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

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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

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   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 (600F 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,

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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

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  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

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   400
-vO
I

 I
O
   300
   200
            293 kWt(1 x 106) Btu/hr load
            589 K (600F) 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

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   300 r
            233 kW, (1 x  106) Btu/hr load
            589 K (600F) air preheat
            15% primary  air
            15% excess air
            6yaw
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

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   350,
    300
 Q.
 Q.


 1
    250
    200
    750
         ' 233 kW< (1 x 106) Btu/hr load
          15% excess air
          15% primary air
          589 K (600F) 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 (600F) 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.

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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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