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Electrostatic precipitators for large power station boilers
Journal of Electrostatics, 8 (1980) 309--324
309
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
ELECTROSTATIC PRECIPITATORS F O R L A R G E POWER STATION BOILERS*
J. DALMON
Central Electricity Research Laboratories, Leatherhead, Surrey (Gt. Britain) (Received February 19, 1979; accepted in revised form July 16, 1979)
Summary The basic principles of electrostatic precipitation are given and it is shown how these apply to large installations collecting ash from pulverized fuel fired boilers in power stations. The problems of meeting the current emission limits of 115 mg/m 3 are discussed. Some new ideas which are being developed to improve precipitator performance are described.
1. Principles The separation of particles from a gas stream in an electrostatic precipitator is accomplished by giving the particles a charge, then passing them through an electric field where they are accelerated towards an oppositely charged collecting surface by coulomb forces. The captured particles are temporarily retained on the collector until a layer is formed. This layer is then periodically transferred out of the gas stream into a hopper. Thus, the process may be conveniently considered in four stages. These are: (1) Charging. (2) Transport. (3) Deposition. (4) Removal. 1. I Charging Particles in a gas stream normally carry an electric charge. However, this is t o o small by several orders of magnitude for effective electrostatic precipitation. Charging is normally achieved b y passing the particles through a stream of ions originating from a high voltage, direct current corona. The corona is usually of negative polarity, as this gives a lower inception voltage and a higher sparkover voltage than positive corona for most gases in need of cleaning. Thus, the effective operating regime for negative corona is wider. The corona source *Presented at The Institute of Physics (Static Electrification Group) Meeting, Electrostatic Filters and Precipitators, London, December 13, 1978.
310 is usually a small diameter wire, or rather, many wires strung between parallel, earthed plates. Two distinct particle charging mechanisms are recognized: firstly, ion bombardment, which is the dominant mechanism in most applications, and, secondly, diffusion charging, which is important only for very small particles. In the b o m b a r d m e n t process, ions travel along field lines and attach to particles. Eventually, the charge accumulated on the particles will reach a limiting value and further ions will be repelled. The final charge is proportional to both the electric field strength and the surface area of the particles. It is also dependent, to a lesser extent, on the particle permittivity. Diffusion charging is important for particles smaller than about 0.2 ~m in diameter and is the result of random collisions between ions, which share the thermal motions of the gas molecules, and particles suspended in an ionized gas. It is often assumed that the process is independent of the external electric field, but experimental evidence suggests otherwise.
1.2 Transport The velocity with which the particles move towards the collecting electrode is referred to as the drift velocity, or migration velocity and is derived by equating the coulomb forces acting on the particles with the viscous forces due to their motion through the gas. The important points emerging from this relationship are that the larger particles, which have the highest charge, move the fastest and that the greater the field strength the greater the particle migration velocity. 1.3 Deposition Once having arrived at the collecting electrodes, solid particles will adhere by electrostatic and molecular forces. If the particles have a high electrical resistivity, ionic current from the corona discharge will maintain a negative charge on the accumulating dust layer and this will enhance the force of adhesion. However, a positive charge will be induced on conducting particles and particles of low resistivity and this will result in a force tending to repel them back into the gas stream. 1.4 Removal The process of particle accumulation on the collecting surfaces obviously cannot continue indefinitely and steps must be taken to remove the deposited layer out of the gas stream and into a hopper for final disposal. This is achieved by periodically applying an impulse to the collector, the object being to dislodge the layer without significant break-up so that it will fall out of the gas stream as a sheet or as large agglomerates and with the minimum carryover of particles. This process is known as rapping.
311 2. Practice The large electrostatic precipitators used by the C.E.G.B. in their power stations are of the plate type. These take the form of a number of parallel, vertical, earthed plates (or collecting electrodes) through which the dust-laden gas flows horizontally. A row of vertical wires, suspended mid-way between each pair of parallel plates, and at a negative potential of about 50 kV, serve the dual purpose of providing a corona discharge for particle charging and the electric field for particle transport. Typical gas velocities between the plates are 1.5--2 m/s and the appropriate n u m b e r of plates is arranged in parallel, and divided between 3 or 4 casings to accommodate the gas flow, which for a 500 MW boiler is around 700 m 3/s. Within each casing, groups of plates, known as zones, are arranged in series to give a treatment length sufficient for the required dust collection efficiency. Each zone is electrically and mechanicaliy independent. Typical dimensions are: 250 m m between plates, plate heights up to 12 m, plate length 3 m per zone with up to 4 zones in series. The total number of plates in parallel, divided over the 3 or 4 casings, is around 180. The total collecting area is about 4 hectares (10 acres). An average 500 MW boiler will burn some 150 t of coal per hour at full load. The ash content of the coal may be, say, 16%. Thus some 25 t of ash per hour will be produced. Around 80--85% of this ash is carried out of the furnace to the precipitators, which must reduce the load carried by the combustion gases to a concentration below 115 mg/m 3 at S.T.P. as required by the Alkali Inspectorate. This represents an efficiency, on a weight basis, of over 99.3%. 3. Problems In order to achieve this very high collection efficiency, careful attention to detail design is essential, followed by equally careful erection. To maintain the efficiency consistently over long periods of time also requires high standards of maintenance. Large generating units normally run continuously for m a n y months between planned overhaul periods, and unscheduled shut-downs to rectify electrostatic precipitator faults can be very costly. 3.1 Gas distribution One of the major design problems affecting efficiency is the requirement to distribute the enormous quantity of gas being treated evenly over the crosssection of the precipitator. The "effective migration velocity" (e.m.v.), i.e, the net velocity with which particles are transported to the collecting surfaces, is related to the gas velocity, and the optimum value of the gas velocity is 1.8--2 m/s [1]. In parts of the precipitator where the gas velocity is too low, particles may not be carried by the turbulent motion of the gas to the region of high field near the discharge electrodes where they acquire their m a x i m u m
312
charge; consequently, collection is inefficient. On the other hand, high velocities cause scouring of the collected dust layer and particles are re-entrained back into the gas stream. It can be shown that regions of low flow do n o t compensate for regions of high flow and, thus, uneven gas flow means that performance suffers. Flow model tests of projected designs are therefore carried o u t b y manufacturers and are followed by full-scale measurements (on air) when the plant is installed to allow for final trimming by means of guide vanes.
