Recent Advances in Renewable Diesel / Biodiesel technologies

Section 1: Introduction to Renewable Energy

For thousands of years, humanity had little need to harvest enormous amounts of energy. However, the advent of the Industrial Revolution changed this notion with humanity becoming more dependent on fossil fuels to power the modern industrialized society [1]. Over time, an issue arose with the limited supplies of petroleum, coal, and natural gas necessary for powering the machinery integral to an industrialized society [2]. Furthermore, this reliance on fossil fuels is not only unsustainable but also detrimental to the environment, as the combustion of fossil fuels releases enormous amounts of greenhouse gases that are linked to pollution and increasing global temperatures [3].

The global power requirements for electricity generation, transportation, and heating have increased over time, driving up the demand for fossil fuels. Simultaneously, more sustainable methods of energy production are being researched; one such field is that of biodiesel and renewable diesel. These two types of fuel are blended with traditional diesel to create a sustainable option by offsetting fossil fuel usage.
Although biodiesel and renewable diesel are often used interchangeably, they are two separate entities. The main difference between the two is that biodiesel is made using a process known as transesterification, or the conversion of fats into alkyl esters through a reaction between fats, alcohols, and catalysts [4]. On the other hand, renewable diesel is made using hydroprocessing, a process that combines a feedstock with hydrogen and catalysts under elevated temperature and pressure to remove impurities and saturate double bonds, resulting in a cleaner, higher-quality diesel fuel product [4]. The two processes are shown in Figures 1 and 2. Both biodiesel and renewable diesel can utilize biomass as their feedstock, including commonly grown crops like coconuts, rice, switchgrass, and wheat [5]. These feedstock sources, unlike fossil fuels, can be readily replenished.
Figure 1: Flow Chart Detailing the Production of Biodiesel through Transesterification.
Original Diagram from reference [4].
Figure 2: Flow Chart Detailing the Production of Renewable Diesel through Hydroprocessing. Original Diagram from reference [4].

The shift to biodiesel and renewable diesel has already begun in some places across the world including New York City. On November 29th, 2023, New York City Mayor Eric Adams started an initiative to replace conventional diesel with renewable diesel for the city’s fleet of trucks [6]. Similarly, Germany is keen on phasing out diesel for biodiesel and renewable diesel, given the lack of domestically available petroleum, but ample feedstock material [7]. Furthermore, according to the IEA, emerging economies such as Brazil, Indonesia and India are also spearheading the adoption and production of biodiesel, as part of initiatives to incorporate more renewable fuel sources with domestically produced feedstocks [8].

Section 2: Recent Research and Advancements in Renewable Diesel and Biodiesel

Scientists have researched new ways of producing and improving renewable diesel and biodiesel in an attempt to reduce greenhouse gas emissions. Such advancements include improved properties such as cetane number, and innovative methods of deriving new renewable diesel and biodiesel, whether that be new feedstocks or catalysts. These advancements have also exhibited reduced greenhouse gas emissions.

