United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-90/001 Apr. 1990
&EPA Project Summary
Design Report: Low-NOx Burners
for Package Boilers
R.A. Brown, H. Dehne, S. Eaton, H. B. Mason, and S. Torbov
A low-NOx burner was designed for
residual oil-fired industrial boilers
including boilers cofiring nitrated
wastes. The design employs deep
staging to achieve NOX levels lower
than conventional low-NOx ap-
proaches while maintaining an envi-
ronment favorable to high waste
destruction efficiency. A cylindrical
shell precombustor chamber fired
substoichiometrically is to be
retrofitted to the burner opening of
the boiler. Remaining combustion air
would be added through retrofit
sidefire wall air ports. The design is
based on fabrication from lightweight
refractory block modules covered
with a Saffil™ fiber veneer. This
design allows quick thermal
response necessary for industrial
boiler applications which is not
possible with heavy castable or brick
refractory. Design dimensions and
materials specifications are made for
the thermal input capacity range of
15-59 MWa. Annular spool section
modules are installed to fire higher
loads in the range of 15-29 MW, and a
geometric scale-up is used for larger
capacities. Construction and field
evaluation of the burner has not been
performed.
This Project Summary was
developed by EPA's Air and Energy
Engineering Research Laboratory,
Research Triangle Park, NC, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
a Readers more familiar with nonmetric units
may use the conversion factors at the back.
Introduction
The Iow-N0x burner design reported
here was motivated by two regulatory
needs for more effective NOX emission
reduction. The primary need is for
additional emission reduction for new
industrial boilers firing high nitrogen
residual oil. This requirement derives
from Section III of the Clean Air Act,
which directs that New Source
Performance Standards (NSPS) be
periodically reviewed and tightened as
new technology is developed-. The
current standard for residual-oil-fired
industrial package boilers constructed
after July 6, 1984, was set at 172 ng/J on
an NO2 basis that corresponds to about
330 ppm. This emission standard
requires a fairly low degree of emission
reduction from uncontrolled levels
because of a lack of demonstrated
technology applicable to package boilers
without introducing operational problems.
The current burner design effort was
initiated in part to support EPA in the next
series of revisions of the NSPS so that
additional emission reduction technology
would be available.
The second need addressed by the
burner arises from the requirements of
the Resource Conservation and Recovery
Act (RCRA) for disposal technologies
such as thermal destruction as
alternatives to land disposal. Where the
waste has high heat content, cofiring of
the waste in industrial boilers is a
practical means of destruction while
recovering fuel credits from the heating
value. If the waste has high nitrogen
content, however, the NOX emissions
from cofiring may be unacceptably high
unless additional steps are taken for NOX
reduction. Approximately 114 RCRA
Appendix VIII compounds and 30 F and K
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category listings contain significant
nitrogen in the form of nitriles, amines,
nitrobenzene, and cyanides. Thermal
destruction of these wastes currently
presents a challenge to simultaneously
satisfy the intent of both the Clean Air Act
and RCRA. The current burner design is
intended to offer waste generators a
thermal destruction option which is high
in destruction efficiency but low in NOX
emissions in order to treat nitrated
wastes.
Approach
The design goals set for the low-NOx
burner were:
• Fuels:
- High nitrogen residual oil
- Nitrated waste cofired with residual
oil
- Natural gas or distillate oil for light-
off
• NOX emissions:
- 100-125 ppm with residual oil
- 200-300 ppm with nitrated waste
cofiring
* Carbon monoxide: < 35 ppm
• Smoke emissions: < 4 Bacharach
• Capacity: 15-59 MW thermal input
• Turndown: >5:1
• Destruction efficiency with waste
cofiring: > 99.99%
The concept for the burner was
derived from an earlier EPA-sponsored
program in which a deep-staging
prototype burner was designed and
tested for an enhanced oil recovery
(EOR) steam generator firing crude oil.
With this concept, low conversion of fuel
nitrogen compounds to NOX is achieved
by creating a hot, fuel-rich first stage with
relatively long residence time. The high
temperatures and long residence time
serve to drive the kinetics of fuel nitrogen
intermediary species toward recombina-
tion to Ng rather than oxidation to NO.
The earlier EOR burner used a
regenerative design for the precombustor
in which the refractory was cooled by the
incoming combustion air. The
precombustor was fabricated from heavy
refractory bricks. The NOX performance
for this prototype burner was 65 ppm.
