c/EPA
United States
Environmental Protection
Agency
Industrial Environmental Researctv M
Laboratory *
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-81-137 Oct 1981
Project Summary
Pilot-Scale Development of a
Low-NOx Coal-Fired
Tangential System
J. T. Kelly, R. A. Brown, E. K. Chu, J. B. Wightman, R. L. Pam, E. L. Swenson,
E. B. Merrick, and C. F. Busch
A 293 kW, (1 x 10s Btu/hr) pilot-
scale facility was used to develop a
low-NO. pulverized-coal-fired tangen-
tial system. Conventional tangential
system burner and vortex charac-
terization tests defined the major
system design requirements for a low-
NOx system. Given these require-
ments, a burner concept was devel-
oped which achieves low NO, by
directing the fuel and a fraction of the
secondary combustion air into the
center of the furnace, with the remain-
ing secondary combustion air directed
horizontally and parallel to the furnace
walls. The separation of secondary
combustion air in this manner creates
a fuel-rich zone in the center of the
furnace where NO, production is
minimized. This combustion modifi-
cation technique has lowered NO« 64
percent, relative to conventional tan-
gential firing, by injecting 85 percent
of the secondary air along the furnace
walls. Under these conditions, NO
emissions were 180 ppm corrected to
0 percent oxygen. In addition, at these
conditions. CO, UHC, and unburned
carbon emissions were less than 40
ppm, 3 ppm, and 2.4 percent, respec-
tively. These levels are comparable to
conventional tangentially fired pilot-
scale results. Also, the modification
places a blanket of air on the furnace
walls which is beneficial from a wall
corrosion and slagging point of view.
With the modification, oxygen con-
centrations above the burner level
near the furnace wall were 12 percent.
This is nearly three times conventional
tangential pilot-scale system wall
oxygen concentrations. Finally, in
some configurations, the modification
shows a decrease in NO* emissions as
firebox gas temperature is increased.
This characteristic might be benefi-
cially applied in a large-scale system to
reduce furnace volume, and thereby
capital cost, for a given combustion
heat release.
This Project Summary was devel-
oped by EPA's Industrial Environmen-
tal 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).
Introduction
Maintenance of ambient air quality in
the United States requires the restriction
of NOx emissions from stationary
combustion sources. Presently, tangen-
tial coal-fired boilers produce approxi-
mately 10 percent of all stationary
source NOX and consume, in kW-hr,
approximately 9 percent of all fuel used
in stationary sources.1 The significant
NOX emissions from these boilers and
the projected increase in the number of
these boilers make them candidates for
emission control development both in
terms of retrofit and new boiler designs.
During the combustion of pulverized
coal in tangential boilers, NO, is
-------
generated from the nitrogen chemically
bound in the fuel as well as from the
oxidation of atmospheric nitrogen. For
typical bituminous coals, NOXemissions
from the fuel bound nitrogen can be a
significant fraction of the total.2 NOX
emissions have been shown to respond
to combustion modification techniques
which alter oxygen concentration,
residence time, and temperature during
combustion.3 Lowering the oxygen con-
centration surrounding the fuel, either
locally by fuel/air stratification or
globally by limiting the air flow in the
combustion volume, shifts the fuel and
atmospheric nitrogen emission reactions
from predominantly NOX formation to a
balance between NOX and molecular
nitrogen formation.3 In addition, given
sufficient residence time at oxygen
deficient conditions, previously formed
NO, can be reduced to molecular
nitrogen by homogeneous4 and hetero-
geneous5 catalyzed and noncatalyzed
reactions.
Lowering peak temperature under
excess air conditions decreases atmo-
spheric nitrogen NOX formation.6 How-
ever, under very fuel-rich staged
combustion conditions, lowering first-
stage temperature can increase NOX.6
This is due to less fuel nitrogen being
volatilized in the first stage and carrying
over and being converted into NO* in the
oxygen-rich second stage.
Even though imperfectly understood,
these basic relationships between
system parameters and NOX emissions
have been employed to moderately
reduce NO* emissions from tangential
as well as other types of utility boilers.7
Significant further reductions in NOX
from tangentially fired boilers require a
better understanding of the combustion
processes which control NOX forma-
tion/reduction. Therefore, this study
was separated into two phases. The
objective of the first phase was to
develop an understanding of the
processes which control NO* forma-
tion/reduction in pulverized-coal-fired
tangential boilers. Utilizing the results
of the first phase, the objective of the
second phase was to develop and
demonstrate, in pilot-scale, Iow-N0x
combustion modification techniques
that can be retrofitted in old or incorpo-
rated into new tangential boiler designs.
