Diesel Exhaust Fluid (DEF) is an aqueous urea solution used in Selective Catalytic Reduction (SCR) to lower NOx concentration in the exhaust emissions from diesel engines. The solution may also be referred to as AUS32 , shorthand for Aqueous Urea Solution, 32.5%. The solution adhere to quality standards that are maintained in accord with ISO 22241 specifications. Initially the specification was described in DIN 70070. In 2006 the worldwide ISO 22241 standard was introduced, which also stipulates analytical test protocols to verify adherence to specifications, as well as requirements for storage, transport and handling of the fluid.
SCR systems are sensitive to potential chemical impurities in the urea solution, therefore the solvent is de mineralized water. The solution is clear, non-toxic and safe to handle. However, it can corrode some metals and so must be stored and transported in vessels made of materials not so affected. DEF is stored in a tank onboard the vehicle, and injected into the exhaust stream by a metering system at a rate of 3–5% of diesel consumption volume. This low dosing rate ensures long fluid refill intervals and minimizes the tank's obtrusion into vehicle packaging space. An Electronic control unit adjusts the addition of fluid in accord with such parameters as engine operating temperature and speed.
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Selective catalytic reduction (SCR) is a means of converting nitrogen oxides, also referred to as NOx with the aid of a catalyst into diatomic nitrogen, N2, and water, H2O. A gaseous reductant, typically anhydrous ammonia, aqueous ammonia or urea, is added to a stream of flue or exhaust gas and is absorbed onto a catalyst. Carbon dioxide, CO2 is a reaction product when urea is used as the reductant.
Selective catalytic reduction of NOx using ammonia as the reducing agent was patented in the United States by the Englehard Corporation in 1957. Development of SCR technology continued in Japan and the US in the early 1960s with research focusing on less expensive and more durable catalyst agents. The first large-scale SCR was installed by the IHI Corporation in 1978.
Commercial selective catalytic reduction systems are typically found on large utility boilers, industrial boilers, and municipal solid waste boilers and have been shown to reduce NOx by 70-95%. Applications now include diesel engines, such as those found on large ships, diesel locomotives, gas turbines, trucks and automobiles.
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