3.2 Rapping Another problem area is rapping. The objective, of course, is to detach the dust layer with minimum breakup and despatch it to the hopper w i t h o u t reentrainment of particles. C.E.R.L. laboratory investigations have shown that the most effective way of delivering the rapping blow is in the shear direction, i.e. in the plane of the plate and that the fracture takes place within the dust layer, parallel and fairly close to the electrode. With no corona discharge an impulse of ~ 15 g is sufficient to remove most normal dusts from an electrode and the finer the dust the higher the force required. (see Table 1). In practice, rapping is carried out with the precipitator on stream and with full high voltage applied. In this situation, the force necessary to remove the dust layer is at least doubled and with difficult dusts 60 g is more appropriate. Of course, even with 60g some dust remains on the electrodes and the finest particles will not be removed by 1000 g. The rapping impulse is usually applied by some form of mechanical hammer acting on an anvil attached to the electrode support frame. To allow for attenuation in joints and to ensure that the specified 'g' value is attained over all the plate surface, the hammer impact must be considerably higher than 60 g. 3. 3 Dust resistivity and conditioning A consequence of collecting dust in an electric field is that the dust layer itself can create problems by disturbing the stability of the corona discharge. TABLE1 A c c e l e r a t i o n r e q u i r e d t o r e m o v e p r e c i p i t a t e d fly ash f r o m an e l e c t r o d e
Average particle dia (urn)
2.9 8.7 23.3
A c c e l e r a t i o n t o dislodge ash layer (g) Force normal to surface H.T. o f f
F o r c e parallel t o surface H.T. o f f
F o r c e parallel t o surface H.T. o n
> 40 30 21
17 10 2.5
28 32 6
g values are averages o f ~ 12 tests.
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If the dust is highly resistive, that is, above, say, 101° ohm m, then a layer, even a very thin one, on the discharge wires can act as an insulator and can suppress the corona discharge [2]. This adversely affects the particle charging and the collection efficiency. A layer of this dust on the collecting plates can prevent the ionic current leaking away to earth and a high voltage gradient builds up across the dust layer. The layer will eventually break down causing the emission of positive ions into the gas space. This phenomena is k n o w n as back discharge, or back ionization. The positive ions act to reduce the effective particle charge. The break-down in the dust layer also triggers flashover across the inter-electrode gap. Figure 1 shows back ionization spots in a dust layer on a point-plane geometry with flashover whilst Fig.2(a) illustrates the corona glow from the discharge wires of a plate-type pilot precipitator together with back-ionization illuminating the dust surface on the collecting plates. Figure 2(b) acts as a reference. It may be recalled that the charging and collection process is dependent on the applied electric field. Thus, a reduction in flashover voltage causes a reduction in field and a consequent lowering of the e.m.v, and, hence, collection efficiency. The relationship between e.m.v. and flashover is shown in Fig.3 for fly ash treated with various concentrations of conditioning agents (see below) in a pilot precipitator. Now, highly resistive dusts are associated with coals having low sulphur contents, say, below 1.3%. The sulphur in coal produces small amounts of SO3 in the flue gas which combines with moisture to form sulphuric acid. This
Fig. 1. Point-to-plane flashover showing back-ionization glow in a dust layer.
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Fig.2 (a) B a c k - i o n i z a t i o n in a p l a t e - t y p e pilot precipitator. (b) R e f e r e n c e for Fig.2(a). V i e w l o o k i n g u p s t r e a m b e t w e e n electrodes. 0.21 - ADDITIVE O 0.19
0.17
--
SO2
•
HCL
A
SULPHATES
--
A
/
A
0.15 -.;
O • O ~OA ~ ~"A
0.13
0.[I
0.09
~ 42
A 44
A A
A
I
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46
48
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50 52 54 FLASHOVER VOLTAGE, kV
I
I
56
58
Fig.3. Relationship between E.M.V. and flashover voltage for fly ash treated with conditioning agents.
315
coats the particle surfaces and, provided there is enough sulphur in the coal, gives them a surface which is sufficiently conducting to prevent the deleterious effects of high resistivity. About 18% of the coal supplied to the C.E.G.B. has a sulphur content below 1.3% and so could produce a potentially difficult ash.
Methods of overcoming this high resistivity problem include the injection of certain chemicals into the gas stream to coat the particles with a conducting layer. These chemicals are termed conditioning agents. They must be carefully chosen so that they do not cause any obnoxious emissions, are not hazardous to personnel or have any adverse effect upon the plant. Ammonium sulphate solution sprayed into the flue gas has been shown in C.E.R.L. pilot plant experiments and power station trials to be effective [3], and the C.E.R.L. system is being used in a power station in France [4].
3. 4 Discharge electrodes Discharge electrodes are manufactured in a variety of different shapes: round, square plain, square twisted, star, barbed, saw toothed, etc.; and a variety of claims as to their respective merits have been made. However, experiments carried out at C.E.R.L. on a 4-m-long laboratory tubular precipitator showed that, although the voltage--current characteristics of electrode types differed considerably, and particularly so between the spiked and plain wires, for the same power input the e.m.v, was found to be independent of wire shape. (Fig.4). This finding was confirmed by observations in the field, but only for dusts that were not highly resistive. In the case of highly resistive dusts the high current densities produced by barbed types of electrode tend to promote back-ionization. (~
GAS CONDITIONS :- AIR AT 18°C DUST 95 ~m ALUMINIUMOXIDE TUBE DIA. 305rnm
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02
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10 IS 20 SPECIFIC POWER. Wm-2
i
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25
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30 100rnm
4- DISC.CIRCULAR ROD. 6.3ramDIA.
Crs
Fig.4. Relationship between E.M.V. and power per unit area of collector for a tubular experi mental precipitator fitted with various discharge electrodes.
316 Another very important factor in the choice of a discharge electrode is its mechanical strength. The atmosphere in which discharge electrodes operate is corrosive and erosive and they are being constantly subjected to rapping blows which can induce fatigue failures. The failure of but one wire in a precipitator section can, by short circuiting to the collecting plate, put that whole section out of action with a consequent increase in dust emission. This is a serious matter and must be attended to w i t h o u t delay. However, this is not easy as the treatment zone is at a temperature of ~ 130°C and is filled with noxious gases. Shutting down a 500 MW boiler costs in the region of £20,000--£30,000 per day in terms of starting up replacement units, thus a complete shutdown is to be avoided. The problem of removing broken wires w i t h o u t taking the boiler off-load has been overcome by the development of special double isolating dampers, capable of shutting off one of the parallel flows of the precipitator with the boiler on reduced load. Thus, after a suitable cooling and ventilating period, safe access to remove the offending wire is possible.
