United States
Environmental Protection
Agency
Industrial Environmental
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-84-039 May 1984
Project Summary
Pilot Plant Tests of Chloride Ion
Effects on Wet FGD System
Performance
John C.S. Chang
This report summarizes results of pilot
plant testing, from September 1981
through June 1983, of the effects of
dissolved salts on a lime or limestone
flue gas desulfurization (FDG) system at
the Environmental Protection Agency's
Industrial Environmental Research
Laboratory, Research Triangle Park,
North Carolina. Tests were conducted
using a three-stage turbulent contact
absorber (TCA) with a typical gas flow
rate (G) of 465 m'/hr (0.1 MW) and ab-
sorbing slurry chloride ion (Cl~) concen-
trations of 160 - 180,000 ppm.
The FGD processes investigated in-
clude conventional lime/limestone,
magnesia enhanced limestone, dibasic
acid (DBA) enhanced limestone, and
limestone with two-tank forced oxida-
tion. Data indicate that the effects of Cl~
on the performance of the absorber are
a function of cations associated with Cl~
and scrubber operating conditions. The
accumulation of calcium chloride caus-
ed decreased system pH and SO2 re-
moval efficiency, occasional decreases
of slurry settling rate, and increased
gypsum scaling potential. When mag-
nesium was the cation, the increase of
Cl~ concentration improved SO2 removal
efficiency at Cl~ concentrations below
40,000 ppm. No significant effects were
observed using sodium chloride at Cl~
concentrations less than 50,000 ppm.
However, when C\~ concentrations were
greater than 70,000 ppm, SO2 removal ef-
ficiency and system pH declined with
the accumulation of either magnesium
or sodium chloride. Significant de-
creases in S02 removal efficiency were
also observed when lime was used in
the natural oxidation mode and at high
inlet SO2 concentrations. Calcium chlor-
ide had minor effects on the perfor-
mance of a DBA enhanced limestone
scrubber. Most gypsum specifications
required for wallboard manufacturing
were met by washing cake using a pilot
belt filter.
This Project Summary was developed
by EPA's Industrial Environmental Re-
search Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
High chloride ion (Cl'l concentrations in
the scrubbing liquor are possible with tight-
ly closed water loops in flue gas desulfuriza-
tion (FGD) systems. If closed-loop operation
is assumed (i.e., the only water leaving the
FGD system is through evaporation and filter
cake moisture), material balance calcula-
tions, for high chloride coals, indicate total
dissolved solids (IDS, usually the combina-
tion of calcium, magnesium, and sodium
salts) can accumulate to levels exceeding
50,000 ppm*. Forced oxidation intensifies
the TDS accumulation through improved
cake dewatering, and dissolved solids can
reach worst case levels in excess of 150,000
ppm. The origin of the chlorides can be the
coal, as HCI in the flue gas is absorbed, or
chlorides in the makeup water. The latter is
especially important if cooling tower
blowdown is utilized for makeup water. In
order to evaluate the effects of Cl~ on FGD
system performance, a test program was
"One ppm is equivalent to 1 mg/L.
-------
developed to conduct pilot plant tests under
simulated high Cl~ conditions.
Test Facilities
Two process configurations, natural and
forced oxidation, shown respectively in
Figures 1 and 2, were employed for the tests.
The scrubber, located at EPA/IERL-RTP, is
a three-stage turbulent contact absorber
(TCA) with 465 m3/hr flue gas capacity (0.1
MW). No flyash was present in the flue gas,
which was drawn from a gas-fired boiler and
injected with pure S02 and HCI as required.
The oxidizer consisted of a 30 cm diameter
tower containing slurry at a depth of 5.5 m
and was sparged with air from the tower bot-
tom. A bleed stream of the slurry from the
old tank was directed to the clarifier and then
processed by a rotary drum vacuum filter to
remove the precipitated waste slurry. All
filtrate was returned to the scrubber in order
to maintain closed-loop operation. The
system was manually controlled in a feed for-
ward mode by maintaining a constant lime-
stone stoichiometric ratio of moles of reagent
added per mole of S02 absorbed (when lime
was used, constant pH control was utilized).
