by Savannah River Laboratory for US.Energy Research and Development Administration, DP-1475, 1977. (18) Williams, R. C., “San Juan-Four Corners Joint Ambient Air Monitoring Projects”,Public Service Company of New Mexico, oct 1976. (19) Shacklette,H. T., Boerngen, J. G., Runer, R. L., “Mercury in the Environment Surficial Materials of the Conterminous United’ States”, U S . Geological Survey Circular 644, USDI, Washington, D.C., 1971. (20) Lockeretz, W., Water, Air, Soil Pollut., 3 (1974).
(21) Rogers, R. D., “Methylationof Mercury in a Terrestrial Environment”, U.S.Environmental Protection Agency, EPA-600/3-
75-014, 1975.
(22) Anderson, W. L., Smith, K. E., Environ. Sci. Technol., 11, 75 (1977).
Received {or review January 23, 197’8. Accepted February 5, 1979. This report u a s preuiously published by the Ir.S. Environmental Protection Agency as Report No. EPA-600/3-7’7-063,M a y 197’7.
New Regenerable Sorbents for Fluidized Bed Coal Combustion Lawrence A. Ruth’ and Gideon M. Varga, Jr. Exxon Research and Engineering Company, Linden, N.J. 07036
w Calcium and barium titanate (CaTiOs and BaTiOs) and calcium aluminate cement have been found to possess properties which make them attractive candidates for use as regenerable sorbents for SO2 in a fluidized bed coal combustion system. In tests using thermogravimetry, each of these solids was considerably more reactive than the conventional sorbents limestone and dolomite. The titanates also appear to be unique in that they completely retain their activity when cycled repeatedly between sulfating and regenerating conditions. Sorbent pellets formed from calcium aluminate cement were extremely strong and, in a test of this cement conducted in a pilot-plant scale fluidized bed coal combustor, the attrition rate was less than 1/30th of that experienced with limestone. When fly ash was incorporated into calcium aluminate cement pellets, the sulfation rate was increased nearly threefold relative to pellets prepared without fly ash, indicating an effect which may have been catalytic. Fluidized bed combustion is receiving increasing attention as a technique for burning coal, and other dirty sulfur-bearing fuels, cleanly, and a t a cost competitive with conventional combustion methods. In this technique, coal is burned in a fluidized bed of an SO2 sorbent, typically limestone (CaC03) or dolomite (CaCOsMgCO3) in the form of granules several millimeters in diameter. Up to over 90% of the SO2 formed during combustion can thus be captured within the fluidized bed ( I ) . However, problems with fluidized bed combustion, as with most clean power systems, are that the sorbent required and the amount of solid waste produced are both quite large, especially when burning high sulfur coal. A way to minimize these problems is to regenerate the sulfated sorbent, CaS04, for example by the reaction: Cas04
+ CO
-
CaO
+ CO2 + SO:,
Because of their low cost, limestone and dolomite are probably the best sorbents for use in a fluidized bed combustor without a regenerator; however, for a number of reasons, they are much less suited for use in a regenerative system. First, in order to achieve SO:! levels in the regenerator which are high enough to make sulfur recovery practicable, Reaction 1 must be conducted at high temperatures, causing the limestone to deactivate after only a few cycles of sulfur absorption and regeneration. Regeneration of Cas04 without a reducing agent, i.e., by the reverse of Reaction 2, the absorption reaction: CaO
+ SO2 + 0.502
-
CaS04
(2)
is not possible because the equilibrium SO2 concentration is much lower than for Reaction 1.Besides deactivation. attrition
is another problem which makes limestone and dolomite poorly suited for regeneration. Large amounts of sorbent are lost as dust blown from the fluidized bed. A further concern is variation in the ability of limestones and dolomites obtained from different locations to absorb SO2 and resist attrition. The need for improved regenerable sorbents has long been recognized. The thermodynamics of a large number of compounds were assessed in order to identify those compounds which could absorb sulfur at the conditions of temperature, pressure, and gas composition that prevail in a fluidized bed coal combustor and be easily regenerated (2-4). About 30 such compounds, mostly mixed metal oxides, were identified, but experimental results to confirm the thermodynamic predictions were lacking. Experimental studies also were conducted, notably by Argonne National Laboratory ( 5 , 6 ) .The Argonne work consisted largely of depositing a variety of sorbents, e.g., CaO, BaO, N a 2 0 , on supports such as alumina (A1203) and titania (TiO2) pellets, and then evaluating these sorbents for activity and resistance to attrition. The results were not too promising because the sorbent pellets were expensive and contained, at most, only about 20% active material. This paper reports the results of a new experimental approach. Rather than deposit active sorbents on inert supports, we chose to develop sorbents that would be composed entirely of active material, or nearly so. The outcome of our work has been that two classes of materials were identified as highly promising sorbents which we believe warrant further development.
