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.