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Economic and sustainability of biodiesel production—a systematic literature review.

1. Introduction

2. major characteristics of the biodiesel sector.

• The TOP5 producer will be the same in 2029;
• The share of the EU, the USA and Indonesia will slightly decrease;
• Meanwhile, the share of Brazil and Argentina is expected to increase.
• Argentina introduced B10; however, the country exports more biodiesel than its domestic consumption;
• Brazil has the B10 mandate and the country is 100% self-sufficient with almost no exports or imports;
• Biodiesel use in China and India is insignificant;
• The EU’s blending objective for 2020 is B7; however, that may differ at the country level;
• Indonesia has recently introduced B20;
• No country-level mandates in the USA; only Oregon introduced B5 at the state level, while the B20 mandate is delayed for Minnesota.

3. Materials and Methods

• Raw material production;
• Technology and production methods;
• Policy issues;
• GHG emissions savings;
• Non-fuel use (e.g., in power generation);
• Renewable energies and other biofuels within, e.g., jet fuel, ethanol, biobutanol, biogas, biomethane or biomass.

4.1. Country-Related Analyses

4.2. global biodiesel issues, 5. summary and conclusions.

• Food versus feed is an important issue of the first-generation production, especially in some African and Asian countries, and edible raw materials cannot provide a long-term solution;
• The majority of the current biodiesel production is not competitive with the low petroleum prices; however, state support can make biodiesel production economically viable, for example, through tax exemptions and subsidies;
• Wastewater-based microalgae production could be economically feasible due to the free raw material, followed by used vegetable oil;
• Co-products are as important as the major product; therefore, only biorefineries can be successful. Due to the size and weight of the currently used raw materials, distances and logistics must be taken into account during their location planning process;
• Besides production cost, other aspects should be included into any analyses, such as GHG emissions, e.g., via negative CO 2 balance of algal production;
• A wider interpretation of sustainability is better and that should go far beyond the simple economic issues. In other words, sustainability should have other pillars than economic.

Author Contributions

Institutional review board statement, informed consent statement, data availability statement, acknowledgments, conflicts of interest.

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Mizik, T.; Gyarmati, G. Economic and Sustainability of Biodiesel Production—A Systematic Literature Review. Clean Technol. 2021 , 3 , 19-36. https://doi.org/10.3390/cleantechnol3010002

Mizik T, Gyarmati G. Economic and Sustainability of Biodiesel Production—A Systematic Literature Review. Clean Technologies . 2021; 3(1):19-36. https://doi.org/10.3390/cleantechnol3010002

Mizik, Tamás, and Gábor Gyarmati. 2021. "Economic and Sustainability of Biodiesel Production—A Systematic Literature Review" Clean Technologies 3, no. 1: 19-36. https://doi.org/10.3390/cleantechnol3010002

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Recent advances in transesterification for sustainable biodiesel production, challenges, and prospects: a comprehensive review

Sabah mohamed farouk.

1 Chemical Engineering Department, Egyptian Academy for Engineering and Advanced Technology (EA&EAT), affiliated to the Ministry of Military Production, Km. 3 Cairo Belbeis Desert Rd., Cairo Governorate, 3066 Egypt

Aghareed M. Tayeb

2 Faculty of Engineering, Minia University, Misr Aswan Agricultural Rd., EL MAHATTA, Menia Governorate, 2431384 Egypt

Shereen M. S. Abdel-Hamid

Randa m. osman.

3 Chemical Engineering and Pilot Plant Department, National Research Centre (NRC), 33 El Bohouth St., Dokki, 12622 Giza Governorate Egypt

Associated Data

Data available on request from the authors.

Biodiesel, a renewable and sustainable alternative to fossil fuels, has garnered significant attention as a potential solution to the growing energy crisis and environmental concerns. The review commences with a thorough examination of feedstock selection and preparation, emphasizing the critical role of feedstock quality in ensuring optimal biodiesel production efficiency and quality. Next, it delves into the advancements in biodiesel applications, highlighting its versatility and potential to reduce greenhouse gas emissions and dependence on fossil fuels. The heart of the review focuses on transesterification, the key process in biodiesel production. It provides an in-depth analysis of various catalysts, including homogeneous, heterogeneous, enzyme-based, and nanomaterial catalysts, exploring their distinct characteristics and behavior during transesterification. The review also sheds light on the transesterification reaction mechanism and kinetics, emphasizing the importance of kinetic modeling in process optimization. Recent developments in biodiesel production, including feedstock selection, process optimization, and sustainability, are discussed, along with the challenges related to engine performance, emissions, and compatibility that hinder wider biodiesel adoption. The review concludes by emphasizing the need for ongoing research, development, and collaboration among academia, industry, and policymakers to address the challenges and pursue further research in biodiesel production. It outlines specific recommendations for future research, paving the way for the widespread adoption of biodiesel as a renewable energy source and fostering a cleaner and more sustainable future.

Introduction

Fossil fuels are responsible for most of the world’s energy needs, endangering the environment. The consumption of fossil fuel products like coal and petroleum has increased because of ongoing globalization and industrialization (Touqeer et al. 2020 ). Around 580 TJ of energy is needed annually for the entire planet, and a staggering 80% of that energy is supplied by burning traditional fossil fuels. Greater energy use is required by expanding the population and improving accessibility, which increases the depletion of fossil fuel reserves (Martchamadol and Kumar 2012 ). Finding an alternate energy source, like biodiesel, is essential for improving energy security for economic development (Oh et al. 2002 ; Um and Kim 2009 ). Biodiesel is a renewable, sustainable, and biodegradable fuel with low greenhouse gas emissions (Sharma and Singh 2009 ; Lee et al. 2011 ). Oils from plants and animals, as well as other lipids such as triacylglycerides (TAGs), can be converted into biodiesel (Hoekman and Robbins 2012 ). The process of making biodiesel, called transesterification or alcoholysis, typically requires the use of catalysts such as acids, bases, and enzymes (Ong et al. 2014 ). Catalysts can exist in either a homogeneous or heterogeneous phase. A homogeneous catalyst undergoes alcoholysis in the same phase, often liquid, as the reactants. In contrast, a heterogeneous catalyst is in a different phase, often one that is not liquid, from the reactants (Ruhul et al. 2015 ). Nanomaterials have become increasingly important in enhancing the production of biodiesel. The weight of the catalyst, the reaction temperature, and the oil-to-alcohol ratio are all reduced when a nanocatalyst is used (Dhawane et al. 2018 ; Ghosh and Halder 2022 ). It has already been proven that nanocatalysts play a key role in increasing response velocity. Additionally, it significantly lowers the reactants’ activation energy. This study addresses the need to keep up with the rapidly changing field of biodiesel production. By conducting thorough surveys and reviews, researchers can stay informed about the latest advancements and challenges. This approach helps focus research efforts by identifying promising methodologies and comparing their performance. The production of biodiesel encounters several challenges, including costly feedstocks, environmental concerns, and the quest for improved efficiency. Through a meticulous review of relevant literature, researchers can identify these challenges and propose potential solutions. This review serves as a valuable resource, synthesizing information from different sources to provide a comprehensive understanding of the current state of biodiesel production technology. Policymakers and industry leaders rely on up-to-date insights to make informed decisions that shape the future of the biodiesel industry. Concise surveys and reviews effectively communicate the latest information, enabling informed decision-making. Additionally, this review promotes collaboration and knowledge sharing within the biodiesel research community, facilitating the development of new and better technologies. It also educates the public about biodiesel production and its potential benefits, potentially increasing support and encouraging the adoption of biodiesel as a sustainable alternative to petroleum-based fuels. Literary, and in detail, this paper offers a comprehensive review of biodiesel production, encompassing key topics such as feedstock selection, the transesterification process, the recovery and reusability of nanoparticles, the benefits and challenges of biodiesel use in engines, and the significance of techno-economic analysis. It begins with various feedstocks, including used oils, animal fats, algae, and edible and inedible vegetable oils, which are chosen for their potential to produce biodiesel. The review then highlights biodiesel as a promising alternative to fossil fuels, exploring its environmental benefits, renewability, and potential to reduce dependence on fossil fuels. The transesterification process, a crucial step in biodiesel production, is discussed in detail, covering reaction mechanisms, catalysts, influencing factors, and challenges associated with optimization. Additionally, various methods and strategies, including solid supports, magnetic separation, centrifugation, and surface modification, have been explored for the recovery and reusability of nanoparticles as catalysts in biodiesel production. These approaches offer promising prospects for enhancing the efficiency and sustainability of biodiesel synthesis while reducing costs and environmental impacts. The benefits of using biodiesel in engines, including improved lubricity, reduced emissions, and potential performance enhancements, are discussed. Lastly, this review emphasizes the importance of techno-economic analysis in assessing the financial viability and sustainability of biodiesel production, considering factors such as capital investment, operating costs, feedstock prices, and governmental policies. Overall, this comprehensive review serves as a valuable resource for researchers, engineers, and policymakers in the biodiesel industry.

Feedstocks for biodiesel production

Biodiesel can be synthesized from various raw materials, including vegetable oils, animal fats, algae, and waste cooking oil. Each feedstock has its own advantages and challenges regarding availability, cost, sustainability, and energy content. The purpose of using different raw materials is to diversify the sources of renewable energy and reduce dependence on fossil fuels. At least 80% of the current costs of producing biodiesel come from the feedstock (Sayed and El-Gharbawy 2016 ). Given the global food crisis, almost 95% of biodiesel produced worldwide is manufactured from edible oils, which are viewed as unnecessary (Azizian and Kramer 2005 ). As a result, making biodiesel from inexpensive, non-edible oil is the current trend (Balat 2011 ). A few feedstocks, including palm, jatropha, microalgae, coconut tallow, and used cooking oil, stand out for their high productivity in the manufacturing of biodiesel (Gui et al. 2008 ). Figure  1 represents some different feedstocks for biodiesel production.

The choice of raw materials has a significant impact on the production of biodiesel in terms of yield, quality, cost, and environmental sustainability. The fatty acid profile of the raw material plays a crucial role in determining the properties and performance of biodiesel. Oils with high levels of saturated fatty acids produce biodiesel with better cold flow properties, while oils with high levels of unsaturated fatty acids produce biodiesel with better oxidative stability (Suzihaque et al. 2022 ). FFA content significantly impacts the reaction rate and biodiesel yield. Higher FFA content requires pre-treatment steps, such as esterification, to reduce acidity and prevent soap formation, which can hinder the reaction and reduce biodiesel purity (Javidialesaadi and Raeissi 2013 ). The amount of water in the raw material can also affect the transesterification reaction. Excess water can hydrolyze the fatty acid methyl esters produced during the reaction, reducing biodiesel yield. Therefore, feedstocks with low water content are preferred (Atadashi et al. 2012 ). The overall quality of the raw material, including its purity and the presence of impurities, can influence biodiesel production. Impurities like phospholipids, gums, and waxes can interfere with the reaction and complicate downstream processing (Manzanera et al. 2008 ). The environmental impact of raw material sourcing should be considered. Using waste cooking oil or non-edible oils reduces the competition for land with food production and minimizes the environmental footprint of biodiesel production. The price of the raw material is a major factor affecting the economics of biodiesel production. Utilizing inexpensive feedstocks, such as waste cooking oil or locally produced oils, can significantly reduce production costs. The choice of raw material may necessitate adjustments in processing conditions, such as reaction temperature, catalyst concentration, and reaction time, to optimize biodiesel yield and quality. Byproducts generated during biodiesel production, such as glycerol, can be utilized for various applications, adding value to the overall process and reducing waste. The sourcing of raw materials can have social and economic impacts on local communities. Promoting the use of locally available or waste-based feedstocks can stimulate agricultural economies and create employment opportunities (Srivastava et al. 2018 ; Suzihaque et al. 2022 ). In conclusion, the selection of raw materials for biodiesel production requires a careful assessment of their fatty acid profile, FFA content, water content, overall quality, environmental sustainability, cost-effectiveness, and potential for byproduct utilization. By considering these factors, biodiesel production can be optimized for yield, quality, cost, and environmental sustainability. Devaraj et al. ( 2020 ) created biodiesel from used frying oil. At 75 °C, 1 wt% catalyst concentration, 1:6 oil-to-methanol molar ratio, 350 rpm, and 90 min, 97% of the possible biodiesel output was produced. In the pilot plant, biodiesel was also produced under these process conditions, with a 97% yield. Hamed et al. ( 2021 ) converted Afia waste cooking oil (AWCO) into biodiesel fuel. At the ideal working conditions of 60 °C reaction temperature, 3 h reaction time, and 0.4 catalyst concentration, the maximum conversion and yield of biodiesel are 97.54 and 94.935%, respectively. Roy et al. ( 2021 ) reported on the conversion of used frying oil into biodiesel to manage liquid waste. The optimum conditions for the transesterification of WFO were 1:16 oil to methanol by weight, 3 wt% catalyst, 65 °C reaction temperature, and 35 min reaction time. FAME conversion (99.5%) and 96% yield are achieved at this optimal reaction setting. Karmakar and Halder ( 2021 ) researched the transesterification reaction used in supercritical settings to produce biodiesel fuel from fish waste oil. According to calculations, the experimental yield of biodiesel generation under ideal conditions was 94.6%. Al Hatrooshi et al. ( 2020 ) created the fatty acid methyl ester from waste shark liver oil (WSLO). At a methanol-to-WSLO ratio of 10.3 M, a reaction duration of 6.5 h, a temperature of 60 °C, and a catalyst concentration of 5.9 wt%, acid-catalyzed WSLO transesterification achieved 99% FAME conversion. A biodiesel yield of 99.73% was achieved at 60 °C for 1.5 h using a 1.2% catalyst and a 6:1 methanol:oil ratio. In a batch-stirred reactor, Nisa et al. ( 2020 ) conducted an experiment to produce biodiesel from the microalgae Spirulina sp. using 1 wt% (w/w) of palm oil as a co-solvent for methanol and potassium hydroxide. At 60 °C, with a methanol-to-palm oil molar ratio of 10:1 and a palm oil-to-microalgae weight ratio of 5:1, the best biodiesel production of 85.28% was generated. Olubunmi et al. ( 2020 ) converted the bio-oil from dairy scum waste into biodiesel. Using a 9:1 methanol-to-oil molar ratio, 40 min for the reaction, 65 °C for the reaction temperature, and 300 rpm for the mixing speed, the maximum biodiesel yield of 94.8% was achieved. In India, Jain and Sharma ( 2010 ) utilized Jatropha oil for the manufacture of biodiesel under the ideal conditions of a 3:7 (v/v) methanol-to-oil ratio and a 1% (w/w) catalyst concentration for mixing at 400 rpm with H 2 SO 4 and NaOH. The transesterification of pretreated JCO yielded the highest yield of 90.1%. Abdulrahman ( 2017 ) made a biodiesel fuel from used cooking oil and chicken fat. The highest conversion to ester was attained at a methanol-to-oil ratio of roughly 7:1 at 60 °C and an 83% yield, also Al-Mawaali et al. ( 2023 ) utilized discarded animal fats and used cooking oil as a cheap source of feedstock to produce biodiesel. With NaOH acting as a catalyst, the discarded frying oil produced the highest yield of synthesized biodiesel (80.6%), followed by a mixture of waste cooking oil and animal fats (79.3%). Finally, using multiple raw materials as feedstock for biodiesel synthesis helps to diversify energy sources, reduce dependency on fossil fuels, and promote sustainability. Every feedstock source has benefits and drawbacks, and the choice of feedstock is influenced by factors like cost, sustainability, availability, and the needs of the biodiesel production process. Continued R&D efforts are aimed at enhancing the efficiency, cost-effectiveness, and sustainability of biodiesel production from diverse feedstocks.

Feedstock preparation

• Cleaning and drying: The feedstock is cleaned to get rid of pollutants, water, and debris. It could entail processes like sedimentation or filtration. The feedstock is dried to eliminate any last traces of moisture after cleaning (Sait et al. 2022 ).
• Pretreatment: Pretreatment of feedstocks may be necessary in some situations to enhance their quality and appropriateness for the manufacture of biodiesel. Degumming, acid esterification, or neutralization are some examples of pretreatment techniques used to get rid of pollutants, gums, and free fatty acids (Singh et al. 2011 ).

The most popular method of treatment for reducing FFAs is acid esterification. One efficient method for lowering FFAs is glycerolysis, also known as glycerol esterification (Elgharbawy et al. 2021 ). Hayyan et al. ( 2011 ) treated the high FFA percentage in palm oil using sulfuric acid. They used H 2 SO 4 by 0.75 wt%, a methanol-to-oil molar ratio of 8:1, 60 min of reaction time, and 60 °C to successfully reduce the FFA content from 23 wt% to less than 2 wt%. Kara et al. ( 2018 ) examined how various methanol-to-oil molar ratios affected the final FFA% throughout the esterification reaction while maintaining constant values for other parameters. Within 160 min of the reaction, the ideal conditions were reached at a molar ratio of 15:1. The highest conversion of 92.6% was achieved, while the FFA content decreased from 21 to 1.5%. Sadaf et al. ( 2018 ) investigated how three acids HCl, H 2 SO 4 , and H 3 PO 4 affected used cooking oil containing 2.75 wt% FFAs. At 60 °C and a 2.5:1 methanol-to-oil molar ratio, the FFA dropped to 0.33 wt%, indicating that H 2 SO 4 was the most effective catalyst. Sousa et al. ( 2010 ) treated castor oil with a non-catalytic glycerolysis procedure with a glycerol-to-oil mass ratio of 1:1 and an FFA of 2.5 wt% for 2 h at 120 °C. They were able to reduce the FFAs from 2.5 to 0.2 wt%.

Triglyceride content in different feedstocks

Triglycerides are the primary component of biodiesel and are present in various amounts in the feedstocks used in the process. Mono- and diglycerides, free fatty acids, phosphatides, sterols, fatty alcohols, fat-soluble vitamins, and other compounds are among the minor constituents. The amount of triglycerides in the feedstock has a significant impact on the output and caliber of biodiesel produced. Most dietary fats and oils are mostly composed of triglycerides.

Vegetable oils

Various vegetable oils, including canola, sunflower, soybean, and palm oils, are frequently utilized as feedstocks in the manufacturing of biodiesel. High concentrations of triglycerides, which are made up of three fatty acid chains joined to a glycerol backbone, are found naturally in these oils. Vegetable oils, therefore, have a high triglyceride concentration, usually more than 90% by weight. Vegetable oils are ideal feedstocks for the synthesis of biodiesel because of their high triglyceride content. Because refined vegetable oils convert pure triglycerides (TG) to FAME at a high rate and quickly, they are the ideal feedstock for producing biodiesel.

