United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 2771
Research and Development
EPA-600/S7-81-141b Dec. 1981
Project Summary
Evaluation of the Limestone
Dual Alkali Prototype
System at Plant Scholz:
Final Report
J. A. Valencia, J. F. Peirson, Jr., and G. J. Ramans
The limestone dual alkali process,
developed by Thyssen-CEA Environ-
mental Systems, Inc. (until late 1980
under Combustion Equipment Asso-
ciates, Inc. ownership) and Arthur D.
Little, Inc., was tested at an existing
20 MW prototype facility at Gulf
Power Company's Scholz Steam
Plant. The intent of the project was to
evaluate the technical feasibility of the
process at a prototype scale and to
develop sufficient technical informa-
tion leading to the implementation of
the process at a full commercial scale.
Due to budgetary considerations,
the testing period was reduced from 6
to 2 months. The report gives results
obtained during the 2 months of
testing—February and March 1981.
Excellent SO2 removal efficiencies in
excess of 95 percent were achieved;
limestone utilizations were also high,
over 97 percent. The solids properties
ranged from excellent to poor. The
generation of poor solids was the most
significant process problem. The
solids content in the filter cake was
typically 35-45 percent, which was
below the anticipated 55 percent. The
soda ash consumption of 0.29 moles
of Na2CO3/mole of SO2 removed far
exceeded the design consumption of
0.04 moles of Na2CO3/mole of SO2.
Much of this consumption was due to
leaks and other liquor losses in the
system. The mechanical performance
of the equipment, recommissioned
after 3 years of inactivity, was poor
and contributed to the above prob-
lems.
Although the technology appears to
be technically feasible, further testing
is necessary before a conclusive
evaluation is made; some refinement
of the process is still needed.
This Project Summary was devel-
oped by EPA's Industrial Environ-
mental 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
The purpose of this test program was
to evaluate the performance, at a
prototype scale, of the limestone dual
alkali process developed by Thyssen-
CEA Environmental Systems, Inc.(TESI)
(initially Combustion Equipment Asso-
ciates, Inc. (CEA)) and Arthur D. Little,
Inc. (ADL). The overall intent of this
project, however, was not only to
demonstrate the technical feasibility of
the process at a prototype level but to
supply technical and cost information
related to the implementation of the
process at a full commercial scale.
The test program was conducted at an
existing dual alkali facility at Gulf Power
Company's (GPC) Scholz Steam Plant
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near Sneads, FL The 20 MW lime-
based dual alkali system was modified
for operation with limestone. Following
its conversion, the system underwent
an evaluation of its performance with
regard to:
• S02 removal capabilities.
• Raw materials and energy require-
ments.
• Quality of waste material generated.
• Reliability and ease of operation of
the system.
Originally, the formal testing period of
the system was to last 6 months; this
period was, however, reduced to 2
months due to budgetary considera-
tions.
Parallel to this EPA program, the
Electric Power Research Institute(EPRI)
sponsored a program to study and
evaluate the landfill disposal of the
waste cake generated by the limestone
dual alkali system.
The Scholz Prototype System
Limestone Dual Alkali
Technology
The limestone dual alkali technology
is based on the absorption of S02 in an
alkaline sodium solution, followed by
regeneration of the absorbing solution
by reaction with a second alkali,
calcium. These reactions generate
insoluble calcium-sulfur salts which are
discharged from the system as a moist
cake.
S02 is absorbed by contacting flue gas
with a sodium sulfite/bisulfite solution.
The sulfite reacts with the SO2 producing
additional bisulfite according to the
following overall reaction:
SOi + SO2 + H2O - 2 HSOa
During absorption, and to a lesser
extent throughout the remainder of the
process, some sulfite is oxidized to
sulfate:
SOI + 1 /2 O2 -SOJ
The level of oxidation during absorp-
tion is generally a function of the
scrubber configuration, oxygen content
of the flue gas, and the scrubber
operating temperature. At excess oxygen
concentrations normally encountered
in utility power plant operations, the
level of oxidation is expected to be 5-10
percent of the SO2 removed (for medium
and high sulfur coal applications).
