SECOND PROGRESS REPORT
LIME/LIMESTONE
WET-SCRUBBING
TEST RESULTS
AT THE
EPA ALKALI
SCRUBBING
TEST FACILITY
U.S. EPA
OFFICE OF
RESEARCH AND
DEVELOPMENT
PROTOTYPE
DEMONSTRATION
FACILITY
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[I,
This capsule report describes a program con-
ducted by The Environmental Protection Agency
(EPA) to test prototype lime and limestone wet-
scrubbing systems for removing sulfur dioxide
(SO2) and particulate matter (fly ash) from coal-
fired boiler flue gases. The program is being
carried out in a test facility which is integrated
into the flue gas duct-work of a coal-fired boiler
at the Tennessee Valley Authority (TVA)
Shawnee Power Station, Paducah, Kentucky.
Bechtel Corporation of San Francisco is the
major contractor and test director, and TVA is
the constructor and facility operator. This
report describes a series of lime and limestone
reliability tests conducted from March 1973 to
December 1974. An earlier capsule report
(EPA Technology Transfer Capsule Report
No. 4) discussed the results of limestone factorial
tests and initial limestone reliability tests. The
results of an advanced program at the Shawnee
test facility will be presented in future reports.
In a lime/limestone wet-scrubbing system, the
flue gas is contacted (scrubbed) with a slurry of
lime or limestone in water. SC>2 is absorbed into
the liquor, where it reacts with the dissolved
lime/limestone, forming the waste products of
calcium sulfite and calcium sulfate (gypsum).
Particulate is removed in the scrubber by impact
with the slurry droplets.
The Shawnee test facility consists of three
parallel wet-scrubber systems: a Turbulent
Contact Absorber (TCA), a venturi followed by
a spray tower, and a Marble-Bed Absorber.
Each system is capable of treating approximately
10 MW equivalent (30,000 acfm at 300°F) of
flue gas containing 1800 to 4000 ppm of SC>2
and 2 to 5 grains/scf of particulates. Testing of
the TCA and venturi/spray tower is ongoing;
testing of the Marble-Bed Absorber has been
discontinued.
The following tests have been conducted:
• Limestone factorial tests on all three
scrubbers to determine the effects of the
independent variables (e.g., liquid-to-gas
ratio, gas velocity, etc.) on SC>2 and
particulate removal
• Limestone reliability verification tests on
all three scrubbers to define regions for
reliable (scale-free) operation of scrubber
internals
• Lime and limestone reliability tests on the
venturi/spray tower and TCA systems,
respectively, to demonstrate long-term
operational reliability
Test results have shown that scrubber internals
can be kept relatively free of scale if the sulfate
(gypsum) saturation of the scrubber liquor is
kept below about 135 percent. This can be
accomplished by proper selection of slurry solids
concentration, effluent residence time, and liquid-
to-gas ratio.
At the conditions tested, the mist elimination
configuration presently used in the TCA appears
to be successful in handling the problem of mist
eliminator scaling and plugging — the most
significant reliability problem encountered during
the test program. This configuration consists of
a wash tray (Koch Flexitray) followed by a
chevron mist eliminator, both continuously
washed on the underside with a combination of
clarified liquor and makeup water. In a test run
of over 3 months' duration in which the TCA
operated in a closed liquor loop at a superficial
gas velocity of 8.6 ft/sec and a slurry solids
concentration of 15 percent, the system remained
essentially clean.
A run of 3 months' duration is yet to be made
with the venturi/spray tower mist elimination
configuration, which consists of a chevron mist
eliminator with provision for both underside and
topside washing. With the system at a superficial
gas velocity of 6.7 ft/sec and 8 percent slurry
solids concentration, scale formation on the top
mist eliminator vanes has been a constant prob-
lem. Underside washing by itself did not elim-
inate topside scale formation. But when the
underside washing was combined with an inter-
mittent high-pressure topside wash of a single
section of the mist eliminator, that section
remained essentially scale-free. This procedure
seems promising and will be tested further.
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The test facility consists of three parallel
scrubber systems, each with its own slurry-
handling system. Scrubbers are of prototype
size, each capable of treating approximately
30,000 acfm (at 300°F) of flue gas from the
TVA Shawnee coal boiler No. 10. This
corresponds to approximately 10 MW of power
plant generating capacity. The equipment
selected was sized for minimum cost, consistent
with the ability to extrapolate results to com-
mercial scale. Boiler No. 10 burns a high-sulfur
bituminous coal, leading to SC>2 concentrations
of 1800 to 4000 ppm and particulate inlet
loadings of about 2 to 5 grains/scf in the flue
gas.