Animal fats

Fish oil, tallow, lard, and chicken fat are examples of animal fats that can be utilized as feedstocks to produce biodiesel. Comparing animal fats to vegetable oils, the former usually have a lower triglyceride concentration, usually between 70 and 85% by weight. The species of the animal, its nutrition, and the rendering techniques all affect the variation in triglyceride content. Animal fats can still be effectively turned into biodiesel even when their triglyceride level is slightly lower.

Waste oils and greases

The manufacturing of biodiesel can use recycled waste cooking oils and greases from food processing industries, restaurants, and other sources as feedstocks. Depending on their quality and place of origin, waste oils and greases might include varying amounts of triglyceride. The quality and conversion efficiency of biodiesel can be impacted by the presence of contaminants or elevated quantities of free fatty acids in these feedstocks.

Algae and microorganisms

The capacity of algae and specific microorganisms to accumulate large concentrations of lipids (fatty acids and triglycerides) makes them viable feedstocks for the generation of biodiesel. Depending on the species and growing environment, triglycerides can make up a large portion of algal oils, anywhere from 20 to 60% by weight. Table ​ Table1 1 represents the triglyceride content of different feedstocks.

Table 1

Biodiesel as a promising alternative source of biofuel

Among the emerging alternatives, biodiesel has garnered significant attention as a promising biofuel with the potential to reduce reliance on petroleum-based fuels and mitigate environmental impacts. The non-toxic, biodegradable fuel known as biodiesel is made from leftover cooking oil, animal fats, or vegetable oil, and presents several advantages over conventional diesel. Firstly, its renewable nature alleviates concerns over depleting fossil fuel resources. Unlike petroleum diesel, which is extracted from finite underground reservoirs, biodiesel can be produced from continuously replenished sources, ensuring long-term sustainability. Secondly, biodiesel boasts superior environmental credentials compared to diesel. Its burning emits significantly lower degrees of particulate matter, nitrogen oxides, sulfur oxides, and carbon monoxide, all of which contribute to air pollution and its detrimental effects on human health and the environment. Additionally, biodiesel’s lower greenhouse gas emissions make it a promising alternative for reducing the transportation sector’s contribution to climate change (Demirbas 2009 ). Moreover, biodiesel offers economic benefits, particularly for regions with abundant feedstock sources. Local production of biodiesel can stimulate agricultural economies, reduce reliance on imported petroleum, and create employment opportunities. Additionally, biodiesel’s compatibility with existing diesel engines eliminates the need for expensive infrastructure overhauls, facilitating a smoother transition toward a more sustainable fuel source (Fukuda et al. 2001 ). Despite its promise, biodiesel faces certain challenges that need to be addressed for its widespread adoption. One challenge lies in its economic viability. The production cost of biodiesel is currently higher than that of petroleum diesel, primarily due to feedstock costs and processing expenses. However, advancements in production technologies and economies of scale are expected to reduce biodiesel costs over time.

Another challenge involves the availability of suitable feedstocks. While various plant oils, such as soybean, palm, and jatropha, can be used for biodiesel production, concerns have arisen regarding land-use competition for the food industry and the potential environmental effects of broad-scope palm oil plantations. Sustainable feedstock sourcing strategies, such as utilizing waste cooking oil and cultivating oil crops on marginal lands, are crucial to addressing these concerns (Knothe 2010 ; Singh and Singh 2010 ). In conclusion, biodiesel presents a compelling alternative to conventional diesel, offering a combination of sustainability, environmental benefits, and economic potential. Addressing the current challenges related to feedstock availability and production costs will pave the way for biodiesel’s wider adoption and its significant contribution to a more sustainable energy future.

Innovations in biodiesel applications

Biodiesel, a renewable and cleaner-burning alternative to conventional diesel fuel, has gained significant attention and widespread applications in recent years. Its versatile nature allows for various uses across different sectors. In transportation, biodiesel can be blended with petroleum diesel to power vehicles, reducing emissions of greenhouse gases and air pollutants. Moreover, biodiesel finds applications in industrial settings, where it serves as a substitute for petroleum-based fuels in machinery and equipment. The growing interest in sustainable energy solutions has spurred research and development efforts, leading to advancements in biodiesel production techniques, feedstock selection, and engine compatibility. Recent studies (Mobin et al. 2022 ; Tripathi et al. 2023 ) have explored the potential of advanced feedstocks, such as algae and waste oils, to enhance biodiesel production efficiency and reduce environmental impacts. These advancements in biodiesel applications and technology are crucial steps in achieving a more sustainable and greener energy future. Figure  2 shows some of the applications of biodiesel.

Biodiesel applications

Transesterification in biodiesel production

Alcohol and lipids react chemically to produce fatty acid alkyl esters, a process known as transesterification. Triglycerides and alcohol are transesterified to produce FAAE and glycerol. Triglycerides and alcohols combine to form diglycerides in the first stage, which are then converted to monoglycerides and glycerol, each of which yields an alkyl ester (Thangaraj et al. 2019 ). Among the factors influencing biodiesel yield in transesterification are time, temperature, type and concentration of the catalyst, kind of feedstock oil, and alcohol-to-oil ratio. It is possible to reverse the transesterification process. To shift the reaction’s equilibrium in favor of the product’s production, an excess of alcohol is therefore necessary. Alcohols with short chains, long chains, and cyclic chains are all used in this process. The availability, polarity, better reactivity, and inexpensive price of methanol and ethanol, however, make them popular choices (Avhad and Marchetti 2015 ), Fig.  3 illustrates the characteristics and features of the transesterification process used to make biodiesel, and Fig.  4 shows a schematic representation of the biodiesel synthesis path through the transesterification process.

The characteristics and features of the transesterification process used to make biodiesel

Schematic representation of the biodiesel synthesis path through the transesterification process

Triglycerides are changed into diglycerides, monoglycerides, and finally glycerol by a sequence of chemical events called transesterification, which is the process used to produce biodiesel. Alkali catalysts are usually used in a single phase of this process. However, a two-step procedure can be required if the feedstock has large concentrations of water or free fatty acids (FFAs). Fatty acid esters, or acid-catalyzed alcoholysis, are the initial step that turns FFAs into fatty acid esters. The process of transesterification, which turns the fatty acid esters into biodiesel, comes next. Figure  5 displays the schematic diagram for the manufacture of biodiesel in both one and two steps.

Biodiesel manufacturing can be done in two steps ( A ) or one step ( B ) (Fattah et al. 2014 ; Mofijur et al. 2014 )

Chemical or biological catalysts can be used in transesterification reactions that are catalyzed. The classification is shown in Fig.  6 . There are both homogeneous and heterogeneous chemical catalysts. Base or acid catalysts are included in the homogeneous catalyst. The heterogeneous catalyst is made up of nano-, biomass waste-based, base, and acid–base functionalities (Thangaraj et al. 2019 ). The choice of any catalyst is influenced by the following factors: oil quality, FFA content, operating conditions, necessary catalyst activity, cost, and availability (Tacias-Pascacio et al. 2019 ). The advantages and disadvantages of different catalysts are presented in Table  2 .

Different catalysts used in biodiesel production (Rizwanul Fattah et al. 2020 )

Table 2

Pros and cons of utilizing various catalysts in the process of transesterification

Homogeneous catalysis

Homogeneous catalysts are the ones that are most frequently used in the production of biodiesel because they are simple to use and require less time to complete a reaction. When dissolving a homogeneous catalyst, a solvent that is in the same phase as all the reactants is usually utilized (Rizwanul Fattah et al. 2020 ). Bhuana et al. ( 2020 ) used leftover beef tallow and methanol as solvents, in a KOH catalyst to create biodiesel. Ethanol effectively functioned as a co-solvent, lowering reaction time by 61.11% and functioning as a low-polarity active ester exchange agent, which prevented soap formation and increased yield by 3.08%. Karmee et al. ( 2015 ) looked at the transesterification of algal oil using methanol as the solvent. Under identical reactional conditions, HCl outperformed the homogeneous H 2 SO 4 catalyst. Rice bran oil was esterified by Arora et al. ( 2015 ) utilizing sulfuric acid as a uniform catalyst. Studies have been done on how the oil-to-methanol molar ratio (1:5 to 1:30), catalyst concentration (0.15 to 1.0 wt%), and reaction temperature (45 to 60 °C) affect the conversion of FFA. Siddiqua ( 2015 ) transesterified palm oil to create biodiesel. Further observation shows that a mixture of methanol and NaOH at a reaction temperature of 55 °C produced the highest production. Abdulsalam ( 2023 ) assessed the process of turning thevetia peruviana seed oil into biodiesel using two different catalysts (NaOH and KOH). Separate NaOH and KOH pellets were dissolved in methanol to create different catalysts with different amounts of sodium and potassium methoxide, respectively. The NaOH catalyst’s conversion yield was 81.2%, 80.4%, and 89%, whereas the KOH catalyst’s conversion yield was 96.8%, 88.4%, and 84.0. Through the calcination of leftover pineapple leaves, de Barros et al. ( 2020 ) created a unique catalyst. After a 30-min reaction period at 60 °C, 4 wt% of catalyst, a molar ratio of 1:40 for oil to methanol, and an oil-to-biodiesel conversion of above 98%, a high catalytic activity was detected. This activity is likely connected to the 85-wt% presence of alkali/alkali metals (K, Ca, and Mg). Kasirajan ( 2021 ) used two-step procedures to produce biodiesel from Chrysophyllum albidum seed, a non-edible source. Transesterification is performed on oil from Chrysophyllum albidum seeds after esterification employing a homogeneous catalyst of H 2 SO 4 . The maximum oil-to-biodiesel conversion was 99.2 wt% when the ideal situations were achieved, which included a 1:9 oil-to-methanol molar ratio, 1 wt% KOH, 500 rpm, and 40 min at 65 °C. Belkhanchi et al. ( 2021 ) showed that transesterifying used frying oils (UFO) at 18 °C for 60 min of reaction in the presence of methanol using MeOH/UFO 6:1 mol proportion and 1 wt% of KOH yields the best conversion of UFO. Jain et al. ( 2023 ) gave a description of the homogeneous base catalyst-based single-step transesterification process for producing biodiesel from waste cooking oil that contains high levels of free fatty acids and algal oil. According to the findings, a biodiesel yield of 92% may be achieved under optimal conditions, which include a 1.5% catalyst (w/w), a methanol to oil ratio of 21:1, a time of 110 min, and a temperature of 50 °C. Saeed et al. ( 2021 ) investigated S. elongata algal for biodiesel creation. To evaluate transesterification to FAME, zeolitic catalysts, KOH, and HCl were used. KOH produced the maximum biodiesel yield (99.9%), which was obtained under the ideal reaction conditions of a 1.0% catalyst, 60 °C, 4 h, and a 1:4 volume ratio between oil and methanol. Table ​ Table3 3 highlights some of the recently published research on the use of several homogeneous catalyst types for biodiesel synthesis, various feedstock sources, experimental setups, and biodiesel yields.

Table 3

Different types of homogeneous catalysts used for biodiesel synthesis

Heterogeneous catalysis

Heterogeneous catalysts go through different phases or states than reactants. According to Melero et al. ( 2009 ), these are the catalysts that often generate active sites when reacting with their reactants. Greater oil/alcohol ratios and greater temperatures than in homogeneous catalysis are the primary disadvantages of this catalysis. The catalyst’s improved reusability and ease of separation and purification are other advantages. Mohamed et al. ( 2020 ) prepared by quickly pyrolyzing rice straw, a heterogeneous catalyst (RS-SO 3 H) was created. The yield of biodiesel was 90.37%. in ideal conditions: 20:1 methanol: oil molar ratio with a 10% catalyst at 70 °C for 6 h. Choksi et al. ( 2021 ) created a solid acid catalyst using the sulfonation carbonization process from a palm fruit bunch. After that, the catalyst was put through esterification and transesterification processes to produce biodiesel. Utilizing a 4% catalyst, a 21:1 methanol-to-oil molar ratio, and a 60 °C temperature, an optimal yield of 88.5 wt% methyl ester was obtained in 180 min. Aghel et al. ( 2019 ) wanted to improve a pilot-scale microreactor that used kettle limescale to turn used cooking oil (WCO) into biodiesel. The produced biodiesel had a maximum conversion of 93.41% at 61.7 °C, a catalyst concentration of 8.87 wt %, a methanol-to-oil 1.7:3 volumetric ratio, and 15 min. Bhatia et al. ( 2020 ) developed a heterogeneous catalyst to initiate the transesterification of used cooking oil by pyrolyzing waste cork. The greatest conversion (98%) for the heterogeneous catalyst produced at 600 °C occurred at alcohol:oil ratios of 25:1, catalyst loadings of 1.5% w/v, and temperatures of 65 °C. Sahani et al. ( 2019 ) used a solid-base catalyst called barium cerate in the transesterification procedure to produce biodiesel from Karanja oil. To synthesize perovskite barium cerate with maximum phase purity, the calcination temperature was optimized. At 1.2 wt% catalyst, 1:19 oil-to-methanol molar ratio, 65 °C, 100 min, and 600 rpm, karanja oil methyl ester with 98.3% conversion was obtained. Kamel et al. ( 2019 ) utilized the fig leaves that had undergone calcination, KOH activation, and activation. The highest conversion to biodiesel (92.73%) was obtained from fig leaves treated with KOH under ideal conditions (2 h of heating, a 6:1 alcohol/oil molar ratio, 1% catalyst, and 400 rpm). Singh et al. ( 2023 ) produced biodiesel from Jatropha curcas oil using the transesterification technique and calcium oxide. The results of the experiment demonstrate that at a methanol/oil ratio of 12:1, 65 °C, 3 h, and a catalyst concentration of 5 wt%, a biodiesel yield of 81.6% was produced. Carbon spheres were the heterogeneous acid catalyst that Nata et al. ( 2017 ) utilized. A maximum yield of 87% was achieved at 60 °C and 1 h when WCO was used as the feedstock to make biodiesel utilizing a C–SO 3 H acid catalyst. Du et al. ( 2019 ) converted Scenedesmus quadricauda algal oil into biodiesel using a cobalt-doped CaO catalyst. Cao was obtained from eggshells and calcined at 400, 700, and 900 °C. Todorović et al. ( 2019 ) conducted research on canola oil-based potassium-supported TiO 2 for biodiesel generation. At 55 °C for 5 h, with a 6 wt% catalyst and a 54/1 methanol/oil, the highest biodiesel output of > 90% was discovered. Salinas et al. ( 2012 ) created a carbon-based MgO catalyst for castor oil transesterification utilizing the sol–gel method. With a 96.5% biodiesel output at 6 wt% catalyst loading and a 12:1 ethanol/oil ratio at 75 °C for 1 h, the MgO/UREA-800 demonstrated remarkable catalytic activity. Gardy et al. ( 2019 ) made a strong, magnetic core–shell SO 4 /Mg–Al–FeO 3 heterogeneous catalyst with the use of surface functionalization, encapsulation, and stepwise coprecipitation. Utilizing the synthesized catalyst, the transesterification reaction was carried out with the highest possible yield of 98.5% at 9:1 methanol/WCO, 95 °C, and 5 h. Table ​ Table4 4 highlights some of the recently published research on the use of several heterogeneous catalyst types for biodiesel synthesis, various feedstock sources, experimental setups, and biodiesel yields.

Table 4

Different types of heterogeneous catalysts used for biodiesel synthesis

Enzyme-based catalyst

Enzyme-based catalysts are produced from living things that speed up reactions while maintaining the stability of their composition (Amini et al. 2017 ). Extracellular lipases are the enzymes that have been isolated and processed from the microbial broth. In contrast, intracellular lipase remains inside the cell or in its walls of production (Gog et al. 2012 ). One drawback of employing extracellular enzymes as catalysts is the expense and difficulty of the separation and purification procedures (Rizwanul Fattah et al. 2020 ). The efficiency of the bio-catalyzed transesterification process is influenced by the enzyme’s source and the process variables (Aransiola et al. 2014 ). Enzymatic biodiesel production also has the advantages of being simple to remove, operating at a temperature between 35 and 45 °C, producing no byproducts, and allowing catalysts to be reused (Christopher et al. 2014 ). For the transesterification of low-grade fish oil, Marín-Suárez et al. ( 2019 ) used Novozym 435 lipase; the greatest FAEE yield was 82.91 wt% after 8 h, 35 °C, an excess of ethanol, and 1% catalyst. Novozym 435 can be used for 10 continuous cycles with a maximum activity decrease of 16%. Jayaraman et al. ( 2020 ) studied used cooking oil enzymatic transesterification with the use of pancreatic lipase to make methyl ester. The best reaction conditions were discovered to be methanol as the alcohol 3:1 M ratio, 1.5% enzyme concentration (by weight of WCO), 4 h reaction duration, 60 °C, and 88% yield after numerous attempts. Fatty acid methyl ester (FAME) was produced by Choi et al. ( 2018 ) produced FAME from the oil in rice bran by just adding methanol. The 83.4% yield was reached after 12 days under ideal conditions.

Nanocatalysts in transesterification

Nanocatalysts have garnered significant interest in the production of biodiesel. Because of their special qualities, which include a large active surface area, high reusability, better catalytic efficiency, high biodiesel conversion, and sustainability, nanocatalysts can be superior to conventional catalysts (Qiu et al. 2011 ). Since they are easily removed from the final products and retain their catalytic activity even after being reused several times, nanocatalysts are widely sought (Ahmed et al. 2023 ). There are numerous ways to create nanocatalysts. Among the techniques are microwave combustion, chemical vapor deposition, impregnation, and gas condensation (Quirino et al. 2016 ; Ambat et al. 2018 ). Some of the latest works on nanocatalysts for the transesterification reaction are listed in Table  5 .

Table 5

Various nanocatalysts in biodiesel production

Metal-oxide nanocatalysts

The most popular nanocatalysts are those based on metal oxide, and they play a crucial role in maximizing the synthesis of biodiesel. Nanoparticles that will be employed for transesterification catalysis have been created using the oxidized forms of numerous different metals, including Mg, Zn, and Ca (Pandit et al. 2023 ). Jamil et al. ( 2021 ) created highly efficient barium oxide using catalysts made of molybdenum oxide. Optimal conditions include 12 methanol/oil, 120 min, 65 °C, and a 4.5wt% catalyst. The best yield was achieved under these conditions, which resulted in a 97.8% yield. Sahani et al. ( 2020 ) produced biodiesel with a transesterification reaction involving used cooking oil and a mixed metal oxide catalyst made of Sr–Ti. Methanol as the alcohol in an 11:1 M ratio, 1% catalyst, an 80-min reaction period, and a temperature of 65 °C with 98% FAME conversion were found to be the best reaction conditions. In a study conducted by Tayeb et al. ( 2023 ), the production of biodiesel using a CaO catalyst through the transesterification of WCO was investigated. The study determined the optimal reaction parameters to be a WCO/methanol molar ratio of 1:6, a 1% CaO nanocatalyst, a reaction temperature of 70 °C, and a reaction duration of 85 min, which resulted in a 97% biodiesel yield.

