United States
Environmental Protection
Agency
Air and Energy
Engineering Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S2-88/069 Mar. 1989
x°/EPA Project Summary
Fundamental Studies of Dry
Injection of Calcium-Based
Sorbents for SCte Control in
Utility Boilers
G. H. Newton, D. K.Moyeda, G. Kindt, J. M. McCarthy, S. L Chen, J. A. Cole,
and J. C. Kramlich
A research program was con-
ducted to determine the mecha-
nisms which limit the extent of
reaction between sulfur dioxide
(SOa) and calcium-based sorbents
[CaCOa and Ca(OH)2l by measuring
the in situ physical structure and
reactivity of sorbent injected into a
combustion environment for resi-
dence times as short as 35 ms. Four
models of the sulfatlon reaction were
used to guide the research and
interpret the data. The extent of
sorbent utilization was found to be
limited by porosity losses during the
sorbent activation process. In-sltu
porosities a fraction of that theo-
retically possible were measured In
the absence of SO2- At temperatures
below 1000°C, this porosity loss was
determined to be caused by COa*
actlvated sintering. The presence of
SO2 during calcination reduced the
extent of porosity loss and at optimal
temperature sulfation conditions no
loss in porosity was observed. At
temperatures above 1200°C, porosity
losses may result from an increased
rate of thermal sintering or a
decrease in the rate of the sulfatlon
reaction. Calcines from CaCOa suf-
fered greater losses in porosity than
those from Ca(OH>2 which, along
with the larger CaCOa particle size,
accounts for the substantial differ-
ences in SO2 capture between these
two sorbents.
This Project Summary was devel-
oped by EPA's Air and Energy
Engineering Research Laboratory,
Research Triangle Park, NC, to an-
nounce 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
Upper furnace injection of calcium
based sorbents to adsorb SOa has been
studied extensively in recent years. A
complete understanding of the funda-
mental physical and chemical processes
which occur at furnace temperatures has
not, however, been achieved. Previous
studies of the sulfation process and well
established theories have allowed the
rates of external mass transfer, pore
diffusion, and product layer diffusion to
be calculated. These processes coupled
with the chemical reaction, the unknown
physical structure of a calcined sorbent,
and the extremely high rate of the overall
process (the reaction is primarily over
within a few hundred milliseconds) make
it difficult to determine which of these
mechanisms controls the extent of
sorbent utilization. Other studies have
determined the physical structure of
calcined sorbent but only at times much
longer than those available in full scale
utility boilers. This program was designed
to determine the physical structure of
calcined sorbents at these early times so
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that the mechanisms that limit sorbent
utilization can be understood and higher
levels of S02 capture achieved.
Experimental Facilities
Fundamental experiments were con-
ducted in three research facilities: the
Isothermal Reactor (ITR); the Short Time
Reactor (SIR); and the Controlled
Temperature Tower (CTT). Firing rates in
these three facilities range from 9.4 kW
for the ITR to 26.4 kW for the CTT.
The ITR is an electrically heated
drop-tube furnace down-fired by a
premixed flat-flame burner. Sampling
residence times normally range from 100
to 500 ms with isothermal operating
temperatures of 800-1500°C. The STR
is a back-fired furnace which operates
isothermally at temperatures of 930-
1370°C with residence times of 35-400
ms. The STR can operate either
isothermally or under quenched condi-
tions and can fire coal as well as
gaseous fuels. The CTT is a back-fired
reactor designed to simulate the time-
temperature histories of a wide variety of
coal-fired boilers. The CTT can also fire
either gas or coal.
The ITR was used to obtain data on
SC>2 capture as a function of time,
temperature, and sorbent type. This
information was supplemented with data
obtained from the CTT on the influence
of quench rate on S02 capture. To
determine the in-situ physical structure
of calcined sorbent, solids were sampled
from the STR as a function of time,
temperature, SC>2 concentration and
sorbent type. A CO flame was used
during solid sampling to allow sorbent to
be obtained without the presence of H20
in the sampling system while still
providing a sulfation environment similar
to that in a coal or natural gas fired
furnace. Surface area, porosity, and pore
size distribution were determined for
sorbent collected from the STR.
