Chemical Engineering May 2020 - 32

FIGURE 2. NOx formation in industrial-combustion units is dependent on the quantity of excess oxygen
present, the peak flame temperature and the fuel type
(2)
where κ is a constant. Since the
reaction is fast, and the fuel concentration
is limited by the required
stoichiometry,
fuel-bound
NOx
formation may be reduced only
by reducing the excess oxygen or
switching to lower-nitrogen fuels.
The third NOx-formation mechanism
is the Fenimore mechanism,
also called prompt NOx. It is similar
to the fuel-bound mechanism,
except that the
directly
nitrogen comes
from the combustion air.
Since nitrogen radicals are exceptionally
difficult to pare from molecular
nitrogen, prompt NOx from this
mechanism is usually negligible and
is discussed no further here.
NOx-reduction strategies
From the NOx formation discussion
above, we see that NOx may
nearly always be reduced in greater
or lesser measure by reducing
the excess oxygen concentration
and peak flame temperatures, or
by a change in fuels, if possible
(for instance, from high-nitrogen
fuel oil to low-nitrogen fuel oil or
from fuel oil to natural gas). Here
then, are nine case-specific NOxreduction
strategies:
1. Burners out of service
2. Derating
3. Oxygen and combustibles trim
4. Tramp air reduction
5. Steam or water injection
6. Staged combustion
32
7. Alternate fuels
8. Fluegas recirculation
9. Low-NOx burners
Each strategy is discussed in
this article. However, and as with
any change in operating procedure
regarding fired equipment, a full
safety review must be conducted
prior to any operational or physical
change. Fire is inherently dangerous.
Site-specific factors beyond
the scope of this article may impact
safe operation. It is incumbent
upon all practitioners of these
techniques to use care in their application
in order to avoid loss of
life, limb and equipment. If plant
expertise is lacking, one should
seek the assistance of a qualified
combustion professional.
Burners out of service
A burners-out-of-service strategy is
applicable to multi-burner systems
only. As an example, consider a
front-fired boiler having two horizontal
rows of three burners each (three
over three). By simply closing the fuel
valve to the upper center burner, the
fuel is redistributed from six burners
to five, while the air is distributed
equally to all six. This causes the
local stoichiometric air ratio to shift
toward more fuel-rich for the five remaining
burners. This reduces the
local excess air and peak flame temperatures,
thus lowering NOx. The
air register on the unfired burner is
left to operate as usual, bringing the
global air-to-fuel ratio to normal and
providing the balance of air to oxidize
CO generated by the local fuel-rich
conditions. Flames may lengthen
with this procedure. Normally, there
is sufficient excess boiler capacity
and furnace volume to accommodate
this change. Notwithstanding,
care must be taken to verify that no
flame impingement to equipment
occurs as the result of the change,
especially at higher firing rates. Usually,
the center burner at the highest
elevation makes the most NOx and is
a primary candidate for this strategy.
Derating
Sometimes, a plant has excess fired
capacity. For the sake of calculating
emissions, air-quality districts always
presume that permitted fired
FIGURE 3. Different types of NOx-producing equipment may be subject to varying measurement conventions,
depending on their location. For instance, boilers in the U.S. may measure NOx in terms of lb produced per
million Btu of heat released
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