Selective Catalytic Reduction: all one needs to know

08/09/2022

A schematic overview of Selective Catalytic Reduction - header

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.

What is the catalyst used in SCR?

An SCR system uses aqueous ammonia to convert harmful components (NOx) into harmless gasses in the exhaust. In most cases we use AUS40 or AdBlue. AdBlue is the brand name for a urea solution that contains 32.5% urea in demineralised water. The technical name is AUS32. AUS40 is a urea solution with 40% urea that has similar properties.

Why is catalyst reduction required?

The exhaust gases produced by the combustion process in a diesel engine contain nitrogen oxides. A SCR system is essential to ensure compliance with IMO Tier III emission standards.

What is the difference between EGR and SCR?

Exhaust Gas Recirculation (EGR) feeds exhaust gases back into the combustion chamber for recombustion, which only partially reduces harmful nitrogen oxide (NOx) emissions. Selective Catalytic Reduction (SCR) is a post-combustion NOx reduction process that does not affect engine performance.

What types of marine engines can be equipped with SCR?

Emigreen®’s innovative Selective Catalytic Reduction system is compatible with basically all types of modern marine engines. It can be installed on new ships and retrofitted on existing ships.

Jelle Westerhof is sales manager at Emigreen® and applies our industry-leading solutions to the projects of our clients. Feel free to reach out to Jelle for feedback on this article, for questions and for a free consultation.