Carbon nanocatalysts

Nanocatalysts are created from carbon materials, including graphene and reduced graphene oxides (Nizami and Rehan 2018 ). Due to their diverse structural, mechanical, thermal, and biocompatibility qualities, carbon nanocatalysts are good catalysts and have advantageous applications in electrocatalytic devices such as fuel cells and other electro-processing systems. CNTs are often manufactured from graphite sheets that have been wound into cylinder forms. They have a large surface area, measure in nanometers, and are incredibly biocompatible (Rai et al. 2016 ).

Zeolite nanocatalysts

Large exterior surface areas and the hydrophobic nature of nanozeolites increase enzyme access to the substrate. Natural zeolite materials are far less frequently used in commercial industries than synthetic-based products. Commercially available synthetic zeolites such as ZSM-5, X, Y, and beta are used primarily in the production of biodiesel (Abukhadra et al. 2019 ). Using zeolites from NaY, KL, and NaZSM-5, Wu et al. ( 2013 ) produced CaO catalysts that were utilized to catalyze the transformation of methanol with soybean oil. In comparison to pure CaO, the activities of synthesized catalysts were studied. It was discovered that after being supported by zeolites, the CaO catalyst’s activity improved, with the CaO/NaY catalyst showing the greatest performance. Using the CaO/NaY catalyst, methanol-to-soybean oil 9:1 molar ratio at 65 °C with a reaction period of 3 h, and a 3% catalyst were used to produce a 95% biodiesel yield. Firouzjaee and Taghizadeh ( 2017 ) synthesized a CaO/NaY-Fe 3 O 4 nano-magnetic catalyst that was employed for the generation of biodiesel. The ideal methanol-to-oil molar ratio is 8.78, the catalyst loading is 5.19% (30% CaO loaded on the surface nanomagnetic zeolite), and the reaction period is 4 h. The maximum methyl esters obtained are 95.37%.

Distinct behavior of nanocatalysts during biodiesel production

Nanocatalysts have been widely used in biodiesel production due to their high catalytic activity, low cost, and environmental friendliness. The properties of nanocatalysts can vary depending on the preparation method, which can affect their catalytic performance. For example, the size, shape, and surface area of the catalyst particles can influence the reaction kinetics and yield of biodiesel. Recent studies have investigated the effects of different preparation methods on the properties of nanocatalysts for biodiesel production. The preparation method and calcination temperature are important factors that can affect the properties and catalytic performance of nanocatalysts for biodiesel production. Further research is needed to optimize the preparation methods and properties of nanocatalysts to improve the efficiency and sustainability of biodiesel production. We can offer general insights into the variations of nanocatalysts throughout the biodiesel production process, focusing on the following aspects.

Catalyst types: Different generations of nanocatalysts may involve distinct types of materials. For instance, first-generation nanocatalysts might include basic materials, while second- or third-generation may involve more advanced materials like metal oxides, zeolites, or other nanostructured materials.

Particle size: Advances in nanotechnology enable the control of particle size in nanocatalysts. The particle size can significantly impact catalytic activity. Smaller particle sizes may provide larger surface areas and enhanced catalytic efficiency.

Functionalization: The functionalization of nanocatalysts with specific groups or ligands can vary across generations. Functionalization can influence the catalyst’s selectivity and stability during biodiesel production.

Reusability and stability: Reusability and recovery are the two main advantages of using heterogeneous nanocatalysts in the production of biodiesel. The nanocatalyst is recovered and utilized again at each stage of these processes, which include many cycles of producing biodiesel. Nanocatalysts are often recovered via chemical means. The intended product and any byproduct may be easily and quickly recovered from the reaction mixture thanks to heterogeneous catalysts. This type of catalyst eliminates the need for a washing step. The esterification method using nanocatalysts was proposed to have several benefits, including speedier mixing of the reactants and catalyst and easy and rapid separation from the reaction mixture (Pandit et al. 2023 ).

Synthesis methods: The methods used to synthesize nanocatalysts may evolve, affecting their structure and properties. Recent advancements might include greener synthesis approaches or techniques that enhance the reproducibility of catalysts.

In addition to the aspects mentioned, the surface chemistry of nanocatalysts can also vary across generations, affecting their catalytic behavior during biodiesel production. The surface chemistry of nanocatalysts can be modified through various methods, such as surface functionalization, doping, or coating, to tune their catalytic activity, selectivity, and stability. For instance, surface functionalization with organic molecules or inorganic ions can enhance the catalyst’s selectivity for specific reactions or improve its compatibility with the reaction medium. The use of nanocatalysts in biodiesel production also presents some challenges, such as the aggregation, fouling, and leaching of active species. These issues can lead to a decrease in catalytic activity and selectivity, as well as an increase in production costs. To address these challenges, researchers are exploring various strategies, such as surface modification, stabilization techniques, and immobilization methods, to improve the stability and reusability of nanocatalysts. In summary, the distinct behavior of nanocatalysts during biodiesel production is influenced by various factors, including catalyst type, particle size, functionalization, surface chemistry, synthesis methods, and stability. The optimization of these factors can lead to more efficient, selective, and sustainable biodiesel production processes. However, further research is needed to fully understand the underlying mechanisms and to develop new generations of nanocatalysts with enhanced performance and stability.

Transesterification reaction mechanism (alcoholysis)

A large variety of exchange reactions involving oils, fats, and other reactants may be explained by the reaction mechanism. This comprises three processes: (1) transesterification, a rearrangement that yields monoglyceride, diglyceride, or other esters; (2) acidolysis, which involves exchanging fatty acids to produce specific fatty acid products; and (3) alcoholysis, which produces methyl esters in reactions with monohydric alcohols and monyl glycerol in reactions with polyhydric alcohols. Natural vegetable oils, animal fats, and food industry waste oil may all be utilized as source materials for transesterification, a process that produces biodiesel. Methanol, ethanol, propanol, butanol, and pentanol are among the alcohols that can be utilized for transesterification. Because it is a cheap, short-chain, strong polar raw material that reacts rapidly with fatty acid glycerides, methanol is the most widely used of them. Also freely soluble in methanol are base catalysts. A catalytic agent in this reaction might be an acid, base, or enzyme. Base catalysts consist of carbonate, NaOH, KOH, and potassium and sodium alkaloids. Acid catalysts might be hydrochloric, phosphoric, or sulfuric acids. The enzyme lipase is a good catalyst for the esterification of alcohols to fatty acid glycerides. Figures  7 , ​ ,8, 8 , ​ ,9, 9 , and ​ and10 10 represent continuous reversible processes for transesterification reactions; every reaction yields a distinct type of alcohol (Kang et al. 2015 ; Sait et al.  2022 ; Li et al. 2020 ; Oyekunle et al. 2023 ).

Continuous reversible processes of transesterification reactions

Acid-catalyzed alcoholysis reaction mechanism

Base-catalyzed alcoholysis reaction mechanism

Enzyme-catalyzed alcoholysis reaction mechanism

Kinetic modeling’s significance for process optimization

Kinetic models of chemical processes are powerful tools for reactor design. The kinetic models are very helpful in choosing the best reaction conditions (temperature, pressure, mixing rate, etc.) for chemical or biochemical transformations in reactors or bioreactors. This maximizes the formation of desired products with the least material investment and financial resources. This also holds true for the many techniques used to produce biodiesel, such as homogeneous, heterogeneous, enzyme catalysis, and others. One of the most important stages in the development of chemical processes for industrial applications is thought to be carefully thought-out experimental research and the subsequent creation of a kinetic model (Trejo-Zárraga et al. 2018 ). Portha et al. ( 2012 ) were able to decrease the extra ethanol used in the transesterification reaction in a continuous mode. By adjusting the temperature of the second reactor and adding methanol in stages, they were able to enhance the system’s overall performance, as demonstrated by the results of their simulation. Using triolein as a model chemical, the authors conducted experiments and discovered that it was beneficial to convert diglyceride and monoglyceride in the second reactor and the majority of triolein in the first. Additionally, their calculations suggested that to improve reaction rates at this point, it would be prudent to raise the temperature in the second reactor. Additionally, the authors computed internal concentration profiles using a reactor model that included the kinetic model. They discovered the limiting phenomenon in the overall transformation. To get a deeper comprehension of the rates of output and the inhibitory patterns seen in the transformation scheme, a kinetic model may also be strategically employed (Firdaus et al. 2016 ). For instance, a reaction scheme for the enzymatic creation of biodiesel might consider many more reaction stages and, consequently, a greater number of parameters. This adds difficulty to the kinetic model creation process, but once this model is solved, it may be utilized to construct an enzyme-catalyzed reactor and eventually optimize the process. The use of kinetic models, which can faithfully replicate the process at various reaction conditions, is helpful in the field of research and process improvement as it offers guidelines for additional experimental work and helps eliminate potentially fruitless experimental trials. Additionally, models may be utilized to foresee how composition will affect the final product’s quality. A model might forecast, for instance, how the feedstock’s water content or FFA may impact the reaction conversion and, in turn, the biodiesel’s production and quality.

Kinetics of transesterification

Few studies have dealt with kinetic modeling; most of the heterogeneous catalysis research has been on the manufacture and utilization of catalysts. To achieve reaction conditions with inherent kinetics and minimal effects, efforts have been focused on using tiny solid particles. It has been discovered that most heterogeneous transesterifications adhere to a pseudo-first-order model. For instance, Kaur and Ali ( 2014 ) discovered that the ethanolysis of Jatropha curcas L. oil, which were catalyzed by 15-Zr/CaO-700, adhered to a pseudo-first-order rate law. The Koros-Nowak test proved that the transit impacts were insignificant. Lukić et al. ( 2014 ) also discovered a first-order reversible rate law under ideal circumstances for the transesterification of sunflower oil. Table ​ Table6 6 lists some kinetic modeling studies of heterogeneous transesterification.

Table 6

List of some kinetic modeling studies of heterogeneous transesterification

Characterization methods for the assessment of produced biodiesels

Characterization methods for the assessment of produced biodiesel include various analytical techniques to evaluate the quality and properties of biodiesel. These methods are essential for ensuring that biodiesel meets the required standards and specifications for use as a sustainable and efficient alternative fuel source. The American Society for Testing and Materials (ASTM) is a prominent organization that provides authoritative guidelines for biodiesel testing and characterization methods.

The most common characterization methods for assessing produced biodiesel include the following.

Fatty acid methyl ester (FAME) analysis: FAME analysis is a fundamental method for biodiesel characterization, involving the determination of the fatty acid methyl ester content in biodiesel. This analysis is typically performed using gas chromatography (GC) or high-performance liquid chromatography (HPLC) to quantify individual FAME components, which provides valuable information about the biodiesel’s composition and purity.

Viscosity measurement: Viscosity is a crucial parameter for biodiesel quality assessment, as it affects the flow behavior and performance of the fuel. Dynamic viscosity measurements are commonly conducted to determine the resistance of biodiesel to flow under specific conditions, offering insights into its suitability for use in engines and transportation applications.

Oxidation stability testing: Biodiesel’s resistance to oxidation is an important characteristic that influences its shelf life and storage stability. Various methods, such as the Rancimat test and the PetroOXY test, are employed to assess the oxidation stability of biodiesel by measuring its susceptibility to oxidative degradation over time.

Cold flow properties analysis: The cold flow properties of biodiesel, including cloud point and pour point, are critical factors affecting its performance in cold weather conditions. Characterization methods such as differential scanning calorimetry (DSC) and automated cloud and pour point analyzers are utilized to determine these properties, ensuring that biodiesel remains operational at low temperatures.

Acid value determination: The acid value of biodiesel indicates its acidity level, which can impact engine components and fuel system integrity. Acid value determination involves titration methods to quantify the amount of free fatty acids present in biodiesel, enabling the assessment of its corrosiveness and potential impact on engine performance.

Calorific value measurement: Calorific value, also known as heating value, represents the energy content of biodiesel and is crucial for evaluating its combustion efficiency and heat output. Bomb calorimetry is commonly used to measure the calorific value of biodiesel, providing essential data for assessing its energy potential as a fuel source.

Sulfur content analysis: Sulfur content determination is essential for ensuring compliance with environmental regulations and assessing the environmental impact of biodiesel combustion. Techniques such as X-ray fluorescence (XRF) spectroscopy or ultraviolet fluorescence analysis are employed to measure sulfur levels in biodiesel samples.

Glycerol content quantification: Glycerol content in biodiesel must be monitored to ensure compliance with quality standards and prevent potential issues related to fuel stability and engine performance. Analytical methods like gas chromatography coupled with flame ionization detection (GC-FID) are utilized for the accurate quantification of glycerol in biodiesel products.

These characterization methods collectively provide comprehensive insights into the chemical composition, physical properties, stability, and environmental impact of produced biodiesel, supporting quality control measures and regulatory compliance within the biofuel industry.

The recent development in biodiesel production

Biodiesel, a renewable and sustainable alternative to conventional diesel fuel, has seen significant developments in recent years. These advancements have focused on improving the efficiency of biodiesel production processes, expanding feedstock options, and enhancing the overall sustainability of biodiesel as a viable energy source. One notable recent development is the use of advanced catalysts in biodiesel production. Catalysts play a crucial role in the conversion of vegetable oils or animal fats into biodiesel through a process called transesterification. Researchers have been exploring various catalysts, such as solid acid catalysts, enzyme catalysts, and heterogeneous catalysts, to improve reaction rates, reduce energy consumption, and enhance biodiesel quality. These catalysts offer advantages like higher conversion rates, milder reaction conditions, and easier separation of the catalyst from the product (Garcia-Silvera et al. 2023 ). Another significant development is the utilization of non-traditional feedstocks for biodiesel production. While conventional biodiesel feedstocks include soybean oil and rapeseed oil, researchers have been investigating alternative sources such as algae, waste cooking oil, and non-food crops like jatropha and camelina. Algae have gained attention due to their high oil content and ability to grow in various environments. The use of non-traditional feedstocks helps to reduce competition with food production and enhances the overall sustainability of biodiesel (Garg et al. 2023 ). Furthermore, efforts have been made to improve the sustainability of biodiesel production by reducing its environmental impact. This includes optimizing production processes to minimize water and energy consumption, reducing greenhouse gas emissions, and implementing waste management strategies. Additionally, researchers have been exploring the concept of “second-generation” biodiesel, which involves utilizing waste materials, such as agricultural residues and lignocellulosic biomass, to produce biodiesel. This approach not only reduces waste but also maximizes resource utilization (Makepa et al. 2023 ).

Biodiesel engine performance and emissions

Compared to petrodiesel fuel, burning biodiesel releases fewer particulates, carbon monoxide, and unburned hydrocarbons. Since biodiesel is produced using natural resources, its sulfur content is relatively low, which means that when it burns in an engine, it releases less sulfur dioxide into the atmosphere (Rayati et al. 2020 ). All biodiesels and their blends have shown the capacity to enhance gas turbine performance while lowering emissions of carbon dioxide, carbon monoxide, nitrogen oxide, and hydrocarbons under a range of operating conditions. To employ fuels in an engine, one must be aware of their characteristics for combustion. Although fossil fuel-based diesel fuel may not be entirely replaced by biodiesel, it can aid in achieving balanced energy utilization. One benefit is that biodiesel may be used in contemporary engines with little modification. Older vehicles with natural rubber gasoline lines, however, require a few modifications. Rubber fuel lines must be replaced since they will crack when used with biodiesel. On the other hand, an oil or gasoline dilution in the fuel system is possible in a modern vehicle with a DPF (diesel particulate filter). The ability of gasoline to lubricate the fuel injection system is believed to be crucial for diesel engines. The use of diesel–biodiesel mixes can thereby enhance their general lubricity. Additionally, the lower sulfur level of today’s diesel fuel could affect its lubricity because the compounds that provided lubrication are no longer present (Veza et al. 2022 ).

Techno-economic analysis

Techno-economic analysis (TEA) plays a crucial role in assessing the economic feasibility and viability of biodiesel production processes. It involves evaluating the overall costs, revenues, and profitability of biodiesel production, considering various factors such as feedstock costs, capital investment, operational expenses, and market prices. Recent studies have employed TEA to analyze and optimize biodiesel production processes, providing valuable insights for decision-making and process design. One example of TEA in biodiesel production is a study conducted by Zhang ( 2021 ), which evaluated the techno-economic performance of different feedstocks and process configurations for biodiesel production. The analysis considered factors such as feedstock availability, conversion efficiency, capital costs, operating costs, and market prices. The study highlighted the importance of feedstock selection and process optimization in achieving cost-effective biodiesel production. Another study by Tasić ( 2020 ) performed TEA for manufacturing biodiesel from used cooking oil. The analysis included the estimation of capital and operational costs, energy consumption, and environmental impacts. The study demonstrated the economic feasibility of waste cooking oil-based biodiesel production and identified critical parameters affecting the overall economics of the process. Furthermore, a study by Atabani ( 2020 ) conducted TEA for biodiesel production from microalgae. The analysis considered various scenarios, including different cultivation systems and conversion technologies. The study assessed the economic viability of microalgae-based biodiesel production, considering factors such as biomass productivity, lipid content, capital investment, and operational costs. These recent studies emphasize the importance of TEA in evaluating the economic aspects of biodiesel production. By considering a comprehensive range of factors, TEA provides valuable insights into the cost-effectiveness, profitability, and sustainability of biodiesel production processes, helping guide decision-making and process optimization.

Challenges, perspectives, and further research

The homogeneous catalyst has been thoroughly examined, and the literature has addressed several issues. However, heterogeneous catalysts are a very new field of study, and there is now a lot of research being done in this area. The literature has documented many obstacles regarding these catalysts:

• It has been said that the primary issues with heterogeneous catalysts are instability, reduced reaction rate, and short catalyst life.
• It has been reported that solid-base catalysts are FFA, CO 2 , and water sensitive. Through saponification, they destroy the catalyst and render it inactive.
• Because water hydrolyzes the ionic group in solid acid catalysts, leaching and product contamination have been documented.
• It has been documented that during enzymatic transesterification, methanol causes lipase inhibition.
• In the case of nanocatalysts, good performance requires increasing the reaction time under very moderate working conditions. To attain normal reaction times, however, harsh working conditions must be used, which raises energy consumption.

