QUALITY D6751 at 5 ppm for calcium/magnesium, and 5 ppm for sodium/ potassium. “The metals were previously limited by the sulfated ash test to relatively low levels,” says Steve Howell, president and founder of M4 Consulting, former longtime NBB technical director and AOCS Fellow. “With the advent of particulate traps coming in 2007 after ultra-low sulfur diesel was introduced in 2006, we added the separate specs for sodium and potassium, and calcium and magnesium for biodiesel.” These metals were identified since residues can be present in improperly processed biodiesel. “Most values we’ve seen for these in biodiesel are at or near the detection limit of 1 ppm, so it hasn’t really been a big industry focus,” Howell says. “This is especially so over the past few years, with most of the material being made from BQ-9000 companies that monitor closely to make sure their process is working properly.” With more diesel technology changes coming, however, Howell says there’s always more work to do to make sure the biodiesel spec keeps up with evolving diesel technology. “The changes we’ve made are working, [evidenced by the] quality of biodiesel we see in the market.” Any fuel quality issues experienced with biodiesel today are no different than “the normal issues we see with diesel fuel,” he says. Johnson says with the advent of close-coupled SCR filters, critical analyses must determine how biodiesel interacts with these devices. Howell and Johnson both say NBB and OEMs have launched a major test program to investigate this. “The big focus is the metals levels,” Howell says. “It’s currently limited to 5 ppm, but [it will] probably [have to be set] lower.” The introduction of the CSFT 10 years ago was another milestone in the evolution of biodiesel fuel quality. “Biodiesel fuel that met the ASTM specification at the time was still plugging filters under certain conditions,” says Brian Hess, Biodiesel Specs the technical service department In 2006, as diesel aftertreat- manager with Evonik Oil Addiment devices were being intro- tives USA Inc. “The method was duced to reduce PM and NOx, lim- developed to help prevent these its were imposed on trace metals in filter clogging issues in the field.” BIODIESEL MAGAZINE 2019 SUMMER EDITION 22 ice at retail stations, crude oil and the refinery if something upsets their system, or certain additives if they’re inadvertently dosed too high. There are multiple ways fuels can be contaminated.” Other efforts over the years at ASTM to change D975 have included measures to control deposit formations, oxidation and cold flow. “For deposit formations, these are specifically related to deposits in injectors,” Lopes says. “Until now there’s been no tool or method to characterize the propensity of the fuel to cause deposits, but numerous projects over the past decade have [made advances in this area].” He says there is still strong interest in putting stability requirements on finished diesel fuel. “Biodiesel has stability requirements, but when the finished fuel oxidizes, we’re not sure if it’s because of the biodiesel or petroleum diesel,” he says. Biodiesel’s oxidative stability is measured using the Rancimat test, but given the nature of the differences between diesel fuel and biodiesel, this would not be applicable for petroleum diesel, so a different test method and corresponding limits would have to be approved for D975. Lopes says there’s also been a parallel push to make nonmandatory cold flow guidelines mandatory in D975. “Diesel fuel has no mandatory requirements here in the U.S., unlike in the EU or other developed nations,” he says. Of the OEM’s greatest concerns over diesel fuel quality— moisture content, stability, metals, particulate contaminants, and cold flow—biodiesel actually has a specification and limits to address most of those, while diesel fuel does not. “Even though biodiesel is a relatively small component in the finished fuel, it’s the only component that has a filterability limit in the Cold Soak Filtration Test, a metals content limit, and a stability limit, which are some of the concerns OEMs have today,” Fenwick says. “Biodiesel is the only component that begins to address these.”
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He says the test is performed using 300 milliliters (ml) of B100 cooled and held at 40 degrees Fahrenheit for 16 hours. “The B100 sample is then warmed to room temperature and subject to vacuum filtration with a 0.7-micron filter,” Hess says. “All 300 ml of B100 must pass through the 0.7-micron filter in less than 360 seconds. For use as Grade No. 1-B, the biodiesel must filter in less than 200 seconds.” The method was initially a part of the D6751 annex, but it later got its own method—ASTM D7501. “One of the early changes to the method focused on the type of filter employed to improve test reproducibility,” Hess says, “so the current version calls for a specific brand of glass fiber filters.” Evonik’s Viscoplex 10-340 cold flow improver (CFI) has been effective in taking B100 samples from failing the CSFT to passing. “Depending on the biodiesel quality, the CFI can also provide faster filtration times to allow a B100 stock to meet the No. 1-B requirement of 200 seconds,” Hess says. “Viscoplex 10-340 is effective in a wide variety of biodiesel types ranging from canola, soy, tallow and used cooking oil.” The effectiveness of a CFI depends heavily on the type of feedstock. “Less saturated biodiesel feedstocks have better inherent low-temperature properties and will also respond best to cold flow additives,” Hess says. “More saturated feedstocks like soy and animal fat methyl esters have warmer starting cold filter plugging point (CFPP) and pour point values and require different additive compositions to achieve low-temperature improvement.” He says typical canola biodiesel can expect a CFPP improvement of 15 degrees Celsius with Viscoplex 10-340 whereas a soy biodiesel improvement in CFPP of 5 degrees C would be achievable. Hess says Evonik can help producers select the proper CFI and optimize the treat rate for a given B100 or blend, noting that CFI dosages typically range between 250 and 1500 ppm. “Ensuring that B100 meets the CSFT is an important safeguard against potential filter blocking issues,” Hess
says. “Viscoplex CFI technology can fix a B100 that fails the test and improve its performance to fulfill all ASTM D6751 quality requirements.” Stability is another hot-button issue for biodiesel quality, with some arguing that the specification requirements are not stringent enough—even though petroleum diesel has none. In order to demonstrate adequate storage suitability (shelf life), B100 must meet a minimum three-hour induction period (IP) under the Rancimat test as described in D6751, while D7467— the spec for B6-B20 blends—must meet a six-hour IP. Earl Christensen, a senior scientist at National Renewable Energy Laboratory, has overseen many biodiesel stability tests over the years, the most recent and comprehensive of which was completed in 2017. The test pulled 12 B20 samples from 12 different states, with one sample prepared in the lab, for a total of 13 B20s. The samples were aged at 110 degrees F per ASTM standard method D4625 to the IP thresholds, after which they were readditized with BHT, simulating nearly three years of longterm storage. After 32 weeks, simulating two and a half years, most samples were still on spec for six hours. While there is a point after which the fuel shouldn’t be used, if the fuel is readditized soon after it shows signs of degradation, a B20 could easily be stored successfully for three years and remain on-spec. “B20 should be readditized before signs of degradation, which are indicated by peroxides and acids,” Christensen says. “Detectable increases in acids are preceded by the IP reaching a ‘threshold.’ The IP will go down, but the fuel will not change significantly in quality until the IP drops to low values, after which readditization is no longer viable. One happens before the other—IP goes down to a low level, peroxides spike, acids go up to an unacceptable level, and insoluble materials become apparent. The IP is a relative measure of the oxidation reserve. Once that reserve runs out, the IP gets very low and the fuel starts to oxidize. Making sure readditization is done well-above that low IP value allows