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Encyclopedia of Marine and Energy Technology

Selective Catalytic Reduction (SCR)


The Selective Catalytic Reduction system reduces the level of nitrogen oxide in the exhaust gas from the engine by means of catalyst elements and a reducing agent. In the process a reducing agent of an urea water solution is added to the exhaust gas stream. The water in the urea solution is evaporated as the solution is injected into the hot exhaust gas. The high temperature also induces thermal decomposition of the urea ((NH2)2CO) into ammonia (NH3) and carbon dioxide (CO2). Exhaust gas NOx emissions are thereafter transformed into molecular nitrogen (N2) and water (H2O), as they react with the ammonia at a catalytic surface.

The catalytic elements are located inside a metallic reactor structure located in the exhaust gas line. The end products of the reaction are pure nitrogen and water, i.e. major constituents of ambient air. No liquid or solid by-products are produced. The efficiency of the catalytic reduction depends on a number of factors, including the dosage of the reducing agent, the volume of catalyst elements and the exhaust gas temperature. Normally, a NOx reduction level of 90% can be reached.