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 NO_x 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.

Resources

• 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>