Environ. Scl. Technol. 1994, 28, 277-283
Effects of Salts on Preparation and Use of Calcium Silicates for Flue Gas Desulfurization Kurt K. Klnd, Phlllp D. Wasserman, and Gary 1.Rochelle'
Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712 High surface area calcium silicate hydrates that are highly reactive with SO2 can be made by slurrying fly ash and lime in water at elevated temperatures for several hours. This concept is the basis for the ADVACATE (ADVAnced siliCATE) process for flue gas desulfurization. This paper examines the impact of salts on such a system. TWO low calcium fly ashes, from the Shawnee and Clinch River power plants, were examined. The addition of gypsum (CaS04.2HzO) or calcium chloride to the slurry system increased the dissolved calcium concentration, allowing the reaction rate to increase and the maximum surface area to more than double in some cases. This increase came despite a lower solution hydroxide level. The salts also enhanced the reaction of the sorbent with sulfur dioxide. This resulted from the higher equilibrium moisture on the sorbent at any humidity due to the deliquescent properties of some of the salts used (calcium chloride and calcium nitrate). Solids made without the deliquescent salts exhibited equilibrium moisture adsorption consistent with a type-I1 BET isotherm while the deliquescent salts caused hysteresis in the adsorption/ desorption isotherm.
Introduction The ADVACATE (ADVAnced siliCATE) system is a flue gas desulfurization (FGD) technology developed for existing coal-fired power plants and municipal solid waste incinerators (1). This dry duct injection system utilizes a fly ash-based throwaway sorbent to remove sulfur dioxide. High surface area calcium silicate hydrates are made by slurrying Ca(0H)z (calcium hydroxide) with fly ash and recycled sorbents in water at elevated temperatures. The resulting sorbent has the handling properties of a dry powder while maintaining moisture levels of up to 50 76 by weight. This powder is injected into the duct with a large moisture fraction upstream of the particulate control device with sulfur dioxide removal taking place in the duct as well as on the particulate collection device. The material is collected with the fly ash and divided into waste and recycle streams. This proposed process has the advantage of having a low capital cost and a small footprint to ease retrofit of existing plants as no spray drier or scrubber unit is required. This paper examines the impact of salts on such a system. Salts can impact the system in two major ways. The first is by changing the environment for the aqueous activation reaction producing the sorbent. Salts will affect the solution chemistry and also may impact the reaction products observed. Salts can also impact the reaction between the sorbent material and the sulfur dioxide in the duct. Salts will change the moisture level on the solids which can impact reactivity ofthe sorbent material toward sulfur dioxide. The salts examined in this study include calcium chloride, sodium nitrate, sodium chloride, and sodium hydroxide. These were chosen because these salts may be found in an operating system. 0013-936X/94/0928-0277$04.50/0
@ 1994 American Chemical Society
There has been considerable previous work performed on the reaction to produce the ADVACATE material and its subsequent reaction with sulfur dioxide. The aqueous reaction has been shown to be highly dependent on temperature (2,3).A minimum temperature of 85 OC was necessary for any significant sorbent surface area formation and reactivity toward sulfur dioxide. A great increase in reaction rate takes place at high temperature and pressure hydration conditions. The addition of sodium hydroxide can have a beneficial effect on surface area generation rate ( 4 ) and reactivity with sulfur dioxide (5). Most work has been performed with Clinch River fly ash, alow calcium ash, but the general characteristics have been found to hold with various other ashes studied (6,7). High calcium ashes show a different behavior due to the high fraction of highly reactive alumina present (8) and are not considered in this work. Reactivity of the sorbent with sulfur dioxide has been correlated with surface area, and the surface area has been correlated with the ability of the material to carry moisture while maintaining the characteristics of a free-flowing powder (9, 10). This relationship was found to hold regardless of the ash or the starting materials in the reactions considered. The reactivity of the sorbent has also been shown to increase with the humidity of the gas stream and initialmoisture on thematerial (II,I2). These observations lead to the conclusion that increasing surface area, or the ability to carry moisture, of the sorbent will improve the reaction rate with sulfur dioxide. Surface area will be used to measure the extent of reaction in the first part of this paper, and its limitations in measuring the ability of the material to carry moisture will be discussed thereafter. Finally the impact of salts on the reaction with sulfur dioxide and moisture-carrying ability of the sorbent in the duct will be considered. Additional details on much of this work are contained in a thesis by Wasserman (13).
