Recent advances in chemical, thermochemical, and biochemical processing of algae

1 Introduction

Environmental sustainability and the global energy crisis resulting from heavy reliance on fossil fuels are among the most pressing challenges facing humanity today. The combustion of fossil fuels releases significant amounts of greenhouse gases, contributing to global warming. Consequently, researchers worldwide are exploring alternative renewable energy sources. However, using first-generation edible feedstocks, such as sugarcane and corn, for biofuel production has raised concerns over the food-versus-fuel debate. (Gupta et al., 2025). Investigation and development of sustainable and renewable fuels is a severe need of the near future as the finite reserves of fossil fuels are being depleted (Lantemona et al., 2025). Additionally, combustion of these fuels as led to detrimental effects on the environment including pollution and global warming (Galadima and Muraza, 2018). Therefore, recent research has increasingly emphasized alternative energy sources, including biofuels produced from renewable biomass, due to their potential to enhance energy sustainability while reducing CO₂ emissions (Xu and Savage, 2014; Zhou et al., 2020). CO₂ emissions from conventional fuels have proved to be a crucial threat to our environment. Despite the global economy’s continued reliance on and benefits from these fuels, yet they have been a major source of increase in CO₂ level to the atmosphere. According to studies, in 2012 the contribution of fossil fuels to the global CO₂ emission was estimated to be 36% leading to a concern for both fuel producers and companies involved with engine manufacturing (Gul T, 2016; Saladini et al., 2016). The CO₂ emissions has been projected to be doubled by 2035 (Dudley B, 2015) and reach up to 45,000 MT by 2040 (Dudley B, 2015) as depicted in Figure 1.

Gap in demand and supply, and shortage of fuel has resulted in an increase in price of fossil fuels (Franchino et al., 2013). The spike in fuel rates is expected to impart detrimental effects on the leading economies of the world resulting in an inflation in various sectors. So, to fulfill the market requirements, researchers are investigating alternative energy sources such as solar energy, wind energy, bioenergy, and nuclear energy (Majeed et al., 2023). Among these options, bioenergy has received attention as an alternative fuel to conventional fossil fuels and recent efforts are underway to achieve sustainable bioenergy for the long haul (Anto et al., 2019; Koley et al., 2018; Mathimani et al., 2017). According to International Energy Agency in 2018, over the last 15 years the energy intake has grown by 37% globally (Dudley B, 2015). Renewable energy accounted for 66 EJ (Tursi, 2019) which is around 18% of the world’s total energy intake among which 72.3% is added by biomass based bioenergy (Dudley B, 2015). Figure 2 shows latest statistics reported by WBA 2025 report.

Algae are regarded as a promising feedstock for third-generation biofuel production, offering potential solutions to many existing challenges, notably, they possess the capacity to sequester approximately 210 tons of CO₂ ha⁻¹ year⁻¹, a rate 10 to 50 times higher than that of terrestrial forests (Ashour et al., 2024). A summary of the advantages and limitations of various biomass feedstocks is provided in Table 1. Green algae, utilizing photosynthesis, flourish as dominant microorganisms in aquatic environments. When assessing nutrient use relative to biomass yield, algal biomass demonstrates remarkably low nutrient requirements. For instance, producing 1 kg of biomass per hectare requires approximately 40 mg of nitrogen for rapeseed, 45 mg for sunflower, but only about 0.003 mg for algae cultivation (Khoo et al., 2019). In the case of phosphorus, algae also require far less input, with consumption around 0.002 mg/ha compared to approximately 25 mg/ha for rapeseed and 56 mg/ha for sunflower. The biochemical richness of algal biomass, comprising carbohydrates, lipids, and proteins makes it a versatile source for generating a variety of high-value products. Recent advancements in integrated downstream biorefineries have enabled the simultaneous production of algal-based biofuels and biochemicals. Given the increasing global demand for energy and the scarcity of arable land, large-scale algae cultivation in aquatic systems offers a sustainable alternative to conventional bioresource farming (Li et al., 2008). Recent technological progress in photobioreactor design, combined with integrated algae farming and biorefinery approaches, has brought microalgae-based biofuel production to a more advanced and cost-effective stage (Brennan and Owende, 2010). Prior to biofuel conversion, algal biomass represents a tunable biochemical platform whose lipid quantity and fatty acid composition can be strategically enhanced through cultivation and metabolic optimization, thereby directly influencing downstream fuel yield and quality. Recent research has shifted from passive lipid extraction toward active lipid upgrading strategies, in which cultivation conditions are deliberately manipulated to enhance neutral lipid accumulation and tailor fatty acid profiles prior to conversion. Among these, synergistic supplementation of organic carbon substrates (e.g., acetate, glycerol, glucose) under mixotrophic or heterotrophic growth has been shown to significantly increase neutral lipid content and improve fatty acid composition in multiple microalgal strains. This approach enables simultaneous enhancement of biomass productivity and lipid yield, addressing a long-standing trade-off in algal biofuel systems (Kiran and Mohan, 2021). In addition to carbon supplementation, nutrient stress strategies, particularly nitrogen and phosphorus limitation, remain effective triggers for lipid accumulation; however, recent studies emphasize the importance of two-stage cultivation schemes, where biomass growth and lipid induction are decoupled to maximize overall lipid productivity. Such strategies allow high-density biomass generation under nutrient-replete conditions, followed by targeted stress induction to promote triacylglycerol (TAG) synthesis (Liu et al., 2022). Advances in fatty acid profile engineering have further improved biodiesel quality by increasing the proportion of C16-C18 fatty acids while reducing excessive polyunsaturation, which negatively impacts oxidative stability. This has been achieved through optimized cultivation regimes and emerging metabolic and regulatory interventions. Collectively, these lipid upgrading strategies enhance the suitability of algal lipids for downstream transesterification and upgrading, reinforcing the role of algae as a next-generation biofuel feedstock rather than a passive biomass source (Meng et al., 2023).

TABLE 1. Various biofuels and bioproducts being produced from algae types.

Within an algae biorefinery framework, it can also yield a range of high-value products, including pharmaceuticals, biofertilizers, and advanced biomaterials (Budzianowski, 2017). By integrating biofuel and biochemical production, algae biorefineries are approaching commercial feasibility. Algae, as a form of aquatic biomass, encompasses both macroalgae and microalgae, as well as emerging plant species (Tursi, 2019).

• Macroalgae are multicellular organisms reaching up to 60 m in length. Their applications consist of food production and extraction of hydrocolloids.
• Microalgae are microscopic organisms broadly classified into diatoms, green algae, and golden algae. Diatoms are unicellular species, often referred to as brown algae due to their coloration, and typically range only a few micrometers in size. They represent one of the most significant contributors to aquatic biomass. Green algae, on the other hand, are predominantly freshwater species and are a source of starch and oils. Golden algae share similarities with diatoms and are primarily utilized for the production of oils and carbohydrates.

Major biochemical constitutes of algae include lipids, proteins, and carbohydrates that can be transformed to biofuels. Table 1 displays the above algae types that are being used to various bioproducts and biofuels.

Algae is a known as a third generation biomass feedstock which is being exploited to obtain various biofuels such as biogas, biohydrogen, biodiesel, and bioethanol (Zhou and Hu, 2020). Table 2 depicts various products obtained from first, second, and third generation feedstocks along with their advantages and limitations. Feedstocks for third generation of biofuel production are photosynthetic microbes such as cyanobacteria, microalgae, and algae (Prates, 2025). There are numerous advantages of considering algae as a biofuel feedstock including tolerance of algae to tough environments, and it requires less area along with its ability to mitigate CO₂ make it a feasible option for biofuel production (Liu et al., 2017; Schenk et al., 2008). Biofuels based on algae are advanced fuels derived via conversion processes, resulting from oil rich composition of algae and it can attributed with the its ability to photosynthesize (Council of the European Union, 2014). In comparison to photoautotrophic algae which utilize sunlight and inorganic CO₂, other third-generation feedstocks like oleaginous yeasts (e.g., Yarrowia) and filamentous fungi (Mortierella) function primarily as heterotrophs, requiring organic carbon sources such as lignocellulosic hydrolysates or industrial waste to thrive. This metabolic distinction fundamentally alters production efficiency; while algae offer superior areal oil yields on non-arable land, heterotrophic yeasts achieve significantly higher volumetric lipid productivities (e.g., >0.25 g L⁻¹ h⁻¹) due to high cell densities unconstrained by light penetration or diurnal cycles. Furthermore, nutrient strategies differ, as efficient lipid accumulation in yeast and fungi typically demands high Carbon-to-Nitrogen (C/N) ratios (often >30–100) to trigger lipogenesis, whereas algae can often integrate nutrient removal (N, P) directly from wastewater streams (Kurniawan et al., 2025; Lopes da Silva et al., 2023).

TABLE 2. Biofuels, advantages and limitations of various generation feedstocks.

Algae are capable of thriving in a wide range of water types, including seawater, freshwater, and even industrial wastewater. Compared to conventional food crops, their growth rate is approximately 20–30 times faster, and they contain nearly 30 times more oil than first- and second-generation biomass feedstocks (Ullah et al., 2015). Algae is normally a better converter of solar energy as its cells can grow in aqueous suspensions where they have access to sufficient nutrients, water, and CO₂ (Ghadiryanfar et al., 2016). The amount of oil contents it possesses with regard to dry weight makes it a suitable renewable energy resource by utilizing it via various conversion processes (Adeniyi et al., 2018). Although, these conversion techniques are complex and expensive they can be commercially feasible if the by-products obtained from them are properly used. The overall potential and applications of algae show that it is not fully being utilized as it can have an immense impact on the global economy as it is present in terms of millions when compared to other plants (Suganya et al., 2016). Apart from being used for food or non-food purposes it is also used for energy related applications. Here we will discuss the various pathways and methods (Figure 3) that can be implemented to utilize this valuable resource for energy. Regarding water sustainability, terrestrial feedstocks like soybean and corn possess high water footprints estimated at approximately 13,676 and 2,570 L of water per liter of fuel, respectively. In contrast, while open-pond algae cultivation can be water-intensive due to evaporation, the utilization of wastewater or seawater in optimized systems reduces the freshwater requirement by over 90% (to approx. 220–245 kg water/kg biodiesel), effectively decoupling bioenergy production from freshwater scarcity (Thomas et al., 2014).

