NOX adsorber seen able to achieve 435,000 miles mandate

Diesel Fuel News, Dec 9, 2002 by Jack Peckham

San Diego -- A nitrogen oxides (NOx) adsorber designed to meet U.S. EPA's 20072010 heavy-duty highway diesel emissions limits seems able to achieve the equivalent of 435,000 miles of sulfur exposure durability, although thermal cycling durability and particulate matter (PM) fouling resistance still must be proven.

The dual-leg NOx adsorber, by EmeraChem (formerly Goal Line Technologies, see Diesel Fuel News 7/3/00, p7), shows it can achieve the >90% NOx reduction seen required to reach EPA's ultimate 0.2 grams/brake horsepower-hour NOx limit by 2010. EPA requires 435,000 miles durability for 2007 heavy-duty exhaust treatment systems for NOx and PM control.

EmeraChem ran a 500-hour test on a fuel with 10 times the sulfur of EPA's 15-ppm ultra-low sulfur diesel, thus simulating 5,000 hours of 15-ppm sulfur exposure. Still, that's only about half the 10,000 hours exposure assumed to represent 435,000 miles. Nevertheless, EmeraChem feels confident about extrapolating out to 435,000-miles equivalence because its system achieved stable performance even with the high-sulfur fuel.

Sulfur in fuel or lubricant will "mask" NOx storage sites over time, hence all NOx adsorbers must have periodic desulfurization cycles along with periodic NOx adsorption/reduction cycles.

To accelerate the sulfur-masking aging process, EmeraChem used a 150-ppm sulfur diesel fuel -- 10 times the sulfur limit of ultra-low sulfur diesel (ULSD) that becomes the standard EPA highway fuel in 2006.

In one test scheme, EmeraChem paired its NOx adsorber to an 11.9 g/bhp-hr NOx engine. The adsorber achieved from 60% to 95% NOx reduction over a wide temperature range (from 160[degrees]C to 4000[degrees]C), with a 3.6 to 3.8% fuel penalty.

In another test on a 6.9 g/bhp-hr NOx engine, NOx conversion efficiency was restored after 1- or 2.5-minute desulfurization cycles in a roughly 430-5300[degrees]C "desulfurization zone" temperature window.

While "sulfur aging" degraded NOx adsorber performance over time, EmeraChem found that it achieved stability at well over 1,000 hours of "15-ppm sulfur equivalent time," still providing greater than 90% NOx reduction efficiency considered "sufficient performance for compliance" with EPA's limits.

This achievement came despite a 25% loss of total NOx adsorber surface area and 44% loss of active metal surface area during the aging process. Phosphorous and zinc (lube oil metals) may have caused some degradation, as these were found on the upstream face of the catalyst, EmeraChem researcher Jim Parks told the Diesel Engine Emissions Reduction (DEER) conference here, organized by U.S. Department of Energy.

Future work will focus on cost reduction and integration with PM emission controls, Parks said.

Although its catalyst formulation is "optimized for sulfur control, it still covers a broad range of operating temperatures" as will be required to prove NOx limits under EPA test cycles, Parks told us.

EmeraChem didn't report platinum group metal (PGM) loadings on its

catalyst system, a critical cost factor.

"The issue of cost is complicated for this reason. To meet the regulation, the OICA test cycle must be passed, along with transient test cycle and 'not-to-exceed' zone limits, plus 435,000 miles durability. It wouldn't be fair to say anything about the PGM loading in reference to any cost targets until an accurate system cost -- for a system that can pass certification -- is defined," Parks said.