What role does oxygen availability play in controlling FCC regenerator NOx emissions? What regenerator design improvements are recommended for minimising NOx emissions?
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22/07/2007
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J. Houdek, UOP LLC, Mark.Houdek@uop.com
The concentration of excess O2 in the FCC flue gas is very important in setting the overall NOX emissions; the lower the excess O2 in the flue gas, the lower the NOx. The minimum excess oxygen concentration is typically constrained against an afterburn temperature limit for the metallurgy of the regenerator internals. The model response of NOx is about 0.5 order in excess O2. As O2 is reduced, the regenerator dilute phase CO level and temperature increase, which together drive NOX reduction.
When optimising the overall regenerator design, it is important to ensure as homogeneous a distribution of carbon, air and combustion gas as possible across the cross-sectional area of the regenerator. The design of the main air distributor, spent catalyst distributor or deflector, position of the recirculation catalyst standpipe(s), position of the catalyst cooler nozzles and cyclone dipleg discharge orientation (to name a few) are all considered when designing a minimum NOx regenerator. The numerous styles of FCC regenerators in the industry require a firm understanding of the interaction of the design components and NOx reduction kinetics to truly optimise the overall design.
UOP combustor regenerators are excellent for low NOX, and UOP is actively working on further improvements, with a target to meet NSPS. A key reason for the low NOX production of combustors is the staged combustion, which allows NOX produced from the combustion of nitrogen species to be uniformly mixed with the required reducing species (CO and C) and the catalyst at the inlet. This permits operation at low excess O2 (less than 1.0 mol-%) and low platinum content, which are instrumental to low NOx operation, without afterburn control problems and excellent carbon burnout.
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22/07/2007
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Mike Maholland, Intercat, mmaholland@intercatinc.com
Intercat’s understanding of NOx control mechanisms points to oxygen availability as the most significant factor driving NOx emissions in most FCC units. All of the combustion reactions in the regenerator use oxygen, and each have a direct impact on oxygen availability on a local scale. It is by selectively promoting some of these reactions, while at the same time inhibiting others, that NOx additives achieve their effect.
Excessive NOx formation is ultimately the result of imperfect catalyst and air mixing in the regenerator. Design improvements, which directly address this issue, will have a positive effect on NOx reduction. This aspect of NOx reduction is best addressed by the design experts in this field.
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22/07/2007
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Mark Schmalfeld, Engelhard, mark.schmalfeld@engelhard.com
Oxygen availability is critical to the full burn regenerator performances of CO, SOx and NOx additives. Many additives require the presence of oxygen to perform properly. However, we think the actual atmosphere concentration gradients in the regenerator are also critical to controlling NOx emissions.
Up to 40% NOx reduction from a ~65 ppm baseline has been achieved commercially by using CLEANOx. In this example CLEANOx additions up to 2% of fresh catalyst additions were used in a full burn regenerator at ~ 0.4-0.6% excess oxygen. High excess oxygen content is not necessarily required to achieve significant NOx reduction and lowering the total NOx ppm is the more important overall objective.
Engelhard has seen differences in the NOx generation in different regenerator designs using similar operating conditions and feed types. Engelhard would recommend reviewing design improvements with FCC unit technology licensors. Potentially important design factors are the air grid design, catalyst mixing in regenerator, regenerator level variation and specific catalyst introduction to regenerator and return to reactor.
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