While several earlier reviews have addressed algal biomass conversion, many primarily emphasized laboratory-scale feasibility and isolated process efficiencies, with a substantial portion of the literature published prior to 2018. In contrast, the present review is motivated by recent paradigm shifts (2020–2025) in algal bioenergy research, where the focus has moved decisively toward scalability, process integration, and sustainability-driven deployment.

Over the past 5 years, significant advances have emerged in catalyst design, reactor configurations, hydrothermal and chemical looping technologies, and integrated algal biorefinery concepts, alongside the growing incorporation of techno-economic analysis and life cycle assessment to evaluate real-world viability. This review therefore goes beyond descriptive summaries by critically analyzing recent developments, identifying unresolved technical bottlenecks, and synthesizing future research directions aligned with energy transition and carbon-neutrality goals. By emphasizing post-2018 literature and system-level perspectives, the manuscript aims to provide an updated and forward-looking reference for researchers, engineers, and policymakers working on algal biomass conversion technologies.

2 Chemical conversion of algae

2.1 Transesterification

Transesterification is a chemical transformation of algal based biomass to obtain biodiesel (Kandiyoti et al., 2017a). The reaction converts lipid (triglyceride) when reacted with alcohol in the presence of catalyst alkaline, acidic, or enzyme based catalyst to produce fatty acid methyl ester (FAME) with glycerol (Chisti, 2007; Johnson and Wen, 2009; Jorquera et al., 2010; Kandiyoti et al., 2017a; Lee and Lee, 2016; Sheehan et al., 1998; Suganya et al., 2016; Yang C. et al., 2016). This step is crucial as the viscosity of algal oils is comparatively higher than diesel obtained from crude oil, hence this reduces the viscosity of oil obtained algae leading to increased fluidity (IEA, 2016). This create a by-product glycerol which applications in pharmaceutical and cosmetics (Suganya et al., 2016). This method can be described by the which depend on type of catalyst, alcohol type, molar ratio, etc., to obtain a low molecular weight FAME from algal lipid (Kuepker, 2015) as depicted in Equations 1–3.

R 1 C O O C H 2 H O C H 2 R 1 C O O C H 3 (1)

R 2 C O O C H + 3 C H 3 O H ⇌ H O C H + R 2 C O O C H 3 (2)

R 3 O O C H 2 H O C H 2 R 3 C O O C H 3 (3)

Triglyceride Methanol Catalyst Glycerol FAME

With the reduced viscosity and increased fluidity of algae oils, alkyl esters can be blended with diesel for use in unmodified engines (Al-lwayzy and Yusaf, 2013). While Fatty Acid Methyl Esters (FAME) are the standard for analytical lipid profiling, Fatty Acid Ethyl Esters (FAEE) are increasingly regarded as the preferred commercial biofuel product. Producing FAEE via ethanolysis allows for a fully renewable carbon cycle (using bioethanol) and yields a fuel with superior physicochemical properties, including a higher cetane number, higher flash point (safety), and better cold flow performance (lower Cloud/Pour Points) compared to FAME. Furthermore, in the context of cost-effective wet in-situ transesterification, ethanol is often superior to methanol as it exhibits higher solvent power for lipids in the presence of cellular water, thereby enhancing reaction yields without energy-intensive drying. Consequently, while alkaline catalysts can achieve >90% conversion (Lee and Lee, 2016), the industry is shifting toward robust enzymatic or acid-catalyzed ethanolysis to overcome the inhibitory effects of water and free fatty acids common in algal feedstocks (Hallenbeck et al., 2016). This process of obtaining biodiesel from algae can be termed as an equilibrium reaction in which the produced alcohol drives the reaction towards FAME (Skorupskaite et al., 2016). The catalyst mentioned in the above reactions can either be an alkali, acid, or lipase enzyme. Table 3 describes a comparison of these catalysts for the transesterification process of obtaining biodiesel from microbial oil. A study finds that during the esterification process the catalyst can be inhibited by the poor solubility of shot chain alcoholic solvents (Trivedi et al., 2015). Some novel solvents simplify the separation of glycerol, while certain acyl-based solvents can prevent both glycerol microemulsion and formation. After lipid extraction, algal biomass can serve as feedstock for producing bioethanol, animal feed, or pharmaceutical products. Comparing dry and wet in-situ transesterification methods, the wet approach is generally more cost-effective because it eliminates the drying step, which benefits algae biorefinery operations. In contrast, supercritical methods requiring high temperatures and pressures are less practical, making catalytic transesterification the preferred route for algae-based biodiesel production.

TABLE 3. Comparison of various types of catalysts used for transesterification.

FFA, free fatty acids.

Direct esterification can be described as a single stage process involving the simultaneous extraction of lipid contents (Ehimen et al., 2010; Lee and Lee, 2016). In this method the unwashed wet algae is directly fed into the reactor for transesterification (Jazzar et al., 2015). This approach eliminates the need for conventional pretreatments such as degumming and extraction, as the process can tolerate a certain amount of water due to the use of excess methanol in the system, as illustrated in Figure 4 (Salam et al., 2016). Additionally, this method produces higher biodiesel yields compared to traditional techniques (Al-lwayzy et al., 2014) and is crucial in order to achieve to a sustainable biodiesel production method (Hossain et al., 2018). However, a critical limitation of direct esterification is the indiscriminate conversion of high-value polyunsaturated fatty acids (PUFAs) into methyl esters, which possess lower oxidative stability and calorific value compared to saturated esters. Preserving these PUFAs in their triglyceride form for nutraceutical applications, rather than converting them into lower-quality biodiesel, is essential for improving the techno-economic feasibility of the algal biorefinery (Chen et al., 2023).

Conventional transesterification is a two-step process where lipids are mechanically extracted prior to transesterification (Al-lwayzy et al., 2014; Lee and Lee, 2016). In this method pretreatments like degumming and extraction is done extensively via a non-expensive, unreactive non-polar solvent such as hexane (Hossain et al., 2018) as shown in Figure 5 (Salam et al., 2016). This conventional process has the ability to yield refined fuel that is optimum for high speed diesel engines (Salam et al., 2016). The algae water content and the catalyst can be controlled via pretreating biomass feedstock but adversely will affect the 88% of overall production cost (Martinez-Guerra and Gude, 2016). Due to the expensive pretreatment steps this process is both time consuming and energy extensive (Jazzar et al., 2015).

Both of these methods were compared by a study where they produced biodiesel using green algae (Al-lwayzy et al., 2014). Direct transesterification of wet algal biomass was found to produce significantly lower biodiesel yields compared to conventional methods. However, when oil is first extracted from the algae using a solvent, biodiesel yielalgalds can reach up to 98%. Further studies indicate that complete (100%) biodiesel conversion is possible if the biomass undergoes pretreatment with solvents such as hexane or chloroform. A major drawback of conventional transesterification lies in the high cost of biomass pretreatment, particularly drying, which can account for up to 80% of the material preparation expenses (Ehimen et al., 2010). Another study (Ward et al., 2014) suggests that the high energy intensive step of biomass pretreatment can be reduced by employing both ultrasound and microwave methods. Following this method tends to produce 90% FAME with a ratio less than one for energy input and output leading to the decrease in overall cost of the process. To make the process of transesterification economically viable the method was thoroughly investigated and reported by Jazzar et al. (Jazzar et al., 2015) by using Chlorella sp. as biomass feedstock. It was reported that a biodiesel having a standard similar to UNE-EN14214 is possible to achieve via in-situ supercritical transesterification of methanol. Researchers suggested (Skorupskaite et al., 2016) that in-situ transesterification method can prove to be cost friendly if high amounts of oil are obtained from rapid growing algae strains. A study (Salam et al., 2016) states that residue of microalgal for biogas generation can provide the energy needed for biomass pretreatment before in-situ transesterification leading to the reduction of production cost. Whereas, to further reduce the cost other major factors such as reaction time, biomass moisture content, alcohol to oil ratio, stirring rate, and the catalyst type employed should also be taken into consideration. A study (Ehimen et al., 2010) verifies that stirring significantly improves the yield of biodiesel since the reaction media can be agitated intermittently without yield loss leading to saving of energy. Biomass drying plays a crucial role as the equilibrium conversion of FAME is dependent on the moisture of biomass feedstock. The main challenge is the high energy cost of traditional methods, so the solution requires advancing on multiple fronts at once. This means combining energy-efficient conversion processes that bypass costly steps like drying, cultivating superior algae strains with high oil yields, creating a circular system where waste powers the operation, and fine-tuning basic reaction controls to maximize efficiency. Ultimately, the path forward lies in the intelligent synergy of biology, chemistry, and process engineering to create a cohesive and cost-effective system.

