Environ. Sci. Technol. 1988, 22, 1299-1304
PA. Aluminum speciation was made possible through the modeling efforts of W. D. Schecher. The advice and assistance of R. D. Fuller was also greatly appreciated. Registry No. Al, 7429-90-5; H+, 12408-02-5;Ca, 7440-70-2; Mg, 7439-95-4; Na, 7440-23-5; K, 7440-09-7.
Literature Cited Lawrence, G. B.; Fuller, R. D.; Driscoll, C. T. J.Environ. Qual. 1987.16.383-390. Schofield, C. L:; Trojnar, J. R. In Polluted Rain; Toribara, T. Y., Miller, M. W., Morrow, P. E., Eds.; Plenum: New York, 1980; pp 347-366. Hall, R. J.; Driscoll, C. T.; Likens, G. E.; Pratt, J. M. Limnol. Oceanogr. 1985,30, 212-220. Theobald, P. K., Jr.; Lakin, H. W.; Hawkins, D. B. Geochim. Cosmochim. Acta 1963,27, 121-132. Mcknight, D. M.; Feder, G. L. Hydrobiologia 1984, 119, 129-138.
Nordstrom, D. K.; Ball, J. W. Science (Washington,DE.) 1986,232, 54-56.
Driscoll, C. T.; Schafran, G. C. Nature (London) 1984,310, 308-310.
Sullivan, T. J.; Christophersen,N.; Muniz, I. P.; Seip, H. H.; Sullivan, P. D. Nature (London)1986,323, 324-327.
(9) Henriksen,A.; Skogheim, 0. K.; Rosseland, B. 0. Vatten 1984, 40, 255-260. (10) Baker, J. D.; Schofield, C. L. Water,Air, Soil Pollut. 1982, 18, 289-309. (11) Likens, G. E.; Bormann, F. H.; Pierce, R. S.; Eaton, J. S.;
Johnson, N. M. Biogeochemistry of a Forested Ecosystem; Springer-Verlag: New York, 1977; p 146. (12) Lawrence, G. B.; Fuller, R. D.; Driscoll, C. T. Biogeochemistry 1986, 2, 115-135. Lawrence,G. B.; Driscoll, C. T.; Fuller, R. D. Water Resour. Res., in press. Driscoll, C. T. Znt. J. Environ. Anal. Chem. 1984, 16, 267-284.
Dohrmann, Xertex Corp., Santa Clara, CA, 1984. Gran, G. Znt. Congr. Anal. Chem. 1952, 77,661-671. Driscoll, C. T.; Newton, R. M. Environ. Sci. Technol. 1985, 19,1018-1024.
Schecher, W. D.; Driscoll, C. T. Water Resour. Res. 1987, 23, 525-534.
(19) Driscoll, C. T.; Baker, J. P.; Bisogni, J. J.; Schofield, C. L. Nature (London)1980, 284, 161-164.
Received for review July 7,1987. Accepted April 26,1988. This work was supported by the National Science Foundation (Grant BSR-8406634).
Aqueous Reaction of Fly Ash and Ca(OH), To Produce Calcium Silicate Absorbent for Flue Gas Desulfurization Joseph R. Peterson and Gary T. Rochelle" The University of Texas at Austin, Austin, Texas 78712
Fly ash was slurried with Ca(OHIzat 85 O C to produce reactive solids for use in dry processes for flue gas desulfurization. Reacting slurries of fly ash and Ca(OH)z were monitored for dissolved metal concentrations. The solids produced were dried and tested for reactivity toward SOz in a packed-bed reactor at bag filter conditions. The dissolved calcium concentration in the slurry was a very important parameter for solids reactivity. The solids formed in slurries containing 10-100 ppm dissolved calcium were up to 40% more reactive than the solids formed in slurries containing less than 10 ppm or more than 100 ppm dissolved calcium. The dissolved calcium concentration was affected by the NaOH concentration, the ratio of fly ash to Ca(OH)z,the slurry temperature, the fly ash type, and the presence of calcium sulfite.
Introduction Two important dry processes for flue gas desulfurization (FGD) utilize spray drying of Ca(OH)zand the injection of dry, calcium-based sorbent into humidified gas at 60-100 "C (I). In both of these technologies, gas/solids reaction makes a major contribution to SOz removal both in the ductwork and in the particulate removal device, especially if a bag filter is used. The reaction of fly ash with Ca(OH)2 to produce reactive solids for use in these processes has been investigated up to the pilot-plant scale (2-6). The reaction of fly ash with Ca(OHIz is called a pozzolanic reaction and has primary importance in the chemistry of cement (7). The use of fly ash as a reagent material for the production of reactive solids for FGD is very attractive, both economically and environmentally, because fly ash is a waste product from all coal-fired power plants. Jozewicz (2)studied the reaction of fly ash with Ca(OH), to produce reactive solids for use in FGD. In bench-scale experiments using a packed-bed reactor, Jozewicz found 0013-936X/88/0922-1299$01.50/0
that the solids produced by slurrying fly ash with Ca(OH)z were much more reactive toward SOz than was Ca(OH)z. He found that increasing the fly ash loading from 0.5 to 20 g of fly ash/g of Ca(OH)z increased the Ca(OH)zutilization from 17 to 78%. Jozewicz also found that silica was the most reactive component of the fly ash and that the solids reactivity increased with slurry time and slurry temperature. Jozewicz postulated that the rate-limiting step of the reaction of fly ash with Ca(OHI2was the dissolution of silica from the fly ash. In order to increase the reaction rate between fly ash and Ca(OH)2,several researchers have tested additives to the fly ash-Ca(OH)z slurries in an attempt to increase the dissolution rate of silica from the fly ash ( 3 , 4 ) . Since fly ash is primarily a glassy substance, and since it is wellknown that NaOH etches glass, Chu (3)tested the addition of NaOH to the fly ash-Ca(OH)z slurries in order to enhance the reaction of fly ash with Ca(OH),. The addition of 0.08 M NaOH to the fly ash-Ca(OH)z slurry increased the reactivity of the product solids toward SOz from 18 to 65 mol of SOz removed/100 mol of Ca(OH),. The addition of 0.08 M NaOH to the slurry also increased the reactivity of the product solids toward NO, from 1to 4 mol of NO, removed/100 mol of Ca(OH),. Phosphoric acid and ammonium phosphate have also been tested as additives for fly ash dissolution (4))and similar increases in the product solids reactivity were reported. An alternative way to increase the reaction of silica with Ca(OH), is to use a more reactive form of silica. Jozewicz and Chang ( 4 ) have tested diatomaceous earths and clays as sources of silica for the production of reactive solids for use in dry FGD systems. These naturally occurring substances are composed of essentially pure amorphous silica. Although diatomaceous earth was shown to be much more reactive than fly ash, the most likely source for silica comes from the power plant fly ashes, since the cost of the al-
0 1988 American Chemical Society
Environ. Sci. Technol., Vol. 22, No. 11, 1988
1299
ternate sources of silica would be much higher than that of fly ash. The use of the fly ash based sorbents has been sucCessfuly demonstrated on a larger scale during the last few years. In pilot-plant experiments with dry injection systems (5.6). the utilization of Ca(OH), was 25-35% higher when solids produced by slurrying fly ash with Ca(OH), were used instead of hydrated lime. Although the use of these fly ash based sorbents seems to be a very attractive option for dry flue gas desulfurization systems, there are several obstacles that must be overcome for the economical production of these sorbents on a large scale. Due to the low dissolution rate of the silica from the fly ash, long slurry times (12 h), high slurry temperatures (90 "C), and high fly ash to Ca(OH)z ratios (31)are required to obtain reactive solids. These obstacles should be minimized by increasing the rate of dissolution of the silica from the fly ash. This study was undertaken to investigate the production of the fly ash based sorbents when an additive was added to the slurry to increase the rate of silica dissolution from the fly ash. Scope of Work The research presented here focused more on the reaction of fly ash with Ca(OH)z in the slurry and less on the reaction of