What is Selective Catalytic Reduction?
SCR is a simple, cost-effective NOx reduction solution. 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. 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 an established technology.
How does 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).
The most important chemical equation for this process is:
4 NO + 4 NH3 + O2 à 4 N2 + 6 H2O
What are the NOx limits SCR can achieve?
SCR meets IMO Tier III standards and beyond by consistently removing 95% 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. Over 500 marine SCR systems have been installed over the last 20 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. Whilst specific segments are at different levels of technology development, it is anticipated that all will be served with commercially available SCR solutions by 2014.
What are the main components of an SCR system?
The typical main components of SCR Systems are:
- Reducing agent storage tanks (typically between 5 to 50 m³)
- Reducing agent pump or pump station
- Reducing agent dosing unit
- Mixing unit with injection nozzle
- Catalyst housing (reactor) including SCR catalysts
- Control unit
Is there a requirement for official certification for a SCR system?
At present there are no official requirements from Classification Societies for SCR systems (e.g. DNV, GL, Lloyd‘s Register). Therefore no certificates are available. Official certificates in relation to TIER III compliance are expected, at the earliest, in January 2016.
What is the best place to install an SCR system?
For the highest efficiencies it is recommended to install an SCR as close to the engine as possible, so to avoid loss of exhaust gas temperature. After the mixer/injector it is advised that a straight duct of approx. 3 - 4 pipe diameter should be installed.
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.
At what temperature range of an exhaust gas can an SCR system function?
Depending on the type of SCR catalyst and concentration of sulfur in the fuel, the reaction temperature range will be between 280 and 510°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 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.
Are SCR catalysts sulphur resistant?
Yes. For SCR catalysts sulphur is not a catalyst poison. The sulphur content of the fuel only influences the minimum exhaust gas temperature required for a reliable SCR operation.
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.
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.
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.