New aftertreatment system provides alternative to urea SCR

By | April 26, 2010
Engineers with Tenneco Automotive have developed a new hydrocarbon lean NOx catalyst (HC-LNC) system for diesel NOx control using E85 as a reductant. E85 is a renewable fuel made of 85 percent ethanol and 15 percent gasoline. Unlike urea selective catalytic reduction (SCR), which employs diesel exhaust fluid (DEF) comprised of ammonia and water, Tenneco's HC-LNC system will use both diesel fuel and E85 dosing in the exhaust stream to help catalyze NOx emissions into tailpipe-out nitrogen and water.

A Tenneco engineer told Biodiesel Magazine that diesel fuel as a reductant can attain moderate NOx reductions but E85, however, can achieve significantly better conversion numbers. Exactly why E85 works so well for NOx abatement is not fully understood yet, but its oxygen content and volatility are thought to be major factors.

Urea SCR systems typically use copper- or iron-based catalysts, but Tenneco's HC-LNC system employs a unique silver-based catalyst. General Electric is the producer of the new silver catalyst, which recently completed a technology transfer with Umicore for Umicore's substrate design and washcoat application of GE's catalyst formulation. Tenneco has secured exclusive rights to GE's specific catalyst formulation in systems development for a majority of the on- and off-road emissions control markets, except for large locomotive applications.

The intention is to use both diesel fuel and E85 in the same exhaust system and catalyst. Typically, diesel fuel would be used for NOx control under light loads while E85 would come into play during heavier loads and stop-and-go cycles.

Another advantage E85 has over urea is its ability to withstand lower temperatures. Alcohol has a much lower freeze point than DEF, which can gel at minus 11 degrees Celsius. An E85 storage tank on the vehicle would not require all of the heaters and heated lines that DEF tanks need, so the overall complexity of the system could be simplified.

Urea can also leave crystal deposits in the exhaust system-on mixers and the inlet face of the catalyst-a problem E85 will not have.

This type of system, however, would rely on a well-developed E85 infrastructure, something many flex-fuel vehicle owners say is grossly lacking. Biodiesel Magazine's source, however, said he believes the U.S.' E85 infrastructure, as deficient as it may be today, is thought to be better developed at this point than a urea distribution infrastructure.

This could also be the catalyst needed by the ethanol industry to crack the "chicken and egg" conundrum often associated with E85 and compatible vehicle availability. And the synergy of simultaneously developing an E85 and clean diesel fluid infrastructure built around one renewable fuel is very appealing.

Tenneco anticipates its new system will be ready for commercial on-road application as early as 2012, with its eye on off-road applications as early as 2014.

While it may be a tough sell to convince engine makers already committed to commercial production of urea SCR systems in their on-road applications to switch to an E85-compatible HC-LNC approach, it may provide an interesting alternative for companies such as Navistar Inc., which has been outspoken over its refusal to incorporate urea SCR as a means to NOx reduction. The company, instead, has developed and relies on what it calls Advanced Exhaust Gas Recirculation (Advanced EGR), but some experts are skeptical whether Navistar can meet its NOx reduction requirements through Advanced EGR alone.

Other potential outlets for this new HC-LNC system include the myriad of off-road applications, which EPA will also require meeting very strict NOx reductions in coming years.
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