Collecting High Resistivity Dusts and Fumes Laboratory Performance of a Special Two-Stage Precipitator WAYNE T. SPROULL Wesfern Precipifafion Corp., Los Aogefes, Calif.
In a two-stage precipitator, the aerosol is deposited in the second stage on electrodes designed to minimize gaseous ionization and corona current. Theoretically, this should be advantageous in precipitating high resistivity material. In order to realize this advantage in practice, however, three modifications from conventional design are necessary, judging from the experimental and pilot plant results. The results indicate that a two-stage precipitator with these modifications may offer an alternative to the practice of conditioning the gases to reduce the resistivity of the collected material to 10" ohm-cm. or less. The theory underlying these experiments is outlined.
T
HE difficulty encountered in operating an electrical precipi-
tator when the resistivity of the dust deposit on the collecting electrodes is too high, is well known. A paper on the subject by Wolcott (IS) was published as early as 1918. This problem is still an important consideration for the designer and the operator of commercial precipitators collecting aerosols from industrial gases ( 3 , 12). If the electrical resistivity of the material being collected becomes too high, the usual procedure is to reduce it by conditioning the gases. This can be done in some cases by adding moisture. Other possibilities include altering the temperature or adding agents such as acid mist or ammonia in low concentrations. Sometimes this is difficult or impractical for various reasons.
Drift Velocity I n order to measure the effect on precipitator performance of such variables as gas temperature or moisture content, or the resistivity of the precipitated dust layer, a mathematical criterion of precipitator performance is necessary. Such a criterion is the quantity known as the drift velocity, w, in the Deutsch ( 2 ) equation F = l-e-wf
(1)
where F is the fractional efficiency of the precipitator, f is its specific collecting area, which is defined as the total area of the collecting electrodes divided by the gas flow rate, and e is the Napierian log base. Thus, if the precipitator has 70,000 square feet of collecting electrode area and is handling 10,000 cubic feet of gas per second, its specific collecting area f is 7 square feet per cubic feet per second. One may convert this to metric units by multiplying by the number of feet in a meter, namely 3.28, obtainingf = 7 X 3.28 = 23 square meters per cubic meters per second. The importance of the drift velocity w is that it is a direct criterion of the performance of a particular precipitator handling a particular material under particular conditions. For example, suppose a particular precipitator, operating a t a gas flow rate equal to its design value, yields a drift velocity, 20, of 15 cm. per second while collecting dust from gas containing 15% moisture by volume a t 350' F. If w drops to 7.5 cm. per second when the moisture content of the gas drops to 5'35, it follows that the size of the precipitator must be approximately doubled if its efficiency is to be maintained a t its former level while operating a t the same gas flow rate on the drier gas.
940
For collecting materials with resistivity
loll-
1013ohm.-cm.
try a two-stage precipitator, modified by
. ..the abnormal electrical clearance in first stage ... high tension collecting electrodes, for minimum re-entraindesigned ment, in the second stage
. . .reversal of
high polarity, at oneminute intervals in the second stage
Physically, the drift velocity, w, is the average velocity a t which the aerosol particles drift toward the collecting electrode while they are passing through the electric field inside the precipitator. Since the electric field EOnear the high voltage discharge wires or rods is stronger than the electric field E near the collecting electrodes, the drift velocity would not be constant for a given particle, even if the gas were free of turbulence. The presence of turbulence contributes further t o the wide variation of the drift velocities of the ,individual particles. Moreover, the larger particles have a higher drift velocity than the smaller ones, as may be noted from Equation 4. T h e drift velocity, w, appearing in the Deutsch Equation 1, is an average value. Considering an individual particle, however, its drift velocity WI relative to the gas in which i t is suspended, may be derived from Stokes' law E!l w1 = Bxrq where p is the electric charge on the particle in centimeter-gramsecond electrostatic units, T is its radius in centimeters, assuming a spherical shape, 7 is the viscosity of the gas in poises, and E is the electric field strength near the collecting electrode in statvolts per centimeter.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 47, No. 5
Both E and EOin Equations 4 and 6 are limited by the sparkover voltage of the precipitator, and this, in turn, is inverselyproportional t o the absolute temperature. Thus, if the gas temperature is doubled, for example, by increasing it from 560"
