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Ind. Eng. Chem. Prod. Res. Dev. 1983, 22,710-716
Investigation of Some Factors Affecting Nonflame Generation and Suppression of SO3 Mlchael J. Zetlmelsl,' Kevin J. McCarthy, and Davld F. Laurence Petrolite Corporation, Tretoiite Research Laboratory, St. Louis, Missouri 63 1 19
Factors affecting the nonflame generation of SO3 were investigated with a small benchtop simulation of a typical boiler flue. Temperature, amount of catalytic Fe203surface available to the hot gases, and SO2 and O2 levels were all found to have an impact. Coating the catalyst surfaces with alkaline earth oxides was found to suppress SO, levels in the exit gases effectively. The SO, is determined by the MO SO3 = MSO, equilibrium rather than inhibition of SO3 formation by physically preventing contact between SOp and O2 and Fe203 surface. Comparative effectivenessof different materials was thermodynamically predictable on the basis of sulfate stabilities. Silica, which forms no sulfates, was shown to have no influence either on the generation or suppression of SO,, Burner rig experiments demonstrated that Mg can suppress SO, beyond stoichiometric equivalence. It is conjectured that MgO aerosol particles such as exist in a combustion gas stream offer competing surfaces for SO2 adsorption, thus decreasing the catalytic conversion to SO,.
+
Introduction The effects of sulfur oxides, especially SO3, in flue gases resulting from the combustion of sulfur-contaminatedfossil fuels have been studied widely. Chapter 4 of Reid's book gives an excellent overview of investigations involving these sulfur gases (Reid, 1971); of particular concern is the nonflame reaction, SO, + '/,02 + SO3. Thermodynamically the reaction favors the formation of SO3 at lower temperatures and SOz at the higher temperatures (National Bureau of Standards, 1966). The most important consideration to bear in mind from a kinetic standpoint is that a catalytic surface such as Fe203is required for the reaction to come to equilibrium, even at relatively high temperatures, e.g., 400-700 OC (Reid, 1971; Levy and Merryman, 1963). Two well-documented boiler corrosion problems, dependent on SO3 levels, are formation of low-melting alkali iron trisulfates (~600-650"C) and so-called cold end corrosion owing to sulfuric acid condensation in the cooler sections of the boiler. Since the sulfuric acid corrosion problem is the more common of the two, the present study is directed primarily to it. For these reasons there is sufficient economic incentive in many cases to lower the occurrence of SO3in the boiler. In addition, for obvious environmental reasons there is incentive to lower the SO3 exiting from the boiler. Fuel additive treatments with alkaline earth materials have proven effective in lowering flue gas SO3 levels (Reid, 1971). The present study is directed not so much to whether these additives work, but rather why they work. It has long been proposed that the additives coat the catalytic Fe,O, surfaces, e.g. in the superheater section, and prevent penetration of SOzand 0,to the surface where they can more easily react to form SO3,presumably owing to a lower activation energy for the thermodynamically favorable reaction. Neutralization is mentioned only as a secondary effect. This work attempts to assess the relative importance of neutralization and other additive effects on the formation of SO3 in boiler flue gas. Experimental Section Simulator Experiments. The approach was to study the effect of different surfaces and different gas mixtures on the conversion of SOz to SO3in a small simulation of a boiler flue. The apparatus is depicted in Figure 1. Gases were metered with the rotameters into a 2-in. (5.1 cm) 0196-4321 1831 1222-0710$01.50/0