3. 5 Electrical operating conditions There has been considerable research to find the best operating regime for precipitators and to try to separate the influence of voltage and current. It has been established from both experimental and full-scale plant that performance increases with increases in applied voltage and that the best performance is obtained when operating on the verge of flashover. This is an extremely delicate region, as the electric stress within the precipitator treatment zone is continuously changing with changes in gas composition and dust loading. Consequently the flashover voltage is not a fixed quantity. This poses a control problem, because to obtain o p t i m u m performance the voltage must follow the flashover level and the h.t. set is consequently in constant danger of tripping out. If the setting is left to a plant operator, he will naturally set the voltage control to some comfortable position that will cause him least trouble. However, this procedure will not achieve the best performance for the plant and some automatic system is necessary. One m e t h o d counts the number of flashes over a given period, compares the result with a pre-set figure and adjusts the voltage accordingly. However, this presupposes that the correct rate of flashover is known and t h a t the controller can distinguish between a spark and an arc. A spark may be defined as a transient phenomena lasting perhaps for one or two cycles of the mains supply in contrast to the power arc which is sustained until extinguished by some drastic action, such as interrupting the supply for a sufficient length of time for the ionization products to be removed by the gas flow. The object of the automatic control is, therefore, to allow the voltage to rise sufficiently to sense the sparking level but n o t enough to establish an arc. This can be achieved by thyristor control devices which respond very rapidly, within one half of a mains cycle, to a sparkover in the precipitator. When
317 a spark is detected the voltage applied to the precipitator electrodes is immediately lowered by a pre-set a m o u n t and then is raised slowly over about a second or so until another spark is sensed. This is called a ramp recovery. The procedure is then repeated. If the disturbance continues for longer than a half cycle, then this is designated to be an arc and the voltage is reduced to zero for a few cycles then restored rapidly to intercept the ramp recovery as from a spark. Another m e t h o d of control that is widely used is to seek and maintain the voltage at the peak of the voltage--current characteristic of the precipitator. With this method, the primary voltage is automatically raised a preset a m o u n t periodically, say every 5 s, and the effect on the secondary is monitored. If this results in an increase in secondary voltage a further rise is initiated and the process continues until the peak is reached and a rise in primary volts results in a decrease in secondary volts. This indicates that the peak has been found. The controller then backs off and starts the cycle again. In fact, the control point is very similar to the thyrister controller as the peak occurs where flashing begins. 3. 6 Sectionalization It was mentioned earlier that a precipitator is divided into a number of separate mechanical and electrical sections. This is desirable for the following reasons: (1) The operating voltage of each section is determined by the flashover voltage of its weakest part. Therefore, the more sections there are, the higher will be this voltage. (2) A fault on any one section will be detrimental to only that section. Thus a precipitator composed of many small sections should give an overall performance that is higher than if it were supplied from only one voltage source. Or, to look at it from another point of view, a smaller precipitator may be able to achieve a given efficiency if more rectifier sets feeding separate zones are provided. Now, to maintain a constant efficiency, the e.m.v, must rise in inverse proportion to the decrease in plate area and for extensive sub-division to be economically worthwhile the e.m.v, must increase at a substantially greater rate than this to compensate, not only for the cost of the extra rectifier sets, but also the extra control gear, insulators, wiring and power consumed. To find whether the e.m.v, would increase sufficiently with smaller zone sizes an extensive series of experiments was carried out on a power station precipitator covering a 12--1 range of zone sizes. The flashover level, discharge current and power input all increased with smaller zone sizes and a correspondingly higher e.m.v, was achieved. This is illustrated for specific power and e.m.v, in Fig. 5(a) and 5(b) respectively. However, the rate of rise was insufficient to justify the additional costs of very small sections and a plate area per zone of about 3000 m 2 was calculated to give a reasonable compromise between performance and economy.
318 10
--
VOLTAGE SET TO FLASHOVER LEVEL 8 i
6
O
O
4
2
0
(a)
t I I 2000 4000 6000 COLLECTING PLATE AREA PER RECTIFIER, rn2
I 8000
0,14
0.12 7 : 0.10 uJ 0,08
0.06 (b)
O
I I I 2000 4000 6000 COLLECTING PLATE AREA PER RECTIFIER. rn2
O
I 8000
Fig.5. Effect of zone size on (a) specific power and (b) effective migration velocity, for a power station precipitator. 4. Prototypes The increasingly stringent emission regulations has triggered worldwide research into methods of improving dust collection by electrostatic precipitators. Some of these devices are described briefly in the following sections.
4.1 Low turbulence precipitator It can be shown that if charged particles are subjected to an electric field in laminar flow conditions they will be collected in a finite distance. (As distinct from the exponential rate of collection in the turbulent flow of a normal precipitator). Laminar flow requires a plate spacing of less than 1 cm and is not practicable on a power station scale. However, it has been shown with pilot plant testing at C.E.R.L. that charged particles may be collected with high efficiency in l o w turbulence flow [ 5 ] . This is achieved by having parallel plates spaced some 10 cm apart. These are fitted into the last zone of a 3 zone precipitator and take the place of the normal corona-wire system already described (Fig.6). The plates are alternatively.at a high voltage, or are earthed.
Fig.6.
Flow metering venturi
D~hn~ Dust hopper
Section t Flow straigh~lmer Showil~li hmtlontal
Section through
\
o ~
C.E.R.L. low turbulence precipitator.
In(lucid draught fan
inlet
CIwJstand gas
- -
I
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f ~ dust hopp~
Qllick reklameclamps
low turbulence
High field
Ilerlpex top llllrel
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320
The particles arriving at this final zone are already charged by the preceding corona zones. Experiments have shown that the all-plate final zone has an effective migration velocity between 45% and 140% better than the equivalent final zone with corona wires, depending on the degree of rapping.
