U.S. Environmental Protection Agency Industrial Environmental Rf>s>>arch
Office ot Resoat'jh and Development Laboratory
Research Triangle Park. North Carolina 27711
EPA-600/7-77-0503
FINAL REPORT: DUAL ALKALI
AND EVALUATION PROGRAM
Volume I. Executive Summary
Interagency
Energy-Environment
Research and Development
Program Report
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This document is available to the public through the National Technical
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EPA-600/7-77-050a
May 1977
FINAL REPORT: DUAL ALKALI TEST
AND EVALUATION PROGRAM
Volume I. Executive Summary
by
C.R. LaMantia, R.R. Lunt, J.E. Oberholtzer,
E.L Field, and J.R. Valentine
Arthur D. Little, Inc.
Acorn Park
Cambridge, Massachusetts 02140
Contract No. 68-02-1071
Program Element No. EHE624
EPA Project Officer Norman Kaplan
Industrial Environmental Research Laboratory
Office of Energy, Minerals, and Industry
Research Triangle Park, N.C. 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, D.C. 20460
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ABSTRACT
This report presents the results of the Dual Alkali Program
conducted by Arthur D. Little, Inc., (ADL) for the Industrial
Environmental Research Laboratory, Research Triangle Park
(IERL, RTP) of the U.S. Environmental Protection Agency (EPA).
The purpose of the program was to investigate, characterize
and evaluate the basic process chemistry and the various
modes of operation of sodium-based dual alkali processes.
The work was carried out at three levels of investigation:
Task I - Laboratory studies at ADL and IERL, RTP.
Task II - Pilot Plant Operations in a 1,200 scfm
system at ADL.
Task III - Prototype Test Program on a 20-megawatt,
Combustion Equipment Associates (CEA)/ADL
dual alkali system at Plant Scholz, Southern
Company Services, Inc./Gulf Power Company.
Various modes of operating dual alkali systems on high- and
low-sulfur fuel applications were investigated, including:
Concentrated and dilute sodium scrubbing systems
Lime and limestone regeneration
Slipstream sulfate treatment schemes.
In each mode, the objective was to characterize the dual
alkali process in terms of 862 removal, chemical consumption,
oxidation, sulfate precipitation and control, waste solids
characteristics and soluble solids losses.
This is Volume I, the Executive Summary of the final report.
Volume II of the final report covers Tasks I and II, the Lab-
oratory and Pilot Plant Programs; Volume III covers the Pro-
totype Test Program, Task III.
iii
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VOLUME I
EXECUTIVE SUMMARY
TABLE OF CONTENTS
Page
Chapter NO.
ABSTRACT ill
ACKNOWLEDGEMENTS vii
I PURPOSE AND SCOPE 1-1
II DUAL ALKALI CHEMISTRY II-l
III OVERALL FINDINGS III-l
IV FINDINGS - TASKS I AND II, LABORATORY
AND PILOT PLANT PROGRAMS IV-1
A. PILOT PLANT S02 REMOVAL AND
OXIDATION GENERAL IV-1
B. CONCENTRATED MODE WITH LIME REGENERATION IV-1
C. CONCENTRATED MODE WITH SULFURIC ACID
SULFATE TREATMENT IV-4
D. CONCENTRATED MODE WITH LIMESTONE
REGENERATION IV-5
E. DILUTE MODE WITH LIME AND LIMESTONE
REGENERATION IV-8
F. SOLIDS CHARACTERIZATION DILUTE AND
CONCENTRATED LIME REGENERATION MODES IV-11
FINDINGS - TASK III, PROTOTYPE TEST PROGRAM V-l
A. BACKGROUND V-l
B. PROGRAM DESCRIPTION V-l
1. System Design V-l
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TABLE OF CONTENTS (cont)
Page
Chapter No.
V (cont) 2. System Operation V-2
C. SYSTEM PERFORMANCE V-3
1. S02 Removal V-3
2. Particulate Removal V-3
3. Oxidation/Sulfate Control V-4
4.. Waste Cake Properties V-5
5. Sodium Makeup V-6
6. Power Consumption V-7
7. Operability/Reliability Potential V-7
REFERENCES V-13
GLOSSARY V-15
APPLICABLE CONVERSION FACTORS -
ENGLISH TO METRIC UNITS V-17
vi
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ACKNOWLEDGEMENT S
The work under this program was performed over a four-year period from
May 1973 through May 1977, with contributions from many individuals
representing several organizations. Persons involved at Arthur D. Little,
Inc. were:
Principal Investigators
Charles R. LaMantia - Project Manager
Richard R. Lunt - Pilot Plant and Prototype Program Manager
James E. Oberholtzer - Laboratory Program Manager
Edwin L. Field - Data Analysis Manager
James R. Valentine - Chemical Analysis Manager
Contributing Staff
Itamar Bodek
Lawrance 1. Damokosh
Bruce E. Goodwin
George E. Hutchinson
Michael lovine
Bernard Jackson
Indrakuraar Jashnani
C. Lembit Kusik
Stephen P. Spellenberg
Robert A. Swanbon
Frank J. Tremblay
Lawrence R. Woodland
The EPA Project Officer for the entire four-year progam, Norman Kaplan,
made continuing and important technical and management contributions to
the program. Michael Maxwell and Frank Princiotta at EPA, through their
involvement in the review and planning, helped to guide the program over
the four-year period. The earlier part of the EPA laboratory program
was conducted under the direction of Dean Draemel, now at Exxon. EPA
laboratory work was carried on and completed by James MacQueen and Robert
Opferkuch of Monsanto Research Corporation under contract to EPA.
The cooperation and important contributions and support of Gulf Power
Company and Southern Company Services, Inc. (SCS) to the prototype test
program were invaluable. Randall Rush, responsible for coordination of
the program at SCS, made important technical contributions to the test
program and to the preparation of this report, in addition to this con-
tinuing support throughout the program; the value of Mr. Rush's dedica-
tion and commitment cannot be overstated. In addition, we would like to
thank Reed Edwards of SCS and James Kelly of Gulf Power for their on-site
vii
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assistance in the operation of the system. We wish to acknowledge the
cooperation of George Layman of Gulf Power and William Harrison of SCS,
individually and as representatives of their organizations, in making
the prototype system available and for the operation and maintenance of
the system during the program.
The cooperation, support and contributions of Combustion Equipment
Associates, Inc. (CEA) and its personnel were important to both the
pilot plant and prototype test programs. With the cooperation of CEA,
both systems were made available to the program. Tom Frank, the CEA
Project Manager for prototype system, and Richard White, on-site for
maintenance and operations, were importantly involved in the prototype
test program. The cooperation of Richard Sommer is gratefully acknowl-
edged for CEA's participation and support in this program.
viii
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I. PURPOSE AND SCOPE
This report presents the results of the Dual Alkali Program conducted by
Arthur D. Little, Inc., (ADL) for the Industrial Environmental Research
Laboratory, Research Triangle Park, (IERL, RTF) of the U.S. Environmental
Protection Agency (EPA). The purpose of the program was to investigate,
characterize, and evaluate the basic process chemistry and the various
modes of operation of sodium-based dual alkali processes. The work
covered a wide range of flue gas conditions, liquid reactant concen-
trations, and process configurations, including:
concentrated and dilute mode (dilute sodium scrubbing solutions,
active Na"*" concentration below about 0.15M)*
use of lime and limestone for regeneration
sulfuric acid treatment for sulfate control.
Each of the modes was evaluated relative to the following performance
characteristics:
S02 removal capability
oxidation and sulfate formation and control
lime/limestone utilization
waste solids properties
sodium makeup requirements and degree of closed-loop operation
process reliability.
Investigations were carried out at three levels: laboratory, pilot
plant and 20-megawatt prototype. Accordingly, the program was divided
into three tasks:
Task I Laboratory Program In the ADL laboratory program,
experiments were performed on the regeneration of concentrated
sodium scrubbing solutions using lime or limestone,and the use
of sulfuric acid treatment for sulfate removal. Work also
*See Glossary for dual alkali terminology.
1-1
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included characterization of the chemical and physical properties
of dual alkali solids. Work was performed also at IERL, Research
Triangle Park on regeneration using limestone in dilute mode opera-
tion.
Task II Pilot Plant Program Pilot plant work was conducted at
the Combustion Equipment Associates, Inc. (CEA)/ADL pilot facility
in Cambridge, Massachusetts. The following modes of operation were
investigated in the pilot plant program:
concentrated mode using lime for regeneration
concentrated mode using lime for regeneration with slipstream
sulfuric acid treatment for sulfate control
concentrated mode using limestone for regeneration
dilute mode using lime for regeneration.
Task III Prototype Test Program The test program was conducted
on the 20-megawatt CEA/ADL prototype dual alkali system at Gulf Power
Company's Scholz Steam Plant in Sneads, Florida, from May 1975 to
July 1976. The prototype system used lime in a concentrated mode.
The system was operated on flue gas generated from moderately low-
to high-sulfur coals, and with varying particulate loads to the system.
This is Volume I, the Executive Summary of the final report. Volume II
of the final report covers Tasks I and II of this program; Volume III
covers the Prototype Test Program, Task III.
1-2
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II. DUAL ALKALI CHEMISTRY
The chemistry and terminology of dual alkali systems are briefly described
here. A more detailed description is given in Volume II, Chapter III; a
glossary of terms is included in each volume and with this Executive
Summary.
