Power Plant NOx Reduction – SCR vs SNCR Technology Which is better?
December 20, 2018 Welcome
In the face of stringent global legislation from organizations like the U.S. Environmental Protection Agency (EPA) and equivalent regulatory bodies in the European Union aimed at minimizing harmful industrial emissions, utility companies are faced with the challenge of employing the most efficient and cost-effective strategies for compliance.
Current and projected regulations for NOx control in the U.S, EU, and Asia specify NOx emission limits of within 30 – 200 mg/NM3.
Two major processes used to reduce nitrogen oxide (NOx) emissions in power plants are Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR).
This article will outline the SCR and SNCR methods in detail and analyze their benefits for power plant operators to comply with NOx emission regulations.
Selective Catalytic Reduction (SCR)
SCR is arguably the most widely-used technique for NOx reduction. Selective catalytic reduction of NOx is classified as an after-treatment solution for emission control and commonly used in combustion turbines, natural gas engines, and diesel engines which generate pollutants like NOx and carbon particulates post-combustion.
How does SCR work?
In SCR, a reagent (usually 19% or 29% aqueous ammonia, anhydrous ammonia or urea) is injected into the exhaust stream of the combustion machine (turbine or engine) which is maintained at a specific temperature depending on the catalyst used. The heat vaporizes the ammonia.
Nitrogen gas and NOx present in the flue gas stream react with vaporized ammonia in the presence of a catalyst (which speeds up the reaction) to yield diatomic nitrogen (N2), water, and trace amounts of CO2 – harmless products which are expelled from the exhaust pipe. The reagent is optimized by maintaining a near-equal ratio with the NOx to be removed from the flue gas stream.
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Advantages of SCR for NOx reduction
- Suitable for large-scale NOx reduction – up to 90%
- Improves fuel economy in diesel engines
- Can be utilized in a wide range of systems including industrial and municipal plants
- Can be used in addition to Diesel particulate filters (DPFs) to reduce particulate matter emissions in diesel engines
- The set-up is simple to install
Disadvantages of SCR for NOx reduction
- Unreacted ammonia might slip through the SCR catalyst due to over-injection (ammonia slip)
- The catalyst required for the reduction is expensive to replace
- The catalysts are prone to contamination by compounds in the combustion gas
Selective Non-Catalytic Reduction (SNCR)
SNCR is an alternative method for NOx reduction commonly employed in power plants that burn coal, oil, waste, and biomass. SNCR technique involves injecting a reagent into the combustion gases at a high temperature without the aid of a catalyst.
How does SNCR work?
In SNCR, aqueous ammonia or urea is injected into the flue gases from the combustion process at temperatures between 1600°F and 2100°F (870°C to 1150°C) to reduce NOx to nitrogen gas (N2), CO2, and water (H2O). The basic chemical principle is that at a high temperature, ammonia or urea reacts with NOx from the combustion gases to yield nitrogen gas (N2) and water vapor (H2O). SNCR can be used to reduce NOx by about 30 – 70 %.
Below are simplified equations of the reaction using ammonia as the reagent:
4 NO + 4 NH₃ + O₂ → 4 N₂ + 6 H₂O (Eq. 1)
4 NO + 2 NH₂CONH₂ + O₂ → 4 N₂ + 4 H₂O + 2 CO₂ (Eq. 2)
Advantages of SNCR for NOx reduction
- The set-up is simple to install
- Relatively lower capital and operating costs compared to SCR
- Saves costs on expensive replacement of catalysts
- Retrofitting of components in large-scale boiler units is relatively easy, incurring little downtime
Disadvantages of SNCR for NOx reduction
- Requires a narrow range of temperatures for effective NOx reduction
- Not suitable for large-scale NOx reduction
- Retrofitting of components in smaller boiler units is difficult due to space considerations for installing the components
- Prone to ammonia slip which leads to accidental NOx emissions
SCR vs. SNCR: Which is better?
Management’s decision to choose one NOx reduction system over the other depends on a host of factors including cost, efficiency, and compliance with current and future regulations on emissions.
Both SCR and SNCR techniques have relatively higher operating and capital costs compared to simpler methods of NOx reduction like Low-NOx burners (LNBs) and Overfire air (OFA) but are highly-favored for their efficiency. SCR is more expensive than SNCR due to the catalyst specifications but makes up for it in terms of efficiency.
Both SCR and SNCR techniques require a reagent to reduce NOx. However, SNCR requires a much higher flue gas temperature at the point of injection of the reagent. SCR pollution control removes a much larger volume of NOx (~90%) than SNCR (30 – 70%), making it the best option for large-scale reductions in industrial and municipal plants.
Compliance with Emission Control Regulations
While the initial investment for SCR NOx reduction is greater than SNCR, it is more cost-effective in the long-term and compatible with greater levels of emission reduction projected for the future. SNCR is ideal for smaller scale, short-term NOx reductions and might not ensure compliance with future regulations.
IFS Solutions for NOx Elimination
IFS is a leading process skid manufacturer that carries a high-efficiency ‘plug and play’ Ammonia Unloading, Storage, Distribution & Vaporization System for the de-nitrification of flue gas.
Fabricated to ASME standards using the highest quality components and deployed and serviced by a team of experienced engineers, you can rest assured that our selective catalytic reduction (SCR) system is compliant with both current and future emission regulations.
Call IFS at 1-800-795-4068 or contact us online today!