Chemical Engineering May 2020 - 34

1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.2
0.4
0.6
0.8
Dimensionless water injection, lb H2O/lb fuel
FIGURE 4. Water injection is an effective method for reducing NOx levels
this diffusion process is generally
what sets the flame dimensions inasmuch
as the flame chemistry is very
fast but the diffusion is much slower.
Once the diffusion is complete, the
flame ends. Typically, the fuel issues
through orifices and the combustion
air comes through a much
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0.9
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0.7
0.6
0.5
0.4
0.3
0.2
0.1
5
10
15
Dimensionless water injection, lb H2O/lb fuel
FIGURE 5. The benefits of fluegas recirculation (FGR) in NOx reduction are twofold: first, FGR reduces the
starting oxygen concentration; and second, FGR abstracts heat and lowers the flame temperature
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larger surrounding inlet, often modulated
through louvered registers or
air dampers. Conventional diffusion
burners (burners not designed to
minimize NOx) usually mix their air
and fuel as rapidly as possible to
produce short, compact flames. This
was the rule for burners dating from
1
the 1970s or before, many of which
are still in service. While this results
in compact flames, it also results in
high NOx, owing to the high heat
release
over
short
distances
that
Oil fuel
elevates peak flame temperatures
and NOx formation. It is not uncommon
for such burners to generate
100 ppmvdc3% of NOx, or more.
However, temperature is not the
only factor.
Gas fuel
1.2
As shown in Equation (1), NOx is
also affected by the local oxygen
concentration. For example, a boiler
generating 100 ppmvdc when operating
at 3% O2 may generate 150
ppmvdc when operating at 6% O2.
This is not a dilution effect - here,
NOx is consistently reported on a
dry-volume-corrected basis in both
cases; rather, this is an effect of
chemistry. More available oxygen
generates higher rates of NOx formation
according to Equation (1).
Many burners rely on mechanical
linkages between fuel and air valves
to apportion the fuel and air. However,
and in the interest of safety,
these are set conservatively. The
result is that more air is admitted
than is strictly necessary. Oxygen
trim enables the air registers to trend
to a specific target. This minimizes
NOx production.
Oil fuel
Gas fuel
What should the target excess
oxygen be? A general rule of thumb
is 3%, but for well-maintained and
well-behaved boilers, the oxygen
may be set lower, and for poorly
maintained boilers, even 3% may be
inadequate. Lower oxygen reduces
NOx and increases boiler efficiency
because one is minimizing the
amount of excess air to be heated
and ultimately discarded. However,
some level of excess air is always required
for a number of reasons: first,
because mixing is never perfectly efficient,
so some excess oxygen is always
needed. More importantly, one
cannot know from oxygen measurements
alone whether the oxygen is
right or woefully inadequate. Many
incorrectly presume that if the boiler
or furnace runs out of air, the flame
will go out, but that is not the case.
Combustion reactions continue,
even with woefully inadequate oxygen,
producing copious quantities
of carbon monoxide and unburned
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM MAY 2020
Dimensionless NOx
Dimensionless NOx
(NOx with FGR) / (NOx without FGR)
(NOx with water injection) / (NOx without water injection)

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Chemical Engineering May 2020

Table of Contents for the Digital Edition of Chemical Engineering May 2020