Apparatus and Procedures The fly ashes used were obtained from the Clinch River and Shawnee power plants. The coal was bituminous, and the ashes are low calcium class F ashes. The Clinch River fly ash was a sample taken in October 1986 from the Appalachian Power Company's Clinch River Plant. It is from a Virginia bituminous coal. The identical sample was also used in work by Peterson (141, Stroud (IO), Beaudoin (II),and White (25). The Shawnee ash used was from a sample received in July 1991from the Shawnee test facility. The ash is from a West Virginia bituminous coal and was collected from an electrostatic precipitator that followed a cyclone separation earlier in the flue duct. The Clinch River ash had an initial measured surface area of 3 m2/g, and the Shawnee area was 10 m2/galthough this area was biased due to significant char in the ash. The major difference in ash compositions was a larger amount of iron in the Shawnee ash with a corresponding decrease in the aluminum content (Table 1). Environ. Sci. Technol., Vol. 28, No. 2, 1994
277
Table 1. Fly Ash Composition8 component (wt 7%) silica, Si02 alumina, A1203 iron oxide, Fez03 lime, CaO potassium oxide, KzO sodium oxide, NazO
Clinch River 51
26 6 6 3
0.6
Shawnee 51 19 20 3 2 0.4
a Clinch River composition from Acurex (16). Shawnee composition from X-ray fluorescenceanalysis of coal on ignited basis from Acurex. Trace elements not shown.
Sorbent was prepared by slurrying Mississippi hydrated lime with a BET surface area of 7 m2/g [previously used by McGuire (193,fly ash, and the applicable salts in a 600-mL batch reactor at elevated temperatures (90-98 "C). Samples were taken from the reactor with a 60-mL syringe, vacuum filtered to remove free moisture (to about 50% solids content), and dried under mild conditions (70 "C with vacuum or up to 120 "C with no vacuum). Selected sorbent samples were washed with water to remove soluble salts prior to drying. Following vacuum filtration, the filter cake was washed with 100 mL of deionized water, and the samples were dried. Washing samples allowed salts to participate in sorbent formation in the reactor, but removed the additives prior to reaction with simulated flue gas. Rinsed samples were prepared by reslurrying a small quantity of previously prepared sorbent at ambient temperature in a salt solution for 15 min (20mL of solution/g of solid). The sample was vacuum filtered and dried again. Rinsing in a salt solution should not affect sorbent formation, but may affect gas-solid reactions due to salts precipitating onto the surface of the material. Sorbent surface area was determined from BET nitrogen adsorption isotherms. Water adsorption (and desorption) isotherms were measured at 70 "C. This measurement involved taking a 4-g solid sample and exposing it to a nitrogen atmosphere with controlled humidity for a period of time. A Brooks mass flow controller maintained the nitrogen flow rate while water was added with a Cole Parmer pump. The water was heated to 250 "C to ensure it evaporated, and the sample was maintained at 70 "C with a water jacket. The sample weight was monitored every 8 min for a total of eight measurements (13). The equilibrium moisture was determined by using the collected data and assuming an exponential approach to equilibrium occurred. The accuracy of this method was confirmed by experiment. Sorbent was analyzed for reactivity with SO2 by dispersing it in sand and exposing it to a synthetic flue gas that contained 1000ppm of SO2 using a procedure similar to that of Peterson and Rochelle (14) (Figure 1). Moisture was removed by condensing the water in two lowtemperature flasks downstream of the reactor. A SO2 analyzer (Therm0 Electron Series 45), downstream of the reactor, measured exit gas sulfur dioxide content. The reactivity was measured for up to 60 min in a sandbed reactor at 70 "C and 60% relative humidity. The calcium conversion was calculated as the ratio of the moles of SO2 removed (determined by gas-phase analysis) and the moles of calcium hydroxide added to make the sorbent. Lime in the fly ash was not considered. The sandbed reactor approximates the reaction time of material in a flue duct containing a bag house to remove fly ash. The calcium to 278
Envlron. Scl. Technol., Vol. 28, No. 2, 1994
sulfur dioxide ratio is lower than in an operating system, causing the removal of SO2 from the gas stream to be lower in the bench-scale system. Alkalinity conversion (conversionof alkalinity from solid calcium hydroxide to calcium silicates during sorbent preparation) was measured by slurrying a small sample of sorbent in a sugar solution at ambient temperatures in a manner similar to that used by Peterson and Rochelle (14). The calcium hydroxide available was measured by filtering off the solid and titrating the solution with HC1. This amount was subtracted from the amount of calcium hydroxide added to the reaction to determine the alkalinity conversion. The solution ion concentrations were measured by atomic absorbance.
