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Selective Catalytic Reduction (SCR)
Selective catalytic reduction, or SCR, is a method of converting nitrogen oxides (NOx) into carbon dioxid (CO2) and water. Diesel exhaust fluid, or DEF, is added to a stream of exhaust gas where it reacts with the nitrogen oxides, breaking them down into harmless carbon dioxide and water.
Since the late 1970s, SCRs have been in use on large utility, industrial, and municipal solid waste boilers, and SCR usage in these systems has reduced NOx by 70% – 95%. More recently, diesel engines on large ships, locomotives, gas turbines, and large diesel-powered trucks have seen an increase in SCR usage.
Diesel Exhaust Fluid (DEF)
Diesel exhaust fluid, or DEF, is designed for use in SCR systems to reduce nitrogen oxide emissions. It is a urea-based chemical produced from raw fossil fuel sources such as natural gas, coal, and a variety of other petroleum based products by dissolving solid urea of high purity in de-ionized water. It is non-toxic, non-flammable, stable, and requires no special handling.
DEF must be kept totally pure to allow for proper usage, and the concentrations of urea to water must be exact. Any variations in either of these ares can result in engine damage.
How DEF is Used in SCR Systems
Through SCR technology, nitrogen oxide (NOx) reducing reactions take place in an oxidizing atmosphere. These reactions reduce levels of NOx using ammonia as a reductant within a catalyst system. The amonia reacts with NOx to convert it into nitrogen, water and carbon dioxide (CO2). The reductant source is usually Diesel Exhaust Fluid, or DEF. SCR technology alone can reduce NOx by as much as 70% – 90%.