Catalytic Conditioning of Fly Ash Without Addition of SO3 from External Sources Siegfried Kanowski and Robert W.Coughlin’ Department of Chemical Engineering, Lehigh University, Bethlehem, Pa. 18015 ~~
~~
Data and experimental results are presented to demonstrate that catalytic oxidation of SO2a t the low concentrations normally present in flue gas from low sulfur coal can produce SO3 in concentrations sufficiently large to cause good conditioning by significantly lowering the resistivity of fly ash. This was accomplished by disposing catalysts in streams of Sonbearing flue gas and measuring the effects of the gases so treated on the resistivity of carbon-free fly ash obtained from several different coal-fired power stations. Concentrations of SO2 and SO3 were measured by gas sampling and chemical analysis. The effects of moisture and temperature were also investigated.
As more stringent air pollution laws cause many power companies to burn lower sulfur coal under boilers in installations that formerly burned higher sulfur coal, it is discovered that the use of lower sulfur coal can bring with it severe decreases in efficiencies of electrostatic precipitators. Burning low-sulfur coal produces a flue gas which contains lower concentrations of SO2 and correspondingly lower concentrations of SO3 since about 1-5% of the SO2 is generally oxidized further to SO3 in the boiler system. The extent of oxidation depends on the condition of the metallic surfaces and the amount of catalytic dust (e.g., V or Fe) in the boiler. The extent of such oxidation cannot be predicted with any quantitative certainty. As a result of lower concentrations of SO3 in the flue, the electrical resistivity of the fly ash particles is correspondingly lower, with consequent adverse effects on the efficiency of electrostatic precipitation. The character and properties of fly ash, including resistivity, vary widely with factors such as coal burned, design and operation of the furnace, and the rate of steaming of the boiler. A very important factor is the concentration of SO3 in the flue gas which is believed to sorb on fly ash, together with water, and thereby cause lower electrical resistivity of the ash. When high-sulfur coals are burned, sufficient SO3 is usually produced to serve as a natural “conditioning agent” in lowering the electrical resistivity of the fly ash. I t was demonstrated early in the laboratory ( I ) that addition of SO3 to the flue gas could reduce fly ash resistivity by about a factor of 20 with a concomitant decrease i n t h e d u s t content o f t h e gases of about 50%; e.g., a change in removal efficiency from 99 to 99.5%. More recently, tests a t the Saarbergwerke in Germany demonstrated that as little as 5-6 ppm of SO3 could cause a 31% decrease in the dust content of flue gas after filtering through an electrostatic precipitator (2).In the just mentioned tests, a t tests a t a Colorado power station ( 3 )and from a survey of similar work ( 4 ) ,it has been generally found that the addition of 10-20 ppm SO3 by volume to flue gas from lowsulfur coal is sufficient to reduce the electrical resistivity of fly ash from about 10l2 ohm cm to below the critical value (about lo1’ ohm cm) required for normal collection by electrostatic precipitation. The injection of SO3 into flue gas produced by low-sulfur coal is now an accepted method of improving the efficiency of electrostatic precipitation. For example, Public Service Co. of Oklahoma has installed three different kinds of systems on eight or more coal-fired boilers using the designs and technology of three different vendors ( 3 ) .The injection systems
differ from one another in that one may be based on vaporizing H2S04,another may vaporize liquid SO3, and yet another may produce the SO3 by passing SO2 from an external source through a catalytic reactor together with air and injecting the SO3 thereby produced into the flue gas; these methods have in common the fact that they involve the addition of sulfur oxides from an external source into the flue gas.
