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March 6, 2009

SCR Coming to Diesel Vehicles

Kennedy Maize

Power plant managers are familiar with selective catalytic reduction (SCR), a technology to reduce emissions of nitrogen oxides from fossil-fueled generating plants. The regulatory battle over SCR was a key feature of the 1900s, and today SCR is a given in new, and many older, power plants.

Starting in 2010, SCR technology, using a urea reagent, will also be required on new diesel-powered vehicles in utility fleets, ranging from light-duty pickups to 18-wheelers (see photo).


This diesel truck has a urea tank and selective catalytic reduction (SCR) unit. Courtesy: DaimlerChrysler Power Systems

How SCR Works in Vehicles

The urea ((NH₂)₂CO) in a diesel vehicle is held in a separate tank. It’s injected as a mist into the hot exhaust gases, which break down the urea into ammonia, the actual NO_x-reducing agent As it runs through a catalytic converter, the ammonia breaks down the NO_x to harmless nitrogen gas and water vapor. According to Kim Doran, a spokesperson for the North American SCR Stakeholders Group, “The exhaust coming from the tailpipe of an SCR-equipped vehicle may actually be cleaner than the air around it.”

Technology Tussles

All new diesel vehicles will have tanks to hold the urea for injection into the exhaust stream, along with a catalyst, to reduce NO_x emissions, per long-standing Environmental Protection Agency (EPA) rules. The automotive industry and the EPA have battled over NO_x reductions for diesel vehicles for a decade. They ultimately settled on SCR as the technology of choice. But truck-maker Navistar is still arguing for a different technology, exhaust gas recirculation (EGR), that requires no urea. Navistar (nee International Harvester) appears to have lost the battle.

Starting January 1, 2010, all new diesel vehicles in the U.S. must have technology to remove 90% of NO_x emissions, according to the EPA rules. SCR is the industry’s preferred technology. SCR injects “automotive grade” urea into the exhaust gas to scrub NO_x from the tailpipe emissions. This has been the practice in much of Europe for years.

The tech-speak term for automotive urea in the U.S. is “DEF,” which stands for “diesel exhaust fluid.” In Europe, it is AdBlue, a trademarked name. Both are a water solution that consists of 32.5% urea and demineralized water. The technology, says Glenn Kedzie of the American Trucking Association, is the only technology able to achieve the EPA’s requirement for 90% reduction of oxides of nitrogen.

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