4.2 High intensity ionizer This is essentially a device for pre-charging particles before they enter a conventional electrostatic precipitator. It is being developed in the U.S.A. by the Electrical Power Research Institute [6]. It consists basically of a venturi tube with a high voltage electrode in the throat (Fig.7). The electrode is in the form of a sharp edged disc which is INLET CONE INTENSE CORONA
SUPPORT TUBE ~
1
. BULKHEAD
=
~
FLOW
] E
SUPPORT
HV DISCHARGE CATHODE
POROUS ANODE
UST CONE
Fig.7. High intensity ionizer [6].
raised to a potential of 80--100 kV. The diameter of the venturi is around 250--300 mm and intense discharge currents and very high electric fields can be achieved. Values quoted are currents of 2 mA and field strengths of 10--12 kV/cm. This compares with approximately 1 mA for a conventional precipitator zone having a breakdown field of about 3 kV/cm. An array of the devices can be fitted in parallel upstream of an existing precipitator where dusty gas will pass through the venturis at a velocity of a b o u t 25 m/s (80 ft/s). Extensive pilot testing has been carried out and it is claimed that particle charges are 3--5 times higher than in normal precipitators and 40% higher than the maximum predicted by classical theory. This is attributed to very high ion densities and electron charging. The pilot plant testing has shown that particle penetration is reduced by 70% compared with a conventional precipitator. A disadvantage of the high intensity ionizer is that, in spite of the very high gas velocity in the venturi throat, a layer of ash is precipitated. When this ash is highly resistive electrical breakdown occurs at a low voltage and the beneficial effects of pre-charging are diminished. To overcome this, an air-swept anode is being developed in which filtered air is blown through a porous surface to prevent dust accumulating.
321 4.3 Sou thern Research Institu te controlled corona precharger This is being developed in the U.S.A. in an attempt to avoid back-ionization and overcome the problems of effectively charging highly resistive dust [7]. It comprises a mesh screen fixed a b o u t 20 mm from the surface of 30-cm-long earthed plates situated immediately upstream of the first zone of a normal precipitator (Fig.8). Corona discharge wires are fitted in the usual way and are TOSCREEN POWERSUPPLY
TOCORONA POWERSUPPLY TOSCREEN PPLY
Fig. 8. Southern Research Institute pre-charger [ 7 ] . operated at a b o u t 30 kV negative. A bias voltage of around 7 kV negative is applied to the screen and is varied by a feedback circuit from the corona power supply such that it maintains a constant corona current between the discharge wires and the screen. If back-ionization occurs, the positive ions emitted from the dust collected on the earthed plates are intercepted by the grid, and the charging current, consisting of negative ions, in the space between the discharge wires and the grid, is unaltered. The work is still at an earl.v pilot plant stage. 4.4 Masuda pulsed charger In this system, which is being developed in J_apan [ 8 ] , corona originates from a spiked discharge electrode. A third electrode is fixed near the discharge electrode and a pulsed discharge is applied between them (Fig.9). A high d.c. voltage is maintained between the third electrode and the collecting electrode
322 THIRD ELECTRODE
COUNTERELECTRODE ~ / J~ DISCHARGE
Fig.9. Bias controlled pulse charger [ 81. to provide the main field. By suitably biasing the third electrode the discharge current is controlled independently of the main field and back-ionization is said to be prevented. This has reportedly proved successful in the Japanese steel industry.
4.5 The electro-dynamic venturi This originated in France and combines the principles of electrostatic precipitation and venturi scrubbing to form a single, compact dust collecting system [9] (Fig.10). In the E.D.V. process, the gases to be treated are saturated with water in a conditioning tower, passed through the throat of a venturi at velocities of up to 100 m/s where adiabetic expansion takes place and water condenses onto the dust particles. The droplets containing the particles are ionized in a corona discharge during their passage through the venturi tube and are collected b y electrostatic forces onto a curtain of oppositely charged water droplets at the venturi exit. They are then removed as a sludge. Any carryover of water droplets is collected in a simple cyclone. The advantages of the system are a high collection efficiency, which is not adversely affected b y dust properties such as high resistivity and particle size. It is c o m p a c t and simple in operation. The disadvantages are a wet, low temperature plume and a fairly high water consumption. It has been applied operationally to a number of processes including pulverised-fuel-fired boilers. 5. Conclusions
Public awareness has focussed attention on dust emission sources in recent
323
CLEAN GAS1 OUTLET
CLEAN WATER INLET SPRAY NOZZLE
::.:.
DISCHARGE ~l
ELECTRODE
F
DUST+WATEROUT
FINSONCENTRALTUBULAR ELECTRODE
T
DUSTY GASINLET Fig.10. Electro-dynamic venturi [9 ]. years and control regulations have become more stringent. This has led to a renewed interest in the detailed operation of electrostatic precipitators and has provided the incentive for novel developments. The C.E.G.B. continue to carry out research to ensure that the emission limits set by the Alkali and Clean Air Inspectorate are met and is evaluating various novel concepts. The aim is to reduce emissions to a minimum consistent with the reliable, economic production of electricity.
Acknowledgements This work was carried out at the Central Electricity Research Laboratories and this paper is published by permission of the Central Electricity Generating Board.
324
References I J. Dalmon and H.J. Lowe, Experimental investigations into the performance of electrostatic precipitators for P.F. power stations, Proc. Int. Symp. on the Physics of Electrostatic Forces and their Applications, Grenoble, 1960. 2 H.J. Lowe, J. Dalmon and E.T. Hignett, The precipitation of difficult dusts, I.E.E. Colloquium on Electrostatic Precipitators, London, Febr. 19, 1965. 3 J. Dalmon and D. Tidy, A comparison of chemical additives as aids to the electrostatic precipitation of Fly-Ash, Atmos. Environ., 6 (1972) 721. 4 C. Guillon, Improving the efficiency of an electrostatic precipitator by injecting ammonium sulphate in the flue gas, Bull. Inf. Centrales Electriques, No. 85 (1976), p.16. 5 J. Dalmon, The performance of an experimental electrostatic precipitator with a low turbulence zone, Symposium on the Changing Technology of Electrostatic Precipitation, 1974, Institute of Fuel (Australia), Adelaide. 6 O.J. Tassicker and J. Schwab, High-intensity ionizer for improved ESP performance, EPRI J. June/July (1977) 56. 7 D.H. Pontius, P. Vann Bush and W.B. Smith, A new system for electrostatic precipitation of particulate materials having high electrical resistivity, APCA Meeting, Houston, Texas, June 25--29, 1978. 8 S. Masuda, I. Doi, I. Hattori and A. Shibuya. Utility limit and mode of back discharge in bias-controlled pulse charging system, IEEE, 1977, IAS Conf. Record, p. 875. 9 J.F. Vicard, Results and experience with a new high velocity electric precipitator: The electro dynamic venturi, 4th Int. Clean Air Congress-Session V-50, Tokyo, May 19, 1977.
309
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
ELECTROSTATIC PRECIPITATORS F O R L A R G E POWER STATION BOILERS*
J. DALMON
Central Electricity Research Laboratories, Leatherhead, Surrey (Gt. Britain) (Received February 19, 1979; accepted in revised form July 16, 1979)
Summary The basic principles of electrostatic precipitation are given and it is shown how these apply to large installations collecting ash from pulverized fuel fired boilers in power stations. The problems of meeting the current emission limits of 115 mg/m 3 are discussed. Some new ideas which are being developed to improve precipitator performance are described.
