Note: Descriptions are shown in the official language in which they were submitted.
1~9~72
METHOD AND APPARATUS ~OR ELECTROSTATIC DUST PRECIPITATION
Field of the invention
The present inventio~ relates to a method and an
apparatus for effecting elec~rostatic precipitation of
dust from a stream of gas, especially effluent gas from an
industrial process.
Background to the lnvention
Many process industries, for example the manufac-
ture of cement clinker in a rotary kiln, produce dusty
effluent gases, which have to be cleaned, i.e. de-dusted,
before they are discharged to atmosphere. Where the dust
has a small particle size, the collection efficiency of
cyclone collectors is usually too low, and the dust burden
in the effluent gas from the cyclones is usually too high
to be acceptable to the environmental control authorities
and to the local populace; therefore, there is need to
use a device capable of a higher dust-collection efficiency.
j ~ ~he decision usually lies between a wet scrubber, a fabric
filter and an electrostatic precipitator.
Wet scrubbers and fabric filters both suffer from
the disadvantage of a fairly high gas-pressure drop through
them, resulting in fairly high fan power requirement and
consequently high electric power consumption. Wet scrub-
bers suffer the additional disadvantages of a tendency to
produce a saturated, water-mist-laden effluent gas and a
discharge of dust-laden water which often in itself poses
an effluent disposal problem. Fabric filters have the
additional limitations that most fabric filter materials
will be damaged or destroyed if they are subjected to
high temperature gases, very fine dust particles will
tend to clog and make impermeable the fabric and with
humid gases there is a possibility of moisture
condensation on the fabric, which also will render it
impermeable to the flow of gas through it. Despite this,
wet scrubbers and fabric filters often find economic
- application where relatively small vo~ume flowrates of
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effluent gas have to be de-dusted, and when the gas
temperature and humidity are not excessively high.
Electrostatic precipitators are often found to be
the most cost-eifec-tive, economic means of de-dus-ting
exhaust gases from process industries where the gas
volume flow rate is high, where the gas temperature is
uniformly high or may sometimes fluctuate to a high tem-
perature (e.g. in excess of 350C) or where the gas has
a high relative humidity. Electrostatic precipitators
are therefore commonly used in the larger process
industry applications and they can readily be designed
to have very high dust-collection efficiencies.
An electrostatic precipitator comprises9 in essence,
one or more electrical discharge electrodes,which are
raised to a high negative potential, and one or more
collector surfaces, which are at earth potential~ The
gas to be de-dusted is passed between the discharge
electrode(s) and the collector surface(s). An electrical
corona discharge from the discharge electrode(s) causes
the dust particles in the gas stream to acquire negative
electrical charges and those particles, now negatively
charged, are driven by electrostatic force to the earthed
collector surface(s) where they agglomerate and are
removed from the system so that the gas which emerges
from the electrostatic precipitator is substantially
dust-free.
The discharge electrodes are usually wires or
spiked rods. They are maintained at the required neg-
ative potential by means of an electrical transformer
and rectifier set, typically capable of generating a
negative potential of around 60 kilovolts and sustaining
a high voltage, DC current of a few hundred milliamperes.
The overall dust-collection efficiency of the electro-
static precipitator is maintained at or better than the
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required level by maintaining the negative potential of
the discharge electrodes as high as can be attained -
without causing electrical sparking or arcing between
the discharge electrodes and the ear-thed collector
electrodes. This maximum sustainable negative potential
varies according to the design characteristics of the
precipitator and also to the nature and composition of
the dust-laden gas. Sustained sparking or arcing caused
by trying to apply too high a negative potential on -the
discharge electrodes will result in inefficient overall
dust-collection performance of the electrostatic pre-
cipitator and also damage to the discharge electrodes.
A control system is therefore required to maintain an
adequately high negative potential on the discharge
electrodes whilst avoiding excessive sparking or sus-
tained arcing between the discharge and collector
electrodes.
e re¢tifier set generating the required negative
potential for the discharge electrodes usually comprises
a primary transformer, which is fed with alternating
current from the normal line supply (typically 440 volts)
and which produces an output of about 60 kilovolts AC,
which is then fed to a rectifier, which produces 60 kilo- ¦
volts negative DC.
