Biodiesel is a recycled, renewable resource that can be manufactured in the U.S. Although the chemical structure of biodiesel is a bit different than that of diesel refined from petroleum, its use is basically the same as petroleum diesel. Almost 12 million Class 3 to 8 trucks are registered in the U.S., according to R.L. Polk, of which approximately 80 percent are powered by diesel engines. The National Biodiesel Board estimates that the U.S. will produce 242 million barrels of biodiesel from crude oil between 2006 and 2015.
If all diesel fuel had a 20 percent blend of biodiesel, we would see a much more substantial reduction in the use of diesel fuel. For the most part, blends of up to 20 percent biodiesel can be used in any diesel engine. One concern is that biodiesel releases deposits in fuel tanks and fuel systems, which may require filter changes.
Vehicles that have the potential to be fueled by biodiesel include light-duty and heavy-duty vehicles manufactured after 1993, according to the United States Department of Energy (DOE).
Strengths and benefits of biodiesel include:
Relatively low impact to environment – Biodiesel contains virtually no sulfur or aromatics and, when used in a conventional diesel engine, results in reduction of unburned hydrocarbons, carbon monoxide and particulate matter. Its production and use results in 78.5 percent reduction in carbon monoxide emissions as compared to petroleum diesel.
Potential to reduce dependence on foreign supply – According to the National Biodiesel Board (NBB), for every billion dollars spent on foreign oil, the U.S. lost 10,000-25,000 jobs. Biodiesel can be manufactured domestically with vegetable oils using existing industrial production capacity and conventional equipment.
Biodiesel is a renewable alternative fuel suitable for use in most compression ignition (CI) engines. It can be produced from a wide range of vegetable oils and animal fats. The DOE defines biodiesel as “a liquid fuel made up of fatty acid alkyl esters, fatty acid methyl esters (FAME), or long-chain mono alkyl esters.” ASTM International (formerly, “American Society for Testing and Materials”) Standard D6751 – 11a, “Standard Specification for Biodiesel Fuel Blend Stock,” specifies that biodiesel “…shall be mono-alkyl esters of long chain fatty acids derived from vegetable oils and animal fats.”
Biodiesel is typically blended with conventional diesel fuel. The nomenclature for biodiesel references the percentage of biodiesel contained in the blend, as follows: Bxx, where “xx” represents the percentage of biodiesel (e.g., a mixture containing 10 percent biodiesel and 90 percent petroleum diesel is referred to as “B10.”; and pure biodiesel is B100.). In general, biodiesel burns cleaner than petroleum-based diesel fuel, is nontoxic, and is classified as being biodegradable.
Biodiesel can be made from a number of resources including new or used vegetable oils and animal fats. These resources are classified as being biodegradable, nontoxic, and renewable. Currently, in the United States, the primary biodiesel feedstock is soybean oil.
Biodiesel can be produced using any of three common esterification processes:
- Base catalyzed transesterification of the oil with alcohol
- Direct acid catalyzed esterification of the oil with methanol
- Conversion of the oil to fatty acids, and then to alkyl esters with acid catalysis
According to the National Biodiesel Board, the majority of alkyl esters produced today are made using the base catalyzed reaction process because it is the most economic.
Typically, the process utilizes approximately 10 gallons of alcohol (usually methanol) for every 100 gallons of feed stock processed, resulting in an output of approximately 100 gallons of biodiesel and 10 gallons of glycerin – a valuable commodity within the pharmaceuticals and cosmetics industries as well as other markets. In fact, saturation of the glycerin market may become a limiting factor for the production of biodiesel.
The remaining small amount of material produced in addition to the glycerin and biodiesel can be used as a crop fertilizer (around one percent of total process flow).