Results and Discussion Slurry Reaction. The reaction between fly ash and calcium hydroxide has been assumed to be a two-step mechanism with the initial rate-limiting step being the dissolution of the silica and alumina from the fly ash and the second reaction with the dissolved calcium in solution (8). Examination by SEM has shown that the product formed is an amorphous, high surface area material on the surface of the fly ash particle. The reaction product is largely calcium silicates and aluminates. The surface area is most likely due to the formation of CSH (18), an amorphous, hydrated calcium silicate with a Ca/Si mole ratio of between 1and 2. This observation is consistent with the solution chemistry, which shows high levels of dissolved calcium (up to about 20 mM) at early reaction times with dissolved silica and alumina being less than 0.04 mM for typical fly ash/lime reaction systems. The dissolution rate of fly ashes with no product layer has been quantified and found to be approximately first order with respect to hydroxide concentration (18). The dissolved silicate ion reacts with calcium to form the calcium silicate product: Ca(OH), (SiO,),
+ 2H20 + OH-
Ca2'
+ Si(0H);
-
Ca'
+ 20H-
(Si02)z-l+ Si(0H); calcium silicates
This study builds on the work cited previously, emphasizing the role of dissolved calcium in the reaction and the impact of additives on the reaction-forming ADVACATE material. The typical behavior of calcium concentration in solution for the simple fly ash/calcium hydroxide reaction is seen in Figure 2. As the ash dissolves in the basic solution, the potassium and sodium in the ash are released and remain in solution rather than being incorporated into the product material: K,O
+ Ca2++ H20 + 20H-
-
Ca(OH), + 2K'
+ 20H-
This increase in potassium and sodium causes a corresponding increase in hydroxide concentration to maintain charge balance in solution, driving the dissolved calcium concentration down tomaintain its solubility product. The dissolution of calcium hydroxide was effectivelyterminated at about 4 h of reaction time in the example presented. The solution behavior was examined by increasing the fraction of calcium hydroxide added to the reaction. The
-./
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. I -
Figure 1. Sandbed apparatus for measuring sorbent reactivity toward sulfur dioxide. 45 40
35 30
25
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,
-
(from pH)- a-.hydroxide (from pH) '
-hydroxide
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reaction time (hr) Figure 2. Typical solutin chemistry behavior for Clinch River fly ash batch reactions at 98 OC. Fly ashllime reaction consisted of 32 g of Ca(OH), and 128 g of fly ash in 450 mL of water. and me gypsum reaction contained 20 g of Ca(OH),. 80 g of Clinch River fly ash, 60 g of calcium sulffte, and 20 g of gypsum In 450 mL of water. Slurty samples were taken by syringe and immediately finered to separate the solution. Ion concentrations other ihan hydroxide were measured by atomic absorbance. comparison of multiple reactions performed with two fractions of calcium hydroxide L0.25 and0.5 g of Ca(OH)z/g of ash] is made with the approximate surface area generated shown hy the lines in each case (Figure 3). The pointa shown indicate the alkalinity conversions for the respective reaction times. It is apparent that the amount of available calcium hydroxide plays a role in the extent of reaction as measured by the alkalinity utilization. The calcium utilization is approximately 8074 of that added in the 0.25 calcium hydroxide case and 60% in the 0.5 case. The dissolution of calcium hydroxide in each of these cases appears to be stopped when the solution hydroxide concentration reaches about 0.1 M. The system with a larger calcium hydroxide fraction allowed for more dissolution and reaction prior to the increase in solution hydroxide concentration. This effect was examined further by using a lower solids fraction in the reactions and adding sodium hydroxide to maintain a constant hydroxide level in solution during the reaction. Shawnee fly ash was used to examine this behavior as it reacted over a much longer time period, forming a higher surface area material enhancing any differences in the reactions. Figure 4 illustrates the
5 0
5
10
15
reaction time (hr) Figure 3. Alkalinity utilization as measured by sugar dissolutlon with Clinch River fly ashlcalcium hydroxide reactions at 98 OC. The points indicate elkaiinny utilization at two different calcium hydroxkle to ash weight ratios (0.25 and 0.5). The lines show approximate surface area generation for the two cases.