Definition of Coal-Fired
Tangential Systems
Figure 1 illustrates the main features
and flow patterns of a tangentially fired
Vortex Core Zone
rNear-Burner Zone
Jet Interaction
Zone
Air
Secondary
Air
Burner
Figure 1. Tangential boiler sche-
matic.
boiler. Fuel and air are introduced into
the furnace through rectangular reg-
isters located in the four corners. The
bulk of the combustion air enters above
and below the fuel jet as shown in
Figure 1. The jets are nonswirling and
fuel/air mixing is slow relative to front-
wall-fired boilers.
The tangential alignment of the
centerlines of the corner jets to the
circumference of a circle in the center of
the furnace promotes the formation of a
large-scale vortex within the furnace.
Ignition of the fuel is provided by
impingement of hot burnt gases from
laterally adjacent burners and large
scale internal recirculation of combusted
gases. Because ignition occurs primarily
on the vortex core side of the fuel jet (see
Figure 1), combustion is asymmetric in
the horizontal plane.
Pilot-Scale Combustion
Facility
To simulate properly full-scale system
combustion environments, the pilot-
scale facility volumetric heat release,
overall residence time, and furnace exit
gas temperature were matched to
typical full-scale values. Also, burners
and their placement in the firebox were
patterned after full-scale tangential
systems.
The simulation of full-scale firebox
flow patterns and mixing by the pilot-
scale facility was evaluated by com-
paring pilot-scale flow and flame
patterns to corresponding full-sc,
results. In the comparison, similarit
were found for (1) ignition standoff a
character, (2) flame spreading ani
from burners, (3)apparentjetcenterli
angle from corners, and (4) vortex si
Baseline Test Results
As shown in Figure 2, NO* emissi
levels achieved by the pilot-scale facil
at various excess air levels on seve
coal types correspond well with fu
scale results. The matching of the Is
trend with excess air is encouraging
that this, aswellastheabovemention
comparison of flame patterns, may
an indication of the matching of mixii
processes between the full- and pile
scale systems.
During baseline testing, the CO, UH
and carbon loss emissions were lo\
indicating that complete combustion
occurring in the pilot-scale facility.
Jet and Vortex Characteriza-
tion Test Results
To assist in the definition of N
emission control strategies, convei
tional tangentially fired tests wei
carried out to characterize the importai
processes to NO formation/reduction
this system and establish the effect i
design variables on these processes.
During the characterization tests, th
facility was operated at baseline cond
tions. The fuel chosen for all combustio
system definition and modificatio
testing was a Utah bituminous coal. Ir
flame gas and solid samples were take
by a water-quenched sampling probe c
a variety of firebox locations to determin
the relative importance of near-burnei
jet interaction, and vortex zones (se
Figure 1 for zone definitions) on N(
processes. A limited number of samplini
locations (0.076 m below, at, and 0.2"
m above the burner centerline) wen
chosen to characterize these zones.
Staged combustion probing tests
were also run at a lower firebo;
stoichiometric ratio of 0.85 with ar
overall stack excess air level of 1J
percent. These tests provided under
standing of the impact of fuel-ncf
combustion on N0xformation/reductior
processes in the firebox.
Once the important zones to NO
formation were defined, NO was injectec
*NO is the dominant form of nitrogen oxid<
emissions from furnaces and only NO was mea
sured during this test program
-------
600
500
^400
§;300
o
200
roo
Conventional Tangential
Pi lot- Scale
Utah
Western Kentucky
/\ Montana
/V Pittsburgh #8
Full-Scale
Reference 8
Reference 9
293 kW,(1 x 70s; Btu/hr) load
589 K (600°F air preheat)
15% primary air, 6° yaw angle
Air-on-Wall
Configur
Primary ation
Swirl L
Short Slot J
"Cir. Divergent M
Cir. (Cir. Sec.) H
Cir. K
Long Slot I
Cir. Coan. N
12 16
Excess Air (percent)
20
24
28
Figure 2.
Effect of excess air on pilot-, full-scale, and air-on-wall system NO
emissions.
into these zones, and others, to deter-
mine the NO reduction potential of
these zones. The amount of injected NO
remaining in the stack gases quantified
the NO reduction potential of these
zones.
Tangential Burner
Characterization Test Results
Different burner designs were tested
to assess their impact on NO and to
determine the relationship between
burner design parameters and NO.