Modeling
The complexity of the sulfation
reaction requires that models of the
sulfation process be used to define which
process controls the extent of sorbent
utilization. Four sulfation modeling
approaches were considered in this
program: a grain model, a pore tree
model, and two distributed pore models.
Each model assumes that sorbent
particles are spherical and fully calcined
prior to the onset of sulfation. In addition,
an activation model of the calcination
reaction based on the grain model was
utilized.
The activation model was developed to
simulate simultaneous calcination and
surface area loss. It considers CaCOa
decomposition at the CaO/CaCOs inter-
face, diffusion of CC>2 through the CaO to
the particle surface, diffusion of CC>2
from the particle surface to the bulk gas,
and continuous, finite rate surface area
loss for the calcined material (sintering).
The calcination process is represented
by a spherical, shrinking core model with
the intrinsic calcination rate dependent
only on the chemical rate. The sintering
rate is assumed to depend on CC>2
concentration and surface area.
All of the sulfation models include the
following sequential mechanisms: exter-
nal mass transfer; pore diffusion; solid
state diffusion; heterogeneous chemical
reaction; and product layer buildup on
the internal surfaces. Each model, how-
ever, view the physical structure of the
calcine differently.
The grain model treats the CaO
particle as an agglomerate of CaO grains
whose distribution of sizes is set to
match the measured BET surface area.
The pore tree model describes the
sorbent pore structure as a set of trees of
various sizes whose trunks are located at
the particle surface. The size distribution
of the pores is proportional to 1/rp3,
where rp is the pore radius. Two versions
of the distributed pore model were
considered: one viewed the pore
structure as being made up of an
interconnected network of pores, and the
other viewed the pore structure as having
non-intersecting pores. Both distributed
pore models use experimentally deter-
mined pore size distributions.
Results
Influence of Temperature
Capture of SOa for sorbents sulfated in
the ITR and for the Linwood hydroxide
sulfated in the STR fired with a CO flame
shows that a maximum in SO2 capture
occurs between 1100 and 12008C. The
activation and grain models indicate that
the surface area of a sorbent will
increase to a maximum immediately
upon calcination and then rapidly de-
crease to an equilibrium value within a
few tenths of a second. The surface area
of the Linwood hydroxide sampled from
the STR, both with and without SO2
present, does not show this. The surface
area of these samples did not
significantly vary with either time or
temperature. The measured porosity of
these solids, plotted in Figure 1, also did
not vary with time or temperature and are
much less than the porosity theoretical!
possible from Ca(OH)2 (0.49).
The porosity of the calcined sorbei
sampled from the STR with S02 preset
has, however, been reduced from il
original value by the buildup of th
CaS04 product layer within the por
structure. These original porosities, whic
can be calculated when the extents <
sorbent utilization are known, are plotte
in Figure 2. A maximum in calculate
original porosity is observed betwee
1100 and 1200°C which is nearly equ;
to the theoretical porosity of Ca(OH)2. /
lower and higher temperatures th
calculated original porosities were les
than the theoretical value. A compariso
of these calculated original porosities t
the measured porosities without SO
present (Figure 1) reveals that when SO
is present the extent of porosity loss fror
the theoretical porosity is either reduce
or prevented.
This dependency of the calculate
original porosity on temperature and th
presence of S02 allows a new hypothesi
to be proposed:
The process which causes this porosit
loss (sintering) occurs at approximatel
the same temperature as the sulfatio
reaction. Porosity loss and sulfatio
may therefore be viewed as processe
which compete for available sulfatio
sites. When the mechanism whic
causes porosity loss is fast compare
to sulfation, the loss in porosity result
in low sorbent utilization. Whe
sulfation is fast, porosity loss does nc
occur and greater levels of sorber
utilization result.
The dependence of porosity loss o
temperature could be caused by either c
two scenarios. At low temperature
(~970°C) the sulfation reaction is sky
due to kinetic and diffusional limitation;
and a large loss in porosity occurs. /
intermediate temperatures (~1180°C
the sulfation reaction is fast enough t
prevent porosity losses. At highe
temperatures (>1200°C) the increasin
rate of CaS04 decomposition reactio
results in a net decrease in the rate of th
overall sulfation reaction, and porositie
again drop.