1. Principles The separation of particles from a gas stream in an electrostatic precipitator is accomplished by giving the particles a charge, then passing them through an electric field where they are accelerated towards an oppositely charged collecting surface by coulomb forces. The captured particles are temporarily retained on the collector until a layer is formed. This layer is then periodically transferred out of the gas stream into a hopper. Thus, the process may be conveniently considered in four stages. These are: (1) Charging. (2) Transport. (3) Deposition. (4) Removal. 1. I Charging Particles in a gas stream normally carry an electric charge. However, this is t o o small by several orders of magnitude for effective electrostatic precipitation. Charging is normally achieved b y passing the particles through a stream of ions originating from a high voltage, direct current corona. The corona is usually of negative polarity, as this gives a lower inception voltage and a higher sparkover voltage than positive corona for most gases in need of cleaning. Thus, the effective operating regime for negative corona is wider. The corona source *Presented at The Institute of Physics (Static Electrification Group) Meeting, Electrostatic Filters and Precipitators, London, December 13, 1978.
310 is usually a small diameter wire, or rather, many wires strung between parallel, earthed plates. Two distinct particle charging mechanisms are recognized: firstly, ion bombardment, which is the dominant mechanism in most applications, and, secondly, diffusion charging, which is important only for very small particles. In the b o m b a r d m e n t process, ions travel along field lines and attach to particles. Eventually, the charge accumulated on the particles will reach a limiting value and further ions will be repelled. The final charge is proportional to both the electric field strength and the surface area of the particles. It is also dependent, to a lesser extent, on the particle permittivity. Diffusion charging is important for particles smaller than about 0.2 ~m in diameter and is the result of random collisions between ions, which share the thermal motions of the gas molecules, and particles suspended in an ionized gas. It is often assumed that the process is independent of the external electric field, but experimental evidence suggests otherwise.
1.2 Transport The velocity with which the particles move towards the collecting electrode is referred to as the drift velocity, or migration velocity and is derived by equating the coulomb forces acting on the particles with the viscous forces due to their motion through the gas. The important points emerging from this relationship are that the larger particles, which have the highest charge, move the fastest and that the greater the field strength the greater the particle migration velocity. 1.3 Deposition Once having arrived at the collecting electrodes, solid particles will adhere by electrostatic and molecular forces. If the particles have a high electrical resistivity, ionic current from the corona discharge will maintain a negative charge on the accumulating dust layer and this will enhance the force of adhesion. However, a positive charge will be induced on conducting particles and particles of low resistivity and this will result in a force tending to repel them back into the gas stream. 1.4 Removal The process of particle accumulation on the collecting surfaces obviously cannot continue indefinitely and steps must be taken to remove the deposited layer out of the gas stream and into a hopper for final disposal. This is achieved by periodically applying an impulse to the collector, the object being to dislodge the layer without significant break-up so that it will fall out of the gas stream as a sheet or as large agglomerates and with the minimum carryover of particles. This process is known as rapping.
311 2. Practice The large electrostatic precipitators used by the C.E.G.B. in their power stations are of the plate type. These take the form of a number of parallel, vertical, earthed plates (or collecting electrodes) through which the dust-laden gas flows horizontally. A row of vertical wires, suspended mid-way between each pair of parallel plates, and at a negative potential of about 50 kV, serve the dual purpose of providing a corona discharge for particle charging and the electric field for particle transport. Typical gas velocities between the plates are 1.5--2 m/s and the appropriate n u m b e r of plates is arranged in parallel, and divided between 3 or 4 casings to accommodate the gas flow, which for a 500 MW boiler is around 700 m 3/s. Within each casing, groups of plates, known as zones, are arranged in series to give a treatment length sufficient for the required dust collection efficiency. Each zone is electrically and mechanicaliy independent. Typical dimensions are: 250 m m between plates, plate heights up to 12 m, plate length 3 m per zone with up to 4 zones in series. The total number of plates in parallel, divided over the 3 or 4 casings, is around 180. The total collecting area is about 4 hectares (10 acres). An average 500 MW boiler will burn some 150 t of coal per hour at full load. The ash content of the coal may be, say, 16%. Thus some 25 t of ash per hour will be produced. Around 80--85% of this ash is carried out of the furnace to the precipitators, which must reduce the load carried by the combustion gases to a concentration below 115 mg/m 3 at S.T.P. as required by the Alkali Inspectorate. This represents an efficiency, on a weight basis, of over 99.3%. 3. Problems In order to achieve this very high collection efficiency, careful attention to detail design is essential, followed by equally careful erection. To maintain the efficiency consistently over long periods of time also requires high standards of maintenance. Large generating units normally run continuously for m a n y months between planned overhaul periods, and unscheduled shut-downs to rectify electrostatic precipitator faults can be very costly. 3.1 Gas distribution One of the major design problems affecting efficiency is the requirement to distribute the enormous quantity of gas being treated evenly over the crosssection of the precipitator. The "effective migration velocity" (e.m.v.), i.e, the net velocity with which particles are transported to the collecting surfaces, is related to the gas velocity, and the optimum value of the gas velocity is 1.8--2 m/s [1]. In parts of the precipitator where the gas velocity is too low, particles may not be carried by the turbulent motion of the gas to the region of high field near the discharge electrodes where they acquire their m a x i m u m
312
charge; consequently, collection is inefficient. On the other hand, high velocities cause scouring of the collected dust layer and particles are re-entrained back into the gas stream. It can be shown that regions of low flow do n o t compensate for regions of high flow and, thus, uneven gas flow means that performance suffers. Flow model tests of projected designs are therefore carried o u t b y manufacturers and are followed by full-scale measurements (on air) when the plant is installed to allow for final trimming by means of guide vanes.
3.2 Rapping Another problem area is rapping. The objective, of course, is to detach the dust layer with minimum breakup and despatch it to the hopper w i t h o u t reentrainment of particles. C.E.R.L. laboratory investigations have shown that the most effective way of delivering the rapping blow is in the shear direction, i.e. in the plane of the plate and that the fracture takes place within the dust layer, parallel and fairly close to the electrode. With no corona discharge an impulse of ~ 15 g is sufficient to remove most normal dusts from an electrode and the finer the dust the higher the force required. (see Table 1). In practice, rapping is carried out with the precipitator on stream and with full high voltage applied. In this situation, the force necessary to remove the dust layer is at least doubled and with difficult dusts 60 g is more appropriate. Of course, even with 60g some dust remains on the electrodes and the finest particles will not be removed by 1000 g. The rapping impulse is usually applied by some form of mechanical hammer acting on an anvil attached to the electrode support frame. To allow for attenuation in joints and to ensure that the specified 'g' value is attained over all the plate surface, the hammer impact must be considerably higher than 60 g. 3. 3 Dust resistivity and conditioning A consequence of collecting dust in an electric field is that the dust layer itself can create problems by disturbing the stability of the corona discharge. TABLE1 A c c e l e r a t i o n r e q u i r e d t o r e m o v e p r e c i p i t a t e d fly ash f r o m an e l e c t r o d e
Average particle dia (urn)
2.9 8.7 23.3
A c c e l e r a t i o n t o dislodge ash layer (g) Force normal to surface H.T. o f f
F o r c e parallel t o surface H.T. o f f
F o r c e parallel t o surface H.T. o n
> 40 30 21
17 10 2.5
28 32 6
g values are averages o f ~ 12 tests.
313
If the dust is highly resistive, that is, above, say, 101° ohm m, then a layer, even a very thin one, on the discharge wires can act as an insulator and can suppress the corona discharge [2]. This adversely affects the particle charging and the collection efficiency. A layer of this dust on the collecting plates can prevent the ionic current leaking away to earth and a high voltage gradient builds up across the dust layer. The layer will eventually break down causing the emission of positive ions into the gas space. This phenomena is k n o w n as back discharge, or back ionization. The positive ions act to reduce the effective particle charge. The break-down in the dust layer also triggers flashover across the inter-electrode gap. Figure 1 shows back ionization spots in a dust layer on a point-plane geometry with flashover whilst Fig.2(a) illustrates the corona glow from the discharge wires of a plate-type pilot precipitator together with back-ionization illuminating the dust surface on the collecting plates. Figure 2(b) acts as a reference. It may be recalled that the charging and collection process is dependent on the applied electric field. Thus, a reduction in flashover voltage causes a reduction in field and a consequent lowering of the e.m.v, and, hence, collection efficiency. The relationship between e.m.v. and flashover is shown in Fig.3 for fly ash treated with various concentrations of conditioning agents (see below) in a pilot precipitator. Now, highly resistive dusts are associated with coals having low sulphur contents, say, below 1.3%. The sulphur in coal produces small amounts of SO3 in the flue gas which combines with moisture to form sulphuric acid. This
Fig. 1. Point-to-plane flashover showing back-ionization glow in a dust layer.
314
Fig.2 (a) B a c k - i o n i z a t i o n in a p l a t e - t y p e pilot precipitator. (b) R e f e r e n c e for Fig.2(a). V i e w l o o k i n g u p s t r e a m b e t w e e n electrodes. 0.21 - ADDITIVE O 0.19
0.17
--
SO2
•
HCL
A
SULPHATES
--
A
/
A
0.15 -.;
O • O ~OA ~ ~"A
0.13
0.[I
0.09
~ 42
A 44
A A
A
I
I
46
48
I
I
I
50 52 54 FLASHOVER VOLTAGE, kV
I
I
56
58
Fig.3. Relationship between E.M.V. and flashover voltage for fly ash treated with conditioning agents.
315
coats the particle surfaces and, provided there is enough sulphur in the coal, gives them a surface which is sufficiently conducting to prevent the deleterious effects of high resistivity. About 18% of the coal supplied to the C.E.G.B. has a sulphur content below 1.3% and so could produce a potentially difficult ash.
Methods of overcoming this high resistivity problem include the injection of certain chemicals into the gas stream to coat the particles with a conducting layer. These chemicals are termed conditioning agents. They must be carefully chosen so that they do not cause any obnoxious emissions, are not hazardous to personnel or have any adverse effect upon the plant. Ammonium sulphate solution sprayed into the flue gas has been shown in C.E.R.L. pilot plant experiments and power station trials to be effective [3], and the C.E.R.L. system is being used in a power station in France [4].
3. 4 Discharge electrodes Discharge electrodes are manufactured in a variety of different shapes: round, square plain, square twisted, star, barbed, saw toothed, etc.; and a variety of claims as to their respective merits have been made. However, experiments carried out at C.E.R.L. on a 4-m-long laboratory tubular precipitator showed that, although the voltage--current characteristics of electrode types differed considerably, and particularly so between the spiked and plain wires, for the same power input the e.m.v, was found to be independent of wire shape. (Fig.4). This finding was confirmed by observations in the field, but only for dusts that were not highly resistive. In the case of highly resistive dusts the high current densities produced by barbed types of electrode tend to promote back-ionization. (~
GAS CONDITIONS :- AIR AT 18°C DUST 95 ~m ALUMINIUMOXIDE TUBE DIA. 305rnm
03
7
+ •
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0.4
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TIP RAD I.Smm H~,GHT 4.Smm ~------ i~Ji '- 100rnm Crs -
0.3 X
i
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PLAIN SQUAREROD 4 rnmSQ.
X TWISTEDSQUARE ROD. 4mm SD. S4mm PITCH
n
DOUBLE PEG" CIRCULAR ROD 6.3 mmDIA
TIP RAD. S0~rn
f . T H EIGHT 9.5ram
02
G w w
ol
L~DIA.
S I
19m~
O TRIPLE SPIKE. CIRCULAR ROD 6.3 mrn DIA.
100 mm Crs
I
S
I
I
i
I
i
I
10 IS 20 SPECIFIC POWER. Wm-2
i
I
25
i
I
~
DIA 18ram ~
30 100rnm
4- DISC.CIRCULAR ROD. 6.3ramDIA.
Crs
Fig.4. Relationship between E.M.V. and power per unit area of collector for a tubular experi mental precipitator fitted with various discharge electrodes.
316 Another very important factor in the choice of a discharge electrode is its mechanical strength. The atmosphere in which discharge electrodes operate is corrosive and erosive and they are being constantly subjected to rapping blows which can induce fatigue failures. The failure of but one wire in a precipitator section can, by short circuiting to the collecting plate, put that whole section out of action with a consequent increase in dust emission. This is a serious matter and must be attended to w i t h o u t delay. However, this is not easy as the treatment zone is at a temperature of ~ 130°C and is filled with noxious gases. Shutting down a 500 MW boiler costs in the region of £20,000--£30,000 per day in terms of starting up replacement units, thus a complete shutdown is to be avoided. The problem of removing broken wires w i t h o u t taking the boiler off-load has been overcome by the development of special double isolating dampers, capable of shutting off one of the parallel flows of the precipitator with the boiler on reduced load. Thus, after a suitable cooling and ventilating period, safe access to remove the offending wire is possible.
