United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-87/014 July 1987
&EPA Project Summary
Status and Evaluation of
Calcitic S02 Capture:
Analysis of Facilities
Performance
G. D. Silcox, S. L Chen, W. D. Clark, J. C. Kramlich, J. F. LaFond,
J. M. McCarthy, D. W. Pershing, and W. R. Seeker
This study was initiated to character-
ize the current state of knowledge
regarding SO2 capture by dry calcitic
sorbent injection. A key difficulty in
reviewing the literature on the subject
is the apparent diversity in the exper-
imental results, caused by the extreme
complexity of simulating the practical
sorbent utilization process. Variables
include sorbent properties, furnace
experimental parameters, and furnace
equipment (e.g., injectors, sampling
system). Failure to control or report all
possible variables in each experiment
produces a diversity of results.
In this project, the experimental data
on dry sorbent injection are compiled
and critically compared. Sulfation and
activation models are developed and
used to identify sorbent properties and
furnace environment parameters likely
to be of importance. The results of the
examination of data and the model
evaluation indicated areas where orig-
inal experimental work could be applied
to resolve key issues. The experimental
portion of the program developed
information on injection temperature,
quench rate, and sorbent properties.
These results are used to define the
overall optimum injection condition for
a variety of furnace configurations.
This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering Research Laboratory, Research
Triangle 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
This report summarizes the theoretical
and experimental studies conducted by
EER as part of the Combustion System
Similarity Criteria Modeling Study.
Objectives of the study were:
• To compile and compare existing data
on dry sorbent injection for SO2
control as a function of experimental
scale, sorbent type, injection condi-
tions, etc.
• To identify differences in experimental
results and, where possible, to rec-
oncile these discrepancies in terms of
experimental system characteristics.
• To develop overall sulfation models
which include consideration of the
physical and chemical processes likely
to be of major importance in upper
furnace injection of dry calcium-based
sorbents and to validate these models
using existing fundamental data.
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• To use the overall sulfation models in
conjunction with the experimental
data to define the process fundamen-
tals, establish combustion parameters
critical to the overall optimization of
sorbent injection, and define the
optimum sorbent injection conditions.
Thus, the overall purpose of the program
was to characterize the current state of
knowledge regarding S02 capture by dry
sorbent injection by carefully analyzing
the existing experimental data base in
conjunction with appropriate mathemat-
ical models and newly acquired exper-
imental results.
Modeling Approach
The first step in the sorbent injection
process is calcination of the injected
hydrate or carbonate to form porous CaO.
An activation model includes consider-
ation of calcium carbonate decomposi-
tion at the CaO/CaCOa interface, diffu-
sion of CO2 through the CaO to the
particle surface, diffusion of CO2from the
particle surface to the bulk gas, and
continuous finite-rate surface area loss
for all calcined material. The calcination
process is represented by a spherical
shrinking-cjore model with the intrinsic
calcination rate dependent only on the
chemical rate.
Two sulfation models were used
extensively on this program: a grain
model and a distributed pore model. Both
approaches assume that the reactive
particles are spherical and fully calcined
to CaO prior to the onset of sulfation.
Activation
The activation step is of major impor-
tance to the overall sulfation process
because it controls the physical charac-
teristics of the sorbent at the onset of
sulfation; in particular, the availablesur-
f ace area and porosity. Three phenomena
appear to be of major significance in the
overall activation process: (1) the calci-
nation rate; (2) the local development of
high surface area associated with metas-
table CaO; and (3) the sintering of
previously formed CaO catalyzed by HzO,
CO2, and probably sulfate ions with its
associated grain (pore) growth and
porosity loss. Hydrates and carbonates
are found to have markedly different
calcination rates. Coupled with this is the
concurrent sintering or desurfacing of
the sorbent, which.also proceeds at a
rapid rate.
Figure 1 shows activation model
predictions on surface area development
and loss for typical hydrate and carbonate
100
I
40
20 -
0.0001
Figure 1. Activation model predictions.
particles at 1100°C. These data clearly
indicate why the activation process can
have a strong influence on the overall
extent of calcium utilization. The hydrate
particles calcine so rapidly that the rate
of calcination and surface area develop-
ment far exceed the rate of sintering;
hence, there exists regions of very high
surface area. In contrast, the 5/jm
carbonate particle calcines much slower,
and the sintering front can more closely
follow the calcination front through the
particle. At higher temperatures with
even larger carbonate particles, the
model predicts a simple steady rise in
surface area because sintering follows
calcination completely.
Sulfation
Figure 2 summarizes time-resolved
sulfation data for sized, precalcined
sorbents prepared from both hydrates
and carbonates. These data characterize
the sulfation process specifically
because sulfation has been decoupled
from calcination by using precalcined
materials. Initial sulfation is very rapid;
most of .the calcium is utilized in the first
200 ms. Subsequently the process slows
dramatically, and further sulfation is
relatively limited. Thus, some chemical
or physical mechanism rapidly reduces
the calcium availability or accessibility
and limits the overall SO2 capture. The
solid line shows the distributed pore
model predictions for the four different
size particles. In each case the measured
pore size distribution of the injected
material was used to define the pore size
distribution in the model. No parameters
were adjusted to improve the agreement
between the model and the experimental
data. These results suggest that the
distributed pore model can predict the
form and the level of the time-resolved
SOzcapture data. Detailed analysis of the
model predictions indicates that, the
slowing in the overall sulfation rate at
times beyond about 200 ms, the overajl
sulfation rate slows because of a
decrease in both calcium availability
(resulting from catalyzed thermal sinter-
ing) and calcium accessibility (resulting
from mouth closure of the smallest
pores).
Influence of Process
Parameters
Temperature has a strong influence on
calcination, thermal sintering, pore and
product layer diffusion, and probably the
intrinsic chemical reaction. Therefore,
influence of both sorbent injection
temperature and downstream thermal
profile have been studied extensively in
previous programs. The observed capture
levels (at a Ca/S molar ratio of 2 foi
hydrated lime) range from 25 to 65%, anc
optimum injection temperature appears
to be about 1120°C. An absolutely critica
point in resolving the optimum injectior
temperature from the various experi-
ments is evaluating the peak tempera
ture experienced by the sorbent. Foi
example, in some experiments maximurr
utilization was obtained at 1260°C
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20
10
c
o
2 concentration.
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60
SO
40
©
£
a 30
Q.
O
CO
20
10
— Open Symbols: Linwood AH
Solid Symbols: Vicron
Figure 3.
1800'
2000'
I
2200
I
2400
2600
r;oo
7300
/400
7500
Comparison of the impact of quench rate on sorbent infection temperature for
Linwood hydrate and Vicron carbonate.
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60
50
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G. D. Silcox, S. L Chen, W. D. Clark. J. C. Kramlich, J. F. LaFond. J. M. h,,^*,.,,,,
D. W. Pershing, and W. R. Seeker are with Energy and Environmental Research
Corporation, Irvine, CA 92718-2798.
Charles C. Masser is the EPA Project Officer (see below).
The complete report entitled "Status and Evaluation of Calcitic SOZ Capture:
Analysis of Facilities Performance," (Order No. PB 87-194 783/AS; Cost:
$30.95, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
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 45268
Official Business
Penalty for Private Use $300
EPA/600/S7-87/014
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