Experimental Since the reactions which occur as sorbents are converted from oxide to sulfate and back produce weight changes, our approach was to use thermogravimetry as the principal experimental technique for measuring sulfation and regeneration rates. The equipment used was a DuPont 951 thermogravimetric analyzer (TGA), modified for use with reactive gases. The modification has been described in an earlier paper by Ruth et al. (7). The TGA was operated as a differential reactor, i.e., gases flowed past the solid sample at a rate sufficient to preclude any substantial change in gas composition. All work was conducted at a total pressure of 1 atm. Positive identification of sorbents, sulfated sorbents, and regenerated sorbents was accomplished by X-ray diffraction. For carrying out the experiments, it was necessary to choose reaction conditions that were convenient, could be readily reproduced, and were relevant to conditions in a fluidized bed coal combustor. Although a real flue gas would contain SO2, 0 2 , Nz, COz, H20, plus small amounts of CO, NO,, and hydrocarbons, we decided to use only the first three of these components in order to keep the experiments simple and avoid the possibility of effects which could make it difficult to in-
0013-936X/79/0913-0715$01.00/0 @ 1979 American Chemical Society
Volume 13, Number 6, June 1979
715
Table 1. Materials Screened for Ability to Absorb SO1 sulfated
Nap0 a BaO SrO CaO Lap03
slmple oxides did not sulfate
MgO CepO3 Mn304 Tho,
COO NiO
Bi203 Y203
ferrltes, all sulfated
LipAl2O4
BaFedl9 SrFel2Olg
BaA1204 SrAI2O4 Ca3AI2O6
ZnO
other materlals sulfated dld not sulfate
BaCO3 CaC03 CaSi03 BaSiO3 BaZrO3
alumlnates, all sulfated
Ca3SiOs
sulfated
tltanates did not sulfate
Li2Ti03
PbTiO3
BaTi03 SrTi03
CaTi03 CaO-contalnlng composites, all sulfated
(Ca0)3A1203 (Ca0)3A1203-7% NapO (Ca0)3AI2O3-1.1 % Na20 (Ca0)3(Si02~A1203)1/~-14.6 % Na2O (CaO)~(SiOp.A1~03)1~~-0.5% Nap0 (Ca0)3Si02-3% Na20
+
CaO Portland type 1 cement CaO f Atlas Refcon calcium aluminate cement Atlas Refcon calcium aluminate cement Atlas Lumnite calcium aluminate cement Lonestar Fondu calcium aluminate cement reaction conditions: 870-900 "C, 0.1 % SOP, 5 % 02, bal N2, 1 atm pressure Formulas indicate stoichiometric proportions of a Melted on sulfation. materials used to prepare sorbents, not final composition of sorbents.