crease the viscosity of the flue gas from a value of 180 to 300 micropoises, thus still further reducing w. These effects alone are therefore sufficient to reduce the drift velocity from a value such as 30 cni. per second at 100' F. to a value of 5 cm. per second at 600" F. If the gas flow rate were the same for both cases, this could reduce the efficiency from a value such as 98.5% at 100" to about 50% at 660" F. In a practical case, this increase in temperature could logically be expected t o double the gas flow rate because of gas expansion. If this were the case, then the difference in precipitator performance a t the two temperatures would be still greater. On the other hand, adding moisture t o the flue gas will increase the sparkover voltage considerably. At 700" F., for instance, the sparkover voltage of a precipitator can be nearly doubled by increasing the moisture content of the flue gases from 5 t o 30% by volume. These effects are characteristic of the precipitator itself, for thus far it has been assumed that the dust or aerosol has the same physical properties a t any temperature or humidity. Electrical Resistivity of Precipitated Material and Its Effect on Precipitator Operation The electrical resistivity of the dust, however, is a property which does vary enormously with changes in temperature. As stated in an earlier paper (ZO), the experimental resistivity of the precipitated dust layer usually varies by a€actorof about 1,000,000 between 100' and 250' F., or between 250 O and 700' F., usually reaching a maximum value between 150" and 300" F. as shown in Figure 1. On the cool side of this peak, the resistivity may vary by factors as great as 100 times at a given temperature when the moisture content of the gas changes from 5% by volume to lo%, for example. It was explained in the earlier paper how the layer of dust precipitated on the collecting electrode will act as an insulating coat when its resistivity exceeds lo1' ohm-cm., and this causes an May 1955
5
\
I II
I Figure 1.
Typical resistivity graph
For fume from Dwight-Lloyd sintering machines a t a l e a d smelter
objectionable electric charge to accumulate on the exposed surface of the dust layer as the ions, electrons, and charged aerosol particles rain down on it. Under these conditions, the dust layer is usually subjected to a voltage difference of several kilovolts between its upper and lower surface, and since its thickness is ordinarily only a small fraction of an inch, the tiny gas pockets between the particles in the layer break down, and the resulting electrical discharge within the layer produces a copious supply of ions both positive and negative, at the surface where the aerosol is supposed t o precipitate. This seriously impairs the precipitation process. If, in an experiment, the resistivity of the dust layer is increased gradually from loLoto 10" ohm-cm., a conventional precipitator will show a considerably reduced sparking voltage, causing the operator or the regulator device to reduce the operating voltage to avoid excessive sparking. This downward adjustment of the voltage may have to go so far that the corana discharge at the high voltage electrodes practically ceases, this fact being indicated by a severe reduction in the high voltage current. As the resistivity is further increased (by reducing the moisture content of the gas, for example) to values as high as 1OI2 or l O I 3 ohm-cm., the ionization a t the collecting electrode dust-coated surface becomes so intense that the current flowing to the precipitator will increase to values far above those for normal operation. In this case the sparking ceases, but the high voltage will be considerably below the values attainable in normal operation. As an example, an experiment recently conducted in our laboratory precipitator (Figure 2) yielded a drift velocity of 14.8 cm. per second for a test dust precipitated from gas at 350' F. containing 6% moisture by volume as it flowed through the electrode system a t a velocity of 6.3 feet per second. When the gas temper-
INDUSTRIAL AND ENGINEERING CHEMISTRY
94 1
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT rate at which charge is transported by the precipitating dust or aerosol, per unit area of dust layer surfa'ce. Therefore, theelimination of the ionic and electronic bombardment of t h e dust layer should relieve t h e situation considerably. At f i s t glance, i t would appear that this could be accomplished by merely substituting a two-stage precipitator for the single-stage design. Two-stage precipitators have been used for years (8) for air cleaning applications, and a few have been built for industrial gas cleaning. However, difficulties arise when a n ordinary two-stage precipitator is used for such a purpose. I n the first place, the ionic and electronic bombardFigure 2. Experimental precipitator test station ment of the dust layer is eliminated only in the second stage. I n the first stage, which is intended to charge the aerosol particles, there are high ature was reduced t o 220' F., and the moisture content was reresistivity difficulties. The negative ions and electrons from the duced to 1.5%, the drift velocity fell to 3.5 cm. per second. Such high voltage corona discharge rain on the dust layer which prea reduction in temperature would normally increase the drift cipitates on the grounded electrodes, producing so-called backvelocity, instead of reducing it. However; in this case, 220' F. is ionization or back-discharge there (11). This, in turn, produces near the peak of the resistivity curve for the test dust, and its a plentiful supply of positive ions which impart a positive charge resistivity exceeds 1011 ohm-cm. at this temperature, forcing a to a significant fraction of the aerosol particles. The sparking reduction of the average voltage from 25 t o 20 kv. I n this parvoltage may decrease until it approaches the corona threshold, ticular case, the precipitator operation went t o the extreme resisthus preventing the gaseous discharge