With the continuous expansion of the global Marine Gas Nitrogen Oxides (NOx) Emission Control Area Tier III, the design, installation, operation and application of Selective Catalytic Reduction (SCR) systems on ships have received increasing attention. An increasing number of ships are being equipped with SCR systems.

The SCR system is mainly used to catalytically neutralize and reduce nitrogen oxides in diesel engine exhaust gases, ensuring that the exhaust emissions meet the emission requirements.

I. Chemical Reaction Principle of the SCR System

The Selective Catalytic Reduction (SCR) technology involves using an ammonia catalyst to react with nitrogen oxides for denitrification, generating nitrogen and water (ammonia water + nitrogen oxides = nitrogen + water), so that the content of nitrogen oxides in the exhaust gas meets the regulatory requirements (< 2.41 [g/(kW·h)]) [1].

Basic Working Process:

The urea solution (40%) stored in the ship's urea tank is sent to the urea metering unit through the urea supply pump unit. The metering unit adds compressed air and mixes it with the urea solution, which is then sprayed into the exhaust mixing pipe. After the urea solution is fully mixed with the exhaust gas in the mixing pipe, it enters the SCR reaction device, where a catalytic reduction reaction takes place in the catalyst module. Finally, the nitrogen oxides in the exhaust gas are converted into nitrogen and water and discharged into the atmosphere.

The main chemical formulas for the SCR catalytic reduction reaction are as follows:

NO + NO₂ + 2NH₃ = 2N₂ + 3H₂O,

4NO + 4NH₃ + O₂ = 4N₂ + 6H₂O,

2NO₂ + 4NH₃ + O₂ = 3N₂ + 6H₂O.

II. Selection of High and Low Pressure SCR and Comparison of Their Advantages and Disadvantages

According to the installation location of the equipment, the SCR system can be divided into a high-pressure SCR system and a low-pressure SCR system.

The high-pressure SCR system has the SCR catalytic reactor installed between the main diesel engine exhaust manifold and the turbocharger. The SCR exhaust system operates with high-temperature and high-pressure gases, so it is called a high-pressure SCR system, which is mainly used in the main engine SCR system.

The low-pressure SCR system has the SCR catalytic reactor installed behind the diesel engine turbocharger. The SCR exhaust system operates with low-pressure and low-temperature gases, so it is called a low-pressure SCR system, which is mainly used in the main engine SCR system and the generator SCR system [2].

The selection of high and low pressure SCR systems and the comparison of their main advantages and disadvantages are shown in Table 1.

III. Application Instructions for the Marine SCR System

1. System Composition and Working Principle

The SCR system is mainly composed of the ship hull structure urea solution tank, urea supply pump unit, urea metering unit and injection gun, SCR mixing pipe, SCR catalytic reactor, auxiliary equipment, relevant piping systems, valve accessories and the electrical control system, etc. The relevant system diagram is shown in Figure 1.

Figure 1 Principle of the SCR Marine System

Main Working Principle of the SCR:

When the diesel engine operates on Marine Gas Oil (MGO) fuel and the SCR operation mode is activated, the SCR system automatically detects and controls. When the exhaust temperature reaches the operating temperature, the urea pump unit is started to supply the urea solution. The urea solution is proportionally mixed with compressed air in the metering unit and then sprayed into the SCR mixing pipe together. After the urea solution is fully mixed with the exhaust gas, it enters the SCR reactor. The SCR reactor is equipped with a catalyst module. The ammonia (NH₃) in the urea solution and the nitrogen oxides in the exhaust gas undergo a full catalytic reduction chemical reaction in the catalyst module, finally generating nitrogen and water, which are discharged into the atmosphere through the chimney [3].

2. Key Points of the Preliminary Design of the SCR System

The preliminary design of the SCR mainly includes the selection of high and low pressure SCR systems, the spatial layout of the main equipment of the SCR system, the design of the SCR auxiliary system, and the testing of the SCR system and obtaining of relevant certificates.

1) Selection of High and Low Pressure SCR Systems.

Conduct a preliminary selection according to the parameters of the diesel engine. At the same time, combine the engine room space and the structure space of the engine room casing to select between high and low pressure SCR systems, and make a comprehensive selection with reference to the equipment prices.

Currently, for the SCR system of marine auxiliary engines, only the low-pressure SCR system is available for selection, which is mainly determined by the characteristics of the small power of the auxiliary engines and the space of the auxiliary engine models.

For the main engine, both high-pressure and low-pressure SCR systems are available for selection. In the early stage of design, the type of SCR system can be comprehensively selected according to the recommendations of the main engine manufacturer, combined with the parameters of the main engine, the engine room space, the space of the engine room casing and the price.

When selecting the low-pressure SCR system, the exhaust back pressure of the diesel engine needs to be increased. For example, the design exhaust back pressure of the diesel engine turbocharger is increased from 300 mm of water column to 450 mm of water column.

2) Spatial Layout of the Main Equipment of the SCR System.

The main equipment of the SCR system includes the SCR mixing drum, SCR reactor, and the connected exhaust pipes and exhaust valves, etc.