Future research should pay attention to the following recommendations:

• To create novel catalysts with enhanced catalytic performance, more research into waste-derived catalysts is required.
• The creation of highly selective and active heterogeneous catalysts that may be used in industrial settings at a reasonable cost.
• Investigating novel catalyst supports with a network of linked pores of the right size and a selected surface area.
• Investigating the use of waste or biomass as a catalyst source to lower related costs and increase sustainability for solid catalysts that are sold commercially.
• Improving synthesis catalyst production procedures and treatment stages to move the technology from a lab to an industrial setting.
• Maintaining the high basic strength of the synthesis catalyst while improving the shape and sensitivity to FFA and water.
• Additional research is needed into the manufacturing of industrial enzymatic biodiesel as a guaranteed choice for the future.
• To give relevant information about the suitable catalyst, future research on the catalytic mechanism of partial catalysts should examine the features in depth.
• Ways to recover and reuse nanocatalysts that are both economical and energy efficient.
• Kinetic studies of the biodiesel production reaction using the synthesized catalyst should be carried out.

This extensive review delves into the various aspects of biodiesel production and its promise as a sustainable alternative for a greener energy future. The significance of feedstock selection and preparation is emphasized, with effective techniques discussed for optimizing biodiesel production efficiency and quality. Biodiesel has emerged as a versatile and promising alternative for transportation, industrial processes, and energy generation, demonstrating its potential to reduce greenhouse gas emissions and dependency on fossil fuels. The key process of transesterification is thoroughly examined, encompassing the utilization of diverse catalysts, including homogeneous, heterogeneous, enzyme based, and nanomaterials. The unique characteristics and performance of nanomaterials in transesterification are highlighted, offering prospects for enhanced efficiency and selectivity. Understanding the reaction mechanism and kinetics of transesterification is crucial for optimizing the production process. Kinetic modeling is identified as a valuable tool for process optimization, enabling better control and improved production efficiency. Methods for assessing the quality and properties of produced biodiesel are discussed, highlighting the importance of accurate characterization to meet quality standards and ensure compatibility with engine systems. Recent developments in biodiesel production showcase progress in feedstock selection, process optimization, and sustainability. However, challenges related to engine performance, emissions, and compatibility remain obstacles to wider biodiesel adoption. Future research should focus on addressing these challenges through innovative engine technologies, improved fuel formulations, and effective emission control strategies. Techno-economic analysis provides insights into the economic feasibility of biodiesel production, considering factors such as feedstock costs, process efficiency, and market demand. Ongoing analysis and assessment are essential for ensuring the commercial viability and scalability of biodiesel production. In conclusion, biodiesel presents a promising sustainable solution, but its advancement requires continuous research, development, and collaboration among academia, industry, and policymakers. Addressing challenges, pursuing further research, and implementing the recommendations outlined in this review will contribute to the widespread adoption of biodiesel as a renewable energy source, paving the way for a cleaner and more sustainable future.

Author contribution

All authors contributed to the study conception and design. Data collection and analysis were performed by Sabah Mohamed Farouk, Aghareed M. Tayeb, Shereen M. S. Abdel-Hamid, and Randa M. Osman.

Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).

Data Availability

Declarations.

Not applicable.

The authors declare no competing interests.

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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• Open access
• Published: 01 June 2021

An overview to process design, simulation and sustainability evaluation of biodiesel production

• Mustafa Kamal Pasha 1 ,
• Lingmei Dai 1 ,
• Dehua Liu 1 , 3 ,
• Miao Guo 2 &
• Wei Du   ORCID: orcid.org/0000-0003-4094-210X 1 , 3  

Biotechnology for Biofuels volume  14 , Article number:  129 ( 2021 ) Cite this article

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The overwhelming concerns due to over exploitation of fossil resources necessitate the utilization of alternative energy resources. Biodiesel has been considered as one of the most adaptable alternative to fossil-derived diesel with similar properties and numerous environmental benefits. Although there are various approaches for biodiesel production, development of cost-effective and robust catalyst with efficient production methods and utilization of a variety of feedstock could be the optimum solution to bring down the production cost. Considering the complexity of biodiesel production processes, process design, quantitative evaluation and optimization of the biodiesel from whole systems perspectives is essential for unlocking the complexity and enhancing the system performances. Process systems engineering offers an efficient approach to design and optimize biodiesel manufacturing systems by using a variety of tools. This review reflects state-of-the-art biodiesel research in the field of process systems engineering with a particular focus on biodiesel production including process design and simulation, sustainability evaluation, optimization and supply chain management. This review also highlights the challenges and opportunities for the development of potentially sustainable and eco-friendly enzymatic biodiesel technology.

Introduction

Global climate change is threatening the ecosystem worldwide by temperature increase and climate swings. Report published by the Intergovernmental Panel on Climate Change (IPCC) concluded that there is higher probability of about one million species’ extinction if the average global temperature escalates the minimal margin of 1.5 °C [ 1 , 2 ]. Greenhouse gas (GHG) emissions from anthropogenic activities such as burning of fossil fuel to meet the energy requirement are the major contributor to the temperature rise. It is signposted that by 2050, minimum 40% reduction in GHG emissions is obligatory to sustain the average increase < 1.5 °C [ 1 ]. This phenomenon continuously compels the community to search for green alternatives both in energy resources and platform chemicals [ 3 ]. One of the primary substitutes to conventional fuels is biodiesel, which received ample attention [ 4 ]. American Society for Testing and Materials (ASTM) defined biodiesel as “mono-alkyl esters of long chain fatty acids that is derived from animal fats or vegetable oils” with an added requirement of having greenhouse gas emissions at least half of the baseline greenhouse gas emissions [ 4 ]. Biodiesel manufacturing attained extended attention and dramatic growth is observed in last decade as indicated in Fig.  1 [ 5 , 6 ]. The characteristics like, lower GHG emissions, highly biodegradable molecular structure with minimal combustion toxicity, compatibility with existing engines and fuel distribution infrastructure are preferred features for its remarkable industrial growth [ 7 , 8 ].

Yearly increase in biodiesel manufacturing in European Union (EU) from 2007 to 2018

Generally, esterification/transesterification of free fatty acids/triglycerides with alcohol applying catalytic (chemical and biological catalysts) and non-catalytic are the principal reactions in prevailing biodiesel production [ 7 , 9 ]. Among all the catalytic routes, biodiesel production using chemical catalyst is the most commercialized route due to shorter reaction time and high yield [ 10 ]. However, there are some limitations in chemical catalysis such as, catalyst recovery and recycling, excessive amount of alkaline wastewater and complexity of downstream product purification [ 11 ]. Additionally, the chemical catalytic process requires high-quality raw materials to save the process from saponification. Thus high-quality raw materials deliberately affect the process economics and increase the product cost [ 12 ]. Consequently, biocatalytic process has been recognized as a favourable alternative having mild reaction conditions, lesser wastes, easy purification and raw material flexibility [ 11 ]. Utilization of alternative low-cost raw materials such as second and third generation feedstock instead of using vegetable oils offers a potential way to reduce the biodiesel cost [ 13 , 14 ]. Beyond biodiesel, research efforts have been also placed on new generation biofuel production from waste by integrating esterification reactions (enzymatic or chemical routes) with organic acids recovery from various waste resources. In addition, recent research has been also conducted to investigate the combustion performances of biodiesel blends in direct injection diesel engine, biodiesel derived from water hyacinth, palm biodiesel, Garcinia gummi-gutta biodiesel, tamarind biodiesel as well as alternative fuels blended with diesel [ 15 , 16 , 17 , 18 ].

A wide range of feedstock (edible, in-edible oil crops and waste oils, as well as microalgae), diverse reaction and separation conditions, and different types of catalyst make biodiesel manufacturing a complex system, which not only requires empirical work, but also the modelling research efforts. Recent comprehensive reviews by Muhammad et al. and Bhatia et al. and by Ananthi et al. provide very good overview of the research advancement in the biodiesel production including feedstock resources and characteristics, oil extraction and transesterification methods, reactor design and process intensification. To better manage and grasp the complexity of biodiesel manufacturing process, Process Systems Engineering (PSE) offers a solution by focusing on the development and application of the modelling and computational methods. This article shall henceforth a review on state-of-the-art PSE modelling research of biodiesel production and supply chains, and identify the emerging gaps and future research frontiers. Specifically, process design and simulation of different technologies for biodiesel production are compared in Sect. " Process design and simulation ". Section " Sustainability evaluation of biodiesel production " focuses on the sustainability evaluation (economic and environmental aspects) of biodiesel production. Section " Optimization " discusses optimization of biodiesel production system at both processes and value chain design levels, which is followed by concluding remarks and critical perspectives for future research.

Process system engineering in biodiesel production

Despite the research advances and commercialization of quality biodiesel as drop-in biofuel in line with standard specification (EN 14214:2012 or ASTM 6751 12), biodiesel manufacturing still represents a complex system, which not only requires empirical work but also the modelling research efforts. A wide range of feedstock (edible, in-edible oil crops and waste oils), diverse reaction and separation conditions, and different types of catalyst have been used in biodiesel manufacturing (Fig.  2 ). Several modelling tools have been applied to tackle biodiesel manufacturing complexity including process design and simulation, sustainability evaluation and optimization (Fig.  3 ).

Schematic representation of technological choices and feedstock for biodiesel production

General framework for integration of different modelling tool

Process design and simulation

Process simulation is a model-based illustration of physical, chemical, biological, and other unit operations and technical processes in a software. It can be used for the design, development, analysis, and optimization of biodiesel production processes.

The advantages of process simulation are to (a) reduce plant design time by allowing designers to quickly test various plant configurations; (b) improve current processes by answering ‘what if’ questions, determining optimal process conditions and assisting in locating the constraints in the process. The ultimate objectives of using process simulation are to realize faster troubleshooting, online performance monitoring and real-time optimization. A variety of modelling platforms, e.g. Aspen Plus, Aspen Hysys, SuperPro Designer, provide a resource where researchers and engineers can model, simulate, design their processes.

Several challenges arise for researchers when using these modelling platforms. The first challenge that researchers usually face is to define and select the appropriate chemical species taking part in the whole process. Yun et al. [ 19 ] added three different free fatty acids (oleic acid, stearic acid, palmitic acid) and one triglyceride (tri-olein) as model components to simulate the biodiesel production process from waste vegetable oil. A more comprehensive representation of the waste vegetable oil was compiled by Abdurakhman et al. [ 20 ] using five different triglycerides (tri-palmitin, tri-stearin, tri-olein, tri-linolein, tri-linolenin) and five FFA (linoleic acid, oleic acid, palmitic acid, stearic acid, linolenic acid) as model components. It was shown that the use of realistic feed compositions and sensitivity on the changes of composition is highly important to provide a more realistic assessment of the large-scale plant [ 20 ]. Due to the variable composition of biodiesel feedstock, the incorporation of all the components in a process simulator is also a challenging issue. Although several triglycerides with varying fatty acid chains are present in the Aspen Plus databanks but their physical property data are not well managed. Moreover, other components such as enzymes are still non-databank components. These components have usually undefined structures and/or difficult to characterize due to which their incorporation in process simulator is still a challenging issue.

The second challenge for using modelling platforms is to identify the available chemical and physical properties in the database. Modelling the biodiesel production system in these simulators, the NRTL or UNIQUAC thermodynamic models are usually selected due to polar compounds (glycerol, methanol and water) and the non-ideal nature of the transesterification reaction system. Zong et al. [ 21 ] applied chemical constituent fragment approach for the estimation of thermo-physical properties of triglycerides. This methodology was then extended to individual mono- and diglycerides [ 21 ]. In most of the simulation studies conducted for biodiesel production, UNIFAC method were employed which resulted in reliable prediction of the missing NRTL coefficients of trilolein–methanol and triolein–glycerol binary system [ 19 , 22 , 23 ].

Another challenge for using modelling platforms is to integrate solids, batch and custom processing unit modelling [ 24 ]. For example, biodiesel production involves several separation and FAME purification steps in which membrane is one option that can be utilized to obtain the desired product purity and recovery of recyclable materials (e.g. methanol, water, liquid lipase) [ 25 ]. Beside these challenges, several important data need to be gathered prior to process flowsheet design and simulation, e.g. reactor type and catalyst, rate of reaction or conversion, stoichiometry of reaction, process conditions, production capacity, mode of operation, etc. The approach that is often employed in the process design and simulation of production plants starts from reactor selection and proceeding outward by adding separation and recycle system [ 26 , 27 ]. Among these several steps, the reaction procedure and the type of catalysts employed in the transesterification are crucial which determine purity of the product as well as severity of downstream separation and purification steps [ 28 ]. The following sub-sections present the process design and simulation of chemical and enzyme-catalysed biodiesel production processes along with the heat integration studies.

Process design of biodiesel production using chemical catalyst

In the design of biodiesel production process, choice of operation mode for the process is one of the most important decision. Many publications on process design and simulation for biodiesel production are available (Table 1 ). These studies were based on the evaluation of heterogeneous and homogenous chemical catalysis as well as supercritical conditions (non-catalytic) in the context of process economy for a batch and continuous operations. Economic comparison of continuous and batch process for biodiesel production has been published by Sakai et al. [ 29 ]. Different types of catalyst (heterogeneous and homogenous alkali) and purification methods are compared extensively. Results elucidated that batch processes were more expensive than continuous process [ 22 ]. Comparing the behaviour, Fonseca et al. [ 30 ] showed that under the usual operating conditions, single continuous stirrer tank reactor (CSTR) is not capable to achieve the same productivity as batch reactor. However, arrangement of CSTRs in series is a viable pattern for mass production than batch process [ 30 ]. Despite the some advantages of batch processes, continuous process is the only choice for large-scale biodiesel production [ 23 ].

Regarding continuous production of biodiesel, Zhang et al. [ 31 ] attempted to design and simulate theoretical scale industrial plant using Aspen HYSYS. Various chemical catalysts (including homogenous-alkaline and acid catalyst) and feedstock (waste cooking oil and virgin vegetable oil) were used to investigate that how each type of catalyst and feedstock affect the process design. The unit operations included in the process design were transesterification, esterification, recovery of methanol, biodiesel separation and purification with either extraction of methyl esters using hexane or conventional water washing. The techno-economic feasibility of each technological option was evaluated and compared on the basis of material and energy consumption. The simulation results revealed that each process is distinct in their merits and demerits which are highly dependent on feedstock quality and the catalyst employed. Overall, alkali-catalytic process with virgin vegetable oil as a feedstock (Fig.  4 ) is a preferred option having less capital investment, but its operating cost is high because of high-quality feedstock requirement [ 31 ]. Modification in the design was carried out for low-quality oil (waste cooking oil) having high amount of FFAs. In this case, esterification of FFAs catalysed by sulfuric acid was carried out prior to the alkaline transesterification step. Contrarily to alkaline process with acid pre-treatment, the acid-catalysed process (see Fig.  5 ) was found suitable requiring no pre-treatment step. However, in this design, the larger methanol requirement resulted in larger reactor and distillation columns [ 18 ]. In addition, the presence of sulphuric acid requires a stainless steel reactor, which results in higher reactor cost.

Flow diagram of alkali-catalytic route for biodiesel manufacturing using refined vegetable oil [ 31 ]

Flow diagram of homogenous acid-catalysed route for biodiesel manufacturing using waste cooking oil [ 22 ]

Heterogeneous acid-catalysed process and supercritical conditions (non-catalytic process) were also simulated in Aspen HYSYS by West et al. [ 18 ]. The simulation results were used to assess the performance of each process for low-quality feedstock. Results showed preference of non-catalytic (Fig.  6 ) and heterogeneous acid-catalysed process over alkali and homogenous acid-catalysed process due to reduced separation stages which results in lower capital investment. However, the process has high-energy profile due to heating and pumping.

Flow diagram of non-catalytic (supercritical alcohol) biodiesel manufacturing route [ 32 ]

From the above discussion, it is inferred that each process consists of the same process units (including reactors, washing column, distillation columns, heat exchangers and pumps), but the process operation for biodiesel production may differ due to type/purity of feedstock. Moreover, all the simulations studies proved that each process could yield high-quality biodiesel within definite process conditions. However, these simulation studies commonly lack integration of real industrial data, therefore, leading to under or overestimation of some of the simulated results for energy and mass balance. As an example, water consumption (11 kg/h of water is required to produce 1177 kg/h of biodiesel) and waste fractions estimated by Zhang et al. [ 31 ] are unrealistically low when compared to real industrial data (47.5 kg of water for 100 kg of biodiesel [ 23 ]). For any biodiesel process design and simulation, incorporation of actual industrial data is complementary to better analyse and reflect the process performance.

Process design of biodiesel production using biocatalyst

Process design for industrial scale enzyme-catalysed biodiesel production is entirely different from the conventional setup. Enzymes are expensive and slow reacting species as compared to conventional chemical catalysts, but offer much simpler and easier purification scheme. Process design has been carried out by Harding et al. [ 33 ] and Al-Zuhair et al. [ 34 ] for enzymatic biodiesel, but the process lacks in optimization on some points. Sotoft et al. [ 23 ] extended the enzymatic process further by designing co-solvent and solvent-free operations for biodiesel (see Figs.  7 , 8 ). Simulations was carried out in Aspen PLUS to explore how each operation affect enzyme performance and process design as well as the process economics. The solvent-free process was designed using three reactor modules in series with inter-stage separation of glycerol through decanters. This configuration made possible methanol stepwise addition, which is necessary to prevent enzyme deactivation by methanol. While in co-solvent process design, the required yield was achieved by employing only one reactor module. Distillation is used for methanol recovery and product purification in both processes. Solvent-free and co-solvent operations differ in solvent recovery requirements by distillation, which influence the process economy by making the process energy intensive. Zheng et al. [ 35 ] stated that the co-solvent process can be made energy efficient if distillation column is replaced with triple-effect evaporator for solvent and methanol recovery. Complete energy balance shows that enzyme-catalysed process is more energy efficient than alkali/acid and non-catalytic processes [ 35 ]. The co-solvent process was further enhanced by designing the enzymatic process with supercritical CO 2 as a co-solvent [ 36 ]. Using supercritical CO 2 was found more competitive eliminating the need for solvent recovery steps that are necessary in case of organic solvents. Contrarily to these models, Yun et al. [ 19 ] proposed two-step process design for enzymatic biodiesel production. They employed a wiped-film evaporator instead of distillation column to acquire the required purity of fatty acid methyl esters which must exceed 98.5%. A promising conversion efficiency was achieved by adding de-acidification step after transesterification. However, this adds additional cost incurring steps of neutralization and salt removal.