The most likely mechanism respor
sible for this porosity loss is eithe
thermal or C02 activated sintering. EPA'
B. K. Gullett conducted a similar set c
experiments in an electrically heate
nitrogen (CO2 free) flow furnace c
1000°C. The measured porosities (with-
out SO2) and the calculated origin*
porosities (with S02) showed n
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1*
I!
oi o
!«•
x
5
0.25
0.2
0.1
0
0.2
0.1
0
0.2
0.1
•o-
970°C
J270°C
Linwood
CO Flame
Q w/o SO2
_ I
0.1
0.2 0.3
Time (sec)
0.4
0.5
Figure 1. Measured porosities of sorbent sampled from the STR.
significant drop in porosity from the
theoretical porosity of a calcined hy-
droxide within the first 500 ms. Porosity
losses therefore occur only in the
presence of COg, indicating that CCV
activated sintering is responsible for
porosities less than the theoretical value
(at the low temperature range).
The second scenario describing the
dependence of porosity loss on temper-
ature is that at the lower temperatures
(970 °C) the high rate of COa-activated
sintering is responsible for porosity
losses. At the intermediate temperatures
(~11808C) the rate of COa-activated
sintering slows, and porosity losses no
'•>nger occur. At higher temperature
inge, porosity loss is caused by an
increasing rate of thermal sintering rather
than a decreasing rate of sulfation. Data
are not currently available to determine
which hypotheses is correct.
A modeling approach was developed
based on the data obtained in this study.
Pore size distributions from the earliest
time sampled and an average calculated
porosity at each temperature were used
as inputs for the interconnected
distributed pore model. The results,
plotted as solid lines in Figure 3, match
the data extremely well. To predict SOa
capture at the highest temperature
(1270°C), the equilibrium concentration
of 802 above CaSC^ had to be included
in the model, as indicated by the dashed
line.
Influence of Mixing Rate
Mixing of sorbent is known to influence
the extent of 862 capture. A venturi
throat in the ITR provides a completely
mixed stream within 7-10 ms, while a
straight throat doesn't provide complete
mixing until 24-40 ms. Use of the faster
mixing throat results in significantly
increased levels of SOa capture. It was
concluded that slower mixing in an
isothermal furnace affects 862 capture
by:
(1) Delaying the onset of sulfation.
(2) Exposing the sorbent to a distribution
of temperatures. When mixing is slow,
eddies within the sorbent/air jets may
obtain temperatures between the jets'
initial temperature and the furnace
temperature. While at these intermediate
temperatures the sorbent will calcine and
obtain an original porosity (see Figure 2)
different than sorbent which calcines at
the furnace temperature. The distribution
of original porosities will result in a lower
average porosity and lower average
sorbent reactivity.
(3) Allowing incomplete mixing to occur.
Mixing which is not complete at the end
of a furnace may result in eddies with
high levels of sorbent (and depleted of
862) and eddies containing low levels of
sorbent (and high levels of
The ITR fitted with the venturi throat has
extremely rapid mixing and none of these
phenomena are likely to influence SOa
capture. When fitted with the straight
throat, factors (1) and (2) above affect
capture.
Influence of Quench Rate
SOa capture at higher quench rates
result from two mechanisms:
(1) The time available at temperatures
where sulfation occurs is reduced.
(2) A finite mixing time exposes the
sorbent to a distribution of temperatures.
Calcination occurs at a range of
temperatures, and a distribution of
original porosities results. The lower
average sorbent porosity leads to lower
sorbent reactivity and lower 802 capture.
This effect also results in a shift in the
temperature where the maximum 802
capture occurs.
When this second mechanism was
included in the interconnected distributed
pore model, it is able to account for the
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0.5
0.4
0.3
I
0.2
0.1
™ Theoretical Porosity
Calculated
Original
Porosity
w/SOz
Measured
Porosity
w/o SO2
-318-369 ms
I
900 1000 1100 1200
Temperature (°C)
Figure 2. Measured porosities and calculated original porosities.
1300
1400
differences in SOg capture due to
quench rate.
