Cetane boost can cut NOx around 2%: US EPA study

Diesel Fuel News, March 17, 2003 by Jack Peckham

A final U.S. EPA report shows that boosting cetane by five points could cut heavy-duty fleetwide nitrogen oxides (NOx) emissions between 2 to 2-1/2% today, or about one-third the NOx benefit from switching from ordinary EPA diesel to California Air Resources Board (CARB) diesel.

The 2% benefit is seen from boosting a 45-cetane base fuel to 50, while the 2-1/2% boost would come from boosting a 40-cetane base fuel to 45.

The final report, like the draft report (see Diesel Fuel News 6/24/02, Ps) shows that NOx benefit from cetane boost diminishes when the base fuel itself has higher cetane. Adding evermore cetane improver eventually brings diminishing returns.

"On average, cetane improver additives are responsible for approximately one-third of the NOx reductions generated by California diesel fuel, with the remaining reductions being generated by higher natural cetane, lower aromatics and lower density," EPA's final report concludes.

Still, EPA's assumption that cetane boost will have a diminishing NOx benefit from post-2002/2004 low-NOx engines because of widespread use of exhaust gas recirculation (EGR) could be questioned, according to the two peer-reviewers that EPA contracted to critique the study.

Example: It's possible that engine makers will in future develop other control schemes for NOx and particulate matter (PM), as peer reviewer Nigel Clark (University of West Virginia diesel engine/emissions researcher) pointed out.

What's more, the "original definition of cetane number is based on an antiquated engine with low injection pressure, low in-cylinder charge motion and operating conditions atypical of a present-day on-road engine," Clark told EPA. "If cetane rating (by comparison with standards) were measured using a modern engine, with a more meaningful measure of premixed burn, then the difference in effects between natural and additized cetane would be much reduced and correlations would be superior."

Cetane "additization" as EPA defined it for this study doesn't include blending Fischer-Tropsch gas-to-liquids (GTL) diesel or other high-cetane components. Rather, it covers conventional cetane boosters such as 2-EHN and DTBP, in less-than-l% amounts.

EPA excluded all two-stroke and all EGR-equipped engines from the database to calculate fleet-wide NOx emissions benefits with today's fleet. Reason: Neither of these groups of engines accounts for much of the current heavy-duty highway diesel fleet.

EPA figures that future diesel engines will be less "cetane-sensitive" hence the 2% fleetwide NOx benefit seen today would diminish to about 1.4% benefit (highway engines) and 2.1% (non-road engines) by 2007.

While 2% NOx reductions might appear small to some, "the impact on tons of NOx that can be reduced could be large," explains Ethyl fuel additives expert Larry Cunningham. "The ease of use and handling of cetane-improved diesel fuel allows for large volumes of diesel fuel to be treated with additives to achieve sizable tons of NOx reductions. While other alternative fuel technologies can provide larger percentage NOx reductions, their limitations do not allow for widespread use."