The SCR mixing drum and reactor of the auxiliary engine are usually vertically installed in the engine room casing, and the corresponding electrical equipment is arranged nearby.

The mixing drum and reactor of the high-pressure SCR system of the main engine are arranged near the main engine in the engine room. In the early stage of design, the main engine exhaust pipe with the SCR mixing drum and SCR reactor should be designed. Generally, the diameter of the SCR mixing drum is 1-2 m and the length is 4-5 m;

The diameter of the SCR reactor is 2-3 m and the length is 5-7 m. Therefore, the spatial layout of the exhaust pipe should be fully considered in the early stage of design.

Now some main engine manufacturers have developed a high-pressure SCR system integrated with the engine, and the SCR system equipment is completely installed on the main engine. This design saves the engine room space of the shipyard. However, the engine-integrated SCR system will occupy the area of the main engine exhaust manifold, resulting in a narrow maintenance space for the SCR system components, and the main engine with this configuration is currently more expensive.

For the low-pressure SCR system of the main engine, the space behind the turbocharger in the engine room or the space of the engine room casing should be mainly considered, especially for ships with high requirements for the space of the engine room casing, such as ro-ro ships and passenger ro-ro ships.

Other equipment in the SCR system mainly includes the urea supply pump unit, urea metering unit and electrical control box, etc. These equipment have a small space and a low power load, and can be generally considered in the early stage.

3) The SCR system also commonly uses auxiliary systems, such as the air system, AUS40 (Aqueous Urea Solution 40%)  supply system, fresh water system, and electrical control and monitoring system.

The air system is mainly divided into a compressed air system with a pressure of 0.7-0.9 MPa and an instrument control air system with a pressure of 0.7 MPa. The compressed air system is mainly used for mixing the air in the SCR metering unit with the urea solution, blowing the dust in the SCR reactor with compressed air, and providing gas for the catalytic reaction, etc. Since the consumption of compressed air is relatively large, a special SCR air compressor, SCR air bottle and the corresponding compressed air system need to be equipped.

The parameters of the SCR air compressor and SCR air bottle are selected according to the gas consumption parameters of the main engine and auxiliary engine SCR systems. Generally, the selected displacement of the SCR air compressor is 50-300 Nm³/h, and the volume of the SCR air bottle is 3-5 m³.

The SCR instrument control system is mainly used for controlling the opening and closing of the pneumatic exhaust valves of the SCR system. Since the gas consumption is small, it can be directly connected to the ship's control air system.

The urea solution is the main raw material for the SCR catalytic reaction. The ship needs to design a structural compartment (about 30-100 m³) to store the 40% urea solution. Considering the cost, the material of the urea tank is usually a steel structure compartment with special paint. A stainless steel 304 structure compartment can also be used, but the cost will be higher.

The urea solution compartment usually also needs cooling and heating to prevent the crystallization and freezing of the urea solution.

The pipes of the AUS40 (Aqueous Urea Solution 40%) supply system are commonly made of stainless steel 304 pipes. The urea system also needs to consider the filling, storage, supply, drainage, transfer of the urea solution, as well as personal protective equipment such as personal protection and shower eye wash devices.

The fresh water system is mainly used for flushing the urea system and supplying water for the shower eye wash device.

The electrical auxiliary system of the SCR system mainly considers the layout of the electrical control box, the application of monitoring and alarm, and the consideration of the power load, etc.

4) After the matching test between the SCR system and the diesel engine, it is necessary to submit it to the classification society for review and finally obtain the EIAPP certificate of Emission Tier III from the classification society.

There are two ways to obtain the certificate, namely SCHEME A and SCHEME B. SCHEME A is to obtain the certificate through a Factory Acceptance Test (FAT) of the SCR system and the diesel engine. SCHEME B is to conduct a certification test on the ship after the installation of the diesel engine and the SCR system on the ship.

In terms of construction cost and construction time, new ships are generally certified in the form of SCHEME A. For ships of the same main engine series of the same shipowner, only the matching certification test between the SCR system and the diesel engine of the first ship needs to be carried out. The SCR system equipment of the first ship is sent to the diesel engine manufacturer for the FAT test and certification. Subsequent ships do not need to carry out the FAT certification again. After the SCR equipment and the diesel engine are directly installed on the ship, only the system matching function test needs to be carried out.

Refitted ships will choose the form of SCHEME B for certification. After the equipment is installed on the ship, the matching certification test between the diesel engine and the SCR system is carried out on the ship, such as: various alarm point tests; alarm point shutdown tests; urea injection catalytic reaction tests under various loads of the diesel engine; exhaust emission (EIAPP) tests, etc.

The dockside and sea trials of the shipyard are mainly for the function verification test of the SCR system. It should be noted that the fuel used for the SCR test during the sea trial is MGO. Therefore, the SCR system test during the sea trial is generally the last item, that is, after all the sea trial items of the ship are completed and the ship changes the fuel to MGO, the SCR system test is carried out. At this time, the SCR system test is a SCR function test. Under the working conditions of the main engine load of 25%, 50%, 75% and 100% respectively, each working condition runs for 15-30 minutes to verify the function of the SCR system.