Apart from the above mentioned catalytic processes, non-catalytic transesterification presents itself an alternative where a supercritical fluids like CO₂, water, even alcohol is used to transform lipids into biodiesel (Salam et al., 2016). This non-catalytic pathway is carried out under high temperature and pressure. Some advantages of this method include catalyst free process, high purity of biodiesel obtained, less reaction intervals, and it is environmentally benign (Deshpande et al., 2017). Researchers investigated biodiesel generation by using ethanol and methanol under supercritical conditions as the reaction media from Chlorella protothecoides via non-catalytic transesterification (Nan et al., 2015). They reported the optimum yields obtained by using ethanol and methanol were 87.8% and 90.8% respectively. Following this study another report was published where they utilized supercritical CO₂ as the reaction medium (Zhou et al., 2017). In this study two types of algae were considered as biomass feedstock having varying amount lipid contents to compare performance of the continuous transesterification method. Microalgae Chrysophyta (10.5 wt.% lipid) and Chlorella sp (19.8 wt.% lipid) yielded 56.31% and 63.78% of biodiesel respectively.

By equating both dry and wet in-situ transesterification processes it can be established that the wet pathway may result in cost reduction of the process by eliminating the drying step. Whereas transesterification carried out under supercritical conditions is less favorable when compared with the catalytic pathway. Hence it can be said that catalytic transesterification for biodiesel production by using algae as biomass feedstock would be feasible in the future.

Beyond conventional homogeneous alkali/acid catalysts, recent work increasingly emphasizes catalyst architectures that (i) tolerate water/FFA-rich algal oils, (ii) enable catalyst recovery and reuse, and (iii) reduce downstream washing burdens. Among these, metal–organic framework (MOF)-based catalysts have emerged as a promising class due to tunable acidity/basicity, high surface area, and controllable pore environments that can enhance esterification/transesterification kinetics and improve stability under complex feed compositions. MOF-based systems are now being positioned not only as “high-activity” catalysts but as process-enabling materials that can reduce separation complexity and improve life-cycle sustainability when catalyst recyclability is demonstrated (Nguyen et al., 2024). In parallel, nanocatalysts (e.g., nanostructured metal oxides and functionalized nanoparticle catalysts) are gaining traction because nanoscale surface properties can increase active-site exposure and mass transfer at the oil–alcohol interface, improving conversion rates under milder conditions. The key future direction is not “nanoparticles per se,” but engineering nanostructures for reusability and deactivation resistance in realistic algal-oil matrices containing pigments, moisture, and trace impurities (Borthakur and Sarmah, 2025). Regarding biocatalysis, recent advances increasingly focus on immobilized and nano-immobilized lipases, which enhance enzyme stability and reuse while retaining the mild-condition advantages of enzymatic routes (low temperature, reduced soap formation, high selectivity). Notably, nano-immobilization strategies (enzyme anchored on nanosupports) are being explored to improve operational robustness and reduce cost barriers, enabling enzymatic transesterification to be considered for higher-value or integrated biorefinery scenarios (Xia et al., 2024). The most credible pathway to scale is process intensification (e.g., reactive extraction/transesterification or continuous packed-bed systems using recoverable heterogeneous/MOF catalysts and immobilized lipases), coupled with feedstock-aware catalyst selection rather than “one-catalyst-fits-all (Pandey et al., 2024).

2.2 Comparative assessment of catalytic and non-catalytic transesterification pathways

Non-catalytic (supercritical alcohol) transesterification offers the advantage of eliminating catalyst-related separation steps and demonstrates high tolerance to free fatty acids and water content in algal oils. However, these benefits are offset by extreme operating conditions, including elevated temperature and pressure, which substantially increase energy input and capital costs. Consequently, non-catalytic routes are primarily suited for small-volume, high-purity lipid streams or niche applications where catalyst contamination must be avoided (Cheng et al., 2021). In contrast, catalytic transesterification, particularly using heterogeneous and enzyme-based catalysts, exhibits superior energy efficiency and scalability, making it more suitable for industrial deployment. Alkaline catalysts remain effective for low-FFA algal oils, while acid and bifunctional catalysts demonstrate improved tolerance to feedstock variability. Enzymatic transesterification, although currently limited by reaction kinetics and catalyst cost, offers unmatched selectivity and operates under mild conditions, positioning it as a promising route for integrated biorefineries producing high-value co-products (Farouk et al., 2024). From a scalability perspective, catalytic systems enable continuous operation, catalyst reuse, and process integration, whereas non-catalytic systems face material and safety constraints at large scale. Therefore, future research should prioritize robust heterogeneous and immobilized biocatalysts, alongside process intensification strategies that reduce solvent use and enable simultaneous extraction–conversion of algal lipids (Wiatrowski et al., 2022).

3 Thermochemical conversion

Thermochemical conversion methods encompass gasification, pyrolysis, liquefaction, and hydrothermal processing, as shown in Figure 3. The primary challenge in thermochemical processing of algal biomass is the drying step, since the biomass typically contains only 10–30 wt% dry solids. Another significant limitation arises from its composition, as the presence of nitrogen and sulfur can lead to increased emissions of NOx and SOx during processing.

3.1 Gasification

Gasification was initially adopted in the beginning of the 20th century. Dried algal biomass can be transformed to beneficial gas-based products by this method as shown in Figure 6. Gasification is partial oxidation where biomass is converted into syngas (primarily H₂ and CO), which possesses a lower heating value but serves as a critical energetic intermediate. Rather than being emitted as a pollutant, this syngas is captured and utilized as a versatile feedstock for high-efficiency power generation in gas turbines or catalytically converted into liquid transportation fuels (e.g., methanol, Fischer-Tropsch diesel), offering a cleaner energy profile than direct combustion (Sittijunda et al., 2021). The main problem of gasification is the formation of tar in products as it leads to fouling of turbines, pipes, filters or gas engines (Adnan et al., 2018; Adnan et al., 2017a; Adnan et al., 2017b; Adnan and Hossain, 2018). Overall efficiency of microalgae gasification is reduced as a result of tar formation. In thermochemical conversion systems, tar removal and gas cleaning strategies depend on whether dry or wet processing is employed. Dry gas cleaning methods, such as cyclones, ceramic filters, and hot gas filtration units, are primarily used for particulate removal and partial tar mitigation. In contrast, wet gas cleaning techniques, including scrubbers, spray towers, and wet cyclones, are applied to condense and capture tar compounds from product gas streams, often at the expense of increased wastewater generation and energy demand. However, these methods are for removing tar and not for conversion of tar to syngas. Technological improvements including: (a) optimizing operating conditions of the gasifier (b) enhancement in the gasifier design, and (c) employing feasible catalysts can be carried out to minimize the formation of tar.

Gasification is known as a complex thermochemical process that occurs in an air-tight chamber under low pressure. The generated gas is cleaned, and impurities are removed, and trace elements are recovered. Due to the step of removing unwanted pollutants from syngas gasification is an interesting technique, as other processes cannot treat gas with impurities or high contents of heteroatoms. Sulphur in its elementary form can be removed using this method and the cleaned syngas can be processed to for chemical formation of power generation. The purified gas is consisted of various fractions which may vary according to the operating conditions and the type of biomass. Normally the obtained syngas has a calorific value ranging from 4.5 to 6 MJ/m3 which is almost 10%–15% of calorific value for natural gas (Akia et al., 2014). Gasification can be divided into four steps (Portha et al., 2017).

3.1.1 Oxidation

This step is the exothermic stage of gasification and carried out to achieve the required operating temperature to initiate the endothermic step. Oxidation during gasification is performed in the lack of oxygen (Molino et al., 2016). Reactions taking place during the oxidation step are as given below in Equations 4-6. It is evident the products of this step include water, CO, and CO₂. The objective of this step if achieve the required thermal energy which will allow the whole gasification process to take place.

Combustion C + O 2 → C O 2 Δ H = − 394 kJ / mol (4)

Partial combustion C + 1 2 O 2 → C O Δ H = − 111 kJ / mol (5)

Combustion of hydrogen H 2 + 1 2 O 2 → H 2 O Δ H = − 242 kJ / mol (6)

3.1.2 Drying

The moisture present in the biomass is removed during this step above 100 °C by transforming it to steam and completing moisture removal at 150 °C. Heat required for this step is proportional to the amount of moisture present in the biomass and is obtained from the oxidation stage hence drying can be considered as an endothermic process (Molino et al., 2016).

3.1.3 Pyrolysis

This is also an endothermic stage of the process and occurs between 250 and 700 °C. Pyrolysis is known as an anaerobic thermal decomposition which results in solid, liquid residues termed as char and tars respectively as displayed in Equation 7. Char is a solid consisting of high carbon content whereas tar is a complex organic substance which is low temperature condensate. Pyrolysis also produces gaseous products such as CO, CO₂, and H₂ and light weight hydrocarbons known as pyrolysis gas (Roos and Roos, 2010). During this stage high molecular weight chemicals are broken into light weight chemicals.As a result of this step, approximately 12.01 kg of carbon is oxidized using 22.39 m³ of oxygen from air, producing 22.26 m³ of CO and nearly 0.04 GJ of heat (Molino et al., 2016).

A l g a l B i o m a s s ↔ H 2 + C O + C O 2 + C H 4 + H 2 O g + T a r + C h a r (7)

3.1.4 Reduction

This is another endothermic step of the overall gasification process where gas phase reaction take place. In this step products obtained from the previous stages react which lead to formation of final syngas. Blow provided equations describe the reactions taking place during reduction (Zhang et al., 2013).

Boudouard reaction

C + C O 2 ↔ 2 C O Δ H = 172 kJ / mole (8)

Character reform

C + H 2 O ↔ C O + H 2 Δ H = 131 kJ / mole (9)

Water gas reaction

C O + H 2 O ↔ C O 2 + H 2 Δ H = − 41 kJ / mole (10)

Methanation

C + 2 H 2 ↔ C H 4 Δ H = − 75 kJ / mole (11)

Equations 8 and 9 are endothermic, occurring due to elevated temperatures, while Equations 10 and 11 are exothermic, taking place at lower temperatures. Overall, the process remains endothermic, requiring an external energy input. The temperature during the reduction stage is a key factor influencing the composition and characteristics of the resulting syngas and solid residues. Higher temperatures enhance biomass oxidation, thereby reducing the formation of tar and solid residues. Gasification generally occurs between 800 °C and 1,100 °C, extending to 500 °C–1,600 °C when oxygen is present (Shen et al., 2011).