The major criterion for scrubber selection was
the potential for removing both SC>2 and particu-
lates at high efficiencies (SC>2 removal greater
than 80 percent and particulate removal greater
than 99 percent). Other factors considered in
the selection of the scrubbers were (1) ability
to handle slurries without plugging or excessive
scaling, (2) reasonable cost and maintenance,
(3) ease of control, and (4) reasonable pressure
drop.
On the basis of information available in the
literature, the following scrubbers were selected:
• Turbulent Contact Absorber (TCA)
• Venturi followed by a spray tower
• Marble-Bed Absorber
The TCA, manufactured by Universal Oil
Products, uses a fluidized bed of low-density
plastic spheres that are free to move between
retaining grids. The venturi, manufactured by
Chemical Construction Co., contains an adjust-
able throat that permits control of pressure
drop under a wide range of flow conditions.
Although a venturi is ordinarily an effective
particulate removal device, gas absorption is
limited in lime/limestone wet-scrubbing systems
by low slurry residence time. For this reason,
the spray tower was included for additional
absorption capability. The Marble-Bed Absorber,
manufactured by Combustion Engineering Co.,
uses a packing of 3/4-inch glass spheres (marbles).
Because of operating problems with the Marble-
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WASH LIQUOR
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WASH LIQUOR
RETAINING
GRIDS
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CHEVRON MIST
ELIMINATOR
WASH TRA Y
EFFLUENT WASH
TRAY LIQUOR
INLET SLURRY
GASOUT
MOBILE
'PACKING SPHERES
APPROX. SCALE
EFFLUENT SLURRY
Figure 1. Schematic of Three-Bed TCA
CHEVRON MIST
ELIMINATOR
SPRAY
TOWER
INLET
SLURRY
WASH LIQUOR
WASH LIQUOR
ADJUSTABLE
PLUG
VENIURI SCRUBBER ~m~ APPROX. SCALE
EFFLUENT SLURRY
Figure 2. Schematic of Venturi/Spray Tower
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Bed Absorber (i.e., nozzle failure and subsequent
plugging of the bed), testing was discontinued on
this system early in the program. Combustion
Engineering has since developed an advanced
Marble-Bed Absorber which has been operating
reliably in full-scale commercial service at other
locations. Figures 1 and 2, drawn roughly to
scale, show the TCA and the venturi/spray tower,
along with the mist eliminators selected for de-
entraining slurry in the gas streams.
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Figure 3, EPA Test Facility — Typical Process Flow Diagram for TCA System in Limestone Service
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Figure 4. EPA Test Facility - Typical Process Flow Diagram for Venturi/Spray Tower System
in Lime Service
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The test facility was designed to allow a num-
ber of different scrubber internals and piping con-
figurations to be used with each scrubber system.
For example, the TCA can be operated as a one-,
two-, or three-bed unit, and solids separation can
be achieved with a clarifier alone or with a clari-
fier in combination with a filter or a centrifuge.
A typical TCA system configuration used dur-
ing limestone testing and a typical venturi/spray
tower system configuration used during lime
testing are shown in Figures 3 and 4, respectively.
Process details, such as flue gas cooling sprays,
are not shown.
For all configurations, gas is withdrawn from
the boiler ahead of the power plant particulate
removal equipment so that entrained fly ash can
be introduced into the scrubber. The gas flow
rate to each scrubber is measured by venturi
flow meters and controlled by dampers on the
induced-draft fans. Concentration of SC>2 in the
inlet and outlet gas is monitored continuously by
Du Pont photometric analyzers. Inlet and outlet
gas particulate concentrations are measured
periodically using a modified EPA particulate
train.
Control of the scrubbing systems is carried
out from a central graphic panelboard. An elec-
tronic data acquisition system is used to record
the operating data. The system is hard-wired
for data output directly on magnetic tape, and
on-site display of selected information is avail-
able. Important process control variables are
continuously recorded, and trend recorders are
provided for periodic monitoring of selected data
sources.
