Sustainable Biodiesel: Real-World Designs, Economics, and Applications

Sustainable Biodiesel: Real-World Designs, Economics, and Applications offers a unique, integrated approach that combines cutting-edge research results and the day-to-day aspects of biodiesel production at the industrial level. It brings together experienced academics and recognized industry experts to explore the most practical elements of research and discuss the limitations and future needs of the industry. The book critically reviews strategies for implementing biodiesel-based biorefineries, feedstock supply chains, reactor technologies, processes for biodiesel production, and biodiesel combustion, including advanced fuel formulations containing biodiesel. The authors examine biodiesel plants from the point of view of design, operation, quality control, and sustainability, including life cycle assessment (LCA) and life cycle costing (LCC). Policy and regulatory constraints in biodiesel production and commercialization as well as future trends and needs of the industry are also covered.

This book, as a volume of the "Biomass and Biofuels" series, provides researchers and practitioners in the field of biomass and biofuels with a well-rounded understanding of how the technologies developed in the lab can be deployed at commercial scale in a sustainable and cost-efficient way. This allows biofuels researchers to better develop technology that is fit for upscaling in an industrial setting and complies with sustainability goals. Practicing engineers, on the other hand, find in this volume up-to-date information on available technology, the latest advances, and future trends that will inform their decision-making when planning, implementing, and troubleshooting biodiesel-based bioenergy systems.

• Sheds light on the real-world aspects of biodiesel production while also covering the cutting-edge research results in the field
• Integrates design, economics, and sustainability aspects, minimizing the gap between theoretical knowledge and practical expertise, as well as between technical aspects and environmental and economic performances
• Includes realistic examples and case studies of applications of state-of-the-methodologies for life cycle assessment, life cycle impact assessment, and life cycle costing

• Language: English
• Publisher: Academic Press
• Release date: Jun 23, 2023
• ISBN: 9780128204856

Book preview

Preface

Meisam Tabatabaei¹ and Abdul-Sattar Nizami², ¹Higher Institution Centre of Excellence (HICoE), Institute of Tropical Aquaculture and Fisheries (AKUATROP), Universiti Malaysia Terengganu, Kuala Nerus, Terengganu, Malaysia, ²Sustainable Development Study Centre (SDSC), Government College University, Lahore, Pakistan

This book is about biodiesel and aims to offer a comprehensive overview of the fundamentals and technology applications that can enable the transition from research to industry. Biodiesel is arguably the most promising alternative to fossil diesel; however, to meet its full development and technology deployment at the industrial level, the gap between theoretical knowledge and practical expertise, as well as by regulatory constraints and economic viability, should be overcome.

The present book, which is the first book in the series on Biomass and Biofuels, is the result of the collaboration between experienced academics and recognized industry experts who have combined cutting-edge research results and the day-to-day aspects of biodiesel production at the industrial level. The 14 chapters critically review strategies for implementing biodiesel-based biorefineries, feedstock supply chains, reactor technologies, processes for biodiesel production, and biodiesel combustion, including advanced fuel formulations containing biodiesel. In addition, biodiesel plants from the point of view of design, operation, quality control, and sustainability, including life cycle assessment, life cycle impact assessment, and life cycle costing, are discussed. Sustainable Biodiesel: Real-World Designs, Economics, and Applications also covers the policy and regulatory constraints in biodiesel production and commercialization are also covered. In closing, the future trends and needs of the industry are presented and discussed. The book is intended for researchers, practitioners, industrial experts, and students in the field of biomass and biofuels who are interested in obtaining a well-rounded understanding of how the technologies developed in the lab can be deployed at commercial scale in a sustainable and cost-efficient way.

It is expected that the present volume on biodiesel would contribute to minimizing the gap between theoretical knowledge and practical expertise, as well as between technical aspects and environmental and economic performances. We are thankful to the authors of all the chapters for their efficient cooperation and for their readiness in revising the manuscripts. We also would like to extend our appreciation to the reviewers who despite their busy schedules assisted us by evaluating the manuscripts and provided their critical comments to improve the manuscripts. We would like to sincerely thank Dr. Peter Adamson and his team at Elsevier; Ms. Zsereena Rose Mampusti and Mr. Prasanna Kalyanaraman for their cooperation and efforts in producing this book. We are also grateful to Ms. Raquel Zanol, Ms. Joanna M. Collett, Mr. Chris Hockaday, and Mr. Jose Paolo R. Valeroso who were involved in the earlier stages of the development of this volume.

