Dissolution of Limestone in Simulated Slurries for Removal of Sulfur Dioxide from Stack Gases Yong K. Kim, Melvin E. Deming, and John D. Hatfield' Division of Chemical Development, Tennessee Valley Authority, Muscle Shoals, Ala. 35660
Rate of dissolution of limestone in 1% aqueous slurries, such as those used for the removal of SO2 from stack gases, is expressed by the equation 12 = [1.92 (325/d)](6.15 pH)(1.4 - 0.08 Po2) exp (-1450/RT), where k = dissolution rate, % of stone dissolved/min; d = particle diameter of stone, pm; pH = pH of the liquid phase; Pop = 0 2 content, %, of gas; R = gas constant, cal/(mol)(deg); and T = temperature, O K . The activation energy for the dissolution is 1450 cal/mol. The rate of dissolution was increased by decrease in the particle size of the stone, pH and 0 2 content of the gas, and by a rise in temperature. The degree of utilization of the limestone to form sulfite or sulfate generally increased as the rate of dissolution increased. Addition of a weak organic acid, such as benzoic acid, increased the rate of dissolution of the limestone about 10%.
+
Wet limestone scrubbing is one of the most promising processes for the removal of SO2 from stack gases, and TVA is planning a full-scale demonstration plant for test of the process on a 500-MW unit (1, 2). The efficiency of absorption of SO2 by the limestone slurry is low, however, and the utilization of the limestone also is low. Since the limestone is very slightly soluble in the liquid phase of the slurry, the low absorption efficiency may reflect this low solubility, and the overall reaction velocity of SO2 with limestone in the slurry may depend largely on the rate of dissolution of the limestone. The dissolution rate can be influenced by the pH and the temperature, both of which affect the solubility of the limestone and hence the driving force of the dissolution; by the particle size of the stone, which determines the surface area; and by the oxygen content of the carrier gas. The presence of extraneous organic acids in the solution also may influence the dissolution rate of the stone by forming a more soluble calcium compound that can react readily with dissolved SO2 (3, 4). This study was made of the effects on the limestone dissolution rate of the pH of the slurry, the temperature, the oxygen content of the carrier gas, the particle size of the stone, and the addition of benzoic acid.
Experimental Conditions The instantaneous dissolution rate was determined from measurements a t 2349 cm-' of the COz content of the exit gas with an infrared spectrophotometer fitted with a precalibrated 10-cm flowthrough cell with Irtran windows. An ultraviolet spectrophotometer was used to measure at 294.5 nm the SO2 concentration in the exit gas. The limestone was a locally available material, Spring Valley limestone, that had been used in pilot plant tests; it contained 96.1% CaC03, 2.0% MgC03, 1.5% SiO2, and traces of impurities. The limestone was closely sized to three particle size ranges--48+65-mesh, -100- +115-~nesh, and -200+325-mesh-by wet screening to eliminate the fines and aggregated particles. These particle size ranges represent average particle diameters of 325, 140, and 60 pm, respectively, and the respective geometrical surface areas are in the ratio 10:23:54. Gases from commercial cylinders of compressed SO2, Nz,
and 0 2 were used without further purification, and the flow rate of each gas was measured with a flowmeter. The carrier gas was a mixture of 400 cc/min N2 and a predetermined amount of 0 2 in the range 10.5-42 cc/min. The flow rate of SO2 was adjusted manually to maintain a predetermined pH of the slurry in the range 4.5-5.5. In each run, 7 grams of the limestone was added to 700 ml of water in a three-necked flask that was maintained at the run temperature in a water bath. The slurry was stirred mechanically and, within 30 sec of the addition of the limestone, the mixture of carrier gas and SO2 was introduced into the slurry through a coarse fritted-glass diffuser. The pH of the solution rose on addition of the stone, but it was lowered quickly to the desired value by a large flow of SO2 a t the beginning of each run and was maintained thereafter within fO.l pH unit by manual control of the SO:! flow rate. The dissolution rate became very low in 2 hr, whereupon another 7 grams of the stone was added and the dissolution was continued for a total of 5 hr. The slurry remaining after 5 hr was filtered quickly on No. 2 Whatman paper. Sulfite sulfur in the filtrate was determined by iodine titration as soon as possible, and determinations of total sulfur, Ca, and CO2 in both liquid and solid phases were made routinely.
