Chapter 1: Revisiting the Field of Biodiesel: An Overview

Biodiesel, a versatile and eco-friendly biofuel, holds a unique position among the alternative energy sources as it addresses critical concerns related to the environment and energy independence.¹  Derived from biological materials such as vegetable oils and animal fats, biodiesel significantly reduces carbon emissions compared with traditional petroleum-based diesel fuels. The production and utilization of biodiesel require a detailed understanding of its physiochemical properties. These properties encompass a wide range, from viscosity and density to flash point and cold flow characteristics. Each of these properties plays crucial roles in determining the performance of biodiesel as a fuel.^2,3  Understanding these properties is not only a key to successful production but also an essential aspect of its effective application in engines and machinery. Biodiesel has several advantages over traditional diesel fuel, including biodegradability and lower toxicity, which make it a safer and more environmentally friendly option. Its excellent lubricity properties can lead to reduced engine wear and maintenance costs, further improving its demand. Additionally, biodiesel’s superior oxygen content results in cleaner combustion and reduced emissions of harmful pollutants, contributing to decreased environmental pollution.⁴ 

Being a potential alternative fuel, biodiesel is comprehensively linked to various factors such as the selection of feedstocks, the production processes, and post-treatment methods to improve its physiochemical and fuel properties, each of which plays important roles in ensuring its quality, sustainability, and efficacy. The first step to consider is the feedstocks, which can range from vegetable oils to animal fats and even waste cooking oil and other industrial wastes.⁵  The choice of feedstock has implications for fuel properties, food–fuel nexus, waste management and carbon emissions. As the demand for biodiesel grows, the quest for novel and sustainable feedstock sources becomes increasingly important. The next crucial stage involves the biodiesel production process, often achieved through transesterification.⁶  This chemical transformation involves reacting the feedstock with an alcohol in the presence of a catalyst. Through this process, the triglycerides in the feedstock are converted into biodiesel and glycerin. Researchers continually seek to refine and innovate these production methods, exploring alternatives such as homogeneous and heterogeneous catalysts, supercritical fluid transesterification, and enzymatic transesterification to make biodiesel production more efficient and environmentally responsible.^7,8  In addition to the choices of feedstocks and production methods, the post-treatment strategies applied to biodiesel offer many ways to improve its quality and fuel standards.^9,10 

The characteristics of biodiesel involving both its physiochemical and fuel properties play a crucial role in its performance, efficiency, and environmental impact. Biodiesel typically exhibits a slightly higher density than conventional diesel fuel, which has attracted attention owing to its impact on fuel distribution and combustion efficiency. Research has been focused on understanding how density affects fuel atomization and combustion within engines. Advances in computational fluid dynamics (CFD) modeling have allowed researchers to simulate and optimize the effects of density variations.^11,12  The viscosity of biodiesel is known to be higher, which can pose challenges in cold weather conditions. Various feedstock blends and additives have been identified as suitable strategies to mitigate the issues due to viscosity.¹³  Biodiesel exhibits a higher flash point, making it a safer choice for handling and storage. Research in this direction has explored the development of safer, high-performance blends and the impact of flash point on fuel stability.¹⁴  Biodiesel also possesses a higher cetane number, indicating superior ignition quality. Current research is focused on optimizing biodiesel properties to enhance further the ignition and combustion efficiency in various engine types.¹⁵  Biodiesel’s heat of combustion is slightly lower than that of petrodiesel. Researchers are examining the energy content of biodiesel and its implications for fuel efficiency. Innovative analytical methods, such as calorimetry, allow precise measurement of biodiesel’s heat of combustion.¹⁶ 

