United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-87/002 Apr. 1987
Project Summary
Fundamental Studies of
Calcium-Based Sorbents for
S02 Control from Coal-Fired
Boilers
J. A. Cole, J. C. Kramlich, G. S. Samuelsen, W. R. Seeker, and
G. D. Silox
Laboratory-scale controlled-tempera-
ture experiments were used to study
aspects of SO2 capture by calcium-
based sorbents in a flame/gas environ-
ment. Experimental parameters were
sorbent type, temperature, residence
time, and the effects of mineral addi-
tives, or promoters, on sorbent reactiv-
ity. The data revealed that isothermal
capture is greatest at 1000°C, above
which 1000°C sintering of the sorbent
can occur which reduces sorbent uti-
lization. High surface area precalcined
sorbents achieved moderately higher
ultimate utilizations than their parent
carbonates, but their real advantage
was more rapid sulfation at lower tem-
peratures where raw stones were lim-
ited by calcination. At 900 and 1000°C
the time for calcination for carbonate
sorbents was significant. Pressure hy-
drated (Type S) dolomitic limes consis-
tently achieved the highest utilizations.
The results suggest that—at ideal sul-
fation conditions (1000°C, isothermal
residence times greater than 1 s, no de-
activation of the sorbent by coal ash
minerals)—the best calcium utiliza-
tions achievable would be about 25-
30% with the raw limestone tested (Vi-
cron 45-3), about 30-35% with the raw
dolomite tested, and about 40% with
both precalcined dolomite (precalcined
to a surface area of 60 mz/g) and
pressure-slaked dolomitic lime. Adding
Cr2O3, alkali metal salts, and certain
other promoters increased the utiliza-
tion of limestone. CrzO3 effected a fac-
tor of 3.5 increase in utilization after cal-
cination at 1600-1700°C.
This Project Summary was devel-
oped by EPA's Air and Energy Engineer-
ing Research Laboratory, Research Tri-
angle 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
Dry injection of calcium-based sor-
bents is being studied as a way to con-
trol S02 emissions from pulverized-
coal-fired utility boilers. The current
objective for the technology is to
achieve intermediate levels of SO2 re-
moval (50-60%) when retrofit onto exist-
ing boilers, thus serving as a potential
component of an acid rain control strat-
egy.
In the present task, an isothermal
drop-tube furnace and a non-isothermal
flame reactor were used to study as-
pects of SO2 capture in the dispersed
phase under controlled conditions, over
a range of temperatures representative
of a utility furnace environment. The ob-
jectives were:
• To determine the sulfur capture re-
activity of a wide variety of calcium-
based sorbents—including hy-
drates and high surface area
precalcines, as well as carbon-
ates—over a range of temperatures
and residence times. This study in-
cluded assessment of surface area/
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reactivity development (during cal-
cination), surface area decay (due
to sintering and sulfation), and the
sulfation reaction itself.
• To observe physical changes in the
sorbent which could be linked to its
reactivity.
• To study the effects of mineral mat-
ter in either deactivating sorbents,
or enhancing their reactivity.
Experimental
Apparatus
Discussed here are reactors and sor-
bent precalcination apparatus.
Reactors
Two experimental reactors were used
in this program. The principal reactor
used was the isothermal reactor (ITR), a
dispersed-phase, isothermal, drop-tube
furnace. The ITR was down-fired using
S02-doped gaseous fuel, with electrical
wall heating to maintain constant tem-
perature over the reactor length. Sor-
bent particles injected down through
the burner at the top of the unit could
have residence times of about 0.5-3.0 s
by vertically adjusting an isokinetic,
water-cooled, stainless steel sampling
probe extending up through the reactor
bottom.
The second reactor was a non-
isothermal flame reactor, a down-fired
unit which could expose sorbents to
high temperatures (>1200°C) for short
residence times (<0.25 s). The feed sor-
bents were entrained in a fuel/air pre-
mixture prior to injection to ensure
rapid heating to peak reactor tempera-
ture; heat loss through the reactor walls
then resulted in a steep temperature
dropoff.
Solids from the flame reactor were
generally sampled using a dispersed-
phase quartz SO2 reactivity probe. Sam-
ples of dispersed sorbent—calcined in
the flame reactor, but not sulfated—
were collected in the probe, quenched
to 650°C, then exposed to SO2 under
controlled conditions (1100°C, 0.6 s, 6%
S02). This reactivity probe permitted
sorbents, calcined under a range of con-
ditions in the flame reactor, to be sul-
fated under constant conditions, so that
reactivities (in terms of calcium utiliza-
tions) could be compared directly.
Sorbent Precalcination
Apparatus
In order to generate high-surface-area
precalcined materials for testing in the
reactors, a transpirated bed calciner
was developed. The apparatus is a 20-
cm diameter stainless-steel can inside a
large box furnace. The can has a heavy
lid with a single hole for thermocouple
access and to allow sweep gas and C02
to escape. Raw sorbent is spread in a
thin bed on a 400 mesh stainless steel
screen. Sweep gas, N2, is preheated and
passed up through the sorbent bed at
controlled flow rates and controlled
temperatures.
Sorbents
The baseline sorbents used in this
study were Vicron 45-3 (a calcitic lime-
stone having a mean size of 11 (tin) and
D3002 (a dolomite with a mean size of
12 |j,m). These sorbents were also tested
following precalcination: precalcine
surface areas of 40 m2/g were typical for
Vicron (referred to as V40); and 60 m2/g,
for D3002 (referred to as D60). A
pressure-hydrated dolomitic lime
(mean size 1.0 p,m), referred to as Type
S (supplied by Warner), was also tested.
Several other sorbents were also tested.
In most reactor tests, run results were
determined by analyzing sorbent sam-
ples from the reactors. These solids
analyses included measurements for
carbon (carbonate), hydrogen (hydrox-
ide), total sulfur (sulfate) and total cal-
cium, in order to permit determination
of the extent of calcination and the cal-
cium utilization (percent calcium as sul-
fate). Other analyses performed on
some samples included specific surface
area (nitrogen absorption) and pore size
distribution (mercury porosimetry).
Results and Discussion
Non-Isothermal Reactor
The report presents histograms of the
reactivity for eight sorbents flame-
treated in the thermal decomposition
reactor at peak temperatures of 1200
and 1500°C, and then sulfated in the
dispersed-phase reactivity probe at the
standard 0.6 s/6% S02 condition. The
sorbents included two raw limestones,
two dolomites, three hydroxides, and
one limestone precalcined to a surface
area of 34 m2/g. For all sorbents tested,
the reactivity decreased by 25-50%
when the flame temperature was in-
creased. Previous measurements have
indicated that this can be attributed to
decreased specific surface area due to
more rapid sintering at increased tem-
perature. However, the relative reactiv-
ity of the flame-treated sorbents is in-
sensitive to flame temperature: only
two of the sorbents changed positions
relative to each other at the higher tem- {
perature. The limestones were gener-
ally the less reactive after flame treat-
ment, followed by hydroxides; the
dolomites were the most reactive. The
single 34 m2/g precalcine (produced
from Vicron limestone) was found to be
more reactive than the raw limestone
from which it was produced. Thus pre-
calcining provided an increased reactiv-
ity that was not completely lost when
flame-treated for short times (<200 ms).
Isothermal Reactor (ITR)
Sorbent utilizations in the ITR were
determined by sulfur analyses on the
sorbent particles after exposure in the
reactor. Following the testing described
in this report, it was discovered that the
utilizations reported here are generally
high, by a factor as high as 2, as a result
of sulfur uptake by the sorbent in the
ITR solids sampling system. These erro-
neously high utilization values do not
change the basic conclusions of this
study; however, this error should be
borne in mind when the utilization
values reported below are compared
against values from other investigators.
Calcium utilization was measured as
a function of residence time in the ITR
for five sorbents at temperatures of 900,
1000,1100, and 1200°C. In each case the
initial S02 concentration in the burned
gases was 3600 ppm, and the sorbent
feed rate was adjusted to ensure a
calcium-to-sulfur ratio (Ca/S) of less
than 1.0 so that the measured calcium
utilization would not be affected by SO2
depletion in the reactor.
Effects of Residence Time and
Sorbent Types
At 900°C (Figure 1), the utilizations of
the D60 precalcine and the Type S hy-
drate are distinctly the greatest at all
residence times; the V40 precalcine is
the next most reactive. The raw Vicron
and dolomite are the least reactive, es-
pecially at low residence times; the
roughly 0.5 s delay in the onset of sul-
fation for the raw sorbents is undoubt-
edly due to the time required for particle
heatup and calcination at this relatively
low temperature. The precalcines
being already calcined, do not experi
ence this delay, and have almos
reached their ultimate sulfation leve
before the raw stones have even startec
sulfating. The Type S sorbent also ap
pears to experience no delay due to cal
cination (dehydration); the dehydratior
of the hydrate requires a lower tempera
ture, and is less endothermic, than thi
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50
40
30
20
10
2 Type S Warner
D60
- O D3002
£ V40
D Vicron 45-3
H2/Air Flame
3600 ppm SOi
I
I
025
1 25
Figure 1.
