Selective Catalytic Reduction of NOx
Developed during the late 1950s and 1960s, selective catalytic reduction (SCR) provides the most complete removal of nitrogen oxides (NOx) from flue gas streams, using a chemical reducing agent to negate and/or convert pollutants to a non-hazardous emission by converting them to diatomic nitrogen. The catalytic agent—either aqueous or anhydrous ammonia, or urea—is injected into the gas stream before the gases enter a catalytic reactor, similar to the catalytic converter on an automobile.
While the concept of an SCR is simple—that is, using ammonia to reduce NOx, and thus producing nitrogen, N2 and water vapor—success depends on multiple factors. OnQuest experts look at the total air pollution control system to ensure proper functioning in all operating regimes. We design and build SCR systems with an ammonia injection grid (AIG) that evenly distributes the reducing agent throughout the gas stream before entering the catalytic reactor. The injection system which supplies the injection grid is controlled electronically to minimize the amount of un-reacted ammonia in the gas stream after the reactor (ammonia slip).
Design of the air pollution control system is key to the success of the SCR unit. Dusts, mists, and SOx/H2S can all reduce the activity of the SCR catalyst. Dusts and mists can plug the pores of the catalyst support, blocking off the active sites. Sulfur and heavy metals can react with the catalyst, effectively poisoning the catalytic process by forming new compounds and alloys which lack catalytic reactivity. Guard-beds of catalyst support material which have not been dosed with the metallic catalyst are used to polish out the stray materials which bypass upstream dust collection and flue gas treatment steps.
|
