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Researchers Explore New Class of Second-Generation Biofuels for Diesel Engines: Cyclic Oxygenates

Smoke opacity versus cetane number plotted for various 9 wt% fuel oxygen blends at three different EGR levels. Click to enlarge. Credit: ACS

Dutch researchers led by a team from Eindhoven University of Technology suggest that low-cetane C6 cyclic oxygenates—which could be derived from biomass—could perform well as cellulosic diesel blending fuels. A paper on their work was published online 10 September in the journal Energy & Fuels.

The heavy-duty diesel industry is facing a challenge in maintaining fuel economy while meeting more stringent emissions legislation (such as EPA 2010 and Euro 6), the team noted.

Measures used to meet emissions targets such as aftertreatment and exhaust gas recirculation (EGR) systems increase pumping loses and lead to lower engine efficiency. Retarded combustion phasing, while it can reduce engine-out NOx emissions, further lowers the thermodynamic efficiency of the engine.

Beyond 2015, the authors of the paper wrote, trading off fuel consumption for emissions reduction will be unfavorable because the industry will face stringent CO2 legislation as well. Alternative pathways to achieving regulated emissions levels without a fuel consumption penalty are using conventional CI engines with an oxygenated fuel or choosing more revolutionary approaches, such as premixed charge compression ignition (PCCI), at modest engine loads.

While a large number of studies have shown that blending oxygenated hydrocarbons with petroleum diesel can be a very effective route for PM reduction, there is no consensus on the influence of fuel cetane number on the influence of soot emissions.

Michael Boot and his colleagues proposed that low cetane number (CN) oxygenates—specifically cyclohexanone—should hold an advantage over their high CN counterparts via enhanced mixing as a result of both the extended ignition delay and longer FLOL (flame lift-off length).

To test their hypothesis, they prepared a total of 15 blends between various base fuels and neat oxygenates, with fuel oxygen levels ranging from 5 to 15 wt%, for testing in a DAF 9.2L engine with EGR. They found that the difference in PM performance between the various oxygenates (e.g., at 25 wt % EGR) was nearly 2 orders of magnitude.

It can be observed that, the greater than 40% difference in fuel oxygen wt % notwithstanding, the DB-9 and X1-5 [cyclohexanone] blends share a similar PM performance. These results suggest that not only fuel oxygen content but also oxygenate reactivity or CN play a significant role in the PM lifecycle, especially at high EGR levels. For the oxygenated blends at 25 wt% EGR, smoke opacity increases more or less proportionately with the CN.

The X1-5 cyclohexanone blend had a CN of 22; the DB-9 blend a CN of 46.

Smoke opacity (solid line) and NOx (dashed line) vs. increasing ignition delay because of EGR. All blends display a similar reaction to EGR with respect to NOx, but the negative impact of EGR on smoke is not observed for the cyclohexanone blend. Click to enlarge. Credit: ACS

Based on the good results with the cyclohexanone blend, the Eindhoven University of Technology filed three patents on the use of cyclic oxygenates in the combustion process, and is seeking to develop a commercially viable production route for C6 cyclic oxygenates such as cyclohexanone from biomass.

Production. Cyclic paraffins are the most abundant (30-60 wt%) chemical compounds in crude oil, 6 carbon atom rings the most dominant species by far. Distillates contain from about 20 to about 40 vol% of such paraffins.

In other words, an economically viable feedstock for X1 is available at any neighborhood gas station. Even on-site and/or on-board refining is within the realm of possibilities.

Even more ambitious is the production of cyclic oxygenates (e.g., X1) from a biological feedstock, more specifically, from lignocellulosic biomass (also known as plant waste/residue). Principal components of such biomass include cellulose (35-50%), hemicellulose (25-30%), and lignin (15-30%)...all of which are five- and six-membered cyclic oxygenates. Unfortunately, these compounds are large polymers with molecular weights 2-4 orders of magnitude higher than conventional fuels. The production of liquid cyclic oxygenates from such heavy molecules is not straightforward.

—Boot et al. (2008)

Boot and his co-authors noted that several researchers have reported several pathways for the production of cyclohexanone, such as the isolation of phenol from lignin and subsequent conversion (rates in excess of 80%) conversion, or the isolation of guaiacol—the model component from lignin—and its subsequent direct hydrogenation to cyclohexanol.


  • Michael Boot, Peter Frijters, Carlo Luijten, Bart Somers, Rik Baert, Arjan Donkerbroek, Robert J. H. Klein-Douwel, and Nico Dam (2008) Cyclic Oxygenates: A New Class of Second-Generation Biofuels for Diesel Engines? ASAP Energy Fuels, doi: 10.1021/ef8003637>



Here's a way out theory; women won't buy diesel cars because of the odour and stickiness of the pumps at the garage. The benzene fraction makes diesel smell unpleasant compared to petrol. The higher boiling point of other fractions means it clings to the pump handle then onto your skin. Make it smell nice and evaporate quickly.


The NOx and smoke data looks great, both trending down in the same direction. So what is the bad news? Could it be the efficiency reduction?

fred schumacher

This work shows a way to use lignin, the resin which holds plant cells together and forms up to one-fourth of biomass, for something other than boiler fuel. Lignin could form the basis of a biochemical industry similar to the present petrochemical one, but so far it has been the poor stepchild of biofuels research.

Here's an example of what I believe is lignin conversion in nature. Here in northern Minnesota, we get a tent caterpillar population explosion every ten years. The adults live only long enough to mate and lay eggs, which are encased in a hard, foamy, lightweight resin.

This biofoam is a remarkable material. You need the pressure and volume of a fire hose to wash it off. I believe this foam comes from lignin, processed by bacteria in the gut of the caterpillars and stored in their bodies for use by the adults. I've tried to interest chemists in studying this material.

Reed Hanson

The simultaneous NOx and PM reduction is due to the low CN of the fuel. This allows the fuel to mix more before combustion thus lowering the PM and probably lowering the combustion temp, thus lowering NOx. WIth this premixed combustion, the phasing is after TDC and the BSFC is actually as good or better than regular diesel because the combustion is much faster and better phased.

Low CN fuels are the future to meeting low emissions from CI engines. My research is using 93 PON gasoline in a HD CI engine and Im seeing the same results.

fred schumacher

"Low CN fuels are the future to meeting low emissions from CI engines."

Reed -- are you at Minnesota State University Mankato? Why don't you post some more on your research on low CN fuels.

reed hanson

No Im at UW Madison right now doing my MS, I was previouly at MSU Mankato. So far in my research I have been able to get below 2010 EPA HD emissions of .27 g/kw-hr NOx and .013 g/kw-hr at 12 bar IMEP.

michael boot

please feel free to email me if you have any questions on this article or on matter related to it. I also would appreciate links to similar articles on this topic.

Kind regards,

Michael Boot
Eindhoven University of Technology (netherlands)...

michael boot

michael boot

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