Discussion of Results A typical COz evolution curve is shown in Figure 1 (run 29 in Table I). The addition of the first limestone portion resulted in a rapid pH rise and near saturation of the solution with calcite, but the initial rapid introduction of SO2 lowered the pH of the solution to the desired value of 5.0. The COz evolution reached a maximum in about 10 min, decreased fairly rapidly to a minimum, and then increased to a second maximum in about 30 min. The minimum is presumed to indicate the completion of utilization of the easily available surface fraction of the stone, and the subsequent increase in the rate of dissolution may indicate the onset of the precipitation of CaS062H20 by nucleation; this decreases the calcium content of the liquid phase and increases the limestone dissolution rate by the common ion effect. When the second 7 grams of limestone was added after 2 hr, COS evolution increased again to give a much
SECOND PORTION OF LIMESTONE ADDED
1
U
0
I
I
I
2
I
3 T I M E , HOURS
I
4
5
Flgure 1. Evolution of C o n during absorption of SO2 by limestone slurry Volume 9, Number 10, October 1975
949
broader maximum than after the first addition, after which the C02 evolution gradually decreased to a small fraction of the initial rate. The presence of gypsum crystals in the slurry prevented a high degree of its supersaturation, and no minimum and second maximum were observed during the second stage. The dissolution rate of the limestone after 5 hr was very low, and the total utilization of the limestone could not be increased significantly by prolonging the addition of S02. Material balances were obtained from the SO2 and C02 contents of the exit gases and the chemical analyses of the resulting slurries. The relative dissolution rates of the limestone were determined from the amount of CO2 evolved in 60 min (the first 30 min of stage 1 plus the first 30 rnin of stage 2). A correlation was found between the average CO2 evolution maximum (peak height) of the two stages and the C02 evolved in the first 30 min of both the first and second stages (area under curves), and this is shown in Figure 2 and Table I. This correlation provided another estimate of the 60-min COZ evolution from the average peak height. The relative dissolution rate was yS0 of the average of these two estimates, and the results were expressed as the percent of the limestone dissolved per minute, k , by the reaction CaC03
0
:-om
0
0-om
0
L oY o
w
w
w
c v w
L?
N
N
N
m
w
qoo
0
:No0
0
m
+ 2Hf= Ca2+ + H2O + C02
Although the second stage approached steady state conditions more closely than did the first stage, there was little difference in the C02 evolved in the first 30 rnin of each stage, and the total of the two results was taken as the best measurement of k that would apply to large-scale limestone scrubbing operations for the removal of SO2 from stack gases. The degree of utilization of the limestone after 5 hr was calculated from the amount of carbonate remaining in the solid residue and in solution, and by integration of the CO2 evolution curve, Figure 1,for each run. These two measurements of limestone utilization agreed fairly well, and the average values are shown in Table I. The effect of particle size on the dissolution rate at 5OoC
1200 -
i5
w
m c\1 m
d
m
2300 ( v m
0
Loo
0
Loo
0
L O O
0
L o o w m
2
w
N
m
N r n
0 (D
z 1000 0 0
E 3 0
aoo-
w
N
m
>
w
N
600-
0
V
w
-I
a
&
A o
400-
t-
200
-
0
5
IO
15
20
25
MAXIMUM C02 R A T E , C C I M I N
Figure 2. Relationship between rate and extent of the reaction of limestone slurry with SO2 in 60 min. (First 30 min of each stage) 950
Environmental Science & Technology
was measured a t pH 4.5 because of the optimum COz detection range of the infrared instrument; the results are shown in Figure 3. There is a linear relationship between the COP evolved and the geometrical surface area of the stone which can be expressed by the empirical equation
k = 0.33
+ 0.172(S/so)
(1)
where k is the percent of the limestone dissolved per minute and S/So is the ratio of the geometrical surface area of the stone to that of the -48- +60-mesh fraction. Since the geometrical surface area is inversely proportional to the diameter of the particle for the same amount of stone, S/So can be rewritten as doldwhere do = 325 pm, the average diameter of the -48- +60-mesh particles, and d is the average diameter of the other particle size. Equation 1 can then be expressed as
k = 0.33
+ 0.172(325/d)
3
2
4
k = 0.76(6.15 - pH)
(3)
This indicates that the dissolution rate a t p H 6.15 is negligible for this particle size of stone. The degree of utilization of the stone was higher a t pH 4.5 than a t pH 5.0 or 5.5, but the relative effect of the pH on the utilization is smaller than that on the dissolution rate (Figure 4). Both the dissolution rate and the degree of limestone utilization of -200- +325-mesh stone a t pH 5.5 were increased by raising the temperature from 25-65OC (Figure 5). A linear relationship between the logarithm of the dissolution rate and the reciprocal of the absolute temperature was found (Figure 6) and this can be expressed as
k = ko exp ( - E / R T )
(2)
The empirical constants were obtained from the slope and extrapolation of the straight line of Figure 3 to zero surface area. This equation indicates that the finer the particle the higher the dissolution rate, but the degree of utilization of the stone may increase with decrease in particle size in a nonlinear relationship.
