Epa Finally Explains Why Nox/Pm Traps Feasible, Reveals Advanced Catalyst/Trap Concepts

Diesel Fuel News, Jan 22, 2001 by Jack Peckham

Central to U.S. EPA's now-final highway heavy-duty diesel emissions regulation for 2006/07 is an emerging-technology, fuel-sulfur-sensitive, combination trap/catalyst for nitrogen oxides (NOx) and particulate matter (PM) control.

While this system isn't yet commercial, EPA is convinced that engine makers, vehicle makers and catalyst developers will develop a highly efficient, reliable, and reasonably-priced PM/NOx trap that can tolerate ultra-low-sulfur diesel (ULSD) of up to 15 ppm sulfur. For heavy-duty applications, EPA figures this system would cost about $3,000 per vehicle, while a light-heavy-duty system would cost about $2,000.

Three critical reasons why EPA is so convinced of this:

* EPA's discoveries about combined NOx/PM traps in its own lab tests.

* Toyota's plan to commercialize a combined NOx/PM trap on diesel light trucks in 2003 -- and eventually, it hopes, heavy trucks (see Diesel Fuel News 8/14/200, p10). This 80% efficient trap (still not good enough for EPA's 2006/7 emissions limits) depends upon Japan's expected rollout of a 50 ppm ULSD in 2003, possibly followed by an even cleaner fuel later in the decade.

* Highly-experienced, world-leading exhaust catalyst makers assure EPA that all the engineering issues with combined NOx/PM traps have now been identified and can be solved in time for the late-2006 diesel deadlines.

However, many oil refiners and engine makers complain that EPA either lacks or is shielding key evidence backing the agency's overwhelming optimism for a combined diesel PM/NOx trap.

Now -- belatedly -- the reasons behind the optimism are starting to come to light, in U.S. EPA's voluminous "regulatory impact analysis" and "response to comments" reports that accompany its new highway diesel regulation (see: http://www.epa.gov/otaq/diesel.htm).

Using a variety of NOx traps (combined with catalyzed PM traps) donated by Manufacturers of Emission Control Association, EPA has gotten very close to achieving its 2007 highway diesel standards by using a dual-path, twin trap/catalyst system (see Diesel Fuel News 12/25/2000, p10).

Perhaps an even more elegant design (EPA's drawing is reproduced below) would combine the PM/NOX trap into a single canister, partitioning exhaust flow within compartments to allow one portion to regenerate while the other continues to trap, EPA's engineers contend.

A high-performance valve, similar to commercially-proven turbocharger wastegate valves, could re-direct exhaust flows among the partitions, EPA figures.

This scheme would minimize fuel consumption during regeneration, protect the NOx trap and downstream NOx sensors from sooting, improve cold-start NOx catalyst "light-off" times, reduce the total size of the system (and thus allow wider applications in smaller vehicles), and reduce the over-all cost "by allowing the use of a single monolith and housing instead of four or more separate devices," EPA explains.

EPA concedes that with today's state-of-art traps, NOx conversion efficiency drops off at low exhaust temperatures (below 250[degrees]C), as well as temperatures above 500[degrees]C. While most heavy-duty diesels operate within rather than outside these low/high temperature ranges, EPA admits it's possible that its 2006/7 emissions testing cycles could become a problem for compliance.

So, EPA's new rule waives certain "not to exceed" emission test limits at exhaust gas temperatures below 250[degrees]C. On the other hand, EPA says it's confident that diesel NOx catalyst developers will solve high-temperature performance issues promptly, based on previous experience with high-temperature gasoline catalysts. That's especially important because a portion of EPA's new supplemental steady-state emissions test ("SET" test) would measure NOx at exhaust temperatures above 500[degrees]C.

* Thermal Stress Solution Seen

For long-lived, heavy-duty applications, direct fuel injection of reductant into the PM/NOx trap would avoid the thermal stress problems of in-cylinder, post-main injection for creating hydrocarbon reductants, EPA explains.

Heavy-duty engine/turbo/catalyst thermal stress was one of the major criticisms of combined PM/NOx traps cited in a study by AVL less than a year ago (see Diesel Fuel News 4/24/2000, p8). Partly based on its own lab test experience, EPA now believes that PM/NOx trap developers can avoid these problems via post-exhaust-manifold, post-turbo fuel injection, just upstream of the PM trap.

Similarly critical to future NOx trap success is development of a scheme that avoids catalyst sintering from periodic high-temperature desulfurization cycles.

To solve this problem, EPA sees catalyst developers moving toward high-dispersion catalyst designs, along with careful location of PM/NOx traps at an optimum distance from the exhaust, manifold in order to assure temperatures high enough to favor desulfurization and NOx conversion, but not so high as to promote sintering.

Also coming: New catalyst support schemes that prevent sintering. These have already been proven in gasoline engine applications up to 1,000[degrees]C -- considerably hotter than diesel exhaust, EPA notes.