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

-------
                  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

-------
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

-------
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
                                                                                       UNOFFICIAL MAIL
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300

EPA/600/S7-89/004
  000085833   PS
  0  S  EMVia  PROTECTION AGENCY
  BEGICS 5  LIBBABT             C]\/
  230  S  DEABBOEM  STBEEI      Vv
  CHICAGO               1L  60604

-------