NOx Update

For those who have been kicking around biodiesel for awhile now, the latest NREL studies on NOx have been exciting news.

We asked our new Executive Director, Matt Rudolf to get his head around the latest NOx information and to get our website updated. Here is his contribution to the conversation:

NREL NOx Emissions Primer 101

I have been talking with Doug Lawson over at the National Renewable Energy Laboratories about NOx emissions from biodiesel. Doug presented data from NREL studies on NOx emissions (I and II) at the NBB meeting in San Diego in February 2006. These studies bring great uncertainty to the widely held belief that biodiesel use increases NOx emissions.

First off, NREL chassis dynamometer studies measuring NOx emission using biodiesel blends from B2 to B50 and pure biodiesel, B100, have shown a wide variation in NOx emissions from the diesel fuel NOx emissions baseline.

These data show no consistent effect of biodiesel use on NOx emissions, and tend to indicate that the wide variation in the data is likely due to differences in testing and engine technologies, rather than from the use of biodiesel.

This new information seems to be contrary to previous common knowledge on biodiesel NOx emissions that are based on a 2002 study by the EPA. The difference seems to be that EPA testing was done on a lab bench using a motor outside of a vehicle, whereas the NREL study was done using a chassis dynamometer in which testing is done with the motor in the vehicle (in this case a city bus) closer replicating real-world usage. Results from these latest studies indicate a 5% reduction in NOx emissions when using B20, as opposed to the 2% increase reported by the EPA. Results from these tests indicate that at the very least, the jury is still out on whether biodiesel use actually affects NOx emissions.

Apart from all this, Lawson also explained to me the complex issue of the so-called weekend effect which shows that although people drive less on the weekend, ozone levels stay the same or increase in urban areas. Since ozone is created by precursors like NOx and VOCs, this effect is counter-intuitive.

The reason Lawson gives for the Weekend Effect is based in the chemistry of ozone production and in the type of NOx emitted. Current diesel motor NOx emissions are composed of about 95% NO and 5% NO2. The NO actually reacts with ambient ozone in the titration reaction? to produce NO2 and O2. The chemical equation for this reaction is NO + O3 –> NO2 + O2.

When we lower the available NO, ozone produced from hydrocarbon sources other than cars can actually be produced earlier in the day and at a faster rate than normal, leading to higher recorded ozone levels. Whether or not the general public is willing to accept the idea that lowering NOx emissions is actually a bad idea may depend largely on whether NREL air quality investigators can convince EPA air quality regulators that their science is sound and that increased NOx emissions can actually lower ozone production. The chemistry of smog production is a little complex for lay people such as myself to fully understand, but for the moment the public should know of the discussion going on among air quality experts in the field.

Suffice to say that the jury is still out on biodiesel NOx emissions, but we should at the very least stop quoting the increased NOx figures as gospel until the science of what is going on is better understood.

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10 Responses to NOx Update

  1. Lyle says:

    I’d like to contribute a NOx for Dummies section:

    1. More or less NOx from biodiesel?
    The emissions studies done on actual vehicles in real life driving conditions find a 5% decrease in NOx. The emissions study done on a solitary test engine in a lab found a 2% increase in NOx. Most of the emissions I am aware of come from vehicles driving around, not test engines in sitting in fancy garages.

    2. More or less smog from NOx?
    The atmospheric chemistry study of how NOx affects smog formation in cities found that more NOx does not make more smog. This is encouraging because even if there are still people out there who still believe that using biodiesel will increases NOx we can say, “Even if biodiesel did increase NOx (which we haven’t seen evidence of), we can feel good about using it because we now know that it doesn’t increase smog.”


  2. Will says:

    This subject usually comes up for me when it’s a question of whether hybrids are superior to diesels because of their lower emissions. Thus, comparing biodiesel to diesel and showing that their emissions are cleaner for everything but NOx doesn’t answer the question I always get, which is “If we all switched to diesels, wouldn’t our air be dirtier than if we all switched to hybrids?”

    It’s hard to argue that my car is clean when, even on B100, you can see and smell the exhaust of my 2001 TDI. I usually counter by pointing out that getting similar mileage on renewable fuel is better than on fossil fuel, and there is no way to burn renewable energy in any hybrid available today.

    Priuses have near O emissions, as long as you don’t count the emissions at the pump from the more volatile fuel.

    I would like to see someone compare the total emissions, including the emissions during refueling, of a Prius versus a TDI running B100.

  3. tb2 says:

    There is an article over at Green Car Congress about a recent meeting of the American Chemical Society. One of their tracks was Biofuels. The summaries seem to indicate that their presenters were getting emissions from biodiesel that weren’t any better, or were worse than petrodiesel. What gives??

  4. Ken says:

    Interesting NOx update, thanks for the post.

    As for the Prius or any gas car having less emissions, what about catalytic converter pollution? Sure exhaust is filtered/converted but then there are more heavy metals in the air.

    This goes for either diesel or gas cars.

  5. Craig says:

    Many say we will see $3.50/gal this summer. If you factor in Iran, who knows how high it could go. Everyone knows America MUST get off the oil. After September 11, 2001 I expected our President to call on Americans to GET OFF THE OIL. I was expecting a speech like the one JFK gave that motivated us to reach for the moon. As you know, this never happened. Eventually I realized that the only way this is going to happen is for us to do it ourselves. To that end I created this idea and have been trying to make it a reality..

    The EPA is offering a research grant opportunity that I believe is a perfect fit for this idea. I have sent an e-mail to a hand picked list of university professors who have experience with government research projects. I

  6. marc says:

    Tb2 “what gives” at Green Car Congress is that the HCCI engine study you refer to is a deferent engine than a diesel. Can’t compare emission studies directly.

  7. Rupert Crighton says:

    On the issue of NOx. I have read the EPA study show a slight increase in NOx with biodiesel.
    The point that never gets mentioned is that with sulfur free biodiesel (B100) you can install a catalytic converter in the diesel exhaust which will be effective at reducing the NOx. It is the sulfur in petro-diesel which renders catalytic converters inefficient and short lived on diesels.

  8. Why is it never mentioned that a diesel engine (compression ignition engine) is 30 to 40% more themodynamically efficient than the Otto cycle engine (spark fired gasoline engine) ?Even when using LSD #2 diesel, the CIE pollutes less simply because it is more efficient. Using biodiesel is iceing on the cake regardless of the ratio of biodiesel to #2 diesel.

  9. We are from the Philippines, we currently marketing a product called AERO-NOX, it may be a solution to the problem of biodiesels regarding the high emission in NOx. Check out the site…

  10. The single biggest barrier I have to BD (B100) is that I can’t find a retail outlet willing to sell to me! Feel free to email me places to buy B100 in the NYC/Philadelphia, PA area at RHCE_V3 at Yahoo dot com.

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