SCR progress: smart design, better efficiency; new sensors coming to aid clean-diesels

Diesel Fuel News, Sept 29, 2003 by Jack Peckham

Newport, R.I. -- Progress on clean-diesel technical fronts seems to be boosting the chances for urea-selective catalytic reduction (SCR), even though regulatory debate (especially at U.S. EPA) continues to raise doubts about market applications. Examples:

--A relatively compact combination of an upstream catalyzed diesel particulate filter (DPF) with a urea-selective catalytic reduction (SCR) system, dubbed SCRT (see Diesel Fuel News 3131/03, p11), depends in part on oxidizing nitric oxide (NO) to [NO.sub.2]--the same idea for speedy soot oxidation in a DPF.

However, creating "too much" [NO.sub.2] actually penalizes a scheme for promoting low-temperature SCR, as JM researcher Andy Walker explained at Diesel Engine Emissions Reduction (DEER) conference here, sponsored by U.S. Department of Energy.

SCRT "preferentially follows the NO [NO.sub.2] NH3 [ammonia] reaction, since this is the fastest," Walker says. "Therefore as long as the level of [NO.sub.2] coming out of the upstream filter [DPF] system is less than 50%, then you can pretty much guarantee an excellent [NO.sub.2] removal efficiency. So in practice the [NO.sub.2] slip level would be very low."

As expected, California Air Resources Board (CARB) raised the [NO.sub.2] slip issue here. That's because CARB earlier put a limit on NO z slip (20% of all NOx slip) out of catalyzed DPFs, due to concerns about ground-level ozone and worries about possible lung irritation to people nearby.

However, smart SCR/DPF design answers CARB concerns as well as total NOx reduction efficiency concerns, Walker explains.

--50% Break-Point

"If the amount of [NO.sub.2] coming out of the upstream filter exceeds 50% then some of the [NO.sub.2] will be subject to the direct [NO.sub.2] Nh3 reaction, which is very slow," thus penalizing NOx reduction and favoring [NO.sub.2] slip. "This is why the system design is important, to ensure that the NO/[NO.sub.2] split stays favorable," he said.

Some technology developers have shown relatively huge SCR systems, designed to slash high levels of engine-out NOx to EPA's final 2010 limit of 0.2 grams/brake-horsepower hour.

"If you want to get as much NOx conversion as is absolutely possible then the greater the volume, the better," Walker pointed out to us in a post-conference interview. "But good NOx conversions can be obtained

at relatively low SCR/engine swept volume ratios."

For example, an SCRT with three SCR catalysts cut NOx by 92%, while also achieving EPA's 0.01 g/bhp-hr PM limit. Starting with 5 g/bhp-hr engine-out NOx, this SCRT system achieved a 0.4 g/bhp-hr NOx limit--a 92% reduction, one-third the 2007 interim 1.2 grams limit and closing in on EPA's final 0.2 g/bhp-hr NOx limit, even without engine-out NOx reduction modifications.

In field tests on a real-world Class 8 truck (Cummins ISX 15-liter, 450-horsepower engine), an SCRT system achieved 82% NOx conversion efficiency with no degradation over 125,000 kilometers. This system was relatively compact, as the SCR catalyst was 3x swept volume while DPF was less than 2x swept volume, for a total catalyst DPF volume of 75 liters.

To date, 20 "SCRT" equipped heavy-duty trucks are in fleet tests in North America.

--Europe's "Adblue" SCR project (see Diesel Fuel News 717103, p1) continues apace, with urea production commitments secured and key retail infrastructure developments moving forward, as Wolfgang Frank of urea-SCR technology developer, Purem, explained here.

Original equipment maker (OEM) requirements for SCR include 620,000 miles durability, tolerance to huge ambient temperature ranges (-40 to 80[degrees]C), resistance to road shock, urea dosing precision of plus/minus 5wt%, and 310,000 miles of >65% catalytic efficiency for Euro-4, he said.

Recent road and refueling tests in the brutal winters of Finland as well as scorching summers in Spain are showing developers how to optimize SCR systems, he said.

Meantime, four European sites "are now prepared for Adblue production of up to 1 million tons/year--equivalent to 300,000 heavy-duty SCR-equipped trucks annual demand," he said. "Other production sites can be prepared for Adblue production as the market develops."

Univar has already announced its plans for urea storage and distribution units Europe-wide, while Raiffeisen Cooperative will tap its agricultural network to distribute urea. TotalFinaElf likewise has announced plans to open urea refueling in Berlin and Stuttgart this year, he said.

On the vehicle optimization side, improved sensors to detect NH3, NOx and urea are "currently under development," he said.

--DaimlerChrysler SCR developer Michael Schittler revealed that closed-loop control with an ammonia sensor "based on a DaimlerChrysler patent is under development in close cooperation with a leading U.S. component supplier and will be marketed also to our competitors."

The sensor "will considerably increase the SCR catalyst efficiency," by allowing more urea injection without ammonia slip. It also will detect any lack of urea injection, hence warning the driver of an empty urea tank, possibly derating torque, and eventually triggering a "complete engine shut-down," although "details of this are under discussion with [EPA] authorities as well as operators."

--Ford Co-Fueling Scheme