In the absorption section of sodium-based dual alkali processes, absorption
of 862 in sodium sulfite solutions occurs to produce a bisulfite scrubber
effluent solution according to the overall reaction:
Na2S03 + S02 + H20 £ 2NaHS03 (1)
Depending upon the dual alkali mode being used, the feed to the absorber
may also contain some sodium hydroxide (formed in the regeneration section
or used as sodium makeup) and/or some sodium carbonate (used as sodium
makeup to the system). Both sodium carbonate and hydroxide form sodium
sulfite on absorption of S02:
Na2C03 + S02 > Na2S03 + C02 (2)
2NaOH + S02 + Na2S03 + H20 (3)
which is used in further absorption to produce bisulfite. The regenerated
feed solution to the absorber will also contain some level of sodium sul-
fate in solution and may contain some sodium bisulfite if neutralization
is not completed in the regeneration section. Sodium, identified as asso-
ciated with anions involved in S02 absorption reactions, is referred to as
"active" sodium (includes sodium sulfite, bisulfite, hydroxide, carbonate/
bicarbonate). The sulfite/bisulfite content of solutions, or total oxidiz-
able sulfur content, is also referred to as TOS.
Some oxidation of sulfite to sulfate occurs in the absorber due to reaction
of sulfite with oxygen in the flue gas:
2Na2S03 + 02 * 2Na2SOit (4)
The ra,te of oxidation or oxygen transfer in the absorber is a function
of the absorber design, oxygen concentration in the flue gas, flue gas
temperature, and the nature and concentration of the species in the
scrubbing solution.
The regeneration of acid sodium sulfite/sulfate scrubber effluent solutions
can be considered as a sequential reaction first involving neutralization
of the bisulfite using either lime or limestone, to produce a precipitate
of calcium sulfite:
II-l
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2NaHS03 + Ca(OH)2 -» Na2S03 + CaS03 1/2H2OI
+ 3/2H20 (5)
2NaHS03 + CaC03 + Na2S03 + CaS03 l/2H2Oi
+ C02 t + 1/2H20 (6)
In theory , the lime neutralization reaction should go to completion;
complete neutralization of bisulfite is not possible with CaCO.j. Using
lime, the regeneration can be carried beyond neutralization to generate
caustic:
Na2S03 + Ca(OH)2 t 2NaOH + CaS03 (7)
to some equilibrium hydroxide concentration. The usual form of calcium
sulfite produced is the hemihydrate, CaS03 1/2H20.
Depending upon the concentration of sulfite and sulfate and the pH of
the solution, the following reaction for sulfate removal also occurs
simultaneously with neutralization and regeneration reactions (5)-(7)
using either lime or limestone:
(8)
The form of this calcium sulfate was investigated and will be discussed
later in this report.
Thus, the level of sulfate precipitation in the overall scheme is given
by the ratio of calcium sulfate to total calcium/ sulfur salts produced:
mols CaSOi*
Sulfate Precipitation = mols CaS0^ + mols CaS03
Sulfate precipitation is directly related to the ratio of sulfate/sulfite
in solution in the reactor, increasing with the ratio.
If the level of sulfate formation or sulfite oxidation given by:
Sulfate Formation
mols S03 oxidized
mols S02 removed
is matched by the level of sulfate precipitation, then all sulfur removed
from the flue gas can leave the system as a calcium salt and no soluble
sulfate purge is necessary to maintain a sulfate balance in the system.
In practice, even if such a balance is established, the washed calcium
II-2
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sulfite/sulfate salts will contain some soluble sodium salts as well as
soluble fly ash constituents which must be purged and some sodium makeup
to the system will therefore be required. Calcium utilization or yield
in the overall process is defined as:
(mols CaS03 + mols CaSOi,) generated
Calcium Utilization = =r^: x 100%
mol Ca fed
regardless of whether lime or limestone is used.
Regeneration reactions in the "concentrated mode" produce solutions
saturated (or possibly supersaturated) with respect to calcium sulfite;
in the "dilute mode" saturation (or possibly supersaturation) is with
respect to calcium sulfate (gypsum) . For purposes of this program,
dilute operating modes are considered to be those involving solutions
containing active sodium concentrations less than or equal to 0.15 molar
active Na"1", where active sodium is sodium sulfite/bisulfite, carbonate/
bicarbonate or hydroxide. Concentrated modes are those involving solutions
containing active sodium concentrations greater than 0.15 molar active
Na+. Soluble calcium levels in dilute mode regenerated solutions are
quite high compared to levels in the concentrated mode, usually re-
quiring "softening" with carbonate,
Ca4"*" + COf £ CaC03 (9)
to prevent scaling by precipitation of calcium salts in the scrubber.
The sulfite and sulfate concentrations, besides affecting sulfate preci-
pitation, also have a profound effect on the physical properties of the
calcium/sulfur salts and, as a result, on the settling and filtration
characteristics of the solids produced in dual alkali modes.
As an alternate approach to operating at decreased active sodium concen-
trations, high levels of sulfate precipitation can be achieved using
sulfuric acid treatment for precipitation of sulfate according to the
following equation:
Na2SOtt + HaSOi* + 2CaS03 1/2H20 + 3H20 t 2NaHS03 + 2CaSOit 2H20 (10)
*
This reaction is carried out at a low pH (2-3), where sulfite is converted
to bisulfite, thereby bringing calcium sulfite into solution and exceeding
the solubility product for calcium sulfate. This reaction can be applied
on a process slipstream for sulfate precipitation, as required. However,
in the overall process this scheme utilizes sulfuric acid, requiring addi-
tional lime or limestone for ultimate neutralization of this acidity added
to the system and generates an equivalent additional amount of waste calcium/
sulfur salts.
II-3
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III. OVERALL FINDINGS
Based upon results from all three tasks of this program, laboratory
experiments, pilot plant operations, and prototype testing, we have
concluded that both the concentrated and the dilute modes, using lime
for regeneration, can be operated in a closed loop. No purge of soluble
solids is required other than solubles in the liquor wetting the washed
filter cake. These processes can be applied over a wide range of boiler,
fuel, and flue gas design and operating conditions. The concentrated
mode is inherently more reliable, less complex and cheaper than the
dilute mode, but is limited to situations where oxidation rates will
not exceed about 25% of the S02 removal rate. The dilute mode can be
operated at higher oxidation rates.
The performance of these two modes relative to the important performance
characteristics is as follows:
S02 Removal These modes are capable of easily achieving over
SC>2 removal and approaching 99% removal. 862 absorption is easier
to control, especially at higher removal efficiencies, in the con-
centrated mode relative to the dilute mode.
Lime Utilization Utilization easily exceeding 90% and up to 100%
can be achieved in either mode. Utilization increases with reactor
residence time in both modes, but decreases as regeneration is carried
beyond neutralization to the generation of free hydroxide, approaching
the reaction equilibrium limits.
Oxidation and Sulfate Removal In the concentrated mode calcium
sulfate is coprecipitated with calcium sulfite at rates sufficient
to keep up with oxidation levels up to about 25% of the S02 removal.
The sulfate coprecipitates as a mixed crystal with the sulfite in a
manner apparently similar to the coprecipitation observed in direct
lime/limestone scrubbing systems. The dilute mode can be operated to
precipitate gypsum, equivalent to 100% oxidation in the system.
Waste Cake Solids Properties In the concentrated mode, the insoluble
solids content of filter cakes ranges from 45% to 75%. The moisture
content increases as the sulfate content in the loop increases, in-
creasing the calcium sulfate content of the cake. Under similar process
conditions in the concentrated mode, solids properties are improved
using a multistage reactor system. In the dilute mode, insoluble sol-
ids content of filter cakes ranges from 60% to 80% when the scrubber
effluent is oxidized prior to regeneration to produce gypsum as a
waste material.
The soluble solids content in the cake from either mode can be reduced
to the range of 1% to 5% soluble solids (dry cake basis) by washing.
The soluble content of the cake decreases as the concentration of total
dissolved solids in the dual alkali loop decreases; and as the amount
of cake wash water available in the water balance increases (wash
III-l
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water availability increases with decreasing sulfur content of the
fuel) .
The solids generated in each of these modes have good dewatering
properties and result in a filter cake with good handling properties.
The concentrated mode sulfite/sulfate cake has the general consistency
of a moist soil-like material. In limited tests it responded simi-
larly to fixation treatment as other sulfite/sulfate waste materials
from lime/limestone processes. The dilute mode gypsum material also
handled well with a more granular, sand-like consistency.
Sodium Makeup Requirements Since the systems operate in a closed
loop, the sodium makeup requirements are equivalent to the sodium
losses in the cake and would range from 0.05 mols Na2C03/mol of S0£
removed down to about 0.01, depending upon the ability to wash the
cake in the specific application, as discussed above.
Power Consumption When the dual alkali system is operated for
removal only, with no simultaneous particulate removal requirement,
the overall system power consumption would be about 1% of the gener-
ating capacity of the unit being controlled. In a high-sulfur coal
application, 95% 862 removal can be achieved in an efficient two-stage
absorber (e.g. two contact trays) with a total absorber liquid/gas
ratio of under 10 gallons/1000 saturated cubic feet. The scrubber
system pressure drop should be under 10 inches H20.
Reliability Aside from concentration differences, the principal
difference in the operating characteristics between dilute and con-
centrated lime dual alkali systems is that dilute systems operate
at or near saturation in calcium sulfate, potentially reducing the
reliability and ease of operation of the dilute systems. Dilute
systems require the use of carbonate makeup to provide some soft-
ening of the regenerated solution prior to recycle back to the
scrubber. The concentrated mode is inherently quite reliable and
stable requiring no softening for scale control.
No viable approach was developed in this program to enable the use of
limestone for regeneration. The problem which remains to be solved is
the production of solids with good settling characteristics over a wide
range of sulfate, magnesium, and iron concentrations in the scrubbing
liquor. These components tend to reduce the rate of reaction of lime-
stone with sodium scrubbing solutions; solids properties tend to deteri-
orate with a decrease in the limestone reaction rate. However, these
problems may be resolvable.
There are considerable economic incentives for the substitution of lime-
stone for lime In dual alkali processes justifying further work In this
area. Operating cost savings of about 7C/106 Btu can be realized if lime-
stone is used in lieu of lime in high-sulfur coal applications. These
potential savings are equivalent to about 20% of the total annual lime
III-2
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dual alkali operating cost (with lime at $40/ton, limestone at $5/ton,
and including annual capital charges).
