Bulk modulus of biodiesel (BD) described as cause of increase in BD NOx.

[NNadir]

From the upcoming issue of Energy and Fuels (Now available to online ACS journal subscribers as an ASAP version):

"The addition of biomass-derived fuels and synthetic
fuels to diesel fuel basestocks is a means of producing
a cleaner-burning diesel fuel. Blending with oxygenated
or zero-sulfur fuels can lead to reduced particulate
emissions by interfering with the soot formation process
and by decreasing the formation of sulfates. However,
in the case of biodiesel fueling (e.g., “B20”, a blend of
20 vol % biodiesel in diesel fuel), there is a welldocumented
increase of 2%-4% in NOx emissions.1
There is evidence that this increase in NOx emissions
is related to an advance in fuel injection timing. It is
well-known that advancing the injection timing can lead
to an increase in NOx emissions from direct injection
(DI) diesel engines.2
Several researchers have reported an advance in the
fuel injection timing when biodiesel is being used. Choi
et al.3 reported an advance in fuel injection timing, of
0.6 crank angle (CA) degrees, with a 40 vol % blend of
biodiesel."

From the article, it seems that an advance in the injection timing (and I am not an automotive engineer) resulting from a higher bulk modulus in biodiesel as compared to ordinary diesel occurs. This effect seems to derive from changes the speed of sound in the injector system.

If we have any diesel engineers here, I would be interested in comments.

If so, if I understand it correctly, it may be possible to reduce the NOx drawback of biodiesel by building engines with injectors specifically designed for biodiesel burning.

I don't necessarily think that biodiesel is long term viable solution to the world's energy problems, but it does represent a possible "bridging" fuel while better alternatives are developed. A reduction in the primary hazard associated with biodiesel, increased NOx would therefore be a welcome development.

[phantom power]

1. We can take care of NOx with catalytic converters, right?

[NNadir]

2. We can reduce, but not eliminate NOx with catalytic converters, but...

if they were eliminated entirely by catalytic converters it would be hard to account for the brown haze over the ocean off NY and LA.

Englehardt metals in a somewhat self serving proposal - they provide Platinum, Ruthenium, Palladium that are used in such catalysts - once proposed that high end cars have their radiators plated with these metals in hopes of further reducing pollution.

It's important to note that the increase in NOx resulting from biodiesel is not enormous, typically 1 to 4%. Theoretically at least, biodiesel is carbon neutral, the carbon released to the atmosphere is recovered in the plants (usually soybean and in Europe, rapeseed) used to make the biodiesel. Also B100 contains not sulfur and therefore eliminates the important sulfur contamination.

Actually, since No. 2 diesel is equivalent chemically (though not in a regulatory sense) to home heating oil, I think a better and more environmentally interesting application for biodiesel would be in this area. Under conditions of use for heating, NOx is trivial.

[ohio_dem_52186]

4. Conventional catalytic converters do very little for NOx emissions

Here is a summary of technologies for diesels:

just in case:
THC = total hydrocarbons (unburned fuel)
PM = particulate matter (soot)

Diesel oxidation catalysts - take care of CO, THC, and can reduce the mass of PM by oxidizing the HC adsorbed to the carbon particle. (With high sulfur fuel you can actually increase the mass of PM with an oxidatio catalyst, hence one reason for low sulfur fuel.) These catalyst have a peak efficinecy on NOx of about 7%...

Diesel Particulate Filters - The king of DPFs is the wall flow filter. Basically it is a ceramic (cordierite or silicon-carbide) filter. When it gets suitably filled with soot, it is heated up such that the soot ignites and burns out of the trap. Managing this regeneration event is the trick, as the filter can crack.

De-NOx or HC-SCR catalysts - for these catalyst to work, you inject a reductant (i.e. diesel fuel, ethanol, whatever) into the exhaust upstream of the catalyst. The NOx is selectively reduced over the catalyst using the reductant. With diesel fuel, the NOx efficiency on a real cycle would be arounf 50% or 60% with a few percent fuel economy penalty for the reductant

Urea-SCR catalysts - These catalyst are fairly well known, they use a water based urea solution as a reductant for NOx. You can hit above 80% NOx conversion efficiency with these if you have the urea...

Lean NOx Traps - These catalyst chemically store NOx at high efficiencies (>95%). As they fill up with NOx, efficiency decreases. At some point (every 30 sec to few minutes) you need to provide rich exhaust conditions to the catalyst to "regenerate" the NOx from the catalyst and restore efficinecy. This is a tall order for a diesel but can be done. These catalyst are very susceptible to sulfur poisoning and have some durability issues.