The spent scrubbing solution is
reacted with limestone to regenerate
the absorbent. The reaction precipitates
mixed calcium sulfite and sulfate solids,
resulting in a slurry containing up to 5
wt. percent insoluble solids. The re-
generation process involves basically
the following overall reactions:
2 HSOa + CaCOa — SOa + CaS03
•1/2H2Om/2H20
2 HSOS + Na:>SO4 + CaCOs - 2 SO3 +• CaS04
•1/2H2Oi + 1/2H20
Following regeneration, the insoluble
calcium-sulfur salts are separated from
the regenerated liquor. After settling in
a thickener, the insoluble salts are
vacuum-filtered before being discharged
as a moist cake. The clarified liquor from
the thickener is returned to the scrubber,
closing the process liquor loop.
The filter cake is washed to recover
most of the soluble sodium salts in the
mother liquor. A small portion of the
sodium salts, however, remains oc-
cluded in the cake and is lost. Thus,
some sodium carbonate must be added
to the system to make up for these
sodium losses. Typically, the Na2COa
makeup should amount to less than 5
mole percent of the SO2 removed.
The amount of limestone added to the
system for absorbent regeneration is
reduced slightly due to the soda ash
makeup. Under normal conditions, the
limestone feed stoichiometry will be
slightly less than 1.0 mole of CaCO3/
mole of S02 removed.
Description of the Scholz
System
The dual alkali system took flue gas
from the Scholz Steam Plant Unit No. 1 ,
a 40 MW nominal capacity (47 MW peak
capacity) Babcock and Wilcox pulver-
ized-coal-fired power boiler. The flue
gas from the boiler passed through a
high efficiency, sectionalized electro-
static precipitator (ESP) designed to
remove up to 99.5 percent of the
paniculate matter. Part of the gas
discharged from the ESP was directed to
the limestone dual alkali system, which
was designed to handle flue gas flow
rate equivalent to a 20 MW boiler load.
The remaining gas was sent directly to
the stack.
The limestone dual alkali system at
Scholz was a modification of a lime dual
alkali system previously installed and
operated during 1975 and 1976 for a
test program sponsored by Southern
Company and jointly funded with EPA.
The modified dual alkali system con-
sisted of four sections: absorption,
regeneration, waste solids dewaterini
and raw materials storage and fee
preparation. A process flow diagram fc
the dual alkali system for Scholz i
shown in Figure 1.
The absorption system consisted of
venturi scrubber, followed by an absorp
tion tower. An SOz injection system wa
provided to increase, when necessan,
the SO2 concentration in the flue ga
entering the dual alkali system by 2O
ppm or more.
The variable throat, plumb-bob typ
venturi scrubber was designed fo
particulate removal and/or S02 ab
sorption. Typically, a dual alkali systerr
taking flue gas from a particulat
removal device (e.g., ESP) would no
require a venturi scrubber. Such beini
the case at Scholz, the system in fact dii
not require the venturi scrubber. Rathe
than remove it, the venturi scrubbe
was used primarily for quenching an<
saturating the flue gas; these opera
tions, which also contribute to the SO;
removal, would normally be performec
in the absorption tower. A booster far
directed the S02-rich flue gas exitinc
the ESP into the scrubber, where i
contacted scrubbing liquor flowing at ar
L/G of 15-20 gal./IO*5 acf.* The liquo:
was collected at the bottom of the
scrubber in an internal recycle tank. ^
After being quenched and saturatec
with water, the partially desulfurizec
gas entered the bottom of the absorption
tower. Gas passed upward through twc
trays and then through a de-entrainmenl
separator. A spray under the bottom tray
was used to wet the underside of the
tray. The de-entrainme'nt separator was
operated dry, without mist eliminator
wash water. The scrubbing solution,
which was fed to the top tray of the
absorber at an L/G of 2-3 gal./106 acf,
flowed countercurrent 'to the gas
through the tray system. This counter-
flow permitted high SO2 transfer from
gas to liquid phase. The liquor was
collected at the bottom of the absorber
in the internal recycle tank, which
served as the supply for the liquor spray
under the bottom tray.
A bleed from the absorber liquor
recycle line was sent forward to the
scrubber recirculation loop where it was
used to quench the gas and provide
additional SO? removal. A bleed stream
of spent liquor was drawn from the
scrubber recycle line and fed to the
absorbent regeneration system.
(*) To convert from British to metric units: multiplv
gal by 3.79 to yield liters, and multiply cf (ft3; 6y
0 028 to yield m3.
