November 2009

Biodiesel Blends
by Ambrose Hughey, OMA I
Laboratory Manager, Portland, Oregon

Both the feedstock and the process used in the production of biodiesel influence the final product’s various properties. Although the predominant B100 feedstock in the U.S. is soybean oil, several other feedstocks are also used in biodiesel production. Biodiesel feedstocks include a variety of plant oils (such as soy and corn), animal fats, and used cooking oils. With the use of this wide variety of feedstocks, there are potential issues in product control resulting from the varying biodiesel properties. The most common source of biodiesel consumption is through the use of biodiesel blends, as opposed to biodiesel being used as a stand-alone fuel. Biodiesel is blended with conventional middle distillate fuel and used in the same applications in which conventional diesel fuel is typically used. The recent introduction of biodiesel blends in the marketplace has prompted fuel specification considerations.

In October 2008, ASTM D975 (the specification for diesel fuel oils) was revised to allow up to 5 percent by volume of biodiesel, as long as the fuel still meets the requirements outlined in Table 1. Also transpiring in October 2008, ASTM published its first standard specification for biodiesel blends, ASTM D7467. This new biodiesel blends specification covers biodiesel blend volumes B6 to B20. The Table 1 requirements in D7467 are similar to the Table 1 requirements outlined in ASTM D975 for diesel fuel oils, as the majority of these low-level biodiesel blends are composed of the middle distillate fuel oil component. However, with the introduction of biodiesel into the middle distillate fuel, new parameters need to be monitored to evaluate any effects on fuel quality that could result from the biodiesel component.

This article briefly discusses changes to some of the important middle distillate fuel properties when biodiesel blends are produced. This article is intended for general purposes and is not meant to be comprehensive. It is also assumed that both the middle distillate fuel and biodiesel meet their respective ASTM specification D975 and D6751.

Low temperature operability: Biodiesel exhibits reduced cold temperature performance compared to middle distillate fuel; therefore, biodiesel blends will have reduced cold temperature performance. Cold flow additives are often used to improve the low temperature operability of fuels. Low temperature operability is very important as the wax formation can clog fuel filters on equipment. This can lead to fuel starvation and shutdown of equipment. The low temperature operability characteristics of your fuel can be measured by methods such as cloud point, cold filter plugging point (CFPP), and pour point.

40°C Viscosity: Generally, biodiesel has a higher viscosity than middle distillate fuel. However, blends up to B20 have not had issues meeting the viscosity requirements. The D7467 viscosity requirements are the same as D975 viscosity limits.

Water content: Biodiesel has a higher water saturation point than middle distillate fuel and may possess higher levels of water before showing a haze. Water removal from biodiesel also tends to be more difficult. As a result, biodiesel blends may have more dissolved water and complications with water removal. Monitor your fuel and storage tank’s water content to make certain your fuel system does not have excessive water contamination.

Microbial contamination: Most microbial activity occurs at the fuel water interface. With the potential for increased levels of water, there is subsequently an increased concern for microbial activity. Monitoring microbial contamination is important for any fuel storage system, but an increased awareness of microbial contamination should be applied to biodiesel and biodiesel blends. Proper fuel storage tank housekeeping is a chief prevention tool for microbial contamination and its associated problems.

Cetane number: Biodiesel naturally has a higher cetane number than middle distillate fuels. Consequently, biodiesel blends will enjoy improved cetane number over the middle distillate fuel.

Lubricity: Biodiesel possesses excellent lubricity properties, and as little as a B1 blend has been shown to improve the lubricity of middle distillate fuel. Biodiesel blends will have better lubricity performance than the middle distillate fuel portion.

Compatibility: Some materials are not chemically compatible with biodiesel. Some hose material, gaskets, seals, elastomers, glues, and plastics have been shown to have compatibility issues when exposed to biodiesel for a prolonged period of time. Therefore, biodiesel blends could have an increased potential for compatibility issues with these materials as well.

Solvency: B100 has a solvency effect than can promote fuel system problems. In biodiesel blends, the solvency effect is diluted by the diesel fuel, but there is still an increased potential compared to conventional diesel fuel alone. The reduced solvency of the biodiesel blend may facilitate soluble material in the biodiesel portion to become insoluble and drop out of solution, which would increase particulate contamination and filter plugging potential.

Storage Stability: Generally, biodiesel can degrade faster than conventional middle distillate fuel, which should not be problematic for normal use. However, there may be a need for increased monitoring for long-term storage (12 months) of biodiesel blends. For long-term storage, a fuel monitoring program is an important part of an effective long-term storage program. Periodic sampling and analysis will provide the means by which to make important decisions regarding aged fuel, its storage, and its suitability for continued use.

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