United States
                 Environmental Protection
                 Agency	
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
                 Research and Development
EPA/600/S7-89/007  Feb. 1990
vvEPA        Project  Summary
                 Evaluation  of Internally Staged
                 Coal  Burners and  Sorbent Jet
                 Aerodynamics for  Combined
                 SO2/NOX Control in  Utility
                 Boilers: Volume  1.  Testing in  a
                 10  Million Btu/Hr Experimental
                 Furnace
                 B. M. Cetegen, J. Clough, G. C. England, T. R. Johnson, Y. Kwan, and R.
                 Payne
                  As part  of  EPA's  Limestone
                Injection/Multistage Burner (LIMB)
                program, testing was conducted on a
                2.9 MWt (10 million  Btu/hr)
                experimental furnace to explore the
                potential for designing utility coal
                burners to  achieve reduced NOX
                emissions through staging of the
                combustion  air  internally within the
                burner. Such internal staging would
                avoid the need for external tertiary air
                ports, and thus simplify the retrofit of
                such a low-NOx burner into existing
                utility  furnaces.  Testing  also
                addressed the potential for SO2
                removal by injecting calcium-based
                sorbents (such as  limestone) in
                conjunction with coal-fired internally-
                staged burners,  for combined
                SO2/NOX control. Particular emphasis
                was placed upon understanding the
                sorbent jet design parameters which
                could improve the activation and 862
                removal performance of sorbents, by
                controlling sorbent heating rate and
                the peak temperature seen by the
                sorbent. The  sorbent  jet testing
                considered injection both  near the
                burners (using  large, double
                concentric jets), and under upper-
                furnace conditions, remote from the
                burners.
 Testing of  alternative  internally-
staged burner designs showed that—
if a particular retrofit situation offers
the flexibility to increase  the burner
throat diameter, In order to  reduce
velocity—NOX emissions  of 300-500
ppm appear  achievable with  two
secondary air channels and coal
nozzle modifications. This emission
represents approximately the desired
50-60 percent reduction in  the
emissions (typically 500-750 ppm)
characteristic of burners built prior to
promulgation of EPA's initial New
Source  Performance Standards
(NSPS) for large boilers. However,
where  there is no flexibility to
increase the  throat  and  reduce
velocity, then additional steps (e.g., a
baffle in the outer secondary  air
channel  to direct the air  away from
the fireball) are necessary to reduce
NOX to 400-550 ppm, approaching the
reduction objective.
 Sorbent jet  testing confirmed  that
the peak temperature  seen  by the
sorbent is  a  key variable  in
determining sorbent  reactivity. A
peak temperature of  1230-1290°C
(2250-2350 °F) appears to be optimum
for all  five  sorbents tested  (a
limestone, a  dolomite, two  atmos-

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pheric hydrates  and  a  pressure
hydrate).  At this  peak  temperature,
the  most  reactive  sorbent  (the
pressure hydrate) gave  80% SO2
removal  at  a Ca/S  molar ratio  of  2,
and the least reactive (the limestone)
gave 30%.  The apparent effect of
sorbent  heating  rate  was  not
consistent  over  the full  range of
temperatures  tested,   but at the
optimum  peak  sorbent  temperature,
the  higher heating rate  (higher
sorbent  jet velocity)  consistently
produced  somewhat  higher SO2
removals. The  experimental  furnace
was  too  small to  permit an effective
test  of whether double concentric
jets  (with an  annular  air jet
surrounding the sorbent jet)  could
protect the  sorbent from overheating
during near-burner  injection. Testing
of controlled sorbent precalcination
followed by immediate  injection into
the  furnace  showed   little  clear
benefit   of close-coupled   pre-
calcination.
  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).