Burner designs tested were limited to
nonswirling slow-mix designs, such as
those presently used in tangential
systems. Compact intense flames
produced by swirl burners are not
compatible with tangential firing due to
potential corner slagging and deposition
problems
Based on the probing test results and
the known importance of O2 availability
in the fuel jet to NO,6 burner designs
were tested where the exposure of the
fuel jet to combustion air, and the hot
combustion gases which provide igni-
tion, was varied. Also, in some of these
tests the relative position between the
fuel and air was varied to operate the
vortex side of the flame more fuel-rich.
For the burner design tests, three
conventional baseline burners firing on
gas and one experimental burner firing
on coal were used to generate a
conventional tangential system vortex.
Firebox Mixing Tests
Several burner configurations were
tested for the effect of firebox mixing
intensity and jet breakup on NO by
varying the gas burner firing rates while
maintaining the coal burner at a
constant 73 kW, (250 x 103 Btu/hr).
Varying the gas firing rate changes the
vortex strength and the turbulence
intensity in the firebox, which alters the
coal burner mixing processes.
Discussion of Results from
Conventional Tangential
Design Tests
Probing tests showed that near the
burner face, where the bulk of the total
NO is formed, combustion is asymmetric
with ignition, intense burning and peak
NO production occurring on the vortex
core side of the fuel jet in the jet
interaction zone. At this location
approximately 60 percent of the fuel has
been burned and this fraction can be
associated primarily with the fuel
volatiles. Fuel/air mixing in this zone is
enhanced by the crossflow of hot
combustion gases over the fuel and air
jets. Since the initial fuel nitrogen
volatiles see an abundance of oxygen in
this zone, NO formation is very high.
Lifted flame and dispersed fuel jet
burner test results were extreme
examples of how high Ozconcentrations
at the fuel ignition point can lead to high
NO. In addition, this zone has a high gas
temperature, due to reduced wall heat
transfer and high entrained combustion
gas temperature. High temperature
under 02 rich conditions generates
significant atmospheric nitrogen NO.6
As shown by the staged probing and
burner configuration tests, the high NO
production rate of the jet interaction
zone can be reduced by operating this
zone fuel-rich through limits on fuel/air
mixing. Under these conditions the
volatilized fuel nitrogen will be in a more
oxygen-deficient environment and the
fuel nitrogen NO formation reaction will
shift to a balance between NO and
molecular nitrogen formation.3 Also,
atmospheric nitrogen NO formation will
be reduced under oxygen-deficient
conditions.3
Downstream of the near-burner zone,
beyond roughly half the firebox length,
the vortex interacts with the burner jets
and causes the fuel and air to mix
rapidly. Combustion and NO production
in this zone and beyond are dominated
by char burning and the net NO
production is small relative to the near-
burner region. In this zone, the fuel
nitrogen in the char matrix reacts in an
environment which has a much lower
O2 concentration than the near-burner
region where the volatiles react. Also,
-------
previously formed NO concentrations in
this zone are high. In this environment,
NO reduction of the fuel nitrogen to
molecular nitrogen can occur.3 In
addition, previously formed NO can be
reduced by homogeneous reactions
with fuel components4 or by catalytic
and noncatalytic reactions on fuel
particle surfaces.5 These processes
together account for the small net NO
production observed for this zone.
Similar comments also apply to the
above burner elevation zone.
Staged probing tests showed that
operating the downstream burner zone
fuel-rich causes decay of previously
formed NO. In this environment the
homogeneous and heterogeneous NO
decay reactions discussed above over-
whelm any NO production yielding low
stack NO levels.
The effectiveness of the various
combustion zones in decaying previ-
ously formed NO is also clearly demon-
strated by the NO dopant tests. For
conventional combustion conditions,
injecting NO on the fuel jet centerlme
near the burner gives NO reduction
efficiencies of 70 to 80 percent. Away
from the burner and in the vortex,
reduction efficiency is about 50 percent.
In the active zone on the vortex core side
of the fuel jet, explored in the probing
tests, reduction is 94 percent. Belowthe
burner level, NO reductions of 80
percent were measured, whereas above
the burner level reductions were small,
being less than 13 percent.
Under staged combustion conditions,
at a first-stage SR of 0.85, NO reductions
at the burner elevation were a factor of
two better than the unstaged results,
except at a single point for which no
explanation can be given. Below the
burner level, reductions observed were
about the same as conventional un-
staged reductions. Above the burners,
the reductions were significantly better
for the staged conditions.
These results show that NO is most
effectively reduced if the NO is injected
into the active combustion and peak NO
production zone formed by the interac-
tion of hot burnt gases and the fuel jet.