terpret data. Thus, to screen materials for their ability to absorb SOz, we used a temperature of 870-900 "C and a gas containing 0.1 mol % (1000 ppm) SOz plus 5% 0 2 in Nz. For subsequent experiments in which the sorbent was cycled between sulfating and regenerating conditions, the SO2 level during sulfation was increased to 0.25% (2500 ppm), primarily to speed up the reaction and save time. Conditions for regeneration were usually a temperature of 1100 "C and a gas composition of 5 mol % CO in nitrogen. Even though the bed in fluidized combustion is granular in nature, the initial evaluation of sorbents was performed with fine powders. The use of powders was preferred because powders minimize sample variability, permit control of particle size (all samples were -100 mesh), and allow direct comparison of different types of materials without regard to different pellet-forming methods or diffusion effects. The ability of SO2 to diffuse into the complete sorbent sample being evaluated was the prime criterion in determining the quantity of sample powder which could be evaluated a t one time. We found that below about 10 mg, the fraction of material sulfated in a given time did not depend on sample size. For larger samples, the percent sulfation achieved was in part an inverse function of sample size. Subsequent to our evaluation of powders, sorbents were prepared in the form of pellets several millimeters in diameter that could be used in a fluidized bed combustor. The methods used to prepare pellets depended on the type of sorbent and are presented under Results and Discussion. Results obtained in the TGA for powders predicted directionally the results obtained for pellets fabricated from those powders. During a typical run, a small sample of powdered sorbent (5-10 mg), or a sorbent pellet, was heated in nitrogen a t 20 "C/min from room temperature to 900 "C. Sulfation was ini716
Environmental Science & Technology
tiated by introducing the S02/02/N2 atmosphere. At the conclusion of the sulfation experiment, the nitrogen atmosphere was readmitted, the temperature raised to 1100 "C, and regeneration started by introducing the CO/N2 atmosphere. After regeneration, the sample was cooled in Nz to 900 "C, ready for a new cycle to begin. Several of the most promising new regenerable sorbents developed during this program were tested in a pilot-plant scale fluidized bed combustor and regenerator in order to provide the most realistic assessment of performance possible. The combustor was 11.4 cm in diameter, and was operated a t 7.6 atm pressure, 850 "C, 1.0 m fluidized bed depth, and 1.4 m/s superficial gas velocity. The coal burned in the combustor was from a West Virginia mine and contained 2.5% sulfur. The regenerator was a 9.5 cm diameter fluidized bed which was operated a t 5-9 atm and 900-1100 "C. Natural gas was used as the fuel. A detailed description of the combustor and regenerator has been provided by Hoke et al. (8, 9). Testing of sorbents in the fluidized bed combustor was conducted by charging a weighed batch of fresh sorbent into the combustor vessel and feeding coal continuously. As the fractional conversion of the sorbent to sulfate increased, so did the SO2 emissions. After the run, which usually lasted 4 h, the sorbent remaining in the combustor was weighed in order to obtain some measure of the attrition rate. Tests in the regenerator were conducted in a similar manner. A batch of sulfated sorbent was regenerated and the SO2 concentration in the off-gas continuously measured.
Results and Discussion Nearly 40 materials, in the form of fine powders, were screened in the TGA in order to compare their relative abilities to absorb SO2. These materials were those selected from the review of thermodynamic properties noted earlier, those which had structures similar to compounds selected from thermodynamic considerations, and other materials selected by literature review. Table I lists the materials that were screened and identifies those which did and did not react. The pure compounds that were tested included simple metal oxides and carbonates, and mixed metal oxides such as aluminates, ferrates, titanates, and silicates. We also tested composites that contained CaO with Si02 and/or A1203, and combinations of CaO and cement. The composites containing CaO were made by coprecipitation. For example, materials with the empirical formula (Ca0)3(SiOz.A1203)1/zwere prepared by adding solutions of sodium silicate and sodium hydroxide-sodium aluminate to a calcium nitrate solution. The resulting slurry was filtered, washed, dried a t 250 "C, sieved, pelletized, and heated to 1100 "C. Samples with two different levels of sodium were prepared by varying the extent of washing of the filter cake. Composites of CaO/SiO2/A1203 and, particularly, combinations of CaO with calcium aluminate cement (CAC) reacted with SO2 much more rapidly and completely than did either ordinary limestone or pure CaO. Combinations of CaO with Portland cement, which is composed primarily of calcium silicates, reacted less rapidly than the CAC. Sodium oxide, NazO, was present in some of the composites to act as a promoter, but produced varying results. Sodium appeared to have no effect on the sorption rate of materials containing only CaO and alumina but did appear to promote sulfation in materials containing silica in addition to CaO and alumina. This behavior could be related to the acid-base nature of the sorption reaction. Sodium could be expected to be more effective in promoting reaction in more acidic sorbents, such as those containing high levels of silica. However, another explanation could be that a eutectic melt is formed between CaO, SiOz, and NazO. Of course, the presence of sodium in a sorbent has some serious drawbacks: for example, sodium can cause corrosion
