Effects of Biodiesel on Emissions

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Abstract: Biodiesel blends increase NOx emissions in most diesel engines. The increases of NOx can occur for multiple reasons, the relative importance of which is dependent on both the particular engine and its operating conditions. The effect of biodiesel on PM emissions depends on two opposing trends: biodiesel reduces emissions of carbonaceous particulates, while increasing the organic fraction of diesel PM emissions.

NOx Emissions

Engine-Out NOx Increases. Engine-out NOx emissions with neat biodiesel (B100) typically increase by about 10-30% relative to diesel fuel [315][808][458][1200][1412]. With B20 biodiesel blends, increases are typically 1-7% [805][1528]. Others have noted higher increases [6695]. There are also claims that on average, there is no increase in NOx emissions up to blend levels as high as B20 [1529].

The NOx increase depends on the type of the biodiesels feedstock; the highest NOx emissions were reported with the most highly unsaturated fuels (soybean, rapeseed, and soapstock-based) [968]. Biodiesel from more saturated feedstocks, such as animal fats, yields a smaller NOx increase.

The NOx increase also depends on engine technology. According to a study with two heavy-duty engines meeting US 2004 emission standards—one with a common rail and one with an EUI fuel system—the NOx increase effect can be higher in newer engines, where a 30% NOx emission increase was measured with B100 fuel over the FTP cycle [1200]. Similar results have been observed for engine-out emissions from engines fitted with SCR catalysts [6695][6696][6698][6699]. In Figure 1, engine-out NOx emissions with biodiesel blends from the engines with SCR catalysts, 2004 engines and pre-1998 engines are compared. The pre-1998 data is from a 2002 EPA study [817]. NOx increases comparable to the 2004 engines in Figure 1 were also reported by another study with a 2006 model year Cummins ISB fueled with B20 and B50 [1528].

The biodiesel NOx increase has been shown to be test cycle specific. The difference in NOx emissions between diesel fuel and biodiesel blends has been shown to correlate very well with average cycle power regardless of whether the test is carried out on a chassis or engine dynamometer. As average drive cycle power increases, the NOx increase due to biodiesel increases as well, Figure 2 [1528]. This is consistent with steady state tests with biodiesel blends, which show that NOx emission increases with biodiesel are highest at high loads. At low loads, increases are much smaller, and in some cases NOx emissions can be lower than with diesel fuel [1535].