Solving a Great Biodiesel Mystery
by Ron Kotrba (Biodiesel Magazine) The best researchers scratch their heads at rare, isolated cases of storage tank filter clogging with biodiesel blends
The introduction of ultra-low sulfur fuel for on-road diesel in 2006-’07 brought unexpected consequences for biodiesel blending. Refiners began hydrotreating diesel fuel to reduce sulfur to 15 parts per million and, in doing so, changed the solubility characteristics of diesel fuel by removing aromatics and other compounds. “Aromatic compounds are good at making relatively polar things soluble,” says Robert McCormick, a principal engineer at National Renewable Energy Labs. “So when you take them out, that could become an issue.”
Sporadic issues with biodiesel blends clogging filters, both vehicle and dispenser, above the cloud point with on-spec B100 arose, recounts Steve Howell, National Biodiesel Board technical director. In general, clogged filters were found to have high levels of sterol gluccosides and saturated monoglycerides. “That prompted the development of the cold soak filtration test,” Howell says, “as a performance test on B100 to pick up any minor biodiesel components that were precipitating out in the field that were not showing up in the cloud point test.”
…After full implementation, reports of vehicle filter clogging with biodiesel blends ceased by all accounts, but rare, isolated instances of storage tank filter clogging at terminals—above the cloud point—continue to haunt the industry.
…As a result, a useful nonmandatory cold flow appendix was added to the D6751 specification, Howell says, to give users more advice and guidance on when the phenomenon might occur and the potential causes. “With no clear-cut answer, NBB led efforts to involve the leading petrodiesel and biodiesel fuel technical specialists to design a larger set of experiments that could provide a more definitive cause and effect,” Howell says. “That would take some time to design the protocols and secure buy-in, and then secure the samples and funding to execute the work. This work is now in progress, and is estimated to cost over $200,000.” READ MORE