Per the DOE, raw or refined plant oil and recycled cooking greases that have not been processed into biodiesel, are not biodiesel and should be avoided. Plant oils or greases used in diesel engines can, even at low concentrations, cause engine deposits, ring sticking, lube oil contamination and other maintenance problems that may potentially reduce engine life. These problems are caused mostly by the greater viscosity of the raw oils (around 40 mm2/s) compared with that of the diesel fuel, for which the engines and injectors were designed (1.3 to 4.1 mm2/s).
- Current: The total supply of economically viable feedstocks currently available is capable of producing about 1.7 Billion gallons of biodiesel per year. This represents around five percent of the total highway diesel used in the U.S. each year.
- Future: A significant amount of research and development is being conducted to evaluate algae oil as a feedstock for biodiesel production. Algae oil yields per acre are up to 200 times greater than for soy oil (10,000 gallons vs. 50 gallons), and the use of algae has a lesser impact on the food supply chain. Current work indicates that diatoms and green algae are probably the best sources for biodiesel production. As with other bio-fuels, economic factors have a major impact on the overall commercialization process.
Biodiesel is registered as both a fuel (B100) and fuel additive (Bxx) with the Environmental Protection Agency (EPA) and meets clean diesel standards established by the California Air Resources Board (CARB). Pure (B100) biodiesel has been designated as an alternative fuel by the DOE and the U.S. Department of Transportation (DOT).
The National Biodiesel Board has provided the following statement concerning the use of biodiesel to meet Energy Policy Act (EPAct) and Executive Order 13149 Requirements:
Federal agencies can meet up to 50 percent of their AFV acquisition credits by using biodiesel fuel. Under this biodiesel fuel use credits provision, fleets may choose to operate existing diesel vehicles that weigh more than 8,500 lbs. on blends of biodiesel in lieu of purchasing a new alternative fuel vehicle. For each 450 gallons of pure biodiesel purchased and consumed, an alternative fuel vehicle credit is awarded. To constitute a ‘qualifying volume” a 20% biodiesel blend (B20) or higher blend must be used. For example, if a fleet wished to qualify for the credit using 100 percent biodiesel it would need to purchase and use 450 gallons of B100 to receive one credit. Alternatively, if a fleet wanted to qualify for the credit using B20, it would need to purchase and use 2,250 total gallons of the B20 fuel. B20 is also approved as a compliance tool for Executive Order 13149.
- CO2 Emissions: Direct source CO2 emissions for biodiesel (B100) are approximately 48 percent lower than petroleum diesel and the direct source emissions for B20 are approximately 12 percent lower (Source: U.S. EPA).
- Other Emissions: Tests conducted by the U.S. EPA show that biodiesel-fueled vehicles produce 12 percent (B20) to 48 percent (B100) less carbon monoxide; 47 percent (B100) to 12 percent (B20) fewer particulates; and 68 percent (B100) to 20 percent (B20) less unburned hydrocarbons per BTU of energy produced. Oxides of nitrogen (NOX) vary from a maximum of plus 10 percent (B100) to minus 2 percent (B20) depending on the engine in use.
The development of biodiesel as an internal combustion fuel is primarily supported by the National Biodiesel Board, who also provides detailed information on all aspects of biodiesel use on its website.
The National Biodiesel Foundation also supports the advancement of biodiesel as an alternative fuel through outreach, education, research and demonstration activities for the advancement of biodiesel and its co-products.
Specifications – Early development and adoption of biodiesel fuel was hampered by poor product quality and a lack of consistency. This issue has been addressed by the development of ASTM Standard D6751, covering biodiesel fuel blend stock, B100, in Grades S15 (15 ppm sulfur) and S500 (500 ppm sulfur) for use as a blend component with conventional petroleum-based diesel fuel.
Energy Balance – As with other bio-based fuels, there is considerable controversy as to the total energy balance associated with the production of biodiesel (total energy used to produce the product vs. energy content of biodiesel produced). A 1998 analysis by the National Renewable Energy Laboratory (NREL) indicated that biodiesel had a 3.2:1 positive energy balance. Energy Life-Cycle Assessment of Soybean Biodiesel Revisited, an updated analysis published in 2009, takes into account improved production processes, higher agricultural yields, and certain other factors that support an increased positive energy balance of 5.54:1.
