EPA Announces Diesel SCR Compliance Guidelines

Posted on March 30, 2007

In a somewhat surprising move, the U.S. Environmental Protection Agency (EPA) has issued guidance on emission certification procedures for on-road diesels that use selective catalyst reduction (SCR) technology.

While SCR has been used successfully in other applications, such as electric power generation, EPA said this guidance enables on-highway vehicle manufacturers to adapt the technology to light- and heavy-duty vehicles.

Many in the industry had accused, or suspected, that EPA had an anti-SCR bias. Not so, said Margo Oge, director, of EPA’s Office of Transportation and Air Quality, in an interview in the February 2007 issue of Diesel Progress North American .

“Our issue with urea was not a question of is the technology going to work. Rather we had two concerns. The first was we wanted to make sure there was a urea infrastructure in the market. The second, and very important, is that there are compliance mechanisms to assure that the user of the truck or the car would add urea to the system when it’s needed. Because if you don’t add urea, you’re out of compliance.

“It was never really an issue that urea technology would work, can meet the standards? Rather it was the issue of infrastructure and the issue of compliance,” Oge said in the interview.

SCR reduces emissions of the ozone-forming pollutant nitrogen oxide (NOx). It uses a nitrogen containing "reducing agent," (usually ammonia or urea) that is injected into the exhaust gas upstream of the catalyst. Drivers must periodically replenish the agent or else NOx emissions can greatly increase, EPA said in its announcement.

Manufacturers will need to gain approval for their SCR strategies as part of EPA certification. These strategies must address driver warning systems and inducement, system durability and reliability, and reducing agent quality and availability.

EPA’s March 27, 2007 letter can be found at: