In the field of exhaust after - treatment, Selective Catalytic Reduction (SCR) systems play a crucial role in reducing nitrogen oxide (NOx) emissions from stationary sources. As a leading supplier of Stationary SCR System, we understand the importance of the ammonia injection system in the overall performance of the SCR system. This blog will delve into the different types of ammonia injection systems used in stationary SCR systems.
1. Urea - Based Ammonia Injection Systems
Urea - based ammonia injection systems are one of the most commonly used types in stationary SCR systems. Urea, in the form of an aqueous solution (usually 32.5% urea in water, known as Diesel Exhaust Fluid - DEF), is a convenient and safe alternative to pure ammonia.
Working Principle
When the urea solution is injected into the hot exhaust stream, it undergoes thermal decomposition and hydrolysis reactions. The first step is the thermal decomposition of urea into ammonia ($NH_3$) and isocyanic acid ($HNCO$):
$CO(NH_2)_2 \rightarrow NH_3+HNCO$
Then, the isocyanic acid reacts with water in the exhaust to form additional ammonia and carbon dioxide:
$HNCO + H_2O \rightarrow NH_3+CO_2$
Advantages
- Safety: Urea is a non - toxic and non - flammable substance, which is much safer to handle and store compared to pure ammonia. This reduces the risk of accidents during transportation and storage.
- Availability: Urea is widely available in the market, making it easy to source for stationary SCR system users.
- Compatibility: Urea - based systems can be easily integrated into existing stationary SCR systems without major modifications.
Disadvantages
- System Complexity: The decomposition and hydrolysis reactions require a certain temperature range (usually above 180°C) to occur efficiently. This may require additional heating elements or careful design of the exhaust flow path to ensure proper urea decomposition.
- Deposits: In some cases, incomplete decomposition of urea can lead to the formation of deposits on the SCR catalyst or in the exhaust system, which may reduce the system's performance over time.
2. Anhydrous Ammonia Injection Systems
Anhydrous ammonia ($NH_3$) is another option for ammonia injection in stationary SCR systems. Anhydrous ammonia is a pure form of ammonia without water.
Working Principle
Anhydrous ammonia is stored in a pressurized tank. When the SCR system requires ammonia for NOx reduction, the anhydrous ammonia is vaporized and injected directly into the exhaust stream upstream of the SCR catalyst. The ammonia then reacts with the NOx in the exhaust in the presence of the catalyst to form nitrogen and water:
$4NH_3 + 4NO+O_2 \rightarrow 4N_2+6H_2O$
$8NH_3 + 6NO_2 \rightarrow 7N_2+12H_2O$
Advantages
- High Reactivity: Anhydrous ammonia has a high reactivity with NOx, which can lead to more efficient NOx reduction compared to urea - based systems.
- No Decomposition Requirements: Since it is already in the form of ammonia, there is no need for additional decomposition reactions as in the case of urea. This simplifies the injection process and can potentially reduce the system's response time.
Disadvantages
- Safety Concerns: Anhydrous ammonia is a toxic and flammable gas under certain conditions. It requires strict safety measures for storage, handling, and transportation. Specialized equipment and training are needed to ensure the safe operation of the system.
- Regulatory Requirements: Due to its hazardous nature, the use of anhydrous ammonia is subject to more stringent regulatory requirements compared to urea - based systems.
3. Aqueous Ammonia Injection Systems
Aqueous ammonia is a solution of ammonia in water. It is less concentrated than anhydrous ammonia and is also used in some stationary SCR systems.
Working Principle
Similar to anhydrous ammonia, aqueous ammonia is injected into the exhaust stream. The water in the solution evaporates quickly in the hot exhaust, and the ammonia is released to react with the NOx in the presence of the SCR catalyst.
Advantages
- Lower Safety Risk than Anhydrous Ammonia: Compared to anhydrous ammonia, aqueous ammonia has a lower vapor pressure and is less likely to form explosive mixtures. This reduces the safety risk to some extent.
- Easier Handling: It is easier to handle and store compared to anhydrous ammonia, although still more hazardous than urea - based systems.
Disadvantages
- Lower Ammonia Concentration: The presence of water in the solution means that the effective ammonia concentration is lower than that of anhydrous ammonia. This may require a larger volume of the solution to be injected to achieve the same level of NOx reduction.
- Corrosion: The water in the aqueous ammonia solution can cause corrosion in the injection system and the exhaust components over time, especially if the system is not properly designed or maintained.
4. Ammonium Carbamate Injection Systems
Ammonium carbamate ($NH_4CO_2NH_2$) is a solid compound that can be used as a source of ammonia in stationary SCR systems.
Working Principle
Ammonium carbamate decomposes when heated to release ammonia and carbon dioxide:
$NH_4CO_2NH_2 \rightarrow 2NH_3+CO_2$
The released ammonia is then injected into the exhaust stream for NOx reduction.
Advantages
- Solid State: Being a solid, ammonium carbamate is easier to store and transport compared to gaseous or liquid ammonia sources. It also has a relatively long shelf - life.
- Safety: It is less hazardous than anhydrous ammonia and has lower volatility compared to aqueous ammonia.
Disadvantages
- Decomposition Requirements: The decomposition of ammonium carbamate requires a specific temperature range, and the process needs to be carefully controlled to ensure complete decomposition and efficient ammonia release.
- Limited Availability: Ammonium carbamate is not as widely available as urea or ammonia, which may pose challenges in sourcing for some users.
Considerations for Selecting an Ammonia Injection System
When choosing an ammonia injection system for a stationary SCR system, several factors need to be considered:
- Emission Requirements: The level of NOx reduction required by the regulatory authorities will influence the choice of the ammonia injection system. High - efficiency systems such as anhydrous ammonia injection may be necessary for strict emission standards.
- Safety: The safety of the system is of utmost importance. If the installation is in a populated area or a facility with strict safety regulations, a urea - based system may be a more suitable choice.
- Cost: The cost of the ammonia source, storage, and handling equipment, as well as the operating cost, should be taken into account. Urea - based systems are generally more cost - effective in terms of safety and availability, but may have higher operating costs due to system complexity.
- System Compatibility: The ammonia injection system should be compatible with the existing SCR catalyst and the overall stationary SCR system design. Some systems may require specific catalysts or exhaust flow patterns for optimal performance.
As a leading supplier of Stationary SCR System, we have extensive experience in providing customized ammonia injection solutions for various stationary applications. Our team of experts can help you select the most suitable ammonia injection system based on your specific requirements. If you are interested in learning more about our products or would like to discuss a potential procurement, please feel free to contact us. We also offer Marine SCR System for marine applications, which follow similar principles but with specific design considerations for the marine environment.
References
- Johnson, T. V., & Sarangapani, S. (2007). Urea - SCR: a promising technique to reduce NOx emissions from automotive diesel engines. Catalysis Today, 123(1 - 2), 172 - 186.
- Lietzke, S. T., & Stoughton, R. W. (1964). The vapor pressures of ammonium carbamate and mixtures of ammonium carbamate and urea. Journal of Physical and Chemical Reference Data, 3(3), 687 - 700.
- Koci, M., & Harold, M. P. (2009). Modeling of urea - SCR systems for automotive exhaust after - treatment. Chemical Engineering Science, 64(13), 2933 - 2946.