To recapitulate, gasification technology can be viable option to obtain energy from biomass despite the generation of various pollutants such as hydrochloric acid, ammonia, hydrogen sulphide, and tar obtained as by products.

3.1.5 Comparison of gasification reactor configurations and tar control implications

Fixed-bed, fluidized-bed, and entrained-flow gasifiers differ fundamentally in reaction environment, heat transfer efficiency, and tar formation behavior, which directly affects their suitability for algal biomass conversion. Fixed-bed gasifiers operate under relatively simple configurations and are well suited for small-scale, decentralized systems. However, their limited heat transfer and poor tolerance to high ash or salt content restrict their applicability to selected algal strains and low-throughput operations (Kuttin et al., 2025). Fluidized-bed gasifiers provide enhanced mixing and thermal uniformity, making them the most widely investigated option for algal biomass. Their ability to process fine particles and wet feedstocks improves operational flexibility; however, tar formation remains a critical challenge, necessitating secondary tar cracking or catalytic reforming units. Recent research highlights the role of bed material modification and in situ catalysts to mitigate tar generation and improve syngas quality (Inayat et al., 2021). Entrained-flow gasifiers operate at high temperatures and short residence times, resulting in minimal tar production and high carbon conversion efficiency. These systems are best suited for large-scale, centralized facilities, but their stringent feedstock requirements, such as finely ground, low-moisture biomass, pose preprocessing challenges for algae. As such, entrained-flow gasification is most viable when integrated with upstream drying or slurry-based feeding strategies (Zeng et al., 2021). Reactor selection should be guided by scale, feedstock properties, and syngas quality requirements, rather than generalized efficiency metrics. Future research should focus on integrated tar control strategies, including catalytic bed materials and chemical looping approaches, to enhance gasification reliability for algal biomass.

3.1.6 Chemical looping gasification and next-generation tar management

While conventional gasification requires downstream tar cleanup and typically produces N₂-diluted syngas when air is used, chemical looping gasification (CLG) is emerging as a promising next-generation route that can intrinsically improve syngas quality. In CLG, an oxygen carrier transfers lattice oxygen to the fuel in a separate reactor configuration, enabling nitrogen-free or low-nitrogen syngas, with potential reductions in tar formation depending on oxygen carrier properties and staged operation. This shifts tar management from “removal” toward in situ conversion, which is central to improving reliability and reducing balance-of-plant complexity (Fang et al., 2024). Recent studies on biomass CLG highlight how reactor staging, oxygen carrier selection, and feed pretreatment (e.g., torrefaction where applicable) influence syngas composition, tar levels, and overall carbon conversion. From an application standpoint, CLG is particularly attractive when high-quality syngas is needed for downstream synthesis (e.g., Fischer–Tropsch, methanol), or when integration with CO₂ capture routes is pursued (Condori et al., 2024). Research priorities include (i) oxygen carriers resistant to ash/salt deactivation (relevant for algae), (ii) staged CLG designs that couple tar cracking and reforming, and (iii) validated techno-economic comparisons of CLG versus fluidized-bed gasification with external tar reforming to support scenario-based selection (Fang et al., 2024).

3.2 Pyrolysis

Pyrolysis is a thermochemical method that converts biomass feedstock into solid (char), liquid (bio-oil), and gaseous (combustible gases) products through partial decomposition at around 500 °C in the absence of air (Lebaka (L), 2013). This process is popular as biomass drying is not a prerequisite and eventually is energy efficient. Vaporization of volatile compounds from biomass occur at high temperature generating gases which are condensed to liquids through liquefaction. Liquid fuel obtained can either be stored or can be utilized for heating and electricity generation purposes. Furthermore, pyrolysis also generates gas, charcoal, and various other chemical (Kaushika et al., 2016). Figure 7 illustrates the schematic layout for pyrolysis of biomass.

The pyrolysis method can be divided into four steps which occur at various temperatures as described below (Portha et al., 2017).

• Drying of biomass is done at temperatures from 100 to 120 °C.
• Distillation of outlet gases (i.e., CO₂, CO, N₂), methanol, and acetic acid is carried out at 275 °C.
• At temperature from 280 °C to 350 °C exothermic reactions take place which remove complex mixtures of chemicals (i.e., esters, phenols, aldehydes, and ketones), H₂, CO, CO₂, and CH₄ due to breaking chemical bonds.
• Evaporation of volatile components at 350 °C and production of higher amounts of carbon (charcoal residue), CO, and H₂.

Pyrolysis of algal biomass has gained interest due to its high energy efficiency and environmentally favorable characteristics. The process breaks down large, complex molecules into smaller compounds, producing three distinct product phases, similar to those observed in hydrothermal processing (Chen et al., 2015). The three phases are:

• Conventional pyrolysis: Bio-chars obtained at longer reaction time (10–30 s) with lower heating rate (5–10 ^°C/min). Bio-chars can be used both as a fuel and fertilizer for the underground carbon sink (Chaiwong et al., 2013).
• Fast pyrolysis: Liquid (bio-crude oil consisting of tars, heavy hydrocarbons along with water) obtained at short reaction times (2–3 s) with temperatures ranging from 400 to 500 °C. Bio-oils can be used alternatives to fossil fuels or chemicals for various applications (Casoni et al., 2016; Hong et al., 2017).
• Flash pyrolysis: Biogases (i.e., CH₄, CO₂, H₂, C₂H₂, C₂H₄) obtained at shorter reaction time (1–2 s) with a high heating rate (10–600 ^°C/s). Gas obtained from this has an energy density of 50% of natural gas and can be employed for producing chemicals which can be directly used in methanol synthesis (Zhang et al., 2016).

When compared to other conversion techniques, pyrolysis is the most accepting process when dealing with high ash content of algae (Sheehan et al., 1998). However, the oil generated from this has issues relating to stability, acidity, and viscosity (Kandiyoti et al., 2017b) and challenges in disposing ash and carbon (Trivedi et al., 2015). Various stages of algae pyrolysis are favourable when considering the amount of carbohydrate and protein of specific species. A study (Zhang et al., 2016) suggested that algae consisting of rich protein produce more bio-oil composition as they have low amounts of PAHs while algae possessing higher concentrations of carbohydrates are better suited for syngas production. Bio-oil having high levels of nitrogen and oxygen were linked with algae species having higher amount of protein were signified by Sirajunnisa and Surendhiran (2016). The bio-oil had properties such as low thermal stability, low pressure, and low heating value. Bio-oil obtained from this process has a lower heating when compared with petroleum-based diesel but is higher than bio-oil produced from other lignocellulosic feedstock (Skorupskaite et al., 2016). The aforesaid challenges of pyrolysis can be mitigated by decreasing the amounts of nitrogen and oxygen prior to its usage as a fuel for transportation (Sirajunnisa and Surendhiran, 2016). Oxygen content can be reduced by implementing catalytic cracking and hydrogenation (Kandiyoti et al., 2017b) and nitrogen levels can be mitigated to form nitrogenated components by applying hydrotalcite catalyst in catalytic pyrolysis (Milledge et al., 2014).

Algal pyrolysis is uniquely constrained by the protein-rich biochemical composition of many strains, which increases the formation of nitrogen-containing heteroatom species (e.g., nitriles, amides, pyridines) alongside oxygenates. These heteroatoms reduce bio-oil stability and heating value and directly complicate downstream use due to NOx precursor formation and catalyst poisoning during upgrading. A practical pathway to overcome this bottleneck is to treat “high-N bio-oil” as an upgrading-design problem rather than a generic “optimization” problem, where the process is selected based on the dominant nitrogen form and target product slate. Recent studies show that catalytic vapor upgrading using metal-modified zeolites (e.g., Fe-/Ni-modified HZSM-5) can simultaneously promote aromatization and suppress oxygenated/nitrogenated fractions, improving fuel-range hydrocarbon quality (Wang et al., 2024). A second, more scalable route is two-stage pyrolysis–upgrading, where primary pyrolysis is run to maximize volatilization, and a dedicated catalytic bed downstream is tuned for deoxygenation + denitrogenation selectivity. Catalyst selection should be explicitly linked to feed composition: protein-rich algae benefit more from strong-acid zeolitic frameworks with tailored porosity, while ash/salt-rich biomass requires catalyst and reactor designs resistant to mineral deposition and deactivation (Niu et al., 2024).

Pyrolysis is mainly used for producing liquids such as bio-oil which can be applied for power generation in generators. Bio-oil has advantages such as economic production of cost, a higher energy density a neutral balance of CO₂ emissions and when competed with other biofuels (Kaltschmitt, 2013).