N e w Approach to Conditioning Requires N o External Source of S u l f u r We describe here experiments which represent a new approach to fly ash conditioning in that no external addition of sulfur or a sulfur oxide is required. The method is based on causing the conversion to SO3 of a significant portion of the SO2 normally present in flue gas even when low-sulfur coal is burned. The concentration of S O z in flue gas is correlated approximately linearly with the sulfur content of the coal (11); thus, about 700 ppm of SO2 may be expected in flue gas from 1%sulfur coal. The thermodynamic laws of chemical equilibrium predict, however, that when the gas also contains about 5% 0 2 , about 99% of the SO2 can be oxidized to SO3 a t about 750 OF, about 90% a t 950 O F with correspondingly more SO3 a t lower temperatures and less a t higher temperatures. In typical power plant installations burning low-sulfur coal, however, the rate of such oxidation is too low t o provide the , 9 0 3 required for good conditioning of fly ash. The rate of such reaction will depend on the temperature, the concentrations of SOZ, 0 2 , and water vapor as well as on the possible catalytic properties of the surfaces in the system. In practice, often not more than about 1%of the SO2 is oxidized to SO3 in the boiler-flue gas system. This is insufficient for good conditioning when the sulfur content of the coal is not higher than about 1%. It is possible to oxidize to SO3 significant quantities of SO2 present in flue gas a t concentrations typical for installations burning low-sulfur coal, by merely inserting commercial catalysts in the SOz-containing flue gas. We have done this for flue gas containing controlled amounts of SO2 and measured not only the SO2 and SO3 concentrations downstream of the catalyst but also the effect of the flue gases on the electrical resistivity of typical coal fly ash. The results indicate that sufficient SOYcan be formed in flue gas by this simple method, thereby providing good conditioning of fly ash without any external addition of chemical reagents to the flue gas. Experimental A schematic diagram of the apparatus is shown in Figure 1. Flue gas was produced by burning about 20 l./min of natural gas in gas burner 1.The blower 2 (Buffalo Forge Co. Size 31) caused a mixture of about 65% flue gas and about 35% additional air to flow through the heavy insulated pipe (length 2 m, diameter 5.1 cm). This pipe was fitted with openings through which gaseous conditioning agents (at 3) and water (at 4) could be injected continuously into the flue gas. I t was also possible to insert a charge of solid catalyst into pipe 5 and to monitor the temperature in the vicinity of the catalyst using a calibrated thermocouple 6. Further experimental details, including information as to equipment, fly ashes and materials, and assay of sulfur oxides, Volume 11, Number 1, January 1977
67
temperature = 1 9 0 ' C
~
f o r both curves
experiment 5 6
..(
I
1 0 iI ~
a f t . = 4 hr o f conditioning with S O 2 and c a t a l y s t ppm
so3
-
I4 ppm
190-c
J S M L E ?PAIN
Figure 1. Schematic diagram of experimental apparatus
have been deposited with the ACS Microfilm Depository Service.
Results and Discussion The salient details of almost 35 separate experiments are summarized in Table I (deposited with the ACS Microfilm Depository Service) which shows for each experiment the origin of the fly ash, the moisture content of the flue gas, the temperature of the fly ash, and the voltage applied to the fly ash sample. For those experiments in which a catalyst was employed, the identity of the catalyst and the amount of catalyst are shown in the last two columns; another column gives the measured approximate operating temperature of the catalyst. The column titled ppm SO2 has no entry for experiments in which no SO2 was added from an external source; the values of SO2 concentrations given for the other experiments were measured by sampling the flue gas just upstream of the fly ash sample. The column titled ppm SO3 represents a concentration calculated from flow rates when the SO3 was added from an external source (Le., for experiments 26-36 for which no entry appears in the ppm SO2 column). All other concentrations were measured by sampling and chemical analysis of the flue gas as described above, including those