ceramic tube, heated to the desired temperature in a Thermolyne tube furnace, Model F21120, controlled by Thermolyne's Furnatrol Model 13310. The exit gases were analyzed for SO3with the Severn Science SO3 monitor. Levels of SO2 and NO2 were measured on the DuPont Model 411 analyzer and O2levels were measured with the Teledyne Model 980 O2analyzer. A calibration gas mixture of 1.11% SO, in nitrogen from M. G. Scientific Gases was used without further purification. Pure 02,when required, was used as it came from the cylinder (Acetylene Gas, Co., St. Louis, MO; at least 99.6% pure). Air was used as it came to the laboratory from a central compressor after being dried to about 0.8% H 2 0 and passed through a final oil filter. To simulate the abundance of catalytic Fez03 surfaces present in a boiler, three bundles of 13 or 14 lengths of '/&I. schedule 40 pipe (outside diameter = 1.03 cm, inside diameter = 0.68 cm), 3 in. (7.5 cm) long, were placed in the combustion tube, axially to the direction of gas flow. Oil-soluble forms of Mg and A1 available as KI-81 (15% Mg) and KI-91 (14% Al) from Petrolite Corporation were applied to these tubes by soaking the tubes in the additives three times and burning the additive-coated surfaces with a propane torch after each soaking. The result was an adherent glaze of MgO or A1203. Oil-soluble Ca was applied by soaking the tubes in a 50% solution in hexane of an overbased calcium sulfonate from Witco and air drying. Si02was applied by soaking them in a 5% suspension of Cabosil, a fumed silica from Cabot Corporation, in acetone and air drying. An alternative method of applying Si02 was burning the tubes coated with high molecular weight silicones from Petrolite. In experiments where the deposits were analyzed for the mass balance of S all pertinent metals were analyzed by atomic absorption. Sulfate was done by barium perchlorate titration with dimethyl sulfonazo 111 indicator (Budesinsky,1965). In experiments where sulfide analysis was performed the iron sleeves were put into a 2-L bowl, equipped with a ground-glass top with two openings. About 100 mL of concentrated HC1 was added from a dropping funnel through one opening and the whole solution was heated for about 20 min. The evolved gases were collected through the other opening and scrubbed in a concentrated caustic solution (pH >12). The resultant solution was analyzed by potentiometric titration with silver nitrate. 0 1983 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983 711
qgf;l. r--
. , .~ ...: ... i. .. .~ ..... -i ....CEii....~ . ..-.iiiii,-..
j
Figure 1. Schematic for synthetic flue gas system: (A) rotameters for air, Nz,SOz, NO,; (B) tube furnace; (C) 2-in. ceramic combustion tube with tapered end; (D) auxiliary furnace controller; (E) probe for Severn Science SO3 monitor; (F)Console for Severn Science SO3 monitor; (G) DuPont Model 411 SOZ/NOzmonitor.
I
'
n
A' Figure 2. Schematic of boiler rig: (A) pilot light; (B)flame eye; (C) nozzle assembly and secondary air inlet; (D) fireback lining; (E) chromel-alumel thermocouples; (F)cooling water tubes; ( G ) secondary air preheater; (H) catalytic liner section; (I) heat exchanger inlet and outlet; (J) SO3 sampling ports
Burner Rig Experiments. A sketch of the rig used for the burner tests is given in Figure 2. Diesel fuel was burnt at the rate of 29.5 mL/min with secondary air of 12.5 SCFM. A Spraying Systems nozzle (fluid cap. 2050 or 1650, air cap. 140110) was used. The catalytic section was lined with a removable liner of 16 gauge SA330 which, in turn, was lined with a very thin (24 gauge) renewable mild steel liner. (Using the thin mild steel alone gave inadequate insulation to the catalytic section and temperature out of the optimum range.) Twenty feet of 1/4-in. (0.64 cm) 1010 carbon steel tubing was bent into a rectangular heat exchanger so that it would fit into the 8 X 8 X 18 in. catalytic section. Inlet and outlet of the exchanger were brought out the side of the rig. Air was blown through this heat exchanger at a flow rate of 66 L/min during an experiment. A thermocouple was wired to the surface of the exchanger, another touched the wall of the liner, and a third was hung in the bulk gas. The average temperature of this section was about 650 "C. SO3was monitored with the Sevem Science SO3 monitor in early experiments. Later, this method was abandoned in favor of batch samples taken just ahead of the catalytic section and just after it. The technique involved bubbling the gas into 80% 2-propanol-20% water via a gas dispersion tube for a measured period of time with a small bellows pump calibrated to deliver 200 mL/min in some experiments and later opened up to 666 mL/min. A t the end of the measured period of time the sample line was rinsed into the bulk solution and the bulk was brought to
I
I
712
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983
m
15
20
S O
Figure 6. Effect of Oz on SO3 formation for 5.5 X in 4.6 L/min total N, plus O2 at 600 "C.