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SO2/OZ
Outlet
Probe
«Cleaned
I Gas
SOa
Injection
Water
Normal Flow
Alternate Flow
uigure 1. Limestone dual alkali system at Scholz - process flow diagram.
The regeneration system consisted of
five separate reactors in series with a
total design holdup time of approximately
100 minutes. It was designed to provide
the capability of operation in various
configurations. The bleed from the
scrubber system was piped such that it
could be fed either to the first or second
stage reactor. Similarly, the limestone
silo feed chute could feed dry limestone
to either of these two tanks. Thus, the
modified reactor system was capable of
being operated as either four or five
reactor tanks in series with dry lime-
stone feed, or as four reactor tanks in
series with slurried limestone feed
prepared in the first stage reactor.
Furthermore, the three intermediate
reactors had overflows at two levels to
provide the flexibility of varying the
holdup time. Due to the shortened
testing period, however, the regenera-
tion system was operated in only one
mode: five reactors in series with dry
limestone feed to the first reactor. The
first four reactors operated on overflow
(high overflow for the three intermediate
reactors), while the last reactor was on
level control so that the effluent could
be pumped to the thickener.
Clear liquor overflow from the thick-
ener was collected in the thickener hold
tank which acted as surge capacity for
the absorbent liquor feed to the scrubber
system. Process makeup water was
added to this tank to make up for the
total water loss due to evaporation in
the system and moisture in the cake:
The thickened slurry was fed to a
rotary drum vacuum filter. A series of
wash sections were used to wash the
filter cake in order to recover valuable
process liquor. Solids from the filter
cake were discharged to a weigh belt
conveyor for transport to the waste
processing system. The mixed filtrate
and wash liquor from the filter were
returned to the thickener.
Two raw materials were required for
operation of the system—limestone for
absorbent regeneration and soda ash to
make up for losses of sodium salts in the
waste filter cake. Finely ground lime-
stone was received, stored, and fed to
the system in a dry form. Dense soda
ash was used for the makeup soda ash
solution, which was normally prepared
using clarified liquor from the thickener
hold tank; occasionally, river water was
used for preparing this solution but only
when the additional water input did not
upset the system volume balance. The
makeup soda ash solution was fed back
to the thickener hold tank to be mixed
with the regenerated scrubbing solution
and pumped forward to the absorber.
The waste processing system was
installed and operated as part of a
separate test program funded by EPRI. It
is briefly discussed here to indicate its
relation to overall plant operation.
The moist waste cake discharged
from the vacuum filter was mixed in a
pug mill with fly ash and lime. Fly ash
was added at approximately one part of
ash per part of dry solids in the cake.
Lime was added at a rate of 3-5 wt.
percent of the dry solids contained in the
mix of cake and fly ash. The mix
discharged by the pug mill was trucked
to a landfill test area. Mixed material not
meeting specifications or untreated
filter cake—when the waste processing
system was unavailable or inoperative—
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were disposed of in a separate "off-
spec" disposal area.
Operating History
System design and test program
planning began in September 1978,
immediately after EPA awarded the
contract for the project. Recommission-
ing and conversion of the existing
system to a limestone dual alkali system
began in March 1979 and was completed
by March 1980.
Process operations were initiated in
August 1980. In the interim, the system
was operated mechanically to prevent
mechanical deterioration of the equip-
ment. This delay was caused by equip-
ment delivery problems encountered in
the installation of the EPRI waste
disposal system. As the expenses
associated with the mechanical opera-
tion continued to accumulate, it was
decided to start up the system in August
even though the EPRI system was not
yet completely installed. Formal testing
of the dual alkali system did not start
until February 1980 when the S02 and
Oz monitors were finally placed in
operation. During this period—August
20, 1980, to the end of January—the
system was operated for about 57 days
and was down for about 107 days. Of
the downtime in this initial startup and
break-in testing period, 43 percent was
attributable to mechanical problems
(e.g., leak in the thickener tank), 20
percent to process problems (e.g.,
generation of solids with poor settling
characteristics), 26 percent to the time
needed to resolve CEA's bankruptcy-
related issues*, 7 percent to a lack of
raw materials (due to reluctant suppliers
in light of the bankruptcy declaration),
and 4 percent to miscellaneous causes
(e.g., boiler outage).