A small amount of excess reagent was also
added to neutralize absorbed HCI gas dur-
ing some tests.
Granulated or powdered calcium chloride,
magnesium chloride, or sodium chloride was
added to the liquid inventory in order to bring
the system to the desired CT level. HCI was
(at specified times) injected into the inlet duct
at predetermined levels in order to simulate
flue gas generated by the combustion of high
chloride coal. The limestone feed rate was
adjusted manually by a trial and error method
until steady-state stoichiometric ratios were
achieved. Normally, each run consisted of
at least 25 hours of continuous stable opera-
tion, after which scrubber performance and
system conditions at each Cl~ level were
established by averaging operational data
logged hourly.
The gas phase SO2 concentration was
monitored continuously using a DuPont 400
S02 analyzer and occasionally checked by
wet gas titration. Liquid and solid samples
of important species such as calcium, sulfite,
sulfate, carbonate, magnesium, and sodium
were analyzed.
Method of Approach
The effect of Cl" on the FGD system was
studied in three ways:
• All system parameters were held constant
while the concentration of Cl~ in the scrub-
bing liquor was increased.
• At a given Cl~ concentration, either the
liquid-to-gas ratio (L/G) or the limestone
To Stack
Scrubber
SOi
Limestone
Flue Gas =
Make-
up
Tank
o o
PO
1°
Additives
Hold
Tank
Clarifier
Filter
Filtrate
Tank
Figure 1. Flow diagram for single-loop natural oxidation tests in the 0.1 MW limestone pilot
plant.
To Stack
Sump
Tank
Figure 2. Flow diagram for in-loop forced oxidation tests in the 0.1 MW limestone pilot plant.
feed rate was increased until the base case
S02 removal efficiency was obtained.
The Cr concentration was held constant
while system parameters were varied in-
dividually.
For the third case, comparisons of the ef-
fects on S02 removal are difficult since the
base case S02 removal changes significantly
when L/G, limestone stoichiometry, or the
absorber packing height are changed. One
-------
method for comparing S02 removal efficien-
cies is based on the logarithmic ratio of the
fractional S02 remaining in the flue gas at
high Cl~, f , to the fractional S02 remaining
at the base case conditions with no Cl~, fB.
Mathematically, this ratio, R, can be ex-
pressed as a percentage:
R = ln' 1 - f )
x 100.0
(1)
- fB)
For all other tests, effects were evaluated
by examining system performance with and
without Cl~ addition or by quantitatively
determining the L/G or excess limestone re-
quired to return to the base case.
Major areas of testing during the reporting
period are summarized below:
• Cl" effects on the limestone FGD system
utilizing the following salts alone or in com-
bination:
- Calcium chloride
- Magnesium chloride
- Sodium chloride
• Evaluation of methods to regain base case
S02 removal efficiency at high calcium
chloride concentrations including:
- Increasing L/G
- Increasing limestone feed rate
- Adding organic acid
• Effects of calcium chloride on a lime based
FGD system
• Effects of calcium chloride on a magnesia
enhanced limestone FGD system
• Effects of calcium chloride on an organic
acid enhanced limestone FGD system
• Washability evaluation of a gypsum cake
using a horizontal belt filter
Each of these is covered separately in the
following text. By far, most testing was per-
formed in a limestone based system using
calcium chloride and thus will be treated in
greater detail.
Results and Discussion
Cl' Effects on Limestone FGD
Systems
Calcium Chloride
Recycle slurry pH decreased as the con-
centration of calcium chloride increased. It
has been shown that the change in pH
closely follows the calculated equilibrium H*
concentration in the natural oxidation mode
of operation (1). The drop in pH in the forc-
ed oxidation mode was not significant until
the Cl~ concentration exceeded 50,000 ppm.
Calculating the H* concentration during
forced oxidation was not possible due to the
stripping of C02 gas from the slurry. At the
same Cl~ concentration, the pH decrease in
the natural oxidation mode was significantly
more than operation in the forced oxidation
mode.
A decrease in S02 removal efficiency was
observed with the increase of calcium
chloride concentration for both the natural
and forced oxidation modes. As shown in
Figure 3, the decrease was not as severe for
the forced oxidation mode.
