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Intermittent energization on electrostatic precipitators
Journal of Electrostatics, 25 (1990) 55-73
55
Elsevier
Intermittent energization on electrostatic precipitators N. Tachibana Kobe Shipyard and Engine Works, Mitsubishi Heavy Industries Ltd., 1-1-1, Wadasaki-cho, Hyogo-ku, Kobe, Japan
and Y. Matsumoto Takasago Research and Development Center, Mitsubishi Heavy Industries Ltd., 2-1-1, Shinhama, Arai-cho, Takasago, Hyogo, Japan (Received August 1988; accepted May 14, 1989)
Summary It is well-known that back corona seriously affects electrostatic precipitator (ESP) performance in many applications when collecting high resistivity dust. In order to achieve a good performance for high resistivity dust, it is essential for an ESP to be provided with the means to reduce or repress the formation of back corona. As one of the countermeasures to overcome this problem, authors et al. originally developed and commercialized a novel energization method called "Intermittent Energization" (IE), which intermittently blocks the secondary current by a periodical blocking of the a.c. primary current of a T - R (High Voltage Transformer Rectifier) set with thyristors. Parametric tests, which were performed on a pilot ESP at a test facility and fullscale plants for dust of different resistivity levels, showed that IE can effectively mitigate the degradation of collection performance for highly resistive dust and at the same time achieve a marked reduction in power consumption for relatively low cost. In order to understand the fundamental characteristics of IE, this paper firstly describes its principle with the results of the change of appearance of back corona, flying path of the particles, the charge-to-mass ratio of the particles and then introduces an outline of IE with the relationship between migration velocity and intermittent duty for dusts of different resistivities and finally summarizes its effect, features and operational techniques based upon the operational results obtained from various commercial applications.
1. Introduction
The electrostatic precipitator (ESP) is widely used for removing dust from industrial and waste gases. As is well-known, the precipitation efficiency varies in accordance with the property of dust particles especially with resistivity and the condition of gases accompanying them. In particular, a major difficulty in ESP is an abnormal phenomenon called "back corona" which generates on the 0304-3886/90/$03.50
© 1990 Elsevier Science Publishers B.V.
56 collecting electrode surface due to the high resistivity dust. To prevent or suppress this back corona phenomenon, several means such as control of operating gas temperature (e.g. hot-side ESP for coal-fired boiler) [ 1 ], gas conditioning and energization control etc. are taken. Regarding the energy situation in Japan, the change which started from the energy crisis in 1973 gave impetus to a review of fuels. As a result, coal has been again highlighted as a fuel for thermal power generation, and thus the planning and construction of coal-fired power plants are progressing well. However, the electrical resistivity of fly ash varies widely according to the kind of coal and has a significant effect on the performance of ESP, and unfavorably it has its peak in the temperature range (130 ° C to 150 oC ) of the flue gas at the air preheater outlet where an ESP is ordinarily installed. Japan, with few coal resources, is necessarily dependent upon overseas coal, most of which has a low sulfur content (less than 1%) and its fly ash has extremely high resistivity, significantly impairing ESP performance. With this in mind therefore and with the aim of reducing the effect of back corona on ESP performance, intermittent energization (IE) [2] was developed as one of the most economical methods in 1979, and has been applied in many commercial plants (more than 500 IE controller sets) not only in Japan but all over the world [3-5]. Since, with this technology, in addition to performance improvement, reduction of power consumption can also be achieved at the same time, IE is now becoming a commonly utilized technology for power supply to ESPs. This paper describes the background of IE in relation to the result of the observation of the flying path of the particles in an interelectrode gas space with and without back corona, and the results of measurements on the chargeto-mass ratio of the particles. These will outline the characteristics of IE. Test results on a pilot ESP and on full-scale plants with IE for various kinds of dusts are also presented, and the effects, features and operational techniques of IE are summarized. 2. Back corona
Figure 1 shows the voltage distribution in the collecting field between electrodes. A dust layer deposited on the collecting electrode impedes the corona current because increase of the voltage across the layer or corresponding decrease of the voltage across the gas, results in the suppression of the corona current to some extent. The higher the resistivity of the dust layer, the higher the voltage across the layer. When the dust resistivity is extremely high, the voltage across the dust layer on the collecting electrode (Vd) markedly increases and finally electrical breakdown occurs in the dust layer when the electric field strength in the dust layer exceeds its breakdown electric field strength. This phenomenon is called
57 Collecting Electrode Voltage .... VEp
J Dust layer ~.~- . . . . . . . . .-:)
::.~i~ ....:.
Vd . . . . .
{i-".~
...!:/.
Vdo . . . . .
o
J /
High resistivity dust ~ - 0 Discharge ] i - / L (Corona) ', Electrode
•
, ~ ~
/"
,,
Normal resistivity '~ Dust
/'/
_ _ Precipitating_.. Dust layer Space
Fig. 1. Voltage distribution between electrodes.
"back corona" and its onset condition is generally explained by the following formula.
E,~
(1)
where Eds is the breakdown field strength of the dust layer, j and Pd are the current density and resistivity of the dust layer respectively. When the resistivity of the dust layer reaches 1011-1012 ohm cm, the electric field strength begins to exceeds the breakdown field strength (normally 8-12 kV/cm) of the layer. When a strong field strength remains in the gas space, positive ions produced at the points of breakdown are pulled out of the layer into the gas space and spark streamers are formed. When the product ofj and Pd far exceeds E&, the breakdown propagates on almost the entire surface of the layer and abundant positive ions flow out of the layer and move towards the discharge electrodes as a stable back corona like a bluish cloud. Sparking does not occur there because, under back corona, the current goes up sharply to an ordinary transformer rating. 3. Flying path of dust particles under the back corona condition The mechanism of electrostatic precipitation is so understood that particles suspended in the interelectrode gas space are negatively charged by negative ions produced from the corona electrodes and then driven towards the collecting electrodes by the electric force between the electrodes and by electric wind. Therefore, once positive ions produced by back corona are emitted from the collecting electrode side, suspended particles easily lose their negative charge or are charged positively and thus the loss of the driving force towards the
58
Fig. 2. Flyingpaths of particles: {a) with a high resistivity layer on the collectingelectrode; (b) with an ordinary resistivity layer on the collectingelectrode. collecting electrodes or the change in their direction of motion, results in the deterioration of the ESP performance. Therefore, the behavior of suspended dust particles coming into the precipitation field where back corona does and does not exist, obviously differs. Typical flying paths of particles are shown in Fig. 2. Figure 2 (a) shows the paths under the back corona condition while Fig. 2 (b) is of ordinary resistivity dust particles. It is obvious that the flying paths change their pattern in the existence of back corona. T h a t is to say, negatively charged particles approach the collecting plate, however, in the vicinity of the plate, they change their paths as if being pushed back towards the discharge electrode. It appears that the particles repeat this zig-zag pattern throughout the interelectrode gas space and finally leak out of the ESP outlet. The dust layer of this observation has a resistivity of 1013 ohm cm. 4. Principle of intermittent energization
As is described earlier, the most important point in ESP for precipitating highly resistive dust is considered to be the suppression of back corona in order to mitigate a degradation of collecting efficiency. The motive for the develop-
59
0,3
O 0,2
u. 0.1
/
/
/
I
i0
30
2O
40
VoItoge V (kV)
Fig. 3. Voltage-currentcharacteristicsof the precipitation field. (1) Electrodeswithout back discharge. (2) Electrodeswith back discharge. (3) Voltageovershootat rapid voltagerise. ment of IE under those circumstances was in the fact that formation of back corona appears to require an appreciable time, which was confirmed during an investigation on voltage-current characteristics and waveforms of ESP. Figure 3 shows typical VII characteristics of an industrial precipitator with low (1) and high (2) resistivity dust. Once back corona occurs, the current increases sharply (indicated by the solid line (2). When the voltage rises rapidly there is an over-shoot of voltage beyond the highest value of the solid line (2), as indicated by the dotted line (3), and at the next moment, the voltage drops back to the solid line (2) and the current begins to go up. This phenomenon can be explained in terms of the electrical relaxation time in the dust layer [6]. The dust layer can be expressed by the equivalent circuit consisting of a resistance R and a capacitance C as shown in Fig. 4(a). The transient of the voltage across the layer Vd is expressed by the following equation (eqn. 2), when a constant corona current (I--Io) is supplied to the dust layer from time t = 0:
Vd=R'Io(1--e -t/to)
(2)
where to is the time constant (to--CR ) and is calculated by eqn. (3).
