Biodiesel, noted the authors, is the most widely used biofuel in diesel engines, and it is currently the main fuel being used or considered to meet renewable fuel requirements for diesel fuel. In California, biodiesel use accounted for 86% of the alternative fuels consumed by the state fleet in 2010. (Earlier post.)

Studies have shown that biodiesel generally reduces carbon monoxide (CO), particulate matter (PM), and total hydrocarbons (THC) emissions compared to conventional diesel fuel. It has also been reported in many studies, however, that average nitrogen oxides (NO_x) emissions from biodiesel blends can increase with increasing biodiesel content. Although the impact of biodiesel on NO_x emissions has been studied for many years, there is still uncertainty as to how prevalent this effect is, and the specific factors that might be contributing to biodiesel NO_x increases, especially with newer engines.

...Although the impacts of biodiesel on NO_x emissions may be small on a percentage basis, and difficult to quantify, this remains an important issue in regions and cities when poor air quality is a persistent problem...The impacts of biodiesel on NO_x emissions for diesel fuels that are stringently regulated, such as CARB diesel, have not been as extensively studied as for other fuels used throughout the United States.

...This is one of the largest studies to date on biodiesel emissions impacts. The focus of this larger research study was on understanding and mitigating any impact that biodiesel has on NO_x emissions from diesel engines for CARB-like diesel fuels, or diesel fuels with properties consistent with those needed to meet CARB diesel fuel requirements, and understanding the potential impacts of biodiesel on toxic emissions and health effects.

—Hajbabaei et al.

The focus of the paper was on the results of engine dynamometer testing conducted on two on-highway, heavy-duty diesel engines (2006 Cummins and 2007 MBE4000), with a primary focus on understanding biodiesel NO_x impacts. The test fuels included a baseline CARB-certified diesel fuel, biodiesel blends produced from two different feedstocks (one soy-based and one animal-based), and a gas-to-liquid (GTL) and a renewable diesel fuel, with blend levels from 5% to 100%. A range of strategies were also evaluated for the mitigation of any potential NO_x impacts.

The test cycles included the standard Federal Testing Procedure (FTP) for heavy-duty engines and three other cycles derived from chassis dynamometer data: a lightly loaded Urban Dynamometer Driving Schedule (UDDS) cycle; a 40 mph CARB heavy heavy-duty diesel truck (HHDDT) cruise cycle; and a 50 mph CARB HHDDT cruise cycle.

Among the findings were:

• The average NO emissions were higher at a statistically significant level for the B20 and higher biodiesel blends compared to the CARB diesel for all the test cycle/engine combinations, with the exception of the animal-based biodiesel on the UDDS cycle for the 2006 Cummins engine.

• NO_x increased with biodiesel blend level. NO_x increases ranged from 1.5% to 6.9% for B20, 6.4% to 18.2% for B50, and 14.1% to 47.1% for B100, with the magnitude of this effect differing for different feedstocks, engines, and cycles. The soy-based B20 and higher biodiesel blends showed higher increases in NO_x emissions for the different blend levels, test cycles, and engines, in comparison with the animal-based biodiesel blends. Differences between the NO_x percentage increases for the soy-based and animal-based biodiesel were statistically significant for each of the cycle/blend level combinations on both engines.

• NO_x emissions also differed as a function of cycle power for both engines.

• NO_x reductions with the renewable diesel fuel (NExBTL) ranged from .9%-4.9% for R20, 5.4%-10.2% for R50, and 9.9%-18.1% for R100 over all the cycles.

• The GTL diesel fuel showed reductions of 5.2% for the GTL50 and 8.7% for the GTL100. Over the FTP cycle, the NO_x reductions for the renewable and GTL diesels were comparable for the 50% and 100% fuels, but the GTL fuel did not show statistically significant reductions at the 20% blend.

Taking these results in conjunction with other literature studies of CARB-like diesel fuels, it appears that biodiesel likely has a more prominent impact on NO_x when used with CARB-like diesels compared to more conventional US average diesels, at least at B20 or higher levels. For low level B5−B10 blends, that are most likely to be implemented in the near term in meeting regulatory requirements, it is still unclear if or what level of mitigation might be needed in regions such as California that have stringent provisions against any increases in NO_x emissions. Potentially NO_x neutral blends for such regions could include a B5 blend with a highly saturated biodiesel base stock, or combinations of B5 or lower blends with renewable diesel or GTL-like fuels, or with an additive. For biodiesels with a greater propensity for increasing NO_x, such as soy-based biodiesel, further modification to the base fuel might also be possible to offset any potential biodiesel NO increases.

—Hajbabaei et al.

In the coming months, several follow-up studies are planned, including: more comprehensive testing with 5% biodiesel blends; adding additives to reduce NO_x emissions in 20 percent biodiesel blends; and evaluating the impact of biodiesel blends have on emissions from light-duty vehicles, such as passenger cars. This will include the development of certified biofuel and biodiesel blends that can be sold in diesel fuel throughout California.

Resources

• Maryam Hajbabaei, Kent C. Johnson, Robert A. Okamoto, Alexander Mitchell, Marcie Pullman, and Thomas D. Durbin (2012) Evaluation of the Impacts of Biodiesel and Second Generation Biofuels on NO_x Emissions for CARB Diesel Fuels. Environmental Science & Technology 46 (16), 9163-9173 doi: 10.1021/es300739r