The natural oxidation data in Figure 3 was
obtained using a high base case pH of 6.0
and a high limestone stoichiometry of 1.3-1.4
moles of CaCO3 per mole of SO2 absorbed.
Additional tests were performed at a lower
base case pH of 5.4-5.6, summarized in
Table 1, and a series of tests run at high and
low make-per-pass, summarized in Table 2.
The make-per-pass, or the amount of S02
gas absorbed per liter of slurry, was mini-
90
80
I70
.0
Uj
1
o
60
O 50
40
30
G: 465 m3/hr
L/G: 8.0 ///n3
Inlet SOz.' 2500 ppm
Limestone Stoichiometric Ratio 1.2 (forced oxidation)
1.35 (natural oxidation)
Forced Oxidation
O Original Data
• Replicates
Natural Oxidation
30
60 90 120
Cl' Concentration, ppm x 10~3
150
180
Figure 3.
Comparison of natural and forced oxidation SOzremoval efficiencies as a function of
Cl'.
Table 1. Results of Low Base Case pH, Natural Oxidation Tests
Test I.D.
NB-13
N2-4
N5-10
NB-14
N2-HM1
N5-HM5
Inlet
S02
(ppm)
2500
2500
2500
700
700
TOO
cr
Conc.
(ppm)
*
20,000
50,000
*
20,000
50,000
pH
5.6
4.9
4.6
5.4
5.2
5.0
Limestone
Stoich.
Imolarl
1.16
1.13
1.16
1.11
1.07
1.13
L/G
(l/rrf)
8.0
8.0
8.0
3.0
3.0
3.0
SO,
Removal
(%>
68
65
60
77
75
68
/?
from Eq. (11
(%)
100
92
80
100
94
77
* Base Case.
Table 2. Results of High/Low Make-Per-Pass, Natural Oxidation Tests
Test I.D.
NB-8
N5-HM1
NB-9
N5-HM3
NB-10
N5-LM1
NB-11 ,
N5-LM3
Inlet
S02
Ippm)
2500
2500
500
500
2500
2500
500
500
cr
Conc.
(ppm)
*
50.000
*
50,000
*
50,000
»
50,000
Limestone
Stoich.
pH (molar)
High Make-per-pass
5.8 1.2
5.0 1.2
5.9 1. 1
5. 1 1.2
Low Make-per-pass
5.8 1.2
4.9 1. 1
5.9 1. 1
5.4 1.2
L/G
(l/rrf)
11.2
11.2
7.2
7.2
10.7
10.7
4.5
4.7
S0t
Removal
(%)
89
81
92
84
56
47
52
43
R
from Eq.(l)
(%)
100
75
100
73
100
77
too
77
> Base Case.
-------
mized by simply removing the spheres from
the TCA.
Since it is difficult to compare tests at dif-
ferent base case conditions, R from Equa-
tion (1) was calculated for each test and is
presented in both Tables 1 and 2. Figure 4
summarizes the values for R plotted as a
function of the Cl~ concentration for forced
oxidation and the high base case pH natural
oxidation data from Figure 3, as well as the
data from Tables 1 and 2. It is apparent that
R has a linear correlation with the Cl" con-
centration and is independent of inlet S02
concentration, make-per-pass, liquid holdup
in the absorber, and liquid-to-gas ratio.
Figure 4 also indicates that the initial base
case pH is significant, since'tests run with
an initial pH of 6.0 (or a limestone stoi-
chiometry of 1.3-1.4) have a slope signif-
icantly steeper than the tests run with an
initial pH of 5.5-5.9 (or a limestone stoi-
chiometry of 1.1-1.2).
In addition to decreases in S02 removal
efficiency and pH, occasional decreases in
the solids settling rate were observed in the
natural oxidation mode at high Cl". Little ef-
fect of Cl" on solids settling was observed
during the forced oxidation mode. Also, with
the accumulation of calcium chloride, the
relative saturation of gypsum in the scrub-
ber system increased to levels conducive to
scaling; however, scale was rarely observed
in the pilot plant.
