Chemical Engineering September 2022 - 39

Post-treatment solutions
A selective catalytic reduction (SCR) system can help reduce
NOx emissions by as much as 94%. For instance,
if a facility has an uncontrolled burner with 100 ppm
NOx, an SCR system can reduce emissions to 6 ppm
NOx. Another benefit with SCR systems is that by utilizing
some of the burner technologies above, less catalyst
is required.
On a 30-ppm burner, 5-6 ppm NOx is achievable
with SCR, while using less of the catalyst. This solution
works with any fuel if the fluegas temperature
is within the 400-700ºF range. In this temperature
range, the catalyst exhibits its highest level of activity.
The catalyst bed can be designed for small to very
large exhaust flowrates of millions of columns per hour,
and ammonia injection can be varied to accommodate
various fuels for multiple-fuel- and simultaneousfiring
applications.
Note that there are a couple of limitations with SCR.
One is that fluegas flow cannot contain too much particulate
matter. Otherwise, the catalyst will plug up quickly.
There are different catalyst pitches that can be used to
mitigate this problem. Also, the catalyst can be deactivated
in the presence of certain substances, such as
catalyst poisons.
Also, because ammonia is a base, the fluegas acid
concentration must be monitored closely on this type of
system. Below the acid dewpoint, ammonia will combine
with an acid to form a salt, which can plug the catalyst
itself. Figure 7 illustrates how an SCR system works to
reduce emissions and NOx.
In an SCR system, the reaction to ammonia with NOx
will occur naturally at high temperatures, around 1,600˚F.
This reaction decomposes the NOx, NO and NO2 into
harmless water and nitrogen gases.
The introduction of ammonia in the boiler system adds
a new emission that has to be contained - ammonia slip.
This is the unreacted ammonia that finds its way through the
SCR system.
Stack ammonia slip limits are usually between 2 and
10 ppm. Because of the slip limit, the ammonia flowrate
needs to be precisely controlled to match NOx
flowrate. This function is controlled by an ammonia flowcontrol
unit. Also, to minimize ammonia slip, the ammonia
injection pattern needs to match that of the NOx
distribution pattern.
This function of the ammonia is part of the ammonia
injection grid. As the exhaust gas flows through, the
ammonia flow-control monitors the amount of ammonia
going into the ammonia injection-grid area. Inside,
it is mixing with the exhaust gases, going through and
out of the catalyst. Again, with SCR, N2 and H2O are
the byproducts.
The future of combustion
The boiler industry will continue to strive to decrease
NOx emissions. In addition to reducing NOx emissions,
there is emphasis on reducing the carbon footprint.
In the near future, look for technologies to be implemented
within these combustion cycles to not only reCHEMICAL
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duce NOx emissions, but also to reduce the amount of
carbon output from the combustion process. Carboncapture
systems and other technologies are gaining
more traction.
Another focus for our industry is energy efficiency. Higher
efficiency equipment utilizes heat better, which naturally
reduces the amount of fuel needed to be burned, and
hence, emissions (Figure 8).
Lastly, there is also big focus on engineered and renewable
fuels, especially on the hydronics side. Renewable
fuels have less emissions byproduct, which is
helping many companies today achieve their targeted
sustainability goals.
■
Edited by Mary Page Bailey
Further reading
1. Achieving Balance in Combustion, Chem. Eng., August 2022, pp. 14-17.
2. New Ways to Achieve Better Control of Emissions, Chem. Eng., January 2021, pp. 12-15.
3. Low-Cost Techniques for NOx Reduction, Chem. Eng., May 2020, pp. 30-36.
4. Enclosed Combustion Equipment and Technology, Chem. Eng., January 2018, pp. 46-49.
Author
Jonathan Stoeger is an inside sales and application manager,
for the Burner Systems Group of Cleaver-Brooks (221 Law St;
Thomasville, GA 31792; Email: jstoeger@cleaverbrooks.com;
Website: www.cleaverbrooks.com). He has been with CleaverBrooks
for 17 years, and has served on the ASME's CSD-1 Code
committee (Controls & Safety Devices for Automatically Fired
Boilers), and is currently an alternate member of the committee.
Stoeger holds a degree in mechanical engineering from the University
of Wisconsin-Platteville.
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SEPTEMBER 2022
39
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