Figure 4. Effect of varying the amount of catalyst on SO3 levels at different temperatures for 5.5 X L/min SOz in 2.3 L/min air plus 2.3 L/min additional N,.
A
1
a0 TCMrcrmrrCc)
m
m
'
Figure 5. Effect of varying the amount of SOpon SO3formation at different temperatures (2.3 L/min air plus 2.3 L/min additional N2).
sleeves were used in each bundle in order to facilitate removal. The equilibrium amount of SO3 is depicted in the same f i e . Other investigations (Reid, 1971; Reidick, 1980) have reported a similar maximum conversion between 550 and 600 "C and the approach to equilibrium only at much higher temperatures. A whole family of these bell-shaped SO, vs. T curves can be generated by varying the amount of catalyst as suggested in Figure 4,or the SO2 level as in Figure 5. Another variable which affects the conversion to SO3is the O2 level, depicted in Figure 6 for 600 "C. The shape of this curve shows SO3approximately a function of ( 0 2 ) ' / 2 , the same as in the equilibrium expression. These measured values of SO3are 5-7 % of the equilibrium levels.
L/min SO,
The purpose of these preliminary experiments was to establish credibility in our simulation of a boiler flue. Admittedly, there is no simulation which is as good as an actual operating system. Nevertheless, the baseline data presented in Figures 3, 4, 5, and 6 are very similar to observations made in such systems (Reid, 1971; Reidick, 1980). Consequently, the same factors should obtain in this sytem as in an actual operating boiler, although the factors should probably be weighted differently in the two. The next step in our investigation was to look at the effect of coatings, potential catalyst poisons, on the conversion of SOz to SO3. The first experiment was simply to coat the iron tubes with MgO as described above and measure the SO3 produced when SO2/OZmixtures were passed through the combustion tube. A remarkably simple result was observed. No SO3was detected for some time, and SO3 levels slowly increased after a day. A similar experiment with A1203showed an appreciable lowering of the SO, at first, but the levels increased much more rapidly than with MgO. The next step was to quantify the results with coatings by means of a material balance between the amount of SOz input to the system and that accounted for in the coatings or in the exit gas. The data in Table I are a summary of these material balance experiments. Reading across the table there is time, temperature, total SOz input to the system, and then SO2in the exit gases. The next column is SO2 as Sod2on the coated tubes at the end of the run. This value was obtained by extracting the tubes with water and then analzying for Mg2+or Ca2+and Sod2-. The ratio of alkaline earth metal to sulfate was close to that expected for the alkaline earth sulfate in every case. Moreover, these sulfate compounds represented the largest portion of the total SOz with the exception of the unconverted SOz in the exit gases. The amount of SO2as SO, in the exit gases is indeed rather small, and nearly all of the SO, reported in the table appeared only toward the end of the tests. The next to last column of the table reflects that generally about 70% of the total SOz input to the system is accounted for. Relatively small amounts of SO2 became sulfide as indicated by the two examples where this analysis was performed. The presence of S2-was indicated in every experiment by the evolution of H2S when acid cleaning the tubes a t the end of the experiment. In the final column of the table is an estimate of the amount of SO2 that would have been converted to SO, if the tubes had not been coated. This value was calculated from the SO, measured in the exit gases in the baseline experiments, i.e., about 20 ppm (pL/L) for iron.
Ind. Eng. Chern. Prod. Res. Dev., Vol. 22, No. 4, 1983
713
Table I. Material Balance Data for SO, in Benchtop Flue Gas Simulator Using Tubes Coated with Alkaline Earth Oxides or Silica % SO,
test duration, h 47a 27.5' .30a 60 30' 3d 4e
temp, "C 600 550 600 600 600 600 600
total
SO, in
SO,, g
exit gas, g 1.54 1.12 0.65 1.52 1.69 0.196 0.254
3.67 2.15 2.35 4.68 2.35 0.234 0.312
SO, as SO,'., g
SO, as SO,, g
0.80 0.37 0.62 1.87 0.04
0.24 trace 0.035 0.199 0 0.045 0.050
SO, as
sz-,g 0.027 0.216
accounted for 70 69 57 81 74 100 97
baseline SO, as so,, g 0.54 0.24 0.43 0.85 0.02 0.04 0.05
39 mild steel tubes coated with Ca overbase and air cured a 39 mild steel tubes with Mg overbase burnt o n three times. 39 mild steel tubes coated with fumed silica slurry and before use. 39 porcelain tubes with KI-81 burnt o n three times. air dried. e 42 mild steel tubes with a 100% silicone solid fluid burnt o n one time.