Magnesium Chloride
When using magnesium chloride as the
source of Cl", a drop of system pH was
observed at a rate similar to that obtained
during calcium chloride testing, as the Cl"
concentration increased. Figure 5 shows pH
data for all tests as a logarithmic function of
the sum of calcium and magnesium cations.
It is apparent that the magnesium ion and
the calcium ion behave similarly in affecting
the equilibrium pH. The accumulation of
magnesium chloride improved S02 removal
efficiency when the Cl" concentration was
below 40,000 ppm, as shown in Figure 6.
However, a significant decrease in SO2
removal efficiency occurred at Cl" concen-
trations greater than 50,000 ppm. Figure 6
also shows there was no effect of Cl" when
a 50% molar mixture of magnesium chloride
and calcium chloride was used — indicating
that the positive effect of magnesium was
cancelled by the negative effect of calcium
chloride.
As shown in Figure 7, during forced oxi-
dation, no significant effects of magnesium
chloride on S02 removal efficiency were
observed until the Cl" concentration ex-
ceeded 70,000 ppm, after which a slight drop
in efficiency occurred. Also shown in Figure
WO
Key to Test ID's
o/\/B1 through N10-1
o N5-LM1
* N5-LM3
» N5-HM1
O N5-HM3
* FB3 through F18-1
• N2-4, N5-10
• N2-HM1, N5-HM5
Limestone
Stoichiometry
1.3-1.4
1.1
1.2
1.1
1.2
1.2
1.1-1.2
1.1
Forced Oxidation
30
60
90
Figure 4.
Cl 'Concentration, ppm x 10
Ft. from Equation (1), vs. Cl ~ concentration.
6.0
5.9
5.8
5.7
5.6
5.5
5.4
5.3
5.2
5.1
5.0
4.9
4.8
4.7
4.6
oo
Natural Oxidation
Limestone
• CaC/2
D MgCli
• MgClz + CaCIs
A NaCI
A NaCI+CaCI2
O Magnesia Enhanced
•
•
, I
w
50
100
500
1000
Figure 5. Comparison of hold tank pH's as a function of Mg** and Ca** cations.
-------
I
N
O
c/3
90
80
70
60
50
40
30
Natural Oxidation
G: 465 m3/hr
L/C: 8.0 l/m3
Inlet SOz: 2500 ppm
MgClz
30 60 90 120
Cl' Concentration, ppm x 10~3
150
180
Figure 6. Comparison of natural oxidation SOz removal efficiencies as a function of Cr2.
Forced Oxidation
G: 465 m3/hr
L/C: 8.0 l/m3
Inlet SO* 2500 ppm
90
S? 80
70
o
£
rt
O
60
50
40
30
MgCI*
30
60
120
, ppm x W~3
150
w 90
Cl ~ Concentration, Hf,--- - • -
Figure 7. Comparison of forced oxidation SOt removal efficiencies as a function of Cl ~'
180
7 is a more severe effect of the mixed
calcium and magnesium salts due to the lack
of magnesium enhancement during the
forced oxidation mode.
Sodium Chloride
In the natural oxidation mode, no signifi-
cant effects on S02 removal or pH were
observed with the accumulation of sodium
chloride until the Cl" concentration reached
50,000 ppm. The pH data are also plotted in
Figure 5 which indicates that the pH is a
function of the cations Mg++ and Ca** only.
As shown in Figure 8, when the Cl" concen-
tration exceeded 50,000 ppm, drops in S02
removal were observed. When forced oxida-
tion was employed, as shown in Figure 9,
no effects on SO2 removal were observed
until the Cl" concentration exceeded 70,000
ppm.
Methods Used To Counteract
the Effect of Cr on SO2 Removal
Efficiency
Increasing the L/G Ratio
Test results show that, at 40,000 ppm Cl"
and a limestone stoichiometry of 1.3, the
L/G ratio would have to be doubled in order
to regain the base case S02 removal effi-
ciency. Although never tested, the amount
of UG increase should be substantially lower
at lower limestone stoichiometries due to the
aforementioned reduced effects on SOZ
removal when low limestone stoichiometries
are utilized.