050 075 10
Residence Time, s
Calcium utilization profiles lor five sorbents at 900°C
1 5
1 75
calcination of the raw carbonates, so
that (even at 900°C) dehydration ap-
pears to offer no impediment to sul-
fation.
All five sorbents appear to be ap-
proaching an effective "ceiling" on uti-
lization at residence times above about
1 s. This ceiling—at far less than 100%
utilization—reflects a significant slow-
ing of the sulfation rate, presumably
due to increases in pore diffusion resis-
tance (as internal sorbent pores become
blocked due to sulfation and thermal
sintering) and to increases in product
layer diffusion resistance (as the sulfate
layer becomes thicker). The D60 and the
Type S sorbents achieve the highest uti-
lization before leveling off (about 30%);
the V40 and the raw dolomite reach a
lower utilization (less than 20%); and
the raw Vicron had the lowest ceiling of
all (about 10%). The raw Vicron was
only half calcined at 900°C; this is un-
doubtedly one explanation for the low
utilization achieved by the limestone.
Insofar as pore diffusion and product
layer diffusion resistances are responsi-
ble for the apparent utilization ceiling,
the utilizations can be increased by
going to even finer particle sizes than
those tested here.
At 1000°C (Figure 2), all five sorbents
display a dramatic increase in reactivity
compared to that experienced at 900°C.
However, the relative order of reactivity
has changed to Vicron 45-3 < V40
< D3002 < D60 < Type S, reflecting a
large increase in the relative reactivity
of D3002. Both of the raw sorbents still
exhibit some delay in SC>2 uptake due to
calcination, but the delay is not as
severe as that experienced at 900°C. The
Vicron, which was only half calcined at
900°C, is essentially fully calcined at
1000°C.
The apparent ceilings on calcium uti-
lization are much higher for each sor-
bent at 1000°C relative to 900°C, with the
Type S achieving utilizations of over
40% after 1 s. After 1.5 s, the utilization
of the raw dolomite is approaching that
of the D60 precalcine; after 0.75 s, the
V40 utilization levels off at a value per-
haps only 2 to 3 percentage points
above the raw Vicron. Thus, at-this tem-
perature, it would appear that the pri-
mary value of a precalcine would be
where only short residence times are
available, and delays due to calcination
cannot be accepted.
Temperature Effects
Figure 3 summarizes the sulfur cap-
ture of the five sorbents as a function of
temperature. The data shown in Figure
3 were taken from utilization profiles
(analogous to Figures 1 and 2) at the
residence time of 1.0 s. The most signif-
icant aspect of Figure 3 is the appear-
ance of a maximum in the utilization
achieved as a function of temperature.
The location of the true maximum ap-
pears to be very near 1000°C but may be
different for each sorbent. The maxi-
mum is thought to be a result of the
tradeoff between sintering and reaction
kinetics. It is interesting that the opti-
mum temperature is the same for five
different sorbents.
Cr2O3 Addition
Figure 4 shows calcium utilization for
Vicron 45-3 injected into the ITR alone,
and mixed with 6 wt% C^Os- For these
tests, the ITR was operated at a constant
furnace temperature of 1100°C, but the
flame temperatures were varied as
shown, resulting in a ramped tempera-
ture profile from flame temperature to
1100T. All tests employed 3600 ppm
S02. As expected, the utilization of Vi-
cron 45-3 decreased with increasing
flame temperature. This reflects both a
decrease in surface area upon calcina-
tion and, in some cases, a shorter resi-
dence time in the sulfation window
where sulfation will occur with reason-
able kinetics (about 1250 - 1000UC). With
the Vicron/C^Os mixture, however, the
utilization initially increased as the
flame temperature was increased. Sub-
sequently, the utilization decreased
until, at a 1950°C flame temperature, the
utilization was nearly equal to that of
Vicron 45-3 alone. If the ratio of the
Vicron/Cr2O3 utilization to the Vicron-
only utilization is calculated, this ratio is
found to be highest (at a value of 3.5) at
a flame temperature of 1600-1700°C.