Although the earlier design was
appropriate for the EOR steamer, several
changes were needed for the more
demanding industrial boiler application.
The high density refractory in the EOR
burner required a long thermal response
time to keep thermal stresses within
acceptable limits. A faster response was
needed for the industrial boiler for rapid
start-up, shut down, and load following.
For the boiler burner design, this was
approached by using a nonregenerative
design with a lightweight insulating
refractory. This design also gave a better
capacity turndown potential than the
regenerable design.
An additional change was required in
the size of the unit. The EOR NOX design
goal was 65 ppm which necessitated a
precombustor nearly as long as the
radiant section of the steamer. In the
current design, the goal of 100-125 ppm
allows a shorter combustor to be built,
since the combustor length is needed
primarily to provide residence time for
the fuel nitrogen compounds to
recombine to N2 prior to introduction of
stage air. The necessary length was also
reduced by introducing stage air from the
side of the boiler rather than at the
interface of the precombustor and the
boiler. This allows additional residence
time in the boiler prior to staging for
nitrogen reactions to occur.
Changes in the prior design were also
needed in the sophistication of the
control system as well as safety
measures for rapid shutdown.
Burner Design
The general burner design is shown in
Figure 1 for a nominal 15 MW heat input
capacity. For higher capacities, up to 29
MW, additional spools can be inserted to
give added combustor volume. The
burner diameter of 2.1 m was selected to
adapt to the firing wall of package
industrial boilers, and to give gas
velocities in the precombustor typical of
commercial operation with lightweight
refractory.
The combustor length was selected to
give a predicted boiler exit NOX level of
85 ppm (corrected to 3% oxygen). This
target gives a margin of safety in
achieving the design goal of 100 ppm.
NOX as a function of precombustor length
was predicted using test data from the
prior EPA burner development for the
EOR steamer, and fuel nitrogen kinetic
calculations.
For the precombustor stoichiometry of
0.7 and temperature of 1540°C, inter-
polations and extrapolations of the EOR
burner test data snowed a reduction in
total fixed nitrogen concentrations at the
exit of the precombustor to a level of 112
ppm, (at 0% oxygen) for a precombustor
residence time of 400 msec. A further
reduction in total fixed nitrogen to 85
ppm is predicted for the cooling zone
between the exit of the precombustor and
the injection of second stage air at the
stage air ports. The completion of
combustion in the second stage is
predicted to yield 60 ppm NOX (corrected
to 3% oxygen) from the fuel nitrogen
species, and 25 ppm thermal NOX for
total flue gas concentration of 85 ppm <
3% oxygen. For these NOX designl
predictions, the internal combustor length)
needed was 2.7 m for the 15 M\
capacity.
The throat diameter at the transitionl
from the precombustor to the boiler wasl
set at 0.9 m for the 15 MW design tol
maintain exit velocities below 35 m/s.l
This is the desired velocity at full load tol
allow flame shaping and capacity!
turndown similar to conventional register)
burners.
The location of the stage air ports tol
be retrofitted to the boiler sidewalls wasl
selected as the farthest upstream point atl
which the fuel nitrogen kinetics are effec-l
tively frozen and no additional reduction!
would accrue from longer exposure. For!
the nominal 15 MW design, this length!
was 2.1 m from the front:wall. This gives!
a second stage residence time of about!
620 msec. Calculations of CO and smoke!
burnout show that, for this residence time!
and the design temperatures, the CO is I
effectively burned out, and the soot is I
burned to below the particulate emission!
standards.
The burner to be fired at the front end I
of the precombustor is a standard com-1
mercial register burner with adjustable!
louver blades. This burner offers capa-l
bilities for flame shaping and provides I
high turndown ratios. Fuel atomizers and!
jets are specified for residual oil, waste I
fuel, natural gas, and (if needed) distillate I
oil for light-off. Steam atomization is I
specified for both the waste and residual [
oil.
The insulating refractory configuration I
for the precombustor is shown in Figure
2. Section I is Pyro-Bloc® H with a I
maximum design limit of 1340°C. Section
II is Unifelt® XT, a Saffil veneer with a
maximum design temperature of 1650°C
and a recommended continuous use
temperature of 1570°C. Section III is a
Unikote™ S coating with a use limit of
1650°C. This coating is partly to give
structural rigidity prior to curing and [
hardening of the insulating blocks.
