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What role does oxygen availability play in controlling FCC regenerator NOx emissions? What regeneraor design improvements are recommended for minimizing NOx emissions?
 
Answers
28/07/2007 A: Tim Dougan, Grace Davison, tim.dougan@grace.com
The effect of excess oxygen on regenerator NOx formation has been studied extensively. Circulating riser pilot plant testing as well as commercial FCCU data show NOx emissions increase with increasing oxygen up to a point after which NOx emissions level off. The increased oxidizing environment facilitates the formation of NOx by reduced nitrogen intermediate species.
Improving catalyst and air contacting in the regenerator minimizes maldistribution and can help reduce NOx emissions. Improved mixing also generally permits the unit to operate at lower excess oxygen levels and to use less platinum combustion promoter. Both of these variables have been shown to increase NOx.
28/07/2007 A: Adrian Humphries, Albemarle , adrian.humphries@albemarle.com
NOx additives are "oxygen sinks" and are designed to absorb oxygen so that CO in the FCC regenerator preferentially reacts with NO to reduce it to N2 gas. This is the only practical way to reduce NOx in an oxidative environment. Consequently, regenerators with good spent catalyst and air distribution (which require less excess oxygen [< 1%] to overcome these problems) allow NOx additives to function more efficiently. Revamps to improve spent catalyst and air distribution will therefore intrinsically lower NOx emissions.