Influence of Sorbent Type
Ca(OH>2 from different commercial
sources are known to vary in their ability
to capture 802- Capture by a Mississippi
hydroxide in the STR is significantly less
than capture by the Linwood hydroxide.
The calculated original porosities for the
Mississippi hydroxide, plotted in Figure
4, are less for those than for the Linwood
hydroxide. Interconnected distributed
pore model predictions indicate that the
differences in capture by these two
hydroxides result from the slightly larger
particle size of the Mississippi Ca(OH)2
and from the greater porosity loss it
experiences.
The difference in S02 capture between
carbonates and hydroxides was investi-
gated in the STR with a CO flame. It has
been hypothesized that lower levels of
capture by carbonates result from their
large particle size. Calculation of their
original porosities, plotted in Figure 5,
reveals that carbonates are subject to
greater losses in porosity than
hydroxides. Predictions by the inter-
connected distributed pore model
indicate that lower SC-2 capture by
carbonates results (approximately)
equally from their larger particle size and
their greater porosity loss. Analysis of
the porosity data also indicates that
those portions of the carbonates which
calcine most rapidly have low original
porosities while those fractions calcining
more slowly have the theoretical porosity
of a calcined calcium carbonate.
Influence of Physical Structure
Average pore diameters for the
Linwood hydroxide sampled from the
STR without S02 present, varied as a
function of temperature but not as a
function of time. Pore size distributio
corrected to a constant porosity a
plotted in Figure 6 ofor the Linwoi
hydroxide. Above 80 A, the distributio
are about the same while at below 80
the distributions are scattered.
determine the extent that the:
differences make in SOa capture, tl
pore size distributions in Figure 6 we
entered in the interconnected distribut
pore model. The results of the;
calculations, presented in Figure
indicate that the observed differences
pore size distributions have litt
influence on S02 capture. To furth
determine the influence of pore si,
distribution on sulfation pore si.
distributions from the Linwood sorbe
sampled at 970°C and 1270°C, tl
Vicron 45-3 carbonate, the Mississip
hydroxide, and the Fredonia hydroxi*
sampled from a nitrogen reactor
1000°C were corrected to a consta
porosity and used as inputs for tl
model. The predictions exhibit no mo
than 6% variation in SC>2 capture. Tl
variations in sorbent pore size distribute
resulting from different sulfatic
conditions therefore have no significa
effect on SC>2 capture.
Conclusions
The goal of this program was
determine the mechanism(s) which tin
calcium utilization. This was accor
plished by investigating the relationsh
between sorbent physical structure ai
various sulfation parameters. In-si
sorbent physical structure was dete
mined by sampling from the Short Tin
Reactor (STR), an isothermal react
fired with a CO flame. The use of a C
flame allowed sorbent to be sampled
the absence of H2O which was shown
degrade calcined sorbent durir
sampling.
Investigation of the influence
injection temperature on SO2 capture ai
sorbent physical structure revealed th
calcination of sorbent in a combustic
environment resulted in porositi<
dramatically less than theoretical
possible - 0.49 for Ca(OH)2. Tt
presence of S02 was found to reduce tt
extent of or completely prevent porosi
loss, depending on the injectic
temperature. The cause of this porosi
loss was determined to be CO
activated sintering. A hypothesis wj
proposed based on this information:
Porosity loss and sulfation may I
viewed as processes which compe
for available sulfation sites. When tl
mechanism which causes porosity lo
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60
50
3 40
I
Cj
30
20
10
Linwood
CO Flame
Ca/S = 2
t t
-1400 ppmS02
1180°C
Equilibrium Concentration
of SOi Above CaSOt
0.0 0.05
0.10 0.15 0.20 0.25 0.30
Sorbent Residence Time (sec)
0.35
0.40
cining at longer times obtained the
theoretical porosity of a carbonate (0.54).
Modeling indicated that, although
average pore diameters varied as a
function of temperature (but not as a
function of time), the observed variations
in pore size distribution had little effect
on SO2 capture when corrected to a
constant porosity. The use of pore size
distributions from carbonates and from a
sorbent sampled from a nitrogen
environment did not change the
predicted levels of SOa capture.
