Selective Catalytic Reduction (SCR) is a vital process that gets rid of toxic NOx before it can join the atmosphere. Several moving parts must effectively work in tandem to cut down on as much NOx as possible. Read on to learn about factors affecting SCR performance, such as ammonia flow and several noteworthy catalyst variables.
The chemical reaction taking place depends on a balanced supply of the reactant ammonia (NH3). Too little ammonia can limit the amount of NOx that converts into nitrogen gas and water vapor because the ammonia gets consumed before all of the nitrogen oxide is consumed. Adding too much, while it does react fully with the existing NOx, allows for valuable excess ammonia loss. This is referred to as ammonia slip. To balance what’s needed, a trim controller can administer the correct ratio of ammonia to nitrogen oxide. These vary in different industrial facilities, but they commonly use some method for regulating ammonia delivery.
Catalysts in an SCR system enable the reaction to occur without the flame temperature getting too high, upping NOx reduction as a result. Catalysts are typically composed of metals such as titanium or vanadium in conjunction with other substances, and they can be layered on top of one another to boost catalysis. Some factors that complicate catalyst performance, according to a 2017 study by Stephenson, involve the active site becoming compromised by one of several contributing factors. One factor is the disruption of the active site as nearby substances overheat and coalesce into a new shape. This is called sintering, and it’s due to exposure to high temperatures. Ammonia salt deposits can also plug the actives site, which inhibits proper function. The site can also experience more gradual erosion as gases come into contact with it. All these phenomena deactivate the catalysts, and in doing so, they lead to increased ammonia slip and inhibit NOx elimination.
The type of catalyst is another factor that affects SCR performance. Vanadium (Va) and Tungsten oxide (WO3) catalysts are sometimes used, while cerium (Ce) and titanium oxide (TiO2) can be used as well. Ce/TiO2, in particular, can be boosted in the presence of the dopant Holmium to improve SCR performance because these substances allow the reaction to occur at a lower temperature when used together.
To learn more about how ammonia, catalysts, and other factors affect the SCR process and its performance, contact the team at SVI Dynamics.