In this zone, reaction is probably fast
and addition of NO can drive the
reactions from NO* production toward a
balancing of NOX production and reduc-
tion. Another effective reduction zone is
near the burner face at the burner
elevation. In this zone, oxygen is not
abundant and NO is reduced. Below the
burner level, NO reduction is effective
for both staged and unstaged conditions.
Based on burner flame observation
and probing tests, the lower NO found
for reduced firebox mixing is primarily
due to the maintenance of locally rich
zones downstream of the burner face
where NO production from the char is
minimized and previously formed NO
decay is maximized. The same NO
reduction processes which occur under
globally rich conditions for staged
combustion are active in the locally fuel-
rich zones created by burner fuel/air
stratification. Burner tests also showed
that too low a mixing level can result in
loss of ignition and lifted flames. These
flames have high 02 availability at the
fuel ignition and volatile reaction point,
and thereby high NO.
Low-NOx System
Based on the above observations, the
requirements for a low-NO, tangential
system were identified. These are: (1)
initiate burning sooner to minimize Oa
availability at the ignition point, (2)
operate the jet interaction zone fuel-
rich, (3) protect the fuel jet from
dispersion by vortex flow, (4) lower
firebox mixing with the constraint of
Top View
rFuel Rich /-Fuel Lean
positive ignition, and (5) operate
portion of the char burnout zor
oxygen-deficient to get NO decay.
addition to these Iow-N0x requirement
constraints must be applied on \\
system relative to boiler size ar
efficiency, wall corrosion and slaggin
and heat transfer. These constrain
dictate that, to minimize corrosion ar
slagging problems, oxygen-deficiei
combustion gases should not conta
the walls. Also, sufficient time ar
oxygen must be available to fully bur
out the fuel and minimize carbon los
CO, and UHC emissions. Finally, furnac
volume and exit gas temperatures mui
be constrained to those typical c
presently operating units.
Figure 3 presents a top and side vie\
schematic of the Iow-N0« system wit
rich and lean zones identified. Th
major system features are: (1) fue
directed at conventional tangential yav
angle into the center of the furnace; (2
some secondary air, either displace
toward the wall side of the firebox o
surrounding the fuel jet, directet
parallel to the fuel jet; and (3) the bulk o
the secondary air directed along th<
wall at and above the fuel jet elevation
WalJ Air
Secondary Air
Fuel/Primary Hi*
Side View
Wall Air
Primary Air' ij,
Secondary _/
Air
Corner Burner Detail
Figure 3. Low-NO* air-on-wall system schematic.
4
-------
These major system features create
oxygen-deficient conditions in the
active near burner and the char burnout
zone and fuel-lean conditions on the
furnace walls at and above the fuel jet
elevation. These system characteristics
address both the low NO requirements
and operational constraints noted
above.
The corner burners in the pilot-scale
facility were modified to simulate the
low-NOx system illustrated in Figure 3.
Figure 4 schematizes the three burner
designs tested. The System 1 burner
was partially constructed of high
temperature refractory and had variable-
angle-wall air jets which exited roughly
one third the distance along the firebox
side wall. The System 2 burner consoli-
dated the wall air jets into the corner
refractory burner block and added two
more levels of wall air jets to distribute
the wall air vertically in the firebox. The
System 3 burner was designed to
represent how the low NOX system
would be retrofitted into a modern
large-scale utility boiler.
System 1 Test Results
For System 1 testing, the corner
burners in the pilot-scale facility were
modified as shown in Figure 4 to
simulate the low-NOx system illustrated
in Figure 3. Tests were then initiated to
optimize the primary fuel, secondary
and wall jet configuration, placement,
direction, and velocity. As shown in
Figure 5, tests varying the primary fuel
and secondary jet configurations showed
that directing approximately 20 percent
of the secondary combustion air into the
center of the furnace and 80 percent
along the walls, at the fuel jet elevation,
gave the lowest NO for most of the
System 1 primary configurations tested.
At 80 percent air on the wall, probing
results showed that the center of the
furnace is oxygen-deficient with this
zone typically occupying 40 percent of
the firebox at 0 20 m (8 in.) above the
burner level for most configurations
tested. As the wall air mixes into this
oxygen-deficient zone, it typically
shrinks to 20 percent at 0.46 m (18 in.)
above the burner centerline. At this
location, NO formation/reduction
processes are essentially complete and
NO levels are comparable to stack
values.
Vicinity of wall oxygen concentrations
are 10 percent at 0.20 m (8 in.) above
the burner elevation for the Iow-N0x
system versus 4 percent typical of
conventional tangential firing in the
pilot-scale facility.