to=8.85es×pd×lO -14 (s)
(3)
where ~s= specific dielectric constant of the dust layer, Pd = specific resistivity of the dust layer (ohm cm). For instance, to=0.27 when Es=3 and pd=1012 ohm cm. The back corona starting time from the initiation of current supply t~ can be expressed from eqns. (1) and (2) tx=to ln[1/{1--Edd(pdJo)}]
(4)
60
Io(=lp=lav.) t
Vd .
.
.
.
.
.
.
.
.
[v:
(b) Step response for conventional energization
± cT
.... R~
Ip= I°7 ro
V, .
.
i_ L_
.
.
.
.
.
.
.
....
]
(a) Equivalent circuit of a dust layer (c) Step response for intermittent energization
Fig. 4. (a) Equivalent circuit representing the electrical behaviour of a dust layer on a collecting electrode. (b) Step response for conventional energization. (c) Step response for intermittent energization. ( Vdbis the insulation breakdown voltageof a layer.) For example, tl = 0.109 s, when to = 0.27 s, Ed~= 10 kV/cm,jo = 0.3 m a i m 2, es = 3, and Pd = 1012 ohm cm. Thus, in normal cases, tl is long enough to switch thyristors on and off with a simple electronic circuit. The principle of IE is, as mentioned, to control the power supply to E S P by de-energizing prior to the back corona starting time tl and re-energizing when voltage Vd comes down to the initial low level again, by the duty ratio re--T1/ (T1 + T2) where T1 is on- and T2 is off-time. Thus, E S P can be operated with suppression of back corona by applying IE, i.e. repeating energization and de-energization at given intervals. 5. Power supply equipment As mentioned above, the m e t h o d of repeating energization and de-energization at given intervals is the basis of IE. This is graphically represented for the E S P current in Fig. 4(c). Since the current flow time T1 ranges usually
61
Thyristors~
I
} ,L
Periodical Blocking Unit
~
T, T2
1
High voltage transformer _l
"'I
Highvoltage rectifier 2 ~,
ControlCircuit (Conventional)
Fig. 5. Block d i a g r a m o f I E p o w e r supply e q u i p m e n t .
•
>
!
.
.
.... ;...... i....... i....... i....... i....... !....... i............... !................ ....!.......i.......i.......i.......i.......~.......~.......i.......i...............
~ ............ i....... i....................... i....... i....................... i.......
....~;~
~ I~,: Hi.......i: -l.~ :'........~I~ :: N: ~ ........... ~!~........:.~,
Tm i e(50 ms/Dvl) Fig. 6. Wave form o f i n t e r m i t t e n t e n e r g i z a t i o n (two half waves, 7~ = 1/5 ).
f r o m several to t e n s o f milliseconds a n d t h e c u r r e n t p a u s e t i m e T2 ranges usually f r o m several to h u n d r e d s of milliseconds, a c o n v e n t i o n a l p o w e r s u p p l y c o n t r o l l e d w i t h a t h y r i s t o r c a n be utilized as I E w i t h m i n o r m o d i f i c a t i o n s o f t h e c o n t r o l circuit. In Fig. 4, Ip and/ave r e p r e s e n t t h e p e a k a n d average value o f c u r r e n t w i t h I E respectively. Figure 5 shows a b l o c k d i a g r a m o f a p o w e r s u p p l y for IE. T h e m a i n p o w e r lines c o n s i s t i n g o f t h y r i s t o r s , a high voltage t r a n s f o r m e r a n d high voltage rectifiers are t h e s a m e as for c o n v e n t i o n a l e n e r g i z a t i o n systems. T h e o n l y difference is t h e c o n t r o l circuit: it n e e d s to b e modified. A periodical b l o c k i n g u n i t is a d d e d o n t h e signal line f r o m t h e c o n v e n t i o n a l c o n t r o l circuit t o t h e t h y r i s tots. T h i s u n i t blocks a n d passes t h e signal to t h e t h y r i s t o r s periodically so t h a t t h e c u r r e n t t o E S P m a y be s h a p e d as s h o w n in Fig. 4 ( c ) . T1 a n d T2 are
62
adjusted by manually operated knobs or by an automatic control circuit for optimization. The current flow level is automatically adjusted, for example, by using a spark rate control or a current limiting control. For this modification, however, the rearrangement of the printed board is required. Typical waveforms of the output voltage and current recorded on an operating IE power supply equipment are shown in Fig. 6 when the duty ratio rc is 1/5. The duty ratio re is introduced as the ratio of T1/(T1 + T2) assuming T1 and T2 are originally expressed by sets of a pair of half-cycles. For instance, the duty ratio re is 1/5 when T1 = two half-cycles and T2 = eight half-cycles. IE was originally developed as one with full cycle (two half waves) intermittent waves, but recently one with half cycle (one half wave) intermittent waves has been available. This modified method is described later. 6. Test facility and results
As IE seemed to be quite promising in two major areas, i.e. performance improvement and energy saving, investigations and tests were carried out on a pilot ESP to obtain concrete design guides on the effectiveness, operation method, etc.
6.1 Test facility Figure 7 shows the schematic diagram of the test rig of an ESP for a coalfired plant. To provide an accurate simulation, the test rig was carefully designed. It consists mainly of a coal-pulverizer, a furnace, a gas cooling system and an ESP, and burns pulverized coal at a rate of 50 to 60 kg/h. A coal pulverizer produces pulverized coal with the same properties and size distribution Gas Cooling System
Coal Combuster
!
'
LY
"
'
Cool Feeder
Fig. 7. Fly ash precipitability testing facility.
63
as those in commercial plants. The furnace and the gas cooling system are designed to simulate the properties of fly ash exhaust gas of commercial boilers. The ESP has two sections with independent power supplies and uses the same collecting and corona electrodes as in commercial ones. The spacing of the collecting electrodes is changeable from the standard spacing of 300 ram. Further, it can be used for cold and hot-side ESP tests by switching ducts of the cooling system. The total collecting area is F= 5.57 m 2 with a specific collecting are of f= 35 s / m with the standard gas velocity of vg--0.9 m/s. The effect of IE system obtained with the tests is summarized in Table 1.
