What are Nitrogen Oxides (NOx)? Where Do They Come From?
Nitrogen oxide pollution refers to the pollution caused by the release of nitrogen oxides into the atmosphere in gaseous form during the high-temperature combustion of fossil fuels.
These nitrogen oxides are primarily composed of two molecules: nitric oxide (NO) and nitrogen dioxide (NO₂), along with some other molecules at much lower concentrations. These molecules—nitrogen oxides—are important greenhouse gases and play a significant role in global climate change.
Nitrogen oxides are formed during the high-temperature combustion process, where oxygen and nitrogen in the air react. This reaction occurs in internal combustion engines and fossil fuel power plants.
Environmental and Health Issues
Nitrogen oxide gases play a significant role in smog formation. When exposed to ultraviolet radiation from sunlight, nitrogen oxide molecules decompose to form ozone (O₃). The presence of volatile organic compounds (VOCs) in the atmosphere exacerbates this problem; VOCs can also react with nitrogen oxides to form hazardous particulate matter. Unlike the protective ozone layer in the upper stratosphere, ground-level ozone is a serious pollutant.
Under precipitation conditions, nitrogen oxides can generate nitric acid, exacerbating acid rain.
Nitrogen oxides, nitric acid, and ozone easily enter the lungs, causing severe damage to sensitive lung tissue. Even short-term exposure can irritate the lungs of healthy individuals.
For people with conditions such as asthma, even short-term inhalation of these pollutants can be fatal. This air pollution can lead to respiratory diseases such as emphysema and bronchitis. Nitrogen oxide pollution can also worsen asthma and heart disease and is associated with a higher risk of premature death.
The international shipping industry is facing an increasingly stringent regulatory environment, particularly regarding air emission limits. With the International Maritime Organization (IMO) implementing Level 3 nitrogen oxide (NOx) emission limits under Annex VI of the International Convention for the Prevention of Pollution from Ships (MARPOL Convention) on January 1, 2016, ship emission regulations have become more stringent.
According to Article 13 of Annex VI of the MARPOL Convention—Nitrogen Oxides (NOx), NOx emissions from marine diesel engines must be controlled as follows:
NOx emission limits for diesel engines are determined based on the engine’s maximum operating speed (RPM), as shown in the table above. Level 1 and Level 2 limits are universal, while Level 3 standards apply only to NOx emission control areas.
There are two exceptions: engines used only in emergencies, and engines on vessels navigating only within flag state waters. The latter exception applies only to engines for which alternative NOx control measures have been implemented.
Emission Control Areas (ECAs) must comply with Stage 3 NOx emission limits, meaning their NOx emissions must be 80% lower than those of Stage 1 engines. Under these regulations, vessels keeled after January 1, 2016, and operating within U.S. and Canadian Emission Control Areas (ECAs) must comply with the new emission limits.
These emission limits apply to engines with a gross tonnage exceeding 5,000 tons and a power output exceeding 130 kW.
The Phase 3 standards are expected to require specific NOx emission control technologies. These technologies primarily include two options:
1) Using liquefied natural gas (LNG) as engine fuel and employing lean-burn technology.
For example, the systems of Win GD Engines-Winterthur Gas & Diesel Ltd. These systems use a low-pressure LNG injection system to burn LNG in the combustion chamber to reduce nitrogen oxide (NOx) emissions.
2) Various mitigation technologies can be employed, such as introducing water during combustion (mixed with fuel, as clean air (humidified intake air), or within the cylinder), and exhaust gas recirculation (EGR) or selective catalytic reduction (SCR).
This article describes the types, basic operating principles, components, advantages, and disadvantages of marine SCR reactors.
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Basic Working Principle
Selective catalytic reduction (SCR) is a method that uses a catalyst to convert nitrogen oxides in waste gas into diatomic nitrogen and water.
A reducing agent (such as anhydrous ammonia (NH3), ammonia solution (ammonium hydroxide), or urea) is added to the waste gas stream and adsorbed onto the catalyst. When urea is used as the reducing agent, the reaction product is carbon dioxide (CO2).
The chemical equations for the reaction using anhydrous ammonia are:
- 4NO + 4NH3 + O2 = 4N2 + 6H2O
- 2NO2 + 4NH3 + O2 = 3N2 + 6H2O
- NO + NO2 + 2NH3 = 2N2 + 3H2O
The reaction of urea (not anhydrous ammonia or hydrated ammonia) is: 4NO + 2(NH₂)₂CO + O₂ = 4N₂ + 4H₂O + 2CO₂ (in the presence of a catalyst).
