B100 Quality Specification

The specification for biodiesel (B100) is frequently updated, and as of this writing the most current version is D6751-15cε1, summarized in Table 3. For the most up-to-date version of the specification, check the ASTM website (astm.org).

In 2012, a new grade of B100 was added to the D6751 specification. This new grade is referred to as the No. 1-B grade and is a special-purpose grade of biodiesel meeting more stringent purity requirements intended to provide better low-temperature performance. Most biodiesel produced at the time of publication is No. 1-B grade. This grade may be required in certain applications, but it is up to the customer to select the proper grade of biodiesel for their application.

Specification D6751 is intended to ensure the quality of biodiesel to be used as a blend stock in middle distillates, like diesel fuel and heating oil, at 20% and lower levels. Any biodiesel used in the United States should meet ASTM D6751 before blending. ASTM D6751 is based on the physical and chemical properties needed for safe and satisfactory diesel engine and boiler operations. It is not based on the specific raw materials or the manufacturing process used to produce the biodiesel. The finished blend stock must meet the properties specified in Table 3 as well as the following definition from D6751:

“Biodiesel, noun, a fuel comprised of mono-alkyl esters of long chain fatty acids derived from
vegetable oils or animal fats, designated B100.”

This specification was never intended to be applied to B100 to be used as a neat fuel. However, buyers and sellers are encouraged to use ASTM D6751 for the commercial trading of biodiesel (B100) for blending. Other arrangements or specifications can be legally used if the buyer and seller agree as long as they meet pertinent local, state, and federal regulations (EPA sulfur limits, Occupational Safety and Health Administration [OSHA] safety limits on flash point, etc.). However, B100 must meet the requirements of D6751 for blends to be legal fuels under the Clean Air Act fuel registration requirements and to be a legal blending component under many state regulations.

The intent of each quality requirement in Table 3 is described here:

• High levels of Group I and II metals. Sodium (Na),potassium (K), calcium (Ca), and magnesium (Mg)can cause deposits to form, catalyze undesired side reactions, and poison emission control equipment. The Group I and II metals are limited as the combination of metals in each category, Na+K and Ca+Mg. The specification upper limit is 5 parts permillion (ppm), combined, for each pair of metals.Research is ongoing to determine whether these metals limits are adequate for protection of NTDE emission control catalysts and filters.
• Flash point. A minimum flash point for diesel fuel is required for fire safety. B100’s flash point is required to be at least 93°C (200°F) to ensure all the alcohol from production is removed; it is classified as nonhazardous under the National Fire Protection Association code.
• Alcohol. It is critical to ensure that the manufacturer has removed excess alcohol (typically methanol) used in the manufacturing process. Residual methanol in the fuel is a safety issue, because even very small amounts dramatically reduce the flash point, can affect fuel pumps, seals, and elastomers, and can result in poor engine combustion properties. The intent of the alcohol control requirement is to limit volatile alcohols to less than 0.2 percent by weight (wt %). This can be accomplished by meeting a higher flash point requirement of 130°C(266°F) or by measuring methanol content by gas chromatography.
• Water and sediment. This refers to free water droplets and sediment particles suspended in the B100. The allowable level for B100 is set at the same level as for conventional diesel fuel. Poor drying techniques during manufacturing or contact with excessive water during transport or storage can cause B100 to be out of specification for water content.Excess water can lead to corrosion and provides an environment for microorganisms. Fuel oxidation can also raise sediment levels, so this test can be used in conjunction with acid number, oxidation stability, and viscosity to determine if fuels have oxidized too much during storage.
• Viscosity. A minimum viscosity is required for some engines because of the potential for power loss caused by injection pump and injector leakage.This is not an issue for B100, and the minimum is set at the same level as for petroleum diesel.The maximum viscosity is limited by the design of engine fuel injection systems. Higher viscosity fuels can cause poor fuel combustion that leads to deposit formation as well as higher in-cylinder penetration of the fuel spray, which can result in elevated engine oil dilution with fuel. The maximum allowable viscosity in ASTM D975 for No. 2 diesel is 4.1 mm2/s at 40°C (104°F). ASTM D6751 allows for slightly higher viscosity than D975, primarily because the normal viscosity of B100 is slightly higher than that of diesel fuel. Biodiesel blends of 20 vol% or lower should have viscosities between 1.9 and 4.1 mm2/s, within the range allowed by D975.
• Sulfated ash. This test measures the amount of residual alkali catalyst in the biodiesel as well as any other ash-forming compounds that could contribute to injector deposits or fuel system fouling.
• Sulfur. This is limited to 15 ppm to reduce sulfate and sulfuric acid pollutant emissions and to protect exhaust catalyst systems on NTDEs. Biodiesel generally contains less than 15 ppm sulfur. The test for sulfur in fuel (ASTM D5453) should be used for accurate results instead of D2622, which will provide falsely high results caused by test interference with the oxygen in the biodiesel.
• Copper strip corrosion. This test is used to indicate potential difficulties with copper and bronze fuel system components. The requirements for B100and conventional diesel are identical, and biodiesel meeting other D6751 specifications always passes this test. Copper and bronze may not corrode in the presence of biodiesel, but prolonged contact with these catalysts can degrade the fuel and cause sediment to form.
• Cetane number. An adequate cetane number is required for good engine performance. Conventional diesel must have a cetane number of at least 40 in the United States. Higher cetane numbers help ensure good cold start properties and minimize the formation of white smoke. The ASTM minimum limit for B100 cetane number is set at 47 because this is the level identified for “Premium Diesel Fuel” by the National Conference of Weights and Measures. Also, a 47 cetane number has been the lowest cetane number found in U.S. biodiesel, from a wide array of diverse feed stocks. The cetane index(ASTM D976) is not an accurate predictor of cetane number for biodiesel or biodiesel blends because itis based on a calculation that uses specific gravity and distillation curve, both of which are different for biodiesel than for petroleum diesel.
• Cloud point is the most commonly used measure of low-temperature operability. Fuels are generally expected to operate at temperatures as low as theircloud point. The cloud point of B100 is typically higher than the cloud point of conventional dieselfuel. Cloud point must be reported for biodiesel. Low-temperature properties and strategies for ensuring good low-temperature performance of biodiesel blends are discussed in more detail in later sections.
• Carbon residue measures the carbon-depositing tendency of a fuel and is an approximation of the tendency for carbon deposits to form in an engine.For conventional diesel fuel, the carbon residue is measured on the 10% distillation residue. Because B100 boils entirely at the high end of the diesel fuel range and in a very narrow temperature range, it is difficult to leave only a 10% residue when distilling biodiesel. So biodiesel carbon residue specifies that the entire biodiesel sample be used rather than the 10% distilled residue.
• Acid number for biodiesel is primarily an indicatorof free fatty acids (natural degradation products of fats and oils or a component of some biodiesel feedstocks) and can be elevated if a fuel is not properly manufactured or has undergone oxidative degradation. Acid numbers higher than 0.50 milligram potassium hydroxide per gram (mg KOH/g)have been associated with fuel system deposits and reduced life of fuel pumps and filters.
• Free and total glycerin numbers measures the amount of unconverted or partially converted fats/oils and by-product glycerin in the B100. Incomplete conversion of the fats and oils into biodieselcan lead to high total glycerin from elevated mono-,di-, and tri-glycerides. Incomplete removal of glycerin can lead to high free and total glycerin. If the glycerides are too high, the storage tank, fuel system, and engine can be contaminated. Fuels that exceed these limits are highly likely to plug dispenser and/or vehicle filters and cause other problems. One of the major shortcomings of the D6584 gas chromatograph method is its sensitivity to diesel fuel. Diesel fuel components over whelm the column used in the gas chromatograph, making accurate determination of glycerin and glycerides difficult or impossible, and may damage the column. Thus, many laboratories are unable to determine free and total glycerin by this method in samples with even small amounts of diesel fuel.
• Phosphorus content is limited to 10 ppm maximum in biodiesel because it can damage emission control systems. Phosphorus above 10 ppm can be present in some plant oils and recycled greases. At this time, biodiesel produced in the United States generally has phosphorus levels of about 1 ppm.
• The T90 distillation is the temperature where 90%of the fuel has distilled. The specification was incorporated to ensure that fuels have not been contaminated with high boiling materials such as used motor oil or triglycerides. B100 exhibits a boiling point rather than a distillation curve. The fatty acids from which biodiesel are produced are mainly straight chains with 16 to 18 carbons that have similar boiling point temperatures. The atmospheric boiling point range of biodiesel is generally 330°C to 357°C (626°F to 675°F).
• Oxidation stability. Biodiesel can oxidize during storage and handling, leading to the formation of peroxides, acids, gums, and deposits. The minimum oxidation stability requirement is intended to ensure the storage stability of B100 and biodiesel blends in clean tanks.
• Cold soak filter ability was added in 2008 in response to data indicating that some B100 could,in blends with petroleum diesel of up to 20%, form precipitates above the cloud point. B100 meeting the cold soak filter ability requirements does not form these precipitates. This, along with cloud point, is needed to predict low-temperature operability.
• No. 1-B grade. The No. 1-B grade has year-round limits on cold soak filter ability and monoglycerides. These limits ensure that trace components in biodiesel are minimized, while not requiring the measurement of many different compounds that may or may not be present in biodiesel. In particular, the limit on monoglycerides limits the saturated monoglyceride (SMG) content of the biodiesel. The percent of SMG in a B100 will be determined by the percent of saturated FAME.For example, if a B100 is 30% saturated FAME,the monoglyceride in the B100 will contain approximately 30% SMG.

Specification D6751 also includes the following workmanship statement:

“The biodiesel fuel shall be visually free of undissolved water, sediment, and suspended matter.”