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,
-------
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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