American Chemists explore means to reducing the high NOx biodiesel output.

[NNadir]

"Biodiesel is an alternative diesel fuel consisting of the alkyl monoesters of fatty acids. The use of biodiesel offers many environmental advantages over petrodiesel. Biodiesel is made from animal and vegetable fats and oils and is therefore a renewable resource. It is less toxic and more biodegradable than petrodiesel, and its combustion results in a decrease in particulate, hydrocarbon,
and carbon monoxide emissions compared to petrodiesel. However, an increase in nitrogen oxide (NOx) emissions from biodiesel combustion, relative to levels observed from petrodiesel combustion, has been reported by several researchers.1-3 This increase is of concern in areas that are subject to strict environmental regulations, such as air quality low-attainment areas, inner cities, and national parks. For universal acceptance of biodiesel, it is desirable to reduce NOx emissions at least to levels observed with petrodiesel..."

http://pubs.acs.org/cgi-bin/abstract.cgi/enfuem/asap/abs/ef049682s.html

[Massacure]

1. Isn't that what catalytic converters are for?

[NNadir]

2. Yes.

However they are not 100% efficient. Moreover they depend on surface phenomena and therefore a subject to poisoning. This is particularly the case with diesel engines, which have a relatively high output of particulates and, in the case of petroleum diesel or petroleum/biodiesel mixtures (for instance "B20") sulfur. All catalytic converters are platinum metal group based (Platinum, Palladium, Ruthenium, etc.), and all are rendered inactive by sulfur. (Note that particulates are less profound a problem with biodiesel than with petroleum diesel.)

Some years ago Englehard, the catalyst company tried to sell high end buyers on the idea of having platinum coated radiators. The idea was environmental responsibility. The passage of air over the radiator would catalyze the decomposition of NOx and CO. It didn't go anywhere.

[Massacure]

3. I also remember reading something about engine tweaking somewhere.

Cannot remember the details exactly, but it seems that the fuel injection could be timed a little differently to reduce NOx. If the engine has to be modified though, that might mean it would be incompatible with regular diesel.

[NNadir]

4. You are correct.

The combustion properties of a fuel are a function of many variables, including things like bulk modulus (compressibility), vapor pressure, viscosity, surface tension, melting point/cloud point, flash point, heat capacity etc, etc, etc. These physical properties are in turn, closely related to molecular structure. Moreover they can vary with temperature, and in the case of some kinds of conceivable fuels - wood oils come to mind - with the age of the fuel. (Not all fuels have equal shelf lives - the most stable fuels are petroleum based.)

The interesting thing about diesel engines is that the types of fuels that can operate the engine vary considerably over a wide range of molecular structures.

One can control to a certain extent how these properties interplay by varying the physical arrangement of the engine, the size of the injector ports, the speed at which fuels are injected, the precise timing of injection, stroke length, etc. Obviously most engines are now optimized to handle petroleum diesel and not biodiesel. Were engines to be fueled with DME or some other type of fuel, these variables would be yet again different.

Obviously if one is attempting to operate engines with variable fuels, say B100 in summer or in warm climates where issues of flowability and cloud point do not play much of a role, and B20 in winter, when these issues become very important. Whether the B20 is petroleum or FT (Fischer-Tropsch) based, one must engage in a process of combinatorial optimization in selecting the engine design. That is, adjustment for one variable that may minimize problems or drawbacks under one set of circumstances may elevate problems under another set of circumstances. Thus one is seeking to maximize the performance characteristics while minimizing the pollution properties over the average fueling conditions. It is possible than under localized conditions, say cold days, these properties may prove quite poor depending on the engine design chosen but excellent on, say, hot days. One would attempt to choose engine design so that the lifetime environmental impact of the engine is lower over all.