System tests began on February 2,
1981, and extended until March 28,
1981. During this period, the system
was operated for 925 hours (38.5 days)
or 71.4 percent of the time, and it
recorded one completely uninterrupted
period of operation of 431 hours (18
days).
•On October 20, 1980, CEA filed a petition for
financial reorganization pursuant to Chapter 11 of
the U S. Bankruptcy Code Thyssen-CEA Environ-
mental Systems, Inc. (TESI) bought the CEA Air
Pollution Division and administered this EPA
contract starting in early December 1980, assuming
financial responsibilities retroactive to October 21,
1980. The EPA contract for this project was
formally assigned to TESI in April 1981 CEA is
referenced in this report to maintain historic
integrity
Outages during the test period were
due primarily to mechanical problems
(filter repairs) which accounted for 63
percent of the downtime, and to process
problems (resuspension of solids left in
thickener while repairing the filter)
which accounted for 30 percent of the
downtime.
System Performance
Given the limited tests performed,
due to the reduction of testing from 6 to
2 months, a final and conclusive
evaluation of the technology, leading to
its full-scale commercialization, cannot
be made at this time. Nevertheless, a
number of very encouraging qualities of
this technology have been clearly
identified. Problem areas that will
require further refinement have been
identified as well.
S(?2 Removal
The SO2 removal capability was
excellent. During the tests, the system
averaged an S02 removal efficiency of
95.8 percent. S02 removal efficiencies
were easily controlled by simply ad-
justing the pH of the scrubber bleed
liquor. SOz removal efficiencies greater
than 90 percent were obtained by
maintaining a scrubber bleed pH of at
least 5.5. Outlet SOa concentrations of
less than 100 ppm were obtained at
scrubber bleed pH's higher than 5.7;
concentrations less than 50 ppm were
obtained at a pH of 6.0. Inlet S02
concentrations during this period
ranged from 1460 to 3240 ppm and
averaged slightly over 2000 ppm.
Limestone Utilization
Limestone utilization by the dual
alkali system was very good. During the
test period, utilizations in the reactor
train effluent were 85-95 percent of the
available CaCOa in the raw limestone.
As the reaction with limestone continued
in the dewatering system, the final
system utilizations, determined from
chemical analyses of the filter cake,
were 93-100 percent, averaging 97.5
percent.
Oxidation and Suit ate
Precipitation
About 18 percent of the S02 removed
by the scrubbing solution was oxidized
to sulfate as it passed through the
system. As anticipated, most of this
oxidation, as much as 90 percent, took
place in the absorber/ scrubber section.
The capabilities of the system to co-
precipitate calcium sulfate along with
calcium sulfite were adequate to
remove the sulfate formed. The soluble
sulfate concentration in the system
achieved a steady level about 1 M.
Although liquor losses (due to leaks,
spills, and purges to maintain the liquor
volume balance in the system) con-
tributed to the removal of sulfate from
the system, the sulfate concentration in
a tight closed-loop operation is not
expected to rise above 1.2 M, which
appears to be within the sulfate co-
precipitating capabilities of the system.
Waste Solids
The generation of solids with good
settling characteristics was the most
significant process limitation encoun-
tered at Scholz. It essentially accounted
for all of the process-related outages.
Throughout December and February,
solids with excellent settling character-
istics were generated. In contrast, poor
solids were generated in January. The
good settling solids were agglomerates,
roughly spherical in shape; whereas,
the poor solids were fine, needle-
shaped solids. These fine solids, when-
ever carried over in the thickener
overflow in noticeable amounts (>1000
ppm), promoted the formation in the
reactors of more fine and difficult-to-
settle solids. While on some occasions,
the formation of these fine solids was
stopped by process changes, in others it
simply continued to deteriorate, forcing
interruptions in system operation.
Although some mechanical problems
intensified the difficulties in the settling
of solids, the fundamental physical and
chemical reasons for the formation of
poor settling solids are not clearly
understood, and therefore require
further investigation.
The solids appeared, in general, to
have good dewatering characteristics;
nevertheless, the typical solids content
in the cake was 35-45 wt. percent, far
below the anticipated 55 wt. percent.
Severe mechanical problems with the
filter and associated pumps and piping
were the major factors in this short-
coming. Closely associated with this
problem were the high sodium losses in
the waste filter cake.
