Reaction of sulfur dioxide with calcined limestones and dolomites

Apr 8, 1970 - Carman, P. C., Trans. Inst. Chem. Eng. 16,168 (1938). Cooper, F. C., Ph.D. Dissertation, University of Arizona,. Tucson, Ariz., 1967. Co...
0 downloads 0 Views 3MB Size

Bier, M., Bruckner, G. C., Cooper, F. C., Roy, H . E., “Transmission of Viruses by the Water Route,” G. Berg, Ed., Interscience, New York, N.Y., 1967, p 57. Bier, M., Cooper, F. C., “Principles and Application of Water Chemistry,” S. D. Faust and J. V. Hunter, Eds., Wiley, New York, N.Y., 1967, p 217. Bier, M., Moulik, S. P., Proceedings of the Third American Water Resources Conference, Ser. no. 3, 524, 1967. Black, A. P., Chen, C. L., J. Amer. Water Works Ass. 57, 354 (1965). Black, A. P., Singley, J. E., Whittle, G . P., Maulding, J. S., ibid. 55, 1347 (1963). Black, A. P., Walters, J. W., ibid. 56, 99 (1964). Carman, P. C., Trans. Znst. Chem. Eng. 16, 168 (1938). Cooper, F. C., Ph.D. Dissertation, University of Arizona,

Tucson, Ariz., 1967. Cooper, F. C., Mees, Q. M., Bier, M., J . Sanir Eng. Diu. Amer. SOC.Cicil Eng.91, 13 (1965). Dicket, G. D., “Filtration,” Reinhold, New York, N.Y., 1961. D 24. Moulik, S. P., Cooper, F. C., Bier, M., J . Colloid Interface Sci. 24, 427 (1967). Purchas, D. B., Chem. Prod. 20,149 (1957). Riddick, T. M., Chem. Eng. 68 (14), 141 (1961). Smoluchowski, M. von, Bull. Acad. Sci. Cracovie, p 182 (1903). Sperry, D. R., Chem. Metul. Eng. 15, 198 (1916). Sperry, D. R., ibid. 17,161 (1917). 7 r

Receiced for reciew April 8,1970. Accepted April 29,1971.

Reaction of Sulfur Dioxide with Calcined Limestones and Dolomites Kotturi S. Murthi’ Chemical Research Dept., Ontario Hydro Research Division, Toronto 18, Ontario, Canada

Douglas Harrison Chemical Research Dept., Ontario Hydro Research Division, Toronto 18, Ontario, Canada

Raymond K. Chan2 Dept. of Chemistry, University of Western Ontario, London 72, Ontario, Canada

A small rotary-kiln was used to determine calcination parameters of limestones and dolomites, and a fixed-bed reactor to evaluate sulfur dioxide sorption capacity. The results indicated that the capacity of individual samples varied with calcination time and temperature. Sulfur dioxide sorption rates were dependent on reaction temperature and particle size. The role of physical properties such as surface area, porosity, and chemical composition on the sorbent capacity was discussed. The sulfur dioxide sorption capacity of calcined limestones was substantially reduced in the presence of 10% moisture whereas that of calcined dolomites was affected to a smaller extent. The presence of iron oxide in the samples was proved to be beneficial. The efficacy of these samples was demonstrated by dry powder injection in a 100 MW power plant.


here is a growing awareness of atmospheric pollution by sulfur oxides in power plant stack gas, and massive efforts are being made in many parts of the world to develop suitable processes for controlling them. The removal of sulfur dioxide from flue gas by injecting pulverized dry limestone into the boiler was first tried by Wickert (1963) in Germany in a plant-scale test. Since then the process has been tried by numerous investigators. The results obtained in these studies ranged from 10 to 50% sulfur dioxide removal with 1 to 4 times the stoichiometric amount of limestone. Present address, Air Quality Management; Air Pollution Control Div., Ottawa 3, Ontario, Canada. Author to whom correspondence should be addressed. 776 Environmental Science & Technology