I
The effect of pH on the reaction rate for the -200+325-mesh fraction is linear in the pH range 4.5-5.5 and can be expressed as
(4)
where ko = 4.82, E = 1450 cal (activation energy), R = gas constant, and T = temperature, OK. The effect of the concentration of 0 2 in the carrier gas on the dissolution rate at 5OoC and pH 5.5 and for -200+325-mesh particles is shown in Figure 7 . The results indicate a decrease in both dissolution rate and degree of utili-
5
-200 +325
MESH
LL
0 W
I-
a LL
0825
n
I
65
50 TEMPERATURE,
O C
Figure 5. Effect of temperature on rate of reaction of limestone slurry with SO2
I
-100+115 -200+325 PARTICLE S I Z E , MESH
-48+60
Figure 3. Effect of particle size of stone on rate of reaction of limestone slurry with SO2
z
p
k=O.76(6.15-pH)
1.2
-
70 0
8 J z
E
W
-z
- 0.25 k = 4 82 EXP ( - 1 4 5 0 / R T )
P0 W
1.0
1,
LL
o -030
0
v)
E
W
0
I-
s
0.8
z
W
z
t
2
0
- 200t 325 MESH
0
I-
(0
-ae
50°C
0.6
5 % 02
v)
- 0.35
- 2 0 0 + 3 2 5 MESH
W
2
pH 5 5
0
0 I )
5.0 PH
I 5.5
Figure 4. Effect of pH on rate of reaction of limestone slurry with SO2
1 L
3.0
3.I
3.2
3.3
4
IOOO/T
Figure 6. Variation with temperature of rate of reaction of limestone slurry with SO2
Volume 9, Number 10, October 1975
951
- 200 + 3 2 5
w
Combining the effects of the factors that affect the dissolution rate of the limestone-particle size, pH, temperature, and 0 2 concentration-yields the empirical equation
MESH LL
0
k = [1.92
+ (325/d)](6.15 - pH)(1.4 - 0.08 Po2)X exp (-1450/RT)
.4-
-40 I
I
25
2 3
10.0
5.0 7.5 0 2 CONTENT, %, OF G A S
Figure 7. Effect of oxygen concentration on rate of reaction of limestone slurry with SO2
1.2
I
-I
- 200 + 325 MESH
W
l-
a
a 0.82 2
1L
9
p H 5.5 5OoC 5 % 02
-50
z 0
I-
2 -I
-
0 0.6
3 -
$-
t
” c
-40 5
0
3
0.4
I
A
30
zation of stone with increase in 0 2 concentration in the range 2.5-10%. This can be expressed as
k = 0.7 - 0.04 Po2
(5)
where Poz is concentration, %, of 0 2 in the carrier gas. The explanation of this effect is that a higher oxygen content in the flue gas results in faster oxidation of sulfite to sulfate; this produces a higher steady state degree of supersaturation of gypsum that lowers the limestone dissolution rate by the common ion effect. The addition of benzoic acid (0.1 or 0.2% of the weight of limestone slurry) to the scrubbing slurry (Figure 8) increased the rate of the dissolution and the degree of limestone utilization by about 10% a t pH 5.5 with the -200+325-mesh stone. The benzoate ion was the effective agent, and it was as effective when added as the calcium salt as when added as the acid.
952
Environmental Science & Technology
(6)
This equation agrees with the measured dissolution rate of the stone over the range of the study so well that it can be extrapolated with considerable confidence outside the range because of its linear relationship with each factor. Equation 6 indicates that the dissolution rate varies linearly with each individual variable-pH, the reciprocal of the particle diameter, the exponential of the reciprocal temperature, and oxygen content-as was observed experimentally; the constants of each linear relation vary with the fixed conditions of the other variables.
Conclusions The following conclusions can be drawn from these results when the limestone slurry was used for the absorption of SO2 at about pH 5.5. Both the dissolution rate and the degree of utilization of the limestone were nearly proportional to the geometrical surface of the stone. Addition of benzoic acid increased moderately both the dissolution rate and the utilization of the stone. Raising the pH of the slurry decreased the dissolution rate markedly, but decreased only slightly the utilization of the stone. Increasing the 0 2 concentration decreased both the dissolution rate and the utilization of the stone. Raising the temperature increased both the dissolution rate and the utilization of the stone. The utilization of the stone increased with decrease in particle size. It is estimated that the average diameter of the particle should be 30 km or less to obtain 90% or more utilization of the stone under normal operating conditions of the limestone scrubbing process. Prolonged recycling of the slurry would not increase the utilization of the stone significantly. Literature Cited (1) Slack, A. V., Falkenberry, H. L., “Son-More
Questions Than Answers,” Electrical World, Dec. 15,1971. (2) Tennessee Valley Authority, “Sulfur Oxide Removal From Power Plant Stack Gas-Use of Limestone in Wet-Scrubbing Process,” Conceptual Design and Cost Study, prepared f& NAPCA, Contract No. TV-29233A, 1969. (3) . . Hatfield. J. D.. Kim. Y. K.. Mullins. R. C.. “Sulfur Dioxide Removal From Power Plant Stack Gas-Study of the Effect of Organic Acids on the Wet-Limestone Scrubbing Process,” Prepared for EPA. Contract No. TV-344.25A, 1972. (4) Hatfield, J. D., Potts, J. M., “Removal of Sulfur Dioxide From Stack Gases by Scrubbing With Limestone Slurry-Use of Organic Acid,” 2nd International LimeLimestone Wet-Scrubbing Symposium, New Orleans, La., Nov. 8-12, 1971. Receiued for reuiew October 28,1974. Accepted J u n e 2,1975
