Recent Fuel Specification Changes

By Ambrose Hughey, OMA

Lab Manager – Portland, Oregon

In recent years, biodiesel consumption has grown considerably in the United States. Sources indicate that double-digit production increases are expected. This increased production and utilization creates a demand to control these fuel products. As we learn more about biodiesel and biodiesel blends through their production and use, we continue to evaluate the associated fuel specifications in order to implement improvements.

The feedstock and process used in the production of biodiesel influences the final product’s various properties. The predominant B100 feedstock used in the U.S. is soybean oil, while Europe’s major source of biodiesel production is rapeseed oil. Palm oil feedstock is used predominantly in biodiesel production outside the U.S. and Europe. Although soybean oil is the leading feedstock in the U.S., the U.S. also uses a variety of plant oils such as soy and corn, animal fats, and used cooking oils. With several feedstocks used in biodiesel production, there are potential issues in product control resultant of the varying biodiesel properties.

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. ASTM D975 also requires that the biodiesel used in the blending must meet the specification of ASTM D6751, which is the standard specification for B100 to be used as a blend stock for middle distillate fuels. ASTM D6751 was also recently revised to include the cold soak filterability test (CFST) as an annex. ASTM subcommittee D02.14 currently has a work item to publish the CSFT as a standalone standard test method. Cold temperature applications of biodiesel and its blends will be grossly affected by the cold soak filterability of the biodiesel blend stock. Recent studies have shown that components of biodiesel may be soluble at room temperature but, with cooling, become insoluble, not returning to solution even upon return to room temperature. These newly formed insolubles will potentially cause filter-plugging issues.

Also 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 be resultant of the biodiesel component. The Table 1 requirements in D7467 were developed such that blends of on-spec D975 middle distillate fuel and on-spec D6751 B100 should routinely meet D7467 specifications.

ASTM D7467 Table 1, in comparison to D975 Table 1, adds an acid number as a way to monitor the level of acidic constituents in the blends. These acidic constituents can originate from the free fatty acids of the biodiesel component, from the processing acids in the distillate fuel, and from natural occurrence of acid due to the aging process. Another addition is the oxidation stability test, which is already a test parameter outlined in the B100 blendstock specification. This parameter is used to monitor the oxidizing tendency of the fuel, which can be used as an indication of the storage stability of the fuel. The D7467 Table 1 requirement for the distillation 90 percent recovery temperature’s maximum limit is 5°C higher than D975 fuel requirements, rising from 338°C (640°F) to 343°C (650°F).

Another specification change in D7467 in comparison to D975 is the inability to use ASTM D4737 Calculated Cetane Index as an approximation of the cetane number. Cetane engines are few, and the cost for the analysis can be pricey; therefore, the cetane index has been used as a means to estimate the cetane number by employing the API gravity and the 10 percent, 50 percent, and 90 percent distillation boiling points. Both API gravity and distillation are routine tests for fuel laboratories. As biodiesel has a naturally high cetane number along with excellent lubricity properties, ASTM D7467 asserts that each of these tests are not necessary to be performed on biodiesel blends (B6 to B20) when the middle distillate fuel portion of the blend met its respective parameter’s D975 Table 1 requirements. Total and free glycerin were not added to Table 1 of D7467,as there is not yet an appropriate test method for determining these components in biodiesel blends because the middle distillate fuel component causes interference in the detection of glycerin by the currently employed methods. The amount of glycerin in the finished biodiesel blend is currently solely monitored by the analysis of glycerin in the B100 blend stock.