the product solids with SOz. The reactivities of the product solids toward SOz were determined only to serve as an indication of the extent of the reaction of fly ash with Ca(OH),. The reaction of fly ash with Ca(OH), was followed by slurrying the fly ash with Ca(OH), and determining the change in solution cornpasition with time. The dissolved metal concentrations in the slurry were monitored with time by atomic absorption spectrophotometry. The product solids were dried and tested for reactivity toward SOz in a packed-bed reactor designed to simulate bag fdter conditions (3). The product solids were also tested for BET surface area to determine the effect of surface area on solid reactivity. The effects of fly ash type, sodium hydroxide concentration, fly ash loading, and slurry temperature were examined. The effect of calcium sulfite addition to the slurry was examined to simulate the recycling of spent Ca(OH)z. Additional details are available elsewhere (8). Experimental Methods The experimental methods involved in this study included both solution analysis and product solids analysis. These methods are more clearly understood if they are discussed separately. Solids Preparation and Slurry Analysis. Reactive solids were prepared by slurrying fly ash with Ca(OH), in a temperature-controlled, stainless steel beaker. Distilled water was sparged with nitrogen for 30 min prior to reagent addition to remove COz and O2 The NaOH concentration was then adjusted with 5 N NaOH. The reagent Ca(OH), was added before the fly ash to allow for complete dissolution of Ca(OH),. After 15 min the fly ash was added, normally at a loading of 4 g of fly asb/g of Ca(OH), Four types of fly ash, ranging from 4 to 30% CaO (Table I), were used in the experiments. The solids were slurried at either 65 or 85 "C, and the usual water to solids ratio was 201. Slurry samples (20 mL) were taken approximately every hour, filtered with a 0.45-pm membrane filter, and diluted. The diluted filtrates were analyzed for dissolved calcium, silicon, and aluminum by atomic absorption spectrophotometry. Larger slurry samples (100 mL) were taken at 1-,4,9-, and 12-h slurry times, vacuum filtered, and vacuum dried 1300 Environ. Sci. Technd.. Vol. 22.
NO. 11, 1988
F71
. .... Scrubber
Analyzer
FYure 1. Packed-bed reactor apparatus.
at 85 O C . The filtrates were discarded. The dried solids were tested for BET surface area and tested for reactivity toward SOz in a packed-bed reactor. Solids Reactivity. The solids reactivity experiments were conducted in a packed-bed reactor (Figure 1) designed to simulate bag filter conditions (3). The Pyrex reactor (4 cm in diameter and 12 cm in height) was wrapped with heating tape, and the temperature was controlled within 1OC. Flue gas was synthesized by cornbining N2, COz, and SOzfrom gas cylinders with house air. Water was added to the system by a syringe pump and evaporated at 12&150 "C in a stainless steel evaporation chamber before being mixed with the gas stream. The tubing upstream of the reactor was heated to prevent condensation. A PI controller was used to regulate the gas temperature to within 2 "C. The reactor was equipped with a bypass to allow for preconditioning of the solids and to allow the gas concentration to stabilize before starting the experiment. After the reactor, the gas was cooled and the water condensed out by cooling water. A gas sample of 2.5 mL/min was diluted with 2 L/min air and then analyzed by a flame photometric SOz analyzer (Columbia Scientific, Model SA28SE). The SOz concentration was continuously recorded, and the concentration curve was integrated to determine solids reactivity. The reactivity of the sorbents toward SOz is expressed here as 'Ca(OH), conversion" or as "SOz capture". Ca(OH), conversion is defined as the total number of moles of SOz removed after 1 b divided by the initial number of moles of Ca(OH)zin the sorbent. SOz capture is defined as the total number of millimoles of SOz removed per gram of sorbent. The experimental conditions for the packed-bed reador system were chosen to simulate conditions of a fabric filter downstream from a coolside dry FGD process. The synthetic flue gas flow rate was 4.6 L/min (0 OC, 101m a ) with 7% Oz, 10% COz, and the balance nitrogen. The reactor temperature was 66 "C, and the SOzconcentration was 450 ppm. Water was injected a t 12 mol % to give a relative humidity of 55%.