Chapter 1

Biodiesel: the fundamentals

Amna Aqeel¹, Javaria Zafar¹, Pouya Mohammadi²,³, Meisam Tabatabaei²,³, Mortaza Aghbashlo⁴, T. M. Indra Mahlia⁵ and Abdul-Sattar Nizami⁶,⁷,    ¹Institute of Industrial Biotechnology, Government College University Lahore, Lahore, Pakistan,    ²Higher Institution Centre of Excellence (HICoE), Institute of Tropical Aquaculture and Fisheries (AKUATROP), Universiti Malaysia Terengganu, Kuala Nerus, Terengganu, Malaysia,    ³Biofuel Research Team (BRTeam), Terengganu, Malaysia,    ⁴Department of Mechanical Engineering of Agricultural Machinery, Faculty of Agricultural Engineering and Technology, College of Agriculture and Natural Resources, University of Tehran, Karaj, Iran,    ⁵Centre for Technology in Water and Wastewater, School of Civil and Environmental Engineering, University of Technology Sydney, Ultimo, NSW, Australia,    ⁶Sustainable Development Study Centre (SDSC), Government College University, Lahore, Pakistan,    ⁷Center of Excellence in Environmental Studies (CEES), King Abdulaziz University, Jeddah, Saudi Arabia

Abstract

An upsurge in industrialization and the human population has led to increasing energy demands most met by fossil-oriented energy carriers, resulting in the emission of pollutants and greenhouse gases. The environmental and public health damages caused by these emissions have triggered a global transition toward replacing fossil fuels with their renewable and green counterparts. Biodiesel is a promising alternative to fossil diesel and can be used in existing diesel engines with no or minor modifications. Transesterification is the most commonly used process among the various techniques used for biodiesel production. The worldwide boost in biodiesel production has led to the establishment of various methods used in a real-world setting to increase biodiesel yield and transesterification efficiency. This introductory chapter explains the fundamentals of industrial biodiesel production, including statistics, approaches, and costs. Despite improvements over the years, the biodiesel production industry still needs further improvements and advances to enhance the production process and meet increasing global demands.

Keywords

Biodiesel; transesterification; industrial production; feedstock

1.1 Introduction

Energy demands for industrial production processes, logistics, and transportation boost fossil fuel consumption, leading to the emission of pollutants and greenhouse gases (GHG) and environmental issues [1,2]. Consequently, biofuels, as renewable, biodegradable, and alternative fuels, have been developed to replace fossil fuels. Replacing all or part of the fuel consumed with biofuels could mitigate the emissions of GHGs, carbon monoxide, unburned and aromatic hydrocarbons, and particle matters [3,4]. Commonly, biofuels are manufactured from biological or organic materials in solid, liquid, or gaseous states [5]. Technical assessment of industrial biofuel production reveals lower prices, more efficient land use, improved air quality, and minimal water usage compared to fossil-based fuels [6,7]. The biofuels produced from different biological feedstocks are categorized into four groups: first, second, third, and fourth generations [8]. The generations of biofuel are summarized in Fig. 1.1.

Figure 1.1 A schematic presentation of different biofuel generations.

First-generation biofuels are produced from edible feedstocks (i.e., starch- and sugar-containing crops or edible vegetable oils) [9]. Although the global expansion of first-generation biofuels has resulted in a serious debate on food security and land-use change (LUC), their production technologies have reached maturity and are available worldwide [10]. Second-generation biofuels are generated from lignocellulosic feedstock and organic waste resources. Second-generation biofuels, except for biodiesel produced from waste cooking oil (WCO), are not generally industrially profitable yet because they require extensive pretreatments, causing more production costs than their first-generation counterparts [5,11,12]. The feedstock for producing third-generation biofuels includes microalgae and microbial biomass [13,14]. The fourth-generation biofuels have been developed through genetically modified feedstocks, ranging from plants to microalgae and microorganisms. Among the recent research on fourth-generation biofuels is gene modification approaches to boost CO2 adsorption for enhanced photosynthesis [15].

Fatty acid alkyl esters, also known as biodiesel, are eco-friendly and nontoxic biofuels produced through a transesterification reaction [16]. The reaction is carried out using waste or straight vegetable oil, animal fats [17], or microalgal/microbial and short-chain alcohol (i.e., methanol or ethanol) in the presence of a catalyst (acid, base, or enzyme). Glycerol is the byproduct of the transesterification reaction obtained and carries impurities, including catalyst residues, unreacted alcohol, and traces of mono, di, and triglycerides [18–20].

