United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-84/096 Apr. 1985
&EPA Project Summary
Economic Evaluation of a
Sodium/Limestone
Double-Alkali FGD Process
C. D. Stephenson, T. A. Burnett, and R. L. Torstrick
A conceptual design and economic
evaluation of a sodium/limestone dou-
ble-alkali flue gas desulfurization (FGD)
process was prepared based on recent
EPA-sponsored pilot-plant and proto-
type test work. The results of this
evaluation were compared with the
results from a recent forced-oxidation
limestone process evaluation. For a
500-MW new power unit burning 3.5%
sulfur coal and meeting the 1979 new
source performance standards (NSPS),
the estimated capital investments in
1982 costs are $95 million ($190/kW)
for the sodium/limestone double-alkali
process and $103 million ($206/kW)
for the forced-oxidation limestone
process. Estimated first-year annual
revenue requirements in 1984 costs for
these processes are $26 million and
$29 million (9.3 and 10.6 mills/kWh),
respectively. Although the sodium/lime-
stone double-alkali process appears to
be about 8% lower in capital investment,
given the accuracy associated with
studies of this type (±10%), it is un-
certain whether the sodium/limestone
double-alkali process has a lower capital
investment. In terms of first-year and
levelized annual revenue requirements,
the sodium/limestone double-alkali
process shows a 12% and 14% lower
cost than the forced-oxidation lime-
stone process and thus is marginally
less expensive. However, some of the
design assumptions used to generate
the estimated costs for the sodium/
limestone double-alkali process are
based on short-term pilot-plant tests
and are unconfirmed. Therefore, the
economics are more uncertain for the
double-alkali process than for the
forced-oxidation limestone process.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
For the past 15 years, wet limestone-
and lime-scrubbing processes have in
almost all cases held a distinct cost
advantage over other flue gas desulfur-
ization (FGD) processes. However, lime-
stone and lime processes have several
inherent disadvantages; i.e., erosion,
corrosion, and a high liquid recirculation
rate. They were also plagued during the
course of their early development by
operating problems such as scaling and
plugging. These various shortcomings
have added impetus to the continuing
development of alternative FGD process-
es.
One such alternative is the use of
double-alkali (dual alkali) processes in
which the absorption function is sepa-
rated from the precipitation function. A
highly reactive absorbent solution is used
to absorb SO2, which is precipitated as
CaSOa-1 /2H2O and CaSO«-2H2O outside
the absorber. The waste produced is thus
identical to that produced by limestone
and lime processes but the difficulties of
scrubbing with a calcium-based slurry
are avoided.
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During the course of the development
of double-alkali processes in the U.S., a
solution of sodium sulfite has been used
as the absorbent and lime has been used
as the precipitant. The use of lime, due to
its high reactivity, has come to be regard-
ed as a disadvantage because of its cost.
One phase of a continuing EPA double-
alkali development program has been
devoted to the development of a double-
alkali process using limestone instead of
lime. That process was tested recently on
a pilot scale at Gulf Power Company's
Scholz Power Station. The results of that
test program are subject to question
because the pilot plant did not achieve a
long-term, closed-loop operation.
This study is an economic evaluation of
the sodium/limestone double-alkali proc-
ess and an economic comparison of the
sodium/limestone double-alkali process
with a conventional forced-oxidation lime-
stone-scrubbing process.
Design and Economic Premises
The major design premises for this
study are listed in Table 1. The base case
power plant is a new, 500-MW coal-fired
power unit located in the north-central
U.S. The fuel is a bituminous coal with a
heating value of 11,700 Btu/lb* and
containing 3.5% sulfur(dry basis), 15.14%
ash, and 4.0% moisture. The boiler heat
rate is 9,500 Btu/kWh.
The FGD unit includes all the equip-
ment necessary to meet the 1979 new
source performance standards(NSPS)for
SO2 removal. The overall design for the
sodium/limestone double-alkali process
is based on pilot-plant studies at the
Scholz Power Station and bench-scale
work at EPA's Air and Energy Engineering
Research Laboratory (AEERL), Research
Triangle Park(RTP), NC. The design of the
forced-oxidation limestone process is
based on industry data and previous
evaluations by TVA.
The project is assumed to have begun
in mid-1980 with a 3-year construction
period ended in mid-1983. The midpoint
for the capital investment costs was mid-
1982. The annual revenue requirements
are based on 1984 costs.