Figure 3. Interconnected distributed pore model predictions based on calculated original
porosities.
is fast compared to sulfation (at low
temperatures, ~970°C), the loss in
porosity results in low sorbent
utilization. When sulfation is fast
(~1180°C), porosity loss does not
occur and greater levels of sorbent
utilization result. At higher tempera-
tures (>1200°C) the increasing rate of
CaS04 decomposition reaction results
in a net decrease in the rate of the
overall sulfation reaction and porosities
again drop.
It was also noted that, at the higher
temperature range porosity losses may
result from an increased rate of thermal
sintering rather than a decrease in the
rate of the sulfation reaction. A
distributed pore model, which used the
measured pore size distributions and
these porosities, was able to adequately
predict the observed SO2 capture
without the use of adjustable parameters.
Mixing was found to influence BO2
capture by (1) delaying the sulfation
reaction, and (2) altering the extent of
porosity loss by changing the sorbent
thermal history. In an isothermal
environment with slow mixing, transient
temperatures (between a sorbent's initial
temperature and the ambient temper-
ature) may occur resulting in a porosity
loss not directly related to the injection
temperature.
In a quenched environment, mixing
which occurs over a period of time will
expose the sorbent to a distribution of
temperatures and a resulting distribution
of porosity losses. This results in (1) an
average porosity different from the
isothermal case (for a given injection
temperature) and a resulting different
capture and (2) a shift in the temperature
where maximum SOg capture occurs.
The extent of porosity loss and sorbent
particle size were found to be the
primary factors which determine sorbent
reactivity. Different hydroxides experi-
enced significantly different levels of
porosity loss, and carbonates suffered
dramatically greater losses in porosity
than did hydroxides. The degree of
porosity loss in carbonates varied with
time. Those portions of a carbonate
which calcined under 35 ms had a
porosity equal to that of a carbonate
calcined without SC>2 present (the lowest
porosity possible), while portions cal-
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0.5
0.4
0.3
0.2
0.1
0:
Theoretical Porosity
Linwood
ft 1BO°C)
•o—.
<1160°C>
1400ppmS02
CO Flame
0.1
0.2
Time (sec)
0.3
0.4
Figure 4. Calculated original porosities of two hydroxides.
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0.6
0.5
0.4
g 0.3
§.
0.2
0.1
0.0
0
III
Theoretical Porosity
Projected
fj Calculated
-f^ Original (w/SOz
A£ N r
|£ £ Measured (w/o SOii
Vicron 45-3 (1 1 urn)
III
0 0.1 0.2 0.3
Time (sec)
•
•
•
•
• • i
Theoretical Porosity
["] Projected
_ JJ*
• ^VO Calculated
Cj Original (w/SOz
' ^ t^N
Measured (w/o SOi)
7fjm Vicron
ill
0.4 0.1 0.2 0.3
Time (sec)
0.4
Figure 5. Measured porosities of solids sampled without SO2 and calculated original
porosities of solids sampled with SOi corrected for extent of calcination. The
projected original porosities were calculated by assuming that the uncalcined
portion of the collected sorbent would have a porosity of 0.54 upon complete
calcination.
15
W
10
t= 35ms
t= 69ms
t=110ms
t=369ms
I I I I
Linwood
1180°C
I I I I I I I II
i I I I I I I
100
Pore Diameter (A)
1000
Figure 6. Pore size distributions corrected to a constant porosity.
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Linwood, STR
CO Flame. 1180°C
1400 ppmSOz
Ca/S = 2
0.2
Time (sec)
0.3
0.4
Figure 7. Predictions of the interconnected distributed pore model based on pore size
distributions from Figure 6.
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G. H. Newton, D. K. Moyeda, G. Kindt, J. M. McCarthy, S. L. Chen, J. A. Cote, „„«
J. C. Kramlich are with Energy and Environmental Research Corp., Irvine, QA
92718-2798.
Brian K. Gullett is the EPA Project Officer (see below).
The complete report, entitled "Fundamental Studies of Dry Injection of Calcium-
Based Sorbents for SO2 Control in Utility Boilers," (Order No. PB 89-134
1421 AS; Cost: $36.95 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 Center for Environmental Research
Environmental Protection Information
Agency Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S2-88/069
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