Selective Catalytic Reduction

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What is DEF?
Diesel Exhaust Fluid (DEF) is one of the key elements of the SCR process. It is a nontoxic solution of 67.5% purified water and 32.5% urea. When injected into hot exhaust as a fine mist and passed over a catalyst, DEF helps convert NOx into nitrogen gas and water vapor. DEF is stable, colorless and odorless, and meets accepted international standards for purity and composition.


Urea is a compound of nitrogen that turns to ammonia when heated. It is used in a variety of industries, including as a fertilizer in agriculture. It is classified as non-hazardous by EPA. Urea is naturally occurring, but is also commonly made from natural gas. International standards ensure quality is maintained during the DEF production and distribution. The advantage of using SCR is a lower rate of exhaust gas recirculation (EGR) can be used to meet the near-zero NOx emissions requirement, while at the same time achieving greater fuel efficiency than a diesel using massive EGR.

How will operators get and store DEF?
Since DEF is made from common substances - water and urea - a supply base is already in place. A coalition of industries, including truck manufacturers, truckstops, retail fuel facility owners, fuel distributors and urea providers is developing a network of bulk DEF filling stations at truckstops and diesel service stations across North America. In other words, it will be available at the same places as diesel fuel. In addition, DEF will be sold at truckstops, dealerships, parts stores and other retail locations.

DEF can also be stored and dispensed at fleet locations, using bulk and intermediate tanks. DEF freezes to a "slush" type consistency at 12 F, not to a solid, so just as with diesel, stored DEF needs to be protected from extended periods of severe cold.