Increasing the Limestone Feed Rate
Results of testing are summarized in Table
3. As can be seen, there is a significant dif-
ference in the amount of excess alkali re-
quired to restore base case S02 efficiency
between the high and low limestone base
case stoichiometry runs, but the quantity re-
quired in either case is unacceptably high.
Addition of Organic Acid
A mixture of organic acids, DBA, was
added to the absorbing slurry at 50,000 ppm
Cl" to determine the amount of acid required
to restore the base case S02 removal effi-
ciency. As seen in Figure 10, the amount re-
quired is only 9.2 meq/l. Further testing
indicated that DBA is very effective in
counteracting the effects of Cl" with forced
oxidation, requiring only 9.4 meq/l to
counteract the effect of CI" at 160,000 ppm
CI".
Effects of Cl~ on a Lime
Based FGD System
The results of testing with lime are in-
teresting in that the depression in S02
removal was identical to that of the limestone
tests using high limestone stoichiometry, as
shown in Figure 11. The lime tests were run
at constant pH and a stoichiometric ratio ap-
proaching 1.0. Since pH control was used,
only the absorber effluent pH declined when
the Cl" concentration increased. When the
inlet SO2 concentration was lowered to 500
ppm, there was no noticeable effect of Cl"
on the S02 removal efficiency up to 100,000
ppm Cl".
Effects of Calcium Chloride
on a Magnesia Enhanced FGD
System
Addition of magnesia to a conventional
limestone FGD system increases the concen-
-------
o
I
90
80
70
60
50
40
30
G: 465 m3/hr
L/C: 8.0 l/m3
Inlet S02:2500 ppm
Natural Oxidation
A
30 60 90 120
Cl 'Concentration, ppm x 10~3
150
180
Figure 8. Comparison of natural oxidation SOi removal efficiencies as a function of Cl ~2.
90
80
£ 70
.U
1 60
o
I
O 50
CO
40
30
Forced Oxidation
G: 465 m3/hr
L/C: 8.0 l/m3
Inlet S02:2500 ppm
NaCI
30 60 90 120
Cl 'Concentration, ppm x 10'3
150
180
Figure 9. Comparison of forced oxidation SOi removal efficiencies as a function of Cl ~2.
trations of two dissolved sulfite species, S03=
and MgSOJi, which react with the absorbed
S02 and enhance S02 removal. The ac-
cumulation of calcium chloride affects the
magnesia enhanced limestone FGD system
by reducing both S03= and MgSO§ through
chemical equilibrium effects. Pilot plant tests
were conducted to evaluate the effects of
calcium chloride on the performance of a
magnesia enhanced limestone FGD system.
Results indicate that when calcium chloride
was initially added to the system, a sharp
drop of SO2 removal efficiency and a slight
decrease in system pH were observed. The
sharp drop in S02 removal reflected the
reduction of S03= and MgSO§ in the scrub-
bing liquor. As shown in Figure 12, further
increases in the calcium chloride concentra-
tion resulted in less of an effect on S02
removal since the dissolved sulfite species
have been depleted.
Effects of Calcium Chloride
on a DBA Enhanced FGD System
The operation of a DBA enhanced
limestone FGD system has the advantage of
high S02 removal efficiency, and low
limestone stoichiometry. In order to simulate
the DBA enhanced limestone scrubber, the
base case pH was controlled at 5.2, lime-
stone stoichiometry was less than 1.1, and
S02 removal efficiency was 90% for both
natural and forced oxidation tests. Short-
term tests were utilized to evaluate the ef-
fects of Cl" on system performance. Long-
term tests were conducted to evaluate DBA
consumption and decomposition at high Cl"
concentrations. S02 removal efficiency and
system pH decreased in both the natural and
forced oxidation modes. In order to main-
tain the base case S02 removal efficiency,
DBA concentrations had to be increased as
the Cl" concentration increased, as shown
in Figure 13. In addition, plugging and scal-
ing problems were encountered during the
natural oxidation mode: hard scale formed
on the scrubber bottom internals while
operating at Cl" concentrations above 50,000
ppm and a phenomenon similar to "lime-
stone blinding" was encountered while
operating at a Cl" concentration of 120,000
ppm. There was no noticeable effect of high
Cl" on the degradation rate of DBA, but an
increase in the DBA coprecipitation rate was
observed at high calcium chloride concen-
trations during natural oxidation testing as
shown in Table 4.