experiments in which SO2 was added from an external source (and oxidized by means of a catalyst in the system). The concentrations appearing in brackets are felt to be improbable. Figure 2 shows typical graphs for current vs. voltage applied to the fly ash sample in the flowing flue gas. The lower curve refers to ash with no externally added conditioning agent, but with a background concentration of SO3 of about 7 ppm; the upper curve refers to ash which had been conditioned for 4 h at 190 O C in a flue gas containing 14 ppm SOs. Figure 3 shows the different conditioning behavior (current vs. time) of five different fly ashes exposed to equivalent concentrations of SO3 (added from external source) under identical conditions of temperature, voltage, and gas moisture content. Figure 4 provides a similar comparison of the conditioning behavior of the different fly ashes but in terms of resistivity vs. conditioning time. Figure 5 (deposited with the ACS Microfilm Depository Service) presents the same information shown in Figure 4, the only difference being the semilogarithmic scale of resistivity used in Figure 5 rather than the arithmetic scale used in Figure 4. A comparison of Figures 4 and 5 shows that the arithmetic scale of Figure 4 shows the effect of conditioning on resistivity more effectively, whereas the logarithmic scale of Figure 5 magnifies small differences of resistivity among the conditioned ashes (i.e., when their resistivities are 68
Environmental Science & Technology
PotentLb:,
volts
Figure 2. Typical measured current vs. potential imposed on fly ash layer
-
4000 v o l t s , taBh 1 3 9 Y SO3 = 2 5 - 3 5
30 31 32
-
ppm
Portland Titus I T i t u s TI
5x10'
1
6
40
ao
120
200
310
360
440
Time, man
Figure 3. Conditioning behavior of different fly ashes, current vs. time low). In the graphs that follow, only the arithmetic scale will be used. Figure 6 (deposited with the ACS Microfilm Depository Service) provides yet another similar comparison of the conditioning behavior of three additional fly ashes. Figure 7 shows the decrease in resistivity due to conditioning with SO2 in combination with a catalyst. Here, experiments 38 and 47 show that the conditioning behavior with SO2 t catalyst is comparable to the experiments done with SO3 alone, as shown in previous graphs. Especially noteworthy is experiment 42 which shows the effects of switching the flow of SOz on and off; a t the start of this experiment, the fly ash was maintained initially at 205 OC-a temperature too hot for good conditioning with the selected amount of SO3. At the point designated t d , the SO2 flow was shut off and the ash
-
tash
139'C
SO3
2 5 - 3 5 ppm
H20
26
28 29
36
= 8.5%
- Homer C i t y , 4000 volts - Keystone, 4000 volts - m n t o u r , 4000 volts - Front S t r e e t , volts 5000
Figure 8. Comparison of conditioning by SO3 to conditioning with SO2 4- catalyst, resistivity (arithmetic scale) vs. time
0
100
50
200
+
300
T i m e , min
Figure 4. Conditioning behavior of different fly ashes, resistivity
(arithmetic scale) vs. time
. P
1 Ill0
120
ti-,
ture after this (at point t d ) had no manifest effect until the SO2was again switched on (about t = 240 min) whereupon the resistivity fell to values still lower than before, thereby reflecting the lowered temperature as well. Experiments 43 and 44, in which SO3 from an external source was used in large concentrations (340 ppm) as a conditioning agent, are also plotted on Figure 8 for direct comparison with the conditioning of experiments 39 and 45 caused by SO2 catalyst. A comparison of experiments 43 and 44 also shows that increasing the moisture content from 9 to 17% has no material effect on the conditioning; however, substantial decrease in moisture content in the region below 9% can adversely affect the conditioning. Other experiments, not shown herein because of space limitations, show that catalyst alone (without SO2) is ineffective and that several different commercially available catalysts give comparable results. Some general conclusions may be drawn from the various experiments described above. Commercially available catalysts may be used to convert low concentrations of SO2 (-200-1000 ppm) in flue gas to SO3, and the SO3 thereby produced is effective in conditioning fly ash; the greater the concentration of SO2 in the flue gas, the greater the