Capital investment consists of direct
investment, indirect investment, and
other capital charges. The direct invest-
ment is based on equipment costs and
installation costs (e.g., piping, electrical,
and instrumentation). Indirect investment
(e.g., engineering design and supervision.
Table 1. Major Design Premises
Item
Premise
Power plant
Operating schedule
Fuel
Base year
FGD waste disposal
SOz removal efficiency
Paniculate removal efficiency
SOz absorber redundancy
North-central U.S.. SOO-MW coal-fired
boiler, 9,500 Btu/kWh heat rate
165.000 hr. 30-yr life. 5,500-hr first-
year operation
Eastern bituminous coal, 11,700 Btu/lb.
3.5% sulfur, 15.14% ash, 4.0% moisture
Capital investment: mid-1982
Revenue requirements: 1984
Sodium/limestone double-alkali fixation
and landfill
Forced-oxidation limestone scrubbing:
landfill
89%
99.8% (0.03 Ib ofparticulates/106 Btu
heat input)
25% (4 operating trains, 1 spare)
'Readers more familiar with metric units may use the
conversion factors at the back of this Summary.
and construction expense) is estimated
based on the direct investment. Other
capital costs (e.g., allowance for start-up
and modification, and interest during
construction) are estimated from the total
direct and indirect investment. These
preliminary capital investment estimates
are considered to have a -15% to +30%
range of accuracy and to be comparable
within 10%.
Two types of annual revenue require-
ments are projected—first year and level-
ized. Both are based on 5,500 hours of
operation per year at full load (about a
63% capacity factor) and both use a
levelized capital charge. However, level-
ized annual revenue requirements differ
from first-year annual revenue require-
ments in that they take into consideration
the time value of money over the life of
the FGD unit and are calculated using a
10% discount factor, a 6% inflation factor,
and a 30-year economic life.
Process Descriptions
The sodium/limestone double-alkali
process (Figure 1) is similar in design and
operation to utility and industrial sodium/
lime double-alkali processes except for
the additional reaction tanks and the
limestone-grinding equipment. The forced-
oxidation limestone process (Figure 2) is a
generic design based on current tech-
nology, including the use of forced oxida-
tion to produce gypsum. Both systems
have five absorber trains—four operating
and one spare—supplied with flue gas
from a plenum downstream from the
electrostatic precipitator (ESP) and the
boiler induced-draft (ID) fan. In both cases,
ducts provide for emergency bypass of
50% of the flue gas. Process design
conditions for both systems are summar-
ized in Table 2.
Each train of the sodium/limestone
double-alkali system consists of a sieve
tray absorber, a steam reheater, and a
booster fan. The absorbent liquid is a
sodium sulfite solution containing 1.7
mol percent sodium and 0.7 mol percent
active alkali (SOa2" and HSO31 when
regenerated. Bleedstreams from the ab-
sorbers are combined and treated with
ground limestone in two parallel trains of
four reaction tanks operating in an over-
flow mode. The resulting slurry is pumped
to a thickener-filter train where a 55%
solids filter cake is produced. The filter
cake is blended with dry fly ash and lime
and trucked 1 mile to a landfill.
Each train of the forced-oxidation lime-
stone process consists of a spray tower
absorber, a steam reheater, and a booster
fan. The absorbent liquid is an 8% solids
slurry containing ground limestone. The
scrubbing slurry drains from the absorber
into a tank where air is sparged into it to
oxidize the sulfite to sulfate before it
overflows to a hold tank where the
makeup limestone slurry is added. A
bleedstream from the oxidation tank is
pumped to a thickener-filter train where
an 85% solids filter cake is produced. The
filter cake is then trucked 1 mile to a
landfill.
Results
The capital investments and annual
revenue requirements of the sodium/
limestone double-alkali process and the
forced-oxidation limestone process are
shown in Table 3. Fly ash disposal costs
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Partial Bypass
Stack
Solids
Separation
Waste
Disposal
Figure 1. Sodium/limestone double-alkali process flow diagram.