As a first step in determining the applicability of this method, it was decided to assess the sulfur oxide sorption characteristics of several local limestones. There are many possible applications for this work. The limestone injection process can be applied intermittently with a minimum of capital investment for partial desulfurization of the flue gas. Dolomite can be used to prevent high-temperature vanadium oxide corrosion in oil-fired boilers. Magnesium ,oxide reacts with vanadium oxide and raises the fusion temperature of slag from about 970” to approximately 2700°F. Oxides of calcium and magnesium neutralize sulfuric acid vapors in flue gas and thereby reduce cold-end corrosion. A highly reactive lime is preferred, both for the dry injection and for the wet scrubbing methods of controlling sulfur oxide emissions. With these applications in mind, four limestones and five dolomites were chosen for this study. The differences in their sulfur dioxide sorption capacities and the physical and chemical properties responsible for these differences were investigated. From these studies we conclude that some carbonate rocks in Ontario are suitable for desulfurizing flue gas by dry injection. On heating the carbonate samples, calcination proceeds from the outside of the particle inward and carbon dioxide diffuses out (Ingraham and Marier, 1963). As carbon dioxide escapes from the particle interior, porosity is developed because of the differences in the molar volumes. The experimental porosities of calcined limestone and dolomite are 54.4 i 1 . 2 z and 57.2 i 1.6%. respectively (Chan et al., 1970). This porosity has a significant effect on the calcium and magnesium oxide reactivities (Falkenberry and Slack, 1969). The rate of decomposition of either calcium or magnesium carbonate is proportional to the equilibrium carbon dioxide pressure at a given temperature (Boynton, 1966). If we assume flue gas has about 12% carbon dioxide below

1400°F for calcium oxide and 600°F for magnesium oxide, the oxides will recombine with carbon dioxide forming calcium and magnesium carbonates. By elevation of the calcination temperature to 2250-2600°F, the lime sinters. This makes the particles shrink, increasing the bulk density and reducing porosity, surface area, and SO2 sorption capacity. To prevent this, dead burning must be avoided in calcination. Therefore, calcination plays a n important role in developing a reactive lime and magnesia t o capture sulfur dioxide from flue gas by dry injection. The sorption of sulfur dioxide by calcium oxide in the presence of oxygen, and similarly for magnesium oxide, proceeds as follows CaO(s)

+ S02(g) + O.5O2(g)




for which the equilibrium constant at 1 atm is

where X i is the mole fraction of ith component in the flue gas. Calculations for calcium and magnesium oxide in 2 . 7 z oxygen, performed by Reid (1970), are reproduced in Figure 1. At low temperatures the equilibrium concentration of sulfur dioxide is favorably small. However, the time required for equilibrium would be impracticably long because of kinetic considerations. Since the flue gas contains 2700 ppm of sulfur dioxide there exists a n upper temperature limit of desulfurization. The lower temperature limit is chosen arbitrarily as 1 ppm. Thus, the lower and upper temperature limits correspond approximately to 1800-2200°F for calcium oxide and 12001500°F for magnesium oxide. The most striking feature is that above 1500°F calcium oxide is the effective desulfurizing agent. If the kinetics is controlled by heat and mass transfer, the desulfurization has been estimated t o be 3 0 z for 3000 ppm of sulfur dioxide in the flue gas using a 5 : l stoichiometric ratio of dolomite (200 mesh) and sulfur dioxide with a residence time of 2 sec each at mean temperatures of 1600" and 900°F (Walling et al., 1966).

nitrogen through water maintained at 120°F. The composition of the metered flue gas is shown in Table 11. The Beckman Model 315A nondispersive infrared analyzer was used to monitor sulfur dioxide. The full-scale sensitivity of the analyzer was 3000 ppm of sulfur dioxide. Because the absorption frequencies of sulfur dioxide overlap with those of carbon dioxide in the flue gas, compensation was made for this interference. Results and Discussion

Three runs were made with limestone 1211 to study the effect of calcination temperature on SOn sorption capacity. For each run, 40 grams of limestone (-16/+30 mesh) was calcined for 1 hr. Three calcination temperaiures, 1400", 1550", and 1800°F, were chosen for this study. Twenty grams of calcined sample was taken from each batch and exposed in a fixed-bed reactor at 1300°F to a simulated flue gas having 2700 ppm of sulfur dioxide and flowing at a rate of 10 scfh. The exit gas was monitored continuously for SOz with the infrared analyzer. The bed temperature was kept below the calcining temperature in order to study the