Table I. Fly Ash Characterization by Energy Dispersive Spectroscopy fly ash operator coal type sample date
Clinch River Appalachian Power Co. bituminous 10186
CaO
4 27 57 8
A1203
SiOz FeO
Composition, wt % 6 24
54 1
0
Craig Station Colorado Ute subbituminous 7/85
Laramine River Basin Electric subbituminous 12/86
12 25 55 4
30
17 35 6
60 I
..
1000
10;
San Miguel San Miguel Electric Coop. lignite 1985
A
A
6
A
O
I
0.25M
b
o
A 0
b
0
0 0
'0 0
t
0.0
0.1
0.2
0.3
[NaOHl (M)
Flgure 3. Optimum reactivity for fly ash-Ca(OH), solids. Conditions: 4 g of Clinch River fly ashlg of Ca(OH),, 50 g of solids/L of H20. Reactor conditions: 66 OC; 55% RH; 4.6 L of gas/min (7% O,, 10% COP, 83% N,); 450 ppm SO2; 12 mol% H20; 1-h reaction time.
slurry. The calcium silicate system is complicated, and depending upon the ratio of calcium to silica in solution, the ratio of calcium to silica in the product calcium silicates can change (10). The solids formed in the presence of very low levels of dissolved calcium probably have a low ratio of calcium to silica in the product solids, which may explain the solids' low reactivity toward SO2. Therefore, there is a tradeoff: the addition of NaOH to the slurry increases the dissolution rate of silica from the fly ash which increases the product solids reactivity, but it also decreases the concentration of dissolved calcium which lowers the reactivity of the solids toward SO2. Since NaOH is reactive toward SO2 and since some of the NaOH that is present in the slurry is retained on the dried solids, this could explain some of the increase in the product solids reactivity when NaOH is present in solution. However, it was calculated that there was only a 1.5% increase in the alkalinity of the product solids when 0.08 M NaOH was added to the slurry, but the product solids conversion increased from 41 to 52%. Therefore, the NaOH addition cannot account for all of the increase in solids reactivity. This is particularly the case for the slurries to which 0.25 M NaOH was added. The product solids conversion decreased from 41 to 36%, even though the alkalinity of the product solids increased by 4.5%. Effect of Fly Ash Type. Four samples of fly ash were slurried with Ca(OH)2for 12 h at 85 "C and at a fly ash loading of 4 g of fly ash/g of Ca(OH)2to investigate the effect of fly ash type on the reactivity of the fly ash based solids. The fly ashes varied from 4 to 30% calcium as CaO (Table I). The NaOH concentration was varied from 0.0 to 0.25 M and the water to solids ratio was 20:l. When no NaOH was present in the slurry, all of the fly ashes behaved similarly (Figure 4). Within 3 h the dissolved calcium concentration decreased below the Ca(OH), solubility (460 ppm), indicating that there were no Ca(0H),solids left in solution after 3 h. The Ca(OH)2 was probably incorporated into calcium silicate solids on the Environ. Sci. Technol., Vol. 22, No. 11, 1988
1301
600
Table 11. Summary of Reactivity Dataa
Clinch River
500
Concentration
(wm)
P
' A
t
.
n
o
300
-
200
-
100
-
0
20 301
0' 0
Ca(W2 SolubfitY m - I *
b d e p . n * m * A D
0.
