Marine SCR


SCR is a simple, cost-effective and proven NOx reduction solution, capable of achieving the emission limits set by IMO Tier III.

Selective Catalytic Reduction (SCR) technology uses a simple chemical reaction to neutralise the NOx in the exhaust. The catalyst promotes the reaction between NOx and a reducing agent such as ammonia at the relatively low temperatures of an exhaust stream. This process produces harmless nitrogen and water. Highly efficient, SCR is capable of removing up to 99% of the NOx emissions. The technology has found widespread application in reducing NOx from power plants and it is also the industry standard from removing NOx emissions from truck engines. SCR has also been used for marine applications since the 1990’s and is now a well-established technology.

To help you learn more about marine SCR we have answered below some frequently asked questions and challenges.  We have also explored in more depth:


Key publication: Technical and operational capabilities of marine SCR


How does marine SCR work?

SCR has the capabilities of reducing the concentration of polluting nitrogen oxide in the exhaust gases of diesel engines, to below the emission limits set by IMO Tier III (3.4 g/kWh). A reducing agent (gaseous ammonia, aqueous ammonia or aqueous urea solution) is added into the stream of a flue or exhaust gas. The exhaust gases and reducing agent are then absorbed onto a catalyst, upon which the nitrogen oxides are transformed on the catalytic surface into nitrogen (N2) and water (H2O).

How does marine SCR work


What are the NOx limits SCR can achieve?

SCR meets IMO Tier III standards and beyond by consistently removing 99% or more of NOx in the exhaust gas of a marine engine. Consequently, a NOx limit of less than 0.5 g/kWh can easily be achieved.


What is the status of development of marine SCR technology?

SCR is a proven, commercially available technology.  have been installed over the last 30 years. SCR has been successfully applied to a wide range of engine types, utilizing different fuels (of differing sulphur content) and operating over a range of engine conditions. The industry is supported by well-developed, competitive supply chains.  Experience of marine SCR has been gathered in all major market segments including emission control of slow speed engines.


What are the main components of an SCR system?

  • Reducing agent pump or pump station
  • Reducing agent dosing unit
  • Mixing unit with injection nozzle
  • Catalyst housing (reactor) including SCR catalysts
  • Control unit
  • NOx analyser (optional)


Are there any restrictions concerning engine size?

No, there is no limitation. The catalysts are modularly arranged, and every SCR unit can be up-sized. Current IMO NOx Tier III limits in Emission Control Areas apply only to installed marine diesel engines of over 130 kW output power.


At what temperature range of an exhaust gas can an SCR system function?

Depending on the type of SCR catalyst and concentration of sulphur in the fuel, the reaction temperature range will typically be between 270 and 540°C.


Why do I need a catalyst for NOx reduction?

Without a catalyst the reaction of NOx with the reducing agent takes place at temperatures above 800°C. The catalyst lowers the reaction temperature down to the temperature range of the exhaust gas. Therefore, no additional heating is required.


What is the lifetime of an SCR catalyst?

The lifetime of the catalysts depends on various parameters such as the operational profile of the engine, temperature, fuel and lube oil quality. Typically, a lifetime of 4 – 5 years can be assumed. After this period, it is not necessary to exchange the entire catalytic material. This is because the catalysts are arranged in a layered system, which allows for damaged catalysts to be identified, removed and exchanged. The injection of urea solution does not affect the lifetime of the SCR catalyst.


Why do I need a reducing agent?

The reducing agent reacts with NOx on the catalyst, forming harmless nitrogen and exhaust humidity.


What types of reducing agents can be used?

Besides aqueous urea solution, gaseous or aqueous ammonia can be used as reducing agents. Aqueous urea solution with a concentration of 40 % (Maritime Grade Urea Solution) is the typical reducing agent for marine applications. This is because it is easy to handle and non-hazardous. The freezing point of this Marine Grade Urea Solution is 0°C. Trucks and buses used 32.5 % due to a lower freezing point and most marine systems will be able to use 32.5% with none or moderate adjustments, which is more and more used in artic waters by installing an outside hull tank.


Are there any quality requirements for the reducing agent?

Maritime Grade Urea Solution should be used. A commonly agreed standard has been defined by a working group organized by The European Chemical Industry Council (CEFIC). This standard can be downloaded here.

Are there any requirements for storage tanks for the reducing agent?

The hull tank of a ship can easily be converted into a urea solution tank, by surface coating with an epoxy painting.


Is there comprehensive global availability of aqueous urea solution?

SCR Grade Urea is already a global business with well-developed competitive supply chains for all relevant industry segments.  The supply of Marine Grade Urea – will be a part of this market with similar requirements for product quality.  It is anticipated that for the first few years after implementation of IMO Tier III, relatively small volumes of Marine Grade SCR will be required (a very small percentage of total amount of SCR grade urea).  In the event of an increased demand, IACCSEA can confirm that the supporting industries e.g. ammonia manufacture, are well placed to cope with additional requirement.


Are there any restrictions concerning fuel or lube oil quality?

Not particularly. However, depending on the quality of fuel and lube oil the lifetime of the catalysts may vary. Correct lube oil TBN number and adequate dosing can benefit SCR lifetime.


What are the investment costs for an SCR system?

The investment costs are between € 15 and € 70 per kW engine power. This is dependent upon engine size and the number of engines per ship.


What are the running costs of an SCR system?

The running costs are mainly driven by the cost for the urea solution. In general, running and maintenance costs are between € 5 and € 7 per MWh engine power utilized. Engines with SCR systems can be tuned for lower fuel consumption and thereby reduce the running costs.