The numbers clearly do not suggest the blocking mechanism, but rather they indicate simple neutralization. In fact, we make more SO, when the tubes are coated than when they are uncoated (compare the sum of columns 5 and 6 with column 9), but the SO3 produced reacts with the alkaline earth oxide to make sulfate. The SO3 that does appear in the exit gases is generated toward the end of the test when the coatings are exhausted or begin to flake off. If we had a way of renewing the coating, these values would be small throughout. Clearly, there should be no SO-: if the inhibitor worked by preventing formation of SO,. A word is needed about the SO, vs. time profiles for the first four rows of data in Table I. In the experiments where Mg overbase was used to coat the tubes, SO3levels were zero for the first 24 h, then increased to about 10-11 ppm over the next 6 h in the 47-h and 30-h experiments, and stayed at that level in the 47-h experiment. In the 60-h experiment with the Ca overbase, SO3levels stayed close to zero to 46 h and then increased to about 20 ppm over the next 14 h. In order to preclude the possibility that the SOz was reacting with MgO on the tubes to make MgS03 which in turn could be oxidized to MgSO,, an experiment with porcelain tubes as substrate was performed. The fifth line of data in Table I is from this experiment. Only very slight amounts of SO3are produced when porcelain is the substrate, indicating that the sulfate detected in the other experiments is, indeed, dependent on the iron substrate as catalyst. The amount of SO, produced on uncoated porcelain tubes was measured and found to be very small 0.02 g out of a total of 2.35 8). A slight increase of SO, as SO-: above this baseline value is indicated for the coated tube, but the value is nowhere close to that for the coated iron tube. The data suggest that the coating is not impervious to SOz and Oz, but rather these gaseous species can still migrate to the iron oxide surface and make SO3which is immediately converted to SO:-, thermodynamicallya very favorable process. A possible explanation for the production of more total SO3 than in the baseline condition (but far less in the exit gases) is simply LeChatelier's principle. Consider the equilibrium: S02(g) + ' / 2 0 2 + SO,(g) S042-(c). As SO, is converted to S04z- in the crystalline state, the SO,/SO, equilibrium is continually forced to the right. An examination of the thermodynamic stabilities of sulfates at various temperatures and the amount of SO, required to stabilize these sulfates at equilibrium was done next. Consider the general case
-
MSO,
6
MO
+ SO3
The equilibrium constant for the reaction is simply the
/
i I 5oo
rEMPERAr~(o)o
0 90
lid0
Figure 7. Sulfate stabilities as a function of log Pso, and temperature (calculatedfrom data from National Bureau of Standards, 1966; Lay, 1974).
partial pressure of SO,, K = Pso . Using literature data (National Bureau of Standards, 1966; Lay, 1974), comparisons of equilibrium SO, values as a function of temperature for the alkaline earth sulfates are presented in Figure 7. In addition to the alkaline earths, A12(S04),has been included in the same figure since there is some interest in A1 as a cold end corrosion inhibitor. Keep in mind that for this material K = (PsOJ3 because of the stoichiometry of the decomposition reaction: AlZ(SO4),+ Alz03+ 3s03. From Figure 7 one can see that the sulfates increase in stability in the order: A1 < Mg < Ca < Ba. Moreover, one can see from the figure that Pso, (eq) for MgS0, at 900 K is about (2.28 X lov8to be exact). Typically, at this temperature about 10 ppm of SO, is measured when the iron tubes are uncoated. This translates to a parital pressure of 2 X which means that there is a tremendous driving force for any SO, that is formed to go to the sulfate. The experiments summarized in Table I showed no SO, in the exit gases until sometime toward the end of the test duration. It is not clear whether the eventual occurrence of SO3 in the exit gas is a result of exhausting the basic oxides or complete breakdown of the coating by spalling so that there is no longer good contact between the SO3,as it is formed, and the coating.
