Selective Catalytic Reduction (SCR) Systems Information

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Selective Catalytic Reduction (SCR) Systems Information

Selective catalytic reduction (SCR) systems inject ammonia into boiler flue gas and pass it through a catalyst bed where the ammonia and nitrogen oxide gas (NOx) react to form nitrogen and water vapor. Selective catalytic reducers are a post-combustion technology used in coal burning and petrochemical processing applications to reduce NOx emissions. In the United States, selective catalytic reducers (SCR) are often the technology of choice for meeting meet Environmental Protection Agency (EPA) regulations that govern the amount of NOx emissions that can be released into the atmosphere. Other technologies for NOx reduction include low NOx burners, staged combustion, gas recirculation, low excess air firing, and selective non-catalytic reduction (SNCR).


Selective catalytic reducers (SCR) work in a manner similar to the way a catalytic converter works to reduce automobile emissions. A gaseous or liquid reductant (generally ammonia or urea) is added to the exhaust gases before they exit a smokestack. The mixed gases travel through several catalytic layers, causing a reaction between the NOx emissions and the ammonia injection. The reaction converts the NOx emissions into pure nitrogen and water vapors. The benign elements are then released into the air. One common problem with selective catalytic reducers (SCR), however, is that they operate well only within narrow temperature bands. Consequently, control units are required to ensure the exhaust gas temperatures are within the range that will allow the catalytic reaction to occur.


Like selective catalytic reducers (SCR), selective non-catalytic reduction (SNCR) control NOx emissions by using urea or ammonia; however, unlike SCR, SNCR does not require a catalyst. Still, it is the use of a catalyst that allows an SCR system to operate at much lower temperatures (340° C to 380° C or 650° F to 720° F) than a SNCR system (870 °C to 1200° C or 1600° F to 2200° F). As a rule, these lower temperatures accelerate the chemical reaction process. Because of differences in capabilities, SNCR systems are generally less expensive than SCR systems.


There are many applications for selective catalytic reducers (SCR). Some systems are installed at chemical plants or utility power plants. Others are used combined cycle facilities or manufacturing plants. Some selective catalytic reducer systems are reported to achieve NOx reduction of up to 75%.