The actual performance of any particular dual alkali process will vary
depending upon the S02 and oxygen concentrations in the flue gas, the
design of the system and the concentration of sodium solutions used in
the process. Using lime, some version of the dual alkali process can
generally be designed to achieve very desirable performance characteris-
tics in most utility applications.
More specific results and conclusions regarding the various dual alkali
modes and pilot plant operations are given in following chapters.
III-3
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IV. FINDINGS - TASKS I AND II. LABORATORY
AND PILOT PLANT PROGRAMS
A. PILOT PLANT SO? REMOVAL AND OXIDATION GENERAL
The pilot plant S02 removal and oxidation data are specific to the pilot
plant scrubber configuration as influenced by the scrubber operating tem-
perature for the pilot plant flue gas stream. The scrubber operating
temperature of 140-150°F is higher than that normally encountered in
conventional boiler flue gas applications (120-130°F). The elevated
temperature in the pilot plant system tends to decrease S02 removal effi-
ciency due to elevated S02 partial pressures for any given solution, and
tends to increase oxidation rates. However, the purpose of the pilot plant
scrubber and its operations was to provide scrubber effluent with an appro-
priate composition for use in the various dual alkali modes rather than to
generate basic data on S02 absorption using sodium solutions.
Within the above constraints, the scrubber operations did indicate that
802 removal in excess of 90% is easily accomplished over a range of 802
inlet concentrations from 700-2,800 ppm by adjusting the scrubber feed
stoichiometry. To achieve this removal efficiency, a stoichiometry of
1.1 mols of active Na"1" capacity/mol 802 inlet was required at the high
inlet S0£ range; a stoichiometry of 1.3 was required in the lower inlet
S02 range. In any range of S02 concentration, increasing stoichiometry
increased the S02 removal. There was no important apparent effect of
active sodium concentration within a range of 0.2-0.5M or total dis-
solved solids concentration within a range of 5-15 wt %.
Sulfite oxidation is mass transfer limited at active sodium concentrations
above 0.2M with the rate of oxidation increasing with the oxygen content
of the flue gas. At lower active sodium concentrations the oxidation
rate is roughly proportional to the active sodium concentration. The
rate of oxidation decreases with increasing total dissolved solids; by
increasing TDS from 8-15 wt % to 25-35 wt %, the oxidation rate would be
reduced by a factor of 2-3. At lower total dissolved solids in concen-
trated active sodium systems (0.3-0.6M active Na+, 5-15 wt % TDS) sulfite
oxidation can be expected to be on the order of 100-300 ppm equivalent
S02 removal for oxygen concentrations in the flue gas ranging from 4 to
8 vol %.
B. CONCENTRATED MODE WITH LIME REGENERATION
In the concentrated mode using lime for regeneration, calcium sulfate
will coprecipitate with calcium sulfite at sulfate precipitation rates
equivalent to oxidation rates as high as 25% of the S02 removal depending
upon the solution sulfate and sulfite concentrations. Solutions remain
unsaturated with respect to calcium sulfate and have low soluble calcium
concentrations. Process modes can be operated over a wide range of sodium
solution concentrations achieving high S02 removal (greater than 90%)
IV-1
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producing good quality filter cake (45% solids or greater), and containing
low soluble solids (2-5 wt % dry cake basis) with no sulfate purge required.
The performance characteristics of concentrated lime regeneration modes are
summarized in more detail below.
S02 Removal S02 removal efficiencies in excess of 90% were easily
achieved with the removal efficiency a function of sodium solution
feed stoichiometry for any particular absorber design. In all closed-
loop runs the feed stoichiometry (scrubber operating pH) was con-
trolled to ensure better than 90% removal. For a given design, a
slightly higher feed stoichiometry (or operating pH) was required
for high sodium solution concentrations (30-35 wt % sodium salt
solutions) than for moderate concentrations (10-15 wt % sodium salt
solutions) to achieve this same removal efficiency because of the
increase in 862 equilibrium partial pressure with the increase in
sodium sulfite/bisulfite concentration.
Lime Utilization Lime utilization in the range of 95-100% can be
achieved with reactor holdup times of 25 minutes or greater when re-
generating to a pH of 8 or higher. High utilizations can be achieved
at shorter residence times if the regeneration reaction is not carried
beyond neutralization of the bisulfite. Lime utilization decreases
if regeneration is carried much beyond a pH of 12.5.
Oxldation/Sulfate Control At active sodium concentrations above
about 0.2M, calcium sulfate coprecipitates with calcium sulfite upon
reaction of the sodium salt solution with lime. The sulfate/sulfite
content of the precipitated calcium salts is related to the sulfate/
sulfite concentrations in the reactor liquor by the following relation-
ship:
= 0.0365 [ir\ (11)
^idl X ' *eactor
solids ,.
liquor
This relationship describes the coprecipitation phenomenon over the
range of sulfite and sulfate liquor concentrations used in labora-
tory and pilot plant experiments ( [SO"] > 0.2M, [SO"]/[SO|] -0-6).
This method of sulfate precipitation is effective for oxidation rates
up to about 25%. At any given active sodium concentration, high sulfate
precipitation appears to be favored by either partial neutralization of
the absorbent solution or regeneration to pH's well above neutrality
(>11.5), thereby reducing the sulfite concentration in the reactor
liquor and maximizing the sulfate/sulfite ratio in the liquor.
In a properly designed concentrated dual alkali loop, the sulfate/
sulfite ratio will self-adjust at steady-state so that the rate of
sulfate 'precipitation equals the rate of sulfite oxidation. It is
IV-2
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possible to achieve this balance over a wide range of active sodium
and sulfate concentrations in dynamic response to changes in flue gas
rates and oxygen and S02 concentrations.
For dual alkali systems operating with high IDS (in the range of
25-30 wt % sodium salt solutions) oxidation rates can be reduced
by a factor of 2-3 from those encountered at lower TDS levels
(10-15 wt %). At such high TDS levels, the active sodium concen-
tration as well as the sulfate concentration must be elevated in
order to promote effective regeneration reactions and production
of solids with acceptable dewatering properties. As a result,
sulfate precipitation capability is limited.
Solids Properties ~ Single-stage CSTR (continuous stirred tank
reactor) and multistage reactor systems can produce solids, over
a wide range of process conditions, which settle well and filter
to insoluble solids contents of 45 wt % or higher. When using a
single CSTR as the regeneration reactor, solids properties deteri-
orate as the regeneration reaction is carried to a higher pH range
with the degree of deterioration increasing from pH 7.5 to pH 12.
This effect is worse for reactor holdup times of 60 minutes than
for shorter reactor residence times (30 minutes). Using a CSTR,
solids properties also decrease as the sulfate/sulfite ratio in-
creases in the reactor liquor (at higher oxidation rates). In a
single-stage CSTR, it is difficult to produce solids with acceptable
properties (45 wt % insoluble solids) at process conditions consis-
tent with sulfate precipitation and sulfite oxidation rates much
beyond 15%.
Good quality solids can be produced over a wider range of pH and sulfate
concentration using a two-stage reactor system, consisting of a short
residence time reactor (5-10 minutes) followed in series by a longer
residence time second stage (20-40 minutes). This multistage system
produces good solids at pH levels up to about 12.5 and at sulfate/
sulfite ratios required for sulfate precipitation rates equivalent
to about 25% oxidation.
Sodium Losses For a filter cake containing 50% insoluble solids,
the soluble solids content of the cake can be reduced to 2-3% (dry
cake basis) using the amount of filter cake wash water which would
normally be available when operating closed-loop in a high sulfur
coal boiler application. At TDS levels in the range of 10-15 wt %,
2 to 3 displacement washes are effective in reducing the soluble
content of the cake to 2-3 wt %. Of this material, 0.5-1.0 wt %
soluble sodium salts appear to be occluded in the calcium salt
crystals and cannot be washed regardless of the amount of wash water
used. About 2-3 displacement washes are available for high-sulfur
coal applications. At high TDS concentrations (30%), 4 to 5 dis-
placement washes are necessary to reduce solubles to the 2-3% level.
With only 3 displacement washes, solubles losses at high TDS concen-
trations can be expected to be roughly twice those when operating at
10-15 wt % TDS levels in the absorbent solution.
IV-3
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At the lower IDS levels, sodium makeup requirements are on the order
of 2-3% of the total alkali requirements (mol basis). That is,
roughly 2-3% of the sulfur absorbed from the flue gas leaves the
system as sodium salts with the remainder as calcium salts.
From the above considerations, operating a concentrated lime mode
with IDS in the range of 10-15%, the single-stage CSTR can produce
good quality solids (45 wt % or greater) containing 2-3 wt % solubles
at system oxidation rates up to 15%. When using the multistage reactor
system the operability of the process is extended to oxidation levels
in the range of about 25%. Increasing IDS reduces oxidation but re-
quires more wash water to produce the same cake solubles content. At
2 to 3 displacement washes, the solubles content of the cake is pro-
portional to the IDS levels in the system loop.
System Operability/Reliability In concentrated modes using lime
for regeneration, soluble calcium concentrations range from 15-90 ppm
with the calcium concentration generally decreasing with increasing
sulfite concentration. No scaling or deposition of solids was ob-
served in the scrubber loop during any of the concentrated mode
operations. Scrubber operation and 862 removal were easy to control.
The regeneration reaction is stable and easy to control, but should
be kept at a pH below about 8 if operating with a single stage re-
generation reactor. Increasing the IDS level in the system raises
the sodium salt saturation temperature, Increasing the potential
for solid sodium salt crystallization in elements of the system
which are permitted to cool.
C. CONCENTRATED MODE WITH SULFURIC ACID SULFATE TREATMENT
The sulfuric acid slipstream treatment scheme is a technically feasible
and reliable approach for removal of soluble sulfates from dual alkali
systems. The basic chemistry of the treatment process is given in the
following simplified reaction equation:
2CaS03 l/2H20(filter cake) + Na2SOif(system liquor)
(12)
+ H2SO(, + 3H20 -> 2NaHS03 + 2CaSOit 2H20
The treatment produces sulfate in the form of gypsum that can be readily
dewatered to 65 wt % insoluble solids or higher. The scheme adds com-
plexity to any dual alkali mode to which it is applied. The complexity
is reflected in additional capital costs and in increased operating costs
for the sulfuric acid, the additional lime consumed and the additional
solid waste produced.