...........................
..
0.01
0.1 1 NaOH (M) Figure 4. Impact of sodium hydroxide concentration on surface area generation. Shawnee fly ash batch reactions with calcium hydroxide (0.25g of calcium hydroxldelg of ash) at 92 O C . Sodium hydroxide was added to ihe dilute slurry (2.5-5 wt. 56 solMs).
behavior of this ash with varying sodium hydroxide concentration. The points shown are from multiple experiments with the solids fraction low so as to limit the EnVlron. Sol. Tachnci.. Vol. 28. NO. 2. 1994 219
increase in hydroxide concentration during the reaction to less than 20%. This required solid fractions less than 5 % by weight as opposed to about 28% in the Clinch River experiments. The impact of high hydroxide concentration (greater than 0.2 M) and therefore low calcium concentration is apparent at reaction times greater than 5 h. The calcium ion concentration is less than 0.025 mM at all reaction times for reactions with hydroxide concentration greater than 0.2 M. At short reaction times, the beneficial effect of the high hydroxide level to dissolve ash seems to outweigh the limited calcium availability as the area is greater than in the low hydroxide/high calcium cases studied. The high solution silicate and aluminate levels (greater than 7 mM) remain high throughout the reaction, indicating that there is little calcium available for reaction. I t is likely that the high calcium environment only exists near the dissolving calcium hydroxide and that the product precipitates on that particle inhibiting further dissolution. The calcium concentration decreases throughout the reaction time for the low hydroxide reactions despite the approximately constant hydroxide concentration. The other observation that may be made from this data is the insensitivity of the area generation rate to moderate hydroxide concentrations at reaction times greater than 7 h. The reaction was carried out until no further area generation was observedfor the 0.01,0.03, and 0.1N sodium hydroxide cases. The generation of area continued through 14 h at comparable rates independent of the hydroxide concentration. This insensitivity to solution hydroxide concentration was also observed in the Clinch River experiments where the solids concentration was changed by a factor of 2, thereby changing the hydroxide concentration by about two since hydroxide concentration was largely a function of potassium and sodium dissolved from the ash in those reactions. The difference in surface area at short reaction times may be attributed to the increased ash dissolution due the higher bulk solution hydroxide concentration causing an increased reaction rate as proposed by Peterson and Rochelle (8). The subsequent behavior is not as clear. The maximum surface area produced was also dependent on the hydroxide level with a lower final surface area in the higher hydroxide cases. This difference is due in part to a lower calcium hydroxide utilization at high hydroxide concentrations. These observations suggest adding material that will increase the concentration of calcium in solution and limit the increase in hydroxide level during the reaction while still maintaining a basic reaction environment. The additives studied included calcium sulfite hemihydrate, gypsum (calcium sulfate dihydrate), and calcium chloride. The expected reaction product of calcium silicates with sulfur dioxide is the sulfite species with a minimal amount of gypsum present due to the oxidation of the calcium sulfite. The gypsum fraction could be increased by forcing oxidation of a portion of the calcium sulfite. Calcium chloride will be present should the coal used in the process have a significant amount of chloride impurities. Sulfite additives alone were not examined because sulfite has a minimal effect on solution chemistry (18). A comparison of surface area generation for the gypsum and sulfite system and the base fly ashllime case for Clinch River ash is presented in Figure 5. 280
Environ. Sci. Technol., Vol. 28, No. 2, 1994
sulfitelgypsum Ca(OWph/+j
r^
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w-
E m zm
Y
0 '5 ln
10
/:
0.5 Ca(0H)Jash
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.......................................................
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..........................................
5.
................................................
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reaction time (hr) Flgure 5. Surface area generation for Cllnch Rlver batch reactions at 98 O C . System with gypsum compared with simple ash/calcium hydroxide system. The gypsumlsulfite points shown contain calcium hydroxlde to ash weight ratios varying from 0.25 to 1, while the ratios for the calcium hydroxiddash case are identified.
