Hydrocarbon Processing - January 2021 - 30

Sustainability
Capital project options. The following are capital project
options to reduce FCCU NOx emissions.
Regenerator hardware. Upgraded regenerator air
grids, advanced spent catalyst distribution systems and
well-mixed regenerator designs are becoming more
Placid Refining was able to effectively
common to reduce NOx emissions and optimize reuse a NOx reduction additive to reduce
generator performance. The hardware improvements
FCCU NOx emissions by 32%. This enabled
not only improve NOx control, but also provide other
benefits, such as lower CO, lower afterburn and lower
emissions limit compliance without having
regenerated catalyst coke levels. Multiple licensors ofto invest in a sizeable project.
fer enhanced regenerator designs. These designs can
be implemented either through revamps or through
the installation of new regenerators. The various designs have different levels of achievable NOx reduction.
factors that impact NOx formation are the use of CO promoters and the use of antimony (Sb). Most U.S. refiners have
Selective catalytic reduction (SCR). SCR is a process that
switched from platinum-based promoters to palladium-based,
involves the injection of NH3 into the FCCU flue gas followed
but platinum is still widely used across the rest of the world.
by reaction across a catalyst bed. The reaction occurs between
Platinum catalyzes the formation of NOx and will continue
287°C-399°C (550°F-750°F) and produces N2 and water vapor.
to contribute to NOx while present in equilibrium catalysts
The main reaction (1) as the majoirty of FCC NOx is NOx . The
(Ecat). Palladium promoters also generate NOx but to a lower
cost of the system is primarily driven by the SCR reactor, which
must be incorporated in the flue gas system. In some FCCUs,
extent than platinum. In addition, the NOx -generating half-life
feed-forward control is utilized. Flue gas NOx is measured upof palladium is lower than platinum. Sb is often used for nickel
(Ni) passivation, but this can also increase NOx .
stream of the SCR unit and is used to control the NH3 injection
rate slightly above the molar equivalent ratio. Feedback control
measures NOx downstream of the SCR and adjusts NH3 injecNOx reduction options. Multiple strategies are available to
decrease FCCU NOx emissions. These strategies can be dition accordingly. The process has operating costs for NH3 and
vided into two main categories: capital investment projects and
catalyst changeouts and can achieve up to 95% NOx reduction.
non-capital solutions. Most solutions involve some level of onThe reduction chemistry is detailed below:
going operating expense.
4 NO + 4 NH3 + O2 → 4 N2 + 6 H2O(1)
60
2 NO2 + 4 NH3 + O2 → 3 N2 + 6 H2O(2)
NO + NO2 + 2 NH3 → 2 N2 + 3 H2O(3).
50
NOx formation due to consistent catalyst and air mixing. This
avoids pockets with high or low oxygen content. Additional

Flue gas NOX, ppm

40
30

20
10
0
100

200
Feed nitrogen, ppm

300

400

FIG. 2. Impact of feed nitrogen on flue gas NOx.
55
50

Start NH3 to ESP

45
Flue gas NOX, ppm

40
35

30
25

20
15
0

100

150

Days

200

FIG. 3. NOx reduction from ESP NH3 injection.

30 JANUARY 2021 | HydrocarbonProcessing.com

250

300

350

Selective non-catalytic reduction (SNCR). SNCR is like
SCR in that it uses NH3 to react with NOx to form N2 and water.
The process is completed at a higher temperature than SCR, allowing it to be accomplished without a catalyst. The temperature must be maintained between 926°C-1,093°C (1,700°F-
2,000°F) for the reaction to take place; therefore, the SNCR
system is incorporated with the CO boiler. Urea may be used
instead of NH3 due to the higher temperature of SNCR. The
process includes an injection system with air, which is designed
to produce effective mixing. Precautions include NH3 slip at
high injection and NOx breakthrough at low injection. SNCR
can remove up to 50% NOx and is generally a lower capital investment than other NOx projects.
Proprietary NOx emissions reduction technology a. This
proprietary process is a system that combines an ozone generator with a wet gas scrubber to remove NOx . The ozone generator converts supplied oxygen to ozone. The ozone selectively
oxidizes insoluble NOx into soluble nitrogen species that can be
removed in the wet gas scrubber. The process can remove up to
95% NOx , with low flue gas pressure drop. The associated operating costs include oxygen, power supply and caustic.
These four different capital project options-which can
achieve different levels of NOx removal-have both benefits
and precautions. The choice depends significantly on the existing flue gas system and how the different solutions could be
integrated into the existing system.

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Hydrocarbon Processing - January 2021

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