The amount of sulfuric acid required is important since it directly affects
the overall lime requirement. As the sulfuric acid addition rate increases,
the lime rate must increase accordingly for precipitation of the additional
sulfur value added to the system. The maximum efficiency of the treatment
IV-4
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scheme ((mo Is Na2SOi4 removed/mo Is I^SOit fed) x 100%) appears to be practi-
cally limited to a maximum in the range of 60-70%. In order to precipi-
tate sulfate at a rate sufficient to keep up with an oxidation rate of
15% (of the S02 absorbed), the lime feed requirement will be increased
by 25% for a 60% reactor efficiency.
The efficiency of the sulfuric acid treatment is importantly affected
by the calcium utilization achieved in the absorbent regeneration reactor
in the main dual alkali loop. As calcium utilization decreases in the
main loop the efficiency of the sulfuric acid slipstream treatment de-
creases and acid consumption increases to neutralize unreacted lime in
the filter cake. In order to achieve a 50% efficiency in the sulfuric
acid treatment system, calcium utilization in the main dual alkali loop
must exceed 90%,
Because the use of this sulfuric acid treatment scheme may be costly
when applied to systems with high oxidation rates (due to the sulfuric
acid and extra lime requirements), it may be more appropriate for systems
with intermediate levels of oxidation where the rate of sulfate formation
cannot be easily handled in a simpler concentrated sodium mode. The con-
sequences of using the sulfuric acid slipstream treatment approach for
sulfate regeneration should, therefore, be carefully evaluated in terms
of the overall process operation. In many cases, where oxidation rates
are high enough that they cannot be easily handled by normal concentrated
mode operation, other dual alkali approaches, such as the dilute lime
system, might be more promising than a sulfuric acid treatment scheme.
D. CONCENTRATED MODE WITH LIMESTONE REGENERATION
No viable approach was found for use of limestone in a concentrated dual
alkali mode. Through the laboratory and pilot plant efforts allocated
to work on the concentrated limestone mode, we were not able to develop
process parameters and reactor conditions consistent with good limestone
utilization and generation of acceptable quality waste solids. The work
did, however, uncover important factors influencing the limestone re-
generation reaction that indicated promising areas of future work. Un-
like results from work on use of limestone in dilute modes, the potential
for technical success argues for additional work on the concentrated lime-
stone dual alkali mode; especially when the economic incentives are con-
sidered.
*
Limestone is substantially less reactive toward sodium salt solutions
than is lime, even when reacting with relatively acidic scrubber bleed
solutions. The reaction rate of the limestone regeneration reaction is
dependent upon:
nature of the limestone and its particle size;
reactor temperature and residence time;
IV-5
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concentrations of soluble reactants (sodium sulfite,
sodium bisulfite and sodium sulfate); and
the presence, at low concentrations, of trace constituents
such as magnesium and iron, which influence the reaction rate.
Increase in the reaction rate was generally consistent with improvement
in the dewatering properties of the solids produced and with improved
utilization of limestone.
Three limestones, with similar particle size distribution, were examined
Fredonia limestone used in the EPA/TVA Shawnee program; another, locally
available, natural limestone; and reagent grade CaCOa. Of these, the
Fredonia limestone was amorphous, rather than crystalline in nature, and
was considerably more reactive than the other two limestones examined.
The Fredonia limestone, therefore, was used extensively in the laboratory
and pilot plant programs.
Laboratory experiments indicated that increasing temperature importantly
increased the reaction rate. However, the pilot plant was not equipped
for heating the reactors or for heating the reactor feed. As a conse-
quence pilot plant regeneration was performed at a maximum of about 50°C.
The dewatering properties of solids were generally observed to deteriorate
as the regeneration reactor residence time was increased. Increasing the
reactor residence time results in carrying out the reaction closer to the
equilibrium conditions and consequently at a lower driving force and reac-
tion rate. Use of multistage reactor systems, containing several stages
with residence times in the range of 15 minutes, were found to produce
solids with a quality superior to that of solids produced in fewer reac-
tors with the same total residence time. Recycle of solids, increasing
reactor 'solids concentrations from about 2 wt % to 5 wt %, improved
limestone utilization but did not appreciably improve the quality of
the solids.
Under controlled conditions, with a multistage reactor system operating
at about 50°C, it was possible to produce solids with acceptable dewater-
ing properties (45% insoluble solids) and to achieve limestone utiliza-
tions on the order of 75%. However, if the sulfate concentration in the
loop rose above 0. 7M or if the magnesium concentration rose much above
300 ppm, the reaction rate, and the resulting limestone utilization and
solids properties all deteriorated.
Sulfate concentration in the reactor liquor had a much more important,
deleterious affect on the reaction rate and solids properties in lime-
stone regeneration reactions than the similar effects of increased sulfate
concentration observed in concentrated lime regeneration. As in lime re-
generation, the reaction rate is inversely proportional to the ratio of
sulfate/sulfite concentrations in the liquor; but the rate drops dramat-
ically using limestone as the sulfate concentration exceeds 0.7M at TOS
IV-6
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levels of 0.3-0.5M. Operation at lower sulfate/sulfite ratios tends to
limit sulfate precipitation in this mode and limit the range of oxidation
in which limestone regeneration could be operated closed loop.
Calcium sulfate coprecipitates along with calcium sulfite in concentrated
limestone regeneration reactions in an analogous fashion to the coprecipi
tation of calcium sulfate observed in the concentrated lime regeneration.
However, pilot plant data indicate that for the same sulfate/sulfite con-
centrations with the same range of TOS in the feed liquor (i.e., [TOS] Of
0.3-0.5M), lower sulfate precipitation occurs when using limestone, as
given by the following:
/CaSO,A
VT^T/ °-°22
\caso3/
reactor 3 reactor
solids liquor
The sensitivity of the reaction to high sulfate concentrations and the
lower sulfate precipitation rates make limestone regeneration less viable
for closed-loop operation than lime regeneration at higher oxidation rates.
The presence of Mg"*~*" in solution, introduced into the system in varying
amounts depending upon the magnesium content of the limestone, also can
retard the limestone regeneration reaction rate, resulting in poor solids
quality and limestone utilization. This effect becomes pronounced as the
MS"*"*" concentration rises much above a few hundred ppm. Relatively low
magnesium limestones, such as Fredonia limestone (1.0-1.5 wt % Hg as
MgCOs), would result in concentrations on the order of a few thousand
ppm, at steady-state, in a concentrated dual alkali loop.
Laboratory work confirmed that magnesium concentrations could be controlled
by reacting part of the process stream with lime to precipitate Mg(OH>2-
However, such an approach would reduce operating cost savings, requiring
part of the total regeneration to be performed using lime. Use of lime
with limestone would increase the complexity and the capital cost, re-
ducing economic Incentive.
In pilot plant operations, iron from corrosion of unlined steel equipment
was found to have an effect similar to that of magnesium on the limestone
regeneration reaction at pH's below about 6. At higher pH's, Fe(OH>3 ia
highly insoluble, limiting the buildup of iron in solution. By selection
of proper materials of construction and linings and by carrying the lime-
stone regeneration beyond a pH of 6, interference by iron can be eliminated
in concentrated limestone modes.
Future work on limestone regeneration should be directed at increasing
reaction rates at high magnesium levels by increasing sulfite concentra-
tions, reactor temperature and by staging of the reactors.
IV-7
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E. DILUTE MODE WITH LIME AND LIMESTONE REGENERATION
Use of limestone only for the regeneration of solutions in the dilute
mode (less than 0.15M active sodium) is not viable. The limestone
reaction rate decreases as the ratio of soluble sulfate/sulfite increases
in the reactor solutions. At sulfate/sulfite ratios required for adequate
sulfate precipitation in the dilute mode, reaction rates are poor result-
ing in poor limestone utilization and poor solids quality.
Use of lime in combination with limestone in a dilute dual alkali mode
was more viable technically. In this approach, the lime regeneration
was carried out in a second reaction system to promote sulfate precipi-
tation. The limestone/lime process is more complicated than a simple
dilute lime process, resulting in higher projected capital cost. Economic
analysis indicated that operating cost savings which could potentially be
realized in using limestone for part of the regeneration would not offset
the additional capital cost probably required to enable use of the lime-
stone. The dilute lime system, using soda ash for softening, was technically
and economically the most viable dilute mode considered. Conclusions based
upon laboratory and pilot plant investigations of this mode are given below.
A dilute lime mode can be operated in a closed loop with sulfate precipi-
tation keeping up with any level of system oxidation. The system can be
operated with high SO^ removal (90% or higher) and good lime utilization
(90% or higher) to produce high quality solids (60% insolubles or higher)
with low soluble sodium losses (2 wt % soluble solids achievable, dry cake
basis). The process may be more appropriate for low-sulfur coal applications
or in situations where oxidation rates are expected to exceed 25-30% of the
S(>2 removal. The dilute lime mode is somewhat more complicated than the con-
centrated lime mode, involving higher liquid rates and larger reactors and
associated equipment. The process is also potentially less reliable than
the concentrated lime approach.
The regeneration reaction, carried out at low sulfite levels, results
in the precipitation of calcium sulfate (usually gypsum) to produce a
regenerated solution of sodium hydroxide and sodium sulfate with soluble
calcium levels which are, at best, at the saturation level of about 700 ppm
Ca'++. Even with moderate amounts of soda ash makeup (and resulting soften-
ing by precipitation of calcium carbonate) the solutions have soluble
calcium levels in the range of 600-700 ppm with a high potential for
scaling in the system. Close control of scrubber pH is required to
prevent carbonate or sulfite scaling. High scrubber oxidation rates
may create sulfate scaling.
