Tuesday, March 17, 2009

Selective Catalytic Reduction (SCR)

Selective Catalytic Reduction (SCR), a process where a gaseous or liquid reductant (most commonly ammonia or urea) is added to the flue gas stream and is absorbed onto a catalyst. The reductant reacts with NOx in the flue gas to form H2O and N2.

Graphic showing a schematic of a selective catalytic reduction (SCR) system. A diesel engine equipped with a cooling system produces exhaust, which travels through a diesel particulate filter (DPF) toward an SCR catalyst. Before the hot exhaust reaches the SCR catalyst, Diesel Exhaust Fluid (DEF) from a DEF tank with control unit is injected into the exhaust. The mixed exhaust and DEF travel together into the SCR catalyst. Reactions in the SCR catalyst produce nitrogen and water, which are emitted to the environment.

Specialty Catalytic Converters are required to make an SCR system work, the current options being a Vanadium based catalyst, or a catalyst with Zeolites in the washcoat. In some jurisdictions use of a Vanadium based catalyst are frowned on (California) and the Zeolite type preferred, however Vanadium offers better performance.

SCR catalysts function well only within a narrow temperature window, and for OEM installations the Engine Control Unit is specially programmed to keep the exhaust gas temperature in that range
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