27%02 In FLUE GAS (Unlt octivi:y 01 Solids)

Experimen fa1 The selection of local limestone and dolomite samples was made on the basis of two detailed reports by Hewitt (1960, 1964). The term limestone refers to a rock consisting mostly of calcium carbonate, CaC03. Dolomite is the double carbonate of calcium and magnesium, CaMg(CO&. The main undesirable impurities in the carbonate rocks are silica and alumina. These impurities combine with calcium oxide at elevated temperatures (2000°F) to form slags which reduce the amount of available active lime and occlude pores in the quicklime thereby reducing its reactivity (Nicholls and Reid, 1932). Such impurities either occur in the matrix or come from the material in the crevices and other strata excavated along with the limestone. Samples were chosen from quarries in which these impurities are low (Table I). A conventional rotary-kiln calciner was used. A suction pump was attached to one end of the kiln to force air through the lime bed and to sweep out the carbon dioxide. The calcining temperature was maintained within 1 2 0 ° F . The sorption capacities of the carbonate rocks were evaluated with a fixed-bed reactor similar to that used by Harrington et al. (1968). It consisted essentially of gas metering equipment to give a simulated flue gas, a fixed-bed stainless steel reactor, and a heavy-duty furnace with temperature control of =t4"F. The synthetic flue gas was made by mixing gases from cylinders and the moisture was added by bubbling the





Figure 1. Sulfur dioxide equilibrium concentration vs. temperature

Table I. Chemical Compositions of Limestones and Dolomites in Weight Percent Standard limestone analysis MgO RzOJ SiOl

Sample Limestones 1211 1213 1214 1217

547 521 509 551

0 6 1 2 1 3

Dolomites 1212 1215 1216 1218 1219

302 326 305 291 302


2 2 7 1

0 1 2 5 3 8

0 5

0 1 1 0

210 164 215 190 198

0 2 0 1 0

4 0 2 5 8

0 5 4 1



4 4 0 6


435 420 412 44 3 466 436 47 4 441 467

Volume 5, Number 9, September 1971 777

effect of calcination temperature. In the limestone evaluation study, the stone capacity is defined as the quantity of SO2 sorbed at the time when 2 0 z of the inlet SO2passes through the bed (Harrington et al., 1968). [Note: The term capacity is used i n all cases where it refers to amount of sulfur dioxide sorbed under a given condition. The term reactivity refers to a more general situation when it is used for comparing samples regarding their reaction rate and sorption capacity.] If the gas composition and flow rate are kept constant, capacity may be taken as the time that lapses while at least 80% of the inlet concentration of sulfur dioxide is still being sorbed. The relative capacity is greatest for the stone calcined at 1550°F (Figure 2). Based on these findings a calcination temperature of 155O0F and a calcination time of 1 hr were chosen. For the effect of temperature on SO2 sorption, a vitrosil mat impregnated with 200 mg of calcined limestone 1211 (- 100/+140 mesh) was placed in the reactor. Four runs were made at 1200°, 1400°, 1600°, and 1800°F. The samples were analyzed for SO2content after 10 min. The average rate constant, k , is evaluated assuming first-order kinetics

where x o is the initial amount of sample and x is the amount of sample remaining after a time interval t. Figure 3 is the Arrhenius plot which is linear from 1400-1800°F. At 1200'F it deviates considerably from linearity, which suggests that above 1400°F the sulfation of lime appears to be a firstorder reaction. Indeed, Borgwardt (1970) demonstrated that the reaction rate was proportional to SOr concentrations. Since the activation energy changes with temperature, the calculated apparent overall activation energy from the Arrhenius plot will not be significant. Based on these results a reaction temperature of 1700°F was chosen to evaluate relative capacity of the carbonate rocks. Fifteen grams of sample (-16/+30 mesh) was placed in the reactor. The velocity of flue gas was about 1.1 ft/sec with a bed 7/8 in. deep and 1 in. i n diam, giving an average residence time of about 0.067 sec at 1700'F. The outlet gas was continuously monitored for SOz with the infrared analyzer. The time taken for 2 0 z breakthrough was taken as a measure of the capacity of the test stone. The total amount of SO2in sample was determined by wet analysis (Table 111). Figure 4 shows the amount of SO2 sorbed by a 200 mg sample (- 100/+140 mesh) at various time intervals for the limestone 1211 and dolomite 1216 at 1700OF and 2700 ppm of SOz. The initial rate was higher for dolomite but became practically the same for both as sorption continued. The average rate of sorption was estimated to be 2 X lop4 gram of SOn per gram calcined sample per second up to 15 conversion of CaO. Under comparable circumstances, Borgwardt (1910) estimated the rate to be roughly ten times higher. However, one has to bear in mind that the rates vary widely depending on sample origin, physical and chemical properties, and, above all, reaction conditions. There is some correlation between the sorption capacity and the surface area (Table 111). The porosity of the samples was measured by an Aminco 60,000 psi mercury porosimeter. The results of typical pore size analysis performed with this instrument are given in Figure 5 . For calcined dolomite 1212 the pore volume is 0.42 cm3/gram and that of calcined limestone 1211 is 0.32 cm3/gram. The results for nine samples calcined at 1550OF for 1 hr are summarized in Table 111.