needed to charge the partivity region where the current rose from its normal value of 10 ticles. I n order t o overcome these difficulties, however, the twomilliamperes, to 28 milliamperes. stage design may be modified advantageously in two ways. When the gases, at 220" F., were conditioned by adding sulfur dioxide in a concentration of 0.5%, the drift velocity increased 1. The clearance or distance between t h e high voltage and from 3.5 to 5.0 cm. per second, which is still far below the 14.8cm. grounded electrodes in t h e first stage may be increased until it is substantially greater than t h e usual clearance in single-stage per second value attained a t 350" F. with 6% moisture in the gas. precipitators. This improves the electrical stability of t h e first The sulfur dioxide conditioning here brought the resistivity of the stage and makes it possible t o maintain a satisfactory corona dust layer down t o a value around 1011 ohm-cm., and precipitator current under high resistivity conditions. operation was characterized by excessive sparking a t 22 kilovolts. 2. The second stage should be designed t o collect positively charged dust as well as negatively charged dust. This demonstrates that conditioning helps but in some cases does not necessarily bring the operation below the critical resistivity As a n example of the first modification, one might specify a borderline. clearance of 6 inches in the first stage and 4 inches in the second From experiments of this sort, it is possible to plot a curve of the stage. The second modification can be accomplished by protype designated A A in Figure 3, for a n aerosollike cement dust; viding the second stage with high voltage electrodes of the pocket but such a graph is somewhat ambiguous. It implies that the or screen type, similar to the grounded electrodes ( I ). resistivity of the dust can be varied while other conditions are I n designing a single-stage precipitator, it is ordinarily assumed held constant. However, the only way t o vary the resistivity is that all the dust will be negatively charged, and no provision is by adding moisture or conditioning agents, or by changing the made for collecting positively charged dust. I n fact, the only temperature, and such measures may vary the drift velocity in place where it could precipitate in a conventional single-stage ways not connected with the resistivity at all. Another ambiguity is evident from Figure 1. With 10% moisture in the gas, the resistivity of this particular material (lead fume) is the same at 155' as it is at 510' F. Yet the drift velocity and precipitator performance should be considerably better a t 155' than at 510" F., even though the resistivity happens t o be the same at both temperatures in this case. I n spite of such ambiguities, however, the curve A A in Figure 3 presents a general picture of the way the performance of a singlestage precipitator passes through a transition from good below 10'0 ohm-cm. to poor above 10" ohm-cm.
Special Two-Stage Precipitator for Minimizing Effects of High Resistivity The performance drop shown by curve A A in Figure 3 is caused by the excessive electrical charge building u p on the exposed dust layer surface on the collecting electrodes. This charge builds u p from two sources (IO). 1. The rain of negatively charged dust on t h e dust layer surface. 2. The rain of ions and electrons on t h e dust layer, which ordinarily transport electrical charge a t a rate several times t h e 942
0
1
i
I
OAPPlRENT RESISTIVITY
i
OF MATERIAL AS!PllECIPITA~D-OnM.ELNTIMLTERII
io9 io' IO" IO" Io' Figure 3. Typical curves obtained from laboratory tests AA = single-stage precipitator collecting (lime rock) test dust. 66 = special two-stage precipitator IO'
IO'
10'
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 5
AIR POLLUTION design is on the discharge rods or wires which usually have a surface area less than 1% of the area of the collecting electrodes where the negatively charged dust precipitates. Furthermore, the designer usually depends t o a certain extent on the rain of negative ions and electrons t o make the negative dust adhere t o the grounded collecting electrodes and thus minimize the tendency for the dust t o blow off and be re-entrained in the gas stream. Returning t o the modified two-stage design under discussion, it is important t h a t the electrodes in the second stage be free of any sharp edges, corners, or points, because the aim is to eliminate a n y corona discharge in the second stage. This suggests the use of smooth flat plates with well-rounded edges. Such electrodes are unable to retain t h e dust properly after it is precipitated, however, especially when there is no longer any ionic rain t o make i t adhere. One solution is a compromise in the form of a pockettype electrode without projecting edges, or a single or double screen electrode. This applies t o the high voltage system as well as t o the grounded system. The word "screen" here includes not only wire screen but also other materials which are electrically equivalent, such as expanded metal. Figure 4 illustrates one type of electrode which performed well in these tests. Finally, one more problem is encountered if the deposited dust layer has very high resistivity, such as 1012 or 1013 ohm-cm. The voltage which builds u p on the dust layer surface is a function not only of the resistivity and thickness of the layer and the current density b u t also is a function of the time (10). These experiments have confirmed this statement. When a two-stage precipitator built according