3. Production Design and Installation Instructions for Each Equipment of the SCR System

The installation of each equipment of the SCR system is the key point of production design and also the prerequisite for the safe operation of the equipment.

1) The layout of the exhaust pipe is the primary concern of the SCR system. The low-pressure SCR system is designed according to the general requirements of the exhaust pipe behind the diesel engine. The SCR mixing drum and SCR reactor in the system are normally arranged according to the exhaust pipe, and the maintenance space of the equipment is the key consideration.

The exhaust pipe between the mixing drum and the SCR reactor in the low-pressure SCR system is generally made of stainless steel 304 pipes, and other pipes are designed according to ordinary steel pipes. The bolts and pipe insulation are designed according to the common exhaust pipe standards.

Layout Instructions for the High-Pressure SCR Exhaust Pipe.

The high-pressure SCR exhaust pipe mainly refers to the exhaust pipe of the high-pressure SCR system of the main engine, which is a difficulty in the design.

The design temperature of the main engine high-pressure exhaust pipe is usually 450-500 °C, and the design pressure is 0.5 MPa. The thermal deformation of ordinary steel pipes is very large under high temperature and high pressure, and the exhaust pipe should be reasonably arranged; the support bracket at the exhaust elbow should be able to withstand the impact of the resultant force of the exhaust flow in multiple directions;

The material and thickness of the exhaust pipe, the density and thickness of the thermal insulation cotton, the thermal expansion stress analysis of the exhaust pipe and the exhaust pipe bracket, and the selection and layout of the exhaust expansion joint all need to be determined after simulation calculation and analysis by special software.

A finite element model is established through the commonly used ANSYS software, and the thermal state heat transfer, coupling analysis, support reaction force and support reaction moment of the pipes are simulated and calculated. Finally, the overall layout of the exhaust pipe, the exhaust pipe bracket and the expansion joint is determined. Through the analysis, calculation and optimization of each section of the exhaust pipe, the layout design of the entire high-pressure SCR system exhaust pipe is finally completed.

Taking the design of the main engine high-pressure SCR system exhaust pipe of a 62,000 dwt ship as an example.

The main engine high-pressure SCR exhaust pipe and the exhaust pipe bracket are made of stainless steel 304 material. The thickness of the exhaust pipe is 12 mm, and the thickness of the exhaust pipe bracket plate is 15 mm. The direction of the exhaust bracket is strictly supported in the opposite direction of the resultant force point of the gas flow direction. The thickness of the insulation cotton is 150 mm.

Each section of the exhaust pipe is subjected to a hydraulic pressure test of 1.0 MPa in the workshop. After all the high-pressure SCR exhaust pipes are installed on the ship, an airtight test of 0.75 MPa is carried out, including the SCR system mixing pipe, SCR catalytic reactor and the connected expansion joint supplied by the manufacturer, etc., all of which need to be subjected to the airtight test.

2) Production Design and Installation of the SCR AUS40 (Aqueous Urea Solution 40%)  Storage and Supply System.

The SCR urea system mainly includes the urea solution injection system, urea solution storage, transfer and supply system centered on the urea solution storage tank.

The urea solution used in the SCR system is usually a 40% solution, which is weakly alkaline, non-toxic and has certain corrosiveness.

The recommended storage temperature of the urea solution is 0-35 °C.

If the engine room temperature exceeds 45 °C or is lower than 0 °C for a long time, a cooling coil or a heating coil needs to be arranged in the urea solution storage tank.

The material of the urea solution storage tank can be stainless steel or a steel plate with special paint coating. The urea solution storage tank is wrapped with thermal insulation cotton, and a leakage alarm sensor is added to the water collection tray of the tank.

The urea solution storage tank is equipped with high and low liquid level alarms and high and low temperature alarms, all of which are connected to the SCR system monitoring and alarm system. All the sensor materials are at least made of stainless steel 304.

The pipes of the urea system are mainly made of stainless steel 304. The system valves and installation bolts should be at least made of stainless steel 304. The pipe gaskets are usually made of polytetrafluoroethylene gaskets. The drain water trays of the equipment are made of stainless steel water trays or steel paint water trays.

During the filling and use process of the urea solution, sufficient protective measures are required, such as coverall work clothes, aprons, masks, chemical-resistant goggles, protective masks, eye drops, and emergency eye wash devices, etc.

IV. Conclusion

In recent years, many shipyards have designed and installed SCR systems on newly built ships. The design and technical application of SCR systems are becoming more and more mature, and more shipyards and shipowners are aware of and adopting SCR technology.

Although new fuel ships represented by Liquefied Natural Gas (LNG) and methanol are developing rapidly, SCR technology will not be abandoned for the time being. Instead, it will coexist with new fuels and new technologies. Now, many ships burning LNG are also equipped with SCR systems at the same time.

With the continuous emergence of new marine regulations, the gas emission control area will continue to expand, and the technology of the marine SCR system will also continue to change.

It is necessary to continuously accumulate experience and sort out a complete set of application standards for SCR marine technology to make the application of SCR systems on ships safer and more environmentally friendly.