The marine-specific solution designed by Caterpillar includes Selective Catalytic Reduction (SCR) technology to meet Tier 4 Final emission standards with few or no changes from the Tier 4 Interim solution. SCR has less impact on engine efficiency and durability than other emissions reduction solu-tions such as EGR. The new Tier 4 Final solution offers superior performance and an increase in power. Since SCR only impacts emissions there is no alteration to core engine components, no increase in the injector, the cylinder pressure or in heat rejection. It offers total reliability with maximum uptime in the most demanding applications without interruption, and supreme durability with the same long life to overhaul. Installation flexibility brings minimum impact to the vessel design owing to different configurations with multiple mounting options and a multi-position spool piece. As well as reduced emissions, SCR offers improved fuel efficiency, which translates to lower owning and operating costs.
THE 1-2-3 OF SCR
The dosing cabinet mixes the right dosage of compressed air [1A ] and urea [1B] drawn from the the urea tank and transfers it [2A]to the SCR module where it is injected in to the exhaust stream [2B]. In the SCR module  water evaporates and the urea is converted into NH3. When ammonia comes in contact with the SCR catalyst there is a reduction in NOx. The final output is carbon dioxide, nitrogen, water and oxygen.