Partial Bypass
Stack
Figure 2. Forced-oxidation limestone process flow diagram.
are not included in the forced-oxidation
limestone process costs. Since fly ash
disposal is an integral part of the fixation
portion of the sodium/limestone double-
alkali process, a credit for fly ash disposal
is applied to the costs of the sodium/
limestone double-alkali process for com-
parability with the forced-oxidation lime-
stone process costs. The capital invest-
ment and annual revenue requirements
of the sodium/limestone double-alkali
process are both lower than those of the
forced-oxidation limestone process—by
8% and 12%, respectively. Since the
comparative accuracy of these cost esti-
mates is 10%, the capital investment of
the double-alkali process can be regarded
as marginally less expensive.
Capital Investment
The direct capital investments (the cost
of material and labor to install the system)
are compared by process area in Table 4.
It is apparent that the key to the economic
advantage of the sodium/limestone
double-alkali process is the less expensive
S02 absorption area that results from the
reduced pumping requirements made
possible by the low liquid-to-gas (L/G)
ratio (the absorber sizes are similar
because the size is determined largely by
the flue gas volume). The S02 absorption
area costs for the double-alkali process
are 50% of those for the I imestone process
and constitute 24% of the total direct
investment; while, for the limestone
process, they constitute 36% of the total
direct investment. Cost differences in
other processing areas are a much lower
portion of the total. Regeneration area
costs (3%) and fixation area costs (2%) for
the sodium/limestone double-alkali pro-
cess are minor, as are oxidation area
costs (5%) for the forced-oxidation lime-
stone process. The solids separation area
(dewatering) costs for the double-alkali
process are 50% higher; but these, too,
are a relatively small part of the total
direct investment—12% for the sodium/
limestone double-alkali process and 7%
for the forced-oxidation limestone proc-
ess.
Annual Revenue Requirements
A comparison of the major components
of the first-year annual revenue require-
ments is shown in Table 5. Among the
direct costs, the major costs for the
sodium/limestone double-alkali process,
in order of importance, are maintenance,
raw materials, labor, steam, and electric-
ity. For the forced-oxidation limestone
process, they are maintenance, electric-
ity, steam, labor, and raw materials. The
major differences in direct costs between
the processes are in maintenance, elec-
tricity, and raw material costs. Mainte-
nance costs for the sodium/limestone
double-alkali process are 37% lower than
for the forced-oxidation limestone process
as a result of the forced-oxidation lime-
stone process having high maintenance
costs associated with circulating large
volumes of slurry. This also results in
electricity costs for the sodium/limestone
double-alkali process being about half
those of the forced-oxidation limestone
process. Raw material costs are over
twice as high for the sodium/limestone
double-alkali process, however, and are
divided about equally between costs for
limestone, soda ash, and lime for fixation.
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Table 2. Process Design Conditions
Sodium/limestone
double-alkali
process
Forced- oxidation
limestone
process
Absorber type
Superficial gas velocity, ft/sec
L/C, gal./'1000 aft3
Presaturator/underspray
Absorber
Stoichiometry. mol Ca/mol(SOz + 2HCI/
absorbed
Soda ash feed rate, mol NA*/mol SO2
absorbed
Sulfite oxidation, %
Thickener feed solids, %
Thickener underflow solids, %
Filter cake solids, %
Sieve tray tower
9
2/1
5
1.0
0.07
10
1.4
25
55
Spray tower
10
4/0
106
1.4
95
a
40
85
Table 3. Comparison of Capital Investments and Annual Revenue Requirements
Sodium/limestone
double-alkali
process
Forced-oxidation
limestone
process
Capital investment (1982 $)
$ Millions
$/kW
First-year revenue requirements (1984 S)
$ Millions
Mills/kWh
Levelized annual revenue requirements (1984 $)
$ Millions
Mills/kWh
95.2
190.3
25.5
9.3
35.7
13.0
103.1
206.1
29.1
10.6
41.5
15.1
Table 4. Comparison of Direct Investment by Processing Area
Direct investment, 1982 $ thousands
Processing area
Sodium/limestone
double-alkali
process
Forced- oxidation
limestone
process
Materials handling
Feed preparation
Gas handling
SOa absorption
Reheat
Regeneration
Oxidation
Solids separation
Fixation
Services, utilities, and misc.
Landfill construction
Landfill equipment
Landfill credit (fly ash disposal)
Total
2,426
4,506
10,800
1 1.348
3,630
1.506
--
5.493
906
2,437
5,247
1.454
(2.312)
47.441
2,528
4.715
11.281
20.288
3.634
--
2,670
3,679
--
2,928
3.781
1.123
--
56.627
Overall, the sodium/limestone double-
alkali process direct costs are about 13%
lower than those of the forced-oxidation
limestone process because of lower main-
tenance and electricity costs, both a result
of using a highly reactive scrubbing
solution instead of a limestone slurry.