concentration of SO3 produced. It has been clearly established that both the catalyst and the SO2 are required to produce the SO3 required for Conditioning. The greater the temperature of the fly ash, the less effective is such conditioning; this result is in essential agreement with known conditioning phenomena. It is possible to extrapolate the experimental results reported above to a typical coal-burning power plant t o estimate the technical feasibility of in situ catalytic conditioning. There is not space here to show the calculations, but the result is that a volume of only 1 m3 of catalyst should be sufficient to condition all the fly ash from a 100-MW coal-fired power plant. Of course, this is a modest amount of catalyst for a large generating station, but it must be remembered that it will be impossible to cause the fly ash-laden flue gas to flow through a bed of catalyst pellets or spheres since such a bed would probably soon become fouled with fly ash and develop a large pressure drop and plug. Instead, it will be necessary to deploy the catalyst in a way that will prevent accumulation of fly ash by the catalyst. Another alternative would be to prefilter and catalytically process only a portion of the flue gas which would then be mixed back onto the total flow of flue gas to produce the required SO3 concentration. Catalytic conditioning by processing such a slip stream has been previously prescribed (IO). In processing such a slip stream, more SO3 could be produced by simply increasing the residence time of the flue gas in the catalyst bed; in the experiments described above,
140
mi"
Figure 7. Conditioning by SOz in combination with catalyst, resistivity (arithmetic scale) vs. time
temperature was decreased simultaneously; this caused the resistivity to increase immediately, but it is impossible to attribute this increase in resistivity with certainty to either lowered SO3 content or decreased temperature. The SO2 was switched on again when the resistivity had reached about 16 X 10'0 R-cm, and the resistivity then began to decrease quickly; when it had fallen to about 7 X 1O1O, the SO2 was again switched off, resulting in another increase in resistivity. Similar behavior was also observed during experiment 39 in which the catalyst was introduced (point C) into the pipe some minutes after the flow of SO2 was started. Immediately after the catalyst was introduced, the resistivity fell; some time later the SO2 was switched off, thereby causing an increase in resistivity. Experiment 45 began with SO2 flowing and catalyst in the pipe; after the resistivity had fallen and begun to approach a constant value, the SO2 was switched off, thereby causing an increase in resistivity. Lowering the ash tempera-
Volume 11, Number
1, January 1977
69
the residence time in the catalyst bed was only about 0.003 s, about 270 times less than the 0.8 s in a typical SO2 converter for sulfuric acid manufacture. The results and considerations reported above suggest that in the future it may be possible to avoid adding sulfur oxides from external sources to condition fly ash. The use of internal, in situ catalytic conditioning appears to offer considerable promise.
Acknowledgment N. J. Weinstein and H. Hall have shown great interest in this project and contributed many helpful suggestions. Literature Cited (1) White, H. J., Air Repair, 3,79 (Nov. 1953). (2) Schrader, K., Combustion, 22 (Oct. 1970). (3) Electr. World, 22 (June 29, 1970). (4) White, H. J., J. Air Pollut. Control Assoc., 24 (4), 314 (April 1974).
(, 5,) Test Code for Determining the Prouerties of Fine Particulate Matter, ASME, PTC 28,196y5. ( 6 ) Method 8, Fed. Reaist.. 36 (247) (Dec. 23.1971). (7) Fritz, J. S., Yamamura, S. S., Anal. Chem., 27 (9), 1461 (1955).
(8) Corbett, P. F., J . Inst. Fuel, 247 (Nov. 1957). (9) Terraglio, F. P., Manganelli, R. M., Anal. Chem., 34 (6), 675 (1962). (10) Roberts, L. M., U.S. Patent 3,581,463 (June 1,1971). (11) “A Study of Resistivity and Conditioning of Fly Ash”, p 113,
Final Report to EPA Office of Air Programs Under Contract CPA-70-149,Southern Research Institute, Birmingham, Ala., Feb. 1972.
Received for review February 12,1976. Accepted J u l y 28,1976. Financial support provided by the Middle Atlantic Power Research Committee and the National Science Foundation under Grant No. GK-38188.