Section 2.1. Conventional Feedstocks for biodiesels and renewable diesel
One such study conducted by Li et al. investigated the effects of certain soybean variants on the life cycle greenhouse gas emissions of soybean-based biodiesel and renewable diesel. Researchers sought to assess if they could reduce the amount of greenhouse gases emitted in the production, or “life cycle”, of these fuels. To do this, researchers produced renewable diesel and biodiesel from both traditionally used beans (“commodity beans”) alongside newer soybean varieties (“emerging soybean varieties”). The greenhouse gas emissions of each production process were then compared. The soybean varieties used were “high-oleic, high-lipid, high-lipid,  and high-oleic, and high-protein soybeans” [9]. The study also accounted for any factors that may create greenhouse gas emissions [9]. For example, some principal factors considered were CO2 emissions from crop harvesting, fuel production, and the transportation of the fuel. Additionally, energy consumption of the farm, fertilizer production, and the greenhouse gasses produced from the application of fertilizers, herbicides, and insecticides were considered, along with other factors [9]. Figure 3 is a system boundary diagram used to illustrate the internal and external factors affecting the production process, with a breakdown of the main greenhouse gas producing factors throughout the life cycle analysis.
The results of this study shown in Figure 4 indicate that the type of soybean did not result in any major change in the amount of production of greenhouse gasses throughout the soybean’s lifecycle. Regarding biodiesel, the researchers’ findings suggest that “high-oleic soybeans” (HO) and “high-lipid high-oleic soybeans” (HLHO) both reduce greenhouse gas emissions during the conversion phase of the renewable diesel’s life cycle. The conversion phase relates to when the crude fuel parts are formed into usable fuel. According to researchers, this occurs due to high oleic soybean oil’s fewer double bonds when compared to conventional soybean oil, causing reduced hydrogen use during the hydrogenation step. This reduced hydrogen use in turn lowers the gas emissions produced over the lifecycle of HO soybean oil.
Researchers also applied a “market value-based allocation method,” in which they considered the cost of the distinct soybean and its impact on the Figure 4 data. For example, soybean oil is more expensive when bought in bulk (per kg), so by applying the market value allocation method, the emissions calculated would increase [9]. Figure 5 shows these market analysis results and the inherent potential to reduce greenhouse emissions from both biodiesel and renewable diesel. By viewing the data through a cost-effective lens, the effect of alternative soybean variants on greenhouse gas emissions of the soybean-based production of biodiesel and renewable diesel can be better comprehended [9].

Farnesane, a renewable diesel produced from sugarcane, has also been researched extensively as a renewable diesel. Sugarcane is a common feedstock, allowing farnesane to be extremely useful in nations where sugarcane is in abundance. One study led by Costa et al. researched the possibility of using farnesane. The process of producing farnesane includes the genetic modification of yeasts, which are then used to ferment the sugars present in sugarcane. Hydrocarbons are also produced as a secondary product in this process, which can be hydrotreated, making more farnesane [10].
The main goal of the study by Costa et al. was to assess the viability of using farnesane in a single cylinder compression engine constructed to imitate the designs used in rural Brazil for agriculture purposes [10]. To evaluate the practicality of the use of farnesane, Costa et al. analyzed the combustion, performance and emissions of the constructed engine under different pressures [10]. The results found that farnesane underperformed when blended with diesel at intermediate pressures and negatively affected engine performance at higher pressures. However, comparisons to traditional diesel showed farnesane reduced hydrocarbon, carbon monoxide, and particulate matter emissions. Moreover, there were also reduced emissions due to the unique physicochemical properties of the farnesane, such as its cetane number and H/C (hydrogen to carbon) ratio [10]. Another influence in the reduction of emissions is attributed to farnesane’s unique molecules which allow for “favorable vaporization and reactivity speeds” [10]. According to Costa et al., NOx levels were reduced by up to 36% and particulate matter by up to 92% [10]. Overall, this research indicates that the use of farnesane could be a possible greener alternative for small-scale power generation, particularly in rural areas in which sugarcane feedstock is abundant. Though producing impressive results, one avenue of future research is the long-term effects of farnesane on engines, such as clogging, which were not studied in Costa et al.’s experiments.