Flow diagram of solvent-free enzymatic route for biodiesel manufacturing [ 23 ]

Flow diagram of co-solvent enzymatic route for biodiesel manufacturing [ 23 ]

Heat integration

In general, biodiesel production process requires a number of distillation steps for product purification (mainly with non-edible oil as the feedstocks) and to recover the methanol for recirculation. Pinch analysis [ 37 ] is the well-established method for heat integration in the process and optimal design of heat exchanger networks. Sanchez et al. [ 38 ] used pinch technique for heat integration in biodiesel manufacturing from microalgae. An optimal heat exchanger network was designed to reduce the load on external cooling and heating utilities. The simulation results showed that the heating and cooling utilities were reduced by ~ 13% and 11%. Meanwhile, Song et al. [ 39 ] reported that the operational cost of biodiesel from microalgae can be reduced by ~ 41.6% and 22.5% compared to two different reference cases when pinch analysis-based heat integration were performed. Yun et al. [ 19 ] put forward pinch analysis for heat exchanger network design and energy optimization of enzyme-catalysed biodiesel production process. The results showed a reduction in heating requirement by 15.6% compared to non-integrated process. Several other studies utilized pinch analysis for optimal heat and mass integration and found a significant reduction in energy consumption and utility cost [ 40 , 41 ]. However, the thermodynamic approach adopted in these studies lack in configuration of subsystems which fails in guarantying best decisions [ 42 ]. In this regard, Martin et al. [ 42 ] made a contribution by the simultaneous heat integration and optimization approach for optimal process design of biodiesel. Apparently, the temperatures and flowrates were key decision parameters for both the optimization and the heat integration concern that resulted in much lower energy and water consumption with higher overall profit.

Sustainability evaluation of biodiesel production

Sustainability, as alike concept to sustainable development, has been well thought-out to encompass the primary balance of three dimensions: environmental, economic and social, where poor performance related to one could impede performance on the others [ 46 ].

Life cycle sustainability assessment (LCSA) refers to the evaluation of all environmental, social and economic impacts in decision-making processes towards more sustainable products throughout their life cycles. Initiated from life cycle assessment, the life cycle thinking approach has been extended since 2002 to form a LCSA methodology framework, which consists of three pillars (Fig.  3 )—environmental life cycle assessment (LCA), life cycle costing (LCC) and social-LCA. As a systematic and rigorous evaluation framework, life cycle sustainability provides integrative and holistic perspectives for multi-criteria decision on a given process or a system. As generalized in Eq. ( 1 ), LCSA accounts for all input–output flows occurring at each life cycle stage throughout the ‘cradle-to-grave’. Formalized by the International Organization for Standardization, LCA quantifies the environmental footprints associated with all stages of a product, service or process. LCC and SLCA examine the holistic economic aspects and social consequences respectively, evaluating the improvement opportunities of various product systems and processes including biodiesel:

where the variable \(E{I}_{\mathrm{kpi}}\) denotes the total sustainability impacts of a given process expressed as key performance indicator kpi (e.g. global warming potential and economic costs). \(E{I}_{\mathrm{kpi}}\) is determined by the characterization impact factors for input resource r \({(EIf}_{r,\mathrm{kpi}}^{\mathrm{in}}\) ) or emitted compound c ( \({EIf}_{c,\mathrm{kpi}}^{\mathrm{out}})\) and the input or output flows \({(X}_{r,s}^{\mathrm{in}} \mathrm{or} {X}_{c,s}^{\mathrm{out}})\) at life cycle stage s .

Evaluation of sustainability aspects have increasingly been reported for biodiesel production process during the last decade. However, most of the reports focused on the environmental and economic aspects of sustainability while omitting social aspect. The following sub-sections present a detail discussion on the techno-economic and environmental performance of biodiesel production processes.

Economic evaluation

Economic performance is the most imperative factor for evaluating the sustainability of biodiesel production and plays a vital role in industrialization of any process. The higher production cost is the major challenge for biodiesel production scaling-up and its use as an alternate to petro-diesel [ 47 ]. However, an extensive research has been conducted during the past decades concerning the process economics and product cost reduction. These researches elaborated the utilization of different feedstock together with alternative technologies for the production and purification of biodiesel. Most of the studies analysed the total investment required for biodiesel production including fixed capital investment and production cost. Such cost estimation are often based on the process flowsheet and affected by the equipment type and size, construction material, material and energy balance [ 48 ]. Economic analysis can be performed in commercially available softwares such as Aspen In-Plant Cost Estimator or Aspen Icarus Process Evaluator [ 23 , 32 ]. The key variables that determine the economic performance of a given biodiesel production plant include the production capacity, the type of feedstock, and the technological production process.

Production capacity

The production scale is the significant factor that could influence the techno-economic profiles by either decreasing or increasing the unit cost of biodiesel. This was elaborated by analysing the economic performance of biodiesel production plant with varying production capacities. One of such study was carried out by You et al. [ 49 ] for alkali-catalysed biodiesel production using refined soybean oil with three different production scales (8, 30, 100 kilo tons/year). It was concluded that higher capacity led to more attractive ARR (After-tax Rate of Return) with a lower BBP (Biodiesel Break-even Price) and higher NAP (Net Annual Profit). The author also stated that increase in plant capacity gave the same economic effects for soybean oil as well as waste cooking oil. On another hand, Apostolakou et al. [ 50 ] analysed the effect of plant capacity on the economic viability of biodiesel manufacturing using alkali-catalysed process. They found that production scale of 60 kilo tons/year is a threshold, above which, an increase in the production scale could improve the process viability since the production cost of biodiesel could be considerably reduced.

Similar result was reported by Van Kasteren and Nisworo [ 51 ] for supercritical process using used cooking oil with three different plant capacities (8, 80 and 125 kilo tons/year of biodiesel). They found that as the plant capacity increases, the biodiesel cost decreases from 0.52 to 0.17 US $/L. Glisic et al. [ 52 ] analysed the economics of the three different biodiesel production processes and investigated the effect of production scale on the net present value (NPV) of the process. The processes investigated were homogenous alkali-catalysed, non-catalytic transesterification for biodiesel production and catalytic hydrogenation process for diesel production. The authors reported that the plant capacity significantly affected the NPV of all processes. Especially for catalytic hydrogenation process, the NPV increased from 7 to 53.1 million US$ as the plant capacity were increased from 100 to 200 kilo tons/year. They concluded that plant capacity below 100 kilo tons/year (for all the investigated plants) results in negative NPV value after 10 years of project life.

Most recently, Navarro-Pineda et al. [ 53 ] assessed the economics of biodiesel production from jatropha using alkali-catalysed process. They also included the upstream process of jatropha plantation and pellet production from waste cake that is obtained from oil extraction process. The authors found that the biodiesel production cost remains constant when the production capacity was greater than 10,000 m 3 /year. However, at this scale, the plant expenses were greater than the plant income that can only be reversed by higher Jatropha oil yields. Similarly, Kookos [ 54 ] indicated that a plant with annual production capacity > 42,000 tons could produce economically competitive biodiesel utilizing spent coffee grounds as feedstock. As reported by Apostolakou et al. [ 51 ], the unit production cost of chemical-catalysed biodiesel decreases and can be expressed as the function of plant size. A significant decrease in production cost from 0.9 to 0.75 euros/L biodiesel was observed with the increase in production capacity from 0 to 40 kilo tons/year, which was followed by a plateau [ 50 ]. Contrarily, the total capital investment increases proportionally with production size but not linearly. Generally mass production is always cost-effective and most economical and same is the case with biodiesel. This effect of plant size on the total capital investment has been investigated in previous research [ 23 ] where similar trends were shown for co-solvent and solvent-free operation. It was observed that total capital investment varies between 10 and 60 million euros while plant size increases from zero to 1000 million kg biodiesel per year [ 23 ].

Most of the techno-economic studies concluded that the high cost of biodiesel production is mainly credited to the feedstock’s price. An economic assessment study published by Haas et al. [ 55 ] demonstrated that the biodiesel production cost increases linearly with increasing the cost of the feedstocks. They found that the cost of the feedstock is about 88% of the total biodiesel production cost. Thus, there was an increasing research attention on the low-cost feedstock as a measure to reduce biodiesel costs. However, the low-cost resource often represents low-quality feedstock, which incurs additional processing costs due to pre-treatment, separation and purification steps. For example, at industrial scale, the base-catalysed process is the most economically viable option to produce biodiesel from high-quality oils [ 32 , 56 ]. However, it shifts to unfeasible solutions for low-quality oil feedstock (cheaper feedstock) containing high free fatty acids and water contents due to additional energy intensive pre-treatment requirement. A technology capable to process both low and high-quality oil feedstock without any additional pre-treatment steps offers a solution. Supercritical non-catalytic and enzymatic biodiesel production technologies are the examples of such technologies that have the ability to process low-quality feedstock without any pre-treatment requirements [ 19 , 22 , 23 , 32 ].

Alternative technologies and their comparison for economical biodiesel production

The economics of biodiesel production vary with production technologies, which are driven by the number of unit operations and associated costs on equipment and energy consumption [ 47 ]. Alternatively, such economic advantages may also arise due to the relatively cheaper catalyst employed in the process. Moreover, catalyst type is highly important as it defines the type and sequence of production and purification scheme.

Table 2 compares the economic evaluation studies on different catalytic processes for biodiesel production. As previously mentioned, the alkali-catalysed process gives higher yields in shorter reaction time but it is not economically viable option when low-quality oil is considered [ 57 ]. It is limited by the saponification reaction (soup formation) that occurs between catalyst and free fatty acids, resulting in energy intensive downstream purification and making the process unprofitable. Acid-catalysed process avoids the side reactions and can esterify the FFAs to biodiesel. Zhang et al. [ 57 ] showed that acid-catalysed process could give lower production cost, lower biodiesel break-even price and better after-tax-return-rate compared to alkaline process using waste vegetable oil. However, the slow reaction rate, high alcohol requirement with larger reactor size and the corrosion problems imposed by the acid catalyst do have cost implications and makes the process economically unfeasible [ 22 , 32 , 57 ].

Heterogeneous acid-catalysed process could be a promising alternative with economic benefits compared to the homogenous acid-catalysed process. The techno-economic analysis performed by West et al. [ 22 ] showed that the heterogeneous acid-catalysed process has better economic performance (lower production costs and capital investment) compared to the homogenous acid-catalysed process which arises due to easy separation and recyclability of the catalyst, less corrosive nature and absence of washing steps for product purification. However, the slow reaction rate and lower biodiesel yields remain the major issues with acid-catalysed processes. These issues can be addressed by transesterifying the triglycerides with supercritical methanol. Using supercritical conditions give higher methyl ester yield in a shorter reaction time with reduced purification stages which results in very competitive biodiesel prices [ 32 , 51 ] compared to previously denoted processes [ 22 , 56 ]. The study carried out by Lee et al. [ 32 ] further elaborated the economic benefits of supercritical non-catalytic process by estimating the most promising values for discounted cash flow return rate (DCFRR), discounted payback period (DPP), and net present value (NPV) of the plant. However, the high alcohol requirement and extreme operating conditions (350 °C and 45 MPa) [ 22 ] makes the process energy intensive and incur considerable cost to the process.

Another perspective technology is enzyme-catalysed process that is more advantageous [ 23 , 36 ] than chemical and non-catalytic processes in terms of milder reaction conditions, tolerating low-quality feedstock and easy purification of the products. The enzyme-catalysed process can also be carried out in the presence of solvent to increase the enzyme productivity. Sotoft et al. [ 23 ], demonstrated that the enzyme cost that was 50% of the raw materials cost in the absence of solvent was reduced to about 22% when t -butanol was used as a co-solvent. Although, the enzyme cost was significantly reduced but this led to the high production cost due to high energy consumption for solvent recovery. Using supercritical CO 2 as a co-solvent can further improve the profitability of the process by both enhancing the enzyme productivity and eliminating the energy intensive step of solvent recovery [ 36 ]. This was confirmed by Lisboa et al. [ 36 ], reporting the production cost of biodiesel as 0.75 euro/L which is lower than the cost estimated by Sotoft et al. [ 23 ] (EUR 2.35/L of biodiesel) for solvent-free process with similar enzyme productivity and price. For low-quality oil feedstock, the enzymatic process is economically superior than the acid and alkali-catalysed processes in term of capital investment but inferior in operating cost [ 59 ]. This discrepancy was due to the high cost associated with the immobilized enzyme indicating that reusing the enzyme for several batches is needed to reduce the operating cost. Profitability of the process evaluated by net present value (NPV) for assumed interest rate of 13.5% and plant life span of 10 years showed that the enzymatic process is more economically attractive than the alkali-catalysed process [ 44 ]. Generally, the reusability of immobilized enzyme or using cheap biocatalyst (soluble or liquid lipase) are the most important aspects, improvements in which could make enzymatic process economically competitive with chemical-catalysed processes.

Environmental evaluation

Life Cycle Analysis (LCA) has been widely adopted as a tool to evaluate environmental performance of any product or process. In previous LCAs (see Table 3 ), the inventory of biodiesel production derived from computer-aided process were fed into LCA to identify environmental hot-spots contributing to the impacts and evaluate environmental sustainability of biodiesel production. As visualized in Fig.  9 , the inventory including input–output flows are associated with mid-point environmental impact categories and converted to category indictors by using defined characterization factors; the aggregated indicator results provide characterized profiles of the biodiesel systems, which can be further normalized and linked with protection areas (i.e. end-point categories including human health, ecosystem, resource depletion).

Life cycle impact assessment (LCIA) phase

Key methodological challenges in LCA

Biodiesel production can be largely classified as three life cycle stages. Raw material production is the first stage, which includes cultivation, harvesting, transportation and storage of oil seed crops, as well as production and transportation of all the required chemicals. The second stage involves pre-treatment (milling, extraction and purification) of oil feedstock and conversion via esterification/transesterification to biodiesel. The third stage includes storage, distribution and transportation to petrol station, and eventual burning of biodiesel. As summarized in Table 3 , LCA study conducted by Hou et al. [ 60 ] adopted a full well-to-wheel approach by including all relevant processes in the life cycle stages of biodiesel (e.g. production of chemicals and energy, feedstock cultivation and transportation, production of biodiesel and combustion of biodiesel at use phase). However, majority of the surveyed studies adopted well-to-gate approach (see Table 3 ) excluding the step of biodiesel distribution and end use. This approach is useful when the study is conducted to compare different technological pathways for biodiesel production, since the performance of vehicle engine does not change with the fuel combustion produced from different technological routes [ 61 ]. But, when the purpose of the assessment is to compare biodiesel with their fossil substitute, e.g. biodiesel with conventional diesel fuel, the well-to-wheel approach offers better reflection of the overall life-cycle performance where engine plays a role for exhaust gas emissions and ignition performance. Significant reductions in particulate matters, hydrocarbons and carbon monoxide emission are reported which are the profound advantages of biodiesel over conventional diesel [ 62 ].

Functional unit is another important factor which quantify the identified functions of a product system in which all the materials and energy flows and all effects resulting from these flows are related [ 63 ]. Mostly four types of functional unit can be identified in biodiesel LCA which include input-related units, output-related units, unit of agriculture land and year [ 64 ]. In biodiesel LCAs, majority of studies selected functional units based on the output of the product system (e.g. ton of biodiesel, L of biodiesel, MJ of biodiesel) [ 60 , 65 , 66 , 67 ], while few studies used agricultural land and kilometres of transportation service as a functional unit [ 68 , 69 ]. Besides, some studies presented the final results using multi-functional units [ 68 , 70 ]. Ravindra et al. [ 70 ] used input, output and agricultural land related functional units. They used the product biodiesel as the output-related functional unit; for oil extraction functional unit is the production of 1000 kg of oil while functional unit for agriculture stage is per hectare of cropland. Similarly, Zhang et al. [ 69 ] reported two output-related functional units in their study for biodiesel based on the MJ of biodiesel and 1 km of driving distance. The implementation of kilometre of transportation service as a functional unit is better option when the goal is to compare biodiesel and fossil fuels used for transportation. Assessment with multiple functional units avoids biased outcomes and is highly effective for better assessment of any system in diverse scenarios.

Apart from functional unit and system boundary definition, the allocation approach, i.e. partitioning of environmental burdens among the multiple product is of great importance for biodiesel systems [ 63 ]. In biodiesel LCAs, the key allocation concern is between the biodiesel and by-product glycerol. There are mainly four options for adopting the allocation approach namely, null allocation, physical allocation, economic or market value allocation, and system expansion or substitution-based allocation [ 71 ]. Among the biodiesel LCAs surveyed in this review, the choice of allocation is dispersed (Table 3 ). The allocation adopted in most of the biodiesel LCAs were based on the physical properties of the product. Some studies related to biodiesel LCAs adopted the null-allocation approach and assigned all the environmental burdens to the main product biodiesel. However, this approach is not necessarily representative of the actual contribution of the studied products. Different allocation procedures may influence the results of biodiesel LCAs, which should be evaluated by sensitivity analyses [ 63 ]. Castanheira and Freire [ 72 ] analysed the sensitivity of the final LCA results to different allocation approaches in palm biodiesel evaluation. They adopted three different allocation methods (mass allocation, energy allocation and economic allocation) and stated that the environmental impacts estimated with energy and economic allocation were higher than those obtained with mass allocation. Our summary in Table 3 presents a lack of robustness analyses in the biodiesel LCAs, i.e. sensitivity analyses not presented in most of the published work.

A number of research articles have been published on the evaluation of environmental performance of biodiesel and its use by considering various feedstock and alternative production technologies. Following sub-sections discuss in detail the environmental performance of biodiesel utilizing various feedstock and different production technologies.

Environmental performance of biodiesel using various feedstock

A variety of feedstock can be utilized for biodiesel production that offers environmental benefits based on their requirements for agriculture, transportation and several other conditions. The feedstock assessed for biodiesel environmental performance through its life cycle includes first, second and third generation feedstock along with waste oils and fats (see Table 3 ). Hou et al. [ 60 ] conducted a comprehensive LCA of biodiesel from different feedstock (soybean, jatropha, microalgae) and compared the environmental performance with conventional diesel (fossil-derived). Among different feedstock, microalgae come out as more feasible alternative in terms of terrestrial eco-toxicity potential (TEP) and fresh water aquatic ecotoxicity potential (FWAEP) due to lower agriculture inputs. Hou et al. [ 60 ] found that FWAEP that is caused by agricultural process contributed 92%, 43.9% and 91% to the total environmental burden in the life cycle of jatropha, microalgae, and soybean-based biodiesel, respectively. In comparison to conventional diesel, biodiesel performed better in terms of global warming potential (GWP), ozone layer depletion (ODP) and abiotic depletion (ADP), but showed worse performance in acidification, eutrophication, photochemical oxidation, and toxicity [ 60 ]. The better performance of biodiesel in ADP, GWP and ODP is principally due to CO 2 uptake and solar energy from the environment through photosynthesis during the biomass agriculture. In another study, the environmental performance of second-generation biodiesel was compared with waste oil-based biodiesel [ 65 ]. When non-edible oil from jatropha is compared with waste cooking oil for biodiesel production, the latter showed lower environmental impact to all damage categories (climate change, human health and ecosystem quality). The inferiority of jatropha-based biodiesel in environmental performance is attributed to fertilizers, chemicals, water and land requirements for biomass cultivation and harvesting [ 65 ]. However, waste cooking oil-based biodiesel showed severe environmental impact for damage categories of resources (including mineral extraction and non-renewable energy demand). The total burden on the environment was 74% lower in case of utilizing waste vegetable oil as a feedstock compared to jatropha oil [ 65 ].

