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How does SCR work?
Catalysts perform by lowering the temperature required to allow a chemical reaction to occur. In this case, a reduced nitrogen compound (ammonia or urea) is introduced into the air. The "oxidized" chemical (NO or NO2 or both) then reacts with the other nitrogen compound forming elemental nitrogen and water.
The two common sources of neutralizing chemical are either ammonia, which is a gas at room temperature or urea, which is normally used as a solution and must be atomized into the air.
Selective Catalytic Reduction (SCR)
SCR technology is often used where high temperatures, low concentrations or high percentage of NO are encountered.
The catalyst used is normally coated on a ceramic substrate. Various configurations are used including lose pieces of ceramic media and structured honeycomb shapes. The structured shape is the most common because of its lower pressure drop. It is also easier to install and replace.
Different catalysts can operate over different temperature ranges. The minimum range in temperature can be as low as 500°F or as high as 1000°F.
Other temperature considerations include the presence of sulfur dioxide and the concentration of NOx.
At lower temperatures, ammonium sulfate can form. The ammonium sulfate salt can deposit on the catalyst effectively blocking off its activity and reducing the NOx scrubbing efficiency. The catalyst usually be regenerated by heating above the point where the ammonium sulfate becomes volatile again, but you cannot operate in this temperature range continuously.
There is a heat of reaction between ammonia and NOx. Care must always be used when designing an SCR system to avoid excessive temperature rise. Excessive is defined as anything that takes the temperature higher then the design range for the catalyst. Excessive temperatures will rapidly deteriorate the effectiveness of a catalyst.