Table 111. Concentrations of Polycyclic Aromatic Hydrocarbons Found in Commercial Coal Tar concn, mg/g
compound
naphthalene fluorene phenanthrene, anthracene fluoranthene pyrene
0.15 1.2
25 20 19
~~
crograms per liter total) found in the leachate from the test panels, compared to the small amounts (tenths of a microgram per liter) found in the leachate from the storage tank, are also interpreted to indicate the difference in age and exposure of the coal tar surfaces. Residence time for storage tank samples a t the time of collection is estimated to have been about 1 day, compared to 7 days for the test panels. In conclusion, PAHs from commercial coal tar coatings of storage tanks can leach into the water supply, which was the main purpose of this study. These results corroborate analyses by the EPA of leachate trom a storage tank in Pascagoula, Miss. ( 7 ) .However. little information on this subject is found in the literature. Only one of the compounds discussed in this paper. fluoranthene, is among the six PAHs designated by the World Health OrganiLation t o indicate a health hazard from contamination of drinking water: 0.2 pg/L is t h e maximum recommended total concentration for these six PAHs in drinking water (8).With respect to this proposed standard, the levels of fluoranthene and other compounds reported are considered significant. However, all of these compounds are generally acknowledged to have low or negligible carcinogenic activity. No conclusion can be made concerning levels of other PAHs of toxicological interest not reported in this paper, for want of standards to optimize recoveries and GC-MS conditions a t the time of analysis. The diversity of PAHs encoun-
tered in coal tar leachate and the effects of chlorination will be discussed in a subsequent paper (9). The appropriate use of materials such as coal tar in the water supply system is brought to question. The PAH compounds found in leachate samples may in part account for increased mutagenicity levels associated with water distribution ( I O ) . However, a more complete chemical analysis is needed for a representative number of distribution systems, differing in age and design. Hopefully, the results presented in this paper will encourage the acquisition of an adequate data base to fully evaluate the potential health hazard from the use of coal tar materials in the water supply.
Literature Cited (1j U.S. Environmental Protection Agency, “Sampling and Analysis Procedures for Screening of Industrial Effluents for Priority Pollutants”, Environmental Monitoring and Support Laboratory, Cincinnati, Ohio, April 1977. ( 2 ) New York State Department of Health, Division of’ Sanitary Engineering, “Engineering Sanitation Manual, NY State”, Albany, N.Y., 1978rpp 1--12. ( 3 ) U.S. Food and Drug Administration and Environmental Protection Agency, Fed. fiegist., 4 4 , 4 2 775 (July 20, 1979). (4) Junk, G., Richard, J.,Greiser, M., Witiak, D., Witiak, J., Arguello, M., Vick, R., Svec, H.. Fritz, J.,Calder, G., J . Chromatogr., 99, 745 (1974). ( 5 ) hlay, W., Wasik, S., Freeman, D., Anal. Chem., 50,997 (1978). 16) Liiinskv. is’.. Domskv. I.. Mason. G.. Ramahi. H.. Safari. T.. Anal. Chem , i5,952 (19633: ( 7 ) McClanahan. M., EPA LVater S u.~_ u.l vBranch, Atlanta, Ga., private communication, 1977. (8) World Health Organization, “European Standards for Drinking LVater”, 2nd ed.. WHO, Geneva, 1970. (9) Alben, K., manuscript submitted for publication in Anal.
Chem.
(10) Schwartz. D., Saxena, J., Kopfler. F., Environ Sci. Technol , 13, 1138 (1979).
