EPA
TECHNOLOGY
TRANSFER
DOUBLE ALKALI
FLUE GAS
DESULFURIZATION
SYSTEM APPLIED
AT THE GENERAL
MOTORS PARMA,
OHIO FACILITY
U.S. EPA
OFFICE OF
RESEARCH AND
DEVELOPMENT
INDUSTRIAL BOILER
DEMONSTRATION
FACILITY
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This report has been jointly prepared by the
Environmental Research Information Center
(Technology Transfer)'and the Industrial Environmental
Research Laboratory (Research Triangle Park). The
EPA selected Arthur p. Little, Inc. to evaluate the
General Motors' Chevrolet-Parma double alkali system
located near Cleveland, Ohio. This final report,
EPA-600/7-77-005, (or PB 263-469), is available from
the National Technical Information Service, Springfield,
Virginia, 22151, at a cost of $6.00 per copy.
For further information on the General Motors'
double alkali system and other EPA-sponsored pro-
grams, write: j
Utilities and Industrial Power Division
Industrial Environmental Research Laboratory
Research Triangle Park, N.C. 27711
Photographs in thps capsule report were supplied
by the General Motors Corporation, Environmental
Activities Staff.
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View of equipment during construction.
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Chemical mix tanks.
Clarifiers.
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I :
Table 4
Estimated Double Alkali Economics
Total Capital Cost = $3.2 Million (1975$)
Parameter
Scrubber operating costs (1976$)
Total $ $/ton of coal
Capital Charge
Chemicals
Utilities
Solid Waste
Labor
Maintenance
Totals
250,000
26,500
33,800
46,100
113,400
77,200
547,000
6.90
0.73
0.94
1.27
3.13
2.13
15.10
Based upon burning 36,000 TPY coal (i.e., approximately 31% load factor).
Figure 4. Effect of Sulfur Content and
Operating Rate on Operating Cost
18
16 -
12,
v»
<3
5
a
a.
O
10 —
8-
6 —
Sulfur
Content
20
30
T
40
T~
50
T"
60
T"
70
T~
80
90
100
Annual Average Operating Rate, % of capacity
Jl
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The costs of constructing and operating a Double
Alkali System, as they are for any flue gas desulfuriza-
tion system, are quite variable depending primarily on:!
• coal properties and boiler characteristics, i
• whether the system is constructed along with a new'
boiler or retrofitted to an existing power plant, I
• space availability, !
• specific plant requirements such as spare capacity j
or provision for expansion,
• size and operating rate, and ;
• geographic location. j
This section presents the overall economics of
the Chevrolet-Parma system and describes some of ;
the unique aspects of the facility which affect its cost
of operation.
The construction cost of General Motors' Double
Alkali System was 3.2 million, reported in 1975 dollars.
This figure includes all construction-related activities
including engineering which was performed by the
Argonaut Division of General Motors. Table 3 sum-
marizes the major factors which contribute to this
capital investment. One important factor which affects
the capital investment for any retrofit situation is the
space availability for scrubbers and auxiliary equip-
ment. At Parma the retrofit was relatively easy and
most equipment is housed in a large multi-level
building.
The operating costs of the double alkali system
per ton of coal burned are shown in Table 4.
One operating cost factor requiring special note
is the capital charge. Industrial firms will use differing
capital charge rates depending on their own capital
structures and the specific method of financing the
plant. The rate of less than 10% used in this instance
by General Motors is perhaps lower than most. How-
ever, the capital costs are probably somewhat higher
than would be typical of future installations, as they
would not generally incorporate all the operating
flexibility afforded in this first design. Therefore, the
capital charge of $6.90 per ton of coal would be a
reasonable figure in many instances.
Further affecting the capital charge would be the
relative values of installed capacity and annual
operating load. General Motors' Chevrolet-Parma
average annual load factor is 31% of capacity based
on 33 x 106 kg/yr (36,000 tons/yr) coal, if the steam
load were to increase and quantity of coal burned
increased accordingly, the capital charge (per ton of
coal) would drop. Figure 4 generalizes the operating
costs for double alkali to consider the effect of
operating rate (load factor).