Views of the scrubber structure, TCA, spray
tower, and control room are shown in Figures
5, 6, 7, and 8, respectively.
Figure 5. Scrubber Structure
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Figure 6. TCA with View of Fluidized Spheres
during Air/Water Tests
Figure 7. Inspect/on of Spray Tower
Figure 8. Control Room
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The following sequential test blocks were estab-
lished for the test program:
(1) Air/water tests
(2) Sodium carbonate tests
(3) Limestone wet-scrubbing tests
(4) Lime wet-scrubbing tests
The test program schedule from March 1972
through December 1974 is shown in Figure 9.
Detailed test results have been presented in EPA
Progress Reports EPA-650/2-73-013 and
EPA-650/2-74-010. A third report will be issued
in mid-1975.
AIR/WATER TESTS
These experiments, which used air to simulate
flue gas and water to simulate alkali slurry, were
designed to determine pressure drop model
coefficients and to observe fluid hydrodynamics
for all three scrubbers under clean conditions.
SODIUM CARBONATE TESTS
These tests, which used sodium carbonate
solutions to absorb SO2 from flue gas, were
designed to determine coefficients within mathe-
matical models for predicting SC>2 removal.
LIMESTONE WET-SCRUBBING TESTS
The objectives of these tests, in which lime-
stone (CaCO3) slurry was fed to the scrubber
circuit were:
(1) To determine the effect of important
variables on particulate and SO2 removal
(2) To identify and resolve operating prob-
lems, such as scaling and mist eliminator
plugging
(3) To identify regions of reliable operation
of the three scrubber systems, consistent
with reasonable SO2 removal, and to
choose economically attractive operating
configurations from within these regions
(4) To establish long-term operating reliability
for one or more of the scrubber systems
and to develop definitive process econom-
ics data and scale-up factors
A large number of short-term limestone fac-
torial tests, of about 4 hours each, were made on
each scrubber system to accomplish the first
objective. The major independent variables were
gas rate, liquor rate, scrubber inlet liquor pH, and
number of grids and height of spheres in the TCA.
The results of these tests and of the air/water and
sodium carbonate tests are reported in EPA Tech-
nology Transfer Capsule Report No. 4.
A relatively small number of longer term lime-
stone reliability verification tests, of about 3
weeks per test, were made on each scrubber sys-
tem to accomplish the second and third objectives.
In these tests, the dependent variable was the scal-
ing and plugging potential of the scrubber inter-
nals, and the major independent variables were
gas rate, liquor rate, scrubber inlet slurry pH,
effluent residence time, solids concentration in
the scrubber recirculation slurry, and solids
concentration in the discharge sludge. The results
of these tests are reported in Section 4.
Long-term limestone reliability tests, of up to
3 months in duration, were run on the TCA sys-
tem to accomplish the fourth objective. The
results of these tests are reported in Section 5.
LIME WET-SCRUBBING TESTS
The objectives of this test sequence, in which
hydrated lime (Ca(OHJ2) slurry was fed to the
scrubber circuit, were identical to those for the
limestone wet-scrubbing sequence just described.
Originally, the testing was to be divided into the
same three categories: (1) short-term factorial
tests, (2) longer term reliability verification tests,
and (3) long-term reliability tests. Subsequently,
it was decided to begin the lime testing with
long-term reliability tests on the venturi/spray
tower system and to perform the factorial and
reliability verification tests at a later date (after
December 1974). The results of the long-term
reliability tests on the venturi/spray tower system
are reported in Section 6.
ANALYTICAL PROGRAM
Samples of slurry, flue gas, limestone, and coal
were taken periodically for chemical analyses,
particulate size sampling, and limestone reactivity
tests. Locations of slurry and gas sample points
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for the TCA and venturi/spray tower systems
are shown in Figures 3 and 4.
To meet the formidable analytical requirements
of the facility at reasonable costs, equipment was
selected to minimize manpower. For example,
an X-ray fluorescence unit was used for compre-
hensive slurry analyses. All analytical computa-
tions and recording of results were handled by
an on-site minicomputer.
TESTS
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Figure 9. Shawnee Test Schedule
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The primary objectives of the limestone relia-
bility verification tests were to identify and
resolve operating problems and to identify re-
gions of reliable operation of the three scrubber
systems. Emphasis,was placed on solving the
problem of scaling of scrubber internals. The
reliability verification tests averaged approx-
imately 500 hours (3 weeks) each.