Receiced for revieic October 26, 1979. Accepted January 18, 1980
NO, Influence on Sulfite Oxidation and Scaling in LimeILimestone Flue Gas Desulfurization (FGD) Systems Harvey S. Rosenberg” and Henry M. Grotta Battelle Columbus Laboratories, 505 King Avenue, Columbus, Ohio 43201
A laboratory-scale scrubbing system was used to perform semibatch experiments with slaked lime slurries and simulated flue gas to determine the influence of NO, on the solid-phase oxidation of sulfite to sulfate. I t was found that N O acts as an inhibitor and NO2 acts as a promoter of the oxidation of CaS03. NO present a t the same concentration as NO2 is unable to significantly inhibit the oxidative influence of NOz; however, a t a n NO/N02 mole ratio of about 12:1, the promotive action of NO2 is offset and the total oxidation is no greater than it would be without NO, present. Above a mole ratio of about 20:1, the oxidation of sulfite appears to be minimized. Oxidation inhibition merits consideration as a method to avoid sulfate scaling in lime/limestone flue gas desulfurization systems. Crystallization of calcium sulfite or sulfate on scrubber surfaces was the main problem in early lime/limestone flue gas desulfurization (FGD) systems and continues to be a major consideration although considerable improvement has been made. Sulfite scaling can be controlled in lime systems by keeping the p H of the entering liquor below 9. In practice, sulfite scaling seldom occurs in limestone scrubbing because t h e p H is lower and because sulfite supersaturates to such a 470
Environmental Science & Technology
degree that crystallization does not occur until t h e slurry reaches the reaction tank. However, sulfate scaling is much more difficult to control. Unlike sulfite, the decrease in p H down through the scrubber does not help hold sulfate in solution. Thus, if there is a considerable amount of sulfite oxidation in the scrubber, sulfate crystallization can occur. Various approaches have been taken to avoid sulfate scaling ( I ) such as seeding the scrubbing slurry with gypsum ( C a S 0 ~ 2 H z 0 crystals ) to reduce the sulfate supersaturation by providing sites for sulfate nucleation. The gypsum content of the scrubbing slurry can also be increased by oxidizing the sulfite to sulfate a t some point in the scrubber loop. The latter approach has the added advantage of providing a scrubber waste t h a t is easier to dewater and handle for disposal. However, a n alternative approach to avoid sulfate scaling t h a t merits consideration because of its simplicity and reliability is to prevent sulfate formation by inhibiting the oxidation of sulfite. Many methods and additives have been proposed to inhibit the oxidation of sulfite in SO2 scrubbers. Such additives include magnesium oxide ( Z ) , thiosulfate ( 3 ) ,and hydroquinone ( 4 ) .Graefe e t al. ( 5 )discuss the effects of NO, on sulfite oxidation in, principally, sulfite/bisulfite scrubbers. NO,, which is always present in flue gas, is characterized as a n oxidation 0013-936X/80/0914-0470$01 .OO/O
@
1980 American Chemical Society
Vent
t
Figure 1. Schematic diagram of laboratory apparatus for study of oxidation in lime/limes'tone FGO systems: A, analyzers, SO2 and NO,; CTC, constant temperature chamber, 130 O F ; CV, check valve: F, flow meters; FC, flow controller; H, humidifier; M, mixing chamber; P, pressure gauges; R, reactor; S, H202scrubber; SC, stopcocks, three-way: SO, shut-off valves; T, thermometer; WB, water bath, 125 O F
promoter. More work is needed to determine the relative importance of various oxidation inhibitors and promoters in limehimestone FGD systems. The work described in this note was undertaken to determine the effect of NO, on the oxidation of calcium sulfite. A laboratory-scale scrubbing system was constructed and used to perform semibatch experiments with slaked lime slurries and simulated flue gas. A p p a r a t u s and Procedure The apparatuij shown schematically in Figure 1 was constructed to perform the experiments to study oxidation in limehimestone FGD systems. Simulated flue gas was obtained by mixing gases from three cylinders. One contained 66 mol % nitrogen, 24 mol '3% COz, and 10 mol % oxygen; the second contained 0.015--0.15 mol '3% NO, with the balance nitrogen; the third cylinder contained SO2. Several cylinders containing NO, were available so that the ratio of NO to NO2 could be varied over a wide range. The gases from the cylinders were passed through calibrated rotameters equipped with dial thermometers and pressure gauges. 