The fly ash/lime data shown illustrate the area generation from the experiments discussed previously with the gypsum experiments shown for comparison. The weight fractions of reactants for the gypsum experiments were varied so the area is plotted per gram of ash, allowing all reactions to be plotted together. While the area generation for each of the cases is comparable at short reaction times, the addition of gypsum allows the reaction to continue for up to 18 h while enhancing the area generation rate to a small extent. The reason for this behavior is illustrated by the solution chemistry shown in Figure 2. Calcium concentration is maintained above 10 mM throughout the reaction while hydroxide concentration as measured by pH is approximately constant during the reaction period. This behavior is expected due to the buffering effect of gypsum on the solution. Area generation is independent of the presence of sulfite and the amount of gypsum once enough has been added to the reaction to maintain solution chemistry. Reactions performed with smaller amounts of gypsum at long reaction times showed a step decrease in the calcium concentration in solution, indicating that there may be some incorporation of gypsum into the reaction product. Calcium hydroxide availability became limiting in some reactions and was indicated by a decrease in pH with reaction time and alkalinity conversions above 90%. Similar behavior is observed with the addition of calcium chloride to the reaction system. Calcium chloride will dissociate completely and can give solution calcium concentrations of greater than 300 mM if desired. The Shawnee fly ash surface area generation for the three systems discussed is shown in Figure 6. The trend of increasing area with increasing calcium concentration is readily apparent. In each of the cases, as the calcium concentration was increased, the solution hydroxide concentration was lowered. Water Adsorption and Surface Area. A fraction of the salt added to the slurry remains on the surface of the solid after free moisture removal using the experimental protocol described. During drying, salt precipitates from the solution as the water evaporates, filling the pores and decreasing the measured BET surface area of the sample. Figure 7 shows how this can affect the surface area for samples prepared with calcium chloride. One set of samples was washed to remove the salt. Since the calcium
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Figure 6. Comparison of surface area in calcium chloride, gypsum, and calcium hydroxide/fly ash systems. All reactions performed with Shawnee fly ash at 92 O C . Calcium hydroxide to fly ash weight ratio was varied from 0.25 to 1.0 with 20% solids for ail systems. Gypsum to fly ash weight ratio was varied from 0.25 to 0.5. Calcium chloride concentration was 0.6 N.
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surface area (rnz/g) Figure 8. Adsorption and desorption of water at 51 % relative humidity for samples preparedwith chlorides. All samples shown contain calcium hydroxide, Clinch River fly ash, CaS03.0.5H20,and gypsum in a 1/4/ 311 weight ratio. Baseline curve is the adsorption expected with BET theory ( 78, 19). Isotherms were measured at 70 O C .
below) as previously determined for lime sorbents by Klingspor (19, 20):
0
2
4
6
8
1 0 1 2 1 4
slurry time (hr) Figure 7. Effect of water washing solids on measured surface area. Samples prepared from a 1/4/3/1 ratio of Mississippi hydrate, Clinch River fly ash, CaS03.0.5H20, and gypsum at 90 O C , with 0.2 N CaCi2.
chloride is no longer plugging the pores of the solid, the surface area of the washed sample is greater than that of the baseline sample. This suggests that the measured surface area of sorbents prepared with high levels of dissolved salts will underpredict the moisture-carrying capacity of the material. Stroud (9)developed a correlation relating BET surface area to the ability of the sorbent to carry moisture without agglomerating. The method consisted of adding moisture to a solids sample and determining the moisture level at which less than 50% passed through an 80-mesh sieve following 10min of shaking. This measure correlated with the pore volume of the sample as measured using a BET isotherm. In this study, the moisture-carrying capacity of the sorbents prepared with high concentrations of salts were greater than that expected based on measured surface area. Water adsorption isotherms were determined for several samples. These isotherms are the best measure of the sorbents ability to retain moisture in a humid environment. There was a good agreement between adsorption data for samples that simulated product recycle with or without additional sulfate additives and the isotherm predicted from BET theory with a C parameter of 100 (equation