Further to compare environmental impact of a variety of waste feedstock, Dufour et al. [ 73 ] adopted well-to-gate analysis of feedstocks including beef tallow, sewage sludge, poultry fat and waste vegetable oil. The scope of the study was further extended by conducting well-to-wheel analysis of first-generation feedstock (soybean and rapeseed) to compare the impacts of waste oil derived biodiesel with first generation and conventional diesel. When these findings were compared, results elucidated the environmental superiority of FFA-rich materials derived biodiesel compared to both first-generation biodiesel and conventional diesel. While, among FFA-rich feedstock, waste vegetable oil showed better environmental performance in terms of GHG savings [ 73 ]. It can be conferred from the above discussion that waste oils are paramount encouraging feedstock for biodiesel production.

Environmental performance of biodiesel from chemically catalysed technological routes

Besides comparing different potential feedstock, LCAs were also conducted on the perspective of comparing different technological pathways for biodiesel production. One of such study was conducted by Morais et al. [ 74 ] to evaluate environmental viability of biodiesel produced from three technological alternatives including non-catalytic process (supercritical) with propane as a co-solvent, acid-catalysed process, and traditional alkali-catalysed process with acid pre-treatment. For each of the alternative technology, depletion of abiotic resources and marine aquatic ecotoxicity potential were found the most relevant environmental impact categories. Methanol that is used as a raw material in all alternative processes, significantly contributed to the depletion of abiotic resources since it is synthesized from fossil resources. Compared to methanol, ethanol could be a preferred option due to its renewable origin. That is, ethanol is responsible for absorbing significant amount of CO 2 , decreasing significantly the GHG effect of the manufacturing system [ 75 ]. Beside this, non-catalytic (supercritical conditions) route using propane as a co-solvent is relatively more environmentally favourable process [ 74 ]. This is because of the absence of catalyst and its lower steam consumption compared to other process.

While, the acid-catalysed route generally causes the highest environmental impact, mainly due to high energy profile related with methanol recovery operation. Compared to alkali-catalysed process, the supercritical non-catalytic process was reported to reduce the acidification by 754%, abiotic resource reduction by 313%, marine aquatic ecotoxicity by 793%, and global warming by 496% [ 74 ]. When the environmental impact of alkali catalyst (potassium hydroxide and sodium hydroxide) is compared, sodium hydroxide (NaOH) exhibited greater environmental impact on ecosystem quality and human health [ 76 ]. This can be explained by the sodium hydroxide that is an environmental hazardous material as compared to potassium hydroxide (KOH). Moreover, NaOH produces water-soluble salts on neutralization with acid and KOH precipitated to potassium sulphate by reacting with sulphuric acid. Salt precipitation decrease the overall water consumption and discharge of polluted water to environment, while this is not the case in using NaOH [ 23 ].

Environmental performance of biodiesel from enzyme-catalysed technological routes

In contrast to aforementioned studies, many researchers evaluated enzymatic technology for biodiesel production in their LCAs and reported that this technology has potentially lower environmental impact as compared to chemical catalytic technologies. For example, using biocatalyst (phospholipase) for degumming vegetable oils could reduce 44 tonnes of equivalent CO 2 per 1000 tonnes of oil produced because of high efficiency and low raw material requirement [ 77 ]. To further elaborate the environmental benefits offered by enzymatic production of biodiesel, LCAs were conducted to compare enzymatic process with alkali-catalysed process. These studies showed that enzyme-catalysed process outperforms the alkali-catalysed process in each measure of potential impact categories including human toxicity, global warming, and depletion of ozone layer [ 33 , 70 ]. Ravindra et al. [ 70 ] compared the results for both processes based on the single score and final total score. The single score result pointed out that, for both processes, the land use contributes the most to the environmental impact (75% for enzyme-catalysed and 70% for alkali-catalysed). However, the total score indicated less contribution to the total environmental impact by the enzyme-catalysed process [ 70 ]. Using immobilized enzyme instead of free enzyme in biodiesel production was found to further reduce the environmental burden on the processes [ 67 ]. This is because the reuse of immobilize lipase reduces consumption of minerals and carbohydrates needed for its soluble form production.

Overall, the enzymatic production technology provides significant reduction in environmental impacts compared to chemical-catalysed processes. However, photochemical ozone creation, global warming potential, terrestrial ecotoxicity and human toxicity potential are some of the impact categories in which enzymatic process shows almost same contribution as the conventional alkali-catalysed process [ 11 ]. These impact categories can be made lower for enzymatic process when the agriculture stage is avoided and a low-cost waste vegetable oil is used as a feedstock. In a study, it was estimated that for one tonne biodiesel production, 1775, 1633 and 383 kg of CO 2 eq is emitted to the atmosphere by alkali-catalysed, enzyme-catalysed, and enzyme-catalysed using waste cooking oil, respectively [ 11 ]. The latter process shows significant reduction in greenhouse gas emissions. Figure  10 shows greenhouse gas emissions for biodiesel in the surveyed LCA studies in this review (see Table 3 ). Generally, GHG emissions range from 0.51 × 10 –4 to 0.11 kg CO 2 eq/MJ of biodiesel, which is in most cases lower than the conventional diesel ensuring net GHG reductions for using biodiesel as a substitute to petro-diesel. The variation in GHG emissions with the same technology and utilizing the same feedstock can be attributed to the variation in the system boundaries, allocation methods and other methodological assumptions. For most of the cases, enzymatic processes show considerable reduction in GHG emissions compared to chemical-catalysed processes, which is probably due to the decrease in energy consumption. Comprehensively, it is inferred that the enzymatic process is more environmental benign process as compared to the chemical-catalysed processes.

GHG emissions in surveyed biodiesel life cycle studies [ 11 , 29 , 60 , 67 , 73 , 75 ] (conventional diesel [ 68 ])

• Optimization

The biodiesel production is an inherently complex system involving diverse feedstock, a number of technological alternatives, and various separation/purification sequences and conditions that require optimization on several aspects. In such complexity, conflicting design criteria can be concerned such as cost effectiveness and environmental sustainability. This section focuses on the optimization of biodiesel production considering the system complexity and sustainable design criteria, e.g. profit maximization, cost minimization, and environmental impact minimization.

Single-objective and multi-objective optimization

Optimization approach has been applied to biodiesel production system at both processes and value chain design levels, which provides solutions and insights into decision-support. In previous research, as summarized in Table 4 , a range of tools (Aspen Plus/HYSYS, SuperPro Designer, MATLAB, Excel) has been adopted to optimize biodiesel production process for multiple objectives. The methodology in these works is based on the implementation of the process model in the commercial process simulators (e.g. Aspen PLUS/HYSYS, SuperPro Designer) that are coupled with the multi-objective optimization (MOO) algorithm solving the process model for multiple objectives. Patle et al. [ 79 ] used Non-Dominated Sorted Genetic Algorithm (NSGA) executed in Excel. The algorithm was linked to rigorous process simulation in Aspen Plus for MOO of the two different continuous biodiesel manufacturing processes. The link and communication between Excel-based MOO programme and Aspen Plus was established via visual basic application (VBA). The optimization problem was solved for multiple objectives including profit, heat duty and organic wastes. The obtained results enabled them to decide the best production technology for a specific weighting of objectives.

Similarly, Woinaroschy et al. [ 80 ] presented the multi-objective optimization of biodiesel production process considering three objectives (profit, volatile organic emissions, and number of jobs) for optimization. They used multi-objective genetic algorithm (MOGA) implemented in MATLAB that are linked with rigorous process simulation in SuperPro Designer. The Component Object Module (COM) feature of SuperPro Designer and MATLAB Graphical User Interface (GUI) were used to establish the link between SuperPro Designer and MATLAB for the data transfer. In this work, all the three pillars of sustainability (Environmental, economic, and social) were evaluated and optimized simultaneously. The evolutionary algorithms applied in these works perform well and have attractive properties when integrated with the process simulator. However, regarding the complexity of biodiesel production process, careful analysis of numerous problems is needed, including the constraints integration; entire flowsheet initialization; decision variables and their boundaries selection. Moreover, these algorithms face difficulty in applications where the model flowsheet demands a long time for convergence or fails to converge due to some values of decision variables suggested by the optimization algorithm as iterations proceeds [ 81 ]. Sharma et al. [ 82 ] adopted an attractive alternative to overwhelm these curbs and further reduced the computational means used during optimization. They used multi-objective differential evaluation algorithm with taboo list (MODE-TL) for the optimization of biodiesel production process from used vegetable oil [ 82 ]. Generally, the literature summarized in Table 4 , adopted sequential modular simulation approach relying highly on the detailed equations for unit operations, which impedes the smooth application of derivative-based deterministic optimization solvers. Therefore, derivative-free optimization algorithms are mostly attractive due to their performance efficiency in discontinuous, non-differentiable, or highly non-linear expressions.

Biodiesel process synthesis and optimization

Mathematical programming has been widely applied to optimize biodiesel process synthesis problem, considering the economic trade-offs and interactions among subsystems. By far the most systematically considered synthesis problems in biodiesel production is the heat exchanger networks synthesis, separation sequences, and superstructure optimization for alternative technologies [ 41 , 86 , 87 ]. This methodology was implemented by Martin et al. [ 42 ] to perform superstructure optimization of biodiesel manufacturing from microalgae and waste vegetable oil. A mixed integer non-linear programme (MINLP) is formulated and solved for five different technologies including enzymatic catalysis, alkali catalysis, acid catalysis, heterogeneous basic-catalysed, and under non-catalytic conditions (supercritical). The superstructure optimization was performed to find out the best option among different alternatives. The results indicated that when waste vegetable oil is utilized, the enzymatic technology is the best option yielding biodiesel with a production cost of approximately 0.6 US$/gallon, energy and water consumption of 1.9 MJ/gallon and 0.3 gallon/gallon of biodiesel, respectively. Similarly, for microalgae the best production process was the alkali-catalysed with a production price of 0.4 US$/gallon of biodiesel requiring 0.60 gallon/gallon of water and 1.94 MJ/gallon of energy. Similar to Martin et al. [ 42 ], Mansouri et al. [ 88 ] also used superstructure optimization but they included process intensification options in their framework to model biodiesel production from pure and waste palm oil. A more comprehensive superstructure optimization for process synthesis of microalgae based biodiesel was performed by Rizwan et al. [ 87 ]. They included all the possible alternatives for microalgae harvesting, pre-treatment and lipid extraction along with the possible alternatives for the transesterification technologies in their superstructure mapping.

System and value chain optimization

Supply chain management (SCM) is relatively new optimization area that targets to integrate production plants with their suppliers and customers in an effective manner [ 89 ]. For biodiesel production, optimal design, management and integration of supply’s operations, manufacturing as well as distribution activities (entire supply chain) are crucial, to hasten transition towards large-scale and economically sustainable biodiesel [ 90 ]. Generally, biodiesel supply chains are multi-echelon networks including feedstock facilities, feedstock collection and pre-processing facilities, biodiesel production facilities, biodiesel distribution centres, and biodiesel consumers [ 91 ]. In addition, logistic framework is managed to facilitate efficient and substantial material flow between different echelons within the network. The most critical and important decisions for biodiesel supply chain network design are the location and optimum number of facilities, volumes of facilities, technological options, suitable logistics and carriage means, and optimum material flow.

Table 5 summarizes the state-of-the-art literature based on the type of feedstock used, decision variables addressed, uncertainty consideration, optimization approach used. Developing the first-generation biodiesel intensify the food crises. Therefore, design for second-generation biodiesel supply chains from waste oils and non-edible energy crops was focused [ 89 ]. Moreover, researchers carried out supply chain design for hybrid first, second and third generation biodiesel with consideration of land competition between edible and in-edible energy crops [ 92 , 93 ]. An optimal design of biodiesel supply chain using multi-period mixed integer linear programming (MILP) model was developed in Argentina that considers land competition among different feedstock including soybean, sunflower and jatropha [ 92 ]. The results indicated that jatropha serves as a more promising feedstock alternative to edible or more valuable feedstock for biodiesel production.

Biodiesel supply chain has been developed considering different aspects of strategic level such as technology selection, location of facility and capacity determination [ 94 ]. Mixed integer programming (MIP) is mostly applied to solve biodiesel supply chain design and optimization problems. Considering strategic level decision-making, Leao et al. [ 95 ] formulated a MILP mathematical model to design biodiesel supply chain networks in Brazil. The model considered agricultural, logistics, social as well as industrial aspects for biodiesel manufacturing from castor oil. Supply chain networks for biodiesel were also designed for 2nd and 3rd generation feedstocks on the strategic level [ 96 , 97 ]. Hombach et al. [ 97 ] exploited 2nd generation feedstock such as sawmill wastes, agricultural residues, and forest residues in their supply chain model incorporated with European biofuel regulations. To prevent sub-optimal solutions, tactical level decisions (like inventory level and production capacity in different periods) can be incorporated with the strategic level decisions. In this regard, Babazadeh et al. [ 93 ] designed biodiesel supply chain network by integrating both tactic and strategic level decisions in the supply chain model. Apart from minimizing the environmental burden of all the processes involved, the proposed model was effective only in minimizing the cost of biodiesel supply chain from feedstocks supply centres to consumers. As a result, high investment cost is obligatory to reduce the environmental burden. Although the integrated model prevented sub-optimal solution, it increased the level of complexity and in consequence needed more computational efforts than the non-integrated one [ 93 ].

The aforementioned studies mostly presented deterministic models by assuming known parameters in the supply chain network model. However, uncertainty is an intrinsic portion of every genuine system and can seriously pose the decision-making process. Overall, uncertainty of biodiesel supply chains can be classified as process uncertainty, demand and supply uncertainty [ 90 ]. Dal-Mass et al. [ 98 ] considered price uncertainty in designing biomass supply problem by describing distinct scenarios for price variations. In variance to Dal-Mas et al. [ 98 ], Kim et al. [ 99 ] considered all the three categories of uncertainty in biodiesel supply chain optimization. Shayan et al. [ 100 ] presented a two-stage robust MILP model under variant uncertainty sets. The model considered biodiesel demand, cost parameters, uncertainty in resource supply. When the decision-maker needs to cope with uncertainty but without sufficient historical data, the robust programming approach could be applied. In this context, Babazadeh et al. [ 101 ] presented a possibilistic programming approach to design a biodiesel supply chain network sourcing from waste cooking oil and jatropha. They addressed both cost and environmental uncertainties in a novel possibilistic programming, structured as MINLP model.

The above discussion suggests that supply chain optimization has been studied systematically at both tactical and strategic levels. Moreover, cost criteria is the most considered objective function considered so far (Table 5 ). Conversely, the social and environmental apprehensions are often overlooked. Moreover, research challenge in addressing the uncertainties in biodiesel supply chain design remains open. Through a thorough literature review on wider biofuel and bioproduct systems beyond biodiesel, a range of promising supply chain optimization research has emerged which deserves future research attention in biodiesel system optimization:

demand-driven supply chain integration, in particular biodiesel with value-added platform chemicals derived from the same oil feedstock;

supply-driven supply chain integration for multiple oil feedstock streams with similar processability, e.g. terrestrial oil crops integration with algae;

waste value chain design under uncertainty considering the high variance in waste oil stream composition and supply;

sustainable value chain optimization for biodiesel systems considering conflicting sustainability design criteria applying life cycle approaches.

Conclusion/future prospects

Recent developments in biodiesel production suggest that the production of biodiesel offers evident environmental benefits but its economic competitiveness highly depend on feedstock sources, technological choice and production capacity. Further research is necessary in modelling areas to enable a sustainable biodiesel production. Our literature review also highlights several frontiers for future research and developments.

Due to the dominant role of feedstock in cost profiling, the selection of the low-cost feedstock is of importance for the development of economically feasible yet sustainable biodiesel production process. A life cycle approach, which addresses economic and environmental aspects, offers a holistic evaluation to highlight the improvement spaces and screen the suitable feedstock and technology options. Moreover, LCSA accounts for three sustainability aspects and provides systematic insights into decision spaces; LCSA could enable further investigation and decision to be effectively focused on the major hot-spots that can be further optimized to achieve sustainability trade-offs.

The biodiesel production process requires in-depth investigation to tackle multi-scale multi-criteria design challenge. Our literature review suggests that supply chain and process network optimization are generally based on discretized time intervals, which consider process design scenarios. Such approach represents a trade-off between solution quality and computational complexity. Surrogate-based optimization could reduce the computational complexity. Specifically, surrogate modelling techniques could be applied which follow a black-box or grey box approach and use first-principle modes as a source of computational experiments; the generated sample data points can be fit into surrogate functions to represent the accuracy of first-principle modelling and project process performances. This will enable the inclusion of technology alternatives (surrogate models) and resources for biodiesel production in a multi-objective optimization framework, considering decision variables and sustainability criteria at both process and network levels.

Despite the supply chain optimization research, much attention has been placed on the long-term planning. Mid- or short-term production scheduling problems emerged as a research gap in response to recent digital technology and data advances (Internet of Things, Smart of Machinery, Big Data). Such advances enable real-time data collection and have the potential to catalyse transformation of biodiesel refinery towards batch manufacturing modes. Thereby, batch scheduling to enable ‘production-on-demand’ biodiesel refinery represents an interesting direction.

Deterministic optimization has been the research focus, whereas biodiesel optimization under uncertainty emerged as an interesting research direction. Particularly, biodiesel is sourced from natural sustainable resources and relies on policy intervention (e.g. green technology deployment policy); thus, its production is regulated by seasonal variables and other uncertain factors. Under this context, the uncertainty performances of biodiesel production at single sites and multi-sites would be of particular interests. The potential uncertainty indicators include responsiveness and resilience. Responsiveness considers the biodiesel production performances in response to operational uncertainties (e.g. feedstock supply and diesel demand fluctuation); whereas the resilience indicates the system capacity to recover, adapt facing the unexpected external disruption (e.g. natural extreme events or policy intervention). Responsiveness and resilience in biodiesel production design has not yet been explored. By integrating the risk mitigation and resilience-building measures into the stochastic and/or robust optimization, precision decision-making presents a future optimization direction for biodiesel research.

Availability of data and materials

The data supporting the results of the article are included in this manuscript.

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Acknowledgements

The authors express their gratitude to the support from China–Latin America Joint Laboratory for Clean Energy and Climate Change (KY201501004), and Dongguan Science & Technology Bureau (Innovative R&D Team Leadership of Dongguan City, 201536000100033).