The lubricity properties of biodiesel have attracted considerable attention. Advanced research seeks to understand the mechanisms behind this lubricity and develop improved lubrication additives.¹⁷  Tribology, the study of friction and wear, has played a pivotal role in this research. Compared with petrodiesel, biodiesel exhibits a lower energy content per unit volume, which can affect fuel economy. Researchers are exploring innovative methods to enhance the energy content while maintaining the environmental benefits of biodiesel. Importantly, the higher oxygen content of biodiesel results in cleaner combustion and reduced emissions of certain pollutants. Research activities are centered on optimizing biodiesel blends for reduced emissions while retaining fuel quality and stability. Further, the cold flow properties of biodiesel make it challenging in colder climates. This can be addressed by developing suitable feedstock combinations and additives to improve the properties.¹⁸  Biodiesel’s susceptibility to oxidation necessitates research into stabilizing agents and antioxidant additives. Advanced analytical methods, such as gas chromatography–mass spectrometry, help identify degradation products and assess fuel stability.¹⁹  Another important property involves biodiesel emissions, including the emission of carbon monoxide, particulate matter, and unburned hydrocarbons.²⁰  In this direction, ongoing research is exploring engine design modifications and fuel optimization to enhance emission reductions further. Research and development in the field of biodiesel continue to evolve, with an emphasis on optimizing its physiochemical and fuel properties, which play pivotal roles in determining the performance, efficiency, and environmental impact of biodiesel.

Feedstocks are the building blocks of biodiesel, and their selection has an important impact on the entire production process. These feedstocks encompass a broad range of materials, such as vegetable oils, animal fats, waste materials, non-food oilseed crops, and even algae.²¹  Vegetable oils, such as soybean, canola, and palm oil, are derived from crops and represent a significant share of biodiesel production.²²  Animal fats, such as tallow and lard, serve as a potential feedstock for biodiesel.²³  Waste cooking oils and grease contribute to the sustainability of biodiesel production by recycling materials that might otherwise be discarded. Non-food oilseed crops, such as jatropha and camelina, offer novel approaches to feedstock cultivation. Algae, with their potential for high oil content and rapid growth, have emerged as an exciting frontier in biodiesel research.²⁴  However, the choice of feedstock is far from arbitrary and depends on a complex interplay of factors, including regional availability, climate conditions, environmental sustainability, and economic feasibility. Sustainability considerations are paramount, with the need to minimize competition with food production and reduce environmental impact being of utmost importance. Although biodiesel feedstocks offer many advantages, there are also challenges to be addressed. Potential competition with food production, land use change, and energy-intensive cultivation methods are all topics of concern. However, researchers, policymakers, and the biodiesel industry are actively working to find solutions, exploring innovative feedstock sources, sustainable cultivation methods, and responsible land use practices.²⁵ 

Of the various above-mentioned conventional feedstocks, modified feedstocks have also been gaining industrial importance. These modified feedstocks refer to materials that have undergone various treatments or modifications to improve their suitability for biodiesel production.²⁶  These modifications are typically aimed at enhancing the quality and yield of biodiesel while addressing specific challenges associated with the feedstock. One of the best known feedstock modification methods is transesterification,²⁷  which is a process similar to biodiesel production, to convert it into biodiesel-compatible materials. This pretreatment can improve the quality and reduce the need for additional processing. In certain cases, feedstock blends are created by combining different types of feedstocks to optimize their fatty acid composition and other properties.²⁸  Blending can enhance the yield and quality of biodiesel. Hydrogenation is used to reduce the levels of unsaturated fatty acids in feedstocks. This process can improve the oxidative stability of biodiesel produced from highly unsaturated feedstocks such as soybean oil.²⁹  Some feedstocks, particularly waste oils and animal fats, can be preprocessed with specific catalysts to improve their suitability for biodiesel production. These catalysts help remove impurities and neutralize free fatty acids. Emerging feedstocks such as algae can be genetically modified or subjected to microbial treatments to enhance their lipid content, oil extraction efficiency, and overall suitability for biodiesel production.³⁰  Acid-based catalyzed esterification is used to reduce the acid value of feedstocks containing high levels of free fatty acids, making them more amenable to conventional alkaline transesterification processes.³¹  Blending biodiesel feedstocks with petrodiesel in specific ratios can help improve cold flow properties and other characteristics. This approach is common in regions with cold climates. For feedstocks with a high free fatty acid content, deacidification processes are employed to reduce acid levels and make them compatible with standard transesterification. Furthermore, pretreatment steps such as filtration and purification are used to remove impurities, solids, and contaminants from feedstocks to ensure cleaner biodiesel production.³²  Basically, all of these feedstock modifications are critical for overcoming the challenges associated with feedstock quality, impurities, and undesirable properties. By adapting and improving feedstocks, biodiesel producers can enhance the efficiency, yield, and quality of biodiesel while promoting sustainability and environmental benefits.