i.0
A A
~
"
~
"
'
'
'
'
~
'
'
'
'
~
1/4/0/0.0 1/4/0/0.04 1/4/0/0.08 1/4/0/0.16 1/4/0/0.25 1/4/0/0.0 1/4/0/0.25 1/4/0/0.0 1/4/0/0.25 ' 1/4/0/0.0 1/4/0/0.25 l/8/0/0.0 1/10/0/0.08 1/4/4/0.0 1/4/4/0.08 1/4/o/o.od 1/4/O/0.08d 1/4/0/0.25d
fly ash
15.5 13.0
calcium concn, ppm
Ca(OH), convn, %
228 57 28 16
41 41 52 40 36 43 32 44 37 49 55 55 47 44 71 (62)e 43 56 43
CR CR CR CR CR SM SM
12.7 11.8
25.0 6.5 8.3 30.0 19.0 15.5
1
300 1
cs cs
172 1 228 14 168 4 281 65 490 67
LR LR CR CR CR CR CR CR CR
11.7
13.3 18.1
9.0 9.2 4.5 9.2 8.9
12
50 g of solids/L of HzO; 85 "C; slurry time, 12 h. 1/4/4/0.08 = 1 g of Ca(OH)2/4 g of fly ash14 g of CaSO3.0.5Hz0/0.08 M NaOH. 'CR, Clinch River; SM, San Miguel; CS, Craig Station; LR, Laramie River. Slurried at 65 OC. e Corrected Ca(OH)2conversion.
Craig Station Laramie River
I
I
5
surface area, m2/g
slurry compb
Craig Station Laramie River
10 Slurry Time (hr)
15
Flgure 5. Effect of fly ash type on solid surface area. Conditions: 4 g of fly ash/g of Ca(OH),; 85 OC; no NaOH; 50 g of solids/L of H,O.
surface of the fly ash. However, the BET surface areas of the product solids (Figure 5) and the reactivities of the solids increase with slurry times greater than 3 h (2). Therefore, the reaction to produce calcium silicates continues after 3 h but must continue at the fly ash surface between the silica of the fly ash and the Ca(OH)2incorporated into the solids on the surface of the fly ash. With 0.25 M NaOH present in the slurry, the low- and medium-calcium fly ashes behaved differently than the high-calcium fly ash (Laramie River). For the low- and medium-calcium fly ash slurries, the dissolved calcium concentration dropped to 1ppm as silicon entered solution (75-150 ppm Si). For the high-calcium fly ash slurry, the calcium concentration dropped to 15 ppm and no silicon was detected in solution. The reactivities of the solids produced from the low- and medium-calcium fly ashes were less than when no NaOH was present in solution (Table 11). The reactivity of the high-calcium fly ash solids improved with the addition of 0.25 M NaOH to the slurry. The product solid reactivities can be related to the dissolved calcium concentration in the slurries. The solids formed with the low- and medium-calcium fly ashes were formed in the presence of a very low dissolved calcium concentration and therefore had a low reactivity toward SO2. This low reactivity results from the low ratio of calcium to silica in the product solids. The solids formed by slurrying the high-calcium fly ash with Ca(OHI2were formed in the presence of higher dissolved calcium concentration and were more reactive toward SO2,because the ratio of calcium to silica was higher for these solids. Use of High-Calcium Fly Ash. Experiments were conducted with low-, medium-, and high-calcium fly ashes 1302 Envlron. Sci. Technol., Vol. 22,
No. 11, 1988
(mmol S02ig) 0.5
,
,
0
0.0 0.0
0.2
0.1
1/4 4 % CaO 4 % CaO 12%CaO 0.3
[NaOHl (M)
Flgure 6. Optimizing high-calcium fly ash solids reactivity. Conditions: 50 g of solids/L of H,O; 85 O C . Key: 1/4, 1 g of Ca(OH),/4 g of fly ash; 4 % CaO, Clinch River fly ash; 12% CaO, Craig Station; 30% CaO, Laramie River.
to evaluate the feasibility of producing reactive solids without the use of Ca(OH)2. The fly ashes were slurried without Ca(OH)2for 12 h at 85 OC and at a water to solids ratio of 20:l. The NaOH concentration was varied between 0.0 and 0.25 M. Figure 6 shows that the high-calcium fly ash could be activated to produce solids that were very reactive toward SO2. The solids were as reactive, on a weight basis (millimole of SO2 removed/gram of sorbent), as the solids prepared by slurrying low-calcium fly ash with Ca(OH)2. On a weight basis, the low- and medium-calcium fly ash solids were much less reactive than the high-calcium fly ash solids. However, the utilization of the calcium in the fly ash decreased with increasing calcium content of the fly ash (Figure 7). There is an optimum NaOH concentration for the high-calcium fly ash slurries which is similar to that observed for the low-calcium fly ash-Ca(OH), slurries. The slurry analysis data (Table 11) indicate that the level of dissolved calcium concentration is the reason for the optimum NaOH concentration. The solids are most reactive when they are fomed in the presence of 10-100 ppm dissolved calcium. This indicates that there is an optimum
80
Total CaO Utilization
1-
L
Fly Ash w/ 0.25 M NaOH w/Ca(OH)2&NaOH
A
4o
W)
a
20
.
