United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-86/040 Mar. 1987
&EPA Project Summary
Reactivity Study of S02 Control
with Atmospheric and Pressure
Hydrated Sorbents
B. J. Overmoe, J. M. McCarthy, S. L Chen, W. R. Seeker, and D. W. Pershing
The purpose of this study was to
develop an understanding of the factors
that control the reactivity of hydrated
sorbents toward S02 in coal fired
furnaces. The study focused on the
impacts of hydrate properties (e.g.,
particle size, surface area, and chemical
composition) and the furnace tempera-
ture of the injection location. A bench
scale hydrator was used to produce
atmospheric and pressure hydrated
'sorbents, with parameters pertinent to
the hydration process varied. The
chemical and physical properties were
characterized for these and several
commercially available hydrates, and
they were tested for SO2 sorption re-
activity on bench (17.6 kW) and pilot
scale (300 kW) furnaces.
The results of this study indicate that
pressure hydrates generated under well
controlled conditions were more re-
active than commercially produced
atmospheric hydrates. The important
production and operating parameters
for the pressure hydration process in-
cluded the size and composition of the
quicklime, the hydration temperature
and pressure, the rate of water addition,
and the pressure progression during
discharge. Comparison of commercial
atmospheric hydrates showed signifi-
cant differences in reactivity, ranging
from 40 to 58 percent at a calcium to
sulfur molar ratio of 2. All hydrates,
atmospheric and pressure, exhibited a
strong dependence on injection
temperature.
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 docu-
mented In a separate report of the same
title (see Project Report ordering in-
formation at back).
Introduction
The purpose of this study was to
develop an understanding of the factors
controlling the reactivity of hydrates to
capture S02 when injected into a coal
fired furnace. The approach taken was to
generate hydrates, both atmospheric and
pressure, under well controlled conditions
and compare their reactivity with that of
the corresponding quicklimes and parent
limestones. A bench scale hydrator was
designed and constructed to produce
batches (about 9.07 kg each) of atmo-
spheric and pressure hydrated limes.
Parameters pertinent to the hydration
process were varied, and the hydrates
were characterized in terms of surface
area, particle size, and pore size distri-
butions. These prepared hydrated limes,
together with commercially available
hydrates, were tested for S02 absorption
reactivity on both the bench scale (17.6
kW) Control Temperature Tower (CTT) and
the pilot scale (300 kW) Boiler Simulator
Furnace (BSF).
Commercial Atmospheric
Hydrates
In order to determine the relative re-
activity of commercial atmospheric hy-
drates with SO2, several hydrates which
exhibited a variety of physical properties
and chemical compositions were screened
on the CTT. A natural gas flame doped
with H2S was used to obtain an SO2
-------
concentration of 2000 ppm (dry, 0 percent
02). This is similar to the SO2 levels found
in a coal fired system. Previous results
had shown that the sorbent capture
potential was not dependent on the fuel
used. The sorbent was injected at a gas
phase temperature of 1230°C, which has
been shown to be at or near the optimum
injection temperature for several calcium
hydrates. Data obtained for the com-
mercial sorbents at a molar calcium to
sulfur ratio (Ca/S) of 2 are shown in
Figure 1a, with Vicron, a limestone,
shown for comparison. The hydrates ex-
hibited a range of capture from 58 percent
for the Linwood hydrate to 40 percent for
the Colton hydrate. The performance of
all of the hydrates was significantly better
than that of Vicron.
The particle size distributions for the
sorbents are shown in Figure 1b. The
reduced particle size for hydrates relative
to limestone is evident. Although the
most reactive hydrate had the largest
percentage of particles less than 10 urn,
no direct correlation between reactivity
and particle size could be determined for
the atmospheric hydrates. Chemical ef-
fects may have contributed to the rela-
tively poor performances of the Kemikal
and Colton hydrates. Analysis showed
that Kemikal was not completely hydrated,
which is detrimental to sorbent reactivity.
The high level of silicon in Colton likely
led to the formation of calcium silicates,
which prevent reaction between calcium
and S02.
The sorbent injection temperature was
a major concern for the reactivity tests.
This temperature must be high enough to
promote rapid progression of the calcina-
tion and sulfation reactions, but excessive
temperatures could hinder the overall
process due to sorbent particle sintering
or hard-burning of the particle surface
prior to sulfation. The influence of in-
jection temperature on sulfur capture at
a Ca/S of 2 is shown in Figure 2. The
tests were performed on the BSF, with
10 percent of the total air used for sorbent
transport. The sorbent injection location
was moved down the length of the
furnace while a constant exhaust sample
location was maintained, thus the sorbent
had a longer residence time in the furnace
at higher injection temperatures. The data
exhibited a strong dependence on injec-
tion temperature, with an optimum of
1260 to 1315°C observed for all sorbents.
The disparity in capture between the dif-
ferent hydrates found at most tempera-
tures was negated when excessively high
injection temperatures were employed.
O)
I
2»
a
60
50
40
30
20
10
-
Linwood I
Mercer I
| Longview | j
CTT
| Kemikal
-------
c
I
Q.
ST
to
to
I
Q.
o
60
50
40
30
20
JO
BSF
90
. CTT
Figure 2.