3. 5 Electrical operating conditions There has been considerable research to find the best operating regime for precipitators and to try to separate the influence of voltage and current. It has been established from both experimental and full-scale plant that performance increases with increases in applied voltage and that the best performance is obtained when operating on the verge of flashover. This is an extremely delicate region, as the electric stress within the precipitator treatment zone is continuously changing with changes in gas composition and dust loading. Consequently the flashover voltage is not a fixed quantity. This poses a control problem, because to obtain o p t i m u m performance the voltage must follow the flashover level and the h.t. set is consequently in constant danger of tripping out. If the setting is left to a plant operator, he will naturally set the voltage control to some comfortable position that will cause him least trouble. However, this procedure will not achieve the best performance for the plant and some automatic system is necessary. One m e t h o d counts the number of flashes over a given period, compares the result with a pre-set figure and adjusts the voltage accordingly. However, this presupposes that the correct rate of flashover is known and t h a t the controller can distinguish between a spark and an arc. A spark may be defined as a transient phenomena lasting perhaps for one or two cycles of the mains supply in contrast to the power arc which is sustained until extinguished by some drastic action, such as interrupting the supply for a sufficient length of time for the ionization products to be removed by the gas flow. The object of the automatic control is, therefore, to allow the voltage to rise sufficiently to sense the sparking level but n o t enough to establish an arc. This can be achieved by thyristor control devices which respond very rapidly, within one half of a mains cycle, to a sparkover in the precipitator. When
317 a spark is detected the voltage applied to the precipitator electrodes is immediately lowered by a pre-set a m o u n t and then is raised slowly over about a second or so until another spark is sensed. This is called a ramp recovery. The procedure is then repeated. If the disturbance continues for longer than a half cycle, then this is designated to be an arc and the voltage is reduced to zero for a few cycles then restored rapidly to intercept the ramp recovery as from a spark. Another m e t h o d of control that is widely used is to seek and maintain the voltage at the peak of the voltage--current characteristic of the precipitator. With this method, the primary voltage is automatically raised a preset a m o u n t periodically, say every 5 s, and the effect on the secondary is monitored. If this results in an increase in secondary voltage a further rise is initiated and the process continues until the peak is reached and a rise in primary volts results in a decrease in secondary volts. This indicates that the peak has been found. The controller then backs off and starts the cycle again. In fact, the control point is very similar to the thyrister controller as the peak occurs where flashing begins. 3. 6 Sectionalization It was mentioned earlier that a precipitator is divided into a number of separate mechanical and electrical sections. This is desirable for the following reasons: (1) The operating voltage of each section is determined by the flashover voltage of its weakest part. Therefore, the more sections there are, the higher will be this voltage. (2) A fault on any one section will be detrimental to only that section. Thus a precipitator composed of many small sections should give an overall performance that is higher than if it were supplied from only one voltage source. Or, to look at it from another point of view, a smaller precipitator may be able to achieve a given efficiency if more rectifier sets feeding separate zones are provided. Now, to maintain a constant efficiency, the e.m.v, must rise in inverse proportion to the decrease in plate area and for extensive sub-division to be economically worthwhile the e.m.v, must increase at a substantially greater rate than this to compensate, not only for the cost of the extra rectifier sets, but also the extra control gear, insulators, wiring and power consumed. To find whether the e.m.v, would increase sufficiently with smaller zone sizes an extensive series of experiments was carried out on a power station precipitator covering a 12--1 range of zone sizes. The flashover level, discharge current and power input all increased with smaller zone sizes and a correspondingly higher e.m.v, was achieved. This is illustrated for specific power and e.m.v, in Fig. 5(a) and 5(b) respectively. However, the rate of rise was insufficient to justify the additional costs of very small sections and a plate area per zone of about 3000 m 2 was calculated to give a reasonable compromise between performance and economy.
318 10
--
VOLTAGE SET TO FLASHOVER LEVEL 8 i
6
O
O
4
2
0
(a)
t I I 2000 4000 6000 COLLECTING PLATE AREA PER RECTIFIER, rn2
I 8000
0,14
0.12 7 : 0.10 uJ 0,08
0.06 (b)
O
I I I 2000 4000 6000 COLLECTING PLATE AREA PER RECTIFIER. rn2
O
I 8000
Fig.5. Effect of zone size on (a) specific power and (b) effective migration velocity, for a power station precipitator. 4. Prototypes The increasingly stringent emission regulations has triggered worldwide research into methods of improving dust collection by electrostatic precipitators. Some of these devices are described briefly in the following sections.
4.1 Low turbulence precipitator It can be shown that if charged particles are subjected to an electric field in laminar flow conditions they will be collected in a finite distance. (As distinct from the exponential rate of collection in the turbulent flow of a normal precipitator). Laminar flow requires a plate spacing of less than 1 cm and is not practicable on a power station scale. However, it has been shown with pilot plant testing at C.E.R.L. that charged particles may be collected with high efficiency in l o w turbulence flow [ 5 ] . This is achieved by having parallel plates spaced some 10 cm apart. These are fitted into the last zone of a 3 zone precipitator and take the place of the normal corona-wire system already described (Fig.6). The plates are alternatively.at a high voltage, or are earthed.
Fig.6.
Flow metering venturi
D~hn~ Dust hopper
Section t Flow straigh~lmer Showil~li hmtlontal
Section through
\
o ~
C.E.R.L. low turbulence precipitator.
In(lucid draught fan
inlet
CIwJstand gas
- -
I
'
f ~ dust hopp~
Qllick reklameclamps
low turbulence
High field
Ilerlpex top llllrel
J
- Dischaqle w i r e
;ollecttnll gtstll
j
i i I l--1---.-,-
>
320
The particles arriving at this final zone are already charged by the preceding corona zones. Experiments have shown that the all-plate final zone has an effective migration velocity between 45% and 140% better than the equivalent final zone with corona wires, depending on the degree of rapping.