714
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983
Table 11. Experimental Data for SO, Formation and Inhibition Using Low Sulfur Fuel with 1%Sulfur Added (Flue Gas 0. = 7 - 7 . 5 % ) with additive before additive SO, after cat., no. of SO, ahead of no. of SO, ahead of' SO, after cat., flue gas det. cat., ppm (viv) PPm (viv) det. cat., ppm ( v i v ) ppm (viv) SO,, P P ( ~ vh) ~
I
12 6 3
4 3 3
%5
_
19.47 i 24.78 i 18.19 = 14.79 ~16.62 T 15.44 T 15.71 1
1.96' 5.51 0.82 5.34 3.35 4.04 3.37
' All confidence levels 95%.
24.88 i 2.12 30.5 T 5.65 32.87 - 2.44 25.21 I 7.52 22.48 I 1 . 9 3 25.88 i 5.25 22.33 3.85
556-613 556-590 492-502 $22-434 452-532 434-517 439-453 346-379
3b
18.96 i 5.6
20.43
5' 3c
15.19 ? 1.83 16.19 5 1.34
17.54 i 1.80 19.43 ? 6.66
3d
14.34 i 5.08
11.43 ? 1.55
Additive feed rate at 10 gal/lOOO hbl x Q S.
t
4.08
Additive feed rate at 5 gal/lOOO bbl x
% S. d Catalytic surfaces precoated with MgO suspension.
Another indication of the importance of acid-base relationships is the fact that several experiments in which the tubes were coated with A1203gave the predicted results. At 600 "C some SO, was measured immediately, and at 650 and 700 " C the values increased rapidly. The equilibrium SO, for AlZ(SO4),is 1.4 X lo-,, quite a bit higher than the SO, values measured in the to lo-* range. At lower temperatures no SO, was measured, consistent with the much lower equilibrium values for these temperatures, so3(500"C) = 4 X lo-, and so3(400 "C) = 7 X In short, A1203is not as strong a base toward SO, as the alkaline earth oxides and would not be expected to be as effective as a cold end corrosion inhibitor. The results of the A1203 experiment are not given in Table I since this experiment was relatively short and a material balance was not attempted. On the other hand, the experiment with CaO, a considerably stronger base than MgO, showed that CaO was a considerably more effective SO, suppressor than MgO. The coating lasted longer than the MgO coating as indicated by experiment 4 of Table I. Moreover, a smaller percentage of SOz ended up in the exit gases, and a larger This experiment was percentage was converted to SO:-. done to confirm the comparative sulfate stabilities in Figure 7 rather than to suggest Ca as a practical solution to SO, problems, since Ca is known to aggravate deposition in a boiler. Two experiments were done with SiOz coatings, one applied as a slurry of fumed silica and another by burning a high molecular weight silicone onto the surfaces. The data for these two experiments are given in the last two lines of Table I. Since the tubes were thoroughly coated with Si02and SiOzis inert toward SO,, these experiments test the physical barrier hypothesis without ambiguity from neutralization. As indicated by the last two entries in Table I, the results of these SiOz experiments were essentially the same as the results with uncoated tubes, Le., SO, levels of ~ 2 0 - 2 2ppm (0.05 g total for the duration of the tests) and SOz levels of ==lo0ppm. If a barrier to the iron oxide surface was all that was necessary for inhibition of SO, generation, there should have been some lowering of SO, in these experiments. In a few experiments water vapor was injected into the flue gas at a rate of 5.3% by means of a syringe pump. The temperature vs. SO3 profiles were qualitatively the same as observed with the dry gas but the SO, values were 20-30% higher in the wet gas. Moreover, there was no SO, in the exit gas for about 25 h when simulated flue gas containing 5.3% water vapor was passed over iron tubes coated with MgO, just as observed with the dry gas. (Actually, none of the experimentss was done with absolutely dry gases since the laboratory air, which comprised nearly 50% of the mixture, contained about 0.8% HzO.)