In the dilute mode regeneration reaction, there is a high tendency to
produce solutions which are supersaturated in Ca"1"1" (with respect to
gypsum). Using a single-stage CSTR with no solids recycle, calcium
supersaturation levels of 100-200 ppm are easily encountered. Special
design precautions must be taken to prevent supersaturation and the
resulting scaling throughout the system. Supersaturation can be re-
IV-8
-------
duced In a number of ways, by reactor system design and by controlling
conditions of the regeneration reaction:
Increased reactor residence time Allows time for completion of
reaction and desupersaturation. Holdup time of 60 minutes is a
minimum; 90 minutes is preferable.
Solids recycle Increases suspended solids concentration and seed crystal
concentration for reaction and desupersaturation. Recycle of solids
to achieve a concentration of 4% or higher suspended calcium salt
solids is required to eliminate supersaturation in the reactor
effluent.
Oxidation of sulfite in scrubber bleed prior to regeneration Lowers
the concentration of TOS which tends to retard the lime/sulfate reac-
tion when TOS is present in the dilute mode concentration range. Oxida-
tion to TOS concentrations of about 0.02M or lower is desirable.
Multistage reactor configuration Solids generated in a short
residence time first stage provide good seeds for completion of
reaction in a longer residence second stage. Using a multistage
reactor can reduce supersaturation to within about 50 ppm of the
saturation level. Solids recycle is required to completely elimi-
nate supersaturation.
Elimination of supersaturation was achieved in the single-stage reactor,
with 90 minutes' residence time; using solids recycle to the minimum of 4%
suspended calcium salts in the reactor; and with oxidation of the reactor
feed solution to TOS levels of 0.02M or lower. Variation in soluble sulfate
concentrations in the range of 0.50-0.75Mhad no apparent effect on the
level of supersaturation.
Utilizing these design factors in dilute mode with lime regeneration not
only reduces or eliminates supersaturation, but also promotes a good
reaction rate which generally improves the overall process performance
parameters such as lime utilization, sulfate precipitation and solids
properties. More specifically, the performance of the dilute lime mode
relative to the important process performance characteristics is given
below:
S02 Removal S02 removal of 90% is easily achieved especially at
low to medium inlet S02 levels. S02 removal is not as efficient as
in a concentrated dual alkali mode (with the same scrubber configura-
tion) because of the low active sodium concentration. The scrubber
operation is more difficult to control due to the low buffering
capacity of the dilute mode liquors. Higher calcium concentrations
(in the range of 600-700 ppm Ca"1"*") present potential scaling problems
in the scrubbing system. Operation of the scrubber in a high pH
range (9-11) to promote good S02 removal results in some C02 absorp-
tion and potential carbonate scale formation. Increasing active
sodium concentrations to provide more buffering can result in sulfite
scale formation in the pH range of 8-11.
IV-9
-------
Lime Utilization Lime utilization of 90% or higher is achievable
when regenerating to hydroxide concentrations of about 0.1M with
solutions containing sulfate in the range of 0.50-0.75M and using
reactors with a minimum total holdup time of 60 minutes. Utilization
increases as the residence time and sulfate concentration are in-
creased. Solids recycle also helps increase lime utilization. How-
ever, TOS levels in the feed to the reactor should be held to 0.02M
or less (by deliberate oxidation if necessary) to prevent retarding
of the reaction rate by the sulfite.
Oxidation/Sulfate Control Complete sulfate control is possible in
this mode of operation at any rate of oxidation in the system. How-
ever, at very high scrubber oxidation rates, sulfite/bisulfite buffer-
ing is minimal and scrubber pH control becomes difficult. All other
aspects of the process operation are improved by high oxidation rates
(i.e., minimal TOS concentration in the feed to the regeneration
reactor). Deliberate oxidation should be used to maintain TOS levels
below about 0.02M. At sulfate concentrations in the range of 0.52-0.75M
calcium sulfate (usually gypsum) was produced instead of a mixed calcium
sulfite/calcium sulfate crystal, when TOS is maintained at or below
0.02M. At this point the calcium sulfate content of the solids is
no longer limited by the apparent maximum content of 25-30% in the
mixed crystal; 100% calcium sulfate can be produced.
Solids Properties It is possible to produce excellent quality
solids containing 60%-80% insoluble solids. Good solids properties
are- favored by the following conditions:
Low TOS in the reactor feed - less than 0.02M.
High sulfate in the reactor feed - 0.50-0.75M (high end of
range favored).
Solids recycle - improves solids quality but increases the
solids load and ultimately the size of the thickener.
Multistage reactor system - improves solids quality compared
to same total residence time in a single stage.
High reactor residence time - 80% Insoluble solids can be
produced using a 90 minute residence time reactor.
Sodium Losses In any application, increasing the insoluble solids
content of the filter cake increases the effective number of displace-
ment washes for any given amount of wash water available. By pro-
ducing 75% insolubles solids in a high-sulfur coal application,
roughly 5 displacement washes are available (as opposed to 2.5 dis-
placement washes at 50% solids) permitting more effective cake washing;
in low-sulfur coal applications even more wash water can be available.
Consequently, sufficient wash water should be available to reduce the
solubles content of the cake to under 2%; and down to the range of
IV-10
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0.5-1.5% solubles in low-sulfur coal applications. In such
applications it may be possible to wash the filter cake to loss
levels lower than those corresponding to sodium carbonate makeup
levels required for softening of the regenerated liquor. A sodium
carbonate makeup rate of 2-2.5% of the 862 removal rate provides
sufficient carbonate to reduce the Ca"^1" concentration in the regen-
erated liquor by about 50 ppm, providing only minimum softening.
Thus, sodium makeup (and ultimately the losses in the cake) may be
controlled by softening requirements rather than by wash water
availability or cake washability.
System Operability/Reliability The dilute lime mode is inherently
less reliable and more difficult to control than the concentrated
lime mode. When appropriate care is taken to eliminate supersatura-
tion, the calcium levels in the regenerated solution are in the range
of 700 ppm. Only a minimum of softening is provided at low sodium
carbonate makeup levels. Potential for scaling exists in the reactor
system and associated auxiliaries and piping, and in the absorber.
Absorber operation is less effective and more difficult to control
than in the concentrated mode.
F. SOLIDS CHARACTERIZATION DILUTE AND CONCENTRATED
LIME REGENERATION MODES
Limited testing was performed to characterize the basic physical and
chemical properties of ash-free waste filter cakes produced in the two
most successful dual alkali modes piloted concentrated and dilute
active sodium modes with lime regeneration. Testing included: analysis
of major chemical constituents; crystalline morphology via X-ray diffrac-
tion and scanning electron microscopy; unconfined compressive strength
compaction moisture/density relationship; permeability; leaching behavior;
and the effects of treatment with lime (or portland cement) and fly ash
on the physical properties.
The concentrated mode filter cake that was tested was produced in the
prototype system using" the two-stage reactor. The cake was a mixture
of calcium sulfite and sulfate (about 15% calcium sulfate) and contained
55% solids. The crystalline structure of the solids was rosette-like
agglomerates of needles characteristic of the concentrated mode opera-
tion. X-ray diffraction data and chemical analyses indicate that the
calcium" sulfite and calcium sulfate were coprecipitated as a mixed crystal
of hemihydrate salts. There was no evidence of any appreciable amount of
gypsum (CaSOif. 21^0) in the solids.
The dilute mode filter cake was essentially pure gypsum produced in the
pilot plant under conditions of intentional oxidation. The solids crystals
were monoclinic and the filter cake contained approximately 80% insoluble
solids.
The mixed sulfite/sulfate solids had the appearance and physical properties
similar to a silt-like soil and handled much like a moist powder.
IV-11
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The gypsum, on the other hand, was much more grainy and had the consis-
tency of a sandy soil. The unconfined compressive strengths of both
materials were in the range of typical soils, 10-15 psi; and both had
optimum dry densities in the range of 75% solids. The coefficient of
the permeability of the compacted sulfite/sulfate solids ranged from
about 3 x 10~** to 5 x 10~5 cm/second. The permeability of dual alkali
gypsum was 2 x 10~5 cm/second. These values are within the range of
published data on coefficients of permeability of gypsum and sulfite-
rich solids produced in FGD systems.1'2
The treatment of the sulfite/sulfate filter cake was studied using various
mixtures of lime (or Portland cement), filter cake, and fly ash. This work
showed that the concentrated mode solids could be treated in a fashion simi-
lar to the treatment of solids from direct lime and limestone scrubbing
systems with similar effects on the mechanical properties. Testing per-
formed on prototype system concentrated dual alkali solids by IU Conversion
Systems (IUCS) indicate that the coefficient of permeability of treated
filter cake was about 5 x 10~6 cm/second using standard treatment mixes.2
Accelerated leaching tests and elutriate analyses performed on untreated
samples both at ADL and by IUCS showed that the initial and "steady-state"
concentrations of soluble species that can be leached, notably total dis-
solved solids (TDS) and total oxidizable sulfur (TOS), will be very depen-
dent upon the initial conditions and composition of the solids (as affected
by the degree of cake washing, ratio of sulfate-to-sulfite, chloride con-
centration in the gas, etc.) and the manner of solids handling and disposal.
TDS levels in the initial leachate can range from a few thousand ppm to
about ten thousand ppm; and "steady-state" concentrations (after the first
few pore volume displacements) can vary from a few hundred to approximately
two thousand ppm. Similarly, TOS levels can range from essentially nil to
up to 50 ppm. These concentrations are consistent with the range of pub-
lished data for leachates from solids generated in direct lime and limestone
scrubbing systems.
Testing performed by IUCS on the treatment of the filter cake indicated
significant reductions in both initial and "steady-state" levels of TDS
in leachates. Depending upon the type of treatment, reductions of 50%
to 80% were observed.