SCALING OF SCRUBBER INTERNALS
The rate of scaling of the scrubber internals
was found to be sensitive to the supersaturation
of the calcium sulfate (gypsum) in the circulat-
ing scrubber liquor. Results of the limestone
reliability verification tests showed that scrub-
ber internals can be kept relatively free of scale
if the sulfate saturation is kept below about 135
percent at 50°C (i.e., below 35 percent super-
saturation).
The limestone tests showed that, generally,
the sulfate saturation in the scrubber liquor de-
creases (i.e., scaling potential decreases) with
(1) increasing slurry effluent residence time,
(2) increasing solids concentration in the scrub-
ber slurry, (3) decreasing solids concentration
in the discharge sludge, (4) increasing scrubber
slurry pH, and (5) increasing liquid-to-gas ratio.
Figure 10 shows the sulfate saturation of the
scrubber liquor as a function of slurry solids
concentration and effluent residence time for
the TCA reliability verification tests. All these
tests were operated in closed liquor loop. In
these tests, the slurry pH ranged from 5.2 to
6.1, tending to increase with increasing slurry
effluent residence time. Included are two data
points from TCA long-term reliability runs at
10 and 15 minutes residence time.
As seen in Figure 10, the sulfate saturation of
the scrubber slurry was 190 percent at 4.4 min-
utes effluent residence time and 8 percent slurry
solids concentration. Severe sulfate scaling of
the bottommost TCA grid occurred under these
conditions after 500 hours of operation. At 20
minutes residence time and 15 percent slurry
solids concentration, the sulfate saturation was
about 110 percent, and no significant scaling of
the TCA grids occurred after 500 hours of opera-
tion. In the reliability test at 10 minutes resi-
dence time and 15 percent slurry solids concen-
tration, sulfate saturation was about 130 percent,
and less than 15 mils of sulfate scale formed on
the bottommost TCA grid after 1200 hours of
operation.
The calcium sulfate (gypsum) saturations of
the scrubber liquors were obtained with the use
of a chemical equilibria computer program. Using
laboratory-measured liquor compositions, the equi-
libria program calculates the activities of the cal-
cium and sulfate ions. The degree of saturation
is equal to the product of the activities divided
by the solubility product of calcium sulfate at
the specified temperature. Calculations of sulfate
saturations in this report were based on a solu-
bility product for CaSO4 • 2H2O of 2.2 x
TO'5 gmole2/liter2 at 50°C.
\
SPtKCENI SLURRY
SOLIDS CONCENTRA TION
LIQUID-rO-C^AS RAIIO = 60-75 GAI /MCF
PERCENT SOLIDS DISCHARGED^ ^S
I
I
I
5 10 15 20
SLURRY Et I LUENT RESIDENCE TIME, MIN
Figure 10. Effect of Effluent Residence Time and
Slurry Solids Concentration on Sulfate
Saturation for TCA Limestone Tests
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§
tf®a
The major objective of the limestone reliability
tests was to demonstrate long-term (2 to 5
months) operability of the TCA system, with
emphasis on mist elimination and scrubber
internals. This was achieved in TCA Run 535-2,
terminated in December 1974, which ran with
little scale and an essentially clean mist eliminator
over a 3-month operating period (2325 hours on-
stream). A summary of the operating conditions
for this run is given in Table 1.
MIST ELIMINATION SYSTEM
The TCA mist elimination system consists of a
six-pass, closed-vane, stainless-steel, chevron mist
eliminator preceded by a wash tray (Koch Flexi-
tray). (See Figure 1.)
The underside of the mist eliminator was
sprayed continuously (0.3 gpm/ft^) with clarified
liquor diluted with makeup water.
A 2-inch depth of liquor on the wash tray
(0.5 gpm/ft^ combined rate from mist eliminator
sprays and additional clarified liquor) was used to
intercept the solids in the entrained mist. En-
trained droplet solids concentration was reduced
from 15 wt % to less than 0.5 wt %.
Initially in Run 535-2, the underside of the
wash tray was intermittently steam sparged (125
psig, 1 minute/hour). At 2000 hours, the steam
sparge was replaced by a continuous underside
spray (0.3 gpm/ft^) using wash tray effluent
liquor.
