Can you comment on one or more unit-specific cases where additives reduced NOx emissions by up to 80%? In other cases, what conditions existed where "only" a 20% NOx reduction was observed --- and what (if anything) was done to further reduce NOx emissions?
Answers
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22/07/2007
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Mark Schmalfeld, Engelhard, mark.schmalfeld@engelhard.com
Up to 70% NOx reduction from a 200-ppm baseline has been achieved commercially by using CLEANOx. In one example where 70% reduction was achieved, CLEANOx additions at 1.4% of fresh catalyst additions were used in a full burn regenerator at ~ 2-3% excess oxygen with a platinum CO promoter. Also, additional testing was completed using a Low NOx CO promoter as required by the consent decree.
Engelhard has also observed decreases of less than 20% NOx reduction. In one such example, the unit had a ~80 ppm baseline while using CLEANOx at ~1%. This unit was a full burn unit with ~1.5% excess oxygen with a platinum CO promoter. In this case, further work was done using a Low NOx CO promoter as required by the consent decree.
Most US trials are conducted following guidelines described in a consent decree. The guidelines often limit how much can be done to modify the trial or to try additional additive or catalyst changes to improve the performance. These types of additive evaluations can be conducted outside of the consent decree period.
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22/07/2007
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J. Houdek, UOP LLC, Mark.Houdek@uop.com
UOP is familiar with at least one unit in which NOx reduction additives successfully lowered the NOx emission by greater than 80% relative to uncontrolled NOX. In many cases, where less dramatic reductions were observed, other conditions existed that impaired the performance of the NOx additive. Overdosing of platinum-bearing CO promoter is a primary reason for NOX additive ineffectiveness. Excessive platinum drives high yields of NOX from organic nitrogen in soft coke (strippable hydrocarbon), and at the same time consumes the necessary CO reductant while driving the dilute phase temperature down, both of which greatly slow the rate and extent of NOX reduction.
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22/07/2007
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Mike Maholland, Intercat, mmaholland@intercatinc.com
Intercat has quantified NOx reductions of up to 70% using NOx reduction additives alone. Others have reported NOx reductions as high as 80%. There are several factors that impact the level of reduction that can be achieved with a NOx additive, but at this stage, it is not possible to model the level of reduction that can be achieved ahead of time. The chemistry of NOx reduction in an FCCU is extremely complex and not completely understood. NOx formation in the FCC regenerator is a function of several factors including regenerator design, flue gas excess O2, concentration of nitrogen in coke, carbon on regenerated catalyst, and other additives being used in the FCCU. A short 4-8 day commercial trial is still the easiest way to determine the level of reduction that can be achieved for any given FCC unit. Some other factors impacting the level of reduction that can be achieved are as follows:
• Additive concentration: In general, increasing the amount of additive used will result in a greater level of NOx reduction. This, however, applies only up to a point. Increasing NOx additive concentration beyond the point of maximum benefit is a waste of money, and can actually have negative side effects, including making NOx emissions increase again
• Base Level of bed homogeneity: A well mixed regenerator bed will normally be able to achieve lower levels of NOx emissions than a poorly mixed bed. The maximum NOx reduction that can be achieved with an additive will conversely be higher in a poorly mixed regenerator bed. Although NOx additives may well be able to achieve a higher percentage of NOx reduction in a poorly mixed bed, the absolute level of NOx will still normally be higher than that from an equivalent well mixed bed
• Location of NOx formation: If the NOx is primarily being formed in the dense bed, then a high level of NOx reduction can normally be achieved with additives. However, if the NOx is being formed primarily in the dilute phase, then NOx additives will have little or no effect. It is possible to draw an analogy here with the use of CO promoters to control afterburning. When afterburning is caused by inadequate dense phase residence time, CO promoters will have a dramatic effect on afterburning. Where afterburning is caused by channeling in the dense bed (some areas being rich in CO, some rich in O2) then CO promoters will have a much smaller impact on afterburning, as the unburned CO can only burn in the dilute phase when it mixes with the oxygen rich stream of flue gas from the other side of the bed.
Other things that can be done to reduce NOx emissions include:
• Operating at lower levels of excess O2 in flue gas
• Minimising Pt-containing CO promoter use, or using a non-Pt CO promoter
• By using the results of the regression analysis carried out earlier (as previously discussed), it may be possible to manipulate other operating variables to minimise NOx formation. For example, stripper operation, or oxygen injection rates may well have an impact on NOx.
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