Ammonia-SCR demo meets CARB's 70% NOx cut goal

Diesel Fuel News, Oct 27, 2003 by Jack Peckham

California transit operators seem likely to breathe easier thanks to a demonstration project that taps a relatively low-cost combination of ammonia-selective catalytic reduction (SCR) with diesel particulate filters (DPF).

The three-bus demonstration project at Santa Clara Valley Transit Authority (VTA) pairs the KleenAir SCR system with a silicon carbide DPF, both of which work over a wide exhaust temperature range, explains Phil Roberts, president of Extengine, a KleenAir licensee that won the demonstration program contract with VTA.

Over the CARB-mandated UDDS drive cycle test for nitrogen oxides (NOx) and particulate matter (PM), the SCR/DPF combo is slashing NOx by over 90% and PM by 95%, Roberts told us.

This more than fulfills a CARB mandate for a diesel transit bus NOx control demonstration program achieving at least 70% NOx reduction (see Diesel Fuel News 4/1/02, p12; 3/4/02, p13; 2/4/02, p10).

Bonus: The demonstration program will generate the required 1,000 hours (or 50,000 miles) durability data to achieve CARB diesel retrofit "verification" for NOx and PM control. While the demo project is scheduled to run three years, it probably won't take that long to generate the required minimum hours/miles for "verification."

Had bus operators failed to come up with a practical retrofit scheme demonstrating at least 70% NOx reduction, they could have lost their legal fight to pursue the low-cost "diesel path" and would have been forced to convert to costly compressed natural gas (CNG), electric or fuel-cell buses.

Extengine figures the SCR/DPF retrofits would cost around $20,000 per bus.

That's about one-third the incremental cost penalty of a CNG bus once including the very high cost of CNG refueling infrastructure, Roberts says.

"Fleets want the biggest NOx reduction at the lowest cost," Roberts said, pointing to emissions-reduction incentive programs targeting NOx emissions.

CARB imposes an ammonia-slip limit on SCR retrofits, so Extengine decided to put a slip catalyst after the SCR. As measured via a mandatory laser detection technique, this system results in "zero slip," he said.

Besides transit bus applications, this SCR system is also planned to expand into school bus and medium/heavy-duty vocation truck applications as well as off-road applications including irrigation pump engines, he said.

The system avoids permitting problems for large-scale ammonia storage by employing an every-two-weeks tank swapping-out scheme, at fleet vehicle storage sites, he said. For the transit buses, this means swapping out two eight-pound tanks, mounted in the rear in a safe enclosure. Even a rear-end accident couldn't rupture the U.S. Dept. of Transportation (DOT) certified tanks and any leakage would quickly dissipate, he said.