A view of the top of the wash tray and under-
side of the mist eliminator after 1350 hours of
TCA Run 535-2 is shown in Figure 11. It ap-
pears that this mist elimination system can be
operated for a year or more at the run condi-
tions tested.
Future plans include testing the TCA mist
elimination system (1) at increased gas velocity
(10 and 12 ft/sec) and (2) with the wash tray
removed.
SCRUBBER INTERNALS
As demonstrated during limestone reliability
verification testing, scrubber internals can be
Table 1
OPERATING CONDITIONS FOR TCA RELIABILITY RUN 535-2
Operating Time, hr
Gas Velocity, ft/sec
L/G, gal/mcf
Pressure Drop (including Mist Elimination System), in. H2O
Slurry Soiids Concentration, percent
Effluent Tank Residence Time, min
Inlet SO2 Concentration, ppm
Percent SO2 Removal (controlled)
Scrubber InJet Liquor pH
Percent Limestone Utilization (100 x moles SO2
absorbed/moles CaCO3 added)
Percent Sulfate Saturation @ 50°C
Percent Oxidation of Suffite to Sulfate
Percent Solids in Discharge Cake
Dissolved Solids, ppm
2325
8.6
73
6.5
12-15
15
2000-4000
75-88
5.7-6.0
65
110
10-28
35-42
6000
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kept relatively free of scale if the sulfate (gyp-
sum) saturation of the scrubber liquor is kept
below about 135 percent. For TCA Run 535-2,
the sulfate saturation of the scrubber slurry was
110 percent, and less than 20 mils of sulfate
scale formed on the bottommost TCA grid during
2325 hours of operation. This scale growth rate
would not interfere with normal scrubber opera-
tion over a 1-year operating period. Figure 12
shows the bottom bed of the TCA after 1350
hours of Run 535-2.
Until recently, the operating life of the 1]/2-
inch-diameter, 5-gram plastic spheres has been a
significant limiting factor in the long-term relia-
bility of the TCA scrubber. High-density
polyethylene (HOPE) spheres had an operating
life of about 2000 hours before eroding through
and filling with slurry. Thermoplastic rubber
(TPR) spheres showed a weight loss of only 6
percent after 2500 hours. The TPR spheres tend
to dimple, however, and can slip through the
supporting bar-grids presently used in the TCA.
This can be corrected by respacing the bar-grids.
There has been no evidence of significant ero-
sion of the bar-grids in the TCA after more than
5000 hours of operation. The original wire
mesh grids deteriorated during approximately
3000 hours of operation owing to vibrational
wear at the points of contact.
Figure 11. Wash Tray and Mist Eliminator after
1350 Hours of Operation during TCA
Reliability Run 535-2
Figure 12. Internal View of TCA Showing Bottom
Support Grid and Spheres A fter 1350
Hours of Operation during TCA
Reliability Run 535-2
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The major objective of the lime reliability
tests was to demonstrate long-term (2 to 5
months) operability of the venturi/spray tower
system, with emphasis on mist elimination and
scrubber internals. A successful run of long-
term duration has not yet been achieved because
of scale formation on the spray tower mist
eliminator.
MIST ELIMINATION SYSTEM
The spray tower mist elimination system con-
sists of a three-pass, open-vane, stainless-steel,
chevron mist eliminator which has provision for
underside and topside washing (see Figure 2). In
tests of this system at a superficial gas velocity of
6.7 ft/sec and 8 percent slurry solids concentra-
tion, scale formation on the top mist eliminator
vanes (of TOO to 200 mils/month) has been a
constant problem. A variety of washing configu-
rations have been tried in order to alleviate this
problem.
Underside washing only, either continuously
with low-pressure water at 0.3 gpm/ft^ or
intermittently with high-pressure water at 3
gpm/ft^ (9 minutes every 4 hours at 45 psig),
was unsuccessful in eliminating scale formation
on the top vanes.
A combination of topside and bottomside
washing was studied during venturi/spray tower
Runs 609-1 and 610-1. A summary of the oper-
ating conditions for these runs is given in Table 2.