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Pasha, M.K., Dai, L., Liu, D. et al. An overview to process design, simulation and sustainability evaluation of biodiesel production. Biotechnol Biofuels 14 , 129 (2021). https://doi.org/10.1186/s13068-021-01977-z

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A Comprehensive Review of the Impact of Nano-Catalysts on Biodiesel Production

• Published: 02 September 2024

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• Christopher Selvam Damian 1 &
• Yuvarajan Devarajan 1  

This review evaluates the potential of nanocatalysts to enhance the efficiency and sustainability of biodiesel production from nonedible feedstocks such as waste cooking oil, animal fats, and non-food crops. It highlights the economic and environmental benefits of using nanocatalysts, addressing challenges like high costs, low conversion rates, and complex purification processes, while avoiding competition with food resources.

The review provides a thorough analysis of various nanocatalyst types, including metal oxides, magnetic nanoparticles, and carbon-based materials, and their roles in biodiesel production. It also assesses the environmental and economic impacts of using nanocatalysts compared to traditional methods, drawing on recent research and case studies to evaluate their effectiveness, catalytic performance, and ease of separation, along with the challenges of synthesis and handling.

The review finds that nanocatalysts significantly improve biodiesel production by offering larger surface areas, increased catalytic activity, and simplified separation processes. These advantages lead to higher conversion rates, reduced purification complexity, and potential cost savings. However, challenges such as high production costs and the need for safe nanoparticle handling remain.

Nanocatalysts present a promising solution to the challenges of traditional biodiesel production, with the potential to enhance efficiency, environmental friendliness, and cost-effectiveness. Despite current obstacles, such as high production costs and stringent safety requirements, nanocatalysts could revolutionize the biodiesel industry. The review suggests future research should focus on improving nanocatalyst synthesis and recovery techniques, as well as exploring alternative nonedible feedstocks to further improve the sustainability and economic feasibility of biodiesel production.

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Damian, C.S., Devarajan, Y. A Comprehensive Review of the Impact of Nano-Catalysts on Biodiesel Production. J. Biosyst. Eng. (2024). https://doi.org/10.1007/s42853-024-00234-z

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• Published: 12 December 2019

Optimized Biodiesel Production from Waste Cooking Oil (WCO) using Calcium Oxide (CaO) Nano-catalyst

• Tadesse Anbessie Degfie 1 , 2 ,
• Tadios Tesfaye Mamo 1 , 3 &
• Yedilfana Setarge Mekonnen 1  

Scientific Reports volume  9 , Article number:  18982 ( 2019 ) Cite this article

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Biodiesel production from waste cooking oil (WCO) provides an alternative energy means of producing liquid fuels from biomass for various uses. Biodiesel production by recycling WCO and methanol in the presence of calcium oxide (CaO) nano-catalyst offers several benefits such as economic, environmental and waste management. A nano-catalyst of CaO was synthesized by thermal-decomposition method and calcinated at 500 °C followed by characterization using x-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. The XRD results revealed nano-scale crystal sizes at high purity, with a mean particle size of ~29 nm. The SEM images exhibited morphology of irregular shapes and porous structure of the synthesized nanocatalysts. The highest conversion of WCO to biodiesel was estimated to be 96%, at optimized experimental conditions i.e., 50 °C, 1:8 WCO oil to methanol ratio, 1% by weight of catalyst loading rate and 90 minutes reaction time, which is among few highest conversions reported so far. Biodiesel properties were tested according to the American (ASTM D6571) fuel standards. All reactions are carried-out under atmospheric pressure and 1500 rpm of agitation.

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Introduction.

Accessibility of energy sources and climate change are the two biggest challenges that mankind facing in this century. The fast-growing population and the increasing prosperity have led to rapid rise in the energy demand. Human civilization predominantly depends on the utilization of energy, it plays a big role in socio-economic development by improving the standard of living. Energy is vital for the economic development of every country. Every sector of the economy such as agriculture, industry, transport, commercial and domestic sectors require energy 1 , 2 , 3 , 4 , 5 . Fossil fuel-based fuel sources such as petroleum, coal and natural gas have been the predominant sources of energy all over the world for a long time. The majority of the world energy needs, about 81.1% is supplied through petrochemical sources such as coal, oil and natural gas. Nuclear, hydro, biofuel and other renewable energy systems account only 18.9% 6 .

The high energy demand in the industrialized world as well as in the domestic sector had caused environmental pollution problems due to their widespread use of fossil fuels. Fossil fuel combustion has several public health risks and environmental problems which extend to universal and potentially irreversible consequences on global warming 1 , 7 . As a result, the concerns about environmental impacts have increased and trigged the examination of alternative energy sources. Typical forms of renewable energy include wind power, hydropower, solar energy, biomass and biofuels. The contribution of all these resources is important because of the economic and environmental reasons, and biodiesel could be one of the solutions 8 .

Biodiesel is a substitute to diesel fuel derived from the triglycerides of vegetable oils or animal fats. Biodiesel can be produce from various vegetable oils such as palm oil, sunflower, soybean, rapeseed and castor oil using different types of catalysts 1 , 4 , 5 , 9 , 10 , 11 . In this article, the phrase “waste cooking oil” refers to edible oil which has formerly been used for frying in restaurants and hotels, and no longer be used for similar purpose. In most towns in developing countries including in Ethiopia waste cooking oil simply dumping into the environment. This causes serious environmental, social, economic and health problems to the society 11 , 12 , 13 , 14 , 15 . Improper or poor waste cooking oil disposal into water bodies rises the level of organic pollutants in the water. This significantly lowers the water quality and consequently affects the life of fish stocks, other aquatic living things and the surrounding community 11 .

Biodiesel production from WCO is environmentally friendly for it recycles waste cooking oil and gives renewable energy with lower pollution. It substitutes some amount of petrochemical oil import and also lowers the cost of waste management. Biodiesel production from waste cooking oil has three solutions those are economic, environmental and waste management 7 , 8 , 16 , 17 . Heterogeneous catalysts have attracted great attention in recent times for use in the biodiesel production 18 . The need for development of heterogeneous catalysts has risen due to the fact that homogeneous catalysts used for biodiesel production pose some limitations. These drawbacks include; washing of products with water to remove catalyst from the products which results in waste water generation and loss of biodiesel as a result of washing, the use of intensive biodiesel separation protocol, the corrosive nature of the catalysts and impossibility of catalyst reuse. Heterogeneous catalysts have also the advantages of easy separation from the product, reusability and eco-friendly. Calcium oxide nanoparticle has a higher basicity, non-corrosive, can be synthesized with a lower price, lower solubility and easier to handle than homogenous catalysts. In addition to these advantages, its being safe to the ecosystem made it an interesting choice for a catalyst 4 , 5 , 8 , 10 , 11 , 19 , 20 , 21 , 22 , 23 .

The main purpose of this research work is to enhance the production of biodiesel from waste cooking oil feed stock using nanoparticles of CaO and optimizing the major transesterification reaction parameters. In this study, nano sized calcium oxide (CaO) catalyst is synthesized with high purity using thermal decomposition method and characterized using XRD and SEM techniques. The biodiesel production reaction parameters such as WCO to methanol ratio, catalyst dose and reaction temperature were optimized for optimum biodiesel yield at laboratory scale.

Methodology

Waste cooking oil sample preparation.

Waste cooking palm oil was collected from café, restaurants and street fast food sellers in Addis Ababa city which has been used for food frying. The waste cooking oil was settled for 4−6 days at room temperature and pressure and later filtered by sieves of hole size 100 nm to remove any suspended food particles and inorganic residues and followed by heating at 110 °C for water removal.

Nano-catalyst synthesis

CaO nano-catalyst was prepared by thermal decomposition method following the procedure of Zhen-Xing Tang and David Claveau 24 . A nitrate solution was prepared by mixing 11.81 g of calcium nitrate tetrahydrate (Ca (NO 3 ) 2 .4H 2 O) was dissolved in 25 ml of ethylene glycol solution and 2.10 g of sodium hydroxide was added into above mixture under vigorous stirring. In order to get uniform size nanoparticles, after it has been stirred for 10 min, the gel solution was kept about 5 hours at static state. Then it was washed using distilled water followed by vacuum drying. Finally, different sizes of CaO nano-particles were obtained after calcination at 500 °C.

Catalyst characterization

The synthesized catalyst properties were characterized by X-ray diffraction (XRD) for identification of major components and for the determination of crystallite size. XRD analysis was performed with Mini Flex 600 × -ray diffraction (XRD) system with Ni filtered CuKα radiation at λ = 0.154 nm and Scanning electron microscope (SEM) JSM-IT300 LV was used to study the morphology of the synthesized catalyst.

Transesterification process

The biodiesel production from waste cooking oil with methanol in the presence of nano-sized calcium oxide nano-catalyst was done at a laboratory scale. Transesterification reaction is carried out in a flask with overall volume of 300 ml flask was placed on a hot plate equipped with a controlled magnetic stirrer and temperature sensor. Waste cooking oil was preheated to the required reaction temperature before methanol and the catalyst were added into the reaction flask. The calculated amount of methanol to oil ratio was poured into the reactor. Then the CaO catalyst was added in a range between 0.5 to 5% by weight with respect to mass of the WCO, and then the formed reaction mixture was mixed for 10 minutes. 100 ml of waste cooking oil was added and temperature of the mixture was set from 30 to 70 °C, 5 °C interval. Transesterification proceeded under continuous stirring of the reaction mixture for a desired duration.

All transesterification reactions were carried-out at atmospheric pressure with stirring speed of 1500 rpm. Thermometer was inserted into the flask to monitor the reaction temperature. After the completion of the reaction, the mixture was transferred into a separating funnel and allowed to stand overnight. Three phases were formed due to the solid catalyst and glycerol is denser than biodiesel.

Biodiesel characterization

The separated biodiesel was heated above the boiling point of methanol (64.7 °C) to remove excess unreacted methanol. Moreover, very few suspended solid catalysts are removed by settling it for two to three days then the Biodiesel viscosity, specific gravity, water and sediment, total acidity, ash content, sulfur content, Flash Point and Cloud Point were checked according to the American Society for Testing and Materials (ASTM D 6751).

Results and Discussion

Wco characterization.

The frying process changes the chemical and physical characteristics of the oil because in the frying process many chemical reactions were carried like hydrolysis, polymerization, oxidation and material transfer between oil and food. The collected WCO sample physic-chemical properties are reported in the Table  1 .

XRD analysis

As the XRD diffraction intensity (pattern) of CaO nanoparticle were present in Fig.  1 and the 2θ value of the synthesized CaO Catalyst was seen in the range 15–70°.

XRD result of the synthesized CaO nano-catalyst.

As can be seen in Fig.  1 , the sharp spectra revealed high crystallinity of the powder. The sharp peaks were exhibited at 2-theta (2θ) of 32.25°, 37.41°, 43.03°, 53.92° and 64.2°. The crystallite size diameter (D) in nanometer of the CaO nanoparticle were calculated by using Debye Scherrer equation (D = Kλ/β cos θ) and as it was seen in Table  2 the particle size of the synthesized CaO lies between 27.02 nm and 31.21 nm, with the mean crystal size of 29.072 nm.

SEM analysis

The Scanning Electron Microscopy (SEM) analysis was performed at 50 μm, 10 μm and 5 μm magnifications as shown in Fig.  2(a–c) , respectively. According to the SEM images, the prepared CaO nano-catalyst typically comprises irregular shape of particles, porous in structure and possesses active sites. In other words, there were various sizes and shapes of particles, which indicate that the catalyst has bigger surface area for reaction.

( a – c ) shows the SEM images of synthesized CaO nano-catalyst at 50 μm, 10 μm and 5 μm magnification, respectively.

Optimization of reaction parameters

Catalysts loading.

To take in to consideration of the catalyst impact on the biodiesel yield amount a baseline reaction without catalyst were carried at reaction temperature at 50 °C, reaction time at 90 minutes and oil to methanol ratio at 1:8 reactions and it gives no biodiesel. Hence, presence and amount of Catalyst used plays a vital role in the optimization process of biodiesel production in the transesterification reaction. In this research work the amount of catalyst used on the yield of biodiesel amount was investigated by varying the amount of the percentage mass of the catalyst range from 0.5% to 5% w/w with the mass of WCO and keeping constant the reaction temperature at 50 °C, reaction time at 90 minutes and oil to methanol ratio at 1:8 as shown in Fig.  3 .

Influence of the amount of CaO Catalyst (%) on biodiesel yield (%).

As it can be seen from the result the biodiesel yield increases as of the catalyst concentration increases from 0.5 to 1% w/w while further increment in catalyst loading concentration shows a decrement in Biodiesel yield. Accordingly, the optimal catalyst amount was note at 1% w/w catalyst loading with 96% biodiesel yield. The excess catalyst has slightly reduced the biodiesel yield because the excess catalyst amount reaction soap formation also increase and it hinders further biodiesel production 25 . Unlike other heterogeneous nano catalysts nano-CaO were prepared without much effort and it only needs preparation and activation by calcination of the prepared catalyst 26 . In addition, it is not expensive, environmental friendly, easy to handle, low solubility in organic solvents with high basicity and reusability nature 26 , 27 , 28 . Reusability of CaO was not done in this research work. However, in many researches it can be seen that the Nano-CaO catalyst can be reused with no significant catalytic decrement from three 26 to six 28 times.

WCO to methanol molar ratio

The influence of the variable oil to methanol molar ratio on the yield of biodiesel was studied for the ratios 1:4, 1:5, 1:6, 1:7, 1:8, 1:9 and 1:10. The stoichiometric molar ratio of triglyceride to methanol in the transesterification reaction is 1:3. So, 1:4 was taken as the starting value for oil to methanol ratio. Oil to methanol ratio has been varied from 1:4 to 1:10 by keeping constant the reaction temperature at 50 °C, reaction time at 90 minutes and a keeping the 1% optimum value of the catalyst amount with the mass of the WCO and the percentage change in yield has been observed.

As it is clearly shown in Fig.  4 that the molar ratio of oil to methanol has a substantial impact on the yield of biodiesel. When the molar ratio increases from 1:4 to 1:8 the biodiesel yields likewise increases. The optimal oil to methanol molar ratio was determined to be 1:8 with a biodiesel yield of 96%. In order to enhance the rate of methanolysis the amount of methanol must be found in excess to promote the formation of methoxy species on the surface of catalyst. This will shift the equilibrium towards the biodiesel formation. Moreover, the biodiesel yield slightly reduced when the oil to methanol molar ratio was higher than the optimal ratio, 1:8. Furthermore, the presence of excess alcohol in the product affects the quality of biodiesel fuel by reducing its viscosity, density and flash point 29 . Transesterification process yield glycerol as a by-product of the reaction. Glycerol is highly dissolve in the excessive methanol and later hinder the reaction of methanol to reactants and catalyst. Therefore, this makes the separation of glycerol from the product very challenging and allowing the equilibrium to shift in the reverse direction and consequently lowers the biodiesel yield.

The effect of WCO to methanol ratio range from 1:4 to 1:10 on biodiesel yield (%).

Reaction temperature

By keeping Methanol to oil molar ratio and catalyst loading constant on their optimum value (1:8 and 1%) and Varying the reaction temperature from 30 °C to 70 °C has given the result as shown in Fig.  5 . That show that the yield of biodiesel from waste cooking oil at different reaction temperature from 30 to 70 °C.

The effect of reaction temperature on biodiesel yield (%).

The yield of biodiesel increases as of the temperature increases till 50 °C which is the optimal point and it gives 96% biodiesel yield. Above this temperature the yield was decreased sharply and reaches 74% yield at 70 °C. The applied thermal energy must be sufficient enough in order to overcome the diffusion resistance developed within the three phases of the reaction mixture (i.e., oil-alcohol-catalyst). However, applying temperature beyond the optimal range is not preferred. Since as the temperature reaches around the boiling point of methanol, it will rapidly vaporize and produce a large number of bubbles, which hinders the reaction and consequences the decrease in the biodiesel yield.

Reaction time

Varying the reaction time from 30 to 130 minutes and keeping the molar ratio of methanol to oil ratio, the catalysts loading amount and the temperature constant on their optimum value has given the result as shown in Fig.  6 . That shows the yield of biodiesel from waste cooking oil through transesterification reaction at different reaction duration from 30 to 130 minutes.

The effect of reaction time (minute) on biodiesel yield (%).

In the first stages of the transesterification reaction the forward reaction or the production of the biodiesel was fast until it reaches equilibrium. However, the backward reaction was start after reactions carried beyond the optimal reaction duration. Hence, too long reaction duration reduces the biodiesel yield. in conjunction, determining the optimum reaction duration for transesterification is vital and in this research work case the optimum reaction duration was 90 minutes with a yield of 96% biodiesel.

Overall effects of reaction parameters

As shown in Fig.  7 , the experimental results revealed a maximum yield of 96.0% (w/w), therefore it was concluded that the maximum amount of biodiesel yield was gained at 1% (w/w) of catalyst loading, 1:8 oil to methanol molar ratio at 50 °C temperature and 90 minutes of reaction duration.

The overall effect of reaction parameters on biodiesel yield (%).

The produced biodiesel viscosity, specific gravity, water and sediment, total acidity, ash content and sulfur content were check according to ASTM D6571 and it is found to be in a good agreement, see Table  3 . Biodiesel from waste cooking oil could be used as a diesel fuel and for cleaner household energy source for cooking.

A CaO nano-catalyst, with a mean particle size of 29 nm, was synthesized by thermal decomposition method and used as a catalyst for biodiesel production in the transesterification process from WCO. The optimal biodiesel yield of 96% was achieved at optimized reaction conditions i.e., WCO to methanol molar ratio of 1:8, 1 wt. % of CaO nano-catalyst, 50 °C reaction temperature and 90 minutes reaction time. The produced biodiesel viscosity, specific gravity, water and sediment, total acidity, ash content and sulfur content were tested according to the American fuel standards (ASTM D 6571) and found in good agreement of the standard. Biodiesel from WCO could be used as a diesel fuel and for cleaner household energy source for cooking which was considered as renewable energy and environmental recycling process from waste vegetable oil after frying.

Change history

11 april 2023.

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Acknowledgements

This work was financially supported by Addis Ababa University through thematic research project (Grant number TR/012/2016).

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The authors have involved in different stages of this manuscript preparation first of all this manuscript is an MSc thesis work of Addis Ababa University Center for Environmental Science conducted by Mr. Tadesse Anbessie Degfie under the supervision of Dr. Yedilfana Setarge Mekonnen. Hence, the graphs, figures and tables were originally prepared by them. However, Baseline reation, preparation of this manuscript with some literature incorporation, adjustment of data has been done mainly by Mr. Tadios Tesfaye Mamo. Moreover, further refinement of the manuscript has been done by three of us. In this manuscript and research work no other persons were involved except helping us on some technical aspects. Authors would like to acknowledge Addis Ababa University for financial support through thematic research project (nr: TR/012/2016). Hence, we (the authors) want to declare no competing interests.