Factors Impacting Double Alkali System Capital and Operating Costs
Plant Investment
Capacity
^Operating Rate
Process Building
Excess Capacity
Sulfur Removal
Operating Requirements
, $3.2 million (1975 dollars)
Scrubbers: 4 scrubbers @ total equivalent 32 Mw
Reactors and sludge handling; capable of expansion to 5 scrubbers
@ total equivalent 40 Mw
.-^Annual coal bur,n: 107x106 Kg (118,000 tons) capacity
Annual coal burn: 33x106 Kg (36,000 tons) average
Spacious, four-level building houses: absorbers, all other process
, equipment and storage silos excluding the large clarifiers and reactors
Fully-spared vacuum filter
All equipment and piping conservatively designed for this f irst-of-a-
kind installation,
Design: 90% based on 3% sulfur coal
Average operating: 90% based on 2% sulfur coal
Consumptions based on best sustained operation to date
Cost based on local market conditions
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Vacuum filter.
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deliberately to increase oxidation. Consistent with
expectation, the higher oxidation rates produced
improved solids because of the prevalence of gypsurn
crystals.
Entrainment
Isolated instances of high entrainment from the
scrubbers were evidenced by considerable dense misi
emerging from the scrubber stacks and carrying dowln
from the roof to the ground. An entrainment test
showed that, although the carryover was significant,
entrainment of scrubber liquor was within the design
specifications.
A contributing factor to the periodic entrainmert
problem is the location of the demister relative to the
transition piece at the top of the scrubber. Immedi- !
ately above the full-diameter demister pad is a squarje
reducer through which the outlet gas passes into a |
high velocity duct. Inspections of solids buildup on j
the pad indicate that the gas is not utilizing the full
cross section of the demister as it contracts to flow
into the duct. A future design would probably
include larger overall vertical distances both below
and above the demister pad to ensure its effective
operation. Since the demisters are relatively free of
plugging in a double alkali application, there is no
need for continuous washing of the demister pad.
Re//a6;'//ty
During the 21-month test program the scrubber
ran a total of 37.5% of the boiler operating hours and
were available for an additional 23.5%. The maximum
potential then was 61.0% over this period. When I
downtime for major system modifications is excluded,
the total was 77.9%.
These major modifications involved four periods:
• October 7, 1974 to November 12, 1974, when a
number of changes were Fmplemented. Included
were instrument recalibration, cleaning of control
valves, installation of sample petcocks, installation
of cake wash nozzles, and investigation of chemical
mix tank and clarifier overflow plugging problems.
• March 14, 1975 to April 15, 1975, when General
Motors was investigating the sulfite plugging prob-
lem in the scrubbers. Ultimately, the scrubber
operating mode was changed, requiring some
repiping of the recirculating and regenerated
liquor lines.
• June 27, 1975 to September 8, 1975, when open trough
lines were installed in the overflows of both
chemical mix tanks and clarifier number 1, all of
which had plugged seriously at various times.
« March 5, 1976 to March 29, 1976, when a section of
the regenerated liquor line was replaced along with
a new orifice plate. Both were intended to afford
accurate flow monitoring at low flow rates relative
to the original system design. At the same time the
instrumentation on the lime feed line was replaced
to permit better control of the lime stoichiometry.
Although the operation of the General Motors
Double Alkali System has not yet achieved an availa-
bility in the 90% range over several months, General
Motors has gained considerable experience in equip-
ment and process operability and is continuing to
make modifications to improve its plant. This
experience should be a valuable resource to any
prospective user of dilute double alkali technology.
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The double alkali process in general consists of
four major sections, as shown in Figure 1: Absorption
(or scrubbing); Regeneration; Solids Dewatering; and
Calcium Control.
Absorption Section
In the Absorption Section an alkaline solution of
aqueous sodium hydroxide and sodium sulfite are
contacted directly with the dirty flue gas leaving the
boiler. SO2 is removed from the gas by reaction with
these sodium compounds to form additional sodium
sulfite plus some sodium bisulfite:
2 NaOH + SO2 -»-Na2SO3 + H2O
Na2SO3 + SO2 + H2O -*• 2NaHSO3
Simultaneously, some sodium sulfite reacts with the
oxygen in the flue gas to produce sodium sulfate:
Na2SO3 + 1/2 O2 -*Na2SO4
Thus, the scrubber effluent solution consists of a mixture
of Na2SOs, NaHSO3, and Na2SC>4.
Regeneration Sect/on
The capacity of the scrubbing solution to absorb
SC»2 is depleted as it passes through the scrubber.
Therefore, a portion is continuously withdrawn to
the regeneration section to begin reactivating the
absorbent solutions so that it may be reused.
This regeneration is accomplished by reaction
with lime:
Na2SO3 + Ca(OH)2 -*-2 NaOH + CaSO3 +
2 NaHSO3 + Ca(OH)2 — Na2SO3 + 2 H2O + CaSO3l
Na2SO4 + Ca(OH)2 -*2 NaOH + CaSO4|
The reactor effluent, then, is a mixture of soluble
sodium species and insoluble calcium salts.
