Nitrogen oxides (NOx) are harmful air pollutants, primarily formed during combustion. Due to its harmful impacts on human health and the environment, NOx is facing increasingly stringent regulations. This includes limits on emissions.
Two solutions for NOx control are Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR) systems. These solutions are efficient and cost-effective methods of reducing harmful NOx emissions. Keep reading to learn more about SCR and SNCR for NOx Control.
SCR
SCR is a widely used post-combustion NOx reduction technology. The system converts harmful NOx into nitrogen and water. SCRs are highly efficient and can achieve over 80% NOx reduction. Depending on the system, SCRs may be designed to meet higher removal efficiencies. As such, SCRs tend to be utilized in combustion units requiring a higher level of NOx reduction.
Catalysts
SCRs are designed for the chemical reduction of NOx. Incorporating a catalyst improves the reduction reaction rate. When compared to SNCR systems, the catalyst in the SCR system offers higher NOx reduction efficiency and enables the reactions to occur in a lower temperature range. However, this may increase capital and operating costs when compared with the SNCR systems. Increased costs come from both the large volume of catalyst needed for the reduction reaction and the replacement costs of the catalyst and the reagent.
SCR catalysts can be made from active metals or ceramics. Within the porous structure are activated sites where the reduction reaction occurs. Catalyst designs and formulations can vary depending on build and application. A number of factors may go into catalyst selection, including reaction temperature range, flue gas flow rate, and fuel source.
Reagents
The most common reagents used in SCRs are urea and ammonia. Once the reagent is injected into the flue gas, both enter the reaction chamber where the catalyst is located. As both diffuse through the catalyst, the NOx in the flue gas chemically reduces. The resulting nitrogen and water vapor then flow out of the SCR system. Heat drives the chemical reaction process and comes from the flue gas.
There are a number of factors to consider when discussing reagent options, including performance, cost, and safety. The potential hazards of ammonia as a reagent are often weighed against the superior performance and efficiency of the reagent. Additionally, ammonia tends to be less expensive for large-scale applications.
When compared to ammonia, urea is a safer and less hazardous option. However, when considering urea as a reagent, the additional factors may arise. Urea is a corrosive substance that must be stored in reinforced tanks. Piping, tubing, and seals must be compatible with urea. Additionally, urea must be stored in temperature-controlled areas or in a temperature-controlled manner. Unlike ammonia, urea does not need to be kept under pressure.
SCR systems are highly temperature-dependent and must be regularly maintained to ensure optimal performance. Other factors that should be considered include flue gas temperature, space, and the exhaust environment.
ANDRITZ is a leading designer and supplier of a wide range of flue gas treatment systems. With over 30 years of experience and hundreds of installations around the world, ANDRITZ is the partner you can trust for the highest NOx removal efficiencies.
SNCR
SNCR systems, unlike SCR systems, do not use a catalyst. Like SCR systems, SNCR systems are cost-effective and fuel-efficient technologies for stationary source emissions. SNCRs typically have up to 60% removal efficiencies; however, it is possible to achieve over 80% NOx reduction. NOx control depends on a number of factors, including temperature, residence time, reagent injection rate, and the mixing of reagent and flue gas.
SNCR systems can be optimized to meet the needs of a broad range of applications, including industrial and power generation applications. Key features of an SNCR system include its simple design and low capital expenditure. Installation requires a short outage time, and the system does not have solid or liquid waste.
Reagents
As with SCRs, the most common reagents used in SNCRs are urea and ammonia. Unlike SCRs, there is no catalyst. This means that SNCRs rely on higher temperatures for a reaction between the reagent and NOx in the flue gas. Other factors to consider for optimal NOx control include correct reagent distribution that can handle the fluctuations of NOx in the flue gas. Like SCRs, the chemical reaction process reduces NOx to nitrogen and water vapor, which then flows out of the SNCR system.
ANDRITZ is a trusted partner for NOx Control. Our SNCR systems are built to the specifications of our partners. With ANDRITZ, state-of-the-art design, simulation, and technology comes together for optimal NOx control.
Contact ANDRITZ
ANDRITZ is your trusted partner for NOx control and reduction. Our clean air technology delivers proven results across a range of industrial applications.
Contact ANDRITZ today to learn more about our solutions for NOx control.