The entire underside of the mist eliminator and a
small area of the topside (14 ft^) were washed
intermittently (8 minutes every 4 hours) at high
pressure (45 psig) with makeup water at a rate
of 2.7 gpm/ft^ for the underside and 1.0 gpm/
ft^ for the topside. At the termination of these
runs, after 530 operating hours, the washed area
was essentially clean, with less than 1 mil of
solids accumulation, compared with an average
of 70 mils scale buildup on the rest of the top-
side surfaces. Figure 13 shows the topside wash
nozzle and the relatively clean mist eliminator
Table 2
OPERATING CONDITIONS FOR VENTURI/SPRAY TOWER
RELIABILITY RUNS 609-1 AND 610-1
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surface beneath it. It is anticipated that the
chevron mist eliminator can be operated for a
year or more with underside and topside wash-
ing at the run conditions tested.
Carryover of water from the topside sprays
caused reheater overloading during the runs.
This problem might be reduced by a sequential
sectional topside wash or by the use of a second
mist eliminator to catch the entrainment from
the topside sprays. These concepts will be
studied during future testing.
SCRUBBER INTERNALS
As with limestone, the lime reliability tests
ha-ve shown that scrubber internals can be kept
relatively free of scale if the sulfate (gypsum)
saturation of the scrubber liquor is kept below
about 135 percent. Again, this can be accom-
plished with increased slurry solids concentra-
tion and/or with increased effluent residence
time. The lime system was found to differ
from limestone, however, in that sulfate satura-
tion of the scrubber liquor is a strong function
of inlet gas SC>2 concentration (i.e., SC>2 ab-
sorption rate).
The lime reliability tests have also shown that
severe scale formation within the spray tower
does not necessarily limit scrubber operability.
Figure 14 shows a view of the spray tower
internals (looking upward) after approximately
1 month of operation at a scrubber liquor sulfate
saturation of 180 percent. The white gypsum
scale on the scrubber internals did not noticeably
interfere with the spray tower operation.
Initially, nozzles in the spray tower frequently
plugged, but dual strainers installed in the circu-
lating slurry lines greatly reduced this problem.
Stainless-steel spiral-tip nozzles were badly eroded
after about 4300 hours in service. Stellite-tipped
nozzles have shown no measurable signs of erosion
after approximately 4000 hours in service.
Figure 13. Topside of Spray Tower Mist Eliminator
at Conclusion of Runs 609-1 and 610-1
Figure 14. Spray Tower Internals Showing Gypsum
Scale after One Month of Operation at
180 Percent Sulfate Saturation
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This section highlights the operating experience
during both lime and limestone wet-scrubbing
tests at the Shawnee facility. Mist elimination
systems and scrubber internals have been dis-
cussed previously and will not be described in
this section.
CLOSED LIQUOR LOOP OPERATION
Most commercial scrubber systems are required
to maintain a closed liquor loop. A closed
liquor loop is achieved when the raw water input
to the system is equal to the water normally
exiting the system in the settled sludge and in
the humidified flue gas. For lime/limestone wet-
scrubbing systems, the solids concentration in the
settled sludge is normally equal to or greater than
38 percent by weight.
Scaling potential is significantly affected by the
quantity of raw water makeup, and meaningful
reliability data can be obtained only by operating
with a closed liquor loop. Because of excess
water input, closed liquor loop operation was
not achieved early in the test program during
limestone factorial testing. Sources of excessive
water included pump seal water, flue gas
presaturation sprays, and water in the 10 to 20
wt % limestone slurry feed. To reduce water
input, water seals were converted to air purge,
slurry was substituted for water on the pre-
saturation sprays, and the slurry feed concentra-
tion was increased to 60 wt % solids. As a
result of these modifications, all testing in both
lime and limestone systems has been in closed
liquor loop operation since March 1973.
HOT-GAS/LIQUID INTERFACE
During the limestone reliability verification
testing, there was a continual problem of soft
solids buildup at the hot-gas/liquid interface in
the TCA scrubber inlet duct, where the hot flue
gas is cooled by slurry sprays to protect the
vessel's rubber linings. The problem was solved
by selecting the proper size, location, and
orientation of the slurry spray nozzles and by
soot blowing in the direction of the flue gas
flow only.
The venturi scrubber is an extremely reliable
gas-cooling device and does not require presatu-
ration sprays.
REHEATERS
Fuel-oil-fired reheaters with external air supply
and direct combustion in the flue gas stream were
originally installed on the scrubber systems. They
were hard to start, had frequent flame-outs, and
generated considerable soot. Field modifications
to provide an isolated combustion zone and instal-
lation of mechanical atomizing nozzles improved
combustion, but burner flame-out continued to
be a problem.