-
b a A
A
a
A
I
a
I
0
O.OM, 85 "C Calcium Concentration
t
I
At high concentrations of NaOH, the solids formed at 65 "C are more reactive than those formed at 85 "C even though the latter have much higher surface areas (Table 11). Comparison of the dissolved calcium concentrations shows that the solids formed at 65 "C were formed in the presence of higher dissolved calcium than the solids slurried at 85 "C. Therefore, the solids formed at 65 "C probably had a higher calcium to silica ratio in the product solids and therefore were more reactive toward SO2 than the solids formed at 85 "C. When NaOH is not present in the slurry, the dissolved calcium concentration is high (greater than 200 ppm) for both slurry temperatures and the ratio of calcium to silica in the product solids is limited by the dissolution of silica and not by the dissolved calcium concentration. Therefore, for long slurry times, the solids formed at 65 "C will be as reactive as those formed at 85 "C. At shorter times, however, the solids formed at 85 "C will be more reactive than those formed at 65 "C (2) due to the higher dissolution rate of silica at the higher temperatures. Effect of Fly Ash Loading. Clinch River fly ash was used to investigate the effect of fly ash loading [gram of fly ash/gram of Ca(OH)2]on the reaction of fly ash with Ca(OH)> Fly ash and Ca(OH)2were slurried at 85 "C and at fly ash loadings from 4 to 10 g of fly ash/g of Ca(0Hh. The NaOH concentration was varied from 0 to 0.08 M, and the water to solids ratio was 20:l. When no NaOH was present in solution, the solids reactivity was favored by higher fly ash loading (Table 11). The Ca(OH)2conversion for a fly ash loading of 8 was 55% versus 41% for a loading of 4 g of fly ash/g of Ca(OHI2. These results confirm the data by Jozewicz (2) which showed that the solids reactivity increases with fly ash loading. In the absence of NaOH, the dissolved calcium concentration is high (228 and 168 ppm after 12 h for loadings of 4 and 8, respectively) and the solids reactivity is favored by increased fly ash loading since at higher loadings more silica is available to react with the Ca(OH), to form reactive solids. When NaOH is present in solution, the level of dissolved calcium can limit the ratio of calcium to silica in the product solids. When 0.08 M NaOH was present in the slurry, the dissolved calcium concentration was only 4 ppm (after 12 h of slurrying) for a fly ash loading of 10 versus almost 30 ppm for a fly ash loading of 4 g of fly ash/g of Ca(OH),. Table I1 shows that the solids formed at a loading of 10 were less reactive than those formed at a loading of 4 g of fly ash/g of Ca(OH)2[47 and 52% Ca(OH), conversion, respectively]. Effect of CaSOs. Laboratory-produced CaS03.0.5H20 was slurried with Clinch River fly ash and Ca(OH)2to simulate the recycling of spent Ca(OH)2. Samples of Clinch River fly a~h/Ca(OH)~/CaS0~.0,5H~0 at a weight ratio of 4:1:4 were slurried at 85 "C. The NaOH concentration was either 0.0 or 0.08 M. The data show that the addition of calcium sulfite has little effect on the dissolved calcium concentration or on the product solids reactivity toward SO2 when no NaOH is present in solution. However, when 0.08 M NaOH was present in the slurry, the addition of CaSO3.0.5Hz0to the slurry increased the solids reactivity toward SO2. The slurry analysis data (Table 11) showed that the dissolved calcium concentration was higher when CaS03.0.5H20was added to the slurry due to the reaction. CaS03 + 2NaOH + silica calcium silicates + Na2SO3 (1) This reaction is driven to the right by the low solubility of the calcium silicates.