Washability Evaluation of
Gypsum Cake Using a Pilot
Horizontal Belt Filter
The marketability of gypsum produced
from a forced oxidized FGD system is af-
fected by its purity, especially with regard
to dissolved solids. Gypsum generated from
high Cl" FGD systems must be washed
thoroughly to meet the purity specifications
of wallboard manufacturers. A belt filter,
with the potential of three-stage countercur-
rent washing, was utilized to determine the
washability of gypsum cake produced dur-
ing the DBA testwork described previously.
Results of these tests are shown in Figure
14 for both TDS and Cl". Also shown is the
calculated theoretical washing efficiency ob-
tained with ideal washing. These data in-
dicate that the pilot belt filter approaches
ideal washing. Chemical analysis of the
resulting washed cake was performed at
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Table 3. Excess Limestone Required to Restore Base Case S02 Removal
Base Case
% SO,
Removal
80
89
56
92
52
Inlet
SO,
-------
Natural Oxidation
100 • Magnesia Enhanced Limestone
^ G/ 435 m3/hr
g \ L/G:10.0l/m3
.* SO A inlet SO* 2500 ppm
*z \ Mg+: 2000 ppm
1 ^^^
0 70 ' — ^^
""0 10 20 30 40 50
Cl ~ Concentration, ppm x 10~3
Table 4. Comparison of Unaccountable Organic Acid Losses at the RTP Limestone FGD Pilot Plant
Natural Oxidation Forced Oxidation
Ig/hr) (g/hr)
DBA losses at low C/~ 7.4-8.0
concentration
(less than 8,000 ppm)
DBA losses at high C/' 10.4
concentrations *
* Cl' = 30,000 ppm for natural oxidation.
Cl' = 50,000 ppm for forced oxidation.
n JT
14.4
13.9
Figure 12. Effect of Cl ' on SOz removal
efficiency in a magnesia en-
hanced scrubber.
; 7 00
1000
9
* 900
t
3/hr
L/G:8.0//m3
lnlet SO*: 250° ppm
Natural Oxidation
Forced Oxidation
DBA Enhanced Limestone
0 20 40 60 80 100
Cl 'Concentration, ppm x 10~3
Figure 13. DBA concentration required to
reach 90% SO2 removal at
high concentrations of Cl'.
70
Predicted by ideal
countercurrent
washing model
o Washing efficiency based on Cl '
A Washing efficiency based on
water soluble salts
1 2 3
No. of Washing Stages
Figure 14. Performance of a belt filter
with countercurrent washing
and five wash displacements.
Table 5. Comparison of Gypsum Qualities after Belt Filter Washing
Analysis of Sample from Slurry with Cl' of
Required
Quality* 50,000 ppm** 80,000 ppm**
Free Water, %
Combined
Water, %
CaSO3 • 1/2 H20,
CaCO3, %
CaO, %
SO3, %
Na, ppm
K, ppm
Mg, ppm
Cl, ppm
TDS, ppm
10
19.8
% 2
2
31-33
44
75
75
50
120
600
* Specified by U. S. Gypsum for wallboard quality
** Washed with
*** Washed with
5 displacement water and 3-stage
5 disolacement water and 2-staoe
14.4
21.4
1
1
32.6
46.1
5
5
8.0
90.6
136.3
gypsum.
countercurrent
countercurrent
15.4
19.6
1
1
32.6
45.9
5
5
6.0
98.6
147.5
washing.
wash/no.
120,000 ppm***
15.7
20.7
2.1
3.7
33.8
45.0
5
5
8.0
292.4
451.8
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John C. S. Chang is with Acurex Corporation, Research Triangle Park. NC 27709.
J. David Mobley is the EPA Project Officer (see below).
The complete report, entitled "Pilot Plant Tests of Chloride Ion Effects on Wet FGD
System Performance," (Order No. PB 84-167 584; Cost: $19.00, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
•&U. S. GOVERNMENT PRINTING OFFICE: 1984/759-102/938
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