CARB: flow-through filters more reliable for some 'cold' duty-cycle applications

Diesel Fuel News, May 12, 2003 by Jack Peckham

California Air Resources Board finds in tests that catalyzed diesel particulate filters (DPFs) won't always regenerate on many low-exhaust-temperature garbage trucks, with consequent excess backpressure (from soot buildup) and potential DPF damage.

But new "flow-through filters" (FTFs) can tolerate lower temperatures while providing up to 60% particulate matter (PM) reduction in about 80% of garbage trucks, CARB finds. These FTF systems also might be better suited to older (especially pre-1994) trucks with mechanical engine control, the GARB study finds.

FTFs typically employ wire-mesh or "tortuous flow" designs to help oxidize most particles, but are more permeable than wall-flow, higher-efficiency DPFs. FTFs also can employ ceramic foam or metal substrates.

Preliminary estimates are that FTFs would cut PM by about 30-60% (though GARB has yet to issue an official emissions "verification" for FTFs), compared to conventional DPFs (Johnson-Matthey CRT, Engeihard DPX) that GARB says get over 85% PM reduction (arb.ca.gov/msprog/SWCV/SWCV.htm).

According to GARB, the DPX needs average exhaust temp of 225[degrees]C with 10% of duty cycle above 300[degrees]C, while the GRT needs an average 260[degrees]C for 40% of duty cycle. Problem: Many garbage trucks have such "cold" duty cycles that that the GRT would work in only about 35% of garbage trucks and DPX in only about 48%, GARB found.

In contrast, an FTF is estimated to require exhaust temps of only 200[degrees]C for 50% of duty cycle, GARB says, citing a Johnson-Matthey estimate. Since about 80% of garbage truck fleets tested achieve this temperature minimum, it's possible that a much higher percentage of garbage trucks could employ the FTF than a conventional DPF, GARB figures.

Boosting the exhaust temperatures in order to expand applicability of DPFs and FTFs could require close-coupling the catalyst to exhaust manifold or "greater catalysis," GARB says.

Cummins and Cat engines predominate in the garbage truck fleets GARB tested, but most of these engines didn't achieve the minimum exhaust temps for CRT or DPX "passive" filters. Only a Navistar engine could meet the temp requirements for both of the GARBverified DPFs.

"Front and side-loaders appear to be most suitable to application of either the passive DPF of FTF, although a substantial percentage of rear loaders and roll-offs may also find this technology to be feasible," GARB's report says.

GARB first began testing an FTF system - with Glean Diesel Technology's (GDT) platinum-cerium fuel-borne catalyst (FBG) to aid PM oxidation - on Waste Management trucks in metro Los Angeles last fall, and will demonstrate six more FTF systems this spring.

The GDT-FBG system tests (with catalyst/filter systems from Glean Air Systems) involved a diesel oxidation catalyst (DOG), a DOG paired with an FTF, and a lightly-catalyzed (LG) DPF, on six trucks with Gummins engines of 1992 and 1996 model years.

(Separately, GARB has committed funding to test DOG/FTF systems using Johnson-Matthey technology, the report says).

According to CDT chief operating officer Jim Valentine, the FBG/FTF combination has several advantages over conventional DPFs beyond the wider operating temperature range.

Example: The lightly-catalyzed FTF system makes less N[O.sub.2] than a conventional catalyzed DPF. That's an important issue since GARB now limits N[O.sub.2], slip from DPFs to 20% of total N[O.sub.2] slip, in order to minimize near-ground ozone formation.

"While dropping N[O.sub.2] from a baseline of 18% to 5% with our FTF/FBG is attractive, we also showed better PM reduction with our catalyzed wire mesh/FBC system which gave 65% PM reduction and still hit GARB's 20% N[O.sub.2] cap," Valentine told us.

One such version of this FTF system is expected to cost in the $2,000 to $3,000 range, or less than half that of conventional catalyzed DPFs, he said. Bonus: Getting 65% PM reduction from an FTF system installed on older, 0.275 grams/bhp-hr PM engines "is a lot more PM reduced than 85% reduction from a 0.1 g/bhp-hr PM engine," Valentine points out.

What's more, the FTF system is more sulfur-tolerant than conventional DPFs, thus expanding possible applications to areas and countries still lacking ultra-low sulfur diesel (ULSD).

"These type of devices should be good on older, dirtier engines," Valentine said. "We have several running on today's normal sulfur fuel, which appear to be functioning well after about 600 hours of commercial operation, although PM reduction is slightly less than on ULSD. That could be very good for developing countries with limited ULSD."