: 0 08 M, 85 "C
10 0
5
10
15
Slurry Time (hr)
Effect of slurry temperature on slurry composition. Conditions: 4 g of Clinch River fly ashlg of Ca(OH),; 50 g of solidslL of Flgure 8.
HZO.
ratio of calcium to silica in the product solids and that this ratio can be controlled by adjusting the slurry conditions (e.g., NaOH concentration and fly ash type). The presence of an optimum ratio of calcium to silica in the fly ashCa(OH), solids agrees well with the work by Jozewicz (5) which showed that there was an optimum ratio of diatomaceous earth to Ca(OH), for the production of reactive solids for FGD. Effect of Slurry Temperature. Clinch River fly ash was selected to assess the effect of slurry temperature on the reaction of fly ash with Ca(OH)> The fly ash was slurried with Ca(OH), for 12 h at a fly ash loading of 4 g of fly ash/g of Ca(OH),. The NaOH concentration was varied from 0.0 to 0.25 M, and the water to solids ratio was 201. The slurry temperature was varied from 65 to 85 "C. Figure 8 shows the effect of slurry temperature on the dissolved calcium concentration in the slurry. The initial dissolved calcium concentration is higher for lower slurry temperatures due to the higher solubility of Ca(OH)2at lower temperatures. The final calcium concentration is also higher because the solubility of silica is lower at lower temperatures. Since the solubility of silica is lower, less calcium is precipitated from the lower temperature slurries. Figure 3 shows that, for long slurry times (12 h) and in the absence of NaOH, the solids formed at 65 "C have the same reactivity as those formed at 85 "C. These results confirm the data of Jozewicz (2)which showed that for long slurry times the reactivity of the fly ash-Ca(OH), solids was independent of slurry temperature for slurry temperatures greater than 65 "C.
-
Environ. Sci. Technol., Vol. 22, No. 11, 1988 1303
1.6 1.4
1
A.
SO2 Capture 1.2 (mmol SO2/g) 1.0
-
0.8
-
0.6
-
0.4
AU.
1
,
,
...
I
i
10
100
Surface Area (m2/g)
Figure 9. Correlation of solids reactivity. Reactor conditions: 66 "C; 55% RH; 4.6 L of gaslmin (7% 02, 10% GO2, 83% N2); 450 ppm SO,; 12 mol % H,O; 1-h reaction time.
The solids formed in the presence of the higher levels of dissolved calcium had higher reactivity toward SO2 [71 versus 52 % Ca(OH)2conversion, respectively]. This conversion should be cotrected, however, since it does not include the Ca(OH)2produced by the reaction of NaOH with CaS03.0.5H20. The slurry filtrate was titrated with HCl to determine the change in NaOH concentration. Assuming that all of the NaOH reacted to form Ca(OH)2, the corrected reactivity is 62% Ca(OH)2conversion, which is still significantly higher than without CaS03 [52% Ca(OH)2conversion]. Chu (3) found that the addition of calcium sulfite to the slurries of fly ash and Ca(OH)2did not affect the solids reactivity toward SOz. However, since Chu slurried his solids at 65 "C, the dissolved calcium in his slurries was probably much higher than in these higher temperature (85 "C) slurries. Since the dissolved calcium concentration was already high, the addition of calcium sulfite did not significantly affect the dissolved calcium concentration and therefore did not affect the reactivity of the solids toward
sop
Correlation of Reactivity. Figure 9 shows a correlation between the solids reactivity, the BET surface area, and the dissolved calcium concentration in the slurry. The data show the reactivity to be the greatest for the solids formed in slurries containing 10-100 ppm dissolved calcium. The reactivity is seen to generally increase with the surface area of the product solids, but the data show that high surface area solids are less reactive when they are formed in slurries containing low concentrations of dissolved calcium. Conclusions