0 Barium Titanate Calcium Aluminate Cement Conventional Sorbent (Grove Limestone) After Sulfation
c . 1; 0.5
J.G 0.3
0 . ;? 3.1 !I
CYCLE NUMBER Figure 1. Comparison of new regenerable sorbents and limestone
in fluidized bed combustion systems containing gas turbines. Moreover, in our testing, sodium attacked the quartz parts of the TGA. As a consequence, sodium-containing materials were deemphasized. Judged on the basis of sulfation rate, completeness of regeneration to the oxide, and activity maintenance, two classes of sorbents appeared so outstanding that they formed the dual development centers of this program. These sorbents were barium and calcium titanate (BaTiOa and CaTiOx) and calcium aluminate cement (CAC). Figure 1 shows the clear superiority of BaTiOy and CAC to the conventional sorbent Grove limestone (BCR No. 1359). Pellets of all three materials were cycled between identical sulfating and regenerating conditions. The time period for sulfation was arbitrarily selected as 75 min and the fraction of sorbent converted to sulfate in this time period is indicated by the height of the bars in Figure 1. A negligible amount of sulfate remained after each regeneration for all three sorbents. With limestone, the fraction of the calcium converted to sulfate (utilization) declined to 5% after six cycles, but the utilization of CAC was down only slightly, to about 16%,after 25 cycles. On the other hand, BaTiOa actually increased in activity and, after 50 cycles, the utilization was still over 60%. I t should be appreciated from Figure 1that when a sorbent deactivates, it is the sulfation rate, not the regeneration rate, which becomes slower. In fact, for all sorbents studied in this program, regeneration was much faster than sulfation, complete regeneration requiring only about 2 min. Rather than comparing these sorbents on the basis of percent utilization, as was done in Figure 1,a comparison can also be made based on the mass pickup of SO3 per unit mass of sorbent. Table I1 gives this comparison a t the number of cycles indicated for each sorbent. On this basis also, BaTi03 and CAC are both clearly superior to limestone. Barium and Calcium Titanate. Experiments in which BaTiOs and CaTi03 were cycled provided a surprise. All other sorbents studied deactivated noticeably with cycling, whereas these titanates showed no deactivation whatsoever, as Figure 1 shows for Bal'i03. Samples of sulfated and regenerated BaTiOz were analyzed using X-ray diffraction. The products of sulfation were Bas04 and TiO2. After regeneration, BaTiO3 was found, but BaO and Ti02 were absent. Thus, the reactions which occur during sulfation and regeneration can be written as:
Table II. SO3 Pickup per Unit Mass of BaTi03, CAC, and Limestone mass SO3 sorbed per unlt mass of sorbent after no.
sorbent pellet
no. cycles
cycles indicated
50 25 5
0.22 0.09 0.04
BaTiO, CAC
limestone BaTi03 [Bas04
+ SO2 + 0.502
+ Ti02]+ CO
-
-
+
[BaS04 Tion]
BaTi03
+ COZ + SO2
(3) (4)
Similar reactions probably occur with CaTi03. In addition to their novel ability to maintain activity, BaTiOs and CaTi03 appear to possess yet another desirable property: the thermodynamics for regenerating them are much more favorable than for regenerating sulfated limestone. For example, Figure 2 shows that the equilibrium constant for regenerating [Cas04 Ti021 is about three orders of magnitude higher than for CaS04. In practical terms, this means that it may be possible to produce higher SO2 concentrations in the regenerator and, a t the same time, operate the regenerator a t a lower temperature. Energy and sulfur recovery costs could be reduced. Much of the work with the titanates was directed toward the development of pellets with high attrition resistance. Several methods for preparing pellets were tried. The first, which yielded pellets that were hard but did not react, involved pressing BaTiOs powder into pellets and heating a t temperatures ranging from 980 to 1370 "C. Within this temperature range, pellets formed a t lower temperatures were weak and pellets formed a t higher temperature were glassy and unreactive. In the second method, BaTiOB powder was mixed with either 6 or 12 wt % H3P04,pressed into pellets a t 16 000, 33 000, or 65 000 psig, and heated a t 300 "C. This is similar to a technique used in the ceramics industry to pelletize powders. Pellets made with 6% acid and pressed a t the lowest pressure were the most reactive. The pellet of Figure 1 was made by this method. Unfortunately, swelling and weakening of the pellet were noted after cycling. Examination of the pellet under the transmission electron microscope revealed that an apparent loss of crystallinity had occurred during cycling.
+
Volume 13, Number 6, June 1979 717
TEMPERATURF, 9
1400
1600
1800
“F
2000
2400
22OC
7
E
5 Y
0 7
W t
- J n
L
1
(
1200
1
1400
TEMPERATURE, Figure 2.