The control system usually employed steps up by
increments the negative potential towards the maximum
of which the rectifier set is capable, typically 60
lovolts, until excessive sparking or arcing is detected,
then it withdraws or reduces the high-tension voltage
by pre-set increments, until cessation of the excessive
sparking or arcing is detected. Then, after a pre-set
time delay, the control system once again steps up by
increments the high-tension voltage until excessive
sparking or arcing is detected and then it again withdraws
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or reduces the high tension voltage by pre-set incre-
ments until sparking ceases. The cycle is continually
and automatically repeated.
This t~2e of con rol s~stem has -the charac-teristic
that it seeks continually to sustain the maximum possible
negative high tension potential at the discharge electro-
des and hence to maintain the maximum possible overall
collection efficiency of the electrostatic precipitator.
It has, however, two significant disadvantages. Firstly,
because it is continually seeking and then retracting
from a condition of sparking between the discharge and
collector electrodes, there must frequently though
intermittently be some sparking which will, in time,
cause erosion of and damage to the discharge electrodes
so that eventually they will fail mechanically, thus
making it necessary to take the electrostatic precipitat-
or out of service whilst it is repaired and adding to
~- the overall operating cost in terms of repair, labour
and replacement materials and, often, loss of production.
Secondly, this control system is designed continually to
seek the highest possible collection efficiency regard-
less of whether that collection efficiency is higher than
that required, and regardless of the electric power
consumption and cost.
There is usually no practical benefit to be gained,
- in terms of avoidance of dust deposition or nuisance, by
operating the electrostatic precipitator at much more
than its specified performance. Also, in most countries
- the environmental control authorities require that dust
collection equipment be designed, specified and operated
so that the final dust emission will always be less than
a specified figure: it is usually required not to exceed
a stated dust concentration in the outlet gas, usually
expressed in milligrams of dust per Normal cubic metre
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of gas. Because this condition must be always achieved,
the electrostatic precipitator has to be designed so
that it will meet the required maximum dust concentrat-
ion in the outlet gas even under the most unfavourable
operating conditions, hence the eleGtrosta-tic precipit-
ator must be over-designed in respect of the normal or
average operating conditions. The result of -this is
that the electric power consumption o~ the rectifier set
is, for the majority of the operating time, significantl~
higher than is necessary to achieve the maximum dust
concentration in the outlet gas as permitted by the
control authority, and higher than that required to
avoid causing a dust nuisance in the neighbourhood of
the plant.
:It.is known to use the recorded level of dust con-
centration in the outlet gas from a precipitator as a
signal to automatic voltage controllers to reduce or
increase voltage if the recorded dust level goes out-
~.side pre-set upper and lower dust emission limits.
This system is capable of reducing electric power
consumption and damage to discharge electrodes but is
not capable of producing a sufficiently precise control
of power input with variations in the emission level
to achieve maximum power saving and a minimum discharge-
electrode damage because of the difficulty of adjustingvoltage by the small amounts necessary to achieve the
requ.red current changes.
Thus, there is still a need for an improved.method
of reducing both the usage of electric power and the
progressive damage to the discharge electrodes by
sparking and/or arcing, whilst maintaing the dust
burden in gases leaving the precipitator at acceptable
levels.
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Summary of the invention
Surprisingly, it has now been found that signifi-
cantly finer control OI th~ actual power output of a
rectifier se~ associated with an elec-trosta-tic precipit-
ator electrode can be achieved by appropriate adjust-
ment of -the level of current of the rectifier set.
Furthermore, it has been found that this permits an
electrostatic precipitator system to be operated with
reference to only a single, predetermined level of dust
concentration in the effluent gas.