Recent studies have emphasized that pyrolysis of algal biomass presents distinct challenges and opportunities compared to lignocellulosic feedstocks, primarily due to the high protein, nitrogen, and ash content of algae. While fast pyrolysis can achieve high bio-oil yields, the resulting bio-oil is often characterized by elevated nitrogen- and oxygen-containing compounds, leading to poor stability, corrosiveness, and limited direct fuel applicability. These compositional constraints represent one of the principal bottlenecks preventing large-scale deployment of algal pyrolysis for liquid fuel production (Sekar et al., 2021). Beyond bio-oil, increasing attention has been directed toward biochar production from algal pyrolysis as a value-added pathway rather than a by-product. Algal biochar exhibits unique physicochemical properties, including high mineral content and surface functionality, making it suitable for applications such as soil amendment, nutrient recycling, carbon sequestration, and adsorption of pollutants. Recent reviews highlight that biochar valorization can significantly improve the overall techno-economic feasibility of algal pyrolysis systems, particularly when liquid fuels alone are insufficient to justify process costs (Lee et al., 2020). From a process perspective, modern research has shifted toward catalytic and staged pyrolysis configurations to address inherent feedstock limitations. Catalytic pyrolysis and vapor-phase upgrading aim to reduce heteroatom content, improve aromatic hydrocarbon formation, and enhance bio-oil quality. Additionally, integrating pyrolysis with downstream upgrading or hybrid conversion routes (e.g., biochar utilization combined with fuel or chemical production) is increasingly viewed as a pragmatic strategy to overcome single-product limitations (Kumar, 2024). Future scope for algal pyrolysis lies in (i) feedstock-specific process design, where algal strain composition informs reactor conditions and product targeting; (ii) integration of biochar valorization into circular bioeconomy frameworks; and (iii) coupling pyrolysis with catalytic upgrading or carbon-negative strategies to align with sustainability and climate-mitigation goals. These directions move algal pyrolysis beyond yield-centric studies toward system-level viability and application-driven deployment.

3.3 Hydrothermal processing

Hydrothermal is a conversion method where biomass breakdown is done in a thermally compressed aqueous media. Microalgae possessing more than 90% water content can be directly processed through this method once they have been dewatered via gravity. This is one of the main advantages where energy for drying is not required and hence improving the energy balance (Kumar et al., 2018). Hot compressed water serves as a solvent, reactant, and/or as a catalyst for the processing of algal biomass. It is a technique where algae oils are transformed into jet fuel via hydrotreatment (Kuepker B. Commission E, 2015). This is feasible technique for producing synthetic paraffinic kerosene (SPK) which is considered as a bio-jet fuel having enhanced energy efficiency from algae based lipids hence leading to the reduction of emissions in the aviation industry (Yang X. et al., 2016). Algae can be used to generate biofuel via hydrothermal processing in three various steps: liquid (i.e., bio-crude oil), gas (i.e., biohydrogen and biomethane), and solid (i.e., hydro-char) (Biller and Ross, 2014). Corresponding to operating conditions hydrothermal processing can be classified as liquefaction, gasification, and carbonization where the end products vary from each other. Properties of algae-based biofuels depend on temperature and reaction time of the hydrothermal method (Hrnčič et al., 2016). Hydrothermal reaction carried out at mild operating conditions (i.e., 10–40 bar and 180–250 °C) is termed as hydrothermal carbonization and solid hydrochar is obtained as the main product. In a 2012 study, Miller and Ross demonstrated that algal biomass could be processed hydrothermally to produce biofuels in the form of solid hydrochar, liquid bio-oil, and gaseous products such as biomethane or biohydrogen (Heilmann et al., 2010). Biological hydrogen production can be classified into dark fermentation, which occurs in the absence of light, and photofermentation, which requires light. Hydrogen is typically produced via dark fermentation, though pretreatment of algal biomass prior to fermentation has been suggested to improve the hydrolysis of carbohydrate polymers. In contrast, hydrothermal liquefaction operates at much higher temperatures (250 °C–450 °C) and pressures (40–165 bar), producing bio-oil as the main product (López Barreiro et al., 2013). Whereas hydrothermal processing carried out under severe operating conditions (i.e., above critical point of water, i.e., 374 °C and 221 bar) is known as hydrothermal gasification and bio-syngas is the product of the process. Table 4 mentions various operating conditions and products for hydrothermal processing, which indicates that Chlorella and Nannochloripsis are commonly used type of algal biomass feedstock.

TABLE 4. Hydrothermal processing of algal biomass using water in the absence of catalyst.

3.3.1 Hydrothermal liquefaction

A wide variety of flexible feedstock of both fresh or marine water based algae can be utilized via hydrothermal liquefaction (HTL) (Han et al., 2019). Hydrothermal liquefaction (HTL) is distinct from fractionation-based conversion pathways, specifically algae-to-ethanol (carbohydrate fermentation) and algae-to-biodiesel (lipid transesterification), because it utilizes the entire wet algal biomass rather than extracting specific components. Unlike pyrolysis which requires dry biomass, HTL processes wet algae to generate bio-crude (30%–60% of dry feedstock) having a high heating value of around 25–35 MJ/kg, a mixture of H₂, N₂, CH₄, CO₂, CO gases, a solid residue, and an aqueous product rich in organics (Das et al., 2020; Davidson et al., 2019). Using HTL provides advantages over biochemical and thermochemical conversion processes by mitigating the energy required for feedstock drying and providing increased reaction rates with improved separation of various products. HTL can be utilized as both non-catalytic (Cheng et al., 2019; Han et al., 2019) or catalytic (Chang et al., 2015; Qiao et al., 2013) and has shown to operate in both continuous and batch type reactors at medium temperature (>400 °C).

Hydrothermal liquefaction (HTL) typically produces viscous bio-crude oil at temperatures between 245 °C and 460 °C and pressures of 45–160 bar (López Barreiro et al., 2013). In contrast, hydrothermal gasification (HTG) operates under more severe thermal conditions, typically between 400 °C and 800 °C, to produce bio-syngas, surpassing the critical point of water. Biological hydrogen production can be achieved through dark fermentation, which occurs without light, or photo fermentation, which requires light. Dark fermentation is the most widely used approach, though some studies recommend pretreating algal biomass to improve carbohydrate polymer hydrolysis. Ruiz-Marin et al. (2020) further demonstrated that different light wavelengths can be applied to cultivate immobilized algae for biohydrogen production (Ruiz-Marin et al., 2020), The cultivation of Chlorella vulgaris and Scenedesmus obliquus can be promoted by utilizing urban wastewater as a nutrient source. To assess the influence on algal growth and biohydrogen generation, researchers compared the effects of two light wavelengths. Cultures exposed to blue light achieved higher biomass concentrations, while those grown under purple light produced greater volumes of biohydrogen, reaching approximately 128 mL/L compared to ∼61 mL/L under blue light. A summary of previous hydrothermal investigations on algal biomass is provided in Table 3.

Similarly, Kumar et al., 2018, used swine manure dilutions to ferment mixed algae to produce biohydrogen (Kumar et al., 2018). A swine manure dilution ratio of 5 g/L produced the highest algal biomass concentration of 2.57 g/L. However, increasing the dilution ratio resulted in a decline in biomass yield. The harvested biomass was subsequently fermented under dark conditions to generate biohydrogen, with the 5 g/L loading rate achieving a maximum hydrogen yield of 13 mL/g of algal biomass. In a related study, Batista et al. (2015) cultured C. vulgaris and S. obliquus in tubular photobioreactors using urban wastewater as the growth medium. Their findings revealed that S. obliquus and C. vulgaris produced the highest biohydrogen yields per Gram of volatile solids, reaching approximately 57 mL and 41 mL, respectively (Batista et al., 2015).

Recent studies increasingly emphasize integrated HTL–biochemical conversion frameworks, particularly the coupling of hydrothermal liquefaction with dark fermentation. In such systems, HTL is used to convert wet algal biomass into bio-crude, while the aqueous phase by-products, rich in short-chain organic acids, nitrogenous compounds, and soluble organics, are subsequently valorized via dark fermentation for biohydrogen or volatile fatty acid (VFA) production (Leggieri et al., 2022). This integrated approach addresses two critical challenges simultaneously: (i) improving overall carbon utilization by recovering energy from HTL aqueous streams, and (ii) mitigating wastewater treatment burdens associated with HTL processing. Recent investigations demonstrate that appropriate conditioning of HTL aqueous fractions, through dilution, pH adjustment, or inhibitory compound management, can significantly enhance dark fermentation performance (Fuentes-Cortes et al., 2023). From a systems perspective, HTL-dark fermentation integration represents a promising pathway toward circular algal biorefineries, where thermochemical and biochemical routes are synergistically combined to maximize energy recovery, reduce environmental impacts, and improve overall process sustainability (Kılkış et al., 2024).

3.3.2 Hydrothermal carbonization

The process of hydrothermal carbonization (HTC) can be categorized as a thermochemical treatment of wet algal biomass to carry out the conversion of carbohydrates within the biomass to carbon-rich solids (Merzari et al., 2019; Yu Krylova and Zaitchenko, 2018). The concept of HTC can be related to wet torrefaction (WT), both processes are being reported (Bach et al., 2017; Bach et al., 2013; Zhang et al., 2017) to label methods which take place in hydrothermal or compressed hot water mediums at temperatures ranging from 180 °C to 260 °C (Bach et al., 2017; Bach and Skreiberg, 2016; Chen et al., 2012). According to Bach et al. (Bach et al., 2013), HTC is mostly utilized to obtain coal having high carbon content which can be applied as not only a fuel but also as an source of activated carbon, fertilizer and soil enhancer. Whereas WT is used to generate only solid fuels. When in terms of energy efficiency, WT is more crucial than HTC and literature can be found where WT is performed at lower temperatures as compared to HTC. Therefore, studies on WT are important for the understanding of HTC’s mechanism, however a clear distinction should be maintained between these two processes.

Hydrothermal treatment can also be applied for N reduction in the produced bio-oil which was carried out in Nnanochloropsis occulate where N was decreased and carbon was increased (Z et al., 2012). Hydrothermal treatment can display excellent performance mainly in hydrodeoxygenation, improved decarbonization, and fast hydrolysis leading to a increased energy efficiency (Tekin et al., 2014). Heat integration can lead to an energy efficiency of 85% by employing the hot reactor effluent to preheat the inlet stream (Magdeldin et al., 2017). This is an advantage to conventional conversion techniques where dry biomass is utilized in low temperature processing conditions. Therefore, there is a less chance of recovering thermal energy hence affecting assessment of the thermal process. Recent research focused at rectifying high energy requirement of hydrothermal pretreatment by combining it with solar was carried as an alternative process to save energy (Xiao et al., 2019).

