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 April 2007
Features
ROBERT McCORMICK, Ph.D. is leader of the Non-Petroleum Based Fuels activity at the National Renewable Energy Laboratory. NPBF develops renewable fuel blending components to extend petroleum supply, including biodiesel and ethanol. Research areas include biodiesel quality, compatibility with modern engines, pollutant emissions and impact on engine durability.
STEVEN R. WESTBROOK is principal scientist at Southwest Research Institute, where he has been involved in standardized and non-standardized testing of petroleum fuels and lubricants. He is the current chairman of ASTM Subcommittee D02.E0 on Burner, Diesel, Marine and Non-Aviation Gas Turbine Fuels, and chairman of the International Association for Stability, Handling, and Use of Liquid Fuels.

Induction Time

In the EN14112 test a 3-g sample of B100 is heated at 110 ºC while air is bubbled through the sample at a specified flow rate. For hydrocarbon liquids undergoing oxidation, it is typical for there to be a significant time lag between the start of heating/air contacting and the start of formation of the products of oxidative degradation. During this time lag or induction period free radical concentrations increase to a level where sustained reaction can occur. At the end of the induction period the formation of volatile acids such as formic acid is observed. The volatile acids are carried out of the B100 by the air and into a water bath. The conductivity of the water bath is monitored with a sharp increase in conductivity signaling the end of the induction period.

Biodiesel & Biodiesel Blends

ASTM International Committee D02 on Petroleum Products and Lubricants and Subcommittee D02.E0 on Burner, Diesel, Non-Aviation Gas Turbine, and Marine Fuels have been particularly active over the past year in producing new and improved standards for biodiesel and blended biodiesel fuels.

The term “biodiesel” refers to mono-alkyl esters of fatty acids, typically fatty acid methyl esters, that are produced from vegetable oil, animal fat, or waste cooking oil in a chemical reaction known as transesterification. In the United States, soybean oil is the major feedstock, while rapeseed oil is used in Europe and palm oil is a large player in other regions of the world. While there are some users of 100 percent biodiesel, or B100, as a fuel, the properties of B100 are considerably different from those of conventional diesel fuel or heating oil; it is not clear that diesel engines or boilers are fully compatible with B100. Therefore, it is much more common to use biodiesel as a blend with petroleum diesel at levels ranging from 2 (B2) to 20 (B20) percent by volume.

The U.S. Department of Energy has conducted a resource assessment that indicates the potential to produce between 1 and 2 billion gallons (4 and 8 billions of liters) per year of biodiesel within the next decade.1 Additionally, DOE has conducted a life cycle energy analysis that shows a 19 percent reduction in life cycle petroleum consumption and a 16 percent reduction in life cycle carbon dioxide emissions for use of B20 derived from soybean oil.2 The National Biodiesel Board estimates that 2006 U.S. consumption of biodiesel was 225 million gallons.

Standard Specification for B100 Biodiesel

The only ASTM International standard that exists today for biodiesel is D 6751, Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels. An important feature of this specification is that it is not a fuel specification but is for a fuel blend stock that can be used at up to 20 volume percent. To protect the performance and durability of combustion equipment, D 6751 places limits on impurities that can remain from the biodiesel production process. These include methanol, glycerin (a byproduct), unconverted and partly converted feedstock, and the sodium plus potassium that may remain from the caustic used to catalyze the transesterification reaction.

Methanol is one of the reactants used to produce biodiesel. To drive the reaction to completion, it is typical to use 4:1 excess methanol, hence methanol must be removed from the product. Methanol above about 0.2 weight percent is known to be incompatible with some automotive fuel system elastomers and metals.

D 6751 limits methanol by requiring that the B100 meet one of two requirements. In the first option, the flashpoint as measured by D 93, Test Methods for Flash Point by Pensky-Martens Closed Cup Tester, must be above 130.0 °C. In the second option, the flashpoint must be above 93.0 °C and the methanol content must be below 0.2 percent as determined by European Norm EN 14110, “Fat and oil derivatives—Fatty acid methyl esters (FAME)—Determination of methanol content [Analysis of headspace at 90 °C by gas chromatography].” A minimum flashpoint of 93.0 °C ensures that the biodiesel is in a nonhazardous category for transportation purposes.

Glycerin is a byproduct of the transesterification reaction. Glycerin is a viscous liquid that can separate from the B100 or blend to form deposits at the bottom of fuel storage tanks and can cause fuel filter plugging. High levels of glycerin can also lead to fuel injector deposits. To prevent these operational problems, ASTM D 6751 limits free glycerin to 0.020 weight percent maximum.