Table 111. Experimental Data for SO, Formation and Inhibition Using Low Sulfur Fuel (0.1-0.4%) and SO, Injection Just Ahead of Catalytic Section ( 7 .O-7.5% 0,)
so,
SO, injected, mL/min
no. of det.
0 185
1 7
0 285
1 8
285
ahead of cat., SO, after ppm cat., ppm (v/v) (viv) Before Additive 6.41 11.81 21.03 1. 3.91 2.35 8.95 23.69 2 2.26
flue gas, SO,, ppm (vh) 48 526-548 96-103 609-626
With Additive (10 ga1/1000 bbl/% S) 5 14.85 i 609-626 2.71
Burner Rig Data. From these benchtop studies one would conclude that neutralization is the only mechanism whereby MgO lowers flue gas SO,. Data from a few experiments with a small simulation of a boiler, however, show a greater than stoichiometric neutralization of SO, by MgO. The first set of rig experiments monitored the SO, levels just ahead of and just after the catalytic section described above. The data in Table I1 show a 6-10 ppm increase in SO, before and after the catalyst, and the increase will hold up to some statistical scrutiny (95% confidence limits). Furthermore, the additive, even at these remarkably low dosage rates, lowered the SO, levels about 5-10 ppm. The difficulty is that the precatalyst SO, levels are already rather high. Apparently, the rig passage is too small for the flame SO3 to relax back to a lower level before going through the catalytic section as would be the case in an actual full-size boiler (Hedley, 1967). It is desirable to have the difference between the precatalytic and post-catalytic SO, values as high as possible to give a good window to work with in applying additive. Another difficulty is that the reproducibility from run to run is only fair. This variability is caused by variance in fuel sulfur levels from 0.1 to 0.4%, which was determined later after fluctuations in flue gas SO2 were noticed. To obviate this problem of wide variations in SOz going through the catalytic section (and consequently wide variations in SO, levels) and to widen the difference between pre- and post-catalytic SO,, a few experiments have been performed in which SOz was metered into the catalytic section rather than produced by burning sulfur-laden fuel. The data are given in Table 111. In these experiments SO, was first measured while burning an as-received fuel. Then, 285 mL/min SOzwas injected, i.e., enough SOz to give about 550 ppm of SOz in the flue gas. The reader
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983 715
0
10
20
0
TREATMENT T t 3 E ( h r r . )
40
50
Figure 8. Decrease in SO,with time of additive injection (additive rate = 10 gal of KI-81/1000 bbl/% S). Table IV. Comparison of the Molar Decrease in SO, with the Number of Moles of Magnesium Injected
so, decrease, PPm
SO, decrease, mol/min
Mg injected, mol/min
8.94 12.44 4.94 6.45
1.29 X 1.89 X los4 7.49 x 9.78 X
4.71 X 4.71 X 2.36 X 2.36 X
mole ratio SO,/Mg
2.73 4.01 3.17 4.14
will note about a 50 ppm difference in SO2 (lines 1and 3 of the table) between the two experiments where no SO, was added. These two fuels were analyzed for sulfur and found to be 0.1% and 0.35%,respectively. Lines 2 and 4 represent continuation of the experiments in lines 1 and 3 and there is about a 50 ppm difference in SO2, as expected. The precision in SO3determinations within a run and from run to run is much improved over the old configuration. The very last line of Table I11 represents an experiment in which additive was used with the new configuration. Figure 8 is a plot of SO3vs. time of additive injection; it gives the reader a general idea of the time factor involved before the SO3levels out. With this apparatus the values fell rapidly for about the first 10-15 h and then leveled out. This time factor is probably dependent on the geometry of each boiler. A summary of the data from experiments involving treatment is given in Table IV along with the comparison of the decrease in SO3 with the Mg added. A factor of about 3-4 times stoichiometric is indicated for the effectivenesss of Mg. (A sample calculation of this factor is given in the Appendix.) Another indication that the additive works beyond its stoichiometric equivalence is the fact that X-ray diffraction of the deposits taken after several of these experiments indicated the presence of a mixture of MgO and MgSO, with the sulfate being the dominant phase. If neutralization