The Hydrothermal Carbonization (HTC) mechanism involves a complex reaction network (Kambo and Dutta, 2015). Initially, hydrolysis depolymerizes biomass macromolecules (carbohydrates, proteins, lipids) into monomers. For carbohydrates (cellulose/starch), this breaks down into glucose

C 6 H 10 O 5 n + n H 2 O → n C 6 H 12 O 6

Subsequently, dehydration removes water from these monomers, converting hexoses (like glucose) into intermediates such as 5-hydroxymethylfurfural

C 6 H 12 O 6 → C 6 H 6 O 3 + 3 H 2 O

Concurrently, decarboxylation releases carboxyl groups as

R – COOH → R – H + CO 2

For algae specifically, the protein content drives Maillard reactions, where amino acids react with reducing sugars to form nitrogen-containing aromatic chars (melanoidins)

C 6 H 12 O 6 + R – N H 2 → Schiff base + H 2 O

4 Biochemical conversion

This pathway of utilizing biomass deals with the decomposition of carbohydrates which later can be converted to biogas, liquid fuels as well as various bio-based products using bacteria and enzymes acting as biological agents (Mahalaxmi and Williford, 2012). The most commonly used methods of biochemical conversion are fermentation and anaerobic digestion (Brethauer and Studer, 2015).

4.1 Fermentation

Fermentation of organic compounds constitutes series of reactions where simple sugars (i.e., pentoses and hexoses) are converted to ethanol and CO₂ under anaerobic conditions via microorganisms such as yeasts as depicted by Equation 12.

S u g a r s → E t h a n o l + C O 2 + b y − p r o d u c t s (12)

Saccharomyces Cerevisiae are commonly used microorganisms in this process, whereas the feedstock can be categorized into three different groups: lignocellulosic substrates, starch, and sugars. Theoretical yield of this process for producing ethanol and CO₂ is 51.14 g and 48.86 g from 100 g of hexoses and pentoses respectively as shown in Equations 13 and 14.

C 6 H 12 O 6 h e x o s e s → 2 C 2 H 5 O H + 2 C O 2 (13)

3 C 5 H 10 O 5 p e n t o s e s → 5 C 2 H 5 O H + 5 C O 2 (14)

In addition to ethanol and CO₂, carboxylic acids and glycerol are generated as by-products. The yield and quality of these products are influenced by several factors, including pH, reaction medium, reaction time, inoculum, temperature, and the type of feedstock used (Strezov, 2014). Conversion of these sugars can occur through different paths depending on the feedstock. Figure 8 displays a schematic diagram of fermentation using algae as a biomass feedstock. Hexoses such as glucose are converted through glycolysis or by the Embden-Meyerhof pathway (EMP) (Taherzadeh and Karimi, 2008) and pentoses are transformed via pentose phosphate pathways (PPP). Reaction time for hexoses is less as compared to pentoses. Algal sugars such as mannitol, laminarin, and alginate are transformed into ethanol using yeast (Lee and Lee, 2016). This process is also capable of producing butanol and acetone via solventogenesis and acidogenesis (Trivedi et al., 2015). Bioethanol production from algae is possible due to algae’s high carbohydrate content (Sheehan et al., 1998). High concentration of mannitol in the obtained hydrolysates can be used as a cheap organic substrate for producing bioethanol (Yang C. et al., 2016). Bioethanol production capacity when compared with sugar cane is twice for algae (Veillette et al., 2017) and is five times higher than corn. Due to the high starch content present in Chlorella vulgaris (microalgae) is a suitable source for ethanol for which the conversion efficiency has been reported up to 65%.

The viability of producing ethanol by using algae as a biomass feedstock through fermentation was reported by Brennan and Owende (2010) which was based on potential of lipid production. Residue obtained from fermentation could offset cost of the feedstock and result in a cheap product. Literature (Sirajunnisa and Surendhiran, 2016) reports that ethanol production from algae can have an immense impact on the global sustainable development without compromising food production and its supply. With today’s research focus being on developing secondary by-products, the economic stability of producing bioethanol can be amplified (Fasahati et al., 2015). Pretreating biomass can improve bioethanol yields during fermentation; however, this approach is often not economically viable due to the high costs associated with pretreatment reactors. After the conversion process, ethanol is distilled and dehydrated to produce concentrated alcohol, while the residual biomass can either be used as fuel in boilers or directly as livestock feed (Mariod, 2016).

In the absence of lignin, brown algae-based biomass is suggested as one of the most feasible options for producing bioethanol due to its high content of fermentable sugars (mannitol, laminarin) and polysaccharides (alginate). To obtain bioethanol, proper fermentation by specialized microorganisms must be carried out to utilize these diverse carbon sources. Higher yields of bioethanol are possible from brown algae when the carbon source is optimized during fermentation (Slade and Bauen, 2013). Bioethanol production after extracting lipid from biodiesel can be obtained via anaerobic fermentation (Ashokkumar et al., 2017). Commercialization of bioethanol will be dependent upon the development of economical cultivation and processing techniques (Lee and Lee, 2012).

4.2 Anaerobic digestion

This is a multi-step process which is not only utilized for waste management but is also used to generate renewable energy (Zamani, 2015). During this microbial process the algal biomass is degraded in the absence of oxygen (Ward et al., 2014). Anaerobic digestion of algae-derived biomass generates valuable biogases, such as biohydrogen and biomethane, along with a solid–liquid residue known as digestate. The process proceeds through four main stages driven by specific microbial groups. First, Hydrolysis converts biopolymers into monomers, a step typically dominated by bacteria from the phylum Firmicutes (e.g., Clostridium) which secrete extracellular enzymes. Second, during Acidogenesis, soluble monomers are converted into volatile fatty acids (VFAs) by fermentative bacteria such as Bacteroidetes. Crucially, the rapid accumulation of VFAs during this stage can significantly lower the pH; if the system’s buffering capacity is exceeded, this acidification inhibits the subsequent methanogenic populations which are highly pH-sensitive (optimal pH 6.5–7.5), leading to process failure. Third, Acetogenesis converts VFAs into acetate and hydrogen, involving specialized acetogens (e.g., Syntrophomonas). Finally, Methanogenesis produces methane, primarily mediated by archaea such as Methanosarcinales. It is important to note that acetogens and methanogens cannot be independently cultivated due to their obligate syntrophic relationship; the oxidation of VFAs by acetogens is thermodynamically unfavorable (G > 0) and only proceeds when hydrogen is continuously consumed by methanogens to maintain a low partial pressure (G < 0) (McInerney et al., 2008).

Through these pathways, complex organic macromolecules are decomposed into simpler compounds, ultimately yielding biogas and digestate. Typical feedstocks for anaerobic digestion include sewage sludge, municipal solid waste (MSW), agricultural residues, and animal manure (Sharma, 2015).

Technically this process is more viable when compared to other techniques, due to its capacity to generate other sources of energy and products (Sheehan et al., 1998). Anaerobic digestion is capable of fixing high amounts of carbon as this process extracts nutrients from the biomass and returns them into the cultivation system (Hallenbeck et al., 2016). Algae based organic waste can be transformed to biogas by breaking it down to generate CO₂ and mainly CH₄ which can eventually be utilized domestically for cooking and heating purposes and also for electricity generation via gas engines (Lee and Lee, 2016) as depicted in Figure 9. This process can be transform biomass possessing high moisture content and is capable of procuring methane from delipidized algal biomass (Trivedi et al., 2015). Biogas produced from this process has an energy content lower than the heating value of the original substrate. It is particularly suitable for organic wastes with high moisture content, ranging from 80% to 90%. However, the resistant cell walls of algae often make hydrolysis the rate-limiting step, necessitating long hydraulic retention times (HRT). Implementing pretreatment strategies (thermal, mechanical, or enzymatic) disrupts these cell walls, solubilizing intracellular organic matter; this acceleration of hydrolysis significantly reduces the required holding time (retention time) in the digester and increases methane productivity (Passos et al., 2014). A key advantage is that the resulting biogas can be directly used in gas engines and turbines. The overall process electrical conversion efficiency is approximately 21%, with the remaining heat recoverable through a heat exchanger (Córdova et al., 2018).

Biogas produced from this process has an energy content lower than the heating value of the original substrate. It is particularly suitable for organic wastes with high moisture content, ranging from 80% to 90%. A key advantage is that the resulting biogas can be directly used in gas engines and turbines. The overall process efficiency is approximately 21%, with the remaining heat recoverable through a heat exchanger (Ho et al., 2004). Figure 10 depicts anaerobic digestion process highlighting all the stages of the process along with their description is as follows.

4.2.1 Hydrolysis

This initial stage of the process involves breaking down biomass components, fats, proteins, and carbohydrates into smaller molecules such as simple sugars, amino acids, and fatty acids. (Wid and Horan, 2018). Most of the larger molecules are further degraded during the acidogenesis step of the process. By-products such as acetate and hydrogen from this step are utilized at the final stage, i.e., methanogenesis.

4.2.2 Acidogenesis

The second stage of anaerobic digestion involves acidogenic microorganisms further breaking down the products from the hydrolysis step. This results in the formation of CO₂, H₂, H₂S, NH₃, alcohols, volatile fatty acids, and carbonic acids, representing a partial decomposition of the algae-based biomass.

4.2.3 Acetogenesis

In this step acetogenic microorganisms are employed which oxidate products from the previous stage into acetic acid, H₂, and CO₂ which are suitable for methanogenesis (Ehimen et al., 2013).

