4.2 0.2 1.9 53.1 [88]
4. C. pyrenoidosa 20 300 30 Pd/Al2O3 51.2 6.8 11.3 0.7 30.7 42–47 [38]
5. Spirulina platensis - 350 60 Ca3(PO4)2 2 2 5 262 25 12 [79]
6. Dunaliella tertiolecta 10 360 30 ZrO2/SO4 70.30 8.78 3.89 - 17.03 47.19 [91]
7. Pyropia yezoensis 20 400 120 Ru/C 74.21 8.5 6.8 1.81 9.64 70 [92]
8. Schizochytrium limacinum - 430 15 Pt/C 55.24 7.40 7.61 0.56 25.50 - [93]
9. Auxenochlorella pyrenoidosa 20 400 120 Ru/C 74.34 9.62 6.62 0.73 7.07 70 [92]
10. Saccharina japonica 20 400 120 Ru/C 74.91 8.57 4.15 0.4 8.99 70 [92]
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