Introduction
  LIMB  is  a  technology  being
investigated to achieve simultaneous S02
and  NOX control for existing coal-fired
utility boilers. The process envisions the
use of  calcium-based sorbent to achieve
intermediate levels  of BO2 removal (50-
60 percent),  and staged burners for NOX
reduction,  for retrofit applications as a
potential  component of  an acid rain
control strategy.
  Some staged  burners which have been
tested involve the use of external tertiary
air ports to delay fuel and air mixing. In
some cases, it could be difficult to retrofit
such external ports into existing boilers,
due to structural or other  constraints. An
objective  of the current  study was  to
investigate  burner  design approaches
which could achieve the  benefits  of air
staging without  external ports;  i.e., with
the staging  internal to  the burner, in a
manner which would facilitate  retrofit.
  The  sulfur capture performance of a
sorbent is dictated by the surface area it
develops  upon calcination,  upon its
residence time  at sulfation temperature,
and  upon its dispersion  in the furnace.
Injection near the burner would provide
the greatest residence time and the best
dispersion, but could subject the sorbent
to high temperatures which would greatly
reduce its surface area/reactivity.  Initial
testing in this study focussed on sorbent
injection near the  internally-staged
burner, including  use  of large, double
concentric jets which provide a sheath of
annular  air around  the sorbent jet  to
protect it from high temperatures. Later
testing focussed  on conditions repre-
sentative of  upper-furnace  injection,
remote from the burners. The sorbent jet
testing addressed the  ability to  improve
sorbent surface area/reactivity  through
control of peak temperatures seen by the
sorbent, and sorbent heating rate.

Experimental Equipment
  The  testing  was conducted on  a 2.9
MWt  (10  million Btu/hr) experimental
furnace, referred  to  as  the  Small
Watertube Simulator (SWS).  The furnace
was a horizontal cylinder which could be
fired  from  one  end with coal, oil  or
gaseous fuel.  For the  internally-staged
burner tests,  the  modified burner  to be
tested was mounted on the firing end  of
the furnace.  For  the sorbent jet testing,
the hot gas  flow field was  generally
established  using a series of  gas-fired
burners at the firing end, with the jet  to
be tested being   mounted through the
firing wall (co-flowing jet).

Results

Internally Staged Burners for
NOX Reduction
  If an internally staged burner  is to be
retrofit into an existing boiler,  the  ease
with which reduced  NOX emissions can
be achieved  will depend  upon the
flexibility which the host boiler provides
for reducing the  secondary air velocity
through the  burner  (e.g., by  increasing
burner throat diameter).
  If the  boiler permits  retrofit of an
enlarged-throat burner,  then NOX control
capabilities  are suggested in  Figure  1.
The secondary air velocity utilized in the
SWS testing used to generate that figure
was generally 24 m/sec (80 ft/sec), which
is relatively  low.  Two  types  of burner
modifications  were used to  obtain the
NOX performance  indicated in  the figure,
beyond the reduction in velocity:

   Ldual secondary air channels  were
    used. (By comparison,  the original
    pre-NSPS burner  to be replaced in
    retrofit situations will often have only
    a single secondary air channel.)
  2. alternative coal nozzles were used 1
    promote fuel/air staging.
The  alternative  coal  nozzles  whic
appeared to be the most effective, ar
which also gave good flame stability ar
combustion performance  (CO  <  6
ppm), were an axial swirler and a splitte
Also  effective  was  dense-phas
coal/primary air injection (utilizing 0.2 ^
of primary air per kg of coal, about 10'
of the  normal  ratio).  Dense-phas
transport reduced the size  of the  co
pipe  in  the center of the burner,  tht
permitting further  reduction of tr
secondary  air  velocity to 18  m/sec (f
ft/sec). Dense-phase transport might n
be compatible  in some cases with the <
requirements for existing coal mills.
  The burner  modifications  represents
in Figure 1 were capable of reducing N(
emissions  to 300-500 ppm (dry, 0% 0;
This emission with  internally-stage
burners  is  comparable to that achieved
the  SWS  with  a low-velocity low-N(
distributed mixing burner having extern
air ports.   By comparison, pre-NSF
burners  without  staging typically har
emissions  of 500-750 ppm.
  Figure 2 gives results where the retro
does  not  permit enlargement of tl
throat,  and the secondary  air  veloci
must thus  remain at the levels (aroui
58m/sec, or 190  ft/sec) typical of pr
NSPS  burners. This  would be
minimum-flexibility  situation. Burn
modifications  identical  to those used 1
the low-velocity case (dual secondary
channels,  specific  alternative  cc
nozzles) result, in the high-velocity cas
in NOX emissions of  650-850 ppm (upp
shaded  area  in Figure  2). The;
emissions  are comparable to the  ran
observed  in  unmodified  pre-NSF
burners. However, when a conical baf
is installed on  the  lip of  the  inn
secondary  air  sleeve—diverting t
secondary air in  the outer channel aw
from the fireball, thus delaying fuel/
mixing —significant  additional  N
reductions  are achieved. Emissions th
fall to 400-550 ppm (lower shaded area
Figure 2).
  A limestone  (Vicron  45-3)  was inject
at several  locations in the  burner  zc
near the high-velocity  burner: with 1
primary air/coal, with the secondary
and  through  external ports  near  1
burner (at  different injection velocities)
all cases, calcium utilizations  were  I
(about 10%, corresponding  to 20% S
removal at a Ca/S molar ratio of 2). T
low removal was probably due to therr
or coal ash deactivation of  the sorbe