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Degfie, T.A., Mamo, T.T. & Mekonnen, Y.S. Optimized Biodiesel Production from Waste Cooking Oil (WCO) using Calcium Oxide (CaO) Nano-catalyst. Sci Rep 9 , 18982 (2019). https://doi.org/10.1038/s41598-019-55403-4

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Article Contents

1 introduction, 2 theoretical consideration, 3 experimental consideration, 4 results and discussion, 5 conclusions.

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Comparison of the performance and emissions of different biodiesel blends against petroleum diesel

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P. McCarthy, M.G. Rasul, S. Moazzem, Comparison of the performance and emissions of different biodiesel blends against petroleum diesel, International Journal of Low-Carbon Technologies , Volume 6, Issue 4, December 2011, Pages 255–260, https://doi.org/10.1093/ijlct/ctr012

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Biodiesel, an alternative fuel of petroleum diesel, is mainly used to reduce the environmental impact of emissions without modifying engines. This study compares the performance and emissions characteristics of different biodiesel blends with petroleum diesel using an internal combustion engine (Kubota V3300) and following ISO 8178 standards. Two types of biodiesel, type A (80% tallow and 20% canola oil methyl ester) and type B (70% chicken tallow and 30% waste cooking oil methyl ester), were tested in this study. It was found that the performance (mainly torque and brake power) of both biodiesel fuels reduces with increasing blend ratio which can be attributed to lower energy content of biodiesel. Specific fuel consumption increases for both biodiesels compared with diesel fuel, as expected. Some of the greenhouse gas emissions were found to be higher than petroleum diesel, whereas some were lower. Overall, Biodiesel A was found to produce lower emissions across the board compared with diesel and Biodiesel B.

It is well known that petroleum diesels are the major source of air pollutions that create an adverse impact on human health and overall greenhouse gases. Biodiesel has some great benefits over petroleum diesel, such as it produces 4.5 units of energy against every unit of fossil energy [ 1 , 2 ] and also it has some environment-friendly properties such as it is non-toxic, biodegradable and safer to breathe [ 3 ]. Biodiesel is also a clean-burning and stable fuel [ 3 ]. Properties of biodiesel such as oxygen content, cetane number, viscosity, density and heat value are greatly dependent on the sources (soybean, rapeseed or animal fats) of biodiesel [ 4 , 5 ]. Engine performance and emissions depend on the properties of biodiesels. Biodiesel is a highly oxygenated fuel that can improve combustion efficiency and can reduce unburnt hydrocarbons (HCs), carbon dioxide (CO 2 ), carbon monoxide (CO), sulphur dioxides (SO 2 ), nitric oxide (NO x ) and polycyclic aromatic HC emissions. However, brake-specific fuel consumption slightly increases [ 6 ].

Popularity of biodiesel as renewable sources of alternative fuel of petroleum diesel is growing quickly due to increased environmental awareness and the rising price of diesel. It is an earth-friendly choice of consumers that already occupies a great volume of the world's fuel sector due to its clean emission characteristics.

Developments of biodiesel fuels in many countries are driven by the necessity to reduce the greenhouse gas emissions which is the major issue for today's world, and the scarcity of the source of petroleum diesel also enhances the development and production of biodiesel fuel around the world. Biodiesel is generally produced from vegetable oils or animal fats through a chemical process known as transesterification process.

Vegetable oil was first used to run an engine by Rudolf Diesel (1858–1913) who developed the first engine. But sometimes, vegetable oils create adverse effects on engine components which may be due to their different volatility and molecular structure from diesel fuel as well as high viscosity compared with diesel fuel [ 4 , 5 , 7 ]. Currently, this problem is being eliminated by applying different chemical processes such as transesterification, supercritical, catalyst-free process etc., on vegetable oils to convert into biodiesel.

This paper aims to investigate the engine performances (power, torque, fuel consumption) and emissions (unburnt HCs, carbon dioxide, carbon monoxide and nitric oxide) of a diesel engine using two different biodiesels. Two different sources of biodiesel, type A [80% tallow (beef, pork and sheep) and 20% canola oil methyl ester] and type B (70% chicken tallow and 30% waste cooking oil methyl ester), were used for the experimentation in this study. Fuel types such as B5, B10, B20, B50 and B100 are analysed and discussed.

Specifications of Kubota V3300 [ 20 ].

Experimental set-up of Kubota V3300 Indirect Injection, four cylinders naturally aspirated CI engine.

The biodiesel–diesel blends that referred to as B5, B20, B50 and B100 were used in this study, where the percentage ratios of biodiesels are 5%, 20%, 50% and 100%, respectively. Two types of biodiesels were used in this study to blend with petroleum diesel, type A [80% tallow (beef, pork and sheep) and 20% canola oil methyl ester] and type B (70% chicken tallow and 30% waste).

ISO 8178 test procedure was used in this study which is an eight-mode steady-state test procedure that comprises three engine speeds, rated speed, intermediate speed and low idle for testing. The minimum test mode length of each mode is 10 min and emissions are measured in the last 3min of each mode. The engine is preconditioned by warming up the engine at its rated power for 40 min before each test cycle and minimum 50 data were taken for each mode in each test cycle, and three cycles are run per test fuel and then average. Experiments were done at both 2600 and 1400 rpm.

Fuel consumption of biodiesel is expected to be slightly higher than petroleum as density of the biodiesels is higher than petroleum diesel [ 11 ]. Sources of biodiesel greatly influence the engine performance, e.g. the engine fuelled with palm oil biodiesel is more efficient than biodiesel produced from tallow and canola oil [ 12 ]. Biodiesel is likely to produce less power with high fuel consumption than diesel as the gross calorific value (energy content) of biodiesel is lower than petroleum diesel. Blends of biodiesel with petroleum fuel are widely used in the diesel engine [ 13 ]. High viscosity of the fuels causes fuel flow and ignition problems in unmodified CI engines and also decreases the power output [ 11 , 14 ]. The lubricity and oxidative stability of the animal fat-based biodiesels are better than soy-based biodiesel [ 15 , 16 ]. The composition of animal fatty acid methyl esters is different from vegetable fatty acid methyl (ethyl) esters.

The results of performances and emissions of biodiesels tested in this study (Biodiesels A and B) compared with diesel are presented and discussed below.

4.1 Torque and power

Figure  2 shows the torque as a function of diesel and biodiesel blends for both Biodiesels A and B using modes 1 and 5 of the ISO 8178 test procedure. Mode 1 corresponds to the rated speed of the engine (2600 rpm) at 100% throttle, and mode 5 corresponds to the intermediate speed of the engine (1560 rpm) at 100% throttle. These two modes are the only ones that give a good indication of the differences in torque when using biodiesel, as the other modes require the torque to be set to a value (therefore reducing the throttle from 100%) which is the same for all test fuels. It can be seen from Figure  2 that the output torque decreases with increasing blend ratio for both biodiesels. The percentage decrease for both biodiesels at these modes is in the range of 4–5%. A decrease in this magnitude is to be expected, due to the lower energy content of biodiesel. The decrease in output torque at these two modes also affects the power output of the engine, since torque and power are directly proportional when the engine speed is fixed. As a result, the power output will also decrease by 4–5%. A decrease in both power and torque is due to their lower energy content of biodiesel.

Torque comparison for different biodiesel blends [B5(5% biodiesel 95% diesel), B20 (20% biodiesel 80% diesel), B50 (50% biodiesel 50% diesel) and B100 (100% biodiesel)] using Biodiesel A (80% beef, pork and sheep tallow and 20% waste cooking oil methyl ester) and Biodiesel B (70% chicken tallow and 30% waste cooking oil methyl ester).

4.2 Specific fuel consumption

Figure  3 compares the specific fuel consumption for the two biodiesels over the ISO 8178 test procedure. Even though this test procedure is designed to evaluate exhaust emissions, it can also be used in the same way to measure fuel consumption. During testing, the fuel flow rate at each mode was measured, and by using the weighting factors designated in the test procedure, a value for fuel consumption over the duration of the test was found, and averaged over the three tests for each fuel. Since the test procedure requires set values of torque and rpm, fuel consumption should be higher for a fuel with lower energy content.

Fuel consumption comparisons.

From Figure  3 , it can be seen that fuel consumption increases with blend ratio for both Biodiesels A and B. For Biodiesel A, the fuel consumption is 7% higher than diesel and for Biodiesel B, it is +10% higher which indicates that Biodiesel B has lower energy content than Biodiesel A and both biodiesels have lower energy content than diesel.

4.3 Exhaust emissions

Figure  4 compares the NO x emissions for Biodiesel A, Biodiesel B and diesel. Biodiesel A nitric oxide emissions show a decreasing trend with increasing blend ratio, whereas Biodiesel B emissions increase with the blend ratio. NO x emissions can increase or decrease depending on a number of factors such as biodiesel type, engine type and test procedure used. The US EPA reports a 10% increase in NO x emissions for B100 when compared with diesel.

Comparison of NO x emissions.

Figure  5 shows the carbon monoxide emissions for Biodiesels A and B over the ISO 8178 test procedure. Both biodiesels displayed a significant decrease in CO emissions with increasing blend ratio. For Biodiesel A, the decrease is ∼55% and for Biodiesel B, the decrease is ∼30%. This decrease fairly agrees with US EPA [ 4 ] who reported 51% decrease in CO emissions for biodiesel. This decrease could be attributed to the biodiesels having higher oxygen content than diesel which can result in a more complete combustion, leading to less CO in the exhaust stream.

Comparison of carbon monoxide emissions.

The HC emission results for the biodiesels are shown in Figure  6 . It can be seen that both Biodiesels A and B show an increase in HC emissions with increasing blend ratio. Conversely, the US EPA reports that HC should decrease with increasing blend ratio. It should be noted that the HCs measured during testing were very low (<0.002%). This brings into question the validity of these results, since other studies have found significantly higher levels of HCs in diesel exhaust emissions. These low readings could be attributed to a number of factors, one being that the EGA is optimized for measuring petrol engine exhaust emissions, not diesel. Petrol engine emissions contain different HCs to diesel engines, and higher concentrations of HC. If diesel/biodiesel HCs were to be measured accurately, a flame ionization detector would need to be used instead of the infrared sensor that was used for this testing, but this equipment is extremely expensive. Another explanation for inaccurate HC readings is that HC drift was occurring. Drift occurs when the emissions sample point is too far down the exhaust stream, which gives the HCs a chance to break down into other compounds such as carbon dioxide and water vapour. Since the sample point is ∼3m down the exhaust stream on the test rig, it is possible that this is sufficient distance for some of the HCs to break down; therefore, a reduced amount is actually being measured.

Comparison of HC emissions.

Figure  7 shows the carbon dioxide emissions for the biodiesels over the ISO 8178 test procedure. It can be seen that both biodiesels display an increase in CO 2 emissions with increasing blend ratios, although a decrease in CO 2 emissions was expected as CO emissions presented in Figure  5 . For Biodiesel A, the increase is ∼6% and for Biodiesel B, the increase is ∼18% compared with diesel. It is to be noted that CO 2 is a non-regulated emission (i.e. not limited), but is frequently measured when analysing exhaust gas emissions as it gives valuable clues on fuel consumption in dynamometer tests [ 17 ]. Studies have shown that biodiesel can decrease CO emissions up to 51%, whereas it can increase or decrease CO 2 emissions, with the percentage change ranging from −7% to +7% depending on the type of biodiesels [ 18 , 19 ]. In the current study, Biodiesel A clearly agrees with the literature findings both in terms of CO and CO 2 emissions; however, a higher increase in CO 2 emissions was found for Biodiesel B compared with literature findings. This difference can be considered as not very significant, as CO 2 emissions are not regulated. However, the specific reason for increase in CO 2 emission for both the biodiesels studied in this study (i.e. Biodiesels A and B) needs further investigation.

Comparison of carbon dioxide emissions.

4.4 Summary of discussion

The summary of discussion based on the experimental findings is outlined below.

Lower energy content of biodiesel results in the lower performance (torque and power). It shows a decrease in both power and torque for biodiesel fuels.

Emissions of HC and CO 2 from both biodiesels increase with increasing the amount of biodiesel in their blend, whereas CO emission decreases with increasing amount of biodiesel in the blend.

Fuel consumption for Biodiesel B is higher than Biodiesel A, and Biodiesel B has lower energy content than Biodiesel A. This indicates that fuel consumption is higher for fuel with lower energy content.

Biodiesel A has lower exhaust emissions and better performance compared with Biodiesel B.

NO x emission depends on a number of factors such as biodiesel type, engine type and test procedure used. In this experiment, Biodiesel A shows a decreasing trend with increasing blend ratio whereas Biodiesel B shows increasing trend with increasing blend ratio for NO x emission.

Biodiesels having higher oxygen content can lead to less CO emissions with increasing blend ratio due to complete combustion in the diesel engine.

A diesel engine fuelled with biodiesel can make complete combustion due to the presence of oxygen content in the molecule of biodiesel.

Fuel consumption of biodiesel is expected to be higher when engine fuelled with higher density biodiesel.

An engine fuelled with biodiesel containing higher cetane number and higher lubricity is more efficient.

Biodiesel with higher gross calorific value (energy content) produces higher power.

High viscosity of the biodiesel causes fuel flow and ignition problems in engines and decreases in power output.

The results of this study indicated that biodiesel is a more environmental-friendly option than petroleum diesel based on the reductions in CO and NO x in the tailpipe emissions. This comes at the cost of performance, though biodiesel has lower energy content than petroleum diesel. Biodiesel A (the 80% beef, pork and sheep tallow and 20% waste cooking oil methyl ester) was found to have lower exhaust emissions across the board compared with Biodiesel B (70% chicken tallow and 30% waste cooking oil methyl ester). Without knowing more about the exact fuel properties of these two fuels, such as ultimate analysis, it was difficult to draw any definitive conclusions about why emissions were higher for biodiesels. It is recommended that a follow-up study should be completed to further investigate the fuel properties of Biodiesels A and B in order to determine how the differences in chemical properties affect performance and emissions. Once these fuel properties data are obtained, it could be inputted into an appropriate engine simulation programme to analyse theoretical emissions data. If the model was found to be accurate enough, these theoretical data could be compared against the practical data found in this study, which would provide more insight into the performance and emissions of biodiesel fuels.

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Feedstock availability and aviation demand challenge biofuel adoption in shipping

• Maritime energy transition
• Fuel For Thought

Drop-in replacements for fossil fuels may be a cost-effective way of meeting environmental regulations and could extend the life of older vessels, if barriers to adoption can be overcome, according to a new LR report.

As a low-emission alternative fuel with low-CAPEX investment and the potential to expand the operational lifespan of large portions of the world fleet, biofuels may have an important role to play in shipping’s decarbonisation, but feedstock availability and demand competition from other transport sectors pose challenges that will need to be addressed for widespread adoption, finds Lloyd’s Register’s (LR) new Fuel for Thought: Biofuel report .

The report points to biofuels as a path to compliance with environmental regulations for ships for which retrofits to adopt other future fuels are not economically feasible. As ‘drop-in’ replacements for traditional fuels, biofuels require minimal changes to machinery and operations and offer GHG emissions savings of up to 84% compared to traditional fuels. 

The similarities between biofuels and their fossil equivalents, as well as the ability to blend biofuels with traditional fuels, makes adopting biofuels a comparatively straightforward process for shipowners compared to other alternative fuels. Biofuels are generally compatible with existing onboard machinery and fuel tanks, use the same bunkering infrastructure as fossil equivalents, and their similarity to traditional bunkers means training requirements for crew are minimal compared to other future fuels.

The most established products suitable for shipping are Fatty Acid Methyl Ester (FAME) and Hydrotreated Vegetable Oil (HVO), and novel fuels continue to be developed. Ship operators need to be alert to the individual characteristics of any given biofuel.

There are many types of biofuels produced through different processes using a wide range of feedstocks, variables that affect the GHG intensity of a fuel and that can raise operational considerations for machinery. Fuel for Thought: Biofuel details industry standards for FAME and HVO, common considerations for engines and machinery and when using biofuels, and a process for undertaking trials of novel and untested biofuels in marine engines.

The report states that the main challenges for widespread deployment are availability and demand competition from other transport sectors, including aviation, and the investment in biofuel production capacity that will be needed to meet the growing demand from the transport sector. The price of biodiesel blends is expected to rise alongside blending levels as feedstock prices are driven higher by demand.

Tim Wilson, Principal Specialist Fuels Lubes and Emissions, Lloyd’s Register, said : “Biofuels are unique among the future fuels for shipping as the vast majority of the world fleet is equipped with engines that can use them. As a drop-in replacement for fossil fuels, biofuels are an available and affordable method of reducing carbon emissions in the short term without large capital investment. The range of biofuel trials across ship segments and biofuel types reflect a strong level of interest from shipowners in their use onboard.”

Fuel for Thought: Biofuel gathers into one place the most relevant information on the use of biofuels in shipping, serving as a convenient reference for shipowners considering alternative fuel options for their fleets, and for maritime professionals seeking a deeper understanding of the zero-carbon transition. The report combines expertise from LR and other shipping knowledge leaders on topics including the characteristics and operational considerations for biofuels, regulatory drivers for biofuel adoption, techno-economic considerations, fuel quality and availability, and biofuel trials in shipping.

The report builds on the success of earlier Fuel for Thought reports, where LR is creating a one-stop repository for relevant information on all alternative fuels for the maritime industry. The report also contains information from LR’s recently updated Zero Carbon Fuel Monitor , an insight-based assessment of the readiness of biofuels and other zero carbon fuels for maritime applications.

Fuel for thought: Biofuel report

Marine biofuels - alternative shipping fuel.

Zero Carbon Fuel Monitor

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The Economic Inequality & Equitable Growth hub is a collection of research, analysis and convenings to help better understand economic inequality.

JEL classification: G01, G21, N20, N24

Authors: Ricardo Correa, Stephan Luck , and Emil Verner

Why do banks fail? We create a panel covering most commercial banks from 1865 through 2023 to study the history of failing banks in the United States. Failing banks are characterized by rising asset losses, deteriorating solvency, and an increasing reliance on expensive non-core funding. Commonalities across failing banks imply that failures are highly predictable using simple accounting metrics from publicly available financial statements. Predictability is high even in the absence of deposit insurance, when depositor runs were common. Bank-level fundamentals also forecast aggregate waves of bank failures during systemic banking crises. Altogether, our evidence suggests that the ultimate cause of bank failures and banking crises is almost always and everywhere a deterioration of bank fundamentals. Bank runs can be rejected as a plausible cause of failure for most failures in the history of the U.S. and are most commonly a consequence of imminent failure. Depositors tend to be slow to react to an increased risk of bank failure, even in the absence of deposit insurance.

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