/ 4c





Figure 2. Effect of calcination temperature on sorption capacity of limestone 1211

2 1o%?K-1)

Figure 3. Effect of temperature on rate constant of SO2 sorption by limestone 1211


778 Environmental Science & Technology

Table 11. Compositions of Simulated Flue Gas Component

coz 0 2

Nz H20


Volume, With moisture

12.00 3.00 17.13-14.73 7.00-10.00 0.27


12.00 3.00 84.73 ...






0 02


z V

Dolomite 1216




TIME ( M I N 1

From Figure 5 one can see that pores with diameters from 0.1-0.3 p contributed most of the pore volume. According to the method of Rootare and Prenzlow (1967) which related surface areas to porosity data, pores with diameters from 0.1-0.3 p contributed most of the surface area. Since sorption capacity was proportional to surface area, 0.1-0.3 p pores should play an important role in SO2sorption. The effect of moisture was shown in Figures 6 and 7. The addition of 10% moisture was achieved by bubbling the nitrogen content of the synthetic flue gas through water at 120°F. Moisture showed no apparent effect on SO2sorption by dolomites. In the case of limestones, moisture caused little change in the initial sorption rate but, after a lapse of 10 to 20 min, moisture decreased the sorption rate considerably. A possible explanation for this behavior is that, in the case of dolomite, the magnesium oxide does not react at 1700OF but acts as a catalyst transferring sulfur dioxide to active calcium oxide sites, thereby keeping the pore diffusion path free. In the case of lime, the formation of calcium sulfate which has a molar volume of 45.9 cm3/mol vs. calcium oxide 16.8 cm3/mol, and other undetermined phenomena such as the pseudolattice formation in the presence of moisture








dry flue gas with 13% moisture








TIME ( M I N )

Figure 6. Effect of moisture on SO?sorption by dolomites 1216 and 1212

Table 111. Physical Properties of Samples Calcined at 1550°F for 1 Hr SO?

Dolomites 1216 1218 1212 1215 1219


Figure 5. Pore-size distribution by mercury porosimeter

Figure 4. Rate of sulfur dioxide sorption

Sample Limestones 1211 1214 1217 1213



Surface area, mZ/grama

sorption capacity,




24.6 14.7 20.6 13.6

3.10 2.85 2.60 1.65

0.32 0.38 0.34 0.34

30.1 34.2 26.4 20.2 26.1

3,50 3.25 2.80 2.70 2.35

0 40 0.39 0.42 0.41 0.33


Most-probable pore diameter,


density, grams/cm3


0.090 0.17 0.18 0.18

1.61 1.47 1.56 1.56

51.6 55.8 53.1 53.0

0,099 0.096 0.14 0.087 0.085

1.45 1.47 1.41 1.42 1.61

58.0 57.3 59.3 58.2 53.1



'[ Approximately 100 mg of sample was calcined at 1370°F in a C a h n electrobalance for 0.75 hr until calcination \vas complete. Then, the surface area was determined by the standard BET technique with nitrogen at 77°K. The sorption capacity is defined as weight of SO2 sorbed per 15 grams of calcined sample when 20% of Son in the flue gas breaks through the fixedbed reactor.