Both the temperature and the magni-
tude of this maximum in the utilization
ratio agree with data obtained in a
bench-scale boiler simulator furnace.
Additional Minerals
Fourteen additional minerals were
screened to determine their potentials
as sorbent reactivity promoters. The
materials, in 5wt% mixtures with Vicron
45-3, were exposed to 3600 ppm SO2
under the 1100°C isothermal condition
as well as with a 1360°C flame fired into
the ITR at a furnace temperature of
1100°C.
Figure 5 shows the results of isother-
mal tests at 1100°C in bar-graph form.
The open section of each bar is the uti-
lization achieved in 0.92 s; the solid por-
tion represents the additional utilization
up to 1.4 s. The horizontal lines are the
averages of four replicates of the utiliza-
tions measured for unpromoted Vicron
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50
40
30
20
10
Hz/Air Flame
m 3600 ppm SOi
0.25 0.50 0 75 10
Residence Time, s
1.25
1.5
Among the minerals tested, the alkali (
metal salts as a group showed the most
promise as promoters; and lithium, the
lightest alkali, produced the greatest en-
hancement.
Figure 2. Calcium utilisation profiles for five sorbents at 1000°C.
40
30
20
* 1O
O TypeS
£ D60
• D3002
£ V40
• Vicron 45-3
300
1000 1100
Temperature, °C
12OO
Figure 3.
Relative levels of calcium util-
ization at 1.0 s by five sorbents
as a function of isothermal
reaction/calcination tempera-
ture.
45-3 at the two residence times. Down-
pointing vertical arrows adjacent to the
data for Na2S04, K2SO4, and MoS2 show
what their calcium utilizations would be
if the sulfur initially present in the addi-
tives remained with the additive, and
was not released by the additive and
captured by the calcium. For the Li2CO3
mixture, the utilization measured at
1.4 s was lower than that at 0.92 s.
Every additive tested, except MoS2,
caused a net increase in utilization
(compared with unpromoted Vicron)
after 1.4 s. The magnitudes of the in-
creases are not as great as were ob-
served with Cr203; however, all of the
results are above the 95% confidence
limit based on the four replicate sam-
ples of Vicron 45-3 collected at 1.4 s.
The same promoters were tested
under nonisothermal conditions using a
1360°C flame with the ITR walls still at
1100°C. The calcium utilization by un-
promoted Vicron 45-3 was considerably
lower at this condition that an 1100°C
isothermal. This was due in part to ther-
maj deactivation, but also stems from
the substantially shorter time that the
sorbent had in the sulfation window.
The total residence time of the sorbent
in the ITR was 0.6 s; however, much of
this time the temperature was above
1250°C. The percentage improvement in
utilization effected by many of the pro-
moters at 1360°C (relative to unpro-
moted Vicron) was well above that for
the same promoters at 1100°C. How-
ever, at the higher temperature, the
benefits of improved promoter activity
have been offset by increased sintering
(and perhaps by the reduced time in the
sulfation window); as a result, the abso-
lute value of the utilization at 1360°C
never exceeds that at 1100°C for any
one promoter.
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3
I
I
30
20
10
0
Vicron 45-3
1000 1200 1400 1600
Flame Temperature. °C
1800 2000
Figure 4. Calcium utilization of Vicron 45-3 with and without CrzOa as a function of initial
flame temperature. 1100°C wall temperature, 3600 ppm SO*
20
75
I/O
0
Figure 5.
Calcium utilization by Vicron 45-3 in the presence of promoters under isothermal
conditions in the ITR. 1100°C. 3600 ppm SO2. The open portion of each bar
presents utilization achieved in 0.92 s. The solid portion shows the additional
utilization at 1.4 s. For L/'sCOz, the data at 0.92 s were higher than at 1.4 s.
Arrows adjacent to Na^SOt, K^SOt, and MoSi designate the correction for the
sulfur content of the additives.
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J. A. Cole. J. C. Kramlich. G. S. Samuelsen, W. R, Seeker, and G. D. Silox
are with Energy and Environmental Research Corporation. Irvine, CA 92718-
2798.
D. Bruce Henschel is the EPA Project Officer (see below).
The complete report, entitled "FundamentalStudies of Calcium-Based Sorbents
for S02 Control from Coal-Fired Boilers," (Order No. PB 87-152 500/AS;
Cost: $24.95, 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
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 452GO
Official Business
Penalty for Private Use $300
EPA/600/S7-87/002
0000329 PS
U S EHVI8 PROTECTION AGENCY
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