United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-89/004 Nov. 1989
Project Summary
Experimental Study of High
Levels of SO2 Removal in
Atmospheric-Pressure Fluidized
Bed Combustors
D. D. Kinzler and K. R. Drake
Tests were conducted in an atmos-
pheric-pressure fluidized bed com-
bustor (FBC) having a cross-section
of 1 x 1.6 m, for the purpose of
demonstrating high levels of SO2
removal when burning a high-sulfur
coal and feeding limestone sorbent
for SO2 removal. The goal was to
achieve SO2 removals of 90-plus %
with reasonable sorbent feed rates,
through suitable reductions in sor-
bent particle size (to improve reac-
tion kinetics) and increases in gas
residence time (to increase gas/sor-
bent contact time), in a manner
predicted by an existing mathemat-
ical model.
At particle sizes averaging from 800
and 1300 pm (mass mean), and with
gas residence times of 0.5 to 1.5 sec,
the measured SO2 retention levels
ranged from 88 and 98% when sor-
bent was fed at Ca/S molar ratios be-
tween 2 and 3. This result supports
model predictions. Reducing sorbent
particle size and Increasing gas resi-
dence time results in modest in-
creases in SO2 removal over the
range of conditions tested here. In-
creases in flue gas O2 content also
increased removals. Only one of the
three sorbents considered for this
project had the attrition resistance
necessary to permit use in this test-
ing, indicating that some sorbents
will not be suitable for use in dense-
phase FBCs.
Emissions of NOX ranged from 130
to 236 ng/J during these tests. Partic-
ulate emissions following the cyclone
but upstream of the baghouse ranged
from 9 to 35 g/m3; after the baghouse,
at the stack, the particle loading
ranged from 0.4 to 22 ng/J.
This Project Summary was devel-
oped by EPA's Air and Energy En-
gineering Research Laboratory, Re-
search Triangle Park, NC, to announce
key findings of this research project
that is fully documented In a separate
report of the same title (see Project
Report ordering information at back).
Introduction
For FBCs to be competitive with
conventional coal-fired boilers, the FBCs
will have to be able to provide reductions
in S02 emissions comparable to those
possible with conventional boilers using
scrubbers. These comparable reductions
must be achieved at a competitive cost.
Some New Source Performance
Standards (NSPS), which have been
considered or promulgated for various
coal-fired boilers, have envisioned S02
reductions up to 90% with high-sulfur
coals. However, early experimental test-
ing of FBCs had generally focussed on
the earlier NSPS for large steam
generators which had been promulgated
in 1971 (520 ng S02/J). This earlier
standard corresponds to a percentage
S02 reduction of only about 80 to 85%
with a high-sulfur coal. Little experimental
work had been conducted with sorbent
feed rates necessary to achieve
reductions > 90%. Accordingly, there
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Table 1. Text Matrix for Tests i3-28» (Effects of Sorbent Particle Size and Gas Residence Time)
Mass mean
sorbent particle
size (ftm)
Gas velocity
(m/sec)
Bad depth (m)
Residence time
(sec)
Ca/S
0.9
0.9
1.0
2.0b,
2.75"
800
1.4
1.5
2.0,
2.75
1.4
0.9
0.7
2.0,
275
1.4
1.4
1.0
2.0,
2.75
1300
1.8
0.9
0.5
2.0,
2.75»
1.4
0.75
2.0,
2.75b
Conditions for all 16 tests: Illinois No. 6 coal (3.5% sulfur); Greer limestone; bed temperature
844 °C; excess O2 5% (30% excess air); two coallsorbent feed ports; and no carryover recycle.
bThese tests were replicated, to yield a total of 16 tests.
was not a substantive data base in large
pilot fluidized beds (FBs) to confirm how
a requirement for 90-plus % reduction
might impact the design of FBCs, and
their capital and operating costs.
Earlier EPA-sponsored research at
Westinghouse Research and Develop-
ment Center had involved the develop-
ment of a mathematical model predicting
SO2 removal in a FBC, based on sor-
bent/S02 reaction kinetics and on FB de-
sign and operating parameters. Using this
model, it had been predicted that FBCs
should generally be able to achieve high
levels of SO2 removal economically, if
sorbent reactivity is sufficiently great
(e.g., through decreases in sorbent
particle size), and if the gas residence
time in the bed (i.e., the gas/solids
contact time) is sufficiently great, through
a suitably increased bed depth and/or
decreased superficial gas velocity. The
reduced operating costs resulting from
reduced sorbent feed requirements would
more than compensate for increased
capital costs associated with the larger,
deeper combustors that would be
needed.
The purpose of the current study is to
demonstrate that high levels of SO2
removal (>90%) can in fact be routinely
achieved in FBCs with reasonable
sorbent feed rates, if sorbent particle size
and gas residence time in the bed are
appropriately adjusted. This objective is
to be met through a statistically designed
test program on a reasonably large
experimental FBC which has the flexi-
bility to operate over the range of gas
velocities and bed depths needed for this
evaluation.
