Selective Catalytic Reduction


Selective Catalytic Reduction systems for marine diesel engines: what are they and what do they do? A compendium of frequently asked questions about SCR systems.

Selective Catalytic Reduction (SCR) is an exhaust aftertreatment process that minimises the harmful nitrogen oxide (NOx) emissions produced by diesel engines.

The SCR reduces nitrogen oxide (NOx) molecules to molecular nitrogen and water vapour. A nitrogen-based reducing agent, such as ammonia, is used to trigger a chemical reaction in a catalytic converter that turns pollutants into harmless gasses.How does an SCR system work?

An SCR system uses aqueous ammonia to convert toxic nitrogen oxides into harmless nitrogen gas and water vapour. The process can be summarised as follows:

1: A urea and water solution is injected into the exhaust stream
2: As the water evaporates, the urea is converted into ammonia
3: The ammonia bonds with the nitrogen dioxides in the exhaust gases
4: A catalytic converter reduces the toxic components in the exhaust gases to inert nitrogen gas and harmless water vapour
5: The nitrogen and water vapour are safely released into the atmosphere

This process is described in more detail in our article on SCR system for ships.

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