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       1000
        800
        600
           982
                            Furnace Exit Temperature, °C

                     1038        1093        1149       1204
                                            1260
15" Axial Swirler

T-15° Splitter

30° Cone
                                       \
Conventional Nozzles


Dense Phase Nozzle
    o
        400
       200
                       Low Velocity Burner
          1800       1900        2000        2100

                            Furnace Exit Temperature, "F
                                2200
                         2300
Figure 1. Summary of NOX emissions from experimental internally staged burners operating at
         low velocity (flexibility for throat enlargement): effect of alternative coal nozzles.
Thus, it was apparent that the sorbent
would have to be injected either remote
from the burners, or through  jets which
would help  protect  it from this  deacti-
vation.

Sorbent Jet Investigations—
Double-Concentric Jets
  The first  phase  of  the sorbent  jet
investigations addressed  relatively  large
double-concentric  jets, which,  it was
hoped,  might provide  the  necessary
protection of the  sorbent  to  permit
injection near the burner. In this testing,
the  variables  studied  included  the
diameter of  the annular air jet, and the
velocities  of the annular air jet and the
inner sorbent jet. SO2 reductions  were
typically 28-38% at Ca/S = 2. However,
these large jets introduced such a  large
mass of air that they significantly reduced
the background temperature of the  SWS
 jrnace; this thermal effect dramatically
effected the peak temperature and the
                 time/temperature history  seen  by the
                 sorbent, in a manner unique to the SWS
                 experimental system. As a result, it is not
                 possible to assess from these results how
                 effectively double-concentric jets might in
                 fact protect  the sorbent in a large-scale
                 boiler. Because of the impact of the jets
                 on SWS operating conditions, these tests
                 showed  no  consistent  effect  of  jet
                 parameters  (e.g., jet velocities) on cap-
                 ture performance.

                 Sorbent Jet Investigations-
                 Small Jets
                   The  small sorbent jets tested were
                 primarily  5-cm  (2-in.) diameter single-
                 pipe jets, without  the  annular air  jet
                 present in double-concentric jets.  The
                 small jets were tested at conditions
                 (temperatures) representative of injection
                 into the upper furnace, remote from the
                 burners. These small jets gave  higher
                 and  better-controlled sorbent  heating
                 rates, permitting a better study of heating
rate  effects on sorbent activation.  The
test matrix for these tests was designed
to permit separation  of  the effects  of
sorbent peak  temperature and  heating
rate.  Peak  sorbent  temperature  was
controlled  by  adjusting  the SWS
background temperature.  Heating  rate
was controlled by adjusting both the jet
velocity and the background temperature;
a  doubling  of the velocity doubles  the
heating rate, all other factors being equal.
  The effect of peak sorbent temperature
for five different calcium-based sorbents
at constant heating  rate  is  shown  in
Figure 3.  As expected, the SO2 capture
for all of the sorbents tested was found to
be sensitive to peak temperature, with the
higher-reactivity sorbents  showing  the
greater sensitivity. Maximum S02 capture
was   found   with  peak  sorbent
temperatures  of  1230-12908C (2250-
2350"F).  Heating Rate  1  in the figure
corresponds to a calculated 8,000 °C /sec
(15,000 °F/sec), while Heating Rate 2 is a
calculated  19,000°C/sec  (34,000°F/sec).
As in  other studies,  pressure-hydrated
dolomitic lime ("Type  S" in Figure 3) and
pulverized dolomite  were found to be the
most  reactive of the sorbents tested;
calcitic limes which had been hydrated at
atmospheric  pressure  (Colton  and
Longview) had somewhat lower reactivity;
and pulverized limestone (Vicron) had the
lowest reactivity.
  At constant  peak  sorbent temperature,
the effect  of (calculated) heating rate was
not consistent over  the  full range  of
temperatures  tested.  But  at  the
temperatures producing  maximum
capture  (1230-1290°C), the  higher
heating rate consistently produced higher
S02  removals.  Sulfation  modelling
studies indicate  that this increase  in
capture cannot be explained solely on the
basis of the different time/temperature
histories  experienced  by the sorbent in
the different tests; thus, the differences in
heating rate might in fact have been play-
ing a role.
  Sorbent samples were  taken at  the
furnace exit for surface area analysis, to
determine if in fact the  higher  heating
rates  were  generating  higher  surface
areas. These test were made with no SO2
doping of the natural gas being burned in
the SWS,  so that no area  loss would be
occurring due to the sulfation reaction. As
expected,  the  surface areas tended  to
decrease  with   increasing  peak
temperature. But  there was no  observ-
able influence of heating rate on the final
surface area of  sorbent at  the  furnace
exit. This  suggests  that, if  heating rates
did create different areas early in the jet,