Conversely, an often-cited analysis indicated that it takes 27 percent more energy to produce a gallon of biodiesel than the energy produced by the fuel. Biodiesel Energy Balance – a review of this analysis conducted by the University of Idaho – indicates that this negative yield was based on incomplete and/or erroneous data.
Biodiesel is a liquid fuel that is stored and handled using the same basic equipment as petroleum diesel. Some issues may occur with B100 and blends higher than B20 (See Handling and Blend Level Issues).
- Energy density – One gallon of B100 produces approximately 91 percent of the energy in a gallon of #2 petroleum diesel (118,000 BTU vs. 129,500 BTU). One gallon of B20 blend produces approximately 98 percent of the energy in a gallon of pure petroleum diesel (127,000 BTU vs. 129,500 BTU).
- On-Board Vehicle Storage – In general, lower blends (B20 and below) of biodiesel are handled exactly the same as petroleum diesel. A few cases of OEM vehicle fuel tank material incompatibility have occurred in older vehicles, but this is the exception. Higher blend levels may require vehicle modifications to address component incompatibility (See Handling and Blend Level Issues).
- Cetane Number (B100) – 47 minimum
- Fuel Efficiency – In general, the type of fuel being used (diesel vs. biodiesel) does not impact the thermal efficiency of a diesel engine. In addition, the energy content of biodiesel is more consistent than that of petroleum diesel which can vary significantly depending on the quality of the fuel and the blend between #1 and #2 diesel. As a result, while biodiesel (B100) has 9 percent less energy, users of lower blend levels (B20 or less) may see very little or no difference in total fuel economy.
Biodiesel is produced in multiple locations and distributed by truck, train, or barge. Due to the solvent characteristics of biodiesel, distribution by pipeline is not currently viable but tests are being conducted to determine the long-term feasibility of this means of transport. The National Biodiesel Board’s website provides a means to locate distributors by area. Most distributors will deliver pure B100 or product pre-blended to customer specifications.
According to data provided by NREL, the use of biodiesel as a transportation fuel will reduce U.S. petroleum consumption by approximately 95 percent compared to an equivalent petroleum diesel-fueled vehicle.
Lifecycle Greenhouse Gas Reduction
The CO2 associated with biodiesel (and other bio-fuels) is part of a closed loop system. A lifecycle analysis completed by NREL found that carbon dioxide emissions for B100 were 78.5 percent less than those from petroleum diesel, as a result.
Handling and Blend Level Issues
Biodiesel is typically blended with petroleum diesel. At low concentration levels (of up to B5) the mixture will meet ASTM D975 diesel fuel specification and can be used in any application as if it were pure petroleum diesel. Higher concentrations (B6-B20) can be used in most applications as long as the engine manufacturer warrants such use and the fuel meets ASTM D7467 specifications. In some cases (primarily older equipment), it may be necessary to replace components such as fuel lines and seals and in extreme cases even the fuel tanks. B20 is the most commonly used biodiesel blend in the United States because it provides a good balance among material compatibility, cold weather operability, performance, emission benefits, and costs. B20 is also the minimum blend level allowed for compliance with the Energy Policy Act of 1992 (EPAct), which requires the use of renewable fuels and/or alternative fuel vehicles (AFVs) by certain covered fleets.
Methyl esters have strong solvent properties and are commonly used as industrial cleaning solvents. As a result, biodiesel blends have a marked tendency to remove deposits from fuel storage tanks and lines. Before converting a fuel distribution facility to biodiesel, it is recommended that the tanks and lines be thoroughly cleaned. In addition, it will probably be necessary to change the distribution system and the on-board vehicle fuel filters more frequently during the conversion process.