In all physical properties testing performed at ADL, samples were prepared
in accordance with standard soil-mechanics testing procedures. These pro-
cedures required, as a part of the sample preparation, the drying and re-
wetting of the filter cake to achieve a desired solids content. While the
samples were dried at temperatures of 83°C to prevent loss of water of
hydration, there is still concern that the drying/rewetting procedure re-
sulted in some changes in the behavior of the material, particularly in
the case of the rosette-like crystals produced in the concentrated mode
operation. However, the results of these limited tests are believed to
be indicative of the general behavior of the dual alkali solids. More
exhaustive testing on both as-received samples and samples prepared in
accordance with standard soil testing procedures is required to assess
the effects of sample preparation on test results.
IV-12
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V. FINDINGS - TASK III, PROTOTYPE TEST PROGRAM
A. BACKGROUND
The 20-megawatt (nominal capacity) prototype system was designed and built
by Combustion Equipment Associates, Inc. (CEA) and Arthur D. Little, Inc.
(ADL) for The Southern Company Services, Inc. (SCS) and the Gulf Power
Company. The purpose of the test program was to characterize and eval-
uate the performance of the dual alkali process operating in a concen-
trated active sodium mode with lime regeneration. The formal test program
lasted a total of about 14 months and covered a variety of conditions, in-
cluding operating with low-, medium-, and high-sulfur coal. The effects
of both fluctuating gas loads and simultaneous particulate removal were
also tested in conjunction with high-sulfur coal operation.
The operation of the system was evaluated with regard to the following
performance characteristics used to evaluate all dual alkali modes, as
discussed in Chapter I. While the overall operability and reliability
were a principal concern, the system was not intended to be a demonstra-
tion unit to test the ultimate availability of such systems in full-scale
applications. The test program was focused on evaluating the viability
of the process technology and defining process capabilities and limita-
tions. The process reliability and operability were, therefore, of im-
portance primarily as they reflected process chemistry and operational
problems related to process chemistry.
B. PROGRAM DESCRIPTION
1. System Design
The 20-megawatt prototype system was installed at Plant Scholz on Unit
No. 1, a 40-megawatt (nominal capacity) Babcock and Wilcox pulverized-
coal-fired power boiler. The boiler is equipped with a high-efficiency,
sectionalized electrostatic precipitator. The system consisted of basi-
cally three process sections: scrubbing; absorbent regeneration; and
waste solids dewatering. The scrubbing system contained a venturi fol-
lowed by an absorption tower with two trays and a demister. The scrubber
system was designed with the flexibility of operating either in a direct
lime or. limestone scrubbing mode as well as dual alkali. The venturi was
included for testing simultaneous particulate and S0£ removal. Modifica-
tions to the scrubber system following startup of the system provided for
operation of the venturi alone by bypassing regenerated liquor around the
absorber.
The regeneration system consisted of the CEA/ADL two-stage reactor system.
Provisions were made for feeding dry or slurried hydrated lime to either
or both reactors.
The waste solids dewatering system consisted of a thickener and a single
rotary drum vacuum filter equipped with wash sprays. The thickener was
V-l
-------
sized to handle 40. megawatts of capacity in contrast to the scrubbers,
reactor and filter, which were designed for 20 megawatts.
The system was -designed to operate in the concentrated active sodium
mode on medium- and high-sulfur coal. In this mode, sulfate removal
cannot be accomplished by precipitation of gypsum (CaSO^ 2H20); rather,
calcium sulfate is precipitated along with calcium sulfite, resulting in
a mixed crystal of the two salts.
2. System Operation
The prototype system was started up on February 3, 1975 and was operated
over a period of 17 months, through July 2, 1976. The EPA test program
formally began in May 1975 and was completed in July 1976, after which
the system was shut down. This report covers the entire 17 months of
operation, including system startup and shakedown.
The operation of the system can be logically broken down into three
discrete periods as defined by coal composition, flue gas conditions,
and the characteristics of the system operation. The first period, from
February through July 1975, covered system startup and shakedown. During
these first six months of operation the boiler burned low-sulfur coal
(average sulfur content corresponding to approximately 2.6 Ibs S02/MM
Btu). Sulfur dioxide concentrations in the flue gas averaged 1,050 ppm
(range = 600-1,550 ppm) and oxygen levels averaged 7.5% (range = 5.0-
11.0%), conditions well outside the range for which the process was
originally designed. This represented a difficult test for a system
operating in the concentrated active sodium mode because of the rich
levels of oxidation experienced.
In the second operating period, lasting from September 1975 through
early January 1976, the system was tested under relatively stable load
conditions with the boiler firing a combination of medium- and low-sulfur
coals (the average sulfur content of the coal fired corresponded to
approximately 3.1 Ibs 802/MM Btu). During this period the electrostatic
precipitator was maintained in full service. Repairs to the boiler
combustion air preheater and better control of combustion resulted in
improved flue gas conditions in comparison to operations in Period 1.
862 levels in the flue gas during Period 2 averaged 1,250 ppm (range =
800-1,700 ppm), and oxygen concentrations average 6.0% (range = 4.5-
9.5%).
In Period 3, which lasted from March through early July 1976, the system
was tested on high-sulfur coal (average sulfur content in the coal cor-
responding to about 5.7 Ibs S02/MM Btu). Flue gas 50^ levels averaged
about 2,250 ppm (range = 1,500-2,800 ppm), and oxygen concentrations
averaged about 6.5% (range = 4.5-9.0%).
In addition to 10 weeks of operation at relatively stable load, testing
during Period 3 consisted of three weeks of operation at fluctuating
gas loads and two weeks of participate testing. The fluctuating load
V-2
-------
testing involved adjusting the gas flow to the system to four different
levels according to a prearranged schedule roughly representative of
the normal load swings of the Scholz boilers. The average gas rate
handled during fluctuating load testing was 65%, as compared with
85-90% during the stable load periods. Particulate testing was performed
during the last two weeks of the program to evaluate the effects of fly
ash on the system performance (S02 removal, scale formation, oxidation,
and waste cake properties) and to assess particulate removal efficiency
and mist eliminator performance. During these two weeks the operation
of the precipitator ranged from fully activated to completely deactivated.
C. SYSTEM PERFORMANCE
Overall, the performance of the system was excellent. The system
demonstrated high S02 removal efficiency, high lime utilization,
excellent waste cake properties, and very good overall availability.
The various aspects of system performance are discussed below.
1. SO2 Removal
S02 removal efficiencies at Plant Scholz confirm the high S02 removal
capability of sodium solution scrubbing in the concentrated active
sodium mode. With sodium solution scrubbing, achieving a given outlet
S02 level (within the limit of the number of contact stages used) is
essentially a matter of adjusting the operating pH of the scrubber
system (by adjusting the feed forward rate or regenerated liquor pH).
Over the 15 months between April 1975 and July 1976, the average S02
removal using both the venturi and absorber (with two trays) was 95.5%
(for lowr, medium-, and high-sulfur coal); with the venturi alone (low-
sulfur coal only) S02 removal efficiency averaged 90.7%.
For the most part, when both the venturi and absorber were operated
together, the venturi was used principally for quenching the gas, and
the venturi pressure drop was maintained in the range of 4.5 to 7 inches
of water. Under these conditions the pH of the venturi bleed liquor
was maintained between 4.8 and 5.9 to ensure better than 90% S02 removal.
With the low inlet S02 levels of Periods 1 and 2 (600-1,700 ppm) this
resulted in outlet S0£ levels generally ranging from 15 to 100 ppm. At
the higher inlet S02 levels of Period 3 (1,500-2,800 ppm), the outlet
S02 typically ranged from 25 to 150 ppm.
When the venturi alone was used (10-16 inches of water pressure drop),
the bleed liquor was generally maintained at a pH above 5.7 to keep
outlet S02 levels below 100 ppm.
2. Particulate Removal
The particulate removal capability of the scrubber system was tested
with the venturi operated at both 12 and 17 inches of water pressure
drop followed by the absorber containing two trays. Three ranges of
inlet particulate loadings were tested (by partially or wholly
V-3
-------
deenergizing the precipitator): 0.015-0.025 grains/standard cubic foot
dry; 0.030-0.085 grs/scfd; and 2.3-3.6 grs/scfd. In general, outlet
particulate loadings increased slightly with increasing inlet loadings,
as would be expected. Outlet loadings ranged from 0.010-0.015 grs/scfd
at the lowest inlet loadings to 0.024-0.037 grs/scfd at the highest
inlet loadings. However, there was no statistical difference in outlet
loadings between operations at 12 inches and 17 inches of pressure drop across
the venturi throat (undoubtedly due, at least in part, to the two trays).
At these high venturi pressure drops, the S02 removal increased to an
average of about 98% for the particulate test period (using high-sulfur
coal).
3. Oxidation/Sulfate Control
a. Oxidation
Oxidation rates experienced in the prototype system were slightly lower
than those observed in the pilot plant under similar conditions. As in
the pilot operation, though, oxidation in the scrubber circuit accounted
for 80-95% of that throughout the system. The principal variable affect-
ing oxidation was the oxygen content of the flue gas, although the flue
gas rate and the type of coal fired also had some effect.
With the low-and medium-sulfur coals fired in Periods 1 and 2, oxidation
rates in the scrubber system ranged from about 180 ppm equivalent S02
(^230 ppm in the entire system) at 5% oxygen in the flue gas to about
370 ppm equivalent S02 (^420 ppm in the entire system) at 9% oxygen in
the flue gas. The total system oxidation rates correspond to a range
of 20% to 45% of the S02 removed for the average inlet levels for these
periods. However, there was considerably more gas/liquid contacting
provided in the scrubber system (venturi + two trays) than would normally
be incorporated in an absorption system for a low sulfur coal application.
This not only resulted in very low outlet S02 levels (typically less
than 50 ppm) but also unnecessarily high rates of oxidation (as a per-
centage of S02 removed).