Selective Catalytic Reduction (SCR)
This exhaust aftertreatment technology can reduce nitrogen oxides by more than 80%. The principle of SCR is to inject a mixture of urea and water into the exhaust gas using a dedicated catalyst device.
SCR is considered a stand-alone auxiliary exhaust treatment system and therefore does not interfere with the basic design of the engine or the combustion process.
The figure below illustrates an SCR system in which urea reacts with nitrogen oxides in the incoming exhaust gas in the presence of a catalyst, converting them into free nitrogen and water vapor.
The International Maritime Organization (IMO) Marine Environment Protection Committee (MEPC) has published guidelines for the application of selective catalytic reduction (SCR) systems, namely the “SCR Guidelines,” as detailed in IMO resolution MEPC.198(62). Based on configuration, SCR systems can be divided into two categories: one installed between the exhaust manifold and the turbocharger, and the other installed between the turbocharger and the exhaust boiler.
1) High-Pressure SCR System
In a high-pressure SCR system, the reactor is located before the turbocharger. Maintaining exhaust temperatures between 300°C and 400°C during low-load and high-performance engine operation can be challenging.
Therefore, in two-stroke engines, it is best to install the SCR unit before the turbocharger to extend the SCR’s operating range. This has virtually no impact on the engine combustion process.
High-pressure selective catalytic reduction (SCR) units can use heavy fuel oil.
2) Low-Pressure SCR
In a low-pressure SCR unit, the reactor is located after the turbine. Preheating the exhaust gas may be necessary to reach the temperature required for the catalytic reaction at the reactor inlet. This preheating may require a certain amount of power generation.
Components of an SCR Unit
Dosing Unit
The dosing unit consists of an integrated external dosing system containing urea solution and a water tank. The tank capacity depends on the frequency of the ship’s entry into the Class III NOx emission zone and the operating frequency of the SCR unit. For large engines, the urea solution capacity ranges from 4 to 10 cubic meters per megawatt.
Marine-grade urea solution is typically dissolved in water at a concentration of 32%–40%. Urea is a non-toxic, odorless solution and is considered safe for transport and storage at normal temperature and pressure. However, extra caution is advised in low-temperature winter conditions to prevent urea crystallization.
The metering system delivers the reducing agent (urea solution) based on metering signals from the SCR (Speed Control and Monitoring) system and the engine control and monitoring system.
Vaporization/Mixing Unit
After metering the urea, the metering system injects it into the vaporization/mixing unit. The injected reducing agent (urea) evaporates and mixes with the incoming exhaust gas.
The mixing unit is integrated into the engine’s exhaust manifold. The piping is designed and installed using complex flow calculations and extensive testing to ensure thorough mixing of the urea solution with the high-temperature exhaust gas. The mixing unit is typically 2 to 6 meters long and 500 millimeters in diameter, but its dimensions may vary depending on the engine size.
Injection pipes from the metering unit pass below the carburetor. The upper part of the carburetor contains an electronic housing housing a nitrogen oxide (NOx) sensor to monitor the NOx concentration in the exhaust gas, and a back pressure sensor.
Selective Catalytic Reduction (SCR) Chamber
Here, under the action of a catalyst, the NOx in the exhaust gas is converted into nitrogen and water. The SCR contains catalyst carrier strips. The carrier elements operate within a specific temperature range; if the exhaust gas temperature is too high, the elements will be damaged.
If the temperature is too low, the efficiency of the SCR will decrease. The catalyst elements contain vanadium pentoxide (V₂O₅), which promotes the reaction of urea and exhaust gas into nitrogen and water vapor. The volume of the SCR is typically 1.5 to 3 cubic meters per megawatt of installed capacity. Fuel Quality and Selective Catalytic Reduction (SCR) Technology
Fuel Quality and Selective Catalytic Reduction (SCR) Technology
The sulfur content of the fuel and the resulting sulfur dioxide concentration in the exhaust gas are critical parameters to consider during the operation of a selective catalytic reduction system. The temperature of the urea solution must be controlled according to the sulfur content of the fuel.
When the fuel sulfur content is high and the exhaust gas temperature is low (e.g., during vehicle operation), a higher urea solution temperature is required because condensation in the exhaust gas can lead to catalyst carrier corrosion and damage. When the fuel sulfur content is low, a lower urea solution temperature can be used.