4.2.4 Methanogenesis

In the final stage of anaerobic digestion, carbon dioxide, hydrogen, and ammonia are converted into methane and carbon dioxide (Trivedi et al., 2015). Converting algal biomass into renewable gas allows for significant energy recovery from lipid extraction, while retaining sufficient nutrients that can be recycled into fresh growth medium to support further algal cultivation (Ehimen et al., 2013). The obtained products from this stage of the process can be denoted by Equations 15 and 16 known as hydrogenotrophic methanogenesis and acetoclastic methanogenesis respectively (Tursi, 2019).

H y d r o g e n o t r o p h i c m e t h a n o g e n s i s C O 2 + 4 H 2 → C H 4 + 2 H 2 O 15)

A c e t o c l a s t i c m e t h a n o g e n s i s C H 3 C O O H → C H 4 + C O 2 (16)

Anaerobic digestion efficiency can be influenced by several inhibitory factors. In the case of Chlorella vulgaris and Dunaliella tertiolecta, high salinity in the algal sludge has been shown to suppress biomethane generation, while hydrogen production is comparatively unaffected (Lakaniemi et al., 2011). Presence of long chain fatty acids (LCFAs) have also been reported as a hindrance in microbial activity during anaerobic digestion (Ma et al., 2015). When Nannochloropsis salina was used as a biomass source, microbial activity was found to decrease as lipid concentration increased. The buildup of long-chain fatty acids (LCFAs) caused volatile fatty acids (VFAs) to accumulate, which in turn inhibited the metabolic activity of both bacteria and methanogens, thereby reducing the efficiency of anaerobic digestion for biomethane production. Literature (Ehimen et al., 2013) also highlights the impact of pretreatment on methane production during anaerobic digestion of algae where enzymatic processes produce higher amounts of methane as compared to mechanical methods. Thermal pretreatment was reported (Rodriguez et al., 2015) as the ideal pathway to obtain the highest amount of methane from anaerobic digestion with low energy requirements, however mechanical pretreatment is favourable for some specific species of algae. High energy input for the mentioned processes is a crucial issue which requires further investigation. Researchers suggested the possibility of various techniques for higher methane yield with positive energy balances if the pretreatment methods are optimized via the multi-objective optimization technique (Rodriguez et al., 2015). Another study (Marsolek et al., 2014) also recommends thermal pretreatment as a viable option due to the extra heat from algae based biofuel. Heat produced, from other algae transformation techniques, which is suitable for many systems can make up for the energy requited during pre-heating of the digester.

4.2.5 Key inhibitors and solution pathways

For saline algal sludges (particularly marine strains and seawater-grown biomass), inhibition should be treated as an osmotic-stress and enzyme-activity limitation rather than a generic “substrate effect.” Recent evidence indicates methane productivity declines beyond practical salinity thresholds unless the system is redesigned through (i) adaptive acclimation and enrichment of halotolerant consortia, (ii) two-stage anaerobic digestion to decouple acidogenesis from methanogenesis under stress, and (iii) process integration strategies such as partial dilution/co-digestion with low-salinity wastes to maintain ionic strength within a stable operating window (Qian et al., 2025).

For lipid-rich algae, LCFA inhibition should be treated as a mass-transfer + toxicity + syntrophic limitation that can be mitigated by engineering electron transfer and adsorption environments. Breakthrough pathways supported in recent literature include the use of carbonaceous additives (biochar/hydrochar) to (i) adsorb inhibitory intermediates, (ii) stabilize microbial aggregates, and (iii) promote direct interspecies electron transfer (DIET), thereby improving methane yields under LCFA stress (Sun et al., 2025).

These solution pathways also clarify application scenarios: saline systems should prioritize strain/medium choices and acclimated inocula; lipid-rich systems should incorporate staged feeding, co-digestion, and conductive/adsorptive supports to avoid VFA accumulation and methanogenic collapse (Qian et al., 2025).

4.3 Photobiological conversion

This process is most commonly used for producing hydrogen from algae-based biomass. Hydrogen is an environmentally benign energy source possessing a high latent heat. Hydrogen can be photobiologically be produced from microalgae as they possess enzymatic and metabolic capacity to do so (Ghirardi et al., 2000). Eukaryotic type of microalgae under anaerobic conditions can generate hydrogen ions and oxygen during the fixation of CO₂. Hydrogen ions are then utilized by a hydrogenase enzyme to produce hydrogen gas. Potential pathways for hydrogen generation using glucose as a substrate are carried out either with acetate of butyrate production as depicted by Equations 17 and 18.

G l u c o s e + 2 H 2 O → 2 A c e t a t e + 2 C O 2 + 4 H 2 (17)

G l u c o s e → B u t y r a t e + 2 C O 2 + 4 H 2 (18)

Energetically acetate fermentation is more feasible when compared to butyrate fermentation. Theoretical H₂ yield is higher in acetate fermentation than butyrate fermentation. Two-stage photosynthesis in green algae yielded a maximum amount of 200 kgH₂ ha⁻¹d⁻¹ (Melis and Happe, 2001). According to the availability of protons hydrogen can migrate between the product and reactant sides of the reaction as photosynthesis is reversible in nature. Biomass media used for hydrogen production should be carried out in an anaerobic environment as the oxygen is very detrimental towards the hydrogenase enzyme (Amin, 2009). By-products (i.e., butyrate and acetate) may also contribute towards mitigating hydrogen production from microalgae during hydrogen fermentation. Therefore, to obtain optimum hydrogen production measures should be taken to inhibit by-products during generation of biohydrogen.

To address the inherent limitations of photobiological hydrogen production, particularly the “oxygen paradox” where photosynthetic O₂ inactivates hydrogenase enzymes, recent research has categorized production pathways into direct biophotolysis and indirect biophotolysis. In direct biophotolysis, electrons derived from water splitting at Photosystem II (PSII) are transferred directly to the hydrogenase enzyme. While efficient in electron transfer, this pathway is severely limited by simultaneous O₂ evolution. Conversely, indirect biophotolysis decouples these steps by storing solar energy as starch or glycogen during an aerobic phase, which is subsequently degraded under anaerobic dark conditions to produce hydrogen, thereby bypassing direct O₂ inhibition.

Current optimization strategies have moved beyond the traditional sulfur-deprivation protocols to include advanced genetic and metabolic engineering. Genetic modification to create Truncated Light-harvesting Antennae (TLA) strains reduces the optical cross-section of individual cells; this prevents the wasteful dissipation of excess light as heat and allows sunlight to penetrate deeper into the bioreactor, significantly improving the overall solar-to-hydrogen (STH) conversion efficiency of the culture. Furthermore, the development of synthetic consortia, such as microalgal-bacterial co-cultures (e.g., Chlamydomonas with Pseudomonas or Bradyrhizobium), utilizes the high respiration rates of heterotrophic bacteria to scavenge dissolved oxygen, creating a localized anaerobic microenvironment that sustains algal hydrogenase activity without the need for external purging or nutrient stress. Nanotechnology enhancement has also emerged as a promising avenue, where the supplementation of bioreactors with nanoparticles (e.g., metallic or metal-oxide NPs) enhances electron transfer rates and metabolic activity, acting as artificial redox mediators that boost hydrogen yield. Finally, a significant recent breakthrough involves the identification of novel oxygen-tolerant [FeFe]-hydrogenases, such as those from Thermosediminibacter oceani; these enzymes possess unique structural features, including a protective sulfur-containing amino acid cluster, that allow them to retain catalytic activity even in the presence of oxygen, providing a new target for transgenic expression in algal hosts.

5 Downstream processing and biomass fractionation

Effective downstream processing remains the primary economic bottleneck in algal biofuel production, necessitating advanced technologies that can handle high-water-content biomass to bypass energy-intensive thermal drying. Recent industrial strategies in 2024 and 2025 have shifted toward low-energy harvesting protocols, such as bio-flocculation and magnetic nanoparticle-assisted separation, which significantly reduce the energy input compared to traditional centrifugation (Kanwal et al., 2025). Once harvested, the rigid algal cell wall must be breached to release intracellular metabolites. While mechanical disruption remains standard, there is a growing adoption of “green” and selective permeabilization technologies like Pulsed Electric Fields (PEF) and enzymatic hydrolysis. These methods allow for the targeted release of cytoplasmic compounds (such as proteins and carbohydrates) while preserving the integrity of lipids for subsequent extraction, thereby facilitating a more efficient fractionation process (Kopp and Lauritano, 2025).

To maximize economic viability, the industry is increasingly moving toward a cascading biorefinery model, where biomass is fractionated into multiple value-added streams rather than targeting a single fuel product. Innovative liquid-liquid fractionation techniques, such as Three-Phase Partitioning (TPP) and Aqueous Two-Phase Systems (ATPS), have demonstrated high efficiency in simultaneously separating hydrophobic lipids for biodiesel and hydrophilic proteins for food or feed applications (Martins et al., 2025). Furthermore, the use of Switchable Hydrophilicity Solvents (SHS) has emerged as a sustainable alternative to toxic organic solvents, enabling the extraction of lipids directly from wet biomass with high recovery rates and solvent recyclability (Kopp and Lauritano, 2025). The residual carbohydrate-rich biomass, often remaining after lipid and protein extraction, is then valorized through hydrolysis and fermentation into bioethanol or anaerobic digestion for biogas, ensuring complete resource utilization and substantially improving the net energy ratio of the entire production chain.