6.2 Performance improvement The effect of the IE on ESP performance is roughly classified into the following three types. (See Table 1 ). The improvement in collection efficiency is evaluated by the enhancement TABLE1 Effect of IE system
Dust resistivity
VII
Type I
Type 11
Type 11[
High registivity
Medium registivty
Medium-Low (upto 10" 0-cm)
/
/
(10"-10" O " c m )
(10"-10 '= Q ocm)
characteristics
~Y
1
Optimum duty
Enhancement factor
rc-
5
rc-
1.0
1 2
kV
1 3
1
rc- 2
L0
/ '~'~'-
I
L rc
re
re
Enhancement factor (at optimum duty)
1.0- 1.2
1.0- 1.06
0.9- 1.0
Power consumption ratio (at max. performance with MIE, assuming that conventional energization is 100)
10 - 30
30 - 50
50
Coal characteristics
Plant
Low sulfur,low
alkali metal coal • Coal-fired thermal! powe plant • Iron ore sintering plant
General
coal
• Coal-fired thermal power plant
High sulfur, high alkali metal coal
• Coal-fired thermal power plant • Oil-fired thermal power plant • Cement plant • Refuse incineration plant
64
factor H which is defined as the ratio of the modified migration velocity Wk obtained by IE to that for conventional energization. (K= 0.5 in this paper. ) Type I : In the case that back corona occurs due to high resistivity (101' to 1013 ~ cm), the performance is markedly improved by IE and shows maximum performance at a duty ratio of rc = 1/5 or less. Type II : In the case that back corona occurs but not so severely as in Type I due to medium resistivity (101° to 1012 ~ cm), the performance is also improved and has its maximum at a duty ratio of rc= 1/2 to 1/3. Type IIh In the case that the load VII characteristics are normal due to medium to low resistivity (up to 1011 ~ cm), an improvement in performance by IE cannot be expected but performance is not impaired at a duty ofr~= 1/2. The effect of IE on E S P performance can be summarized by saying that it largely depends upon the V/I characteristics associated with the resistivity of the dust.
6.3 Energy savings Regarding energy saving characteristics, power consumption is nearly proportional to the duty of IE as shown in Fig. 8. In the case of Type I in Table 1, which is caused by back corona, IE shows the most effective energy saving characteristics. Although power consumption can also be reduced by decreasing current through decrease of applied voltage in conventional energization, IE provides much superior performance at the same power consumption with less sacrifice 100 .....
Oil-fired thermal power plant /,~ South African coal A /J~ Ic::adir:nS::t~riE9 plant ~ 7 / ' / 1 "
.o 50 E
8
0.5 Duty rc Fig. 8. Power consumption versus duty ratio.
1.0
65
ar I ntermitent
~ / ~
J
==
jss
- ~ ' - ~ - ~ - -- --
Conventional
S
l
0.5
=: LU
I
I
I
I
I
I
'
50
Power consumption ratio
100
(%)
Fig. 9. Comparisonof energysavingfor typicaldata of Type II. of precipitating performance than conventional (see Fig. 9). In this way, the higher the resistivity of dust, the more remarkable the power saving which can be achieved at the same collecting performance. Thus, IE is superior to conventional energization in both energy savings and performance improvement.
6.4 Charge-to-mass ratio of particles The dust particle migration velocity w is considered to be proportional to the product of particle charge q and electric field strength in the precipitation field E. Considering the fact that, with IE, the collecting efficiency of ESP for high resistivity dust increases in spite of a decrease in the interelectrode average potential Vm which corresponds to the field strength E, there must be an increase in q at any rate, likely by the increase in the peak voltage Vp which can be considered to relate to the charging field strength and partly by the suppression of positive ion emission. For the purpose of verifying this assumption, an experimental study on the charge-to-mass ratio q/m of the suspended particles was done with an in-stack Faraday cage device. Figure 10 shows the charge-to-mass ratio q/m on the conventional d.c. and IE with an identical current peak value plotted against the dust mass loading measured at the outlet of ESP in a test rig. As shown in Fig. 10, the curve for conventional energization lies below that of IE. The reduction in q/m in the case of conventional energization is due to severe back corona producing a bipolar ionic field with high positive ion content. But, with IE, the charging performance can be recovered to a similar level to that in normal operation. In addition to the improvement of charging performance, the current density distribution on the collecting electrode could also be improved with IE [ 7].
66
10-4I
~ .2
~. 10.5 ~-
~
~ Intermittent \\\~ Energtzation \o\x~>x~r c =I/3 \
2
\\ \
ConventionaiX\ . _ DC SUpPly \T%-I.O
10-6
\
\\ \
10-7
I 0,i
I
I
I
]
0.5
~, I
\
t ~ i
,0
Dust mass looding (g/mBN)
Fig. 10. Charge-to-mass ratio for different particle loading on fly ash of the same coal.
The application of IE effectively suppresses most of the extremely high current density values and eliminates most of the electrical power consumption that is wasted in back corona. With degraded electrical operating conditions it produced an increase in plate area receiving current density values judged be to useful for particle charging and collection. 7. Applications to full scale plants and operation results
Since the first commercial application of IE in 1979, more than 500 sets of IE control units have been put into operation for ESPs installed after that time and for existing ESPs as retrofits. IE shows remarkable power saving effects in all applications, and in addition improvement in performance for high resistivity dusts. The effect can be classified roughly into the following two categories: Category I : This case is with back corona due to high dust resistivity (10111013 ~ cm) such as iron-ore sintering plants and low sulfur coalfired boilers. E S P outlet dust burden decreases by 30 to 50% of that of conventional energization, and there is a power saving of 50 to 95%. In this case, IE shows a marked effect in both collection performance and power savings.
67
Category IT: This case is with medium dust resistivity (less than 1011 ~ cm) such as medium and high sulfur coal-fired boilers, oil fired boilers, kraft pulp recovery boilers and cement plants where no back corona occurs. There is a saving in power consumption of almost 50%, while the collection efficiency is kept nearly the same or is slightly lower as compared with that of conventional energization.
7.1 Coal-fired boiler Precipitability of fly ash depends primarily upon the resistivity related to sulfur and alkali metal content in coal. Most of the imported coals to Japan containing low sulfur, have high resistivity and significantly impair ESP performance. More than 50 sets of ESPs equipped with IE control units have been operating successfully for fly ash from more than 80 kinds of coals. Four typical operating results are shown in Fig. 11; they are taken from a series of tests on a utility boiler burning various kinds of coal. In the case of coal B and C, belonging to category I, with severe back corona, significant ESP performance enhancement is achieved by IE. For coal B, the enhancement 1.6 Coal B
1.4
1.2 :lZ
1.0 E c Lu
0.8 Coe 0.6
0.4
.~
Coal D
I/ 5 I/I0
I)5 I/8 1~2
1/1
Duty ratio 7"c
Fig. 11. Operating results of E S P performance improvement in relation to duty ratio for a coalfired boiler.
68 factor is about 1.5 at the duty ratio of 1/15 and its power consumption is reduced to only 5% of that with conventional energization. On the other hand, in the cases of coal A and D, belonging to category II, there is no improvement in collection performance while the power consumption can be reduced significantly.
7.2 Oil-fired boiler Dust from an oil-fired boiler has comparatively low resistivity. However, IE has been effectively applied for power saving.
7.3 Kraft pulp recovery boiler IE applied to ESPs for recovery boilers only has a power saving effect because the dust has medium resistivity. However, IE shows an attractive effect of dislodging the sticky and bulky dust by electrode's rapping.
