United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-81 -154 Dec. 1981
Project Summary
Study of Automatic Control
Systems to Maintain Constant
Percentage SCh Retention in a
Pressurized FBC
K. J. Daniel, S. D. Finnigan, and R. M. Reinstrom
The Clean Air Act Amendments of
1977 indicate that future emission
standards for SO2 should be based on
a percentage reduction (comparing
sulfur emissions with sulfur feed). In a
pressurized fluidized-bed (PFB) boiler,
sulfur feed (as determined by coal
sulfur content and feed rate) and
sulfur removal effectiveness (as deter-
mined by the reactivity and feed rate
of dolomite/limestone sorbent) vary
continually during PFB plant opera-
tion. The purpose of this study was to
assess the feasibility of using some
type of automatic control system to
maintain a constant percentage sulfur
removal in a PFB system as variations
occurred in key variables, such as coal
sulfur content and sorbent reactivity.
To conduct this feasibility study, a
transient model of a PFB power plant
was developed and validated for
studying methods of controlling bed
SO2 absorption characteristics. To
accomplish this, the transient equa-
tion for the population distribution as
a function of size and utilization was
solved. The model uses TGA rate data
for 1337 dolomite as a function of
size, utilization, and temperature. The
kinetic data is integrated over the
instantaneous population distribution
to determine the instantaneous SO2
absorption rate constant.
This transient model was used to
assess the potential of alternative
automatic control strategies for
achieving constant percentage SO2
retention during changes in load, in
coal sulfur content, and in sorbent
reactivity in a PFB. Goals were to
minimize costs and sorbent require-
ments. Of the control strategies
considered, the preferred option iden-
tified in this assessment continuously
monitored the sulfur content in the
feed coal, and adjusted the sorbent
feed rate to maintain a constant
sorbent-to-coal-sulfur feed ratio. This
strategy does not instantaneously
change this sorbent-to-sulfur ratio to
account for short-duration changes in
sorbent reactivity, since the inertia of
the large mass of partially spent
sorbent in the bed prevents instanta-
neous changes in the sorbent/sulfur
ratio from being effective. However,
long-term variations in reactivity are
accommodated through a feedback
loop which measures retention and
adjusts the sorbent-to-sulfur set point
accordingly. Such a control strategy
minimizes, but does not eliminate, the
need for excess sorbent feed to ensure
that the PFB does not exceed the
specified percentage SO2 reduction
on a 30-day average.
A prompt neutron activation tech-
nique appears to be the most promis-
ing for the required continuous
monitoring of coal sulfur content.
However, such a system is develop-
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mental, and has not been commercial-
ly demonstrated.
This Project Summary was develop-
ed by EPA's industrial Environmental
Research Laboratory. Research Tri-
angle Park, NC. to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
The Clean Air Act Amendments of
1977 indicate that the future emission
standards for S02 should be based on a
percentage reduction. In a conventional
power plant using flue gas desulfunza-
tion (FGD), percentage reduction of
sulfur can, in principle, easily be moni-
tored and controlled. Sulfur dioxide
(SOi) concentration can readily be
measured before and after the FGD unit
to determine the percentage of SOz
absorbed by the process. Because no
components have inherently slow
response, control of the system is rela-
tively straightforward.
In the future, power plants using a
fluidized bed will encounter a different
situation. Rather than monitor only
gas concentrations to determine per-
centage reduction, coal sulfur content
and feed rate will need to be monitored.
Moreover, the mass of absorbent in a
bed represents a large inertia that must
be controlled. Changes in the reactivity
of the sorbent feed also will affect the
control system.
This system examined the control of
sulfur capture in fluidized beds with
emphasis on pressurized fluidized beds
(PFBs). Specifically, the study examined
how changes in load and coal sulfur
content affect sulfur capture in a PFB
combustor. In addition, various methods
of controlling bed sulfur absorption
properties that minimize sorbent usage
and maintain a constant percent sulfur
removal were examined.
The study involved developing a
transient model for the PFB power plant,
validating the model by comparison
with experimental data, and using the
model to evaluate various control
strategies. Emphasis was on phenom-
enological modeling of the sulfur
capture processes in the PFB. However,
the model also includes representations
of all major equipment (e.g., gas and
steam turbines) in the PFB/combined-
cycle power plant.
The transient rate of sulfur capture is
determined from the transient distribu-
tion of dolomite particles in the PFB, as a
function of size and utilization. The
model solves for this transient distribu-
tion by integrating the distribution with
chemical rate constants (also a function
of size and utilization); the instanta-
neous rate of sulfur adsorption is
obtained. The model was validated
using both transient and steady-state
data from the experimental PFB Mini-
plant (Reference 1); the comparison of
the model and the data indicates good
agreement.
Results
Several interesting results were
obtained from the model. The response
of sulfur retention to a change in a Ca/S
ratio shows a strong dependence on the
magnitude and direction of change.
However, the response is essentially
independent of whether the change is
made in the calcium feed rate or the
sulfur feed rate.
Also, in spite of rapid changes in
sulfur input to the bed, the change in
retention was very slow. Retention
changes slowly because the rate of the
sulfur capture reaction in the bed isfirst
order with respect to SOa concentration
and also because the inventory of
dolomite in the bed represents a large
inertia.