Section 2.2. Novel Feedstocks for biodiesels and renewable diesel
Another potential material capable of producing renewable diesel is octenol. Octenol is a chemical compound naturally produced in plants and fungi to attract a variety of insects, including mosquitoes. The use of conventional methods for processing renewable diesel lowers properties, including the heat of combustion and cetane number [11]. Siirila et al. investigated the viability of using octenol to create a more effective renewable diesel. To do this, a renewable diesel was produced from octenol, which was then compared to traditionally used diesel as well as renewable diesel options (Table 1). To create the renewable diesel from octenol, a three-step process consisting of dehydration, cycloaddition, and hydrogenation is implemented. Properties such as density, NHOC, kinematic velocity, and flash point are all important properties of fuel, integral in the determination of applications for the fuel, as well as its transport and operating conditions. In this case, researchers compared octenol-based diesel to conventionally used diesel. The results of this study exemplify that the resultant fuel (octenol-based renewable diesel) has superior properties as compared to traditional options.
As shown in Table 1, one such example of superior properties is the improved gravimetric net heat of combustion (NHOC) [11]. The NHOC of the octanol-based renewable diesel is also 11% higher than that of soy biodiesel and 1.3% higher than petroleum diesel [11]. This results in the octanol-based renewable diesel having more desirable thermodynamic properties. Another notable improved property of the octenol-based diesel is the higher cetane number. These heightened properties suggest that new renewable diesels could be used over conventional options; however, Siirila et al. acknowledge that research on this octenol-based renewable diesel is still relatively new.
While these results are promising, one issue preventing the mass production of this renewable diesel is the non-commercialization of the reaction involving linoleic acid. While this is a major roadblock, advances in synthetic biology will soon allow for the enzymatic oxidation cleavage of linoleic acid [11]. Siirila et al. believe that once this problem is overcome, there is enormous potential for renewable diesel, as the steps to make octenol-based diesel are rather simple compared to other options. This makes renewable diesel a prospective route towards making advanced renewable fuels [11].
Another study by Hussain and Biradar investigated the use of jatropha oil in the production of hydrogenated renewable diesel. The researchers then the properties of this new renewable diesel. In order to produce the renewable diesel, hydro-deoxygenation is done using a nickel molybdenum or cobalt molybdenum catalyst and hydrogen gas [16]. The jatropha oil is then typically put under 40 to 90 bars of pressure and turned into paraffins (waxes) [16]. Final products are then put through fractional distillation, and the products are separated into their constituents [16].
According to Hussain and Biradar, properties such as cetane number, flash point, and pour point of the jatropha oil diesel  were higher than that of petroleum oil [16]. These heightened properties are significant, for higher cetane number and flash point are both properties related to ignition. A higher cetane number coincides with a more complete ignition, while flash point results in a higher temperature being needed for the vapors of a product to ignite in the presence of an ignition source. Table 2 compares these physiochemical properties, as well as many others of different diesels with the jatropha diesel.
Conversely, the properties of kinematic viscosity and thermal stability were approximately the same as those of the base hydrogenated renewable diesel and traditional diesel [16]. While the kinematic viscosity and thermal stability did not surpass traditional diesel, the fact that the jatropha oil diesel is on par with what is currently used makes it a viable option that should be considered in the future.
Neem seed oil has also been considered as a potential feedstock for the production of biodiesel. According to Dash et al. neem biodiesel is prepared from neem seed oil and the basic catalyst, sodium hydroxide [17]. The study researched neem seed biodiesel’s ideal properties and conditions [17]. When optimized, it was found that the maximum yield of the neem seed oil biodiesel was 96% and that the cetane number was 52 [17]. Comparing this cetane number to traditional diesel (48 for the purposes of this study [17]),  it is clear neem seed greatly improves the cetane number of its biodiesel product. Additionally, the acid value was found to be extremely low at 0.09 mg KOH/g [17]. To get these properties, the ideal conditions of a 1:9 ratio of oil to alcohol must be met [17]. Furthermore, the ideal temperature to maximize yield was found to be 65oC [17]. The optimal reaction time  is two hours  with mixing of the biodiesel at 600 rpm [17]. This will give the aforementioned 96% yield [17]. The remaining mixture should be separated using a separatory flask. After 6 hours, the glycerol will remain at the bottom of the flask, while the crude biodiesel should be at the top [17]. Pure neem seed biodiesel can be obtained by washing the crude biodiesel with water, then heating the product in an oven at 105 ± 1oC [17]. The authors believe utilizing neem seed oil as feedstock in tandem with other appropriate feedstock could result in “continuous and smooth production of biofuel” [17].
Another study by Akinwumi et el., studied the use of polyhydroxyalkanoates (PHAs) to make biofuels. In their study, Akinwumi et.al produced two biofuels and designated them as 3-hydroxybutyrate methyl ester (3HBME) and 3-hydroxyalkanoate (3HAME), respectively. Methyl esters can be used in the transesterification process as substitutes for petroleum oil. According to researchers, methyl esters “are strong candidates that could stand as alternatives for developing biofuels in the fuel additives market” [18]. The processes used to make 3HBME and 3HAME give it select properties similar to that of gasoline and biodiesel, however Akuwumi et al. found that the octane and cetane numbers of PHA-based biofuel are lower than that of other biofuels. This is due to the use of a short chain PHA known as polyhydroxybutyrate (PHB) [18]. The short length of PHB is suspected to be the cause of the depreciated cetane and octane numbers of PHA based fuels. Therefore, Akinwumi et al. argue that using longer chain PHAs to produce biofuel should enable improved physicochemical properties such as cetane and octane numbers [18]. This statement is based on the research of Al-Mashhadani et al. which found that longer and more saturated carbon chains typically yield higher cetane numbers in biofuel blends [19]. This is yet another avenue which researchers may continue to explore ways to improve biodiesel, renewable diesel and their properties.
The search for potential feedstocks often has researchers searching in unexpected places. One such feedstock is the Zagros oak for the production of biodiesel. The cost and abundance of a feedstock is vital to its viability. As such, Zagros oak is the ideal candidate, particularly in the forests of Iran [20]. Researchers Maleki and Talesh used Co@CuO  nanoparticles as a catalyst to make a new biodiesel from Zagros oak oil in gel form (QBLO), and documented the characteristics of the produced biodiesel [20]. A summary of the properties of the QBLO-based biodiesel is found in Table 3:

Maleki and Talesh describe the purpose of each property. Kinematic viscosity determines the engine efficiency and gives an indication of how much sediment and smoke gets built up in the engine [20]. Density influences the fuel properties such as the injection velocity and its flow [20]. Cloud and pour point are also significant in determining the cold flow of the fuel, which are important in low temperature applications [20]. Flash point influences the combustion conditions while cetane number influences the combustion quality. By combustion quality, Maleki and Talesh refer to emissions produced during combustion [20]. By comparing the properties of QBLO-based biodiesel to ASTM standards, the researchers believe that QBLO is a suitable feedstock to produce biodiesel. Along with its cost and availability, QBLO is a highly suitable feedstock [20]. Its molecular make up of fatty acids also make it extremely suitable for biodiesel production [20].
However, Meleki and Talesh assessed their new biodiesel effects on a CI engine. QBLO-based biodiesel was blended at different ratios and assessed for performance in the CI engine. In regards to environmental effects, the concentration of CO2 and NOx increased as the percentage of QBLO-based biodiesel increased. Differently, concentrations of CO and UHC were both decreased as the percentage of QBLO-based biodiesel increased [20].
While seeming to be a step in the wrong direction regarding the greenhouse effect, regarding engine performance, increasing the QBLO-based biodiesel percentage increased the BSEC (brake specific energy consumption) and EGT (exhaust gas temperature) increase. Furthermore, the results found that the BTE (Brake thermal efficiency) of the engine decreases [20]. All of these are positive qualities sought after in biodiesel, indicating that QBLO is a promising new feedstock that can be utilized in the production of biodiesel.