7.4 Iron-ore sintering plant IE applied to ESPs for iron ore sintering plants gives, without exception, both remarkable improvement in collection efficiency and power saving. Operation results from 7 retrofit plants are plotted in Fig. 12. The performance measurements made twice in summer and in winter in the same plant, showed interesting results, namely that the duty ratio at maximum performance changes depending on the ambient atmosphere. As for the power savings, a reduction from 296 kW to 33 kW has been achieved in the most remarkable case. 2.5
2.0 -r-
1.5
I,.LJ
1.C
0.5 1/30 1j201/"15 lJ10 17 5 1/' 3 1.~2 171 Duty Ratio )'c Fig. 12. Operating results of ESP for iron-ore sintering.
69
8. Optimum operation technique For the purpose of optimizing ESP performance, methods related to IE have been developed. 8.1 Full or half cycle IE
In addition to the full cycle IE (a pair of two half-cycles) that was originally developed as the base of IE, half-cycle intermittent waves have now become available. Comparison of the enhancement factors in waveforms between full and half-cycle IE is shown in Fig. 13. Particularly when the dust resistivity is extremely high, half-cycle IE is often more effective than that with full cycle, because, in the case of full cycle, back corona would occur more easily than with half-cycle. Figure 14 shows one of the most effective operating results of half-cycle IE obtained at the same plant as in Fig. 11.
Original wave (commercial frequency)
;
~
i
t
r
l
Rectified current wave
i
V
t
I
I
I
I
i
~
I
V
k
I
I
!
L
l
I
I
,'^,A,'^, A I t
I
L
I
* t
L ~ i
I I
i
,A~ l I I
t t
I i i
V
Full cycle Intarmittent wave (~,,=1/2)
,,,
t
( Y , = 1/ 1)
A I
i
!!!!:
Full cycle Intermittent wave ( y , = 1/ 3)
Half cycle Intermittent wave ( Y , = 1/ 3)
!!,
i
I
I
,~
I
i
Half cycle Intermittent wave ('(, = 1/ 5)
Fig. 13. Primary current wave forms of full-cycle and half-cycle intermittent energization.
7O
8.2 VpX Vm adjustment The migration velocity of particles w is roughly expressed as follows:
(5)
w~c VpX Vm
where Vp = peak applied voltage, Vm= mean applied voltage. In industrial elec1.6
"r
1.4
~
1.2
+
Half cycle
=
?
F
~
•
1.0
0.8
-
~
1/20
1/10 1/9
~
1/5
1/3
'
1/2
1/1
Duty ratio Yc
Fig. 14. Comparison of enhancement factor in wave forms between full- and half-cycle IE.
80 Coal A
60 ,-# \ v
40
20
/ ,
I
1000 Vp • V m (KV 21
Fig. 15. Relation between wk and Vp- Vm.
I
I
!
2000
71
DeNOx
Air Preh
Fig. 16. Flow and control model of an E S P emission control system.
trostatic precipitation, the modified particle migration velocity Wk is used instead of w. The square root of Wk is proportional to w. Figure 15 shows the relation between Wk and Vp× Vm obtained at the same plant as in Fig. 11. It can be seen that there is a fairly good correlation between Wk and Vp X Vm. Therefore the product of Vp × Vm, which can be maximized by the adjustment of IE parameters such as the duty ratio, will be better used as a practical index for optimizing SP performance. 8.3 E S P emission control system Recently, in the integrated air pollution control system consisting of removal of nitrogen oxide, sulfur oxide and flying particles, demands to control the dust burden at the E S P outlet to an optimum or a certain constant value are increasing. One solution for it, is control of the duty ratio of IE using a micro-processor and feedback signals from an opacity meter installed at the ESP outlet. The flow and control scheme is shown in Fig. 16. In this system, ESP emission is controlled to be within a fixed range by changing the duty ratio from 1/61 to 1/3 in accordance with control signals associated with boiler operating conditions, such as change of boiler load or soot blowing etc. [3-5 ]. 9. Summary
Intermittent energization (IE), a novel method for energizing an electrostatic precipitator (ESP) which has been originally developed and commercialized by the authors et al., is outlined.
72
Parametric investigations of IE on a pilot ESP and full scale ESPs were performed, and the features of IE recognized through the investigations and experiments can be summarized as follows: (1) IE is one of the most economical means for preventing or suppressing back corona in the ESP. (2) The major effect of IE is to increase ESP performance for high resistivity dust and to reduce power consumption. (3) The performance enhancement effect of IE differs depending upon the dust properties of each application. In iron-ore sintering plants, the dust of which is a typical example of extremely high resistivity, the maximum improvement of the enhancement factor is 2.5, and in low sulfur coal-fired boilers, 1.5. For medium resistivity dust produced from coal-fired boilers, soda-recovery boilers of paper mills, oil-fired boilers and cement plants etc., IE does not improve the performance, however, it does save more than 50% in power consumption at the duty ratio 1/2. (4) As for the energy saving characteristics, power consumption is nearly proportional to the duty ratio of IE. (5) Intermittent supply of current can increase the peak value of charging voltage Vp in general, while the average voltage Vm decreases with decrease of the duty ratio. The increase of Vp with IE may be considered as an index of degree of back corona suppression. (6) Half cycle IE is often more effective in improving ESP performance, particularly in the case of extremely high resistivity dust, than full (two half) cycle even at the same duty ratio. (7) With IE, the dust charging performance in ESP for high resistivity dust is recovered to a similar level to that of an ESP in normal operating conditions. (8) The enhancement factor of the migration velocity changes in a similar way to Vp × Vm at the change of the duty ratio of IE. Therefore, the product of Vp × Vm will be a better practical indication of the optimum efficiency in industrial ESPs. (9) Regarding ESP emission control in an integrated air pollution control system, IE can be used to keep an optimum and/or fixed constant value of ESP outlet dust burden to stabilize changes of boiler operation conditions such as change of boiler load, soot blowing etc. Notation
C E Eds ~s
F
/
equivalent capacitance of dust layer electrical field strength breakdown field strength of dust layer specific dielectric constant of dust layer collecting area specific collecting area
73
H I
Ip /ave J Pd q R rc T1
T~ to tl
Yd Y. Ym W Wk
enhancement factor of modified migration velocity current peak value of current with IE average value of current with IE current density specific resistivity of dust layer particle charge equivalent resistance of dust layer intermittent duty ratio time of power on time of power off relaxation time constant of dust layer back corona starting time voltage across dust layer peak value of applied voltage average value of applied voltage theoretical migration velocity of particles modified migration velocity of particles
Acknowledgements The authors are indebted to emeritus Prof. Masuda of Tokyo University for his valuable advice on the phenomenon of back corona in electrostatic precipitators and they would also like to express sincere gratitude to colleagues, especially to Mr. T. Ando for his cooperation.
References 1 N. Tachibana, Y. Matsumoto, N. Teramura, K. Tashiro and N. Sakamoto, Mitsubishi Juko Giho, Vol. 15, No. 4, July 1978. 2 T. Ando, N. Tachibana and Y. Matsumoto, A new energization method for electrostatic precipitators: Mitsubishi intermittent energization, Proc. 4th Symp. on the Transfer and Utilization of Particulate Control Technology, Houston, Texas, Oct. 1982. 3 T. Ando, U.S. Patent, 4,410,849, Oct. 18, 1983. 4 T. Ando, G.B. Patent 2,096,845B, June 5, 1985. 5 T. Ando, France, Patent 8,107,121, Aug. 26, 1985. 6 H.J. White, Industrial Electrostatic Precipitation, Addison-Wesley, Reading, MA, 1983, pp. 325-326. 7 E.C. Landham, J.L. DuBard, M.J. O'Brien and C.V. Lindsey, The effect of high voltage waveforms on ESP current density distribution, Record of IEEE/IAS Annual Meeting, Atlanta, Oct. 1987.
