Chemical Engineering September 2022 - 36

ppm NOx is achievable when combining
multiple technologies. Today,
even 5 ppm NOx is achievable in certain
circumstances.
High excess air
FIGURE 4. The first step in the fuel-injection method involves high excess air and heat transfer
The high excess air method increases
the mass flowrate of excess air to
cool the flame, similar to the FGR
solution. To be effective, it is necessary
to control the flame-front position.
Also, it is very important in this
method for the fuel-and-air mixture
to be premixed prior to burner ignition.
Otherwise, the flame will move
up and flow into a fuel-rich condition.
Once that happens, the burner
will go into displacement, and the
flame temperature will start to rise
again, counteracting the benefits
of premixing.
Because the fuel and air mixture
FIGURE 5. The air-injection method begins with sub-stoichiometric combustion
portion of the head) have a single,
large grill in them, called staged injectors.
These injectors
direct
the
fuel jets on the outside of the main
flame combustion.
Larger injection orifices produce
larger jets that travel further into the
furnace itself. Mixing more slowly before
igniting on the main flame produces
a larger flame envelope that
helps to increase the radiant heat
transfer. More heat transfer from the
boiler to the furnace and into the
flame itself yields more heat and increases
NOx emissions.
Fuel staging by itself can reduce
NOx by about 20-25%. For example,
for a 100-ppm NOx standard uncontrolled
type of burner with fuel staging
in place, NOx emissions can be
reduced to 75-80 ppm.
Induced fluegas recirculation
Induced fluegas recirculation (FGR)
is one of the most common and efficient
methods to reduce thermal
NOx (Figure 3). The process takes
cool, inert fluegas and mixes it with
combustion air to increase mass
flow through the flame front, thus re36
ducing
the temperature and thermal
NOx formation.
Figure 3 demonstrates how the
fluegas is pulled out before it reaches
the economizer. On a small firetube
with higher temperatures, recirculated
fluegas will be pulled off and mixed
with the air. From there, with the fan,
it will be introduced and mixed with
the fuel into the combustion process.
This will minimize the flame temperature
to reduce thermal NOx and lower
burner emissions.
This technique is used in many
combustion engines today. It can
be called exhaust gas recirculation
(EGR). Basically, it is a simple solution.
By adding a little water, the system
cools down. It can be likened to
putting fluegas in the system to help
cool down the flame and keep NOx
emissions low.
For larger-capacity boilers, such
as IWTs, FGR will typically be taken
off after the economizer. On larger
boilers, it is preferred to have slightly
cooler temperatures for FGR.
NOx can be reduced by 90% when
utilized with other burner technologies.
With internal technology, 9-10
must be premixed and control of the
flame position is required, a high excess
air strategy can only be used
with certain types of burners.
Utilizing this type of technology,
NOx can be reduced to 7.5-9 ppm.
A major benefit of high excess air
is achieving low emissions without
utilizing FGR.
Two-step combustion
The last of the combustion technologies
is two-step combustion, of
which there are two types: two-step
fuel-injection and two-step air-injection.
The processes are somewhat
similar with the main difference being
that two-step fuel injection utilizes a
lean premix combustion in the first
step, and in two-step air injection,
the first step involves a very fuelrich
combustion.
The first step in the fuel injection
method consists of high excess air
and heat transfer (Figure 4). All of
the combustion air goes through the
burner, whereas only a portion of the
fuel ges goes through, which results
in very lean combustion.
In the second step of the combustion
process, additional fuels are injected
into the combustion products.
For this to be effective, some amount
of heat must be removed prior to the
first step and before subjecting the
second step to fuel. The heat generally
can be removed by a radiant
heat exchanger to the furnace walls.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
SEPTEMBER 2022

http://WWW.CHEMENGONLINE.COM

Chemical Engineering September 2022

Table of Contents for the Digital Edition of Chemical Engineering September 2022