OK
Equilibrium constants for sulfation and regeneration of CaTiOB
and CaO Titanate pellets with high attrition resistance and good activity were produced by the third method. Powdered BaTiOs, or CaTiOB, was mixed with 20%, by weight, of bentonite clay, and water was added t o make a paste which was extruded through a 3.2 m m diameter glass tube. Pellets averaging 3 mm in length were cut from the extruded strip, dried, and heated to promote hardening. In an attempt to understand why the titanates were such good SO2 sorbents, a number of techniques were used t o 0.4
Without
physically characterize sulfated and regenerated samples. Unfortunately, optical and electron microscopy, BET surface area, and porosimetry have, so far, failed to reveal any clues. However, the fact that the titanates have the well-known perovskite structure may be connected t o their activity. Materials with this structure have recently found use as catalysts (10). The activity of BaTi03 and CaTiOs could be, in part, attributable t o a catalytic effect. Calcium Aluminate Cement. CAC is a commercially available cement used in high-temperature applications. It consists mostly of CaA1204, and minor amounts of iron oxides, SiOn, and Ti02. From CAC, sorbent pellets can be made which are hard and tough and yet have better activity and activity maintenance than limestone. CAC pellets were prepared by a number of techniques, including extrusion, pressing, and granulation. Extruded pellets were made by mixing the cement powder with water, stirring to make a paste, and extruding through a glass tube. The resulting pellets were then humidified to promote proper curing and heated in order to form “ceramic” bonds and produce maximum strength. Pressed cylindrical pellets were prepared by using a pilling machine to press the dry powder. The dry powder pellets were then sprayed with a mist of water until saturated, and then humidified and heated. Spherically shaped CAC pellets were prepared by a granulation-like procedure in which a thin layer of the cement powder was placed on a flat tray and sprayed with water droplets. The tray was then shaken gently, as if sieving. Particles of cement wetted by the spray served as granulation nuclei. Larger spheres could be formed by spraying with more water and adding more cement powder. CAC was found to be an exceptionally versatile raw material in that the activity for absorbing SO2 could be improved through the use of additives. For example, since the absorption of SO2 is diffusion controlled, it was reasoned that increasing the porosity of the cement would increase its activity. One way of increasing porosity is to incorporate burnable powders into the cement. Figure 3 shows the effect of adding 1 and 5% carbon black to CAC and subsequently burning out the carbon leaving pores. Fly ash has been used as an aggregate for CAC in applications involving high temperatures. Since fly ash generally contains carbon, we considered that fly ash could be used as a burnable powder t o increase porosity and activity, and increase strength a t the same time. However, the magnitude of
Carbon Black
with 1%Carbon Black O(C-72) With 5% Carbon Black O(C-72)
0 bY
2 _I
0.:
Lo 3
r
r
b0
w 0 w CT
> z 0
r
0.2
After Sulfation
z
2 bu
s LL
0.1
A R r Regeneration -
C 2
3
4
CYCLE NUMBER
Figure 3.
718
Comparison of cycling performance of extruded CAC pellets with and without added carbon black
Environmental Science & Technology
No FVyyash
01
2 CAC/Flyash (by v o l a )
- 0CACIFlyash CAC/2 F l y a s h
After Sulfation
r
r
Regeneration 1
2
3
4
NUMBER Figure 4. Comparison of cycling performance of extruded CAC pellets with varying amounts of added fly ash CYCLE
the activity increase was unexpected. Figure 4 shows the effect of adding various amounts of fly ash to CAC. After four cycles of sulfation and regeneration, the utilization of a pellet prepared with one part CAC and two parts fly ash by volume (about 50% CAC by weight) was nearly three times that of a pellet prepared without fly ash. In another experiment, a pellet of CAC prepared without fly ash was rolled in fly ash in order to coat the surface of the oellet. The utilization of the coated pellet was about 70% higher than the uncoated pellet. This result suggests that the action of fly ash on the sulfation of CAC may be a chemical effect, and is perhaps catalytic. Fe20s, which is a component of fly ash, has been shown to be catalytic to the sorption of SO2 by CaO ( 2 1 ) . Fluidized Bed Tests. Cylindrical pellets of CaTi03, prepared by extruding a wet mixture of the titanate powder and 20 wt % bentonite clay, and spherical pellets of CAC, prepared by granulation, were tested in the fluidized bed combustor and regenerator. These experiments were the first we know of in which SOs sorbents, not based on limestone, were used in coal-burning fluidized bed equipment. Results of the tests with CAC were very promising. The rate of attrition of this sorbent was less than 0.3%/h during a 4-h run. By comparison, the attrition rate of limestone would have been expected to be about lO%/h. The fraction of SO2 released by the coal combustion which was retained by the bed of CAC averaged 67%, which was less than that which can be realized with limestone. However, the sorbent tested contained CAC only and did not include any promoters such as carbon black or fly ash. Furthermore, the dense sorbent fluidized poorly, primarily because of the high superficial gas velocity required to fluidize the sorbent and the small diameter of the combustor. Higher retentions of SO2 could probably be achieved with smaller sorbent particles or a larger combustor. In the combustion run with CaTi03 sorbent, sulfur retention declined as the run proceeded from a high initial value of 98%. Attrition resistance was good, better than limestone but not as good as CAC.