Biodiesel production methods can be broadly categorized into catalytic, non-catalytic, and biocatalytic approaches. One of the industrially important catalytic methods includes the transesterification process, which involves the reaction of triglycerides (typically from vegetable oils or animal fats) with an alcohol (commonly methanol or ethanol) in the presence of a catalyst, such as sodium hydroxide or potassium hydroxide. This process is also known as a base-catalyzed reaction, which breaks down the triglycerides into biodiesel (methyl or ethyl esters) and glycerin.^33,34  It is known for its efficiency, high yield, and compatibility with a wide range of feedstocks. While less common than base-catalyzed transesterification, acid-catalyzed methods use acid catalysts (such as sulfuric acid) to convert feedstocks into biodiesel. This approach is particularly useful for feedstocks with a high free fatty acid content.

As one of the emerging techniques, heterogeneous catalysis involves using solid catalysts, such as zeolites or metal oxides, to facilitate the transesterification reaction.³⁵  This method simplifies the separation of biodiesel and glycerin and can be more environmentally friendly. To improve the reaction rate further, supercritical methods have been developed. For instance, the supercritical methanol transesterification reaction utilizes supercritical methanol as both the solvent and reactant.³⁶  The supercritical conditions basically enhance the efficiency of the transesterification reaction. Similarly, microwave energy can also be used to heat the reaction mixture, accelerating the transesterification process and reducing reaction times.³⁷  Likewise, ultrasonic waves are also employed to enhance the mixing and mass transfer during transesterification, making the reaction more efficient.³⁸ 

In the different types of non-catalytic methods, the pyrolysis process involves heating a feedstock, such as triglycerides, at high temperature in the absence of oxygen.³⁹  This process breaks down the feedstock into biodiesel, bio-oil, and other by-products. It is more commonly used for bio-oil production but can also yield biodiesel. An interesting method, known as in situ transesterification, is carried out directly within the source material, such as microalgae or animal fats, without prior extraction of the lipid content.⁴⁰  Other industrially important methods are based on biocatalytic techniques, which include enzymatic transesterification.⁴¹  This process utilizes enzymes, such as lipases, as catalysts for transesterification. Enzymatic biodiesel production is known for its mild reaction conditions, specific substrate selectivity, and reduced energy consumption. For the effective implementation of these methods, especially for large-scale applications, biodiesel production relies on a variety of reactors and processing equipment to facilitate the chemical reactions and separation processes involved in the conversion of feedstocks into biodiesel. The selection of the reactor is crucial for achieving efficient and high-quality biodiesel production.⁴²  The choice of reactor depends on factors such as the scale of production, feedstock type, and process efficiency goals. Industrial biodiesel facilities often utilize a combination of various reactors and equipment to optimize production processes and ensure the reliable production of high-quality biodiesel.

The aim when choosing potential feedstocks and suitable production methods is to produce biodiesel with improved physicochemical properties, combustion efficiency, and overall environmental performance. However, controlling these characteristics is not limited to just those two possibilities, and it can be likely be possible through the application of post-treatment techniques to the produced biodiesel. One of the important strategies involves the addition of antioxidants and stabilizers to biodiesel, which enhances the oxidation resistance and storage stability of the product.⁴³  These additives also help in lowering the cloud point and improving low-temperature operability. Further, blending the biodiesel with petrodiesel can improve cold weather operability and reduce NO_x emissions.⁴⁴  Testing the fuel properties of biodiesel is crucial to ensure its quality, performance, and compatibility with engines and existing infrastructure.^45,46  These properties include density, viscosity, flash point, cetane number, cloud point, pour point, acid value, sulfur content, water and sediment content, glycerin content, oxidative stability, ash content, iodine value, and copper corrosion.