Volume 5, Number 9, September 1971 779

T i r e In Mln.


120 100


'. Limestone 1214

W '\\,












5 S




-i (



Limestone 1211


-dry flue gas - - - - - w i t h 10%moisture

1 0



80 Time in Min.

Figure 7. Effect of moisture on SO2 sorption by limestones 1214 and 1211



E 60n






DO,omite 1216

$ 1



with 015% iron oxidewithout iron oxide



(Glasson, 1958), plug the pores and inhibit the diffusion of sulfur dioxide into the matrix. This hypothesis accounts for the lower sorption capacity of limestones with sulfur dioxide in the presence of moisture. A special hand-picked sample of dolomite 1216 having an iron oxide content of 0.15% by weight was selected. Although the iron oxide content is low, the iron in the ferric state has diffused uniformly into the sample matrix. Sulfur dioxide sorption test at 1700OF was made on this sample after calcination. A similar experiment was carried out with a sample from the same quarry without iron oxide. Comparison of the sorption curves given in Figure 8 shows that the sample with traces of iron oxide sorbed more sulfur dioxide at all times throughout the experiment. The explanation for increased sorption is that calcium oxide reacts with sulfur dioxide initially to form calcium sulfite. The concentration for sulfur dioxide in equilibrium with calcium sulfite at 1700OF is 1.3 X IO5 ppm whereas for calcium sulfate the equilibrium concentration of sulfur dioxide is only 0.24 ppm in 2 . 7 z of oxygen (Walling et al., 1966). On this basis, any impurity that promotes the oxidation of sulfur dioxide to trioxide will enhance the sorption capacity. The traces of ferric oxide in the sample probably catalyzed the formation of calcium sulfate. The relative capacity of nine samples in a fixed-bed reactor will serve as a guide but does not necessarily predict accurately the behavior under the dynamic conditions of a boiler. In a boiler the temperature of the gas stream decreases in a complex fashion and the thermochemical reaction time is short (about 2 sec). Above all, the concentration of sorbent and the relative velocity between sorbent and SO? are small. Therefore, it is difficult to make use of the available lime effectively. For dry injections in a 100 MW power plant the desulfurization was 40-55% for 200-mesh limestone 1211 injected at 1930°F with 3.3 times the stoichiometric amount of lime. Similarly, when dolomite 1216 was injected at 2050OF with 1.9 times the stoichiometric amount of equivalent lime, the desulfurization was 20-40 %. These results are comparable to those of Goldschniidt (1968) which are given in Figure 9. However, there are many uncontrollable variables such as load fluctuations, gas velocity, etc. which can affect the results. Comparison with results in the literature has to be viewed with caution. Conclusions


0 Q




_1 3

v1 W 0



W z K u W I


Thus, the observations made in this study lead us to the following conclusions : Southern Ontario dolomites and limestones are suitable for the sorption of sulfur dioxide by dry injection into a boiler. The presence of 10% moisture reduces the reactivity of limestone considerably. However, this may not be applicable when the reaction time is of the order of seconds instead of hours. The decomposition of limestone or dolomite leads to a porous oxide structure because of the differences in the molar volumes of carbonates and the oxides. This porous structure varies with type of limestone or dolomite and calcining conditions, but the porosity in the matrix allows sulfur dioxide to penetrate into the interior of the particle by pore diffusion. Acknowledgment

The authors wish to thank C. H. Clark, Chemical Research Department, and J. H. Waghorne, Ontario Hydro, for encouragement received to carry out this project. 780 Environmental Science & Technology

Literature Cited Borgwardt, R . H., ENVIRON. SCI.TECHNOL. 4,59 (1970). Boynton, R. S . , “Chemistry and Technology of Lime and Limestones,” Interscience, New York, N.Y., 1966, p 134. Chan, R. K., Murthi, K. S . , Harrison, D., Can. J. Chem. 48, 2972 (1970). Falkenberry, H . L., Slack, A. V., Chem. Eng. Progr. 65, 61 (1969). Glasson, D. R., . I Appl. . Chem. 8,793 (1958). Goldschmidt, K., Chem. Ing. Tech. 40, 1085 (1968). Harrington, R. E., Borgwardt, R. H., Potter, A. E., Amer. Ind. Hyg. Ass. J . 29, 152 (1968). Hewitt, D. F., “The Limestone Industries of Ontario,” Ontario Department of Mines, Industrial Mineral Circular no. 5 (1960) and no. 13 (1964).