With the high-sulfur coal in Period 3, the absolute rate of oxidation in
the scrubber system Increased slightly. At 5% oxygen in the flue gas,
oxidation in the scrubber system averaged about 200 ppm equivalent S02
(>v/250 ppm throughout the system); and at 9% oxygen, oxidation in the
scrubber ran slightly over 500 ppm (i>550 ppm throughout the system).
These higher oxidation rates, though, represent lower percentages of
oxidation in terms of S02 remdved. For the average S02 removal in
Period 3 these oxidation rates correspond to about 10% and 25% of the
S02 removal, respectively.
As would be expected, the absolute rate of oxidation (mols/min) decreased
with reductions in gas flow, although the percentage of S02 oxidized
increased slightly. No effect of fly ash on oxidation was apparent
during the particulate testing period.
V-4
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b. Sulfate Precipitation
Precipitation of calcium sulfate measured in the reactor system showed
that calcium sulfate could be coprecipitated with calcium sulfite at
levels as high as 25% of the total calcium sulfur salts, indicating
that the system was capable of keeping up with such levels of oxidation.
The correlation of sulfate/sulfite content of the precipitated calcium
salts to sulfate/sulfite concentrations in the reactor liquor was
found to be:
I
- 0.031
reactor solids
reactor liquor
This degree of sulfate coprecipitation corresponds to about 85% of that
observed in the pilot plant.
There was also a slight decrease in the calcium sulfate/sulfite ratio
in the filter cake in comparison with that in the reactor product solids.
The data indicate about a 15% decrease in the sulfate content between
the reactor and filter. This is probably due to some dissolution of
calcium sulfate during the long holdup in the thickener.
Overall, the sulfate formation oxidation/sulfate precipitation data
show that the system is capable of keeping up with oxidation rates of
up to 25% of the S02 removedoxidation rates much higher than those
anticipated for most medium and high sulfur coal applications. And
the operation at the widely fluctuating conditions demonstrated the
stability of the system chemistry and its ability to "self-adjust" to
handle any oxidation rate up to 25% without operator intervention. As
oxidation changed, the ratio of sulfate to active sodium in the liquor
changed accordingly to increase or decrease the amount of calcium sul-
fate precipitated.
4. Waste Cake Properties
a. Solids Content
The solids content of the waste filter cake varied from 41% to 77% of
the total cake weight. In general, the solids content of the cake
varied with calcium sulfate content (decreasing with increasing cal-
cium sulfate levels) and with variations and upsets in the filter
operation. During stable load conditions the average solids content
of the filter cakes produced in each successive operating period in-
creased from 48% in Period 1 (low-sulfur coal) to 54% in Period 3
(high-sulfur coal). The inclusion of fly ash during simultaneous
particulate removal in Period 3 increased the average solids content
to about 57%. These averages include periods of minor filter upsets
and partial loss of vacuum.
V-5
-------
Under most all conditions the cake had the appearance and handling
properties of a moist soil.
It was easily transferred from the storage pile to dump trucks using
a front-end loader for transfer to the disposal pit.
b. Solubles Content
Wash efficiency tests performed on the prototype filter verified pilot
plant results regarding the washability of the cake. The results show
that the soluble solids levels in the cake can be readily reduced to
2-3% (dry cake basis) under controlled filter conditions using a wash
ratio of about 2.5 (gals wash water/gal water occluded in the cake).
The solubles level actually achieved on a continuous basis, though,
were higher due to.the limited capacity of the spray nozzles, system
upsets, and inadequate operator attention to cake washing. Soluble
solids levels in the cake throughout the program ranged from as low as
1.2% to as high as 12% of the dry cake weight, depending upon the degree
of washing and the solids content of the filter cake. The average losses
estimated for each operating period based upon cake analyses and overall
material balances ranged from 4% (Period 2) to as high as 8% (estimated,
Period 1). The average solubles losses, though, were biased upward by
the fluctuating wash conditions. Long periods of adequate cake washing
were more than offset by short periods of poor cake washing (due to oc-
casional high rates of cake withdrawal required with the single filter
to compensate for filter downtime, and Inattention to wash water rates).
5. Sodium Makeup
The rate of sodium makeup to the system in comparison to the estimated
sodium value losses in the filter cake provide a measure of the degree
of closed-loop operation (as well as accountability in the overall
material balances). Soda ash feed rates were closely monitored only
during Periods 2 and 3. During Period 2 soda ash makeup rates repre-
sented about 8% of the total SOa removal (mols NaaCOa/mol AS02) compared
with about 4.5% soda ash requirements based upon cake losses. The
difference is attributed to pump seal leaks, a small thickener leak that
developed during Period 2, and errors in the overall material balance.
Entrainment losses of sodium (in entrained liquor) were negligible. As
measured in both December 1975 and June 1976, entrainment losses were
equivalent to less than 0.1% of the AS02 (as soda ash required). During
Period 3 the material balance on sodium was almost completely closed.
The soda ash required to makeup for cake losses was about 7% of the SC>2
removal (using an average wash ratio of 1.8) versus a soda ash feed rate
of 8% of the 862 removal.
While soda ash requirements were slightly higher than desired due
primarily to inadequate control of the filter operation, the relatively
small soda ash requirements and the degree of closure in the soda ash
material balance reflect a tight, closed-loop operation.
V-6
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6. Power Consumption
Since the process included a venturi scrubber as well as additional
pump capacity for operation in a direct lime or limestone mode, the
power consumed in the dual alkali mode was greater than that which
would be consumed in a system designed specifically as a dual alkali
system. When operating at or near design gas flow, the system power
consumption averaged 2.5-3.0% of the boiler output. Correcting for
the additional pressure drop included with the venturi and the unneces-
sary pump capacity, the power consumed by the equipment actually required
for this application was about 1.0% of the capacity of the boiler.
7. Operability/Reliability Potential
a. Availability
While the system was not operated for the purpose of achieving a high
availability figure, the availability record of the system is impres-
sive. Over the 17 months of operation the system logged more than
7,100 hours of operation, which corresponded to an overall availability
of slightly higher than 70%. Most of the downtime occurred between the
operating periods and resulted from equipment problems of a mechanical
nature or problems caused by operation of the system well outside the
design condition. The availability during the operating periods
averaged about 90%. This availability is impressive, particularly in
light of the fact that the only spare equipment was pumps (and replace-
ment parts for unspared equipment were minimal); and that the system
was called upon to operate about 70% of the time at conditions outside
those for which it was designed.
The longest single outage (1,460 hours) occurred between Periods 1 and
2. During the end of Period 1, oxygen levels in the flue gas were
running in the range of 8-10%, with inlet S02 levels depressed to
850-950 ppm. Because of the resulting high oxidation levels (as a
percentage of S02 removal) the system was allowed to drift- into a
dilute active sodium mode, a mode for which it was not designed. The
result was precipitation of gypsum and formation of some gypsum scale
in the reactor tanks and piping. At the same time, mechanical problems
in the scrubber required a shutdown of the system, and it was decided
to await repair of the preheater and higher sulfur coal prior to restart
of the system. There was also some delay in replacement parts, so the
system remained down from mid-July through mid-September 1975. Such
delays would not normally be encountered in full-scale applications
with adequate sparing of equipment and maintenance of a reasonable
inventory of spare parts.
Between Periods 2 and 3 the boiler was shut down for scheduled main-
tenance The system remained out of service an additional month
following boiler startup, again due to delays in shipments of replacement
parts and equipment being overhauled.
V-7
-------
b. Equipment Performance
Most of the problems encountered with equipment and Instrumentation
during the course of the test program were mechanical In nature and
reflected design or fabrication oversights commonly associated with a
prototype system. All but a few were resolved during the course of
the test program by simple operational adjustments and/or equipment
modifications.
Equipment
The most significant equipment problems encountered In the system Involved
the filter, vessel linings, scrubber control and block valves, and solids
buildup In the first-stage reactor. Collectively, these accounted for the
bulk of mechanical-related downtime and maintenance.
The filter was the largest source of problems In the prototype
system, but the problems resulted In few system outages due to
the solids holdup capacity In the thickener, which allowed
sufficient time for most filter-related maintenance work.
Normally, filters do not require an inordinate amount of
maintenance. However, a large part of the filtration equipment
in the prototype system was fabricated out of fiberglass and
plastic both because of the anticipated corrosion problems
from the high chloride levels achieved in the tight, closed-
loop operation (3,500-11,000 ppm Cl ), and to minimize the
cost for the short-term prototype test program. Fiberglass
is not as sturdy as stainless steel, and there were failures
at stress points in the construction as well as erosion of
some of the plastic and fiberglass parts. Most of the problems
occurred during Period 1 and the early part of Period 2. Modi-
fications of the filter drum and tub by plant personnel, and
overhaul of the filter drum by the manufacturer between Periods
2 and 3 either eliminated the problems or reduced them to
routine, low-level maintenance.
Erosion, cracks and pinholes occurred in glass flake linings
various vessels in the system. Cracks and pinholes occurred
in the absorber recycle tank and the thickener floor and
walls. These were patched during interim periods and did not
recur during the remainder of the test program. Erosion
of lining occurred beneath the agitator in the second-stage
reactor vessel and on the liquor redistribution shelf in
the venturi. These linings were also patched in the interim
between Periods 2 and 3, and the venturi tangential nozzles
modified. No further erosion at either location was observed.
There was also deterioration of the lining in the area of the
quench zone at the gas inlet to the venturi. The cause of
the failure may have been a combination of factors including
poor application, inadequate surface preparation, and
V-8
-------
severity of temperature and chemical attack. This failure
suggests that corrosion-resistant metal alloys may be most
suitable in such areas.
Erosion and "debonding" of rubber linings occurred in control
and block valves in the scrubber system. These failures were
traced to the high degree of throttling to the control flow.
(The valves were sized to accommodate the higher flows asso-
ciated with direct lime slurry scrubbing.) These valves were
replaced with 316 stainless steel valves prior to Period 3 and
no further erosion or debonding occurred in the valves. There
was also no corrosion or erosion of the 316 after the three and
one-half months of service in Period 3.