were the only mechanism, all of the MgO would have formed MgS04, and there would be no MgO remaining. On the other hand, obviously a good bit of SOz is getting through to catalytic surface and making SO3, Although neutralization is strongly indicated in both the benchtop test and the burner rig test, indications of beyond stoichiometric lowering of SO3 in the burner test are not easy to explain. Estimates of residence times in the catalytic section are about 1 s in the rig compared to 2 s in the simulator. One reference to residence times in the literature claims 4-6 s (Rolker, 1973). Catalytic surface area to gas volume ratios are estimated at about 5100
cm2/18000cm3for the rig compared to 1572 cm2/456 cm3 for the simulator. Consequently, the chance of an individual gas molecule contacting a catalytic surface is considerably greater in the simulator. Considerations of velocities and surface area to volume ratios will explain the overall difference in conversion efficiency between the two sets of apparatus, but really does not say anything about specific additive effects. The difference in the efficiency of MgO to lower SO3 must be in the method that the material was applied. In the rig the MgO exists as an aerosol of very fine particles, and stoichiometric enhancement is observed. In the benchtop simulation the catalyst has a tight, adherent coating of MgO, and simple neutralization is observed. Consider the fact that heterogeneous catalysis of SOzto SO3depends on the adsorption of SO, on the catalytic surface to form an activated complex, thus lowering the activation energy for the conversion of SO2 to SO3. Oxygen also needs to adsorb but it is in such abundance compared to SOz that it is not likely the limiting species. With the SOz and Oz side by side on the surface, there is a lower energy pathway for the necessary electron exchange and chemical bond making and breaking between SO, and 02.Consequently, it seems reasonable that tiny particles of MgO in the gas stream with their rather strong basicity would give competing surfaces for the SO,to adsorb, lowering the incidence of adsorption on the catalytic surfaces. Meanwhile, some of the SO,does migrate through the MgO which soon coats the catalytic surface, is converted to SO3,and reacts to form MgS04just as in the benchtop simulation. Consequently, MgS04is the major phase in the coatings of the burner rig. If this competing surface hypothesis is correct, one would expect very finely divided particles of MgO with high surface area per unit weight to be more effective than coarser particles in lowering the conversion of SO, to SO3. The hypothesis needs to be tested in further experiments. Conclusion In summary, the data presented in this paper support neutralization as a key mechanism whereby boiler additives based on Mg successfully suppress boiler flue gas SO3levels and consequent corrosion of cooler parts of a boiler owing to sulfuric acid condensation. Nevertheless, in burner rig experiments Mg lowered SO3about three to four times its stoichiometric equivalence. The difference in the results can best be explained by the manner in which the additive was applied, Le., as a tight adherent glaze of MgO in the simulator and as an aerosol of finely divided MgO particles in a burner rig. Such finely divided particles could provide a competing surface for SO, adsorption and desorption, consequently lowering SO2 adsorption and conversion to SO3 at the catalytic surface. Appendix Comparison of amount of SO3removed with amount of Mg injected. 1. Total gas going through rig secondary air = 12.5 SCFM X 28.3 L/SCF = 353.75L of air/min fuel consumed (29.5 mL/min) contributes another 18.93 L/min
-
CHz + 3/20z HzO + CO, 29.5 mL of CH,; 23.6 g; 1.69 mol = 37.86 L/min 2.53 mol Oz = 56.78 L/min 94.64 L/min
Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 716-718
716
1.69 mol each of H 2 0 and C02 gives 75.71 L/min net change 18.93 L/min Total exhaust gas = 372.68 L/min. 2. Calculate number of moles of SO, represented by an 8.99 ppm drop in SO, 8.84 X lo4 L of SO, X 372.68 L/min = L of exhaust 3.29 X L of SO,/min
4. Calculation of ratio 1.31 X
mol of S03/min
4.71
mol of Mg/min
X
= 2.73
Registry No. SO2, 7446-09-5; SO3,7446-11-9; FeZO3,1309-37-1; MgO, 1309-48-4; A1203,1344-28-1; CaO, 1305-78-8.
