Algal Biomass for Sustainable Clean Fuels

Algal oil has emerged as a promising renewable resource for the sustainable production of biofuels and high-value biochemicals. With growing interest in alternative energy sources and environmentally friendly chemical production, algae have become a viable feedstock owing to their high lipid content, fast growth rate, and ability to thrive on wastewater and non-arable land. Conventional methods of extraction often suffer from high energy demands, low yield, and environmental concerns. This chapter explores novel techniques for the efficient extraction of algal oil, including supercritical fluid extraction, and microwave (MW)-assisted extraction. These innovative approaches enhance lipid recovery, reduce solvent usage, and preserve oil quality, making them more suitable for large-scale biotechnological applications. Beyond extraction, the valorization of algal oil for the production of biodiesel, bioplastics, surfactants, lubricants, and platform chemicals is explored, emphasizing its role in integrated biorefinery systems. The chapter emphasizes the importance of sustainable sourcing, advanced extraction technologies, and downstream processing to harness algal oil as a key bioresource for the circular bioeconomy. By combining innovative extraction techniques with broad biotechnological utilization, algal oil holds significant promise as a versatile and eco-friendly alternative to fossil-derived fuels and chemicals.

Energy availability and security are crucial for a nation’s social and economic development. Microalgae-based biofuel production is a viable alternative to non-renewable energy sources, such as biodiesel, bioethanol, and biogas. However, due to obstacles and high production costs, the conversion of microalgal biomass to biofuel is not yet marketed. Combining the processes of producing biofuel, bioproducts and biogas from microalgae is becoming more popular. The biorefinery approach is an environmentally friendly method of converting biomass into chemical and fuel products. It combines equipment needed for biomass conversion with the conversion process itself, providing a self-sustaining approach for producing biodiesel and high-value byproducts like bioethanol or biogas. About 60-70% of leftover biomass waste is produced as a byproduct of biodiesel extraction from microalgae. Anaerobic digestion (AD) can recover additional energy beyond the microalgal oil itself. Residual rice straw biowaste (RS) is a viable bulk biomass for the anaerobic co-digestion (A-CoD) process. This study investigates the potential application of RS in conjunction with defatted CG12 and GS12 microalgae cultures for A-CoD. The strategy offers a low-cost, zero-waste, sustainable energy-generating model that can be effectively implemented for the biorefinery/scale-up method.

Algal biomass is globally distributed and holds significant potential for valorization, a process that enables the creation of valuable products for applications such as energy storage. This is particularly important given the ability of algal biomass to help mitigate climate change by reducing greenhouse gases and treat wastewater. Due to its high lipid and protein concentrations, algal biomass is particularly suitable for thermochemical processes such as pyrolysis, torrefaction, combustion, gasification, and hydrothermal liquefaction (HTL), which are used to produce biochar for various biomass-based energy storage applications. It is crucial to optimize thermochemical processes to understand the role of pretreatment, the effects of process parameters, and the catalytic enhancement of algal biomass. The unique hierarchical microstructure, high surface area, and excellent mechanical properties allow them to create carbon-based supercapacitor electrode material. The basic mechanism requires immediate research to provide clear guidance and develop an optimized methodology, at both the laboratory and industrial scales. However, there are technical challenges associated with scaling up thermochemical methods to a larger scale, despite the benefits to the environment, economy, and society, which can lead to a circular economy.

Biomass derived from various sources represents a renewable and carbon-neutral alternative to fossil feedstocks for the sustainable production of fuels and chemicals. Various biomass conversion processes are practiced worldwide to produce targeted fuels and chemicals considering various factors such as biomass type, source, quantity, economic, and environmental factors. Among the various biomass conversion processes, the thermochemical conversion route is considered promising because fresh and waste biomass collected from different sources can be processed directly or after pretreatment. Various thermochemical processes used for both fresh and waste biomass are torrefaction, pyrolysis, gasification and hydrothermal liquefaction. These thermochemical processes transform fresh and waste biomass to solid, liquid, and gaseous fuels/feedstock, which are further converted to valued products. The thermophysical properties of biomass and conversion process parameters strongly influence the properties, quality, and yield of the desired products. The effects of these parameters of different thermochemical processes are briefly discussed in this chapter. This chapter also discusses the role of different types of catalysts in altering the properties and yield of thermochemical conversion products based on selected literature.

