Hydrocarbon Processing - January 2021 - 29

Special Focus

Sustainability
P. HOBBINS, Placid Refining Co., Port Allen, Louisiana;
H. BLAIR, Johnson Matthey, Arvada, Colorado; and
T. HOCHHEISER, Johnson Matthey, Orange County, California

FCC NOx reduction methods: Complying with
regulations without capital investment
As fluid catalytic cracking (FCC) nitrogen oxides (NOx )
emissions regulations become increasingly strict, refiners are
driven to find the most economic compliance option. Multiple capital and non-capital project options are available to
mitigate NOx emissions. Selective catalytic reduction, selective non-catalytic reduction, a proprietary NOx removal technologya and regenerator hardware modifications are discussed
in this article for capital project opportunities. A carbon monoxide (CO) promoter, optimization of oxygen (O2) and CO,
feed nitrogen reduction, flue gas ammonia injection and NOx
additives are also covered for non-capital project options.
These solutions are discussed, including their NOx reduction
potential, and how they can be integrated into existing regenerator or flue gas systems.
The successful NOx reduction strategy at Placid Refining
Co. is also described. This strategy includes the utilization of
an additive for NOx reduction, combined with a non-platinum
CO promoter. In addition, process variables are optimized to
reliably control both NOx and CO emissions.
FCC NOx emissions regulations have been gradually tightening. In many countries, NOx emissions regulations have existed for decades, while others are just beginning to implement
NOx limits. The range of NOx limits is substantial. For example, new FCC units (FCCUs) in India are limited to 260 parts
per million (ppm), while many FCCUs in the U.S. have limits
in the range of 25 ppm-75 ppm. Based on proposed legislation,
Southern California anticipates NOx regulations reaching as
low as 2 ppm. A possibility exists for regulations on hydrogen
cyanide (HCN) emissions that would come into force sometime within the next several years.
With more stringent emissions limits, refiners must determine how to comply in the most efficient way possible. The
following work examines several methods of NOx control that
utilize both capital investment and non-capital strategies. FCC
NOx chemistry is also discussed, along with a case study from
Placid Refining Co.'s refinery in Port Allen, Louisiana.

NOx (FIG. 1). The amount of nitrogen that is converted to NOx
is highly dependent on regenerator operation.
Many variables impact the formation of NOx in the regenerator. The type of feedstock will influence the amount of NOx
formed. Processing heavier feeds (e.g., coker gasoil, residue or
deasphalted oil) can increase regenerator NOx formation. Alternatively, hydrotreating FCCU feed reduces the amount of
feed nitrogen, leading to lower NOx in the regenerator. Regenerator operating conditions impact NOx formation, as well. In
full-burn regenerators, higher excess oxygen leads to increased
NOx formation. Operators can minimize oxygen to minimize
NOx formation; however, low amounts of O2 will increase CO
emissions. Therefore, a balance must be achieved.
In partial-burn regenerators, there are three contributors
to CO boiler NOx emissions: NOx formed in the regenerator, NOx formed in the CO boiler from regenerator flue gases
HCN and NH3 , and NOx formed in the CO boiler from air N2.
In the regenerator, HCN and NH3 oxidize to form N2 and NOx
species, but this reaction is limited by oxygen availability. Consequently, reduced nitrogen species are present in the regenerator flue gas. In the CO boiler, these reduced nitrogen species
are readily converted to NOx . The third source of NOx is from
thermal oxidation of N2 in the CO boiler burners. Thermal
NOx can be minimized with lower flame temperatures and by
optimizing air, fuel gas and flue gas mixing.
Regenerator design also has a major bearing on NOx formation. Well-mixed or counter-current regenerators help limit
NO, N2O, NO2
CO, NH3, HCN?
O2

is converted to coke. Coke comprises carbon, hydrogen and
contaminants, including nitrogen. Typically, 40%-50% of
feed nitrogen is converted to coke. In the regenerator, the nitrogen in coke reacts to form reduced nitrogen species [HCN
and ammonia (NH3 )], some of which is oxidized to N2 and

H2O
Amines (unstripped products)

HCN

Aromatic N (coke)

NOx chemistry. In most FCCUs, 5 wt%-9 wt% of the feed

NH3

N0X

FIG. 1. Regenerator nitrogen reaction pathways.
Hydrocarbon Processing | JANUARY 2021 29

Hydrocarbon Processing - January 2021

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