High levels of unconverted or partly converted feedstock (vegetable oil, waste cooking oil, or animal fat) are known to cause fuel injector fouling and cylinder deposits in diesel engines, reducing engine life. These materials can also cause short-term operational problems at low temperatures in both diesel engines and boilers. Chemically, these materials are mono-, di-, and triglycerides (or more properly, acyl glycerides). Their chemical structure involves a glycerin “backbone” connected to one, two, or three fatty acid chains by ester linkages (see Figure 1). To prevent operational problems caused by these components, ASTM D 6751 limits total glycerin to 0.240 weight percent. Total glycerin is the sum of free glycerin and bound glycerin. Bound glycerin is the glycerin bound in the mono-, di-, or triglyceride molecules.

Most biodiesel production schemes use sodium hydroxide (caustic) or potassium hydroxide to catalyze the transesterification reaction. The residual catalyst must be removed to avoid excessive fuel injection system wear, deposit formation, and contamination of engine lubricant. ASTM D 6751 limits sodium plus potassium to 5 ppm maximum as determined using EN 14538, “Fat and oil derivatives—Fatty acid methyl esters (FAME)—Determination of Ca, K, Mg and Na content by optical emission spectral analysis with inductively coupled plasma (ICP OES).” Additionally, biodiesel can become contaminated with calcium and magnesium from the use of hard water to extract (or wash) impurities, or from the use of certain adsorbents to remove impurities. ASTM D 6751 also limits calcium plus magnesium content to 5 ppm as determined by EN 14538.

A requirement recently added to ASTM D 6751 is oxidation stability. Oxidation can lead to the formation of corrosive acids, as well as to the formation of gums and deposits that can cause operational and durability issues. Biodiesel contains polyunsaturated fatty acid chains that can be significantly more susceptible to oxidation during storage, handling and use than is the case for conventional petroleum diesel. Acidity has long been limited in D 6751 by limiting the acid number. During 2006, the acid number limit was reduced from 0.80 mg KOH/g to 0.50 mg KOH/g (determined by ASTM D 664, Test Method for Acid Number of Petroleum Products by Potentiometric Titration). Oxidation stability is also limited by requiring a minimum three-hour induction period on EN 14112, “Fat and oil derivatives—Fatty acid methyl esters (FAME)—Determination of oxidation stability (Accelerated oxidation test),” known as the oil stability index. Commercial biodiesel samples can have a wide range of stability because of different amounts of polyunsaturated fatty acids, but also because of differing levels of natural antioxidants. We estimate that roughly 25 percent of the B100 in the U.S. market today will require the addition of antioxidant additives in order to meet the three-hour induction period requirement (see sidebar above left).

Several other changes have also been made to D 6751 recently. These include the allowance of derived cetane number as determined by D 6890, Test Method for Determination of Ignition Delay and Derived Cetane Number (DCN) of Diesel Fuel Oils by Combustion in a Constant Volume Chamber, as an option for cetane number, with the cetane engine method, D 613, Test Method for Cetane Number of Diesel Fuel Oil, remaining as the referee method. Additionally, the automatic cloud point method D 5773, Test Method for Cloud Point of Petroleum Products (Constant Cooling Rate Method), is now allowed as an option for the manual cloud point, D 2500, Test Method for Cloud Point of Petroleum Products. A number of new or modified requirements for ASTM D 6751 have been under consideration by Subcommittee D02.E0 and various working groups or task forces. Currently, D 6751 limits water and sediment as determined by ASTM D 2709, Test Method for Water and Sediment in Middle Distillate Fuels by Centrifuge, to 0.050 volume-percent maximum. The subcommittee is considering replacing this requirement with a 500 ppm limit on dissolved water as determined by the Karl Fischer method (D 6304, Test Method for Determination of Water in Petroleum Products, Lubricating Oils, and Additives by Coulometric Karl Fischer Titration) and a limit on particulate contamination using a method under development. Additionally, the subcommittee is examining the replacement of D 1160, Test Method for Distillation of Petroleum Products at Reduced Pressure, distillation with a simulated distillation method. The inclusion of D 7039, Test Method for Sulfur in Gasoline and Diesel Fuel by Monochromatic Wavelength Dispersive X-ray Fluorescence Spectrometry, as an alternative method for the determination of sulfur by D 5453, Test Method for Determination of Total Sulfur in Light Hydrocarbons, Spark Ignition Engine Fuel, Diesel Engine Fuel, and Engine Oil by Ultraviolet Fluorescence, is also under consideration.