Figure 2 shows the effect of excess air
at the optimal 80 percent air on the wall
for several primary configurations. As
shown in Figure 2, the best configura-
tion was a coannular primary/secondary
air configuration. In this configuration
the circular primary is surrounded by an
annular passage that contains 20
percent of the secondary combustion
air. Flame observation showed that this
configuration had the smallest amount
of fuel dispersion prior to entering the
fuel-rich core zone. Various length slot
primaries, although initially burning
much sooner than the other configura-
tions, had fuel dispersion problems. The
dispersed fuel would burn in fuel-lean
zones and yield high NO. Circular
Top View
Wall Air
A nnular (fuel air)
Primary
Annular
(fuel air)
Primary
Front View
System 1
Ja
Front View
System 2
Primary Air and Fuel
Figure 4.
Wall Air
-Annular Air (fuel air)
Center Air
x system burner types.
5
-------
400
-vO
I
I
O
300
200
293 kWt(1 x 106) Btu/hr load
589 K (600°F) air preheat
15% primary air
15% excess air
6° yaw
Configurations*
Primary Sec.
Q-tf
O-'
A-J
O-AT
&- L
O- M
O- N
0 - O*
cir.
long slot
short slot
cir.
cir. swirl
cir. diff. •
cir. coan.
cir. coan.
*AII with slot wall air
**With wall air directed 10° off
of furnace wall
cir.
slot
slot
slot
slot
slot
slot
slot
I
I
I
I
50 60 70 80 90 100
Secondary Airflow Along Wall (percent)
Figure 5. Configurations H through 0 NO results with percent wall air.
primaries having swirling or diverging
flow also yielded higher NO at 15
percent excess air.
System 1 tests with a variety of wall
jet inclination angles showed that the
lowest NO levels are achieved when the
wall jet flow is horizontal. As the wall jet
inclination angle is varied from the
horizontal, the wall jet is directed out
from behind the fuel jet which, due to
the lack of fuel jet shielding, might be
more easily entrained and mixed into
the fuel-rich core. The decrease in
fuel/air separation caused by this
mixing might be the reason for the
observed NO increase when the wall jet
is inclined with respect to the horizontal.
Besides yielding minimum NO, direct-
ing the wall air jet horizontally is
desirable for multiburner level firing.
For a system with several burner levels,
directing the wall air upward or down-
ward with respect to the fuel jet might
result in undesirable burner-to-burner
air jet interactions.
In addition to varying the inclination
angle of the wall jets, one test considered
the effect of directing the wall air at 10°
away from the furnace wall. As shown
in Figure 5, directing the wall air 10° off
of the wall (configuration 0) increased
the minimum NO approximately 25 ppm
over the 0° (configuration N) wall jet
angle results. In addition, firebox
probing tests showed that 02 concen-
trations near the wall 0.20 m (8 in.) and
0.46 m (18 in.) above the fuel tube
elevation were decreased for the 10° off
the wall angle case. The higher wall O2
concentrations and the lower minimum
NO levels for the case where the air is
directed along the wall make this the
optimal jet orientation.
Comparison of the best Iow-N0x
concepts results with conventional
pilot- and full-scale tangential firing
results in Figure 2 shows that tl
System 1 concept reduces NO emissioi
by roughly 60 percent and lowers \\
sensitivity of NO to excess air. Th
reduced sensitivity may be a result
the more diffusive burning nature of th
fuel-rich core.
Combustion characteristics for th
low-NOx System 1 are not marked
different from conventional pilot-sea
tangential firing. Carbon monoxidi
UHC, and percent carbon in flyash leve
for this system are <36 ppm, <9 ppn
and <3 percent, respectively, versu
conventional pilot-scale tangentiz
firing results of <22 ppm, <1 ppm, an
<7 percent. These NO reductions an
good combustion efficiency are achieve
while increasing vicinity of wall oxyge
concentrations to 10 percent near th
burner elevation. This oxygen blanket
ing of the wall is beneficial from a wal
corrosion and slagging point of view.
An additional feature of the low-NO
System 1 configuration is the improve
ment in NO emissions as temperaturt
rises. Figure 6 shows that as gai
temperature is increased for tw<
different air-on-wall system burne
configurations, NO decreases. This
attractive emission behavior with
temperature might be used beneficially
to reduce boiler size and capital cost foi
a given heat release.
Also shown in Figure 6 is the NO
reduction caused by decreasing load al
a fixed gas temperature. As discussed
previously, reducing load decreases
firebox mixing thereby maintaining rich
zones in which NO is minimized.