(1)(3.35 X lo-,) L/min
1.29 X mol S03/min (0.0821) (300 K) 3. Calculation of moles of Mg injected. Putting 16.68 mL of KI-81 (15% Mg) in 1 L of fuel and injecting this at a rate of 9.5 mL/min gives about the desired rate. (Actually this is slightly above 10 gal/lOOO bbl because the original dilution was for a fuel use of 35 mL/min.) 16.68 mL of KI-81 0.5 mL 0.9 g XXX L min mL 15 g of Mg - 1.13 g of Mg 100 g of KI-81 min 4.71 X mol of Mg min n=
L i t e r a t u r e Cited Budesinsky, B. Anal. Chem. 1965, 37(9),1159. Hedley, A. B. J. Inst. Fuel 1967, 40, 142. Lay, K. W. J. Eng. Power 1974, 134. Levy, A.; Merryman, E. L. J. Eng. Power 1963, 229. National Bureau of Standards, "High Temperature Properties and Decomposition of Inorganic Salts. Part 1. Sulfates", NSBD7-NBS7, 1966. Reid, W. T. "External Corrosion and Deposits"; American Elseview Publishing Company, Inc.: New York, 1971. Reidick, H.; Reifenhauser, R. Combustion, 1980, 17. Rolker, J. VGB Krsftwerkstechnik 1973, 53, 333.
Received f o r review February 16, 1983 Accepted July 14, 1983
COMMUNICATIONS A New Water Disinfectant; A Comparative Study The N-chloramine compound 3-chloro-4,4dimethyC2-oxazolidinone (agent I) has been compared with several other antimicrobial agents as to its efficacy as a bactericide for treatment of water. The other agents tested included 1,3dichloro-5,5dimethylhydantoin,N-chlorosuccinimide, commercial HTH, and bleach (sodium hypochlorite). The species of bacteria included in the comparative study were Escherichia coli, KlebsEela pneumoniae , Roteus vubris, Salmonella chlolera-suis , Salmonella typhimurium, Serratia marcescens, Enterobacter cloacae , Staphylococcus aureus, Staphylococcus epidermidis , Pseudomonas aeruginosa , and Sphaerotilus natans In general, agent I requires somewhat longer contact time at a given concentration than do the other agents for complete kill, but it is much more stable in water solution than are the other agents.
.
Introduction
The most widely employed commercial disinfectant for drinking water is chlorine gas (Symons, 1977). While this agent is certainly an adequate disinfectant, it suffers several serious limitations. Being an extremely toxic gas, a ruptured metal cylinder or pipe in a treatment plant or a derailed railroad tank car transporting the gas could cause casualties. Chlorine gas has been shown to chlorinate organic impurities in water to produce toxic trihalomethanes (Vogt and Regli, 1981). Furthermore, chlorinated water remains disinfected for only a few hours due to rapid loss of total chlorine through vaporization from the water and reaction with foreign matter; chlorine is not a particularly stable agent in water. Other gaseous disinfectants such as ozone (Rice et al., 1981) and chlorine dioxide (Hoff and Geldreich, 1981) are being employed in some municipal treatment plants, but these agents are also toxic gases and suffer many of the same limitations as does chlorine. Clearly it would be desirable to develop a new water disinfection agent which would be of low volatility (e.g., a solid), stable in water and dry storage, and nonreactive 0196-4321/83/1222-0716$01.50/0
with organic impurities. Several agents which are being used for disinfection of small water supplies fit some of these qualifications. These include household bleach (5% NaOCl), calcium hypochlorite, trichloroisocyanuric acid, and hydantoin derivatives. Although these agents are adequate disinfectants, none of them is particularly stable in water or dry storage (Worley et al., 1983a,b). In fact, the storage of dry agents can be hazardous. Improperly stored calcium hypochlorite can lead to spontaneous fires, and bleach must be protected from light and high temperatures (White, 1972). It has been stated that N-chloramine agents do not react appreciably with organic material to produce toxic trihalomethanes (Vogt and Regli, 1981). However, it has also been suggested that chloramines are much weaker bactericides than is chlorine gas (Hoff and Geldreich, 1981). We believe that this generalized statement has been made prematurely and that there are N-chloramine agents which are strong disinfectants and which possess other attributes which render them of possible commercial use as allpurpose water disinfectants. One such agent, 3-chloro4,4-dimethyl-2-oxazolidinone (henceforth referred to as 0 1983 American
Chemical Society
