OEMs want biodiesel metals measured

By Nicholas Zeman | January 19, 2010
Posted February 15, 2010

Inductively coupled plasma (ICP) analysis is the standard technique for metal detection in biodiesel, which usually exhibits only faint traces of sodium and potassium-two elements that can compromise catalysts and diesel particulate filters (DPF) by causing ash buildup. Teresa Alleman with the National Renewable Energy Laboratory mentioned that Horiba, with its Ultima 2 instrumentation, has been able to improve detection into the parts per billion range, even recognizing sodium leached from glass storage devices.

Another NREL study, "Impact of biodiesel ash loading on DPF performance," collected 100 samples of ASTM biodiesel. The study, led by NREL engineer Aaron Williams, concluded that with the metal content found in biodiesel, it is highly unlikely that ash build-up would result. "No biodiesel produced right now has these levels of metals." Williams said at the 2010 National Biodiesel Conference & Expo in Grapevine, Texas, last week. Nevertheless, NREL asks, does the ASTM spec for alkali metals need to be lowered to satisfy catalyst and diesel particulate filter manufacturers?

The original equipment manufacturing (OEM) community and emission controls firms are especially interested in knowing metal limits. "We wanted to see how low we could go," Alleman said. "If we reported that there was less than 1 ppm in biodiesel, [stakeholders] wanted to know: 'Well does that mean .9 or .09? Can you qualify that any better?'"

Therefore, the knowledge this work looks to provide is the level at which recommended ash clean intervals might need to be shortened or if the durability of a DPF might be compromised by sodium and potassium. To study long-term ash accumulation and thermal aging, NREL spiked its biodiesel samples with 26 times the ASTM limit and ran the fuel through DPFs at accelerated speeds and temperatures. There was ash build-up on the catalysts inside DPFs, however, biodiesel samples contained levels far below what NREL theorized it would take to ultimately compromise the DPFs.
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