1000 1100 1200 1300 1400 1500
Injection Temperature, °C
Influence of injection temperature.
1600
Reactivity Tests
As shown in Figure 2, the pressure
hydrate offered higher capture than any
of the commercial atmospheric hydrates,
and exhibited a similar dependence on
injection temperature. In Figure 3, data
obtained on the CTT at an injection
temperature of 1230°C with coal as the
fuel are presented. The pressure hydrate
(PH) performed significantly better than
the quicklime from which it was produced,
and also yielded consistently higher cap-
ture than the commercial atmospheric
hydrate (AH) obtained from the same
supplier. A consistent level of capture
was observed for pressure hydrates from
three facilities which used the same
conditions for hydration.
Hydration Impact on
Sorbent Properties
The influence of hydration on physical
properties of the quicklime is shown in
Figure 4. The pressure hydration process
results in a finer particle size, as shown
in Figure 4a, as well as a larger pore
volume, shown in Figure 4b Both of
these factors dimmish diffusion effects
and result in enhanced sorbent reactivity
The large peaks in Figure 4b at a pore
diameter greater than 01. Mm can be
attributed to interparticle voids rather
than actual pores
Pressure hydrates were more reactive
than quicklime or atmospheric hydrates
One apparent advantage of the pres-
sunzation process is that the quicklime
hydrates more readily than under atmo-
spheric pressure Commercial manufac-
turers generally recycle the atmospheric
hydrate or remove unwanted material to
attain a product hydrated in excess of 90
percent Pressure and atmospheric hydra-
tion of a Longview quicklime partially
hydrated by atmospheric moisture yielded
93 and 80 percent Ca(OH)2, respectively
The sorbents were screened for reactivity
on the CTT at an injection temperature of
1230°C with the results shown in the left
half of Figure 5 The different relative
amount of hydrate present could account
for the increased reactivity of the pressure
hydrate The right half of Figure 5 is a
80 -
70
o>
CJ
Q.
-------
100
SO
I 60
I
I 40
20
EER Lmwood PH
100 50 10 5 1
fa) Equivalent Spherical Diameter, nm
05
0.05 001
fb) Pore Diameter, fjm
0.005
0001
\\ve to quicklime Both of these factors
enhance the sulfation reaction by dimin-
ishing diffusion effects The reactivity of
pressure hydrates was dependent on
several hydration parameters which in-
cluded the quicklime particle size and
extent of hydration, the mixing rate of
quicklime with water, the discharge
method, and the process pressure,
temperature, and water stoichiometry A
deviation from the baseline condition for
any of these variables resulted in de-
creased reactivity Reactivity did not cor-
relate with hydrate surface area, but did
correlate with the surface area of the
calcined hydrate, with an increase in
reactivity associated with a higher surface
area Hydrated limes show considerable
promise for reduction of SO? from boilers,
with reductions in excess of 50 percent
possible
Figure 4. Influence of hydration on particle and pore size distributions
resulted in the more reactive sorbent due
to a correspondingly finer hydrate. The
slower mixing rate between water and
quicklime associated with a long water
addition time and non-isobanc discharge
of the hydrated product were also detri-
mental to sorbent reactivity. Analyses
showed that the more reactive sorbent
exhibited a finer particle size distribution.
While this is not the only property which
controls sorbent reactivity, it is a physical
property of primary importance
Conclusions
The results of this study supplied in-
formation on the reactivity of various
calcium based sorbents toward SO2 m a
coal fired furnace and determined factors
which influence the reactivity of calcium
hydrates Hydrated sorbents were judged
superior to limestone and quicklime for
the capture of SO2 from exhaust gases,
with the capture at a molar Ca/S of 2
ranging from 40 to 58 percent for a
series of commercial atmospheric hy-
drates. The variations in capture were
due to both chemical and physical dif-
ferences between the sorbents. A pres-
sure hydrated sorbent produced under
controlled conditions offered slightly
higher capture. The performance of hy-
drates was strongly dependent on the
gas phase temperature at the injection
location with an optimum of about
1260°C Hydrates showed a decreased
particle size and increased porosity rela-
4
-------
50
40
Q.
8
2 30
Q.
20
JO
EERPH
EERAH
Commercial AH
EERPH
Ca/S
Figure 5. Influence of hydration on reactivity (Longview).
-------
60
SO
o
to
30
20
10
i i
Coarse
Quicklime Size
i i
8Min.
30 /M/n.
Water Addition Time
i i
Isobsric
Non-lsobaric
Discharge Method
12 12
Ca/S
Figure 6. Impact of hydration process variables.
B. J. Overmoe, J. M. McCarthy, S. L. Chen, W. /?. Seeker, and D. W. Pershing
are with Energy and Environmental Research Corporation, Irvine, CA 92718-
2798.
Brian K. Gullett is the EPA Project Officer (see below).
The complete report, entitled "Reactivity Study of SO* Control with Atmospheric
and Pressure Hydrated Sorbents," (Order No. PB 87-129 250/AS; Cost:
$18.95, 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 Center for Environmental Research
Environmental Protection Information
Agency Cincinnati OH 45268
Official Business
Penalty for Private Use S300
EPA/600/S7-86/040
0000329
------- |