Section 2.3: Advancements in Biodiesel-New Catalysts
When producing biodiesel, many different catalysts have been experimented with for the transesterification process. In a study done by Yang and Yu, research was conducted on the use of zeolites-inorganic crystalline materials [22]. Zeolites are crystalline materials with a structure composed of “TO4 tetrahedrons”, typically with a tetrahedral atom such as silicon, aluminum, or phosphorus [21,22]. Researchers are most interested in their structure, which is built with many tunnels consisting of many equally spread micropores on a molecular scale [22].  Yang and Yu explain that they are interested in this microporous structure because the catalytic activity and efficiency of traditional solid base catalysts are hampered by their low porosity, low surface area, and restricted volume of activation sites [22]. Compared to traditional solid base catalysts, zeolites have many benefits such as having a higher surface area, being generally more stable, and an adjustable basicity [22]. Furthermore, Yang and Yu found that using solid-base catalysts in these reactions could be a preferable way of producing biofuels due to solid-base catalysts having a lower environmental impact and a simpler production scheme compared to that of traditional catalysts [22]. Researchers also discussed synthesis methods including ion exchange, impregnation, and one-pot synthesis methods (Figure 6). Ion exchange synthesis occurs when alkaline cations are added to the structure of the zeolite creating basicity of the zeolite due to it lowering the electronegativity [22].
Furthermore, the impregnation method involves adding solvated alkaline metals into the zeolite. The zeolite is
then thoroughly calcinated or heated, causing the solvated alkaline metal to break down, and creating basic sites of
basic zeolites [22].
Lastly, researchers explain that the one-pot synthesis method entails incorporating metal compounds to manipulate the zeolite’s acid-base reaction [22]. To do this, all compounds involved must undergo pretreatment or hydrothermal synthesis.
While Yang and Yu were optimistic about the future use of zeolites, they acknowledged challenges that prevent the mainstream adoption of solid-base catalysts. One such challenge was the need for improved stability and activity of solid-base catalysts. This refers to the narrow and small pore size, which poses a distribution problem as the necessary molecules struggle to get into the active sites in the pores. These molecules can be viscous and bulky, so the small pores and narrow paths end up decreasing catalytic rates [22]. Researchers explained that “traditional solid-basic catalysts often suffer from poisoning or leaching of basic sites due to impurities or moisture in the reaction medium” [22]. Meaning, the catalyst often stops working after a few uses due to poisoning. Few reactions can occur due to the porous structure of basic zeolites, resulting in long reactant paths. By outlining the challenges of using basic zeolites in transesterification, the study conducted by Yang and Yu aimed to advance future efforts at researching ways to utilize basic zeolites in the production of biofuels.
As previously discussed, Meleki and Talesh used nanocatalyst Co@CuO  to improve the yield of QBLO-based biodiesel. In their study, they go in depth about the process to derive Co@CuO , the characterization of Co@CuO , and the effects Co@CuO  has on the transesterification process. The process to make Co@CuO involves the mixing of 0.5 M Copper Acetate, 0.5 M Cobalt Acetate, and 0.5 M citric acid solutions, all of which were dissolved in 50 ml of deionized water [20]. The solution is then mixed at 25°C for 45 minutes. Copper acetate is then slowly added to the cobalt acetate solution drop by drop while stirring was done for 60 minutes at 25°C [20]. Citric acid is then combined into the resultant solution drop by drop and stirred for 100 minutes at 90°C [20]. This solution is then used to make a gel, which is dried at 100°C for 90 minutes in an oven and calcined at 200°C in a furnace for 180 minutes [20]. Researchers used numerous methods to characterize the surface features of Co@CuO such as XRD, FTIR, Raman, BET and many more. Furthermore, the Co@CuO nanoparticle improved the biodiesel yield to 95.75% when placed in ideal conditions. The nanoparticle catalyst also proved to be extremely reusable as it was able to be applied for 7 rounds of production. Overall, results show that the use of Co@CuO is effective with QBLO and could be used as a “superior recyclable solid nanocatalyst in the transesterification reaction” [20].
Section 3.0: New Methods of Measuring Properties of Biodiesels and Renewable Diesels  
Beyond just researching new methods of developing new renewable diesels and biodiesel, researchers also study methods of grading both renewable fuels and traditional fossil fuels. This section covers proposed methods of grading or testing new biofuels and traditionally used fossil fuels.
One study by Huang et al., proposes a system of measuring the standard enthalpy and exergy of fuels. In order to validate this new system, Huang et al. compared the results from the new system to five preexisting, established models. The proposed system, called the unified thermodynamic reference system, ensures thermodynamic properties are maintained throughout each model. The high accuracy prompted the researchers to estimate the energy characteristics of thirty-nine prospective fuels such as “future fuels, including biodiesel, alcohol fuel, cellulosic fuel, marsh gas, and municipal refuse” [23]. Huang et al. “established these models under the thermodynamic reference system, ensuring the thermodynamic consistency and the reliability of prediction results” [23]. The appeal of these new models is that they are easier to execute and do not require the individual components of the fuel. Instead, these models only require the “value of the standard combustion heat” [23].
Polikarpov et al. also devised a new method of analyzing fuel properties with low volumes. Typically, one liter of any given fuel would be required to evaluate any properties of said fuel [24]. This novel approach only requires 15 mL of any given fuel to evaluate its properties [24]. The way this new method of analysis works is by using “dilution and autoignition resistance via measurement of ignition delay to estimate the octane number or cetane number of a new blend stock” [24]. By estimating the octane and cetane numbers of a potential renewable biodiesel, researchers can get a general idea of whether this fuel is worth investigating or not. This new method of evaluation could be especially helpful in evaluating the properties of expensive renewable diesels and biofuels [24]. Furthermore, this newly reduced amount of volume required speeds up the research process for inexpensive fuels as well. This is because fuels that do not meet desired properties can be dropped early in favor of other potentially viable fuels.