Ingraham, T. R., Marier, P., Can. J. Chem. Eng. 41, 170 (1963). Nicholls, P., Reid, W. T., Trans. Amer. Soc. Mech. Eng. 54, 167 (1932). Reid, W. T., J. Eng. Power Trans. ASME Ser. A 92, 11 (1970). Rootare, H. M., Prenzlow, C. F., J. Phys. Chem. 71, 2733 (1967). Walling, J. F., Cherry, R. H., Jr., Levy, A,, “Fundamental Study of Sulfur Fixation by Lime and Magnesia,” Final Rept., Battelle Memorial Institute, Contract no. PH 86. 66-108 (June 30, 19661, p 21 and Appendix. Wickert, K., M i f f . Ver. Grosskesselbesitzer 83, 74 (1963). Received for review June 29, 1970. Accepted March 30, 1971.

Relation of Airborne Nitrate to Telephone Equipment Damage Harold W. Hermance,’ Charles A.

Elmer J. Bauer, Thomas F. Egan, and Harold V. Wadlow3

Bell Telephone Laboratories, Inc., Holmdel, N.J. 07733

AirL..cuyac rr.ess corrosion cracking of nickelbrass telephone parts-particularly relay wire springs in the Los Angeles area. A survey of nitrate accumulation on equipment was made in Calif6rnia and other locations. Nitrate deposits were transferred to paper disks and determined spectrophotometrically with chromotropic acid. Nitrate deposition correlated with relay failure and a rating was established t o estimate the degree of danger to equipment. A correspondence to the general patterns of smog and air pollution existing in California was indicated. Several eastern locations are marginal and some nitrate-caused component problems have occurred, but no relay failures. Air filtration and humidity control provide protection against the nitrate-caused failures. ...llyLcy


n 1959 nickel-brass wire springs supporting the contacts on relays in telephone equipment in the Los Angeles

area began to break. This breakage occurred where the wires under a moderate spring stress emerged from the molded plastic base of the relay (Figure 1). A study of the breakage showed that it resulted from cracks developing in those springs which normally had a positive notential. Figure 2 shows a typical spring failure on a relay in use only two years. The exposed areas of the flat nickel-brass surfaces of the clamp plates of the failed relays (Figure 1) had a fogged appearance. The springs, their plastlc bases, and other parts of the relays were layered with fine dust. These deposits were noticeably hygroscopic when relative humidity exceeded 50Z. Analyses of the soluble salts in dust samples collected from Los Angeles and New York air samples are listed in Table I, along with available corresponding data from the National Air Sampling Network ( U S . Dept. HEW, 1962, 1966). Los Angeles area dusts are characterized by their high nitrate ion content. As evidence accumulated, it appeared that the failure of the relay

springs was due t o a stress corrosion cracking of the nickelbrass wire under the influence of an applied potential in the presence of the ionic contaminants present in dust layers between springs carrying opposite potentials. Previous laboratory studies (Hermance, 1966; McKinney and Hermance, 1967) confirmed that hygroscopic nitrates, such as zinc or ammonium in particular, cause stress corrosion cracking of anodic nickel-brass wires under stress. Aqueous extracts of Los Angeles dust samples produced this nitrate type of stress cracking which is different from that caused by nitric acid vapors (Uhlig and Sansome, 1964) or ammonia (Evans, 1960). Salts of other anions under the same conditions did not cause cracking. At the nitrate concentration levels found in Los Angeles (up to about 15 &cm2 of surface area) an applied positive potential was necessary. Cracking without an applied potential could occur if the surface nitrate concentrations were significantly larger than 15 pg/cm-. On the basis of these findings, a cupro-nickel alloy, which did not stress crack in this manner, was substituted for the nickel-brass in new relays. Laboratory studies (McKinney and


Present address: R.D. no. 1, Box 483, ForkedRiver, N.J. 08731. a To whom correspondence should be addressed. Present address: LePassC, Gravieres-Les Vans, Ardeche, France.

Figure 1. Wire spring relay Volume 5 , Number 9, September 1911 781