United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
.
//
Research and Development
EPA-600/S2-84-019 Mar. 1984
Project Summary
Limestone Scrubber Slurry
Automatic Control Systems
Patrick H. Garrett
In the case of environmental processes,
consistent reduction in the variance of
controlled variables with large through-
put processes is of particular interest.
Accordingly, this report utilizes current
understanding of limestone scrubbers
for flue gas desulfurization (FGD) to
develop an effort into the optimization of
automatic control for the recirculating
slurry processes. To this end the
acknowledged methods of mathematical
modeling, computer simulation, and
experimental proofing are applied to the
design of slurry limestone addition,
slurry density, and absorber liquid-to-
gas ratio control systems.
Three automatic control methods are
analyzed for dense limestone feedrate
to the recirculating slurry including
experimental results. Stoichiometric
control is based on material balance
considerations, but is compromised by
the lack of slurry reaction measurements.
Control of pH is geared to accommodate
variations in the slurry reactions, but
requires a high slurry reaction gain
(ApH/A limestone feedrate) for stable
and responsive control. Stoichiometric-
assisted pH control offers additional
process disorder reduction than either
Stoichiometric or pH control separately,
but its additional complexity is warranted
only under conditions of low slurry
reaction gain. Absorber liquid-to-gas
ratio control for minimizing scrubber
energy requirements while maintaining
an SOz exit target is also designed based
on a feedforward SO2 removal law and
slurry pump selection.
This Project Summary was developed
by EPA's Industrial Environmental Re-
search 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
information at back).
Introduction
Early attempts to control .limestone
scrubbers involved combinations of
liquid-to-gas ratio, limestone feedrate,
and recirculating solids which would
maximize S02 removal while minimizing
the risks of internal scaling. However,
an incomplete understanding of observed
effects prevented the achievement of
desired goals. More recently the charac-
terization and operating experience with
these processes has reached a level of
maturity to better support a productive
effort into the optimization of their control.
The application of automatic control to
environmental processes such as lime-
stone-alkali wet scrubbers is especially
promising because of its potential for
increased reliability, economy of opera-
tion, and consistent reduction in the
variance of controlled variables with
varying operating conditions and opera-
tor proficiency. Implementation of solu-
tions to the major obstacles to limestone
scrubbing, the latter including internal
scaling and insufficient SO2 remov-
al, in practice can be achieved only through
the disorder reducing capabilities of pro-
cess automation.
The effectiveness of limestone scrub-
bing depends primarily on the ability to
accommodate and regulate the manifold
factors associated with the recirculating
slurry. This problem is all the more a
challenge because of the few useful
slurry process measurements and actua-
tor inputs available, and the economic
considerations associated with the
energy and feedstock requirements of
large scale scrubbers. These consequen-
ces prompted the use of process model-
ing, computer simulation, and control
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system experimental testing for the
development, design, and detailed analy-
sis of the recirculating slurry control
systems presented in this report.
Conclusions and
Recommendations
Automatic control of the recirculating
slurry is essential for successful operation
of limestone scrubbers for reliability, S02
removal, and efficiency. Three control
methods have been analyzed and specified
for the addition of dense limestone to the
slurry, which constitutes the most
difficult scrubber control action to realize
effectively because of competing slurry
factors.
Stoichiometric control of limestone
addition is widespread in application,
unconditionally stable, and meaningfully
based on slurry and inlet-gas material
balance measurements. However, this
control method utilizes dead reckoning
based on feedforward measurements,
and is lacking in its visibility of influential
slurry factors which can result in signifi-
cant errors in the limestone feedrate.
Therefore, its utility as the sole determiner
of limestone addition is compromised and
(therefore) not recommended.
Control of the pH of limestone feedrate
is geared to accommodate slurry factors
which are invisible to Stoichiometric
control, primarily because slurry pH mea-
surement represents an agglomerate of
the slurry reactions. However, the imple-
mentation of this control method requires
a rigorous design for stable and responsive
performance, since the process intervenes
between the controller measurement and
limestone feed. Performance is improved
with an antiwindup batching controller
which prevents further integration of the
error signal when the output actuation
signal has limit cycled. A nonlinear pH
controller is not required for limestone
reagent because of the highly buffered
dissolution and limited range of neutrali-
zation reaction pH.
A narrow-range 4.5 to 6.5 pH probe
was found to increase sensitivity and
control responsiveness, and cascade flow
control was used to compensate for
disturbances in the limestone feed
system. Responsive pH control is achieved
for pH set points (<5.8) that result in a
high slurry reaction gain (A pH/A limestone
feedrate). Consequently, this is the
recommended limestone feedrate design
when conditions of high reaction gain are
maintained during scrubber operation.
The combination of stoichiometric-
assisted pH control utilizes the most
independent process measurements to
0.40 —
0.35.
0.30 - -
~l
s
0.25-
I
0.20- •
0.15
0.10
I
Stoichiometric
I Assisted pH
Control Recommended
5.0
5.2
5.4
5.6
5.8
6.0
Scrubber Inlet pH
Figure 1. Reaction gain versus slurry pH.
determine the limestone feedrate, and is
optimum in the sense that further
reduction in the slurry process disorder is
available than either method is capable of
individually. Deadband pH and integrat-
ing Stoichiometric ratio control additions
necessary to realize improved perform-
ance result in increased system complex-
ity, and is warranted only under the
conditions of low slurry reaction gain
shown in Figure 1.
An analysis of limestone slurry liquid-
to-gas (L/G) ratio control for minimum
scrubber energy requirements, while
simultaneously maintaining an SOz exit
target, resulted in the design of a feedfor-
ward control system. This system uses
inlet SOa and flue gas volumetric flow
measurements with a removal efficiency
law and slurry pump selector logic for
continuously determining the minimum
required L/G ratio. Adding an outlet SO2
measurement to compensate the feedfor-
ward L/G determination for errors in the
exit target was found to produce control
system instability under all conditions
and (therefore) is impractical. These
results are summarized in Table 1.
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Table 1. Limestone Scrubber Slurry Automation Research Milestones
Element Modeled Simulated
Tested
Highlight
pH control of
limestone feedrate
Stoichiometric limestone
feedrate control
Stoichiometric assisted
pH control
Absorber liquid-to-gas
ratio control
Limestone slurry
density control
Slurry buffer
additives control
X
X
X
X
X
Recommended control method, but requires
high reaction gain (A. pW/A limestone
feedrate).
Rapid control response to inlet changes,
but errors due to no slurry measurements.
Maximum slurry disorder reduction, but
complexity warranted only for low reaction
gain.
Energy savings based on feedforward control
using inlet SOa Unstable using outlet SOz.
Maintains optimum condition for solids
crystallization and scaling prevention.
Ratio control determined from percent of
limestone feedrate.
Patrick H. Garrett is with the University of Cincinnati, Cincinnati, OH 45221.
D. Bruce Harris is the EPA Project Officer (see below).
The complete report, entitled "Limestone Scrubber Slurry Automatic Control
Systems," [Order No. PB 84-148 766; Cost: $11.50, 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:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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