Carb re-analysis shows DPF [NO.sub.2] slip not so big a problem

Diesel Fuel News, Feb 18, 2002 by Jack Peckham

Los Angeles -- The combination of ultra-low-sulfur diesel (ULSD) with a catalyzed diesel particulate filter (DPF) still looks like a winning combination for clean-diesel, as California Air Resources Board (CARB) found that the slight amount of nitrogen dioxide (N[O.sub.2]) slip from a DPF isn't such a big concern after all.

CARB had issued a warning last fall that systems such as Johnson-Matthey's "CRT" DPF that oxidize NO to N[O.sub.2] for soot oxidation might cause a slight increase in ozone emissions. But a more thorough follow-up study presented to CARB's International Diesel Retrofit Advisory Committee (IDRAC) this month shows that N[O.sub.2] slip is much less of a concern.

"A modest increase in the diesel N[O.sub.2] fraction has more benefits than disbenefits," CARB's John DaMassa told the IDRAC meeting here.

Bottom line: CARB is likely to approve a tolerance level for N[O.sub.2] slip for DPF filter retrofits fairly soon, although the issue hasn't yet been finalized.

The new CARB analysis shows that ozone levels wouldn't change at all if massive retrofits on diesel engines caused even a 25% boost in the proportion of N[O.sub.2] slip from diesel tailpipes. All NO converts to N[O.sub.2] anyway in the atmosphere, but the impact on near-ground ozone is greater if N[O.sub.2] is the major part of NOx tailpipe emission.

On the other hand, N[O.sub.2] beneficially oxidizes toxic particulate matter (PM) in a DPF, as well as other toxic gases in diesel exhaust. The net impact is an improvement in public health. This decrease in so-called "primary" PM more than offsets any increase in "secondary PM" from the post-tailpipe conversion of NOx to particulates, DaMassa explained.

Whether a slight increase in tailpipe N[O.sub.2] has any health impact upon the occupants of vehicles near the emission isn't known, CARB said in a response to a question raised by Natural Resources Defense Council attorney Gail Ruderman Feuer here. CARD promised to search for scientific advice on this issue and present findings at future IDRAC meetings. NRDC lobbies for a ban on the purchase of clean-diesels and instead pushes compressed natural gas (CNG) mandates.

Sweden likewise found increases in [NO.sub.2] slip with CRT systems, but ambient [NO.sub.2] in city centers is still declining regularly since 1986, according to Lennart Erlandsson of Sweden's Motor Test Center (MTC). Likewise, "VERT" diesel PM filter project director Andreas Mayer pointed out here that "it doesn't matter if a little more [NO.sub.2] comes out the tailpipe" compared to the huge net health benefits of DPFs plus ULSD.

Likewise, BP's ULSD marketing manager Chuck LeTavec pointed out that the combination of a catalyzed PM filter and ULSD delivers a "dramatic reduction in toxic emissions."

What's more, DPF developers are moving to avoid and minimize [NO.sub.2] slip, according to a presentation by Manufacturers of Emission Control Association (MECA) here.

While certain catalysts might cause higher [NO.sub.2], "catalysts can be formulated to decrease precious-group metal [PGM] loadings to decrease [NO.sub.2]," MECA spokesman Dale McKinnon said. "And you can decrease the size of a DPF when your field experience shows what's really needed for certain engines and duty cycles."

Addition of fuel-borne catalysts (FBCs) such as platinum, cerium or iron also will minimize the formation of [NO.sub.2] compared to catalyzed filters with relatively high PGM loadings, he pointed out. Exhaust temperature thermal management schemes (such as electronic fuel injection strategies) also can allow for optimized exhaust catalysts that minimize [NO.sub.2], he pointed out.

In certain situations, fleets can use diesel-water emulsions that would slash all NOx emissions including [NO.sub.2]. The coming deployment of exhaust gas recirculation (EGR) also will minimize Nox/[NO.sub.2], as would selective catalytic reduction (SCR) and lean-NOx catalysts. Repowering old equipment with modern, low-NOx engines also will help, McKinnon pointed out.

While BP's ULSD fleet tests with a variety of vehicles using DPFs found an average 31.7% increase in the [NO.sub.2] portion of NOx emissions, "a lot of the PM filters in demonstration programs are over-designed with aggressive catalyst formulations," he said. What's more, [NO.sub.2] slip will become less of an issue for original equipment manufacturer (OEM) diesel engines that will appear fairly soon, perhaps as early as some time next year.

Other potential increases in "secondary" emissions from DPFs would be reduced along with lower sulfur levels in fuel, he said. "We found that if you eliminate sulfur then you fix the [slight] increase in nitro-PAH [polycyclic aromatic hydrocarbon]" on the cold-start portion of the federal test procedure (FTP). Nitro-PAH doesn't increase on the hot portion of the cycle.

It's possible that the fuel-derived sulfuric acid collected on test lab sampling filter pads downstream of DPFs promote the nitration of PAH. So this phenomenon could be a sampling artefact rather than a real-world emission problem, McKinnon explained.