System 2 Test Results
The burner configuration used in
System 2 testing is shown schematically
in Figure 4. This system differs from the
initial configuration in that four wall
jets, instead of two, are used and these
jets are confined to the corner burner
blocks. Operating the four levels of wall
air in this configuration defines the
emissions and efficiency benefits of
distributing the wall air vertically.
Figure 7 presents the variation of NO
levels with percent of secondary com-
bustion air on the wall for the System 2
configuration. Each curve represents a
different vertical distribution of air flow
between wall air ports 1a, 1b, 2, and 3
as defined in Figure 4. The SRs achieved
at each wall air level as a result of the
vertical distribution of air are given in
Figure 7. Configurations 1b and 1ab
denote tests where air was flowing only
-------
300 r—
233 kW, (1 x 106) Btu/hr load
589 K (600°F) air preheat
15% primary air
15% excess air
6°yaw
Primary*
Configuration
Long Slot y |
Cir. Coan. (j$)
Load kWt
293
o
D
220
•
Burner jets as viewed from the center
of the furnace. Filled area is fuel
entrv and open area is secondary air
entrv.
7/00 7750 7200 7250
Firebox Tower Gas Temperature (K/
Figure 6. NO versus gas stream temperature for air-on-wall low A/Ox system.
in port 1b or air flow was equally split
between ports 1a and 1b, respectively.
Also included in Figure 7 are the optimal
results from System 1 testing.
The cases where SR,, SR2, and SR3
are 1.15 do not have wall air flowing in
ports 2 and 3. Therefore, these cases
are comparable to System 1 configura-
tions where wall air ports 2 and 3 are
absent. As shown in Figure 7, System 1
gives the lowest NO results and System
2, with only the 1 b jet operational, gives
the highest levels. The percent air on
the wall at minimum NO falls between
80 and 85 percent for these cases.
Differences in wall jet configurations
and the separation between the fuel and
wall air jets probably account for the
differences in NO between these cases.
Even though System 2 NO results are
marginally higher under these condi-
tions, this configuration is preferred,
since the wall air jets are confined to the
corners and this approach would be
easier to retrofit into a full-scale boiler
than the System 1 burner.
For both configurations 1a and 1ab,
the Figure 7 results show that when
wall air is distributed vertically to ports 2
and 3, NO levels decrease with the
minimum NO point shifting to higher
levels of percent air on the wall. In these
cases, the vertical separation of the fuel
and air is creating a larger and more
fuel-rich zone at the fuel entry elevation
where NO production is minimized.
The minimum NO achieved with
System 2 is lower than System 1 levels
and represents a 65 percent reduction
from baseline tangential system levels.
Combustion efficiency during these
tests was excellent with CO, UHC, and
percent carbon in flyash emissions
being less than 40 ppm, 3 ppm, and 2.4
percent, respectively. These levels are
comparable to System 1 and conven-
tional tangential system results.
Probing tests at the wall for SR! = 0.88
showed 15, 12, and 12 percent oxygen
at 0.20 m (8 in.) and 0.46 m (18 in.)
above the fuel tube elevation, respec-
tively. As indicated 'previously, main-
taining a high oxygen concentration on
the wall is beneficial from a wall
corrosion and slagging point of view.
System 3 Results
The System 3 burner design repre-
sents how the low-NOx concept might
be retrofitted into an existing modern
large-scale tangentially fired boiler.
Both high and normal primary and
center air velocity cases were tested at
wall air angles from 31° to 45° and at
primary/center air angles of 0° and 8°
with respect to the diagonal. The high
velocities were produced by inserts in
the primary and center air ports.
Figure 8 compares the System 3
burner results with and without inserts
to System 1 and 2 results as a function
of percent wall air. The System 3 burner
results were taken over a range of
primary and wall airyawangles. System
3 burner NO levels decrease with
increasing percent wall air as observed
for System 1 and 2 burner results. The
System 3 results with inserts cor-
respond most closely with System 2
results and System 1 (configurations I,
N, and K) results. (See Figure 2 for
primary configuration designations.)
The System 3 results without inserts
correspond most closely with System 1
(configurations H and G) results.
Correspondence of NO results were
achieved for cases which had similar
primary to wall air velocity ratios.
As in System 1 and 2 testing. System
3 CO, UHC, and carbon loss emissions
were very low. Carbon burnup was
above 99.9 percent for all the System 3
tests.