where P is the water vapor pressure, Po is the water saturation pressure, V' is the adsorbed water/surface area, V', is the monolayer adsorbed water/surface area, and C is a constant related to the heat of adsorption and condensation. The type-I1 BET isotherm does not predict water adsorption for samples prepared with chloride. The presence of chloride salts increases the affinity of the solids for water so that the assumption of simple physical adsorption on a surface does not hold. The effect of chloride (calcium or sodium) on sample moisture content is shown in Figure 8. The baseline curve represents the behavior of samples prepared without chlorides. This increase in equilibrium moisture for a given humidity can greatly enhance the sulfur dioxide-calciumsilicate reaction as demonstrated by Jozewicz et al. (9). Reaction with Sulfur Dioxide. The SO2 reactivity of the Clinch River solidswas tested in the sandbed reactor. I t was expected that samples prepared with chloride would have a higher reactivity than those without chloride due to the increase in moisture content. Figure 9 shows sandbed reactivity for samples prepared with CaC12 as well as simulated recycle samples prepared in water. Other salts such as sodium sulfite had only a minimal effect on reactivity. NaN03 showed the same increase in reactivity as CaClz while NaOH caused a moderate increase in expected reactivity. The maximum calcium conversion was about 70% in all cases, regardless of surface area or slurry time. Tests were made to determine whether the increase in sorbent reactivity observed in the calcium chloride case was due to an effect of chloride on solids formation or the presence of the salts on the sorbent surface. If the enhancement in reactivity of the CaClz samples was caused by an effect during calcium silicate formation, then washed Environ. Scl. Technol., VoI. 28, No. 2, 1994
281
0
8
A
ip+
0
e 0 0
0.2N CaC12
0
O L '
"
0
"
2
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'
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'
Water Washed Samoles
6 8 slurry time (hr)
10
Slurrv time 50.5 Hours o 0.5-2Hours A 22-4Hours
12
E
Y
Figure 9. Samples preparedwith 0.2 N CaCI2exhibitenhanced reactivity with sulfur dioxide when the salts were present. When the salts were removedby washing, the reactivity of the sorbent was roughly equivalent to that of the baseline samples. Reactivity was for 1 h at 70 OC, 60% relative humidity, and 1000 ppm SO2.
samples should also exhibit the enhancement. If the effect of the calcium chloride is due to the presence of the salts, then the reactivity of the washed samples should be equivalent to that of the baseline samples. The reactivity of washed samples in Figure 9 were roughly the same as the reactivity of baseline samples, proving that the effect of calcium chloride is due to the presence of the salt itself and is not an effect on calcium silicate formation. This is consistent with the small difference in surface area between the baseline and the samples that were washed. The difference in adsorption behavior is also seen in the adsorption/desorption isotherms for calcium chloride sorbents. These sorbents exhibited a hysteresis not present in the other sorbents, indicating behavior due to more than simple porosity formation. This hysteresis has also been observed with deliquescent salts as additives to a simple lime sorbent (21, 22). An attempt was made to find the minimum quantity of calcium chloride required to produce the maximum reactivity (70% calcium conversion) at low slurry times. A set of samples were prepared with calcium hydroxide, Clinch River fly ash, CaS0~0.5H20,gypsum (1/4/3/1 mass ratio), and the calcium chloride content was varied. Maximum reactivity for a given reaction time was seen with samples made with CaC12 concentrations as low as 0.1 N. Some enhancement in reactivity was seen with as little as 0.03 N CaC12. Figure 10 gives the sandbed data at low slurry times verses calcium chloride content. A set of samples were rinsed in a 0.2 N CaClz solution and reacted in the sandbed to determine the beneficial effect of calcium chloride on various sorbents. These samples should have CaClz on their surface, but would not have gained any beneficial effects of slurrying with CaC12. Table 2 shows the improvement in reactivity after rinsing. Conclusions The reaction between calcium hydroxide and fly ash to form calcium silicate solids is highly dependent on the solution calcium concentration under the conditions examined. This was observed by increasing the calcium hydroxide fraction in the reaction, thereby increasing its availability, and by adding species such as gypsum and calcium chloride to increase the solution hydroxide level. 282
Environ. Scl. Technoi.. Vol. 28. No. 2, 1994
I
0
0.2
"'
'
0.0
'
0.2
' '
' ' '
0.4
' ' '
0.6
' '
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CaCI, (N) Figure 10. Effect of calcium chloride concentration on sorbent reactivity with SO2.Samples were preparedwitha 1/4/3/1 weight ratioof calcium in a CaCl2 hydroxide/Chch River fly ash/CaSO3-0.5H20/CaS0~2H2O solution at 90 OC. Calcium conversion was measured for a 1-h reaction in the sandbed at 70 O C and 60% relative humidlty in a synthetic flue gas containing 1000 ppm SO2.