t o these principles was operated with very high resistivity dust such as powdered lime rock a t 220" F., at a moisture content of less than 2% in the gas, the high voltage current t o the second stage was initially less than 1 milliampere. This gradually increased until after 40 or 50 seconds it amounted to 4 or 5 milliamperes, followed by sparking and decrease of the voltage. This phenomenon is probably in part due to the charged dust particles raining on the precipitated dust layer, and in part due t o the existence of a slight ionic current through the gas, despite the use of nondischarging electrodes. When the resistivity is high enough, these two factors will produce excessive charging of the dust layer, despite the use of nondischarging electrodes. This difficulty is eliminated b y reversing the polarity of the high voltage applied t o the second stage at suitable intervals (9). I n these experiments, a sequence such as 10 seconds positive and 50 seconds negative for each minute was suitable for the precipitation of the rock dust unde'r these conditions, where its resistivity is about ohm-cm. The reversing sequence is easily set u p as desired by means of the clock-controlled reversing switch shown in Figure 5. Details of Operation of Special Two-Stage Precipitator Several questions remain to be considered. Should the polarity always be p u t through a reversing cycle once each minute? Should both stages be energized from the same power supply? If so, what is the effect of operating the first (charging) stage on positive polarity part of the time? How about operating - the two stages on separate power supplies? These experiments indicate that if t h e resistivity of the aerosol does not
May 1955
Figure 4.
Second-stage electrode system
exceed lo1' ohm-cm., the polarity need not be reversed at all. The elimination of the rain of negative ions and electrons is a n advantage sufficient to permit continuous operation a t this resistivity with negative direct current. If the resistivity of the aerosol reaches ohm-cm., then it is impossible to maintain the voltage on the second stage at its usual high value unless the polarity is p u t through a cycle (from negative t o positive and back to negative) about once each minute. With such reversal, the average voltage, as read by a kilovoltmeter, could be maintained at about 33 kilovolts with a test dust when its resistivity was close t o 10l2 ohm-cm., the electrical clearance in the second stage being 4 inches. Without polarity reversal, the average voltage under these conditions drops t o a value of about 20 kilovolts, meanwhile a heavy corona discharge develops. At a resistivity of 1OIs ohm-cm., it is necessary t o p u t the polarity through a cycle as often as once every 5 or 10 seconds, in order to maintain the voltage on the second stage at the usual high value. This is the upper resistivity limit t h a t can be handled with the equipment t h a t has been tested. Both stages may be energized from' the same power supply, if the gas temperature is above 400" F. Electrically, the characteristics of the first stage resemble those of a point-to-plane gap, At room temperature such a gap behaves very differently with the point positive than with the point negative. However, these experiments show t h a t this difference gets smaller and smaller as the temperature increases, until at 700" F. the difference is practically negligible. Therefore, one may arbitrarily pick 400 F. as the temperature where the inferiority of positive polarity becomes too small t o justify a n extra power supply for the first stage. I n fact, above 400' F., results with high voltage alternating current on the first stage are practically as good as those with negative direct current. Below 400' F., the first stage should be energized with negative direct current. This borderline, being arbitrary, is not sharp. Alternating current might be used at 375" F., for the first stage, - . for example, without Figure 5. Magnetic reversing switch serious loss in efficiency. Alternating (above), controlled by electric clock current is not suitable for the second stage, however, under any conditions. (below) O
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
943
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT Separate power supplies for the two stages are advisable if the precipitator must handle high resistivity material ( 10l2 ohm-cm. or higher) a t low temperatures such as 250' F.
exceeds ohm-em., the special two-stage design now being developed may offer important advantages.
Acknowledgment Conclusions Figure 3 is somewhat ambiguous because precipitator performance is strongly affected by such variables as gas temperature, irrespective of the resistivity of the dust. Within the limitations of this rather loose interpretation, then, the curve BB in Figure 3 is the best representation available to illustrate the performance of the modified two-stage design described. If the resistivity of the material, as precipitated, is below 10'0 ohm-em., or if it can be reduced below this value easily and cheaply by conditioning the gas, then a single-stage precipitator offers certain advantages. On the other hand, if the resistivity of the material as precipitated is between 10" and 10lsohm-cm. most of the time, then the new design incorporating the special features described offers certain advantages, judging from laboratory and pilot scale tests. Single-stage designs offer two advantages over the two-stage design when the resistivity of the material being collected is below 10'0 ohm-cm.