After the combustion runs were completed, the sulfated CAC and CaTi03 were regenerated. Although no attempt was made during these initial tests to operate a t conditions which would produce maximum levels of SO2, the SO2 levels produced with both sorbents were over twice the maximum (equilibrium) levels that could have been obtained when regenerating sulfated limestone.
Preliminary Assessment of the Economics of New Regenerable Sorbents In order to determine if there is any reasonable chance that the new regenerable sorbents could be cost competitive with natural sorbents, we made a simple comparison of the cost of using limestone vs. the new sorbents, CAC and the titanates, in a fluidized bed combustion process with sorbent regeneration. On the basis of this preliminary comparison, we believe that neither CAC nor the titanates should be eliminated from consideration as prospects for commercial use. The comparison takes into account only the preparation cost of the sorbents, which is approximately $180/ton for the CAC and $600-1600/ton for the titanates. These costs are quite conservative and consider only established methods of preparation. There is, as the cost figures suggest, more uncertainty in the cost of the titanates than CAC. The main reason for this uncertainty is that there are a number of “recipes” available for preparing titanate pellets and an optimal formulation has not yet been determined. The result of the limited economic comparison between the new sorbents and limestone is that CAC and the titanates would need lifetimes of about 15 and 50 times, respectively, that of limestone or dolomite to be economically competitive. Based on experimental results obtained in this program, there is a chance that these lifetimes are attainable. It should be emphasized that only sorbent preparation costs were considered in comparing the titanates and CAC with limestone. The result may be that we are giving an unfair Volume 13, Number 6, June 1979
719
advantage to limestone, since preparation cost is the area in which the new sorbents compare least favorably. In other areas, for example, regeneration and sulfur recovery, one would expect a significant economic advantage for the new materials. Hence, the required sorbent lifetimes given above are quite conservative, and shorter lifetimes should result if all costs are taken into account.
Acknowledgment
Conclusions
Calcium aluminate cement and calcium and barium titanate have shown surprising potential as SO2 sorbents in tests conducted in a laboratory TGA and in a fluidized bed pilot plant. CAC can be made into pellets which, in a fluidized bed, are much more resistant to attrition than the conventional sorbent limestone. Although CAC is a more active sorbent than Grove limestone, its activity can be considerably improved by increasing the porosity of pellets made from the cement, or by using promoters. Fly ash was found to sharply increase the activity of CAC, perhaps by catalyzing the oxidation of SO2 to SO3. Possibly, the fly ash that is present in a fluidized bed coal combustor may also affect the absorption of SO2 by limestone. Calcium and barium titanate were unique among all sorbents tested because they did not deactivate when cycled between sulfating and regenerating conditions. With other sorbents, the rate of sulfation became slower with cycling and the fraction of sorbent utilized declined with each succeeding sulfation. Physical characterization of sulfated and regenerated samples did not reveal why the titanates maintain their activity. The explanation, if it can be found, may shed light on the mechanism for deactivation of other sorbents as well. Based on a preliminary estimate of sorbent preparation costs and performance data now available, there is no reason to eliminate either CAC or the titanates as prospects for commercial use. However, a much more thorough cost analysis is needed to accurately predict the real commercial potential of these materials. A modest amount of additional experimental work to further define the technical merits of these materials also appears justified. Future experiments should be conducted in a fluidized bed arranged so that the sorbent can be cycled between sulfating and regenerating conditions, and so that measurements can be made of SO2 retention by the sorbent, SO2 levels produced during regeneration, and attrition rates.