Various blending strategies have been considered as a potential and simple approach to improve the fuel properties of biodiesel. In this process, blending biodiesel with conventional diesel fuel (petrodiesel) is a common practice used to enhance the properties and performance of the fuel.⁴⁷  For instance, a blend containing 5% biodiesel and 95% petrodiesel is called B5, which is a common entry point for biodiesel blending and is often used in many diesel engines without modification. A blend containing 20% biodiesel and 80% petrodiesel (B20) is suitable for many diesel engines and is mandated or recommended by some regulations.^48,49  Pure biodiesel (B100) can be used in certain specially modified diesel engines or blended with petrodiesel to create lower-level blends. Adjusting the biodiesel content in the blend based on the climatic seasons is also one of the flexible strategies. In this process, higher biodiesel percentages (e.g. B20) may be used in the warmer months, whereas lower blends (e.g. B5) are preferred in colder weather to avoid cold-weather issues. This route paves the way to tailor the biodiesel blending level to the specific needs of the application, local climate, and engine requirements. This approach allows flexibility to optimize performance and emissions. Some fuel retailers offer different biodiesel blends at the pump, allowing consumers to choose the blend that suits their needs. Alternatively, biodiesel can be used as an additive to improve the properties of petrodiesel. Even small percentages of biodiesel can enhance lubricity and reduce emissions of the petrodiesel.⁵⁰ 

The use of organic or natural additives in biodiesel production can be a sustainable and environmentally friendly way to enhance a fuel’s properties and performance. These additives include tocopherols, which improve the oxidative stability of the biodiesel, reducing the risk of fuel degradation over time.⁵¹  Castor oil and soybean oil can be added to biodiesel to lower its cloud point and pour point, improving cold-weather performance.⁵²  Small amounts of animal fats, such as tallow, can be used to enhance the lubricity of biodiesel, reducing wear and tear on engine components.⁵³  Coconut oil contains natural lubricating properties and can be added to improve lubricity.⁵⁴  Lecithin is a natural emulsifier that can help improve the stability of water and biodiesel emulsions, reducing the risk of phase separation.⁵⁵  Green-tea extract has natural antimicrobial properties and can be used as a biocide to prevent microbial growth in biodiesel.⁵⁶  Citrus terpenes can act as natural detergents to help keep fuel system components clean and free from deposits.⁵⁷  Aloe vera extract can be used as a natural antifoaming agent in biodiesel to reduce foam formation during handling and storage.⁵⁸  Olive oil has natural anticorrosive properties and can be used as a corrosion inhibitor in biodiesel to protect fuel system components.⁵⁹  Beeswax can act as a stabilizer in biodiesel, helping to maintain the integrity and consistency of the fuel over time.⁵⁹  Natural colorants such as chlorophyll can be used to give biodiesel a green tint, making it easily distinguishable from petrodiesel.⁶⁰ 

Similarly to natural and organic additives, inorganic and nanoadditives have also gained importance in the treatment of biodiesel to improve its properties.⁶¹  These additives include the nanozeolites, which can be used as catalysts in transesterification reactions to improve biodiesel production efficiency.⁶²  Metal oxide nanoparticles can serve as catalysts in esterification reactions to reduce the free fatty acid content of feedstock oils. Silica nanoparticles can be used to stabilize emulsions of biodiesel and water, reducing phase separation and improving combustion efficiency.⁶³  Metal oxide nanoparticles can also act as emulsifiers to improve the stability of water-in-biodiesel emulsions.⁶⁴  Cerium oxide nanoparticles have been applied as a fuel additive and demonstrated to reduce emissions of particulate matter and improve the combustion efficiency of diesel engines running on biodiesel.^65,66  Iron nanoparticles can be used as combustion catalysts to enhance the combustion of biodiesel, reducing emissions.⁶⁷  Nanoscale noble metal catalysts can be used for the hydrogenation of unsaturated fatty acid methyl esters, improving the cold flow properties of biodiesel.⁶⁸  Nanostructured zeolites can be used to remove impurities such as sulfur compounds from biodiesel, enhancing fuel quality and reducing emissions. Nanodiamonds can be added to biodiesel to enhance lubricity, reducing engine wear and improving fuel efficiency.⁶⁹  Tungsten disulfide nanoparticles are another additive with lubricity-improving properties.⁷⁰  Adding aluminum oxide nanoparticles to the coolant can improve the heat transfer in engines running on biodiesel, increasing engine efficiency.⁷¹  Similarly, antioxidant nanoparticles can improve the oxidative stability of biodiesel, preventing fuel degradation over time.⁷²  Overall, additives in biodiesel can provide several advantages by improving various fuel properties. These additives are used to enhance the quality, performance, and compatibility of biodiesel, making it a more viable alternative to traditional diesel fuel.