In addition to being corrosive to the process equipment, basic catalysts cause soap formation, requiring washing the products. Water-washing involves water separation and is followed by biodiesel drying and wastewater recovery processes, raising the production cost [21,22]. The base-catalyzed reaction is also sensitive to the oil’s acidity because the presence of free fatty acids (FFA) further induces soap formation (saponification), necessitating more intensive downstream treatment, such as water-washing. The soap formation also causes the loss of biodiesel yield, hinders the separation efficiency of biodiesel and glycerol, and results in a larger volume of waste generation [23]. The glycerol manufactured is generally separated by gravitational settling down and, afterward, needs purification to meet the standard purity requirements of the target market (i.e., biomedical, cosmetics, or food industries) [20].

1.2 The emergence of the biodiesel industry

The first report on biodiesel production dates back to 1937 when the Belgian chemist Georges Chavanne (University of Brussels) was granted a Belgian patent 422877 entitled Procedure for the transformation of vegetable oils for their uses as fuels [3,24]. He introduced palm oil ethyl ester obtained by acid-catalyzed oil transesterification as a fuel for diesel engines. Then, the preliminary tests were conducted on an urban bus operating on biodiesel that served the commercial passenger line between Brussels and Louvain in 1938. The results showed that the bus performance was reasonable [25].

After more than 40 years, the first industrial design of a biodiesel production plant emerged during the 1980s at an agricultural college in Austria. In the following decade, biodiesel was commercially produced in Europe and the United States [26]. Many fleet executives and diesel engine manufacturers faced the first challenge: the uneven quality of biodiesel from different feedstocks, which led to the development of the ASTM Standard D6751 by ASTM International in 2001 [27].

The past 20 years have observed increasingly rapid advances in industrial biodiesel production to reduce dependence on fossil fuel imports and emit fewer carbon emissions and less air pollution than fossil fuels [26]. For instance, industrial biodiesel production increased from 1 billion L in 2001 to 6 billion L in 2009 in the United States [26]. According to the recent statistics reported by the website of Biodiesel Magazine, on December 14, 2021, the production of 65 operational plants increased to more than 9.6 billion L, while 14 plants with a capacity of 8.2 billion L were under construction and nine plants with a total capacity of 8.3 billion L had been proposed [28].

Plant-based biodiesel emits only the equivalent amount of GHG previously absorbed by the plants as CO2 during the growth period (photosynthesis). Although the biodiesel production and transportation processes also release GHG, biodiesel derived from photosynthetic microalgae and nonedible oil crops could lead to 98% and 91%–95% emission reduction compared to fossil diesel, respectively [29].

The first biodiesel usage as aircraft fuel dates back to October 2, 2007, when a 39-year-old Czechoslovakian jet fighter (i.e., Delfin L-29) was fueled with B100. This flight took place in Reno, Nevada, USA, at 5181 m (17,000 ft) without any significant performance loss compared to conventional jet fuels [30,31]. Douglas Rodante as president of Green Flight International, a promotion and marketing organization, conducted this test flight to promote the adoption of environmentally friendly fuels by the aviation industry [31]. According to the experiments designed, different fuel blends were prepared for each flight test by gradually increasing the inclusion rate of biodiesel produced from French-fry grease from 20% to 100% into the jet fuel (i.e., kerosene known as Jet-A) [30,32]. The engine speed was reduced by 2% when fueled with B100 compared to Jet-A. As stated by Rodante, this flight did not carry out at full power but achieved an acceptable amount of power that was not an issue in climb performance [32]. NASA has confirmed that replacing 50% of aviation fuel with biofuel reduces air traffic particulate emissions by 50%–70% [33].

1.3 Biodiesel specifications and standards: history and the state of the art

According to the ASTM standard, biodiesel is a fuel comprised of mono-alkyl esters of long-chain fatty acids derived from vegetable oils or animal fats. By definition, the diesel fuel produced by novel processes in which biomass is converted into synthetic gas at superheat conditions followed by condensation is not acceptable as biodiesel fuel. The fuel produced using such processes, owing to the application of biomass (cellulosic material, vegetable oils, or animal fats) as feedstock and the fact that the products’ characteristics are similar to those of fossil diesel fuel, is called green or renewable diesel fuel [27].

