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HybridType Scrubber

Hybrid Type SCR Catalyst

SCR Process Description

Specifically, hot flue gas leaving the economizer section of the boiler is ducted to the SCR reactor. Prior to entering the reactor, NH3 is injected into the flue gas at a sufficient distance upstream of the reactor to provide for adequate mixing of the NH3 and flue gas. The NH3 and NOx react as they pass through the catalyst. The flue gas leaving the catalytic reactor enters the air preheater where it transfers heat to the incoming combustion air. Provisions are made for ash removal from the bottom of the reactor since some fallout of fly ash is expected. Ductwork is also often provided to bypass some flue gas around the economizer during periods when the boiler is operating at a reduced load. This helps maintain the temperature of the flue gas entering the catalytic reactor.
A small amount of NH3 exits the reactor unreacted – this unreacted ammonia is termed “ammonia slip.” The allowable ammonia slip is a critical design factor, along with the required deNOx removal percentage. High ammonia slip is an indicator of an unhealthy SCR system, generally caused by one or more factors, such as poor ammonia, NOx, or flow distributions, fouling, or catalyst deactivation. Ammonia slip is problematic for coal-fired boilers because it contributes to fouling In the air preheater due to the formation of ammonium bisulfate (NH4HSO4), and to ammonia contamination of the fly ash which may disrupt sales.