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       7000
           982
       Furnace Exit Temperature, °C


 1038         1093        1149
                                                          1204
                                    1260
                          30" Swirler
                          (dense-phase)
         1800
1900
2000
2100
                                                         2200
2300
                            Furnace Exit Temperature, "F
Figure 2. Summary of NOX emissions from experimental internally staged burners operating at
         high velocity (no flexibility for throat enlargement): effect of baffles in secondary air
         channel.
these differences were gone by the time
the sorbent reached the furnace exit.


Testing of a Close-Coupled
Precalciner
  A calcination vessel was installed near
the SWS, to determine if a highly active
sorbent  could   be  generated  by
controlled,  high-heating-rate  precalci-
nation  of  the sorbent,  followed by
immediate  injection of  the calcined
material into a furnace before the surface
area could decay. Four different versions
of the precalciner were used to study the
effects  of   calciner  temperature  and
                    residence  time,  and  the possible
                    influence of chromium  in  the  calciner
                    refractory  as a reactivity promoter.  The
                    S02 capture results  did not show  any
                    improvement with  use of the calciner,
                    compared to allowing the  sorbent  was
                    allowed to bypass the calciner, except in
                    one case. Surface area measurements on
                    sorbent taken  from both the jet issuing
                    into the SWS, and the SWS exit, showed
                    that the  sorbent was  only partially
                    calcined in  the precalciner, with  final
                    calcination  being  completed  in  the
                    furnace. Surface area at the end  of the
                    furnace was the same with or without the
                    precalciner.

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    TOO
             7204
                         Peak Temperature, °C

                           1260          1315
              1371
      Peak Temperature, °C

1204          1260          1315
1371
     80
  S
  CM-  60

  n

  GO
  re
  O

  03
  3
  C-J
  o
  CO
    20
Heating Rate 1

  O   Vicron


  £   Cotton Lime


  Q   tongwew L/me

      Dolomite


      Type S
                                                                                                     Heating Rate 2
           2200          2300         2400           2500                 2200          2300          2400


                       Peak Temperature (measured), °F                         Peak Temperature (measured), "F


Figure 3. Effect of peak sorbent temperature on SO2 capture at different sorbent initial heating rates.
                                                                           2500

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   8. Cetegen, J. Clough, G. England, T. Johnson, Y. Kwan, and R. Payne are with
    Energy and Environmental Research Corp., Irvine, CA 92718-2798.
  D.  Bruce Henschel is the EPA Project Officer (see below).
  The complete report, entitled "Evaluation of Internally Staged Coal Burners and
    Sorbent Jet Aerodynamics  for Combined S02//VOX  Control in Utility Boilers:
    Volume 1. Testing in  a 10 Million Btu/Hr Experimental Furnace," (Order No. PB
    89-207 955/AS; Cost: $31.00, 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:
           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/S7-89/007
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