More than two billion gallons of biodiesel are consumed yearly [34]. Biodiesel can be blended with fossil diesel at any ratio, generating stable biodiesel–diesel mixtures with properties nearly identical to those of fossil-based diesel fuel [35]. Table 1.1 shows the parameters that must be fulfilled to attain quality standards for biodiesel (fatty acid methyl esters [FAMEs] and fatty acid ethyl esters [FAEEs]) and diesel fuel in Europe, Germany, and the United States.

Table 1.1

aIn summer 0°C, spring and autumn −10°C, and winter −20°C.

Even though biodiesel is widely accepted as a substitute for fossil-based diesel fuel, it is important to ensure the properties of biodiesel in neat or blended forms, that is, antifoaming properties, cetane number, chemical structure, oxygen content, cold flow properties, conductivity, and corrosion, would align with the international standards. The antifoaming properties of neat biodiesel (B100) are superior to fossil diesel, facilitating the vehicles’ fueling process without the risk of foam leaks or overflows, ensuring safety and efficiency. Compared to standard fossil diesel fuels with a cetane number between 40 and 52, biodiesel typically has a cetane number between 45 and 70. The higher the cetane number, the greater the ignition qualities. The fatty acid profile of the original oil or fat affects the cetane number of biodiesel. The more saturated the fatty acids are, the higher the cetane number could be [37]. Biodiesel typically consists of C12 to C24 alkyl esters. In contrast, diesel is a complex amalgam comprising hydrocarbons ranging from C12 to C25, including paraffin, naphthenes, aromatics, and various sulfur and nitrogen-carrying organic compounds.

Moreover, while biodiesel mainly comprises straight-chain hydrocarbon esters (e.g., methyl esters of stearic acid, oleic acid, linoleic, and linolenic acid), diesel comprises ring structures such as ring structures aromatic compounds [38]. Biodiesel contains 11% oxygen (hydroxyl (–OH) group in the esters’ structure), allowing steady combustion despite its lower energy content against fossil diesel, while these functional groups render biodiesel polar. The polarity confers all other attributes of solvency, detergency, wettability, and conductivity. On the contrary, diesel fuel lacks oxygen [39].

Each component of diesel fuel has its specific crystallization temperature, resulting in a gradual solidification process. Neat biodiesel, on the other hand, is a much simpler mixture containing relatively few components, resulting in one or two elements predominating; thus, solidification becomes much more rapid and difficult to control at the crystallization temperature of biodiesel [40]. Unlike gasoline, neat biodiesel has outstanding conductivity, more than 500 pS/m, and as a result, it minimizes the danger of static-induced sparks and ignition of flammable materials [35].

1.4 Biodiesel production yield

Global biofuel production in 2020 is illustrated in Fig. 1.2A. The top country is the United States producing more than 1.3 exajoule, followed by Brazil with 34% lower production. The third leading country is Indonesia, with 283 petajoule production, approximately one-fifth of the US production. The top 10 biodiesel industrial producers worldwide in 2019 are demonstrated in Fig. 1.2B. Indonesia is the first biodiesel mass producer, with 7.9 billion L/year, followed by the United States and Brazil, with a production of 6.5 and 5.9 billion L, respectively.

Figure 1.2 Leading countries based on (A) biofuel and (B) biodiesel production worldwide [41]. Source: Adapted from Statista. http://www.statista.com.

Soybean is the most important oilseed feedstock for producing biodiesel and green diesel (renewable diesel) in the United States. As shown in Fig. 1.3, since 2014, soybean oil has been used as biodiesel’s feedstock more than renewable diesel fuel’s feedstock. However, the proportion of soybean used for renewable diesel production has increased over time. Moreover, only a small amount of soybean oil has been employed for other biofuels.

Figure 1.3 The trend of renewable diesel and biodiesel fuels produced from soybean oil in the United States in recent 8 years [42].

According to the production data reported for 2021 (Fig. 1.3), biodiesel fuel production accounted for 70% of the total soybean-based fuel market. The development outlook for renewable diesel capacity for 2022 indicates that raw material prices will remain high, leading to competition between biodiesel and renewable diesel manufacturers [42].

1.5 Biodiesel chemical production process

The procedures applied to use oils as fuel include direct use or blending of oil and diesel, microemulsions, pyrolysis or thermal cracking, and transesterification. The most commercially used method is transesterifying animal fats or vegetable oils with alcohol to produce biodiesel [43].