6 Life cycle assessment (LCA) and sustainability considerations of algal biofuel pathways

While algal biomass conversion technologies show significant promise at the process level, their environmental viability must be evaluated through life cycle assessment (LCA) to capture upstream and downstream impacts. Recent LCA studies emphasize that algal biofuel sustainability is highly sensitive to cultivation energy demand, nutrient sourcing, harvesting and dewatering requirements, and conversion efficiency, rather than conversion yield alone (Ramachandra and Saranya, 2022). Comparative LCA analyses indicate that algae-based biofuels can achieve substantial greenhouse gas (GHG) emission reductions relative to fossil fuels only when integrated with low-carbon inputs, such as wastewater-derived nutrients, recycled process water, and waste CO₂ streams. Standalone algal cultivation systems relying on synthetic fertilizers and fossil-derived electricity often exhibit unfavorable energy return on investment (EROI) and higher life-cycle emissions (Kalan et al., 2023). Among thermochemical pathways, hydrothermal liquefaction (HTL) has demonstrated comparatively favorable life-cycle performance due to its ability to process wet biomass, thereby avoiding energy-intensive drying steps. In contrast, pyrolysis-based systems frequently require additional upgrading and external energy inputs, which can negatively influence net GHG performance unless coupled with renewable heat or biochar valorization strategies (Collet et al., 2014). Biochemical conversion routes, such as anaerobic digestion, show strong LCA performance when algal residues are utilized for biogas production and nutrient recycling, enabling circular resource flows. Recent studies increasingly highlight the importance of integrated biorefinery configurations, where co-production of fuels, electricity, and high-value coproducts improves overall environmental and economic sustainability (Kumar et al., 2023). Overall, recent LCA literature underscores that the sustainability of algal biofuels is system-dependent rather than technology-dependent, and future deployment must prioritize process integration, resource recycling, and region-specific optimization to achieve meaningful climate and energy benefits.

7 Technical challenges and future directions

Despite significant progress in algal biomass conversion technologies, several intrinsic biochemical characteristics of algae, combined with process-level and scale-dependent constraints, continue to limit their large-scale deployment. Addressing these challenges requires a transition from isolated process optimization toward mechanism-driven, system-integrated solutions that align feedstock properties with conversion pathways and end-use objectives. In chemical conversion routes, particularly transesterification-based biodiesel production, the heterogeneity of algal lipids remains a major bottleneck. Variations in free fatty acid (FFA) content, moisture levels, and co-extracted pigments and proteins significantly affect catalyst performance and downstream separation. Conventional alkaline-catalyzed transesterification remains efficient at laboratory scale but suffers from soap formation, catalyst deactivation, and wastewater generation when applied to high-FFA algal oils (Santos et al., 2021). Recent studies indicate that future progress lies in catalyst–feedstock matching, rather than universal catalyst development. Advanced heterogeneous catalysts, including nanostructured metal oxides, bifunctional acid-base catalysts, and metal-organic framework (MOF)-derived catalysts, have demonstrated improved tolerance to high FFA and moisture contents while enabling catalyst recovery and reuse. Enzyme-catalyzed transesterification, although currently limited by cost and reaction rates, is increasingly viable through immobilization strategies, protein engineering, and solvent-free systems, positioning it as a promising route for high-value or decentralized applications (Farouk et al., 2024; Keshta et al., 2025). Future research should prioritize (i) scalable heterogeneous catalyst systems, (ii) integration of lipid extraction and conversion into single-pot or reactive extraction processes, and (iii) techno-economic optimization under realistic algal strain compositions rather than model oils.

Thermochemical routes, particularly pyrolysis and gasification, are challenged by the high nitrogen, oxygen, and ash content inherent to algal biomass. Protein-rich algae generate bio-oils with elevated nitrogen-containing heterocycles and oxygenates, which reduce fuel stability, calorific value, and compatibility with existing fuel infrastructures. These limitations cannot be resolved by operational optimization alone (Kutluk and Kutluk, 2025). Recent advances emphasize catalytic intervention at the vapor phase, where metal-modified zeolites and multifunctional catalysts enable simultaneous deoxygenation and denitrogenation. Two-stage systems, combining primary pyrolysis with downstream catalytic upgrading, have emerged as a practical pathway to decouple volatilization efficiency from product quality control. Importantly, future thermochemical strategies must move toward application-specific product targeting, distinguishing between fuels, aromatics, and nitrogen-containing specialty chemicals (Niu et al., 2024). In gasification, reactor configuration plays a decisive role. Fixed-bed systems remain suitable for small-scale, low-ash feeds, whereas fluidized-bed and entrained-flow gasifiers offer superior heat and mass transfer for large-scale operations but require effective tar mitigation strategies. Emerging research on chemical looping gasification presents a promising future direction by enabling inherent tar cracking, nitrogen dilution avoidance, and improved syngas quality (Zhu et al., 2025).

Biochemical conversion routes, especially anaerobic digestion, are constrained by salinity stress, ammonia toxicity, and long-chain fatty acid (LCFA) inhibition, particularly for marine and lipid-rich algal strains. These inhibitory effects are often treated empirically, yet recent studies clarify their mechanistic origins in osmotic stress, syntrophic imbalance, and electron transfer limitations (Zupančič et al., 2023). Emerging mitigation strategies emphasize process re-engineering rather than dilution-based fixes. Two-stage anaerobic digestion systems decouple hydrolysis/acidogenesis from methanogenesis, enhancing stability under saline or lipid-rich conditions. Additionally, the use of carbonaceous additives (biochar and hydrochar) has gained prominence due to their multifunctional roles in LCFA adsorption, microbial aggregation, and promotion of direct interspecies electron transfer (DIET) (Sun et al., 2023). Future research should prioritize (i) adaptive microbial consortia development, (ii) conductive material-assisted digestion, and (iii) integration of anaerobic digestion with upstream pretreatment or downstream nutrient recovery to improve system resilience and economic viability.

A critical future direction across all conversion pathways is the integration of algal cultivation with conversion processes. Coupling algae production with wastewater treatment and CO₂ capture not only reduces cultivation costs but also reshapes conversion pathway selection based on nutrient and salinity profiles. Furthermore, hybrid systems combining biochemical and thermochemical routes, such as digestate pyrolysis or hydrothermal carbonization of residues, offer pathways toward near-zero-waste biorefineries (Yadav et al., 2021). Overall, advancing algal biomass utilization requires a shift toward holistic, strain-to-product frameworks, where biological composition, reactor design, catalyst selection, and final application are co-optimized. Such integration represents a decisive step toward translating laboratory advances into commercially and environmentally sustainable algal bioenergy systems (Mehmood et al., 2024).

A major omission in many algae conversion discussions is that cultivation conditions directly determine conversion feasibility (lipid/protein ratio, ash/salt burden, moisture content), and thus conversion should increasingly be framed as cultivation–conversion co-design rather than sequential unit operations. In particular, integrating algae production with wastewater treatment provides dual benefits: nutrient recovery and reduced cultivation cost, while simultaneously producing biomass that can be routed toward wet-tolerant conversion routes such as hydrothermal liquefaction (HTL). Recent pathway studies demonstrate the feasibility of producing upgraded fuels from wastewater-grown algae via HTL, and system-level analyses increasingly evaluate HTL integration with wastewater treatment plants using combined techno-economic and life-cycle frameworks (Kumar et al., 2025). Similarly, “deep integration” with carbon management is advancing beyond generic CO₂ uptake statements to a CCUS framing: algae are being evaluated as platforms for carbon capture and utilization, including coupling to flue-gas streams and designing cultivation systems around carbon source characteristics. Recent reviews synthesize innovations in algae-based CCUS and emphasize the importance of bioprocess design and microbiome/cultivation optimization for practical deployment across different carbon-emission scenarios (Li and Yao, 2024). The most impactful research direction is to demonstrate integrated “circular” operating windows where wastewater nutrients and captured CO₂ are inputs, and conversion residues (digestate/biochar/hydrochar) are recycled to stabilize digestion, improve cultivation, or enhance soil amendment value thereby aligning the challenge–solution chain with measurable system performance metrics (energy return, carbon intensity, and operating cost) (Karka et al., 2024).

8 Conclusion

Algal biomass represents a pivotal third-generation feedstock that surpasses the limitations of terrestrial energy crops through its superior photosynthetic efficiency, non-competition for arable land, and capacity to thrive in non-potable water. While individual conversion pathways, ranging from chemical transesterification for biodiesel to biochemical anaerobic digestion for biogas, have demonstrated technical feasibility, their standalone implementation faces significant techno-economic barriers, primarily driven by the high energy intensity of upstream harvesting and downstream dewatering. The path toward commercial viability lies fundamentally in the transition from single-product fuel production to an integrated multi-product biorefinery model. As highlighted by recent Techno-Economic Analyses (TEA), the high production costs of algal biofuels can only be effectively offset by the cascading valorization of biomass into high-value co-products, such as polyunsaturated fatty acids (PUFAs), pigments (e.g., astaxanthin, phycocyanin), and recombinant proteins, prior to energy conversion. This “whole-crop” utilization strategy ensures that the high capital expenditure of cultivation is amortized across a diverse product portfolio. Furthermore, Life Cycle Assessments (LCA) confirm that the sustainability of these systems is heavily dependent on process integration. Coupling algae cultivation with wastewater treatment for nutrient recycling (phycoremediation) and utilizing industrial flue gas for CO₂ supplementation can significantly improve the Net Energy Ratio (NER) and reduce the water footprint compared to conventional crops. Future research must prioritize the development of “wet” processing technologies, such as hydrothermal liquefaction (HTL) and wet-extraction methods, to circumvent the prohibitive energy penalty of biomass drying. Ultimately, the successful scale-up of algal biorefineries will depend on optimizing these synergistic technologies to establish a truly circular, carbon-negative bioeconomy that is both environmentally benign and economically resilient.

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Footnotes

Edited by: Apip Amrullah, Hiroshima University, Japan

Reviewed by: Hector Alfredo Calderon, National Polytechnic Institute (IPN), Mexico

Jingchun Yan, Nanjing Tech University, China

Saeed Uz Zafar, Carepro Biosciences Private Limited, India