Non-Thermal Plasma Catalysis - Use of a plasma to generate active radicals that are then used as NOx reductants over a catalysts. Still in development. Currently have prohibitive power consumption.

[ohio_dem_52186]

3. Automotive Engineer Here - a bit late...

I just had a conversation with a graduate student on just the topic you are mentioning.

Common-rail diesel injection systems operate at 1600 bar (soon upwards of 2000 bar). At this pressure, a small change in the bulk modulus can have a significant change in the injection characteristics.

A modern diesel fuel injection system consists of a fuel rail (a cylinder) that stores a small amount of high pressure fuel. The rail has a feed tube that goes to each injector. A current is used to actuate a solenoid that leverages the high fuel pressure to open a valve at the tip of the injector and discharge the fuel into the cylinder.

Because of the high pressure, it turns out that Biodiesel causes the injectors to open sooner than with straight diesel. A recalibration of the injector map (in software only) to call for slightly retarded injections can recover the NOx emissions.

SUMMARY - If the research holds true, then the only change needed is a change to the injection maps (tables of numbers in software)

[NNadir]

5. Thanks for the lucid explanation.

Thus conceivably one could theoretically reduce the NOx emissions for a mixed fuel (various biodiesels/Petroleum Diesel/x mixtures) by use of a small computer and a device that measures the bulk modulus.

[ohio_dem_52186]

6. Actually easier than that:

The computer is already on-board. Any modern engine is computer controlled already.

As for the bulk modulus, I think that there is probably a way to come up with an "estimator" that can predict %bidodiesel without the need for a sensor. An "estimator" is basically an algorithm that uses existing engine measurements to estimate an unmeasured input.

[ohio_dem_52186]

7. I was curious and took a quick look, they already have

%biodiesel sensors:

http://www.me.iastate.edu/biodiesel/Technical%20Papers/ASAE%20Paper%20No.%20026084.pdf

[NNadir]

8. Thanks again. Excellent material.

Under these conditions it's somewhat surprising that the biodiesel/NOX problem hasn't been better addressed. It would be interesting to know the type of surface that might exist for fuel composition/NOx/particulates.

As an engineer, what is your opinion on the range over which such adjustments can be made by software alone? I think on a capacity basis biodiesel is at best a means of ameliorating but not solving the energy difficulties the world is facing. My own fuel of choice, DME, a room temperature gas, has a very low bulk modulus as a pressurized liquid, less than half of petroleum diesel fuels. I am not certain that DME or DME mixtures could be used without engine modifications. (DME also has a problem with lubricity.)

[ohio_dem_52186]

9. Probably possible to get the NOx down to normal diesel levels.

From what I have read it is possible to get the NOx down to conventional diesel levels with minimal effort.

It is possible to do nearly anything with the injection timing calibration you want to. Modern injection systems can make multiple injections in one combustion stroke at precisely controlled times and durations. It is really as simple as entering in the desired start of injection and the desired length of injection in a table.

Don't get me wrong, it is a lot of work to optimize the calibration, requiring several weeks of engine testing to get to a solid calibration. But it is the same process the diesel engine manufacturers go through when developing a diesel fueled engine (as opposed to biodiesel.)

I agree with you on biodiesel. Of all of the renewable fuels we have at our disposal right now, Biodiesel is the one that makes the most sense. We still need to get our diesel's cleaner though...

[NNadir]

10. Then, if I interpret you correctly, one can go over a wide range

of values with existing engines.

According to the originally cited paper, the bulk modulus of DME is 450 MPa at 6.47 MPa pressure and 38C, whereas the bulk modulus of B100 at the same pressure and temperature is around 1670 MPa, compared with 1580 MPa for B100 and 1470 for Petro diesel. Thus DME is a much more different fuel than B100. If you have any comments, they would be appreciated.

I confess to being ignorant of many aspects of diesel engine design and performance. To what extent is the lubricity of the fuel an important factor in engine performance and longevity? My reading suggests that the lack of lubricity is an important drawback to the use of DME in extant diesel engines, even if it is true that on pollution and sustainability grounds DME is a far superior fuel to either biodiesel or petro diesel. (Zero particulates for instance, owing to the absence of C-C bonds in the fuel. Nearly unlimited accessibility from nuclear energy makes DME a fuel which is scalable to almost any desirable level with reasonably low environmental impact.)