It appears that both of these prob-
lems—low solids content of the cake
and high sodium losses—are not
inherent to the limestone dual alkali
technology, but were rather caused by
the mechanical condition of the equip-
ment used.
4
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Soda Ash Consumption
Soda ash consumption was excessive,
amounting to about 0.29 mole of
Na2C03/mole of SO2 removed, far
above the design value of 0.04 mole of
NaaSOs/mole of SOz removed. Soda
ash is fed to the system to make up for
'normal sodium losses in the liquor
entrained in the filter cake. At Scholz,
however, not only were the sodium
losses high in the filter cake, but much
sodium was lost through leaks, spills,
and liquor purges needed to maintain
the liquor volume balance in the system.
(Large amounts of seal water were
needed to keep worn out pumps in
operation which, coupled with heavy
rains, were very taxing on the limited
surge capacity available.)
Again, this excessive soda ash con-
sumption problem does not appear to be
inherent to the limestone dual alkali
technology.
Power Consumption
Power consumption ranged from 2.5
percent (0.53 MW)—at flue gas rates
equivalent to a boiler load of 21 MW—to
5.3 percent (0.42 MW)—at an equivalent
load of 8 MW. In a typical limestone dual
alkali application, where flue gas is
taken from an ESP and the use of a
venturi is not required, one would
expect the power consumption to be
much lower—more like 1 -1.5 percent of
the power generated at full boiler load.
Process Operability
Process operability, unlike equipment
or mechanical operability, refers to: the
ease with which the system can be
operated and controlled, the ability of
the system to adequately respond to
varying conditions, and the ability of the
system to tolerate upsets in process
chemistry due to mechanical problems
or operator oversight.
In general, process operability was
good after the first 2 or 3 days of stable
operation. It was during these initial
days, following any restart of the
system, that problems with process
operability were encountered. All of
these related to the generation of solids
with poor settling characteristics.
Once the system reached a stable
operation, such as during December or
February, process operability was very
good. The system was able to easily
accommodate variations in inlet S02
concentrations of as much as 500 ppm,
by simply adjusting the feed forward
rate of regenerated solution to the
absorber/scrubber to maintain a con-
stant bleed pH. Variations in boiler load
and thus in the amount of gas processed
were accommodated in the same
fashion.
Upsets to process chemistry were
also handled well by the system. These
upsets included the carryover of fly ash
in the flue gas due to a malfunction in
the ESPs, the gross overfeeding of
limestone due to operator oversight,
and occasional limestone and soda ash
outages lasting from 1 to 5 hours.
Mechanical Performance
The mechanical performance of the
equipment and its associated instru-
mentation represented a major source
of problems at Scholz: 43 percent of
system downtime during the start-up
and break-in and 59 percent of the
downtime during testing were due to
mechanical problems. Not only was
system availability affected by these
mechanical problems, but also system
operating conditions were affected. An
example of these limitations was the
inability of undersized thickener under-
flow pumps to handle slurry with more
than 15 wt. percent solids, which
required the dilution of the 20-25
percent thickener underflow slurry.
Although some of the problems were
caused by the limited capacity of an
existing piece of equipment, most of the
mechanical problems were due to
equipment failures. The age of the
equipment and its condition, even after
recommissioning, were, undoubtedly,
primary causes of these failures.
Conclusions
System performance, summarized
above, leads to the following conclu-
sions:
• The limestone dual alkali process
appears to be technically feasible.
• Further testing is required to
reinforce the above conclusion and
to develop sufficient process in-
formation needed for full-scale
commercialization.
It is also recommended that laboratory
or small pilot plant tests, to better
understand the generation of solids
with good settling characteristics, be
performed prior to any further prototype
scale testing.
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J. Valencia andJ. Peirson are with Arthur D. Little, Inc.. Acorn Park, Cambridge,
MA 02140; G, Ramans is with Thyssen-CEA Environmental Systems, Inc..
555 Madison Avenue, New York, NY 10022.
Norman Kaplan is the EPA Project Officer (see belowj.
The complete report, entitled "Evaluation of the Limestone Dual Alkali Prototype
System at Plant Scholz: Final Report," (Order No. PB 82-110 685; Cost: $12.00.
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--559-092/3362
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