4.2 High intensity ionizer This is essentially a device for pre-charging particles before they enter a conventional electrostatic precipitator. It is being developed in the U.S.A. by the Electrical Power Research Institute [6]. It consists basically of a venturi tube with a high voltage electrode in the throat (Fig.7). The electrode is in the form of a sharp edged disc which is INLET CONE INTENSE CORONA
SUPPORT TUBE ~
1
. BULKHEAD
=
~
FLOW
] E
SUPPORT
HV DISCHARGE CATHODE
POROUS ANODE
UST CONE
Fig.7. High intensity ionizer [6].
raised to a potential of 80--100 kV. The diameter of the venturi is around 250--300 mm and intense discharge currents and very high electric fields can be achieved. Values quoted are currents of 2 mA and field strengths of 10--12 kV/cm. This compares with approximately 1 mA for a conventional precipitator zone having a breakdown field of about 3 kV/cm. An array of the devices can be fitted in parallel upstream of an existing precipitator where dusty gas will pass through the venturis at a velocity of a b o u t 25 m/s (80 ft/s). Extensive pilot testing has been carried out and it is claimed that particle charges are 3--5 times higher than in normal precipitators and 40% higher than the maximum predicted by classical theory. This is attributed to very high ion densities and electron charging. The pilot plant testing has shown that particle penetration is reduced by 70% compared with a conventional precipitator. A disadvantage of the high intensity ionizer is that, in spite of the very high gas velocity in the venturi throat, a layer of ash is precipitated. When this ash is highly resistive electrical breakdown occurs at a low voltage and the beneficial effects of pre-charging are diminished. To overcome this, an air-swept anode is being developed in which filtered air is blown through a porous surface to prevent dust accumulating.
321 4.3 Sou thern Research Institu te controlled corona precharger This is being developed in the U.S.A. in an attempt to avoid back-ionization and overcome the problems of effectively charging highly resistive dust [7]. It comprises a mesh screen fixed a b o u t 20 mm from the surface of 30-cm-long earthed plates situated immediately upstream of the first zone of a normal precipitator (Fig.8). Corona discharge wires are fitted in the usual way and are TOSCREEN POWERSUPPLY
TOCORONA POWERSUPPLY TOSCREEN PPLY
Fig. 8. Southern Research Institute pre-charger [ 7 ] . operated at a b o u t 30 kV negative. A bias voltage of around 7 kV negative is applied to the screen and is varied by a feedback circuit from the corona power supply such that it maintains a constant corona current between the discharge wires and the screen. If back-ionization occurs, the positive ions emitted from the dust collected on the earthed plates are intercepted by the grid, and the charging current, consisting of negative ions, in the space between the discharge wires and the grid, is unaltered. The work is still at an earl.v pilot plant stage. 4.4 Masuda pulsed charger In this system, which is being developed in J_apan [ 8 ] , corona originates from a spiked discharge electrode. A third electrode is fixed near the discharge electrode and a pulsed discharge is applied between them (Fig.9). A high d.c. voltage is maintained between the third electrode and the collecting electrode
322 THIRD ELECTRODE
COUNTERELECTRODE ~ / J~ DISCHARGE
Fig.9. Bias controlled pulse charger [ 81. to provide the main field. By suitably biasing the third electrode the discharge current is controlled independently of the main field and back-ionization is said to be prevented. This has reportedly proved successful in the Japanese steel industry.
4.5 The electro-dynamic venturi This originated in France and combines the principles of electrostatic precipitation and venturi scrubbing to form a single, compact dust collecting system [9] (Fig.10). In the E.D.V. process, the gases to be treated are saturated with water in a conditioning tower, passed through the throat of a venturi at velocities of up to 100 m/s where adiabetic expansion takes place and water condenses onto the dust particles. The droplets containing the particles are ionized in a corona discharge during their passage through the venturi tube and are collected b y electrostatic forces onto a curtain of oppositely charged water droplets at the venturi exit. They are then removed as a sludge. Any carryover of water droplets is collected in a simple cyclone. The advantages of the system are a high collection efficiency, which is not adversely affected b y dust properties such as high resistivity and particle size. It is c o m p a c t and simple in operation. The disadvantages are a wet, low temperature plume and a fairly high water consumption. It has been applied operationally to a number of processes including pulverised-fuel-fired boilers. 5. Conclusions
Public awareness has focussed attention on dust emission sources in recent
323
CLEAN GAS1 OUTLET
CLEAN WATER INLET SPRAY NOZZLE
::.:.
DISCHARGE ~l
ELECTRODE
F
DUST+WATEROUT
FINSONCENTRALTUBULAR ELECTRODE
T
DUSTY GASINLET Fig.10. Electro-dynamic venturi [9 ]. years and control regulations have become more stringent. This has led to a renewed interest in the detailed operation of electrostatic precipitators and has provided the incentive for novel developments. The C.E.G.B. continue to carry out research to ensure that the emission limits set by the Alkali and Clean Air Inspectorate are met and is evaluating various novel concepts. The aim is to reduce emissions to a minimum consistent with the reliable, economic production of electricity.
Acknowledgements This work was carried out at the Central Electricity Research Laboratories and this paper is published by permission of the Central Electricity Generating Board.
324
References I J. Dalmon and H.J. Lowe, Experimental investigations into the performance of electrostatic precipitators for P.F. power stations, Proc. Int. Symp. on the Physics of Electrostatic Forces and their Applications, Grenoble, 1960. 2 H.J. Lowe, J. Dalmon and E.T. Hignett, The precipitation of difficult dusts, I.E.E. Colloquium on Electrostatic Precipitators, London, Febr. 19, 1965. 3 J. Dalmon and D. Tidy, A comparison of chemical additives as aids to the electrostatic precipitation of Fly-Ash, Atmos. Environ., 6 (1972) 721. 4 C. Guillon, Improving the efficiency of an electrostatic precipitator by injecting ammonium sulphate in the flue gas, Bull. Inf. Centrales Electriques, No. 85 (1976), p.16. 5 J. Dalmon, The performance of an experimental electrostatic precipitator with a low turbulence zone, Symposium on the Changing Technology of Electrostatic Precipitation, 1974, Institute of Fuel (Australia), Adelaide. 6 O.J. Tassicker and J. Schwab, High-intensity ionizer for improved ESP performance, EPRI J. June/July (1977) 56. 7 D.H. Pontius, P. Vann Bush and W.B. Smith, A new system for electrostatic precipitation of particulate materials having high electrical resistivity, APCA Meeting, Houston, Texas, June 25--29, 1978. 8 S. Masuda, I. Doi, I. Hattori and A. Shibuya. Utility limit and mode of back discharge in bias-controlled pulse charging system, IEEE, 1977, IAS Conf. Record, p. 875. 9 J.F. Vicard, Results and experience with a new high velocity electric precipitator: The electro dynamic venturi, 4th Int. Clean Air Congress-Session V-50, Tokyo, May 19, 1977.