Biodiesel’s versatility in utilizing various feedstocks, from crops to waste materials, underscores its potential to reduce our reliance on finite fossil fuels. By adopting this renewable and environmentally friendly alternative, we take a step closer to mitigating climate change and diminishing our carbon footprint. The production of biodiesel, marked by its comprehensive chemistry and innovation, demonstrates the intersection of science and sustainability. As we have seen, it offers a wide range of possibilities for customization, enabling biodiesel to meet the unique needs of different industries and applications. With post-treatment and quality control measures, biodiesel can be refined to meet the highest standards, ensuring not just compliance with regulations but also optimal performance and durability in engines and equipment.

As emerging techniques, computation and machine learning play a crucial role in the biodiesel industry, helping researchers, producers, and policymakers make more informed decisions, optimize production processes, and develop sustainable practices.^73,74  Machine learning models can analyze vast datasets to identify the most suitable feedstocks for biodiesel production based on factors such as availability, cost, and environmental impact. Computation can be used to optimize the growth conditions of feedstock crops, such as adjusting irrigation and fertilizer levels, to maximize yield and quality.⁷⁵  CFD simulations can model and optimize the transesterification process, leading to better conversion rates and reduced energy consumption.⁷⁶  Machine learning can be used to monitor and control various parameters during biodiesel production, such as temperature, pressure, and catalyst concentration. Machine learning algorithms can automate the analysis of biodiesel quality, improving the accuracy and speed of quality control processes.⁷⁷  Computation can be used in spectroscopy and chromatography data analysis to determine key properties such as viscosity, density, and acid value.⁷⁸  Life cycle assessment (LCA) models powered by computation can estimate the environmental impact of biodiesel production, considering factors such as greenhouse gas emissions and land use changes. Machine learning can also help identify opportunities for reducing the carbon footprint and other environmental impacts of biodiesel.^79,80 

The future of biodiesel holds significant promise and importance in the broader context of sustainable energy, environmental conservation, and efforts to mitigate climate change. As technology evolves, there will be an increasing emphasis on non-food feedstocks, such as algae, for biodiesel production.⁸¹  Utilization of waste materials, such as used cooking oil, animal fats, and agricultural residues, will continue to gain traction, promoting sustainability and waste reduction. Research into advanced methods such as hydroprocessing and algae cultivation will lead to the development of second- and third-generation biodiesels, which offer better properties and reduced environmental impact.⁸²  Biodiesel production will become more integrated into circular economy models, where waste streams are recycled into biofuels and co-products, reducing environmental impact and improving sustainability.⁸³  The advances in catalysts, enzyme technologies, and reactor designs will continue to improve the efficiency and cost-effectiveness of biodiesel production processes. Biodiesel blending will become more common, and infrastructure development will facilitate its use in various industries and regions. Biodiesel will play a role in reducing emissions in aviation and maritime transport, where electrification is challenging owing to energy density requirements.⁸⁴  Biodiesel may have a role in energy storage and grid support, especially in remote areas or regions with limited grid access. Ongoing research⁸⁵  will uncover new uses and applications for biodiesel, potentially in areas such as space exploration, where traditional fuels are not viable.

The authors acknowledge with thanks the Department of Science and Technology, India, for funding support through the project DST/SEED/SUTRA/2020/71(G) and DST-INSPIRE faculty award DST/INSPIRE/04/2016/002227.