55
Elsevier
Intermittent energization on electrostatic precipitators N. Tachibana Kobe Shipyard and Engine Works, Mitsubishi Heavy Industries Ltd., 1-1-1, Wadasaki-cho, Hyogo-ku, Kobe, Japan
and Y. Matsumoto Takasago Research and Development Center, Mitsubishi Heavy Industries Ltd., 2-1-1, Shinhama, Arai-cho, Takasago, Hyogo, Japan (Received August 1988; accepted May 14, 1989)
Summary It is well-known that back corona seriously affects electrostatic precipitator (ESP) performance in many applications when collecting high resistivity dust. In order to achieve a good performance for high resistivity dust, it is essential for an ESP to be provided with the means to reduce or repress the formation of back corona. As one of the countermeasures to overcome this problem, authors et al. originally developed and commercialized a novel energization method called "Intermittent Energization" (IE), which intermittently blocks the secondary current by a periodical blocking of the a.c. primary current of a T - R (High Voltage Transformer Rectifier) set with thyristors. Parametric tests, which were performed on a pilot ESP at a test facility and fullscale plants for dust of different resistivity levels, showed that IE can effectively mitigate the degradation of collection performance for highly resistive dust and at the same time achieve a marked reduction in power consumption for relatively low cost. In order to understand the fundamental characteristics of IE, this paper firstly describes its principle with the results of the change of appearance of back corona, flying path of the particles, the charge-to-mass ratio of the particles and then introduces an outline of IE with the relationship between migration velocity and intermittent duty for dusts of different resistivities and finally summarizes its effect, features and operational techniques based upon the operational results obtained from various commercial applications.
1. Introduction
The electrostatic precipitator (ESP) is widely used for removing dust from industrial and waste gases. As is well-known, the precipitation efficiency varies in accordance with the property of dust particles especially with resistivity and the condition of gases accompanying them. In particular, a major difficulty in ESP is an abnormal phenomenon called "back corona" which generates on the 0304-3886/90/$03.50
© 1990 Elsevier Science Publishers B.V.
56 collecting electrode surface due to the high resistivity dust. To prevent or suppress this back corona phenomenon, several means such as control of operating gas temperature (e.g. hot-side ESP for coal-fired boiler) [ 1 ], gas conditioning and energization control etc. are taken. Regarding the energy situation in Japan, the change which started from the energy crisis in 1973 gave impetus to a review of fuels. As a result, coal has been again highlighted as a fuel for thermal power generation, and thus the planning and construction of coal-fired power plants are progressing well. However, the electrical resistivity of fly ash varies widely according to the kind of coal and has a significant effect on the performance of ESP, and unfavorably it has its peak in the temperature range (130 ° C to 150 oC ) of the flue gas at the air preheater outlet where an ESP is ordinarily installed. Japan, with few coal resources, is necessarily dependent upon overseas coal, most of which has a low sulfur content (less than 1%) and its fly ash has extremely high resistivity, significantly impairing ESP performance. With this in mind therefore and with the aim of reducing the effect of back corona on ESP performance, intermittent energization (IE) [2] was developed as one of the most economical methods in 1979, and has been applied in many commercial plants (more than 500 IE controller sets) not only in Japan but all over the world [3-5]. Since, with this technology, in addition to performance improvement, reduction of power consumption can also be achieved at the same time, IE is now becoming a commonly utilized technology for power supply to ESPs. This paper describes the background of IE in relation to the result of the observation of the flying path of the particles in an interelectrode gas space with and without back corona, and the results of measurements on the chargeto-mass ratio of the particles. These will outline the characteristics of IE. Test results on a pilot ESP and on full-scale plants with IE for various kinds of dusts are also presented, and the effects, features and operational techniques of IE are summarized. 2. Back corona
Figure 1 shows the voltage distribution in the collecting field between electrodes. A dust layer deposited on the collecting electrode impedes the corona current because increase of the voltage across the layer or corresponding decrease of the voltage across the gas, results in the suppression of the corona current to some extent. The higher the resistivity of the dust layer, the higher the voltage across the layer. When the dust resistivity is extremely high, the voltage across the dust layer on the collecting electrode (Vd) markedly increases and finally electrical breakdown occurs in the dust layer when the electric field strength in the dust layer exceeds its breakdown electric field strength. This phenomenon is called
57 Collecting Electrode Voltage .... VEp
J Dust layer ~.~- . . . . . . . . .-:)
::.~i~ ....:.
Vd . . . . .
{i-".~
...!:/.
Vdo . . . . .
o
J /
High resistivity dust ~ - 0 Discharge ] i - / L (Corona) ', Electrode
•
, ~ ~
/"
,,
Normal resistivity '~ Dust
/'/
_ _ Precipitating_.. Dust layer Space
Fig. 1. Voltage distribution between electrodes.
"back corona" and its onset condition is generally explained by the following formula.
E,~
(1)
where Eds is the breakdown field strength of the dust layer, j and Pd are the current density and resistivity of the dust layer respectively. When the resistivity of the dust layer reaches 1011-1012 ohm cm, the electric field strength begins to exceeds the breakdown field strength (normally 8-12 kV/cm) of the layer. When a strong field strength remains in the gas space, positive ions produced at the points of breakdown are pulled out of the layer into the gas space and spark streamers are formed. When the product ofj and Pd far exceeds E&, the breakdown propagates on almost the entire surface of the layer and abundant positive ions flow out of the layer and move towards the discharge electrodes as a stable back corona like a bluish cloud. Sparking does not occur there because, under back corona, the current goes up sharply to an ordinary transformer rating. 3. Flying path of dust particles under the back corona condition The mechanism of electrostatic precipitation is so understood that particles suspended in the interelectrode gas space are negatively charged by negative ions produced from the corona electrodes and then driven towards the collecting electrodes by the electric force between the electrodes and by electric wind. Therefore, once positive ions produced by back corona are emitted from the collecting electrode side, suspended particles easily lose their negative charge or are charged positively and thus the loss of the driving force towards the
58
Fig. 2. Flyingpaths of particles: {a) with a high resistivity layer on the collectingelectrode; (b) with an ordinary resistivity layer on the collectingelectrode. collecting electrodes or the change in their direction of motion, results in the deterioration of the ESP performance. Therefore, the behavior of suspended dust particles coming into the precipitation field where back corona does and does not exist, obviously differs. Typical flying paths of particles are shown in Fig. 2. Figure 2 (a) shows the paths under the back corona condition while Fig. 2 (b) is of ordinary resistivity dust particles. It is obvious that the flying paths change their pattern in the existence of back corona. T h a t is to say, negatively charged particles approach the collecting plate, however, in the vicinity of the plate, they change their paths as if being pushed back towards the discharge electrode. It appears that the particles repeat this zig-zag pattern throughout the interelectrode gas space and finally leak out of the ESP outlet. The dust layer of this observation has a resistivity of 1013 ohm cm. 4. Principle of intermittent energization
As is described earlier, the most important point in ESP for precipitating highly resistive dust is considered to be the suppression of back corona in order to mitigate a degradation of collecting efficiency. The motive for the develop-
59
0,3
O 0,2
u. 0.1
/
/
/
I
i0
30
2O
40
VoItoge V (kV)
Fig. 3. Voltage-currentcharacteristicsof the precipitation field. (1) Electrodeswithout back discharge. (2) Electrodeswith back discharge. (3) Voltageovershootat rapid voltagerise. ment of IE under those circumstances was in the fact that formation of back corona appears to require an appreciable time, which was confirmed during an investigation on voltage-current characteristics and waveforms of ESP. Figure 3 shows typical VII characteristics of an industrial precipitator with low (1) and high (2) resistivity dust. Once back corona occurs, the current increases sharply (indicated by the solid line (2). When the voltage rises rapidly there is an over-shoot of voltage beyond the highest value of the solid line (2), as indicated by the dotted line (3), and at the next moment, the voltage drops back to the solid line (2) and the current begins to go up. This phenomenon can be explained in terms of the electrical relaxation time in the dust layer [6]. The dust layer can be expressed by the equivalent circuit consisting of a resistance R and a capacitance C as shown in Fig. 4(a). The transient of the voltage across the layer Vd is expressed by the following equation (eqn. 2), when a constant corona current (I--Io) is supplied to the dust layer from time t = 0:
Vd=R'Io(1--e -t/to)
(2)
where to is the time constant (to--CR ) and is calculated by eqn. (3).