Supplementary Material Available. Experimental details, ( 4 pages), Table I ( 1 page, details of almost 35 experiments),and Figures 5 and 6 ( 2pages, conditioning behaviors of different fly ashes) will appear following these pages i n the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper only or microfiche (105 X 148 m m , 24X reduction, negatiues) containing all of the supplementary material for the papers in this issue may be obtained from the Business Operations Office, Journals Department, American Chemical Society, 1155 16th S t . , N . W., Washington, D.C. 20036. Remit check O F money order for $4.00 for photocopy or $2.50 for microfiche, referring to code number ES&T- 77-67.
Potential Hazards Associated with Spray Drying Operations Joseph Epstein’, George T. Davis, Leslie Eng, and Mary M. Demek Environmental Research Division, Headquarters, Edgewood Arsenal, Aberdeen Proving Grounds, Md. 21010
To illustrate the potential hazards in spray drying of salt solutions, the mechanism of formation of the organophosphorus nerve gas, GB, from the spray drying of solutions of GB fully decontaminated by reaction with sodium hydroxide is described. GB is formed in two phases of the spray drying operation: the distillation of the aqueous phase and the thermal decomposition of the salts remaining after all water has been removed. In both phases the formation of GB takes place according to the equation:
/O P-OH /
H3c\ H,C,
+ HF
GB
+
H20
:HCO’ H,C’ The acidity is provided by carbon dioxide from the burning of the natural gas and/or fuel oil used to provide heat for spray drying, In the case of the fuel oil, sulfur dioxide is also emitted. This provides even more acidity to the solution and produces more GB. The mechanistic studies provide the bases for minimizing the production of GB.
Spray drying of solutions of salts for the removal of water is a common engineering practice. The possible hazard associated with formation of physiologically active materials in spray drying operations has not, to the writers’ knowledge, been cited. In studies on the spray drying of completely detoxified solutions of the toxic nerve gas, isopropyl methylphosphonofluoridate (GB), it was discovered that GB was being resynthesized in extremely small, but still detectable, amounts. Although the quantities formed were extremely low and only emitted into the atmosphere in permissible concentrations, the mechanism of formation was of interest, since a knowledge 70
Environmental Science & Technology
of the mechanism could form the bases for sound engineering practices which would ensure that only concentrations a t permissible levels were being emitted into the atmosphere. In this paper we present the mechanism of GB formation from completely detoxified solutions. The mechanistic studies suggest that there is a potential of formation of relatively large quantities of physiologically active materials such as anhydrides and acyl halides in the spray drying of some salts.
T h e Process Under S t u d y The demilitarization of GB consists of a decontamination of the toxic material by hydrolysis in strong aqueous alkali followed by spray drying of the resulting brine solution. The decontaminated brine contains a precipitate consisting mainly of sodium fluoride with some of the sodium salt of isopropyl methylphosphonic acid (IMPA). The solution contains, in addition to the sodium salts of IMPA and hydrogen fluoride, unused sodium hydroxide (1.5-4%), some dissolved tributyl amine (which had been added previously to prevent GB deterioration), and diisopropyl methylphosphonate, a common impurity in the manufacture of GB. The effluent from the spray drying operation, after water scrubbing, is emitted into the air, which is continuously sampled and monitored for anticholinesterase activity (1).The concentration of an anticholinesterase in the air stream must never exceed 0.0003 pg/l. (0.3 ng/l.). It was established by mass spectral analysis of air samples, mass spectral analyses of material isolated from aqueous collection bubbler samples, and hydrolysis rate characteristics of the anticholinesterase collected and isolated from the bubbler that the anticholinesterase was GB ( 2 ) . Suggestions that the GB expelled into the atmosphere was from residual GB dissolved in the “decontaminated” solution or occluded in the solids were rejected by the authors in that concentrations as high as 0.25 pg/l. in the brine solution (analyses of the GB brine showed less than 0.25 pg/l.; in our