Experimental Equipment
The experimental FBC consists of a
carbon steel shell lined with castable
refractory to inside dimensions of 1 x 1.6
m. The unit can burn from 55 to 250 kg
coal per hr. In-bed temperature is
controlled by an air-cooled tube bundle.
Crushed coal and sorbent are premixed
and fed near the bottom of the bed. Flue
gas leaving the combustor first passes
through an overbad heat exchanger to
reduce temperature, then through a cy-
clone and a baghouse to remove panic-
ulate. The baghouse is a reverse-jet
pulse type containing 93 m2 of Nomex
cloth.
Test Program
The test program consisted of two
segments. In the first segment (Tests 1
through 12), testing was carried out with
one vs. two coal/sorbent feed ports, and
with and without carryover recycle. The
purpose was to determine how these
parameters should be set for the remain-
der of the testing. The second segment
(Tests 13 through 28) was designed to
investigate the effects of sorbent particle
size and gas residence time (i.e., the
relationship of bed depth and gas
velocity). The test matrix for this second
segment is shown in Table 1, covering:
two sorbent particle size distributions
(mass means of 800 and 1300 urn); three
superficial gas velocities (0.9, 1.4, and
1.8 m/sec); two bed depths (0.9 and 1.4
m); and two sorbent feed rates (Ca/S
ratios of 2.0 and 2.75), expected to
provide reductions in the vicinity of 90-
plus % at these test conditions. The gas
velocities/bed depths were selected to
give nominal gas residence times in the
bed ranging from 0.5 to 1.5 sec.
Usually, 6 hours of steady state
operation was maintained for each test
condition. During that time, SO2, O2,
C02, and CO were monitored con-
tinuously in the flue gas, and grab
samples for NOX were taken. We
chemistry of SO2 and NOX (EPA Method
6 and 7) was measured once each run t
confirm the results from the instrument;
A cascade impactor was used t
determine the particle size distributio
upstream of the baghouse, and EP<
Method 5 was used to determine particl
mass loading in the duct downstream c
the baghouse. Coal, limestone, fly asf
and bed material were sampled fc
chemical analysis and determination c
size distribution, as appropriate.
The tests were all conducted burnin
Illinois No. 6 coal (3.5% sulfur) and usin
Greer limestone. Of the three sorbenl
considered for this project, Greer was th
only one having sufficient resistance t
attrition/elutriation. Bed temperature wa
held at 844°C, and excess air wa
generally held at 30% (5% exces
oxygen), although there were som
limited, unavoidable variations. The tesl
in the second segment (Tests 13-2J
were conducted with two feed ports an
without carryover recycle. The results c
Tests 1-12 showed no significant benef
either to sulfur retention or to combustio
efficiency, of operating with one vs. tw
feed ports, or with or without recycle; th
selected options gave the best centre
over freeboard temperature.
Results
The S02 retentions observed durin
the 28 tests ranged from 88 to 98%. A
expected, within the range of condition
tested here, the highest retention level
were generally achieved with the greate;
gas residence times (i.e., with deep bed
and low gas velocities), the highes
sorbent feed rates, the smaller sorber
particle size, and the highest levels (
excess air. Results of a multiple lines
regression analysis show that the SO
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retention increased: by about 2% as gas
residence time was increased over the
tested range; by about 2% as sorbent
particle size was decreased; and by 3%
as excess oxygen increased from 5.0 to
6.1% excess. As expected, the sorbent
feed rate had the dominant effect
(increasing SC>2 retention by 6% as the
Ca/S increased from 2 to 3). The addition
of fly ash recycle and increasing the
number of feed ports from one to two,
each resulted in a 2% increase in
retention.
The fact that Greer limestone was the
only sorbent of three candidates which
had sufficient attrition resistance for these
tests illustrates that some sorbents will
not be suitable for use in dense-phase
FBCs.
NOX emissions during the 28 tests
ranged from 200 to 300 ppm, or 130 to
236 ng/J. The highest NOX levels were
measured at the greatest excess air
values. NOX also tended to be higher
when the S02 concentrations were
lowest.
Particulate mass loadings upstream of
the baghouse (after the cyclone) ranged
from 9 to 35 g/m3, with 15 to 25% of the
particulate smaller than 10 pm.
Particulate mass loadings at the stack
(downstream of the baghouse) generally
ranged from 0.4 to 22 ng/J. Particulate
emissions below the NSPS of 13 ng/J
were generally achieved for baghouse
air-to-cloth ratios less than about 1.2
m3/min/m2.
U.S. GOVERNMENT PRINTING OFFICE: 1989/748-012/07186
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D. D. Kinzler and K. R. Drake are with FluiDyne Engineering Corp., Minneapolis,
MN 55422.
D. Bruce Henschel is the EPA Project Officer (see below).
The complete report, entitled "Experimental Study of High Levels of SO2 Removal
in Atmospheric-Pressure Fluidized-Bed Combustors," (Order No. PB 89-194
1871 AS; Cost: $21.95, 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
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