to=8.85es×pd×lO -14 (s)
(3)
where ~s= specific dielectric constant of the dust layer, Pd = specific resistivity of the dust layer (ohm cm). For instance, to=0.27 when Es=3 and pd=1012 ohm cm. The back corona starting time from the initiation of current supply t~ can be expressed from eqns. (1) and (2) tx=to ln[1/{1--Edd(pdJo)}]
(4)
60
Io(=lp=lav.) t
Vd .
.
.
.
.
.
.
.
.
[v:
(b) Step response for conventional energization
± cT
.... R~
Ip= I°7 ro
V, .
.
i_ L_
.
.
.
.
.
.
.
....
]
(a) Equivalent circuit of a dust layer (c) Step response for intermittent energization
Fig. 4. (a) Equivalent circuit representing the electrical behaviour of a dust layer on a collecting electrode. (b) Step response for conventional energization. (c) Step response for intermittent energization. ( Vdbis the insulation breakdown voltageof a layer.) For example, tl = 0.109 s, when to = 0.27 s, Ed~= 10 kV/cm,jo = 0.3 m a i m 2, es = 3, and Pd = 1012 ohm cm. Thus, in normal cases, tl is long enough to switch thyristors on and off with a simple electronic circuit. The principle of IE is, as mentioned, to control the power supply to E S P by de-energizing prior to the back corona starting time tl and re-energizing when voltage Vd comes down to the initial low level again, by the duty ratio re--T1/ (T1 + T2) where T1 is on- and T2 is off-time. Thus, E S P can be operated with suppression of back corona by applying IE, i.e. repeating energization and de-energization at given intervals. 5. Power supply equipment As mentioned above, the m e t h o d of repeating energization and de-energization at given intervals is the basis of IE. This is graphically represented for the E S P current in Fig. 4(c). Since the current flow time T1 ranges usually
61
Thyristors~
I
} ,L
Periodical Blocking Unit
~
T, T2
1
High voltage transformer _l
"'I
Highvoltage rectifier 2 ~,
ControlCircuit (Conventional)
Fig. 5. Block d i a g r a m o f I E p o w e r supply e q u i p m e n t .
•
>
!
.
.
.... ;...... i....... i....... i....... i....... !....... i............... !................ ....!.......i.......i.......i.......i.......~.......~.......i.......i...............
~ ............ i....... i....................... i....... i....................... i.......
....~;~
~ I~,: Hi.......i: -l.~ :'........~I~ :: N: ~ ........... ~!~........:.~,
Tm i e(50 ms/Dvl) Fig. 6. Wave form o f i n t e r m i t t e n t e n e r g i z a t i o n (two half waves, 7~ = 1/5 ).
f r o m several to t e n s o f milliseconds a n d t h e c u r r e n t p a u s e t i m e T2 ranges usually f r o m several to h u n d r e d s of milliseconds, a c o n v e n t i o n a l p o w e r s u p p l y c o n t r o l l e d w i t h a t h y r i s t o r c a n be utilized as I E w i t h m i n o r m o d i f i c a t i o n s o f t h e c o n t r o l circuit. In Fig. 4, Ip and/ave r e p r e s e n t t h e p e a k a n d average value o f c u r r e n t w i t h I E respectively. Figure 5 shows a b l o c k d i a g r a m o f a p o w e r s u p p l y for IE. T h e m a i n p o w e r lines c o n s i s t i n g o f t h y r i s t o r s , a high voltage t r a n s f o r m e r a n d high voltage rectifiers are t h e s a m e as for c o n v e n t i o n a l e n e r g i z a t i o n systems. T h e o n l y difference is t h e c o n t r o l circuit: it n e e d s to b e modified. A periodical b l o c k i n g u n i t is a d d e d o n t h e signal line f r o m t h e c o n v e n t i o n a l c o n t r o l circuit t o t h e t h y r i s tots. T h i s u n i t blocks a n d passes t h e signal to t h e t h y r i s t o r s periodically so t h a t t h e c u r r e n t t o E S P m a y be s h a p e d as s h o w n in Fig. 4 ( c ) . T1 a n d T2 are
62
adjusted by manually operated knobs or by an automatic control circuit for optimization. The current flow level is automatically adjusted, for example, by using a spark rate control or a current limiting control. For this modification, however, the rearrangement of the printed board is required. Typical waveforms of the output voltage and current recorded on an operating IE power supply equipment are shown in Fig. 6 when the duty ratio rc is 1/5. The duty ratio re is introduced as the ratio of T1/(T1 + T2) assuming T1 and T2 are originally expressed by sets of a pair of half-cycles. For instance, the duty ratio re is 1/5 when T1 = two half-cycles and T2 = eight half-cycles. IE was originally developed as one with full cycle (two half waves) intermittent waves, but recently one with half cycle (one half wave) intermittent waves has been available. This modified method is described later. 6. Test facility and results
As IE seemed to be quite promising in two major areas, i.e. performance improvement and energy saving, investigations and tests were carried out on a pilot ESP to obtain concrete design guides on the effectiveness, operation method, etc.
6.1 Test facility Figure 7 shows the schematic diagram of the test rig of an ESP for a coalfired plant. To provide an accurate simulation, the test rig was carefully designed. It consists mainly of a coal-pulverizer, a furnace, a gas cooling system and an ESP, and burns pulverized coal at a rate of 50 to 60 kg/h. A coal pulverizer produces pulverized coal with the same properties and size distribution Gas Cooling System
Coal Combuster
!
'
LY
"
'
Cool Feeder
Fig. 7. Fly ash precipitability testing facility.
63
as those in commercial plants. The furnace and the gas cooling system are designed to simulate the properties of fly ash exhaust gas of commercial boilers. The ESP has two sections with independent power supplies and uses the same collecting and corona electrodes as in commercial ones. The spacing of the collecting electrodes is changeable from the standard spacing of 300 ram. Further, it can be used for cold and hot-side ESP tests by switching ducts of the cooling system. The total collecting area is F= 5.57 m 2 with a specific collecting are of f= 35 s / m with the standard gas velocity of vg--0.9 m/s. The effect of IE system obtained with the tests is summarized in Table 1.