The System 3 tests showed that the
parameters dominating NO formation
were percent wall air and primary/center
versus wall air velocity ratio. These
parameters, as well as wall air angle,
determined whether the burner flames
would scrape the furnace walls. At high
percent wall air and wall air angle, and
low primary/center to wall air velocity
ratio, flames were observed to scrape
the furnace walls. This condition is
undesirable because of potential tube
wastage and carbon loss problems in
full-scale systems. Lowering percent
wall air and wall air angle, and increas-
ing primary/center versus wall air
velocity ratio alleviated this problem to
some extent. It should be noted that
System 1 and 2 burners had much lower
wall air jet velocities than the System 3
burner and did not experience any wall
flame scraping problems.
System 1, 2, and 3 test results show
that the Iow-N0» system can significantly
reduce NOX while maintaining good
combustion efficiency and wall oxygen
conditions. With the System 3 burners,
some test conditions resulted in flames
scraping the furnace walls, which can
be alleviated to some extent by reducing
wall jet velocity and thereby the angular
momentum of the vortex to levels
characteristic of System 1 and 2 results.
Conclusions
Through extensive pilot-scale testing,
considerable progress has been made m
-------
350,
300
Q.
Q.
1
250
200
750
' 233 kW< (1 x 106) Btu/hr load
15% excess air
15% primary air
589 K (600°F) air preheat
"""~^»**
^Conf. N
0
Conf. S
Ib
0
1
lab
B
A
SR<
1.15
1.02
0.88
SR2
1.15
1.08
1.02
SRi
1.15
1.15
1.15
I 1
I
60
70 80 90
Secondary Air on Wall (percent)
100
Figure 7. NO variation with percent air-on-wall for vertical wall air configuration.
the development of a low-NOx coal-fired
tangential system. Based on coal-fired
pilot-scale tangential system burner
and vortex characterization tests, the
requirements for a Iow-N0« tangential
system were identified. These are: (1)
initiate burning sooner to minimize Oa
availability at the ignition point, (2)
operate the fuel/jet vortex interaction
zone fuel-rich, (3) protect the fuel jet
from dispersion by vortex flow, (4) lower
firebox mixing with the constraint of
positive ignition, and (5) operate a
portion of the char burnout zone
oxygen-deficient to get NO decay. In
addition to these low-NOx requirements,
constraints must be applied on the
system relative to boiler size and
efficiency, wall corrosion and slagging,
and heat transfer. These constraints
dictate that, to minimize corrosion and
slagging problems, oxygen-deficient
combustion gases should not contact
the walls. Also, sufficient time and
oxygen must be available to fully bi
out the fuel and minimize carbon lo
CO, and UHC emissions. Finally, f urnc
volume and exit gas temperatures mi
be constrained to those typical
currently operating units.
Given the above requirements, a lo
NOx system was defined. The ma
system feature is the dividing of t
secondary combustion air betwe
injection into the center of the furna
and injection along the furnace wal
The delayed mixing of wall air into t
vortex causes a fuel-rich combusti
zone to develop in the center of t
furnace, minimizing fuel and atm
spheric NOX formation. Primary, se
ondary, and wall jet configurations a
flowrates strongly influence the effe
tiveness of the system to lower N
emissions. Testing showed that the bt
results are achieved with (1) wall >
directed horizontally and along tl
furnace wall, (2) wall air flow equal to
greater than 80 percent of seconda
combustion air flow, (3) primary/se
ondary air coannular configuration, ai
(4) wall air vertically distributed at ai
above the fuel entry location. At optim
parameter settings, NO reductions of (
percent from conventional tangenti
system levels can be achieved
comparable combustion efficiency.
addition, wall oxygen concentration
the burner level is significantly ii
creased over conventional tangenti
firing. This increase is beneficial fro
wall corrosion and slagging points
view. Also, in some configurations, th
system shows a decrease in N(
emissions as temperature is increase*
This characteristic could be beneficial
applied to reduce furnace volume for
given heat release.
Recommendations
In preparation and in parallel to
demonstration of the low-NOx system t
a modern coal-fired tangential boilei
additional pilot-scale testing is requirec
This pilot-scale testing is needed to (1
help define burner design and tes
conditions to be explored in full-seal
testing; (2) define emissions an
efficiency performance for a range c
burner design parameters, as well a
the demonstration burner design condi
tions; (3) define emissions and efficienc
performance for a range of character
istic boiler coals and alternate fuels, a
well as the demonstration boiler fue
type; and (4) define additional concept
which have a high potential to furthe
-------
500
400
I
300
200
TOO
O
System 3
System 2 — 7"
System 1
Primary
Angle
0°
6°
6°
31°-45°
45°
45°
293 kW: (1 x 10s Btu/hr) load
589 K (600°F) air preheat
15% primary air
15% excess air
20 30 40 50 60 70
Air Flow Along Wall (percent)
80
90
100
Figure 8.
