Aromatics might aid NOx traps; hydrogen reformate may finesse remaining fuel-quality concerns

Diesel Fuel News, Oct 13, 2003 by Jack Peckham

Newport, R.I. -- The automakers' World Wide Fuel Charter (WWFC) doesn't have anything nice to say about aromatics in diesel fuel, because of aromatics' tendency to contribute to particulate matter (PM) and nitrogen oxides (NOx) emissions.

As a result, for "advanced" emission-control areas, the WWFC recommends a 15% aromatics limit, or about half the roughly 30%, average level in U.S. diesel fuel.

Likewise, California Air Resources Board (CARB) pooh-poohs aromatics, setting a default 10% limit--although allowing higher-aromatics "equivalency" recipes utilized by most refiners. CARB's rationale: aromatics limits cut nitrogen oxides (NOx) emissions, at least in some older engines--and EPA gives CARB some crucial "state implementation plan" (SIP) emissions credits for "CARB diesel."

Yet ironically, aromatics might play a key role in regenerating nitrogen oxides (NOx) traps, if this technology emerges for U.S. EPA 2007/2010 diesel emission controls.

At the Diesel Engine Emissions Reduction (DEER) workshop (sponsored by U.S. Department of Energy) here, researchers reported various on-board fuel-reforming schemes to create NOx reductants, desulfate NOx traps, or enhance the performance of lean-NOx catalysts.

Several papers pointed out that it's possible to crack broad-range U.S. diesel fuels (with fairly typical aromatics levels) into useful NOx trap reductants, either through late in-cylinder injection, or in-pipe fuel injection followed by cracking in oxidation catalysts or plasma reactors. Other researchers instead aim to reform a slipstream of diesel fuel into hydrogen ([H.sub.2]) and carbon monoxide (CO), not only for NOx trap regeneration but also to aid cold-start emissions reduction. Fuel reformers some day might be used to feed [H.sub.2] to an auxiliary power fuel-cell, too.

None of the DEER papers here specifically recommended slashing aromatics content in diesel fuel, although one paper pointed out that ultra-low-aromatics Fischer-Tropsch diesel might enhance the performance of certain lean-NOx retrofit catalysts (see Diesel Fuel News 9/15/03, p6).

Some highlights from DEER sessions that could have been sub-titled, "Diesel Reform School:"

--Oak Ridge National Laboratory (ORNL) reported heavy-duty NOx-trap fuel-reforming results from cooperative research with International Truck & Engine and Delphi.

These researchers used in-pipe fuel injection upstream of different size oxi-cats, followed by a catalyzed diesel particulate filter (DPF) and a NOx adsorber. All reported results were for the very challenging rated-load, not-to-exceed (NTE) test condition, with exhaust temperatures of 600[degrees]C.

Fuels tested included a 20% aromatics, ultra-low sulfur diesel (ULSD) version of CARB diesel, as well as another ULSD fuel with a very different aromatics level and a much narrower range of hydrocarbon species.

Researchers found that a 20% aromatics ULSD--twice the 10% default limit of CARB diesel, but legally allowed under CARB "equivalency" rules--delivered a broader range of useful cracked hydrocarbons to the NOx adsorber than the alternative narrow-cut ULSD.

Oxi-cats promote formation of propene and other light HCs--readily utilized by a NOx adsorber, as ORNL researcher Brian West explained here.

"Light alkenes and mono-aromatics are readily utilized by the NOx adsorber, while branched alkanes are less preferred," West said. That prompted West to pose the question: "Are aromatics good?"

Given that the NOx adsorber utilized these compounds well, and that tailpipe hydrocarbon emissions were below EPA's NTE limit of 0.21 grams/brake horsepower-hour, the answer seems to be, "yes," at least at this high-temperature (600[degrees]C) condition.

The oxi-cat also reduced fuel penalty (compared to just injecting diesel fuel in front of the DPF) by about 10%, for equivalent NOx reduction, the tests showed.

While this fuel-injection/catalyst scheme achieved 70% NOx reduction (at 600[degress]C rated load NTE condition), that by itself wouldn't be enough to hit EPA's 2010 NOx NTE limits, West showed.

Rather, NOx levels would have to be under 1.0 g/bhp-hr engine-out for this exhaust treatment system to hit EPA's 2010 requirements. At 2.0-grams NOx engine-out, the system would need >85% NOx reduction efficiency, probably with a larger, improved adsorber, more frequent regeneration and the "best" reductant. More: These tests only covered a small part of what's required for emissions certification.

--From a separate ORNL project, researcher Shean Huff showed test results of incylinder fuel reforming (in a 1.7 liter, common-rail Mercedes light-duty diesel) through several engine control schemes, using a modified engine controller.

By throttling intake air, controlling EGR and manipulating the pulse-width of the main fuel injection event and/or adding a post-injection event, Huff showed that engine-out hydrocarbons, hydrogen and CO can be tailored to suit a NOx adsorber catalyst.

In a post-injection timing sweep, the ORNL research team measured hydrogen as high as 4% in raw exhaust, decreasing as post-injection timing was delayed. The CO:H2 ratio was consistently 2:1 for all strategies investigated so far. The only fuel used in tests was the 20% aromatics ULSD-CARB fuel. (Cont. p3)