Table 2. Effect of CaClz on SO*-Solid Reactions
sample date 7/3/91 3/8/91 8/1/91
aqueous reaction time (h)
additive [normality]
calcium conversion ( % ) a base rinsed with case 0.2 N CaClz
4
Ca(N0)S r0.21 39.2 77.2 NaZS04 [0.21 40.4 67.4 none 38.9 57.9 Conversion was for 1-h reaction in the sandbed at 70 "C and 60% relative humidity. 2 2
The hydroxide level in solution acted to increase the reaction rate at short reaction times but had a negative impact on calcium availability at longer batch reaction times. The optimum reaction environment was high calcium, moderately basic with little hydroxide change over the reaction period. Gas-solid reactivity has been correlated with the moisture content of the sorbents. Moisture content of the solids is dependent on surface area, the presence of deliquescent salts on the surface of the solids, and the relative humidity. Solids that were not prepared with chlorides exhibit a type-I1 BET water adsorption isotherm. Because reactivity is a function of moisture content, for these solids, reactivity is also a function of surface area. Some deliquescent salts induce hysteresis in the water adsorption isotherms of sorbents. Salts that cause hysteresis enhance the reactivity of the sorbent. These salts include CaC12, NaC1, and NaOH. Samples prepared with chlorides are highly reactive with SO2 regardless of slurry time and sample surface area. This may be a result of an increase in the moisture content for these solids. Acknowledgments This work was funded through grants from the Texas Advanced Technology Program (TATP) and the Environmental Protection Agency (EPA).
Literature Cited
Hall,B. W.; Singer, C.; Jozewicz,W.; Sedman, C. B.; Maxwell, M. A. J. Air Waste Manage. Assoc. 1992,42, 103-110. (2) Jozewicz, W; Rochelle. G. T. Enuiron. Prog. 1986,5, 219224. (3) Jozewicz, W.; Chang, J. C. S.; Sedman, C. B.; Brna, T. G. JAPCA 1988,38,1027-1034. (4) Peterson, J. R. M.S. Thesis, University of Texas at Austin, 1987. (5) Chu, P.; Rochelle, G. T. JAPCA 1989, 39, 175-179. (6) Peterson, J. R.; Rochelle, G. T. Enuiron. Sci. Technol. 1988, 22, 1299-1304. (7) Singer, C.; Jozewicz, W.; Sedman, C. B. Suitability of Available Fly Ashes in ADVACATE Sorbents. Presented a t EPRI 1991 SO2 Control Symposium, Washington, DC, Dec 2-6,1991. (8) Peterson, J. R.; Rochelle, G. T. Proceedings of First Combined FGDand Drys02 Control Symposium, St. Louis, MO; EPRI GS-6307; 1989; 7-147-7-163. (9) Jozewicz, W.; Rochelle, G. T.; Stroud, D. E. Reaction of Moist Calcium Silicate Reagent with Sulfur Dioxide in Humidified Flue Gas. Presented a t EPRI 1991 SO2 Control Symposium, Washington, DC, Dec 2-6, 1991. (10) Stroud, D. E. M.S. Thesis, University of Texas a t Austin, 1991. (11) Beaudoin, S. P. M.S. Thesis, University of Texas at Austin, 1990. (12) Johnson, H.I. M.S. Thesis, University of Texas at Austin, 1992. (1)
(13) Wasserman, P. D. M.S. Thesis, University of Texas at Austin, 1992. (14) Peterson, J. R.; Rochelle, G. T. Proceedings of 1990 5'02 Control Symposium; New Orleans, LA; EPRI GS-6963; 1990; P-3-P-24. (15) White, G. W. M.S. Thesis, University of Texas at Austin, 1989. (16) Development of ADVACATE Process, Progress Report No. 1; Acurex Corp., Report under EPA Contract 68-02-4701, 1989. (17) McGuire, L. M. M.S. Thesis, University of Texas at Austin, 1990. (18) Peterson, J. R. Ph.D. Dissertation, University of Texas at Austin, 1990. (19) Hines, A. L.; Mattox, R. N. Mass Transfer-Fundamentals and Applications; Prentice-Hall: Englewood Cliffs, NJ, 1985. (20) Klingspor, J. Doctoral Dissertation, Lund Institute of Technology, 1986. (21) Riuz-Alsop, R.; Rochelle, G. T. Fossil Fuels Utilization; ACS Symposium Series 319; American Chemical Society: Washington, DC, 1985; pp 202-222. (22) Ruiz-Alsop, R. Ph.D. Dissertation, University of Texas at Austin, 1986.
Received for review April 16, 1993. Revised manuscript received October 11, 1993. Accepted October 20, 1993.' Abstract published in Advance ACS Abstracts, December 1, 1993.
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