During the development of the biodiesel production industry, several approaches and techniques were assessed to transesterify oleaginous feedstocks into biodiesel, that is, catalytic routes and noncatalytic routes [44,45].

Catalysts are categorized into homogeneous catalysts, heterogeneous catalysts, and biocatalysts (enzyme) [46–48]. Although homogeneous catalysts cause a faster reaction and require lower loading than heterogeneous catalysts, one of the adverse attributes of homogeneous catalysts is that they cannot be separated from products. On the other hand, heterogeneous catalysts, although recyclable, are more expensive and result in lower yields [48].

Among homogeneous catalysts are base catalysts, that is, alkali metal-based hydroxides (e.g., sodium or potassium hydroxide), alkali metal-based oxides (e.g., sodium and potassium methoxides), and carbonates. Base catalysts have high activity in transesterification [49]. Though metallic hydroxides have lower efficiency than alkoxides, they are utilized frequently due to their lower price [48]. Sulfonic, sulfuric, and hydrochloric acids are among the conventional homogenous acid catalysts employed for biodiesel production [50]. Although homogeneous acid catalysis is insensitive to FFA content and can be used for low-quality oil feedstock such as WCO with high water and FFA content, they have lower catalytic efficiencies, leading to low yields [51]. One main advantage of acid catalysis is the absence of saponification occurrence [48].

Among the catalytic processes, alkali-based transesterification is most common at the industrial level [52], despite its drawbacks such as being time-intensive, wastewater and soap production, susceptibility to reactant ratio, and requiring more downstream physical and chemical treatments like separation, neutralization, and purification stages [53]. Heterogeneous base catalysts, which are easier to handle than homogenous catalysts in terms of the abovementioned downstream stages, can also be used to produce biodiesel; however, these catalysts are generally costlier and lead to lower yields [54]. Moreover, catalyst leakage and recycling challenges also limit the application of homogenous catalysts.

The transesterification reaction mechanism with a base catalyst to produce biodiesel is shown in Fig. 1.4. Transesterification is a nucleophilic reaction in which triglycerides and alcohol produce FAME/FAEE and glycerol through three consecutive reversible reactions. As shown in Stage 3 in Fig. 1.4, triglyceride is first converted to diglyceride. Afterward, diglyceride is converted to monoglyceride, and finally, monoglyceride to glycerol, while one mole of FAME is also produced in each stage. Fig. 1.5 presents the esterification reaction through which the carboxylic acid groups of FFAs react with an alcohol to produce esters [55].

Figure 1.4 The transesterification reaction converting triglycerides to biodiesel and glycerol in the presence of a base catalyst.

Figure 1.5 The esterification reaction converting fatty acids to alkyl esters (biodiesel).

Heterogeneous catalysts are divided into acid, base, and acid/base catalysts. The objective of heterogeneous base catalysts is to reduce the drawbacks of homogeneous base catalysts, that is, saponification that prevents glycerol separation from the methyl esters produced. The heterogeneous base catalysts include alkaline earth and alkali metal-based catalysts (e.g., metal-based oxides; CaO, MgO, SrO, and BaO), mixed metal-based catalysts (e.g., La2O3 in ZrO2 and CaMgZn mixed oxides in Na2CO3), transition metal-based catalyst (e.g., titanium oxide [TiO2] and zinc oxide [ZnO]), hydrotalcite-based catalyst (e.g., Mg–Al hydrotalcite), and waste-based catalysts (e.g., CaO obtained from the calcium contained in waste materials) [48]. Compared to homogeneous acid catalysts, heterogeneous acid catalysts have lower corrosive, toxic, and environmental impacts [56]. However, higher catalyst loading and reaction temperature requirements, on the one hand, and longer reaction time, on the other hand, are the drawbacks of this class of catalysts, limiting their application [57]. As categorized by Rizwanul Fattah et al., heterogeneous acid catalysts include cation-exchange resins, Heteropoly acid derivatives, sulfonic acid-based catalysts, and Sulfated oxide-based catalysts [48].

For biodiesel production from high FFA oils, the heterogeneous acid–base catalytic reaction leads to esterification and transesterification reactions with slight soap formation [58,59]. An acid catalyst is required to esterify the FFA content of oil with an alcohol such as methanol before transesterification. Then the basic catalyst drives the transesterification of triglyceride with methanol. Zirconia, its derivatives, and Zeolite-based catalysts [48] are acid/base catalysts used in biodiesel production.

