Selective Catalytic Reduction

Selective catalytic reduction, or SCR, is an emissions control process that converts nitrogen oxides in exhaust or flue gas into harmless nitrogen and water, by injecting a reductant, typically anhydrous or aqueous ammonia or a urea solution, that reacts with the nitrogen oxides on the surface of a catalyst. The Engelhard Corporation patented ammonia based nitrogen oxide reduction in the United States in 1957, Japanese and American researchers spent the early 1960s developing cheaper and more durable catalysts, and IHI Corporation installed the first large scale SCR system in 1978. Modern SCR systems remove 70 to 95 percent of nitrogen oxide emissions from utility and industrial boilers, and comparable systems on modern engines cut nitrogen oxides, particulates and hydrocarbons by as much as 95 percent compared with earlier uncontrolled engines. The technology is now standard equipment on heavy duty trucks, diesel locomotives, gas turbines and automobiles built to meet current emissions standards.