Chemical Engineering May 2020 - 36

Air staging uses a complementary
approach, where the air is divided
into early and late portions. Staged
combustion of any kind tends to
lengthen the flame and radiatively
cool it while lowering local oxygen
concentrations, reducing NOx. It is
often the case that existing burners
may be modified to stage a portion
of their combustion by a combustion
engineer skilled in the art. One
may also buy a low-NOx burner
that already makes use of stagedcombustion
principles.
Steam or water injection
Sometimes, NOx is very close to,
but not quite below, a mandated
level. For example, suppose a furnace
produces 43 ppm on some
occasions, but that NOx regulations
mandate 40 ppm or below. One may
replace the burner with a low-NOx
version. But sometimes the expense
may be avoided by injecting a small
portion of steam or water. Either will
reduce the concentration of oxygen
by dilution and reduce NOx (Figure
4). Steam also has about double the
heat capacity of air, meaning it will
reduce the flame temperature - another
important factor in NOx reduction.
Water injection has an additional
advantage in that the latent heat of
vaporization
will
carry even more
heat away from the flame. Actually,
these techniques can substantially
reduce NOx. The reason for using
them only for more marginal adjustments
is because they carry an efficiency
penalty - additional heat is
being carried out of the stack in the
form of water vapor rather than being
transferred to the process and because
too much steam or water can
quench the flame. Using only enough
steam or water to reduce NOx by
10% or so often keeps this efficiency
penalty to a percent or two, which is
tolerable for industrial boilers. If fuel
costs are not a significant part of the
production cost, NOx may be lowered
by greater amounts.
Alternative fuels
Sometimes, higher nitrogen-containing
fuels may be swapped for
fuels with lower nitrogen levels - for
example, using lighter rather than
heavier fuel oils, or swapping light
36
fuel oil for natural gas. Many burners
are equipped to burn both liquid and
gaseous fuels, so such a switch may
be possible.
Fluegas recirculation
One source of inert diluent is the fluegas
itself. Generally, some portion of
the fluegas is plumbed to the air inlet
to reduce oxygen concentration in
the windbox from 21% to some lower
level, say 18%. Most conventional
burners can tolerate inlet air containing
15 to 20% fluegas by volume, and
most air fans have sufficient excess
capacity to draw air from the stack
to an inlet, providing that the recirculating
conduit is appropriately sized.
Fluegas recirculation (FGR) can make
significant reductions in NOx even
when used on conventional rather
than low-NOx burners (Figure 5). The
reductions occur, first because FGR
reduces the oxygen concentration
and second, because fluegas that is
cooler than the exhaust will also abstract
heat and lower the flame temperature.
Additionally, the increased
mass flow acts to homogenize the
flame, increasing mixing and further
reducing peak flame temperatures.
These effects conspire to significantly
reduce NOx.
Low-NOx burners
Low-NOx burners (LNBs) are a
ready-made solution and can obtain
sub-40 ppm NOx quite easily. Indeed,
there are low-NOx burners operating
in conjunction with FGR that
achieve sub-10-ppm NOx levels. Of
the nine options considered here,
this is the most expensive, albeit with
certain advantages. First, manufacturers
stand behind their low-NOx
burners with performance guarantees,
and one should not purchase
them otherwise. Second, LNBs may
be purchased turnkey. In that case,
remedies may be specified for failure
to meet performance and schedule.
This is especially important when
performance delays will result in real
revenue loss, such as for missioncritical
units.
Implementation
A good combustion consultant can
often
identify
initial
problems and
corrective actions from an initial inspection.
Then the unit should be
characterized to correlate NOx emissions
against firing rate and oxygen
concentration. Nowadays, portable
NOx measurements are reliable and
relatively inexpensive, and many
emissions-testing companies are
available for this type of work. A characterization
will provide a baseline, let
you know how far away you are from
your required target and inform your
choice of NOx-remediation strategies.
Once you have your baseline, it is advisable
that facilities start with the lowest-cost
NOx-reduction options and
work forward. Deciding which option
is the least expensive is somewhat
case-specific.
However,
strategies
such as tramp air reduction, burners
out of service for multiple-burner
units and steam and water injection
are relatively easy to try. Oxygen and
combustible trim systems are costeffective
and likely to pay for themselves
in short order. Practical strategies
for particular facilities are usually
not difficult to determine. Once they
are implemented, the unit is retested
to validate the fix. Most of these techniques
will result in NOx reductions
of 50% or more, and combinations
of strategies are likely to result in
further reductions.
■
Edited by Mary Page Bailey
Editor's Note: This article follows a previous article
written by the author, Low-Cost Techniques Reduce
Boiler NOx, which was published in Chemical Engineering
in February 1993. Figures 4 and 5 here have been
adapted from the 1993 article. See the online version
of this article at www.chemengonline.com for additional
related content.
Author
Joseph Colannino is principal
and CEO at Colannino Consultants,
LLC (Email: colanninoconsultants@
gmail.com), a firm specializing in
leading process teams to profitable
innovation. Colannino is a registered
chemical engineer in the state
of California with more than 30
years in the hydrocarbon and
chemical processing industries. He
has more than 100 patents pending and granted, and
many of his innovations are still in revenue-producing service.
Colannino has authored or co-authored several
books in combustion, pollution control, and experimental
design. He received his B.S.Ch.E. from the California Polytechnic
University at Pomona and holds a M.S. degree in
Knowledge Management from the University of Oklahoma
with an emphasis in R&D organization. He was formerly
head of research and development for John Zink Co., LLC
and CTO for a Seattle-based combustion startup. Now
CEO of Colannino Consultants, Colannino consults on
combustion and pollution control and teaches industrial
courses on experimental design, process optimization and
improving R&D.
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