6.2 Performance improvement The effect of the IE on ESP performance is roughly classified into the following three types. (See Table 1 ). The improvement in collection efficiency is evaluated by the enhancement TABLE1 Effect of IE system
Dust resistivity
VII
Type I
Type 11
Type 11[
High registivity
Medium registivty
Medium-Low (upto 10" 0-cm)
/
/
(10"-10" O " c m )
(10"-10 '= Q ocm)
characteristics
~Y
1
Optimum duty
Enhancement factor
rc-
5
rc-
1.0
1 2
kV
1 3
1
rc- 2
L0
/ '~'~'-
I
L rc
re
re
Enhancement factor (at optimum duty)
1.0- 1.2
1.0- 1.06
0.9- 1.0
Power consumption ratio (at max. performance with MIE, assuming that conventional energization is 100)
10 - 30
30 - 50
50
Coal characteristics
Plant
Low sulfur,low
alkali metal coal • Coal-fired thermal! powe plant • Iron ore sintering plant
General
coal
• Coal-fired thermal power plant
High sulfur, high alkali metal coal
• Coal-fired thermal power plant • Oil-fired thermal power plant • Cement plant • Refuse incineration plant
64
factor H which is defined as the ratio of the modified migration velocity Wk obtained by IE to that for conventional energization. (K= 0.5 in this paper. ) Type I : In the case that back corona occurs due to high resistivity (101' to 1013 ~ cm), the performance is markedly improved by IE and shows maximum performance at a duty ratio of rc = 1/5 or less. Type II : In the case that back corona occurs but not so severely as in Type I due to medium resistivity (101° to 1012 ~ cm), the performance is also improved and has its maximum at a duty ratio of rc= 1/2 to 1/3. Type IIh In the case that the load VII characteristics are normal due to medium to low resistivity (up to 1011 ~ cm), an improvement in performance by IE cannot be expected but performance is not impaired at a duty ofr~= 1/2. The effect of IE on E S P performance can be summarized by saying that it largely depends upon the V/I characteristics associated with the resistivity of the dust.
6.3 Energy savings Regarding energy saving characteristics, power consumption is nearly proportional to the duty of IE as shown in Fig. 8. In the case of Type I in Table 1, which is caused by back corona, IE shows the most effective energy saving characteristics. Although power consumption can also be reduced by decreasing current through decrease of applied voltage in conventional energization, IE provides much superior performance at the same power consumption with less sacrifice 100 .....
Oil-fired thermal power plant /,~ South African coal A /J~ Ic::adir:nS::t~riE9 plant ~ 7 / ' / 1 "
.o 50 E
8
0.5 Duty rc Fig. 8. Power consumption versus duty ratio.
1.0
65
ar I ntermitent
~ / ~
J
==
jss
- ~ ' - ~ - ~ - -- --
Conventional
S
l
0.5
=: LU
I
I
I
I
I
I
'
50
Power consumption ratio
100
(%)
Fig. 9. Comparisonof energysavingfor typicaldata of Type II. of precipitating performance than conventional (see Fig. 9). In this way, the higher the resistivity of dust, the more remarkable the power saving which can be achieved at the same collecting performance. Thus, IE is superior to conventional energization in both energy savings and performance improvement.
6.4 Charge-to-mass ratio of particles The dust particle migration velocity w is considered to be proportional to the product of particle charge q and electric field strength in the precipitation field E. Considering the fact that, with IE, the collecting efficiency of ESP for high resistivity dust increases in spite of a decrease in the interelectrode average potential Vm which corresponds to the field strength E, there must be an increase in q at any rate, likely by the increase in the peak voltage Vp which can be considered to relate to the charging field strength and partly by the suppression of positive ion emission. For the purpose of verifying this assumption, an experimental study on the charge-to-mass ratio q/m of the suspended particles was done with an in-stack Faraday cage device. Figure 10 shows the charge-to-mass ratio q/m on the conventional d.c. and IE with an identical current peak value plotted against the dust mass loading measured at the outlet of ESP in a test rig. As shown in Fig. 10, the curve for conventional energization lies below that of IE. The reduction in q/m in the case of conventional energization is due to severe back corona producing a bipolar ionic field with high positive ion content. But, with IE, the charging performance can be recovered to a similar level to that in normal operation. In addition to the improvement of charging performance, the current density distribution on the collecting electrode could also be improved with IE [ 7].
66
10-4I
~ .2
~. 10.5 ~-
~
~ Intermittent \\\~ Energtzation \o\x~>x~r c =I/3 \
2
\\ \
ConventionaiX\ . _ DC SUpPly \T%-I.O
10-6
\
\\ \
10-7
I 0,i
I
I
I
]
0.5
~, I
\
t ~ i
,0
Dust mass looding (g/mBN)
Fig. 10. Charge-to-mass ratio for different particle loading on fly ash of the same coal.
The application of IE effectively suppresses most of the extremely high current density values and eliminates most of the electrical power consumption that is wasted in back corona. With degraded electrical operating conditions it produced an increase in plate area receiving current density values judged be to useful for particle charging and collection. 7. Applications to full scale plants and operation results
Since the first commercial application of IE in 1979, more than 500 sets of IE control units have been put into operation for ESPs installed after that time and for existing ESPs as retrofits. IE shows remarkable power saving effects in all applications, and in addition improvement in performance for high resistivity dusts. The effect can be classified roughly into the following two categories: Category I : This case is with back corona due to high dust resistivity (10111013 ~ cm) such as iron-ore sintering plants and low sulfur coalfired boilers. E S P outlet dust burden decreases by 30 to 50% of that of conventional energization, and there is a power saving of 50 to 95%. In this case, IE shows a marked effect in both collection performance and power savings.
67
Category IT: This case is with medium dust resistivity (less than 1011 ~ cm) such as medium and high sulfur coal-fired boilers, oil fired boilers, kraft pulp recovery boilers and cement plants where no back corona occurs. There is a saving in power consumption of almost 50%, while the collection efficiency is kept nearly the same or is slightly lower as compared with that of conventional energization.
7.1 Coal-fired boiler Precipitability of fly ash depends primarily upon the resistivity related to sulfur and alkali metal content in coal. Most of the imported coals to Japan containing low sulfur, have high resistivity and significantly impair ESP performance. More than 50 sets of ESPs equipped with IE control units have been operating successfully for fly ash from more than 80 kinds of coals. Four typical operating results are shown in Fig. 11; they are taken from a series of tests on a utility boiler burning various kinds of coal. In the case of coal B and C, belonging to category I, with severe back corona, significant ESP performance enhancement is achieved by IE. For coal B, the enhancement 1.6 Coal B
1.4
1.2 :lZ
1.0 E c Lu
0.8 Coe 0.6
0.4
.~
Coal D
I/ 5 I/I0
I)5 I/8 1~2
1/1
Duty ratio 7"c
Fig. 11. Operating results of E S P performance improvement in relation to duty ratio for a coalfired boiler.
68 factor is about 1.5 at the duty ratio of 1/15 and its power consumption is reduced to only 5% of that with conventional energization. On the other hand, in the cases of coal A and D, belonging to category II, there is no improvement in collection performance while the power consumption can be reduced significantly.
7.2 Oil-fired boiler Dust from an oil-fired boiler has comparatively low resistivity. However, IE has been effectively applied for power saving.
7.3 Kraft pulp recovery boiler IE applied to ESPs for recovery boilers only has a power saving effect because the dust has medium resistivity. However, IE shows an attractive effect of dislodging the sticky and bulky dust by electrode's rapping.
7.4 Iron-ore sintering plant IE applied to ESPs for iron ore sintering plants gives, without exception, both remarkable improvement in collection efficiency and power saving. Operation results from 7 retrofit plants are plotted in Fig. 12. The performance measurements made twice in summer and in winter in the same plant, showed interesting results, namely that the duty ratio at maximum performance changes depending on the ambient atmosphere. As for the power savings, a reduction from 296 kW to 33 kW has been achieved in the most remarkable case. 2.5
2.0 -r-
1.5
I,.LJ
1.C
0.5 1/30 1j201/"15 lJ10 17 5 1/' 3 1.~2 171 Duty Ratio )'c Fig. 12. Operating results of ESP for iron-ore sintering.