Solids Dewatering Sect/on
To separate the solids from the liquor, the reac-
tion products are taken to the Dewatering Section,
which serves three major purposes: (1) to remove
all traces of insoluble solids from the regenerated
scrubbing liquor to avoid the chance of solids
plugging the scrubber; (2) to concentrate the solids
to a low level of moisture to minimize the tonnage of
waste material; and (3) to wash the waste solids to
reduce the soluble solids content and minimize
sodium losses and makeup requirements. With low
solubles content the solids have minimal impact on
the environment when landfilled, and sodium is
conserved within the double alkali process.
The major pieces of equipment included in the
Dewatering Section are a clarifier to separate the
solids by gravity from the regenerated liquor and a
rotary vacuum filter to further concentrate the solids
from the clarifier underflow and wash them before
they are discharged for disposal.
The filtered solids are usually washed with fresh
water to remove the solubles from the filter cake to
as low a level as possible. Since there is no liquid
purge from the system, the quantity of this fresh
water makeup is limited by the need for water in the
system to replace evaporation losses in the scrubber.
Calcium Control Section
Although the insoluble calcium is effectively
removed from the regenerated liquor in the Solids
Dewatering Section, some soluble calcium remains.
Depending on process conditions, this soluble
calcium can combine with SO2 or CO2 from the flue
gas to produce CaSOs or CaCOs- These compounds,
if present in sufficient quantity, can then precipitate
and build up on surfaces within the scrubber and
seriously affect its operation.
The concentration of soluble calcium depends on
the operating mode of the system. In the "concen-
trated mode", the concentration of the active species
is greater than 0.15 molar and the concentration of
the soluble calcium is quite low. However, in the
"dilute mode", the total molarity of the active species
is less than 0.15 molar and the soluble calcium
concentration is considerably higher, requiring
softening with carbonate, _
Ca++ + C03 3= CaC03l
The insoluble CaCO3 is removed from suspension in a
second clarifier of a design similar to that used in the
Solids Dewatering Section. The clarified overflow is
then returned to the Absorption Section while the
concentrated solids are sent to the Dewatering Section
to be filtered along with the sulfur salts.
The Parma Facility
Boilers
The steam plant at Parma contains four boilers,
two with a nominal steaming capacity of 27,000 kg/hr
(60,000 Ibs/hr) and two of 45,000 kg/hr (100,000
Ibs/hr). They are spreader stoker fired with traveling
grates and operate with variable excess air rates in the
100% range. The two larger boilers are equipped with
economizers with resultant lower flue gas tempera-
tures than the smaller boilers. Each boiler was equip-
ped with existing mechanical dust collectors for
primary particulate control. Normal burning of
medium and high sulfur (2-3%) eastern coal plus
occasional lower sulfur waste oil results in flue gas
generally containing 800-1300 ppm by volume of SO2.
Absorption System
Figure 2 is a flow schematic of the General Motors
dilute mode system. In the scrubber, SO2 removal is
effected by contact of saturated flue gas with a sodium
solution circulating through the scrubber at a rate of
2.7 liter/m3 (0.02 gal/ft3) of flue gas.
Each scrubber was installed to control the emis-
sions from its respective boiler, i.e., no provision was
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Table 1
Summary of Operational Double Alkali Scrubbing Processes
Owner
Location
Size No.
Mw Equiv. Boilers
Startup Date
General Motors Corp.
Caterpillar Tractor Co.
Caterpillar Tractor Co.
Parma,! Ohio
Joliet, 111.
Mossville, III.
Gulf Power/Southern Services, Inc. Sneads;, Fla.
32
18
57
20
March, 1974
September, 1974
October, 1975
February, 1975
Table 1 summarizes the double alkali systems wh^ch have been operated in the United States.
Clean
Flue Gas
Figure 1. Generalizec
Double Alkali Flowsheet
Scrubber
Absorption
Regeneration
Solids Dewatering
Calcium Control
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Much of the emphasis regarding the application
of flue gas desulfurization processes has been in
connection with utility coal and oil fired power plants.
However, a significant growing segment of high sulfur
fuel consumption is in the industrial production of
steam and electric power. Industrial use of coal, other
than utility and metallurgical, was about 61 megatonnes
(67 million tons) in 1975 in the United States. This
represents 12% of the coal consumed for all purposes
in that year.
