Chemical Engineering September 2022 - 34

Cover Story
Burner Technologies
and Concepts: Meeting
Emissions-Reduction Goals
To meet bold sustainability targets, new burner technologies and control configurations are
being developed to help significantly reduce NOx emissions
Jonathan
Stoeger
Cleaver-Brooks
IN BRIEF
NOX TYPES
FUEL STAGING
INDUCED FLUEGAS
RECIRCULATION
HIGH EXCESS AIR
TWO-STEP COMBUSTION
ADVANCED CONTROLS
POST-TREATMENT
SOLUTIONS
THE FUTURE OF
COMBUSTION
M
any industry-leading businesses
have set bold sustainability
goals for emissions reduction
and energy efficiency driven by
factors such as climate change, the Paris
Agreement and various national, state and
local regulations.
In the U.S., California has always led the
way in emissions reduction. It was the first
state to mandate an emissions standard of
30 parts per million (ppm) for oxides of nitrogen
(NOx), which subsequently decreased
to 15 ppm NOx and to then just 9 ppm NOx.
Now, some counties in California are enforcing
a 2 ppm NOx requirement on larger industrial
watertube (IWT) boilers. Other states
will be following suit with 30 ppm or even
lower NOx emissions requirements.
The boiler industry continues to introduce
advanced technologies and solutions that
reduce both NOx and CO emissions (Figure
1). Today, there are various combustion
technologies and firing styles, depending
on application, fuel and firing size, that
are available for traditional and renewable
fuels. These newer offerings coupled with
advanced control systems ensure high turndown,
a consistent fuel-to-air ratio and repeatability
in combustion.
Turndown is a concept that
is important to understand. Regarding
boiler systems, it is a
ratio that identifies a system's
high-fire and low-fire settings.
For example, a 100-million
Btu/h burner with 10:1 turndown
will reach high fire at 100
million Btu/h and low fire at 10
million Btu/h. The main benefit
of high turndown for a boiler
system
is
reduced
cycling,
which reduces the energy consumption
and thermal stress of
a boiler.
34
FIGURE 1. Lower NOx and CO emissions requirements are forthcoming, and
burner technologies are evolving to help achieve these goals
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By reducing cycling, less fuel is needed
and fewer emissions escape into the atmosphere.
Turndown is highly dependent
on the burner design. Many high-turndown
burner systems require an advanced control
system - typically, these either involve parallel
positioning or fully metered systems.
NOx types
It is first critical to understand that there are
three different types of NOx.
* Thermal NOx is based on temperature
and makes up most of the NOx formed during
combustion
* Prompt NOx is formed when molecular
nitrogen in the air combines with the fuel in
fuel-rich conditions
* Fuel-bound NOx is based off nitrogen in
the fuel and is a natural occurrence during
combustion
The type of NOx that is reduced most with
burner technology is thermal NOx. This can
be achieved by lowering the temperature
of the flame or by eliminating prompt NOx,
some of which can be reduced by optimizing
the fuel-to-air mixture in the burner. There
are different types of burner technologies

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