EPA slides back into familiar territory with biofuels

Alternative fuels Strategies

In late May, EPA released its proposed determination that no additional measures are required to mitigate adverse air quality under the updated Renewable Fuel Standard (RFS2). This decision rests on an anti-backsliding study which evaluates the nationwide formation of criteria air pollutants that harm human health, including nitrous oxides (NOx), particulate matter (PM), carbon monoxide (CO), and hydrocarbons (HC). Unfortunately, the study relies on outdated data and fails to reflect the situation today.

In addition to bolstering energy security, biodiesel has been praised for its greenhouse gas (GHG) reduction potential and air quality benefits as assessed under the RFS. We’ve written extensively on the dubious claims of GHG reductions from first-generation biofuels and have also evaluated the effects of biodiesel on criteria air pollutant formation. Plenty of additional research exists on this subject area – in an independent analysis, we identify over 130 biodiesel exhaust emission studies conducted over the last thirty years which examine biodiesel combustion performance relative to conventional fuel. Taking into account this past research, it’s well established that biodiesel increases emissions of NOx compared to conventional diesel, but it is also understood that it reduces emissions of other major pollutants, such as PM, CO, and HC. On balance, EPA has previously concluded that combusting biodiesel blends is no worse for air quality than conventional diesel.

In its anti-backsliding study, EPA relies on the findings of two previous studies – its MOVES assessment and its 2010 RFS regulatory impact analysis – both of which are now outdated. These analyses rely almost entirely on data published before 2007, ignoring dozens of more recent studies and advancements in emissions control strategies. The EPA justifies this omission by arguing that it’s not clear that modern exhaust aftertreatment technologies, such as diesel particulate filters (DPFs), affect conventional pollutant emissions from biodiesel combustion. While this is true, it’s also not clear that these aftertreatment technologies don’t affect biodiesel emissions. But more importantly, there have been two other significant changes the industry has undergone since around 2007: the adoption of ultra-low sulfur diesel (ULSD) under the “Heavy-Duty Engine and Vehicle Standards and Highway Diesel Fuel Sulfur Requirements” rulemaking in 2006 and an industry shift to common-rail fuel injection technology around the same time.

The implications of these changes on emissions formation in are described our comments submitted in response to the anti-backsliding study. Specifically, building off a previous ICCT analysis, we calculated the biodiesel emissions effect, or percent change relative to baseline diesel fuel, from data reported throughout the literature. When the results using data from pre-2007 studies are compared to the results using data from the later studies which EPA omitted, the differences are striking.

As shown in the figure below, there is a 58% difference in the biodiesel NOx effect in a regression analysis when including data from older versus newer studies. For pure biodiesel, older studies predict a 6.4% increase in NOx while we find a 10.2% increase in NOx emissions when analyzing the newer studies.

Figure 1

We also assessed changes in PM, HC, and CO emissions when combusting biodiesel blends compared to conventional diesel. When only looking at the older studies, it appears that biodiesel blends significantly reduce emissions of these pollutants compared to conventional diesel. But we find no statistically significant difference between biodiesel blends and conventional diesel when assessing the data from only newer studies.

Collectively, we found that blending biodiesel substantially increases the NOx emissions from diesel vehicles and does not reduce emissions of other pollutants. It probably is true that biodiesel didn’t worsen air pollution in the 80s, 90s, and early 2000s when those earlier studies EPA cited were published. Relative to older emission standards and sulfur content limits, the combustion advantages of biodiesel may have outweighed its adverse air quality impacts. But things have changed, and EPA needs to recognize that.