During low-load operation, water vapor condensation in sulfur-containing exhaust gas leads to the formation of solid ammonium bisulfate. Therefore, a sufficiently high exhaust gas inlet temperature must be maintained to prevent ammonium bisulfate from condensing on the catalyst carrier elements.
Condensation severely reduces the reduction performance of nitrogen oxides and causes blockage and increased back pressure due to carbon buildup within the reactor.
Soot Blowing System
A soot-blowing system is installed to prevent contamination of reactor components. This system uses compressed air at a pressure of 7 bar to remove soot.
Selective Catalytic Reduction (SCR) Sensor Unit
A nitrogen oxide (NOx) sensor measures the NOx concentration before the SCR and turbocharger.
The reactor chamber also contains an exhaust NOx sensor and an external temperature sensor.
Ventilation System
When the SCR is overloaded (i.e., the engine is in secondary mode), the ventilation system expels exhaust gases from the reactor to prevent exhaust gas buildup and soot formation. In secondary mode, fresh air is introduced into the reactor.
When the SCR is not operating, the ventilation system closes the reactor using a reactor shut-off valve.
A reactor throttle valve is located at its outlet.
When the NOx concentration is at secondary levels or the selective catalytic converter (SCR) malfunctions, a reactor bypass valve bypasses the reactor.
During partial engine operation, a cylinder bypass valve can be used to bypass clean air to the turbocharger to increase exhaust temperature.
For engines equipped with a selective catalytic converter, the auxiliary blower disconnect/shutdown settings are slightly different. When the auxiliary blower disconnects under low load, a slight delay is introduced to prevent a sudden drop in exhaust temperature.
Similarly, when the auxiliary blower disconnects and the engine load increases, the exhaust temperature often rises suddenly. To prevent this, the cylinder bypass valve (CBV) first opens to increase the temperature gradually, then the blower stops operating, and finally the CBV closes according to the engine load.
Advantages and Disadvantages of Selective Catalytic Reduction (SCR) Systems
To ensure continuous removal of nitrogen oxides and prevent clogging, special attention must be paid to exhaust temperature.
Advantages
- Meets Tier 3 NOx standards.
- Highly effective NOx removal (60-90%) under most engine loads.
- Selective catalytic reduction (SCR) systems have over 300 installation cases, with an increasingly robust reference base.
Disadvantages
- Relatively high initial investment cost.
- Limited NOx removal rate under low engine loads.
- Compared to other Tier 3 NOx emission solutions, using urea requires the installation of a urea tank, which may require frequent refills (higher costs).
- May increase fuel consumption (approximately 1%).
- Using urea in Emission Control Areas (ECAs) will incur additional costs.
Maintenance
- NOx sensor replacement – Sensor lifespan is approximately 2000 hours.
- Substrate catalyst replacement in the SCR reactor (lifespan approximately 1000 hours). Catalyst element lifespan is highly dependent on the sulfur content of the fuel. NOx efficiency must be checked annually to meet Tier 3 NOx standards.
If NOx treatment efficiency is below 70%, all catalyst elements must be replaced according to the manufacturer’s instructions.
- The sootblower consists of a compressed air cylinder, a sootblower diaphragm valve, and a pressure switch. All pressure hoses must be inspected and maintained.
- The dosing system consists of a urea solution tank, liquid filter, dosing pump, nozzle assembly, flow meter, valves, and pressure switch.
- Weekly maintenance includes filter cleaning, and monthly maintenance includes injector inspection. All components must be inspected every six months to check their performance.
Benefits for Ship Owners/Operators
The benefits of implementing Tier III-compliant nitrogen oxide (NOx) technologies extend far beyond meeting emission regulations. Demonstrating a company’s commitment to sustainable operations is becoming increasingly important.
Other benefits include direct economic gains, as major ports offer significant port fee discounts. The Environmental Shipping Index (ESI) is a commonly used performance indicator for measuring the environmental impact of shipping, and major ports also use this index to calculate port fees.
Implementing Tier III compliant technologies can improve an ESI score by approximately 5 points compared to Tier II technologies.
For example, the following ports offer port fee discounts when using Tier III technologies:
- Los Angeles: US$2,500 per call (ESI > 50)
- Hamburg: €1,500 per call (ESI > 50)
- Rotterdam: 20% discount for Tier III technologies
- Antwerp: 10% discount for ESI > 31