The present study has investigated how the slurry conditions influence the reactivity of the solids produced by slurrying fly ash with Ca(OH),. The dissolved metal concentrations in the slurries were monitored with time by atomic absorption spectrophotometry, and the product solids were tested for reactivity toward $ 0 2 by using a packed-bed reactor. The influence of the slurry temperature, the fly ash type, the fly ash loading [gram of fly ash/gram of Ca(OH)z],the presence of CaS0,.0.5H20, and the NaOH concentration in the slurry were examined. The data showed that the Concentration of dissolved calcium in the slurry greatly affected the reactivity of the solids produced by slurrying fly ash with Ca(OH)2. The
1304
Environ. Sci. Technol., Vol. 22, No. 11, 1988
solids were most reactive when they were formed in slurries containing 10-100 ppm dissolved calcium. The dissolved calcium concentration was affected by the NaOH concentration, the fly ash type, the fly ash loading, the slurry temperature, and by the presence of CaS03. The influence of the dissolved calcium concentration on the product solids reactivity suggests that there is an optimum ratio of calcium to silica in the product solids and that this ratio can be controlled by controlling the slurry conditions. The addition of NaOH to the fly a ~ h - C a ( 0 H )slurries ~ was tested in order to increase the dissolution rate of silica from the fly ash, which is the rate-limiting step of the reaction of fly ash with Ca(OH)2. The data showed that the NaOH increased the dissolution rate of the fly ash, but the NaOH also decreased the dissolved calcium concentration in the slurry due to the common ion effect of the hydroxide ion on the solubility of Ca(OH)2 and to the increased solubility of silica which precipitates the dissolved calcium as calcium silicates. This tradeoff leads to an optimum NaOH concentration in the slurry. This optimum concentration depends on the fly ash, the fly ash loading, the presence of CaS03, and on the slurry temperature. The present study has also shown that reactive solids could be prepared by using high-calcium fly ash alone [i.e., no Ca(OH), added to the slurry]. These solids were as reactive toward SO2as those produced by slurrying lowcalcium fly ash with Ca(OH)2. Registry No. SOz, 7446-09-5; Ca(OH)2, 1305-62-0; NaOH, 1310-73-2; CaS03, 10257-55-3; Ca, 7440-70-2.
Literature Cited (1) Rhudy, R.; McElroy, M.; Offen, G. Proceedings of the Tenth Symposium on Flue Gas Desulfurization, Atlanta, GA; U.S. Environmental Protection Agency. U.S.Government Printing Office: Washington, DC, 1986; EPA-600/9-87-004a. (2) Jozewicz, W.; Rochelle, G. T. Environ. Prog. 1986, 5, 218-223. (3) Chu, P.; Rochelle, G. T. Proceedings of the Tenth Symposium on Flue Gas Desulfurization, Atlanta, GA; U.S. Environmental Protection Agency. U.S. Government Printing Office: Washington, DC, 1986, EPA-600/9-87-004a. (4) Jozewicz, W.; Chang, J. C. S. Presented at AIChE Spring National Meeting, Houston, Tx, 1987. (5) Jozewicz, W.; Jorgensen, C.; Chang, J. C. S. Proceedings of the Tenth Symposium on Flue Gas Desulfurization, Atlanta, GA; U.S. Environmental Protection Agency. U.S. Government Printing Office: Washington, DC, 1986; EPA-600/9-87-004a. (6) Blythe, G.; Bland, V.; Martin, C.; McElroy, M.; Rhudy, R. Proceedings of the Tenth Symposium on Flue Gas Desulfurization, Atlanta, GA; U.S. Environmental Protection Agency. U.S. Government Printing Office: Washington, DC, 1986; EPA-600/9-87-004a. (7) Idorn, G.; Henriksen, K. Cem. Concr. Res. 1984,14,463-470. (8) Peterson, J. R. Thesis, The University of Texas at Austin, 1987. (9) Karlsson, H. T. Klingspor, J.; Linne, M.; Bjerle, I. J. Air Pollut. Control Assoc. 1983, 33, 23-28. (10) Greenberg, S. A.; Chang, T. N.; Anderson, E. J. Phys. Chem. 1960, 64, 1151-1157. Received for review August 20, 1987. Accepted M a y 20, 1988. This work was supported by DOE Grant No. DE-FG22085PC81006.