Low sulfur fuels are becoming less available to
industrial consumers, and synthetic fuels will not be
available on a significant scale for many years. New
federal regulations may require even users of low
sulfur coal to reduce sulfur emissions by requiring the
best available control technology. Therefore, it is
likely that flue gas desulfurization (FGD) will be the
most widely implemented technique for control of
sulfur dioxide (SO2) emissions.
FGD processes are generally categorized as
either "regenerate" or "throwaway" processes. The
regenerable processes absorb SO2 and recover its
sulfur value for sale in some form, usually as sulfuric
acid, elemental sulfur, or liquid SO2- On the other
hand, throwaway processes produce a solid waste
material consisting of calcium sulfite and sulfate for
disposal. The choice between these approaches
depends on the comparative economics including the
local market conditions for byproduct sulfur values
and the cost of sludge disposal.
There are two types of throwaway systems pro-
ducing solid wastes: (1) direct contact lime or lime-
stone scrubbing; and (2) double (or dual) alkali.
Generally, double alkali is capable of high SO2
removal efficiencies and better alkali utilization and
may be more economical in many high sulfur coal
applications. Further, if on-site sludge disposal is not
possible, the superior dewatering properties of the
double alkali waste would enhance its transportability
and disposability.
Application of the double alkali concept evolved
in part as a result of successful sodium scrubbing
applications in the early 1970's such as those at
Nevada Power Company and General Motors'
St. Louis Assembly Plant. To minimize the difficulty of
treating and disposing of the sodium liquor waste and
to conserve its sodium value, the double alkali process
regenerates the active SO2 absorbing species by
reaction of spent absorbent with lime, producing
insoluble calcium-sulfur waste salts. Thus, no liquid
stream (other than the liquor wetting the washed
solids) needs to be purged from the system.
Double alkali systems may be operated in either
a dilute or concentrated mode. The concentrated
mode generally allows lower flow rates with a con-
sequent reduction in plant investment and operating
cost. Further, this mode requires no special precau-
tions to prevent scaling in the scrubber. However,
operation in a concentrated mode is limited to
applications where the oxidation rate of absorbed
SO2 is less than about 25%. This may be a constraint
common among industrial boilers — particularly older
ones — operating with relatively high excess air
and/or burning low or medium sulfur coal. On the
other hand, operation in a dilute mode is not con-
strained by an upper level of oxidation; in fact, high
oxidation rates usually enhance the properties of the
waste sludge in the dilute mode.
In 1969 General Motors began pilot operations of
its Double Alkali SO2 Control System to determine
the applicability of stack gas scrubbing to its industrial
powerhouses. General Motors chose Double Alkali
Scrubbing on the basis of four criteria:
• high potential reliability of process and equipment
• simplicity permitting freedom of powerhouse
operation
• byproduct readily disposed
• economics competitive with other options
Using data from its two-year pilot plant develop-
ment program, General Motors designed and con-
structed a complete industrial-scale demonstration
plant capable of handling the emissions from all four
coal fired boilers at the Chevrolet-Parma plant near
Cleveland, Ohio. The Parma steam plant has a com-
bined steam generating capacity of 145,000 Kg/hour
(320,000 Ibs/hour). The design provides for incorpora-
tion of a future, fifth 36,000 Kg/hour (80,000 Ibs/hour)
boiler by addition of a single additional scrubber.
The prototype system was placed in operation in
March, 1974. After an initial startup period, General
Motors and EPA began a cooperative program in
which Arthur D. Little, Inc., as contractor to EPA,
observed the extended operation of the facility from
August, 1974 to May, 1976 and tested it intensively
over three one-month periods.
This report describes the results of the test pro-
gram at Parma. In summary, the system has demon-
strated a consistant 90% SO2 removal capability.
Operating reliability has improved during the test
program after some difficulties, principally in the early
months of operation. A more detailed description of
the test program results are available in an EPA report
entitled,- "Evaluation of the General Motors Double
Alkali SO2 Control System" (EPA-600/7-77-005)
published January, 1977 (NTIS Report No. PB 263-469).
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Scrubber recycle tanks.
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EPA
TECHNOLOGY
TRANSFER
DOUBLE ALKALI
FLUE GAS
DESULFURIZATION
SYSTEM APPLIED
AT THE GENERAL
MOTORS PARMA,
OHIO FACILIW
U.S. EPA
OFFICE OF
RESEARCH AND
DEVELOPMENT
INDUSTRIAL BOILER
DEMONSTRATION
FACILITY
EPA-625/2-78-016
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