69
8. Optimum operation technique For the purpose of optimizing ESP performance, methods related to IE have been developed. 8.1 Full or half cycle IE
In addition to the full cycle IE (a pair of two half-cycles) that was originally developed as the base of IE, half-cycle intermittent waves have now become available. Comparison of the enhancement factors in waveforms between full and half-cycle IE is shown in Fig. 13. Particularly when the dust resistivity is extremely high, half-cycle IE is often more effective than that with full cycle, because, in the case of full cycle, back corona would occur more easily than with half-cycle. Figure 14 shows one of the most effective operating results of half-cycle IE obtained at the same plant as in Fig. 11.
Original wave (commercial frequency)
;
~
i
t
r
l
Rectified current wave
i
V
t
I
I
I
I
i
~
I
V
k
I
I
!
L
l
I
I
,'^,A,'^, A I t
I
L
I
* t
L ~ i
I I
i
,A~ l I I
t t
I i i
V
Full cycle Intarmittent wave (~,,=1/2)
,,,
t
( Y , = 1/ 1)
A I
i
!!!!:
Full cycle Intermittent wave ( y , = 1/ 3)
Half cycle Intermittent wave ( Y , = 1/ 3)
!!,
i
I
I
,~
I
i
Half cycle Intermittent wave ('(, = 1/ 5)
Fig. 13. Primary current wave forms of full-cycle and half-cycle intermittent energization.
7O
8.2 VpX Vm adjustment The migration velocity of particles w is roughly expressed as follows:
(5)
w~c VpX Vm
where Vp = peak applied voltage, Vm= mean applied voltage. In industrial elec1.6
"r
1.4
~
1.2
+
Half cycle
=
?
F
~
•
1.0
0.8
-
~
1/20
1/10 1/9
~
1/5
1/3
'
1/2
1/1
Duty ratio Yc
Fig. 14. Comparison of enhancement factor in wave forms between full- and half-cycle IE.
80 Coal A
60 ,-# \ v
40
20
/ ,
I
1000 Vp • V m (KV 21
Fig. 15. Relation between wk and Vp- Vm.
I
I
!
2000
71
DeNOx
Air Preh
Fig. 16. Flow and control model of an E S P emission control system.
trostatic precipitation, the modified particle migration velocity Wk is used instead of w. The square root of Wk is proportional to w. Figure 15 shows the relation between Wk and Vp× Vm obtained at the same plant as in Fig. 11. It can be seen that there is a fairly good correlation between Wk and Vp X Vm. Therefore the product of Vp × Vm, which can be maximized by the adjustment of IE parameters such as the duty ratio, will be better used as a practical index for optimizing SP performance. 8.3 E S P emission control system Recently, in the integrated air pollution control system consisting of removal of nitrogen oxide, sulfur oxide and flying particles, demands to control the dust burden at the E S P outlet to an optimum or a certain constant value are increasing. One solution for it, is control of the duty ratio of IE using a micro-processor and feedback signals from an opacity meter installed at the ESP outlet. The flow and control scheme is shown in Fig. 16. In this system, ESP emission is controlled to be within a fixed range by changing the duty ratio from 1/61 to 1/3 in accordance with control signals associated with boiler operating conditions, such as change of boiler load or soot blowing etc. [3-5 ]. 9. Summary
Intermittent energization (IE), a novel method for energizing an electrostatic precipitator (ESP) which has been originally developed and commercialized by the authors et al., is outlined.
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Parametric investigations of IE on a pilot ESP and full scale ESPs were performed, and the features of IE recognized through the investigations and experiments can be summarized as follows: (1) IE is one of the most economical means for preventing or suppressing back corona in the ESP. (2) The major effect of IE is to increase ESP performance for high resistivity dust and to reduce power consumption. (3) The performance enhancement effect of IE differs depending upon the dust properties of each application. In iron-ore sintering plants, the dust of which is a typical example of extremely high resistivity, the maximum improvement of the enhancement factor is 2.5, and in low sulfur coal-fired boilers, 1.5. For medium resistivity dust produced from coal-fired boilers, soda-recovery boilers of paper mills, oil-fired boilers and cement plants etc., IE does not improve the performance, however, it does save more than 50% in power consumption at the duty ratio 1/2. (4) As for the energy saving characteristics, power consumption is nearly proportional to the duty ratio of IE. (5) Intermittent supply of current can increase the peak value of charging voltage Vp in general, while the average voltage Vm decreases with decrease of the duty ratio. The increase of Vp with IE may be considered as an index of degree of back corona suppression. (6) Half cycle IE is often more effective in improving ESP performance, particularly in the case of extremely high resistivity dust, than full (two half) cycle even at the same duty ratio. (7) With IE, the dust charging performance in ESP for high resistivity dust is recovered to a similar level to that of an ESP in normal operating conditions. (8) The enhancement factor of the migration velocity changes in a similar way to Vp × Vm at the change of the duty ratio of IE. Therefore, the product of Vp × Vm will be a better practical indication of the optimum efficiency in industrial ESPs. (9) Regarding ESP emission control in an integrated air pollution control system, IE can be used to keep an optimum and/or fixed constant value of ESP outlet dust burden to stabilize changes of boiler operation conditions such as change of boiler load, soot blowing etc. Notation
C E Eds ~s
F
/
equivalent capacitance of dust layer electrical field strength breakdown field strength of dust layer specific dielectric constant of dust layer collecting area specific collecting area
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H I
Ip /ave J Pd q R rc T1
T~ to tl
Yd Y. Ym W Wk
enhancement factor of modified migration velocity current peak value of current with IE average value of current with IE current density specific resistivity of dust layer particle charge equivalent resistance of dust layer intermittent duty ratio time of power on time of power off relaxation time constant of dust layer back corona starting time voltage across dust layer peak value of applied voltage average value of applied voltage theoretical migration velocity of particles modified migration velocity of particles
Acknowledgements The authors are indebted to emeritus Prof. Masuda of Tokyo University for his valuable advice on the phenomenon of back corona in electrostatic precipitators and they would also like to express sincere gratitude to colleagues, especially to Mr. T. Ando for his cooperation.
References 1 N. Tachibana, Y. Matsumoto, N. Teramura, K. Tashiro and N. Sakamoto, Mitsubishi Juko Giho, Vol. 15, No. 4, July 1978. 2 T. Ando, N. Tachibana and Y. Matsumoto, A new energization method for electrostatic precipitators: Mitsubishi intermittent energization, Proc. 4th Symp. on the Transfer and Utilization of Particulate Control Technology, Houston, Texas, Oct. 1982. 3 T. Ando, U.S. Patent, 4,410,849, Oct. 18, 1983. 4 T. Ando, G.B. Patent 2,096,845B, June 5, 1985. 5 T. Ando, France, Patent 8,107,121, Aug. 26, 1985. 6 H.J. White, Industrial Electrostatic Precipitation, Addison-Wesley, Reading, MA, 1983, pp. 325-326. 7 E.C. Landham, J.L. DuBard, M.J. O'Brien and C.V. Lindsey, The effect of high voltage waveforms on ESP current density distribution, Record of IEEE/IAS Annual Meeting, Atlanta, Oct. 1987.