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It still smells good: B20 Biodiesel emissions show no NOx increase.
Clayton Bodie Cornell's picture
Filed on Jan 3, 2007 at 12:27 PM PST
By Clayton Bodie Cornell
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An important stumbling block to the
incorporation of B20 biodiesel
(20% biodiesel/80% diesel) into the nation's fleet of heavy-duty diesels has been concern over nitrogen oxides (NOx), a group of pollutants initially found to increase in biodiesel exhaust. In 2002, Environmental Protection Agency (EPA) tests concluded that B20 causes a 2% increase in NOx when burned in standard heavy-duty diesel engines, though this figure approaches 10% for higher biodiesel blends. Nitric oxides are produced when nitrogen from the air is drawn into the hot combustion chamber and reacts with oxygen. NOx contribute to acid rain, smog, and ground-level ozone formation. Generally speaking, an acceptable "green" fuel should reduce pollution, not increase it, and running city fleets on B20 where smog is already endemic could be a bad idea.

Good news for biodiesel advocates came from an October 2006 report from the National Renewable Energy Laboratory (NREL). The report not only reviewed the EPA's work, but also tested 8 heavy-duty diesel vehicles including several transit buses, school buses, Class 8 trucks, and a motor coach. NREL researchers found that smaller vehicle testing showed no significant impact on NOx emissions, while larger trucks varied by engine model:

Our study shows that the NOx impact of B20 varies with engine design, such that some engines show a small increase while others show a small decrease. The EPA's 2002 review was based on a data set made up primarily of data from one engine model that produces a small NOx increase. EPA uses these data to draw a general conclusion for on-highway engines that B20 causes a 2% increase in NOx," McCormick said. "The chassis dynamometer testing along with careful review of previously published data suggest that their conclusion is not correct, and that on average B20 has no effect on NOx. According to the report, some state governments had previously considered banning B20 due to concerns over NOx emissions. These new findings may relieve those concerns and increase the proportion of diesel fleets using the renewable fuel blend.

For the vehicles tested, NREL's experiments also found that B20 caused an average reduction of 16% to 17% for Particulate Matter (PM, i.e. soot) and Carbon Monoxide (CO) emissions, respectively*. Interestingly enough, there was no significant change in emissions between regular diesel and B20 for vehicles equipped with a diesel particulate filter, underscoring the importance of newer technology in reducing diesel-engine emissions.

*Biodiesel also contains no sulfur (a major factor in acid rain caused by sulfuric acid) and, depending on how it’s produced, causes a significant decrease in net CO2 emissions. Higher blends of biodiesel such as B50 or B100 emit 2-3x less CO, PM, and unburned hydrocarbons. To see a complete emissions profile, visit the National Biodiesel Board (NBB).

Photo Credit: National Renewable Energy Laboratory