Paving Biodiesel’s Way for Greater Use in Advanced Diesel Systems

by Ron Kotrba (Biodiesel Magazine) Biodiesel’s use in heavy-duty diesel applications continues to grow in California thanks in large part to the state’s Low Carbon Fuel Standard, but how will California’s Mobile Source Strategy—a plan to further cut NOx emissions by another 90 percent compared to the already low NOx-emitting diesel engines of today—affect the use of biodiesel?

Various approaches are being evaluated to cut NOx from California’s diesel engines by 90 percent, said Timothy Johnson, a fellow with the Society of Automotive Engineers and one of the leading experts on the topic.

In 2006, stringent limits were imposed on trace metals in ASTM D6751, the biodiesel quality specification, at a maximum of 5 parts per million (ppm) of calcium and magnesium, and 5 ppm of sodium and potassium. This was done to allay concerns from OEMs, which were launching a new generation of diesels equipped with sophisticated aftertreatment systems to significantly reduce particulate and NOx emissions per federal regulations. In 2007, many new diesels featured DPFs to meet particulate regulations, and by 2010, when NOx emissions regulations went into full swing for on-road trucks, most new trucks featured a diesel oxidation catalyst, DPF and SCR.

These particular metals were identified as their residues can be present in improperly processed biodiesel, with sodium and potassium from the catalyst and calcium and magnesium from fuel washing.

The durability requirement for modern diesels is 435,000 miles, which is why OEMs are concerned over impurities in fuels, including trace metals in biodiesel.

“Diesel engines are changing dramatically, and we have a lot of work to do,” Howell said, referring to ensuring that biodiesel quality specifications keep up with evolving diesel systems. “The changes we’ve made are working, [which is evidenced by the] quality of biodiesel we see in the market.” He said any fuel quality issues experienced with biodiesel in the market today are no different than “the normal issues we see with diesel fuel.”

Johnson said with the advent of close-coupled SCR filters, critical analyses must determine how biodiesel interacts with these devices. Howell and Johnson said NBB and the OEMs launched a major test program to investigate this. “The big focus is the metals levels,” Howell said. “It’s currently limited to 5 ppm, but [it will] probably [have to be set] lower. We are conducting research to allow biodiesel to grow through 2050. The future is bright, but only if we do this work.”

CARB notes the need for additional federal action. “As vehicles purchased outside the state account for a majority of the heavy-duty vehicle miles travel in the South Coast Air Basin on any given day, federal NOx engine emission standards are necessary in order to achieve emission reductions from vehicles operating in California that were purchased outside the state,” the agency states on its website.

While the purpose of NOx reduction is to improve air quality, the reduction of greenhouse gas emissions such as CO2 is aimed at mitigating climate change. If governments, the public and markets demand low-carbon transport, then biodiesel is solution that is available today.

Biodiesel for on-road transport has many specifications already: ASTM D6751 is the B100 spec for blendstock; ASTM D975, the diesel fuel spec, allows for up to 5 percent biodiesel; and ASTM D7467 was established for B6-B20. But to facilitate biodiesel’s role as an even greater carbon reducer in transport, specs for B21-B99 must be developed. READ MORE