Hydrothermal liquefaction of algal biomass holds the potential to derive bio-oil and valuable byproducts, meeting the worldwide demand for green energy. The process uses wet algae biomass to convert it to bio-oil without an energy-intensive drying step. This chapter covers the HTL process fundamentals, mechanism, and process conditions which affect bio-oil output and quality. Algae comprised of lipids, proteins, and carbohydrates, is a prospective and future feedstock because of its high productivity and energy density. These complex molecules are thermochemically broken down into reactive components to bio-oil, aqueous phase, gaseous products, and solid residues. The by-product aqueous phase can be employed for recovery of platform chemicals and microalgae cultivation. The gases can be used to generate energy and bio-char can be used for soil amendment and water treatment and as activated carbon precursors for anaerobic digestion. This chapter also addresses the difficulties related to processes such as the complexities in designing the reactors, problems with scalability, and the need to refine oil to reach commercial fuel requirements. Despite greater beginning costs of process than traditional biofuel production technologies, economic studies suggest the potential for sustainability and efficiency. Future research should be conducted on improving the bio-oil quality, producing affordable catalysts, and scaling up process at industrial level. This chapter represents the detailed assessment of bio-oil from algae to an energy future and provides paths to generate value-added by-products in a biorefinery manner.

The utilization of hydrothermal liquefaction (HTL) to convert microalgal biomass into bio-crude oil presents a promising pathway for sustainable biofuel production. HTL operates under moderate temperature and pressure, enabling the conversion of wet biomass into biocrude without the need for energy-intensive drying processes. However, raw HTL biocrude contains high levels of oxygen, nitrogen, and sulfur, necessitating further refining to enhance its stability and fuel properties. Various extraction methods, including organic solvent extraction, water extraction, supercritical fluid extraction, ionic liquid extraction, and membrane separation, are employed to isolate valuable fuel fractions. Additionally, blending HTL biocrude with petroleum-based fuels or biofuels improves its compatibility with existing energy systems. NiMo- and CoMo-catalyzed hydrotreating is a standard upgrading route for removing oxygen and nitrogen to obtain hydrocarbon-rich fuels. Although high processing costs, catalyst deactivation, and bio-oil instability remain significant challenges, continued progress in extraction, refining, and upgrading technologies is steadily improving the feasibility of HTL biocrude as a sustainable fuel.

This chapter explores the potential of algae-derived biocrude as a sustainable alternative to fossil fuels when upgraded, addressing the global need for lower-carbon transportation solutions. Hydrothermal liquefaction (HTL) is highlighted as an emerging and efficient process for converting algae into energy-dense biocrude, utilizing its desirable high lipid content, rapid growth, and carbon sequestration capabilities compared to other HTL feeds. The persisting challenges of algal biocrude, including its high oxygen as well as nitrogen content, are discussed alongside upgrading techniques such as hydrotreatment, catalytic cracking, and esterification, among other experimentally emerging methods. Strategies like blending and co-refining with petroleum refinery feeds during upgradation are shown as cost-effective approaches to enhance biocrude quality, overcome technical/economic challenges, and accelerate integration into existing refinery infrastructure. The chapter also examines the technoeconomic and environmental implications of algae biocrude production and upgrading, emphasizing the need for advancements in catalysts, renewable hydrogen production, and by-product recycling. While economic and technical hurdles remain due to its infancy, algae biocrude-derived fuels offer significant promise in reducing greenhouse gas emissions, enhancing energy security, and contributing to the growing bioeconomy. By combining technological innovations with supportive policies and incentives, algae-derived fuels can play a pivotal role in achieving a sustainable energy future.

A rise in carbon dioxide (CO₂) emissions is driving a transition away from non-renewable fossil fuel toward greener, sustainable biofuel. Algal biomass has emerged as a promising third-generation biofuel feedstock due to its high lipid content, rapid growth, and ability to be cultivated without competing with food crops. Algae can be grown on non-arable land using saline or wastewater, providing co-benefits like waste remediation and CO₂ sequestration. Conversion of algal biomass into biofuels via hydrothermal liquefaction (HTL) in subcritical water is gaining popularity, because unlike pyrolysis and conventional processes, HTL can directly process wet algae without energy-intensive drying. Subcritical water conditions during HTL process accelerate the breakdown of biomass, yielding an energy-dense biocrude that can be upgraded to transportation fuels. HTL is complex thermochemical process, and algal biomass is also an heterogonous feedstock. This chapter compares algal biofuels via HTL with other conversion pathways and examines kinetic modeling to understand complex reaction mechanisms and maximize biofuel yields. Review of evaluating the economics and environmental performance of the process is also key for the commercialization of the HTL process. Thus, this chapter also provides a detailed review of techno-economic analysis and life-cycle assessment studies are also reviewed to evaluate economic feasibility and environmental impacts.