With the lower overheads, which are
based on direct costs, and lower capital
charges, the annual revenue require-
ments of the sodium/limestone double-
alkali process are 12% lower than those
of the forced-oxidation limestone process.
Energy Consumption
A comparison of the total energy con-
sumption for each process is shown in
Table 6. Steam, diesel fuel, and electricity
are shown in Btu equivalents.
The energy consumption in the sodium/
limestone double-alkali process is only
71% of that for the forced-oxidation
limestone process. Almost all of the
difference is due to the significantly lower
electrical consumption of the sodium/
limestone double-alkali process.
Conclusions and
Recommendations
The sodium/limestone double-alkali
process is 8% lower in capital investment,
12% lower in first-year annual revenue
requirements, and 14% lower in levelized
annual revenue requirements than a
comparable forced-oxidation limestone
process. Although these economics may
be only marginally attractive relative to
limestone scrubbing (given the accuracy
associated with this preliminary-grade
economic evaluation), the sodium/lime-
stone double-alkali process offers the
potential for high SO2 removal efficiencies
at low absorber L/G ratios and for fewer
maintenance problems with scaling, plug-
ging, and erosion because calcium com-
pounds are primarily confined to the
regeneration area.
The sodium/limestone double-alkali
process is less energy intensive than a
forced-oxidation limestone process—pri-
marily because of the lower electrical
consumption of recirculating pumps in
the scrubbing area.
More development work on the sodi urn/
limestone double-alkali process needs to
be undertaken to demonstrate that both
the design assumptions used in this study
are achievable and the problems asso-
ciated with a previous pilot-plant work at
Scholz can be overcome with only minor
(and relatively inexpensive) modifications.
Both of these areas of uncertainty could
be minimized with additional pilot-plant
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Table 5. Comparison of Annual Revenue Requirement Components
Annual revenue requirements. 1984 $ thousands
Sodium/limestone Forced- oxidation
double-alkali limestone
process process
Raw materials
Limestone
Soda ash
Lime
Total labor
Utilities
Steam
Electricity
Other
Maintenance
Landfill credit
Analysis
Direct costs
Overheads
Operating and maintenance costs
Level/zed capital charges
Total first-year annual
revenue requirements
915
990
794
1.357
1.321
1.096
234
2.715
(312)
105
9,215
2.319
1 1.534
13,987
25.521
1.214
1.269
1.356
2,133
192
4,285
105
10.554
3.395
13.949
15.151
29.100
Table 6. Comparison of Process Energy Consumption
Energy input, 109 Btu equivalent
Process
Electricity*
Steam0
% of plant
capacity
Diesel fuel0 I gross)
Sodium/ limestone double alkali
Limestone scrubbing
281.4
547.7
397.3
407.7
11.6"
14.9
2.6
3.7
'9.500 Btu/kWh.
"757.9 Btu/lb (based on recycle of condensate to power plant).
'144,000 Btu/gal.
"includes credit for fly ash disposal.
or prototype(3 to 30 MW equivalent) work
which Includes a long-term demonstra-
tion run. As a result of the unconfirmed
design assumptions mentioned above,
the estimated economics are considered
potentially less accurate for the double-
alkali process than for the forced-oxida-
tion limestone process.
Metric Conversions
British units are used throughout this
Summary for the reader's convenience.
Readers more familiar with the metric
system are asked to use the following
conversion factors:
British Multiplied by Yields Metric
Btu
ft
ft3
gal.
Ib
mi
1.054
0.3048
28.32
3.785
0.4536
1.609
kJ
m
liters
liters
kg
km
: 1985 — 559-111/10809
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C. D. Stephenson, T. A. Burnett. andR. L Torstrick are with the Tennessee Valley
Authority. Muscle Shoals. AL 35660.
Norman Kaplan is the EPA Project Officer (see below).
The complete report, entitled "Economic Evaluation of a Sodium/Limestone
Double-Alkali FGD Process," (Order No. PB 85-169 886/AS; Cost: $11.50,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Z
Official Business
Penalty for Private Use $300
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