'The rotameters were operated at 15 psig, and flow controllers were used to reduce the pressure to atmospheric conditions. The line from the cylinders containing SO2 and NO, had a check valve after the flow controllers to prevent backflow firom the cylinder containing oxygen. After leaving the flow controller, the oxygen-containing stream passed through a preheat loop in a water bath and t,hen through a water-bubbler humidifier, where the stream was saturated wit,h E120 before entering a mixing chamber. The Son- and NO, -containing stream also entered the mixing chamber which was immersed in the water bath. From the mixing chamber, the gas stream flowed to a manifold containing three three-way stopcocks so t h a t the flow could be directed either to the reactor and then to the SO2 and NO, analyzers or vicls versa. T h e stopcock manifold permitted checking of the SO2 and NO, content of the inlet gas to the reactor periodically during the course of a run. T h e SO2 and NO, content of the outlet gas from the reactor was monitored continuously except for short periods when the inlet concentrations were being checked. The SO2 analyzer was an Inter-
national Biophysics Corporation Series NS-300, which has a fuel cell detector specific for SOz. The NO, analyzer was a Beckman Model 315A infrared analyzer equipped with a thermal converter for decomposing NO2 to NO. T h e NO2 content of the gas stream was determined by difference between readings with the converter on- and off-line. The reactor was also immersed in the water bath, which was maintained at 125 OF, and the entire flow system, between the flow controller and the vent, was enclosed in a plastic cabinet maintained at 130 OF. The gas entered the 100-mL reactor through a hollow glass stirring rod and was dispersed through a small opening a t the bottom of the rod. A set of three glass propeller blades was located at the bottom of the rod to ensure good mixing. The gas left the reactor through a small packed bed of fiberglass screen in order to remove any entrained liquid. The reactor was equipped with a fitting for sealing in a p H electrode and a port for withdrawing samples of the scrubbing liquor. Semibatch experiments were performed to determine the mole percent oxidation of sulfite to sulfate in the solid phase as a function of time. Mole percent oxidation is defined as the moles of CaS04-2H20 divided by the sum of the moles of CaSO4-2H20 and CaSO&H20 in a given sample of solids from the reactor. The scrubbing reagent consisted of a 5% slurry of hydrated lime. The effect of fly ash and chloride was simulated by adding 100 mg of the former and 250 mg of CaCl2 to the scrubber slurry. The gas flow rate and stirrer speed were held constant a t 5 L/min and 1000 rpm, respectively. T h e simulated flue gas had the following approximate composition: constituent
vol 70
constituent
vol 70
76.9 so2 0.25 COZ 11.1 NO 0-0.080 0 2 4.7 NOz 0-0.080 HrO 7.0 T h e SO2 and NO, analyzer readings and reactor p H were recorded continuously on strip-chart recorders. Samples of scrubber slurry were withdrawn from the reactor after 10,30, and 60 min and at the conclusion of a run (usually 90 min), and the solid phase was analyzed for sulfite and sulfate. Prior t o analysis, the solid phase was separated from the liquid phase by a simple draining. Subsequent checks on the sulfite and sulfate content of the liquid phase revealed t h a t the residual liquid in the solid sample had a negligible effect on the analytical results. Sulfite was determined by adding a measured sample to a n excess of acidified iodine solution and back-titrating with a standard sodium thiosulfate solution. Sulfate was determined by acidifying a measured sample, boiling the solution to evolve SOz, and precipitating Bas04 for weighing. These analytical techniques cannot distinguish between sulfite and bisulfite or sulfate and bisulfate. N2
Experimental Results Experimental runs were made with the apparatus described above using varying amounts of NO and NO2. Typical flue gas from large pulverized coal-fired boilers contains about 800 ppm of NO, with an NO/N02 mole ratio ranging from 9 to 19 (6). The experimental results are summarized in Table I. Oxidation of sulfite to sulfate after 30 and 60 min of run time is listed. The oxidation results indicate t h a t NO acts as an inhibitor and NO2 acts as a promoter either catalytically or as a direct oxidant. The inhibitory effect of NO is not adequate to suppress the promotive effect of NO2 at the lower end of the range of mole ratios of NO to NO2 found in flue gas. The effect of the NO/N02 mole ratio on the oxidation of sulfite to sulfate after 60 min is shown in Figure 2. The error in the experimental results shown in this figure is estimated to be about 5% for the absolute value bf the mole percent oxidation and about 10% for the mole ratio of NO to NO2. These error estimates are based on the precision of the wet chemistry techVolume 14, Number 4, April 1980
471
8
:
5 -
6
I 5
‘0
I
I 15
IO
25
20
30
35
40
,
50
45
Male Salio o f NO t o NO2
Figure 2. Effect of mole ratio of NO to NO2 on oxidation of sulfite to sulfate: ( 0 )800 ppm of NO,; (0)400 p p m of NO,; (X) 75 ppm of NO,
Table 1. Oxidation of Sulfite in LimeILimestone Scrubber Simulationa NO^ concn, ppm run no.
NO
NO2
1
700