Status of Specifications for Biodiesel Blends

During the fall of 2006, Subcommittee D02.E0 issued a ballot for a proposed B5 biodiesel blend specification. The ballot was to modify the ASTM D 975 diesel fuel specification to allow for addition of up to 5 percent B100 to a petroleum diesel as long as the resulting blend still met all the requirements of D 975. The ballot received several negatives based on concern over a potential low-temperature operability issue. During the last two winter seasons, there have been some biodiesel blends that produce a precipitate at temperatures above the cloud points of the individual blend components. In all of these cases, both blend components met all the requirements of their respective specifications. The negatives were withdrawn on the condition that Subcommittee D02.E0 establish a task force to investigate the issue. That task force is already at work and is trying to complete its study before the next committee meeting in June 2007. Further action on the proposed B5 specification is on hold pending the findings and recommendations of the task force.

Subcommittee D02.E0 is also working on a separate specification for biodiesel blends with concentrations between 6 percent (B6) and 20 percent (B20). This specification is separate from D 975 because of the higher allowable biodiesel concentrations and the need to add or modify requirements as compared to D 975. This ballot received some negatives and resolution of the negatives is proceeding. At this time, Subcommittee D02.E0 expects that the findings of the B5 low-temperature task force discussed above will also apply to the B6-B20 specification. Therefore, the subcommittee plans to receive the report from the task force before balloting for final approval of the B6-B20 specification.

Heating oil, as specified in D 396, Specification for Fuel Oils, is another area where Subcommittee D02.E0 is working with biodiesel. BioHeat? is the name given to blends of biodiesel and petroleum heating oil. Although research in some areas remains to be completed, most stakeholders agree that blends up to 5 percent should give acceptable performance in typical use. Low-temperature operability is less of a concern with heating oil since most heating oil tanks are located in basements or otherwise indoors. Material compatibility and storage stability seem to be the two largest concerns regarding the use of biodiesel as heating oil. Work in these areas by government, industry and trade organizations is under way. Subcommittee D02.E0 plans to consider all study results during the development of a specification for biodiesel/heating oil blends.

Subcommittee D02.E0 also has jurisdiction over D 2880, Specification for Gas Turbine Fuel Oils. These fuels are used in non-aviation, industrial gas turbine engines such as power generators and marine propulsion. Biodiesel is already in use in several of these applications. There are reports of using even B100 in some instances. Subcommittee D02.E0 expects to start working on standards for these applications within the next year, depending on the demand from pertinent stakeholders.

An important component of any biodiesel blend specification will be a method for determining biodiesel content. Subcommittee D02.04 on Hydrocarbon Analysis has successfully balloted a test method for determination of biodiesel in diesel fuel oil using mid IR. An ASTM interlaboratory study just concluded will provide precision to be included in the test method for the next ballot at the main committee level.

Subcommittee D02.E0 and Committee D02 have made considerable progress with biodiesel issues since the biodiesel industry first requested a standard specification a little over 10 years ago. We now have a specification for B100 and are close to approving three blend specifications. Members of the subcommittee have worked tirelessly with members of several other subcommittees to obtain inclusion of biodiesel and biodiesel blends in the scope of most of the test methods used in these specifications. More is needed but the work continues. Subcommittee D02.E0 expects to be working on other biodiesel standards in the near future, including a proposed B100 fuel specification (recall that D 6751 is a blend stock specification.)

The use of biodiesel in the U.S. and around the world is increasing rapidly. It has been a significant challenge to keep up with the demand for standards to accompany the rapid growth of this new fuel. However, because they recognize the value of consensus standards, all interested parties have worked within the ASTM process to develop standards useful to all. //

References

1 Tyson, K.S.; Bozell, J.; Wallace, R.; Petersen, E.; Moens, L. Biomass Oil Analysis: Research Needs and Recommendations. NREL/TP-510-34796. Golden, CO: National Renewable Energy Laboratory, June 2004.
2 Sheehan, J.; Camobreco, V.; Duffield, J.; Graboski, M.; Shapouri, H. An Overview of Biodiesel and Petroleum Diesel Life Cycles. NREL/TP-580-24772. Golden, CO: National Renewable Energy Laboratory, May 1998.