Conclusion  

Research of renewable diesel and biofuels is continuing to advance every year as innovative ideas are assessed and shared within the scientific community. The aforementioned are just a few of the many promising ideas that are continually being evaluated from various angles, leading to innovative new ways of producing new sustainable energy sources. The prospect of reducing greenhouse gas emissions through replacing traditionally used soybeans with soybean variants is quite promising, and may even be produced on an even larger scale in the future, leading to fewer greenhouse gas emissions. Being made from innovative sources, the properties of these new renewable diesels and biodiesels are also extremely desirable, incentivizing the replacement of conventional fossil fuels. Experimentation with farnesane, octenol, and other innovative feedstock sources, such as jatropha and neem seed, as well as methods to make renewable diesel, are just a handful of many ongoing studies. New catalysts such as basic zeolites and Co@CuO  are researched and are also extremely promising. Additionally, researchers are not only seeking new types of sustainable renewable diesel and biodiesel, but also new ways of evaluating these diesels. Overall, the future that renewable diesel and biofuels bring is one that is green and eco-friendly, propelling humanity into the future without the dependence on copious amounts of fossil fuels for energy.

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Authors

Dr. Raj Shah is a Director at Koehler Instrument Company in New York, where he has worked for the last 25 plus years. He is an elected Fellow by his peers at IChemE, AOCS, CMI, STLE, AIC, NLGI, INSTMC, Institute of Physics, The Energy Institute and The Royal Society of Chemistry. An ASTM Eagle award recipient, Dr. Shah recently coedited the bestseller, “Fuels and Lubricants handbook”, details of which are available at ASTM’s LongAwaited Fuels and Lubricants Handbook 2nd Edition Now Available https://bit.ly/3u2e6GY.
He earned his doctorate in Chemical Engineering from The Pennsylvania State University and is a Fellow from The Chartered Management Institute, London. Dr. Shah is also a Chartered Scientist with the Science Council, a Chartered Petroleum Engineer with the Energy Institute and a Chartered Engineer with the Engineering council, UK. Dr. Shah was recently granted the honourific of “Eminent engineer” with Tau beta Pi, the largest engineering society in the USA. He is on the Advisory board of directors at Farmingdale university (Mechanical Technology), Auburn Univ (Tribology), SUNY, Farmingdale, (Engineering Management) and State university of NY, Stony Brook ( Chemical engineering/ Material Science and engineering). An Adjunct Professor at the State University of New York, Stony Brook, in the Department of Material Science and Chemical engineering, Raj also has over 650 publications and has been active in the energy industry for over 3 decades. More information on Raj can be found at https://bit.ly/3QvfaLX
Contact: rshah@koehlerinstrument.com
Dr. Vikram Mittal, PhD is an Associate Professor in the Department of Systems Engineering at the United States Military Academy.  His research interests include energy modeling, technology forecasting, and engine knock. Previously, he was a senior mechanical engineer at the Charles Stark Draper Laboratory.  He holds a PhD in Mechanical Engineering from MIT, an MS in Engineering Sciences from Oxford, and a BS in Aeronautics from Caltech. Dr. Mittal is also a combat veteran and a major in the U.S. Army Reserve.
Mr. David Forester recently retired after 44 years’ experience in the fuel and refining additive business. He has over 35 US patents on development of diesel and jet fuel additives, refinery antifoulants, and other refinery and process related additives. He has designed, implemented and/or automated many fuel test methods, including many ASTM standards that have resulted in new additive products, reformulations, and improvements to diesel fuel additive products. He served as an ASTM D02.14 Subcommittee officer for 25 years. He received numerous ASTM awards including the Award of Merit.
Mr.  Victor Zhang, Mr. William Streiber, Mr. Gavin Thomas are part of a thriving internship program at Koehler Instrument company in Holtsville, and are students of Chemical and Molecular Engineering, Stony Brook University  ,Stony Brook, New York  where Dr.’s Shah and Mittal are on the external advisory board of directors.