Buildup of product solids occurred in the first reactor through-
out the test program. Through adjustments made to the reactor
system and simulation of the operation in the ADL pilot plant,
the cause of the problem was traced to poor agitation and
operation during severe upset conditions (e.g., gross over-
feeding of lime). While the buildup was never serious enough
to cause a shutdown, it did require occasional cleaning.
Improved agitation and better process control should reduce
such buildup to, at worst, a semi-annual maintenance item.
Such maintenance would not require system shutdown in large-
scale systems where parallel reactor trains can be used, or
the first reactor temporarily bypassed.
Instrumentation
Instrumentation problems primarily involved pH units, level transmitters,
and the soda ash feed solution control system.
The flow-through pH probes originally installed in the system
were prone to plugging and/or erosion and failure of probe
tips. The flow-through unit in the reactor system was replaced
with an immersion unit, which proved to be much more reliable.
Modification of take-off lines for flow-through units in the
scrubber system and increasing the flow rate minimized problems
with these units.
The level transmitters originally installed were unreliable
and required an inordinate amount of instrument maintenance.
These were eventually replaced with Foxboro units, which
proved to be much more reliable and less prone to failure
of critical parts.
A number of difficulties were encountered with the soda ash
feed control system, some of which were related to the wide
turndown range for which it was designed. None of the problems
affected the operability of the system, since continuous,
accurate control of makeup soda ash is not required to replace
V-9
-------
the small sodium losses in the cake. The principal impact
of the difficulties in the feed control system were in the
accuracy of the material balances on sodium.
c. Ease of Operation
Ease of system operation was assessed throughout the program either during
planned tests of system capabilities under differing conditions or indirectly
through inadvertent process upsets and equipment malfunctions. The planned
testing included:
stable operation with low-, medium-, and high-sulfur coal;
fluctuating load testing (30-100% of design gas rate) with
high-sulfur coal; and
simultaneous particulate removal with high-sulfur coal.
The results of this testing have been discussed.
Indirect measures of the system operability were also obtained during
upset conditions. Upset conditions encountered included:
wide, short-term fluctuations in inlet 802;
wide swings in inlet oxygen concentration;
inadvertent substitution of limestone for lime in the
chemical storage silo;
gross under-and overfeeding of lime; and
short-term outages of the filter, first-stage reactor
and various instruments with continued operation.
Operation during both the planned variations in system conditions and
upsets served to demonstrate the basic stability of the system and the
inherent ability of the concentrated lime mode dual alkali technology
to withstand sudden (and extended) changes in operating conditions
without loss of performance. Of particular note is the fact that close
control of pH throughout the system is not required to ensure high S0£
removal efficiency and prevent scaling. In fact, during some extended
periods lasting up to a few days in length the system flows and makeup
chemical feed rates were set by inlet and outlet SC>2 and trimmed
according to pH's of samples taken from the reactor and scrubber twice
per shift.
As oxidation rates changed (due to changes in inlet 862 or oxygen con-
centration of the flue gas), the system chemistry adjusted accordingly.
The ratio of sulfate-to-active sodium in the system liquor simply shifted
to effect the appropriate rate of calcium sulfate precipitation required
(up to about 25% oxidation).
V-10
-------
Similarly, operator errors in setting system flows or makeup chemical
feed rates rarely had any immediate effect on system performance, and
the effects were usually completely reversed simply by re-establishing
proper system conditions.
d. Scale Potential
Due to the low calcium concentrations maintained throughout the system,
there was little potential for scale formation. Other than the deposi-
tion of solids In the first-stage reactor (previously discussed) the only
occurrence of scale formation in the system was the precipitation of cal-
cium carbonate in the absorber during two extended periods when the scrubber
system was inadvertently operated well outside the specified pH range. The
calcium carbonate was completely dissolved within a few hours by returning
the system to normal operating conditions and had no effect on system per-
formance in any way.
The low scale potential, particularly in the scrubber system, is evidenced
by the operation of the mist eliminator in the absorber. The mist elimi-
nator was operated without any wash sprays (fresh water or liquor) for the
last two operating periods (4,600 hours). No deposit of solids or scale of
any form could be found on the mist eliminator following completion of the
test program. Similarly, there was no deposit of solids on the reheat gas
distributor downstream of the mist eliminator.
V-ll
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REFERENCES
1. Leo, P. P., and J. Rossoff. Control of Waste and Water Pollution
from Power Plant Flue Gas Cleaning Systems: First Annual R&D Report,
EPA-600/7-76-018, October, 1976.
2. Edwards, R. Personal Communication. I.U. Conversion Systems to
Reed Edwards of the Southern Company Services, Inc., April, 1976.
V-13
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GLOSSARY
Active Sodium - Sodium associated with anions involved in S02 absorption
reactions and includes sulfite, bisulfite, hydroxide and carbonate/
bicarbonate. Total active sodium concentration is calculated as
follows:
tNa+Jactive = 2 x ([Na2S03] + [Na2C03]) + [NaHS03] + [NaOH] + [NaHC03]
Active Sodium Capacity - The equivalent amount of S02 which can be theoreti-
cally absorbed by the active sodium, with conversion to NaHS03.
Active sodium capacity is defined by:
capacity - [Na2S03] + 2 x [Na2C03] + [NaOH] + [NaHC03]
Calcium Utilization - The percentage of the calcium in the lime or lime-
stone which is present in the solid product as a calcium-sulfur salt.
Calcium utilization is defined as:
mols (CaS03 + CaSOij) generated
Calcium Utilization = x 100%
mol Ca fed
Concentrated Dual Alkali Modes - Modes of operation of the dual alkali
process in which regeneration reactions produce solids containing
CaS03'%H20 or a mixed crystal containing calcium sulfite and calcium
sulfate hemihydrates, but not containing gypsum. Active sodium con-
centrations are usually higher than 0.15M Na+ in concentrated mode
solutions.
CSTRContinuous Stirred Tank Reactor - A well-agitated, baffled reactor
vessel having a uniform concentration of species throughout. At
any time the concentrations in the effluent from a CSTR are equiva-
lent to those within the vessel.
Dilute Dual Alkali Modes - Modes of operation of the dual alkali process
in which regeneration reactions produce solids containing gypsum
(CaSOit 2H20). Active sodium concentrations are usually lower than
0.15M Na+ in dilute mode solutions.
V-15
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Sulfate Formation - The oxidation of sulfite. The level of sulfate forma-
tion relative to S(>2 absorption is given by:
mols 863 oxidized
Sulfate Formation - - x 100%
mol SC>2 removed
Sulfate Precipitation - The formation of CaSOt^XHgO in soluble solids.
The level of sulfate precipitation in the overall scheme is given
by the ratio of calcium sulfate to the total calcium-sulfur salts
produced :
mols
Sulfate Precipitation
mol CaSOjj
IDS Total Dissolved Solids - Equivalent to the sum of all soluble species.
TOSTotal Oxidizable Sulfur - Equivalent to the sum of all sulfite and
bisulfite species.
V-16
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APPLICABLE CONVERSION FACTORS
ENGLISH TO METRIC UNITS
British
Metric
5/9 <°F-32)
1 ft
1 ft2
1 ft3
1 grain
1 in.
1 in2
1 in3
1 Ib (avoir.)
1 ton (long)
1 ton (short)
1 gal
1 Btu
°C
0.3048 meter
0.0929 meters2
0.0283 meters3
0.0648 gram
2.54 centimeters
6.452 centimeters2
16.39 centimeters3
0.4536 kilogram
1.0160 metric tons
0.9072 metric tons
3.7853 liters
252 calories
V-17
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TECHNICAL REPORT DATA
(Please read InOnietions on the reverse before completing)
. REPORT NO.
EPA-600/7-77-050a
2.
3. RECIPIENT'S ACCESSION-NO.
4.T.TLE ANDSUBT.TLE FmAL REPORT: DUAL ALKALI TEST
AND EVALUATION PROGRAM; Volume I. Executive
Summary
5. REPORT DATE
May 1977
6. PERFORMING ORGANIZATION CODE
7AUTHORCS'C.R.LaMantia, R.R.Lunt, J.E.Oberholtzer,
E. L. Field, and J. R. Valentine
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Arthur D. Little, Inc.
Acorn Park
Cambridge, Massachusetts 02140
10. PROGRAM ELEMENT NO.
EHE624
11. CONTRACT/GRANT NO.
68-02-1071
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final: 5/73-4/77
14. SPONSORING AGENCY CODE
EPA/600/13
is.SUPPLEMENTARY NOTES IERL_RTP project officer lor this report is Norman Kaplan,
Mail Drop 61, 919/549-8411 Ext 2915.
16. ABSTRACT
Volume I of the report is an executive summary of the results of a three-
task program to investigate, characterize, and evaluate the basic process chemistry
and the various operating modes of sodium-based dual alkali scrubbing processes. The
tasks were: I, laboratory studies at both Arthur D. Little, Inc. (ADL) and IERL-RTP;
II, pilot plant operations in a 1200 scfm system at ADL; and m, a prototype test pro-
gram on a 20 MW dual alkali system at Plant Scholz. Dual alkali system operating
modes on high and low sulfur fuel applications investigated included: concentrated and
dilute dual alkali systems, lime and limestone regeneration, and slipstream sulfate
treatment schemes. For each mode, the dual alkali process was characterized in
terms of SO2 removal, chemical consumption, oxidation, sulfate precipitation and
control, waste solids characteristics, and soluble solids losses.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Grou|>
Air Pollution
Alkalies
Sodium
Scrubbers
Desulfurization
Sulfur Dioxide
Calcium Oxides
Limestone
Sulfates
Tests
Pilot Plants
Prototypes
Air Pollution Control
Stationary Sources
Dual Alkali Process
Plant Scholz
13B
07D
07B
07A
08G
14B
131
18. DISTRIBUTION STATEMENT
'Unlimited
19. SECURITY CLASS (ThisReport}
Unclassified
21. NO. OF PAGES
45
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
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