Biodiesel can even be used as a fuel in higher blends, or even as pure biodiesel (B100), if proper precautions are taken. B100, and higher blend levels such as B50, require special handling and may require equipment modifications. These issues can potentially be managed with heaters and/or by changing engine seal and gasket materials. However, because the level of special care needed is high, the NREL and the DOE do not recommend the use of high-level biodiesel blends, except where human exposure to diesel particulate matter (PM) is elevated and health concerns merit the additional attention to equipment and fuel handling. Always consult your engine or combustion equipment manufacturer for further information about procedures before using biodiesel blends higher than B20.
For more information on this subject, read the Biodiesel Handling and Use Guide published by NREL.
Most major diesel engine manufacturers have stated that the use of biodiesel blends at various levels (ranging from B5 to B20) will not void their parts and workmanship warranties. Some engine companies have already specified that the biodiesel must meet ASTM D-6751 as a condition, while others are still in the process of adopting D-6751 or developed their own set of guidelines for biodiesel use prior to the approval of D-6751.
For more information, visit the National Biodiesel Board website.
Cold Weather Issues
B100 is more prone to cold weather issues than petroleum-based diesel, but blending can help remedy this issue. A 20 percent blend (B20) usually raises the cold weather properties of biodiesel by 3 to 10 F (pour point, cloud point, cold filter plugging point). Conventional low-temperature operability additives can be used with blends to improve cold weather operations and other cold weather techniques such as blending with #1diesel; the use of engine block or fuel filter heaters on the engine are also effective.
Long-term Storage Issues
All fuels, including biodiesel, are subject to oxidation. In biodiesel, fuel aging and oxidation can lead to high acid numbers, high viscosity, and the formation of gums and sediments that clog filters. It is recommended that biodiesel be used within six months of purchase to avoid operational problems. If a biodiesel blend is to be held for an extended period of time (four months or longer), the use of a fuel stabilizer is recommended.
Biocides are recommended for both conventional and biodiesel fuels in situations where the growth of algae or other biological contamination has been a problem. Such contamination is usually the result of water contamination and should be controlled as quickly as possible. Standard biocides used with petroleum-based diesel fuels work equally well with biodiesel.
- Reduced greenhouse gas emissions
- Reduced emissions of unburned hydrocarbons (HC), carbon monoxide (CO), sulfates, polycyclic aromatic hydrocarbons, nitrated polycyclic aromatic hydrocarbons, and particulate matter (PM)
- Higher Cetane number
- High energy density
- Domestically sourced - Reduces the use of imported petroleum fuels
- High level of fuel lubricity
- Blend level may be an issue in mixed fleets due to OEM warranty issues
- Limited availability in some areas
Lifecycle Cost Factors
- No differential in vehicle costs for blends of B20 or less
- Minimal, if any, difference in infrastructure costs
- No significant driver or vehicle maintenance issues
Current pricing varies significantly from area to area due to feed stock costs, transportation costs, and regional demand. The DOE Alternative Fuel Price Report provides current pricing by geographic region. Other potential factors (e.g., carbon credits, tax incentives, etc.) will have a significant impact on the future economic viability of biodiesel as an engine fuel. The DOE Website provides a list of current Federal Incentives and Laws for Biodiesel. In addition, biodiesel incentives and laws vary by state. The DOE Website provides a list of Biodiesel Incentives and Laws by state.
Other Information Resources
Biodiesel Handling and Use Guide (NREL publication)
Effects of Biodiesel Blends on Vehicle Emissions (NREL study)
Effects of Increased Biofuels on the U.S. Economy in 2022 (United States Department of Agriculture report)
EPA Lifecycle Analysis of Greenhouse Gas Emissions from Renewable Fuels (EPA proposal)
Fuel Quality and Performance Guide: A Troubleshooting Checklist For Diesel Fuel, Biodiesel and Bioheat Users (National Biodiesel Board guide)
On-Road and In-Laboratory Testing to Demonstrate Effects of ULSD, B20, and B99 on a Retrofit Urea-SCR Aftertreatment System (NREL study)