Comparison of System 1, 2. and 3 burner NO variation with percent total
wall air.
reduce NOX under good combustion
conditions.
These pilot-scale results will signifi-
cantly broaden the full-scale low-NOx
data base obtained in the demonstration
tests. The pilot-scale results will provide
information on how the system will
perform for other boiler designs and
different fuels, which is useful in
assessing the retrofit potential of the
system to a range of boilers. It will also
aid full-scale demonstration program
burner design and test condition selec-
tion, thus maximizing the value of the
demonstration test data. Lastly, it will
provide additional low-NOx concepts
and understanding of the low-NO*
system. This information will be useful
in upgrading the low-NOx system for
future applications to full-scale boilers.
To maximize the usefulness of the
pilot-scale data, testing conditions must
closely simulate combustion conditions
in the large-scale demonstration boiler.
Parameters such as firing intensity, gas
temperatures, residence time, and
burner and firebox mixing must be
properly scaled between the pilot- and
full-scale facilities for proper simulation
of full-scale performance. Presently, the
pilot-scale facility simulates these
parameters with a single burner level
firebox arrangement. However, modern
large-scale boilers have multiple burner
levels. The interaction of the burner
levels influences emission and combus-
tion performance. To properly simulate
large-scale boiler performance in the
pilot-scale facility, the system must be
modified to incorporate at least two
burner levels. With the two burner
levels, the effect of multiburner level
interactions could be simulated and its
impact on emission and combustion
performance assessed. In addition, the
multiple burner level interactions could
be optimized to yield minimum NOX
under good combustion conditions.
Therefore, to ensure proper full-scale
boiler simulation and to increase the
flexibility of the pilot-scale facility, two
burner levels must be incorporated into
the facility for the recommended pilot-
scale test program.
References
1. Lim, K.J., L.R. Waterland, C.
Castaldini, Z. Chiba, and E.B.
Higginbotham. Environmental As-
sessment of Utility Boiler Combustion
Modification NOX Controls. EPA-
600/7-80-075a (NTIS PB80-220957),
April 1980.
2. Habelt, W.W. The Influence of the
Coal Oxygen to Nitrogen Ratio on
NOX Formation, presented at 70th
Annual AlChE Meeting, November
1977, New York, NY.
3. Macek, A. Seventeenth Symposium
(International) on Combustion, The
Combustion Institute, 1978, p. 65.
4. Wendt, J.O.L, C.V. Sternling, and
M.A. Matlovich. Fourteenth Sym-
posium (International) on Combus-
tion, The Combustion Institute,
1973, p. 897.
5. Gibbs, B.M., F J. Pereira, and J.M.
Beer. Sixteenth Symposium (Inter-
national) on Combustion, p. 461, The
Combustion Institute, 1976.
6. Brown, R.A., J.T. Kelly, and P.
Neubauer. Pilot Scale Evaluation of
NOX Combustion Control for Pul-
verized Coal: Phase II Final Report.
EPA-600/7-79-132 (NTIS PB
299325), June 1979.
7. Breen, B.P. Sixteenth Symposium
(International) on Combustion, The
Combustion Institute, 1976, p. 19.
8. Selker, A.P. Program for Reduction
of NO, from Tangential Coal-Fired
Boilers, Phases II and lla EPA-
650/2-73-005a and b (NTIS PB
245162 and 246889), June and
August 1975.
9. Crawford, A.R., E.H. Manny, M.W.
Gregory, and W. Bartok. The Effect of
Combustion Modification on Pol-
lutants and Equipment Performance
of Power Generation Equipment. In
Proceedings of the Stationary Source
Combustion Symposium, Volume III.
Field Testing and Surveys, EPA-
600/2-76-152c (NTIS PB 257146),
p. IV-3, June 1976.
-------
J. T. Kelly, R. A. Brown, E K. Chu, J B. Wightman, R. L Pa/77, E. L. Swenson, and
E B. Merrick are with Acurex Corporation, Mountain View, CA 94042, C F.
Busch is presently with Utah Power and Light, Huntington, UT 84528.
David G. Lachapelle is the EPA Project Officer (see below).
The complete report, entitled "Pilot-Scale Development of a Low-NO*Coal-Fired
Tangential System," (Order No. PB 81-242 513, Cost: $18.50. subject to
change) will be available only from-
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
10
U. S. GOVERNMENT PRINTING OFFICE: I98I/559 -092/3309
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