Us Epa's Pm/Nox Trap Hits 2007 Limits, But Needs More Work

Diesel Fuel News, Oct 15, 2001 by Jack Peckham

At its engine test laboratories in Ann Arbor, Mich., U.S. EPA has successfully demonstrated that a combined catalyzed trap system for particulate matter (PM) and nitrogen oxides (NOx) can indeed hit EPA's very tough 2007 limits for heavy-duty highway diesels on a 3 ppm sulfur fuel. But the trap combo still has yet to prove it can hit all of EPA' s requirements.

The agency continues research and development (R&D) on a combined PM/NOx trap in order to show that engine makers and the oil industry together should be able to achieve the 0.01 grams/brake horsepower-hour PM limits and the 0.2 g/bhp-hr Nox limits for 2007 engines, when running on ultra-low-sulfur diesel (ULSD) of no more than 15 ppm sulfur.

EPA earlier showed that a prototype, non-optimized PM/Nox trap almost hit the 2007 limits even with today's conventional 4 g/php-hr NOx engine (see Diesel Fuel News 1/22/01, p8; 12/25/2000, plo).

Since then, EPA has found it can get inside 2007 NOx/PM limits by substituting a more modern 2002/2004 2.5 g/bhp-hr Nox engine with exhaust gas recirculation (EGR), as many engine makers plan to introduce next year. These new results are reported in a paper (SAE 2001-01-3619) that was to have been delivered to Society of Automotive Engineers last month (cancelled due to terrorist attacks in the U.S.)

Combining the 2.5 grams NOx engine with EPA's prototype PM/NOx trap put emissions well within the 2007 limits, at least on the hot-start portion of EPA's Federal Test Procedure (FTP): 0.13 g/bhp-hr NOx and less than 0.002 g/bhp-hr PM. The engine/trap system also can hit emissions limits "that are consistent with the 2007 SET [Euro steady cycle Supplemental Emission Test] requirements," EPA says.

* Almost Fits NTE, Too

Although specific not-to-exceed (NTE) emissions zone limits weren't tested, "NOx emissions for all of the SET points within the NTE zone were within the 1.5-times multiplier with the exception of the peak-torque point," which might have to be corrected by additional engine calibration and a variable-geometry turbo, EPA said.

While EPA was able to regenerate the NOx trap with secondary hydrocarbon injection, it hasn't yet developed a complete desulfurization cycle for the NOx trap. Problem: Even on the 3 ppm sulfur ULSD used in the test, this trap eventually would be fouled by sulfur. This will require periodic high-temperature desulfurization, at some unknown penalty to fuel economy, emissions and trap durability.

What's more, EPA has yet to show results with a more typical 15 ppm sulfur ULSD, hasn't proven its 435,000 miles durability requirement, and doesn't show how to hit EPA's $3,000 cost target for a heavy-duty Nox/PM trap, given the rather high precious-group metals (PGM) loadings indicated for its prototype system.

To solve these and other issues, EPA says it's continuing R&D on desulfurization cycles, thermal durability of the NOx trap, catalyst systems integration, cold-start emissions control, and repackaging/downsizing of the components for optimization.

The current prototype seems oversized, EPA says, as the particle filter volume is "likely three to four times the necessary volume," and the NOx adsorber is "approximately 20 to 35% larger than what has been reported for SCR systems" for heavy-duty engines, although its NOx trap shows higher efficiency than SCR.

Still, the system used to generate data published in the new SAE paper has only 200 hours of history, using 900 gallons of 3 ppm sulfur ULSD. This compares to the many thousands of hours that such a device would have to survive on a heavy-duty truck, with higher-than-3-ppm sulfur fuel exposure, unless a practical on-board sulfur-trap device comes along.

Fuel economy penalty for periodic NOx trap regeneration is about 1.5%, EPA said. Hydrocarbon slip during these regeneration cycles is avoided by a diesel oxidation catalyst "similar in volume and PGM loading to oxidation catalysts used for urea-slip [ammonia] control in SCR systems," EPA explained.