Four candidate strategies to control
bed retention were identified and
evaluated. The first was to change the
size distribution of the absorbent feed to
the bed. Because smaller particles have
higher reactivity, the amount of sulfur
absorbed by the bed can be controlled by
changing the size distribution of the
bed. However, because of the large
amount of mass in the bed, the size
distribution of the bed changes very
slowly. Consequently, the response
with this method of control is too slow to
be practical. Moreover, reducing the
size distribution of the feed could cause
additional elutriation which would limit
the maximum excursion that could be
handled by this method of control. This
type of control would also not be able to
take advantage of downward excur-
sions in coal sulfur content. This
method of control, thus, had several
disadvantages and no clear advantage.
The second strategy for control
studied was to adjust the reactivity of
the absorbent fed to the bed. This
method would also have a slow
response due to the large bed inventory.
In addition, if a highly reactive dolomite
were being used to achieve low steady-
state Ca/S ratio, there would be little
control margin to accommodate
transient excursions. Moreover, this
system could not take advantage of
downward excursions of coal sulfur
content. Because of these shortcom-
ings, this method is not recommended.
The third strategy investigated con-
sidered blending lowsulfurfuel with the
coal feed to maintain a constant rate of
sulfur feed to the bed. This system will
ensure nearly constant percent sulfur
retention if the reactivity of the dolomite
is constant. In addition, this system
minimizes the excess dolomite feed
required. There are no technical bar-
riers to implementing this control
system; however, it is prohibitively
expensive when compared to a system
that uses a sufficient excess of dolomite
feed to ensure that emission limits are
met.
The fourth strategy investigated is the
preferred method. This method
maintained the dolomite feed rate in
proportion to the rate of sulfur entering
the bed. By exercising the model it was
found that 862 retention can be con-
trolled within 1-1/2 percentage points
by this method. Control of the dolomite-
to-sulfur ratio in this manner would not,
by itself, make the necessary adjust-
ments if sorbent reactivity were to vary;
accordingly, a feedback loop is included
to measure retention and adjust the
dolomite-to-sulfur set point as neces-
sary to accommodate long-term
variations in reactivity. Efforts to
instantaneously adjust the dolomite-to-
sulfur ratio, to follow short-term
changes in sorbent reactivity, would not
be effective, since the inertia of the
large inventory of partially spent
sorbent in the bed would prevent
instantaneous changes in the
dolomite/sulfur ratio from having a
significant impact.
This strategy reduces excess dolomite
use to a minimum and shows a signifi-
cant potential cost savings compared to
a system using a constant excess dolo-
mite feed to ensure compliance with
emission limits. One possible disadvan-
tage of this system is that elutriated
fines may increase due to increased
dolomite feed. While available data does
not indicate that this will be a significant
disadvantage, further study is recom-
mended.
Because the preferred strategy
requires measurement of the sulfur
content of the coal feed to the bed, a
review was conducted of on-line coal
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sulfur measurement techniques. This
review indicated that, although many
techniques are available, the one that
appears most suitable is prompt
neutron activation analysis of the coal
feed. This measurement technique is
still being developed. The added cost of
this device was included in the econom-
ic analysis.
The methods identified for maintain-
ing constant percentage sulfur capture
are applicable for use with atmospheric
fluidized-bed (AFB) combustors without
fines recycle, and may be applicable to
AFB systems that incorporate fines
recycle. However, recycle of fines is not
incorporated in the model and,
therefore, no conclusion can be drawn
regarding the effects of fines recycle on
the results presented in this study.
Recommendations
To determine the economic attrac-
tiveness of the proposed control
method, the typical time-dependent
sulfur variations in coal feed must be
defined. Specifically, the size of excur-
sion, the duration of excursion, the
ramp rate of change, or frequency
spectrum must be determined. This will
aid in determining the margin in
dolomite feed rate that is required to
control variations in sulfur emissions.
More accurate economic trade-offs can
then be obtained.
The recommended control strategy
requires that the dolomite-to-sulfur
ratio remain constant. This implies that'
the dolomite feed be increased and de-
creased in response to changes in coal
sulfur content. To reduce this method to
practice, experiments should be per-
formed to determine the effect of
changing feed rate on attrition and
elutriation.
Moreover, the model should be
extended to study advanced AFB con-
cepts that rely on the recycle of elutri-
ated fines to the bed to significantly
improve sulfur capture. The extension
of the model could aid in the evaluation
of various effects and options such as
attrition rates, recycle rates, cyclone
capture efficiency, freeboard height,
coal sulfur contents, and load control.
References
1. Hoke, R. C., et al., "Mmiplant and
Bench Studies of Pressurized
Fluidized-Bed Coal Combustion:
Final Report," Exxon Research
and Engineering Co. (EPA-600/7-
80-013, NTIS No. PB 80-188121),
January 1980.
K. J. Daniel. S. D. Finnigan, and R. M. Reinstrom are with General Electric Co.,
1 River Road.- Schenectady, NY 12345.
D. Bruce Henschel is the EPA Project Officer (see below).
The complete report, entitled "Study of Automatic Control Systems to Maintain
Constant Percentage SO 2 Retention in a Pressurized FBC," (Order No.
PB 82-110 693; Cost: $15.00, 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:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
*
U S GOVERNMENT PRINTING OFFICE, 1981 — 559-017/7405
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