1. T h e aerosol is continuously charged all t h e way through the precipitator, and thus each particle carries a maximum charge all t h e time, whereas with t h e two-stage design, t h e aerosol is charged only intermittently as i t passes through the first stage or stages, assuming a plurality of precipitator sections in series. 2. After the dust is precipitated, t h e rain of negative ions and electrons makes i t adhere t o t h e collecting electrode, reducing erosion or re-entrainment losses. However, when the resistivity of the material, as precipitated,
The assistance of others in performing the experimental work was essential, and is gratefully acknowledged by the author. Special mention is due to Y. Naltada, W. J. Homan, H. Copeland, J. D. Roehr, and E. R. Johnsen.
Literature Cited Anderson, E. (Western Precipitation Corp.), U. S. Patent 2,275,001 (March 3, 1942). Deutsch, W., Ann. Physik, 68, 343 (1922); 9, 262 (1931). Heinrich, D. O., Eng. and Boiler House Rev. (June 1953). Reprints available from Simon-Carves, Ltd., Cheadle Heath, Cheshire, England. (4)Ladenburg, R., Ann. Physik (5), 4, 863 (1930). (5) LaMer, V. K., "Filtration of Monodisperse Electrically Charged Aerosols," Columbia University Report NYO-514, pt. 111, 6, June 30, 1952. ( 6 ) Lipscomb, W. N., Rubin, T. R., and Sturdivant, J. H., J. A p p l .
Phys., 18, 78 (1947). (7) Rohmann, H., 2. Phys., 17, 253 (1923). (8) Schmidt, W. A. (International Precipitation Co.), U. S. Patent 1,343,285 (June 15, 1920). (9) Siemens-Lurgi-Lodge-Cottrell, British Patent 398,724 (Sept. 21, 1933). (10) Sproull, W. T., and Nakada, Y., IND. ENG.CHEM.,43, 1350 (1951). (11) White, H. J. (Research Corp.), U. S. Patent 2,249,801 (July 22. 19411. (12) White, H. J., Roberts, L. M., and Hedberg, C. W., Mech. Eng., 72, 873 (1950). (13) Wolcott, E. R., Phys. Rev., 12, 284 (1918). RECEIVED for review September 1 , 1954.
ACCEPTED December 27, 1954.
Fly Ash Separators for High Pressures and Temperatures J. 1. YELLOTT AND
P. R.
BROADLEY
Locornofive Development Commitfee, Bituminous C o a l Research, Inc., P. 0 . Box 225, Dunkirk,
N. Y.
The development of fly ash separators for use at pressures to five atmospheres and temperatures to 1300" F. has been a vital part of the Locomotive Development Committee program. When bituminous coal is used as the fuel irl open-cycle gas turbines, the combustion products pass through the turbine blades, and high efficiency ash separators must be used to prevent blade erosion. The coal particles are burning as they enter the separator and consequently a continuous blowdown is essential to prevent sintering o f the particles and damage to the separators. The paper describes the test procedure by which continuous discharge separators have been developed with efficiency high enough to prevent turbine blade erosion.
T
HE Locomotive Development Committee (L.D.C.) of Bituminous Coal Research, Inc., is composed of the presidents of the following companies: Baltimore and Ohio Railroad Co.; Chesapeake and Ohio Railway; Illinois Central Railroad; Louisville & Nashville Railroad Co.; New York Central System; New York, Chicago, and St. Louis Railroad Co.; Korfolk and Western Railway Co.; The Pennsylvania Railroad; Virginian Railway; The M. A. Hanna Co.; Island Creek Coal Co.; Pittsburgh Consolidation Coal Co. ; Pocahontas Fuel Co. ; Sinclair Coal Co. Colonel Roy B. White, Chairman of the Board of the Baltimore and Ohio Railroad Co., has been the chairman of the 944
committee since its organization in 1944. The L.D.C. has been engaged since 1946 in developing a direct-fired, open-cycle coalburning gas turbine power plant. The project has progressed from small scale laboratory tests of components t o the point where a 4250 hp. locomotive-type turbine has been operated for more than 1000 hours with bituminous coal as its only fuel (6-7). One of the most important aspects of the project was the development of highly efficient fly ash separation equipment for use a t gas turbine operating conditions-1300' F. and 75 pounds per square inch absolute. This paper presents some of the results of the separator development program.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No. 5