Enzymes such as microbial lipases (EC 3.1.1.3) have been widely used to catalyze the transestrification of many oil feedstocks. This approach is not sensitive to the concentration of FFA in oils and can be used to perform both esterification and transestrification simultaneously (a one-step process). The main parameters affecting the yield of enzymatic biodiesel synthesis are in-solvent or solvent-free characteristics, the molar ratio of oil to alcohol, and temperature. Other subparameters (i.e., water activity, pH of enzyme’s microenvironment, and the highest permissible glycerol concentration in reaction products) can also be named. Despite their advantages, enzymes (lipases) are not economically viable at the industrial level yet, and more research into developing more economically accessible synthetic lipase must be conducted to realize the industrial application of enzymes in the biodiesel production industry [60].

Ultrasonic- and microwave-assisted reactors have also been used to achieve a faster reaction rate and higher biodiesel yields. However, despite higher biodiesel conversion rates achieved in a more environmentally friendly approach, the high energy consumption of these techniques compared to catalytic methods is the crucial challenge in their scaling up. The supercritical transesterification process is conducted at high temperature and pressure conditions and overcomes the disadvantages of the catalytic approaches. However, this process is comparatively much costlier than its catalytic counterparts, hindering its widespread application.

1.6 Biodiesel production cost

Fig. 1.6 demonstrates the variations in the mean price of diesel, B20, and B100 in the United States from 2000 to 2021 in USD/GGE (US dollars per gasoline gallon equivalents) as well as the underlying reasons leading to such ups and downs [67]. As illustrated, the price of B20 has been comparable and, during some periods, even slightly lower than diesel fuel.

Figure 1.6 Average retail fuel prices in the United States. *The numbers on the graph represent the original reference of the event (¹ [61], ² [62], ³ [63], ⁴ [64], ⁵ [65], ⁶ [66]). **The three factors are (1) the OPEC+ agreeing to limit crude extraction; (2) refiners reducing production rate and shuttering some plants; (3) demands rising as travel and business restrictions were lifted. Source: Adapted from US Department of Energy, [Online]. https://afdc.energy.gov/data.

One of the main obstacles to industrial biodiesel production and commercialization is the higher price of biodiesel compared to diesel fuel in some parts of the world [68]. The biodiesel price is principally related to the cost of raw materials consumed in the production process, corresponding to around 70%–95% [69] or 75%–80% [70] of the overall production cost. Nonedible and waste-based feedstocks are promising opportunities to reduce biodiesel prices [71].

Although chemically produced biodiesel is cheaper than its enzymatically produced counterpart, the associated costs could be comparable, considering the higher environmental footprint [70]. Enhancing lipase performance through robust lipase preparations and improving its long-term recyclability, on the one hand, and developing cheaper enzymes, on the other hand, can further facilitate the real-world application of biocatalysts [72,73].

1.7 Biodiesel industry facts and figures

Internal combustion engines are one of the major agents emitting CO2, the main GHG that causes global warming. Biodiesel in diesel engines can significantly decrease GHG by 78% on a life cycle basis [74]. Biodiesel is generally considered a cleaner-burning direct replacement for fossil-diesel fuel. In a discursive essay entitled Biofuels may not be as green as we have been told, published on a science webpage in February 2022, three critical questions were raised; Why are biofuels considered green? Are biofuels actually better for the environment? and So, are biofuels all they are cracked up to be? The author noted that the rise of the biofuel industry dates back to the turn of the 21st century when many governments worldwide were trying to find ways to control carbon emissions [75]. He also stressed that besides biodiesel’s known disadvantages, there are some other discussable drawbacks in its production processes and usage, which should be considered in contrast with biodiesel’s environmental benefits and financial explanation.

Although CO2 is reported as the least harmful GHG from a public health perspective [76], it is one of the most harmful ones from the environmental and climate change perspectives, and it will not be reduced by using biodiesel [77,78], because biodiesel’s main feedstocks, i.e., oil crops, release thousands of tons of carbon dioxide during their growth period. NOx is frequently reported to be increased by biodiesel consumption [79–84], is responsible for several global crises like acid rains, and has a significant global warming potential, orders of magnitude higher than CO2. Biodiesel releases more carbonyl emissions like formaldehyde, acetaldehyde, acrolein, acetone, propionaldehyde, and butyraldehyde than neat fossil diesel [74]. Although biodiesel combustion emissions also contain hazardous pollutants such as volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), and heavy metals, they are lower than diesel fuel