A fuel-oil-fired external combustion reheat sys-
tem was installed on the venturi/spray tower sys-
tem in March 1974. This unit has performed
satisfactorily with high reliability for over 4000
operating hours.
FANS
Erosion, corrosion, pitting, scaling, etc., have
been negligible on all three fans. Operation has
been with 125°F flue gas reheat to give a fan
inlet temperature of 250°F.
PUMPS
The major pumps used in alkali slurry service
at the Shawnee test facility are rubber-lined
variable-speed centrifugal pumps. In general, the
rubber linings have shown excellent erosion-
corrosion resistance and have remained in good
condition. The original pumps had water-sealed
packing, but were converted to air-purged pack-
ing during a boiler outage in February 1973.
LININGS
The neoprene rubber linings on the agitator
blades and in the spray tower, TCA, process water
hold tanks, pumps, and circulating slurry piping
have usually been found to be in excellent con-
dition, except for slight wear on some of the
rubber-coated agitator blades. Hairline cracks
have been noted in the glass flake lining on the
effluent hold tanks and clarifiers, but the cracks
did not appear to penetrate the entire thickness
of the lining.
WASTE SOLIDS HANDLING
The test facility is equipped to study alterna-
tive methods of waste solids dewatering and dis-
-------
posal. Separate clarifiers are provided for each
scrubber system. A belt-type rotary-drum vacuum
filter and a horizontal solid-bowl centrifuge are
common to the three systems.
The venturi/spray tower system has a 20-foot-
diameter clarifier, while the TCA unit is 30 feet
in diameter. The solids concentration in the
underflow of the larger TCA unit has approached
the expected final settled sludge concentration
(approximately 38 wt %), but the underflow
from the smaller unit has averaged only about
25 wt %. To achieve closed liquor loop opera-
tion, the smaller clarifier has to be used in series
with the filter or centrifuge.
Under normal operations, the belt-type rotary-
drum vacuum filter produces a filter cake con-
taining 50 to 55 wt % solids from limestone and
45 to 50 wt % solids from lime slurries. Filter
operation has been significantly hampered by
the short life (usually less than 260 hours) of
the filter cloth.
The continuous solid-bowl centrifuge produced
a cake with 55 to 65 wt % solids from limestone
slurry, and the centrate solids averaged 0.5 to 1.0
wt %. However, erosion made a major repair of
the unit necessary after about 1400 hours of
operation. It was concluded that the centrifuge
was not an acceptable solids dewatering device
for the Shawnee test conditions.
INSTRUMENTS
Two types of pH meters have been used in
slurry service: (1) in-line flow-through meters
and (2) submersible electrode meters. The
performance of the in-line flow-through meters
has been unsatisfactory because of the erosion
and high rate of failure of the glass cells and the
frequent plugging of the sample lines. The
submersible electrode meters have been free of
such problems during approximately 9000 hours
of operation.
Operating experience has been obtained with
three types of density meters in slurry service:
(1) radiation meters, (2) differential pressure
(bubbling tube) meters, and (3) vibrating U-tube
meters. The radiation meter has a continual
calibration shift which is accelerated by scale
formation. The gas line on the differential
pressure meter plugs frequently and requires
significant maintenance, but the meter is accurate
when clean. The vibrating U-tube meters were
installed in September 1973 in two locations.
The performance of this type of density meter
has thus far been encouraging.
Slurry flow rates have been measured by both
magnetic and differential pressure (both orifice
and Annubar) flowmeters. Performance of the
magnetic flowmeters and the orifice flowmeters
has generally been adequate. Annubar meters
plugged frequently and required excessive
maintenance.
Operating experience with control valves in
slurry service has generally been unsatisfactory.
Severe erosion and frequent plugging result from
the throttling operation. This has been observed
with stainless-steel plug valves, stainless-steel
globe valves, and rubber pinch valves. Satisfac-
tory and trouble-free flow control has been
experienced only with variable-speed pumps.
For further information:
Detailed progress reports, EPA-650/2-73-013
and EPA-650/2-74-010, are available from the
National Technical Information Service,
Springfield, Va. 22151.
A third detailed report is currently being
prepared. If you wish to be notified when this
report is available write:
Technology Transfer
Environmental Protection Agency
Washington, D.C. 20460
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