Selective Catalytic Reduction (SCR) is a means of converting nitrogen oxides (NOX) with the aid of a catalyst into nitrogen (N2) and water (H2O). 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 NOXby 70-95%. More recent applications include diesel engines, such as those found on large ships, diesel locomotives, gas turbines, and even cars.
SCR is the dominant technology used to meet NOX emissions standards for heavy-duty diesel vehicles in the United States, Europe, and Japan. In developing markets such as China and India, which are planning to implement Euro IV standards, SCR is also the manufacturers preferred technology option. It allows for NOX control with little or no fuel-economy penalty, and can permit manufacturers to continue marketing the same engine/after-treatment combination even as emission standards become tighter over time.
But SCR poses unique implementation challenges stemming from its reliance on a reducing agent (typically aqueous urea, though others can be used), which is injected into the exhaust gas upstream of the catalyst, and which must periodically be replenished. SCR requires an extensive urea delivery infrastructure for geographically dispersed mobile sources and robust fail-safes to ensure that drivers properly fill onboard urea tanks. Under certain circumstances SCR-equipped vehicles in use may also pose problems with off-cycle and unregulated emissions due to the temperature dependence of catalytic activity, improper urea dosing, catalyst poisoning, and the formation of catalytic by-products.