The front and rear end plates of the I
precombustor use a more complex
structure to accommodate the burner |
throats and the need for greater thermal
conduction away from the wall. The throat I
is fabricated from castable refractory that
has sufficient strength to be self-
supported. The castable will also be
anchored by studs welded to the front
plate and to the boiler front wall tubes. A
lightweight castable refractory is placed
behind the castable plastic refractory in
the front plate assembly. This provides
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Burner End
Section
Boiler and End
Throat Section
Oil X*
Burner
Location
Boiler
Location
Secondary
Air
Figure 1. The combustor section.
thermal insulation while the castable
refractory and the studs conduct heat
away from the localized hot spots on the
front face.
Conclusions
The initial objective of this project was
to design, fabricate, install, and evaluate
the EPA low-NOx heavy oil burner for an
industrial package boiler in the 59-74 MW
thermal input range. Early in the project,
this objective was revised in two ways.
The capacity range for the burner was
lowered and broadened because (1)
almost no industrial boilers in the 59-74
MW range were firing residual oil to serve
as a demonstration, and (2) most
operators who could use the burner for
low-NOx firing of nitrated waste had
boilers in the 15-29 MW thermal input
range. The design range was accordingly
changed to 15-59 MW.
The second revision of the objective
was to adapt the design to the industrial
boiler application with the more stringent
requirements for faster thermal response.
In addressing these objectives, the
design development effort in the current
project led to selection of the lightweight
refractory precombustor/burner design
with boiler sidefire air. Although this
burner was not fabricated and evaluated
in the field, the following conclusions on
the design are based on the process
engineering analyses done as part of the
design development.
Emissions Performance
The estimated performance of the
design based on previous test results and
kinetic estimates is 85 ppm at 3%
oxygen. The emission goal selected for
this program was 100-125 ppm, so the
final design should give sufficient margin
to meet the emission performance
demands of the intended market.
Burner Performance
The use of the lightweight refractory
should allow thermal response during
start-up, shutdown, and load swings
comparable to conventional burners and
should not constrain industrial boiler
applications. The burner is smaller, much
lighter; and cheaper than the earlier EPA
oil burner design. The control logic
during start-up is more complex due to
the two-stage operation, but not out of
line with new boilers with advanced low-
NOx systems.
Market Niche
The burner addresses a need for both
high nitrogen residual oil firing and
cofiring of nitrated wastes. Conventional
low-NOx burners for high-capacity pack-
age boilers firing high nitrogen residual
oil cannot lower emissions below the 250-
350 ppm range without causing flame
impingement, combustible emissions, or
instabilities. With waste cofiring, the
unique hot first stage used in the burner
design offers simultaneous high waste
destruction efficiency and low-NOx
emissions. This burner avoids the prob-
lem with other burners where measures
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Pyro-Bloc Plus
Module
III
Figure 2. Combustor cross section, Pyro-Bloc Plus module arrangement.
taken to lower NOX are counterproductive
for high waste thermal destruction.
Reliability
The present design uses lightweight
refractory to improve thermal response
and reduce failure risk from thermal
shock. The design increases the risk,
however, of mechanical failure and
overtemperature.
Scale-Up
Burner design data are given over a
4:1 capacity range. The general
configuration, stoichiometric ratios,
residence times, and temperatures all
remain nearly identical across that range,
however. Therefore, there is not expected
to be significant performance variations
with scale-up.
Developmental Status
Although the burner concept is derived
from the earlier EPA oil burner, many
differences in materials and configuration
were made to address the industrial
boiler market. Thus, the first prototype
field evaluation will have a larger
component of shakedown and
confirmatory testing than that for a simple
hardware scale-up..
Nonmetric Equivalents
Readers more familiar with nonmetric
units may use the following conversion
factors:
Yields
Metric
°C
cm
m
MW
ng/J
Pa
tonne
Multiplied by
9/5°C + 32
0.391
3.281
3.414
0.0023
0.0040
1.102
Nonmetric
°F
in.
ft.
1Q6Btu/hr
lb/106 Btu
in. H2O
ton
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R.A. Brown, H. Define, S. Eaton, H. B. Mason, and S. Torbov are with Acurex
Corporation, Mountain View, California, 94039
William P. Llnak is the EPA Project Officer (see below).
The complete report, entitled "Design Report: Low-NOx Burners for Package
Boilers," (Order No. PB 90-159 898/AS; Cost: $23.00, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4850
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S7-90/001
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