The objectives of this work were to identify a practical accelerated oxidative stability test method and to define a reasonable, data-based, stability minimum requirement. The biodiesel (B100) samples show a broad distribution of stability on accelerated tests, with EN14112 induction time results ranging from less than 1 h to as much as 12 h and ASTM D2274 total insolubles ranging from less than 1 mg/100 mL to nearly 18 mg/100 mL. The accelerated test data indicate that if the B100 stability is above roughly a 3 h induction time, blends prepared from that B100 appear to be stable on the induction time and D2274 tests. The D4625 long-term storage results for B100 indicate that most biodiesel samples, regardless of initial induction time, will begin to oxidize immediately during storage. If induction time is near or below the 3 h limit, the B100 will most likely go out of specification for either stability or acid value within 4 months (4 weeks on the D4625 test). Even B100 with induction times longer than 7 h will be out of specification for oxidation stability at only 4 months, although these samples may not have shown a significant increase in acidity or in deposit formation. The 3 h B100 induction time limit appears to be adequate to prevent oxidative degradation for B5 blends in storage for up to 12 months and B20 blends for up to 4 months. The results indicate that B100 stability is the main factor that affects the stability of B5 and B20 blends, independent of diesel fuel aromatic content, sulfur level, or stability. Synthetic antioxidants were highly effective at preventing acid and insoluble formation during storage.

Oxidation stability is one of the most important properties of fatty-acid methyl esters (biodiesel) used as a blend stock with petroleum diesel to make biodiesel blends. Even so, at the time of this writing, there is no oxidation stability requirement in ASTM Standard Specification D6751, the specification for biodiesel. The primary reason for the absence of an oxidation stability requirement is a lack of agreement on which oxidation stability test method to specify. The European biodiesel specification incorporates the Rancimat test as a measure of oxidation stability. The Rancimat test is one of the methods under consideration for the ASTM specification along with several other stability tests traditionally used for petroleum fuels.The purpose of this project was to compare and evaluate several candidate test methods. The information gathered from this work will be available to aid in the selection of a test method for inclusion in D6751.

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Executive SummaryIn support of the U.S. Department of Energy Fuels Technologies Program Multiyear Program Plan goal of identifying fuels that can displace 5% of petroleum diesel by 2010, the National Renewable Energy Laboratory (NREL), in collaboration with the National Biodiesel Board (NBB) and with subcontractor Southwest Research Institute, performed a study of biodiesel oxidation stability. The objective of this work was to develop a database that supports specific proposals for a stability test and specification for biodiesel and biodiesel blends. B100 samples from 19 biodiesel producers were obtained in December of 2005 and January of 2006 and tested for stability. Eight of these samples were then selected for additional study, including long-term storage tests and blending at 5% and 20% with a number of ultra-low sulfur diesel (ULSD) fuels. These blends were also tested for stability. The study used accelerated tests as well as tests that were intended to simulate three real-world aging scenarios: (1) storage and handling, (2) vehicle fuel tank, and (3) high-temperature engine fuel system. Several tests were also performed with two commercial antioxidant additives to determine whether these additives improve stability. This report documents completion of NREL's Fiscal Year 2007 Annual Operating Plan Milestone 10.1.The B100 samples examined show a broad distribution of stability on accelerated tests, with oil stability index (OSI) or Rancimat induction time results ranging from less than 1 hour to more than 9 hours and ASTM D2274 total insolubles ranging from less than 2 mg/100 ml to nearly 18 mg/100 ml. The accelerated test data indicate that if the B100 stability is above roughly a 3-hour induction time, blends prepared from that B100 appear to be stable on the OSI and D2274 tests.The D4625 long-term storage results for B100 indicate that most biodiesel samples, regardless of initial induction time, will begin to oxidize immediately during storage. If induction time is near or below the 3-hour limit, the B100 will most likely go out of specification for either stability or acid value within 4 months. Even B100 with induction times longer than 7 hours will be out of specification for oxidation stability at only 4 months, although these samples may not have shown a significant increase in acidity or in deposit formation. The 3-hour B100 induction time limit appears to be adequate to prevent oxidative degradation for both B5 and B20 blends in storage for up to 12 months.For tests that simulated fuel tank aging and high temperature stability, we conclude that stable B100 (longer than 3 hours induction time) leads to stable B5 blends. For B20, the results are less definitive, but provide considerable evidence that B100 with induction time of at least 3 hours produces stable B20 blends, but the test cannot differentiate between intermediate and highly stable samples for acid number increase or sediment formation under these worst-case test conditions. Additional work is required to confirm this finding and to...

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