For their many useful suggestions, we wish to thank Andrej Macek and George Weth of the U.S. Department of Energy, Pic Turner of the U.S. Environmental Protection Agency, Henry Phillips of Fluidized Combustion Co., Irving Johnson of Argonne National Laboratory, Joseph Yerushalmi of the City University of New York, Thomas Wheelock of Iowa State University, and Rene Bertrand and Ronald Hoke of Exxon. The assistance of T. C. Gaydos and D. T. Ferrughelli with the experimental arrangements is also gratefully acknowledged. Finally, we wish to thank the many people from industry who supplied raw materials and made suggestions for preparing sorbents. L i t e r a t u r e Cited (1) Hoke, R. C., Bertrand, R. R., Nutkis, M. S., Ruth, L. A., Gregory, M. W., Magee, E. M., Loughnane, M. D., Madon, R. J.,Garabrant, A. R., Ernst, M., “Miniplant Studies of Pressurized Fluidized-Bed Coal Combustion: 3rd Annual Reaort”. EPA-600/7-78-069. Ami1 1978, pp 46-51. (2) Lowell, P. S., Parsons, T. B., “Identification of Regenerable Metal Oxide SO? Sorbents for Fluidized-Bed Coal Combustion”. EPA 650/2-75-665, July 1975. (31 Cusumano, J. A,. Levy. R. B.. “Evaluation of Reactive Solids for SO;! Removal During Fluidized-Bed Coal Combustion”, final report, EPRI TPS76-603, Oct 1975. 14) Newbv. R. A,. Keairns. D. L.. “Alternatives to Calcium-Based SO7 ‘Sorbents’for Fluidized Bed Combustion: Conceptual Evaluation’? EPA-600/7-78-005.Jan 1978. (5) Snyder, R. B., Wilson, W. I., Johnson, I., Jonke, A. A.,“Synthetic Sorbents for Removal of Sulfur Dioxide in Fluidized Bed Coal Combustors”, ANL/CEN/FE-77-1, June 1977. (6) Pearce, T. A,, Conner, J. C., “Sulfur Dioxide Removal from Fluidized Bed Combustors”, report prepared by Dow Chemical Co. (Texas Division) under ANL Contract No. 31-109-38-3268, Aug 1975-0ct 1976. (7) Ruth, L. A,, Squires, A. M., Graff, R. A,, Enuiron. Sci. Technol., 6, 1009-14 (1972). (8) Hoke, R. C., Bertrand, R. R., Nutkis, M. S., Kinzler, D. D., Ruth, L. A,, Gregory, M. W., “Studies of the Pressurized Fluidized-Bed Coal Combustion Process”, Annual Report, EPA-600/7-76-011,pp 80-13 1. (9) Hoke, R. C., Ruth, L. A., Shaw, H., Combustion, 46,6-12 (Jan 1975). 110) Voorhoeve. R. J. H.. Johnson. D. W.. Jr.. Remeika. J. P.. Gallagher, P. K., Science, 195,827-33 (1977). (11) Yang, R. T., Shen, M., Steinberg, M., Enuiron. Sci. Technol., 12,915-8 (1978).
Received for revieus October 30, 1978. Accepted February 12, 1979. This Lcork was supported by Grant No. AER75-16194 from the R A N N Program of the National Science Foundation.
Particulate Zinc, Cadmium, Lead, and Copper in the Surface Microlayer of Southern Lake Michigan Alan W. Elzerman” Department of Environmental Systems Engineering, Clemson University, Clemson, S.C. 29631
David E. Armstrong and Anders W. Andren Water Chemistry Program, University of Wisconsin-Madison,
The surface microlayer (SM) of natural waters has received increased attention in relation to fluxes and accumulation of certain organic and inorganic materials a t the air/water interface (1-3). Trace metals have been of interest due to their tendency for wide distribution and possible synergistic and toxic effects. Results from previous investigations indicate that particulate matter is important to surface accumulation of trace metals ( 4 , 5 ) . A t the same time, the significance of 720
Environmental Science & Technology
Madison, Wis. 53706
inputs of atmospheric particulate matter to natural waters has been recognized ( 2 ) ,especially for waters such as southern Lake Michigan which are located near industrial areas (6, 7 ) . Also, bubble flotation of trace metals followed by ejection into the atmosphere upon bubble breaking has been established as a mechanism affecting trace metal distribution (2,8). Despite recent advances, our present knowledge of surface enrichment mechanisms and the sources and nature of par0013-936X/79/0913-0720$01.00/0 @ 1979 American Chemical Society