Accordingly, the present invention provides
a method of effecting the precipitation of dust from a
gas stream, in which the gas stream is passed through
one or more electric fields each of which is generated
by an electrode means having an associated rectifier
set; the concentration of dust in the gas downstream of
the said electric field(s) is monitored by dust-monitoring
means which generates a data signal indicative of said
concentration; and the intensity of at least one electric
field is adjustable by control means in accordance with
the data signal; characterised in that the concentrat-
ion indicated by the data signal is compared with a
predetermined target concentration and, if a difference
is found, the current of at least one rectifier set is
altered to produce a variation in electric-field intensity
in the sense required to reduce said difference.
The invention also provides an apparatus for
effecting the precipitation of dust from a gas s-tream,
which comprises means for defining a path for the gas
stream that passes through at least one electrostatic
precipitator, which precipitator comprises electrode
means adapted to generate one or more electric fields
within the precipitator, each such electrode means
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having an associated recti~ier set; dust-monitoring
means positioned downstream of the electrostatic
precipitator(s) and adapted to generate a da-ta
signal indica~ive of the eoncentration of dus-t in the
gas; and control means associated with at least one
of the said rectifier sets and re~ponsive to the data
signal; characterised in that the control means is
adapted to compare the concentration indicated by the
data signal with a predetermined targe-t concentration
and, if a difference is found, to alter the current of
at least one rectifier set to produce a variation in
electric-field intensity in the sense required to
reduce said difference.
Brief description of the drawings
15Figure 1 is a block diagram indicating the main
components of an exemplary apparatus according to this
invention.
- -Figure 2 is a purely diagraMmatic representation
of the manner in which voltage varies with the current
in a rectifier set.
Description of the preferred embodiments
Figure 1 illustrates, by way of example, an electro-
static precipitator system comprising two electrostatic
precipitators 1 arranged in parallel. Each is a three-
field precipitator, the fields being indicated by A,B and C. Each field is determined byitsrespective dis-
charge electrodes(not shown) and earthed collector sur-
faces (now shown), these components being of conventional
construction. Dust-laden gas is conveyed from industrial
process plant (e.g. a rotary cement kiln or a cement-
clinker mill) to the electrostatic precipitators 1 via
gas pipe 2, branches of which communicate with the inlets
3 of the precipitators 1.
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The gas outlets 4 of the precipitators 1 are
provided with gas pipes that converge to a single outlet
pipe 5 which leads to a chimney or stack (not shown~.
An op~ical density rneter 6 is installed in the gas out-
5- let pipe 5, which meter is calibrated to measure the
instantaneous concentration of dust in the effluent gas
from the precipitators and to produce an analogue data
signal in elec-trical form.
A microprocessor-based control system 7 is pro-
vided, which control system is adapted to receive the
data signal:.from the optical density meter 6 and to
compare the emission level represented by that signal
with a predetermined target value. The target value is
preferabIy chosen to be significantly below the maximum
emission level permitted by the environmental control
authority. For example, the maximum dust concentration
typically allowed by a control authority is lOOmg/m3
I - and, in such a case, the target value for the emission
could be set at 80mg/m3. (These emission values may be
contrasted with the typical dust concentration in the
- gas stream in the pipe 2, which may be of the order of
lOg to lOOOg/m .) The control system 7 will generally
; . be provided with an operator panel8 onwhich both the
:: present emission level and the target emission level
are displayed ~convenientIy in digital form using
~ ; liquid-crystal diodes or light-emitting diodes). The
-~` panel 8 may also be provided with switch means whereby
an operator may alter the target value for the emission.
When the control system 7 detects a difference
between the actual emission level and the target
emission level, it acts to alter the collection ef-
ficiency of the precipitators in the direction required to
reduce that difference. Thus, if the actual dust
34'72
concentration in pipe 5 exceeds the target level, then
the efficiency of the precipitators must be increased;
if the actual concentration is below the target level,
then t`ne collection ef~iciency of the precipitators is
reduced. In other words, the control system con-tinually
seeks to bring the actual emission to the target value.
In order that the comparison between the actual
concentration and the target concentration may be
effected, the individlal calibration curve of the part-
icular optical density meter 6 is stored within thememory associated with the microprocessor in the control
system 7. The comparison yields a digitised signal for
both display purposes and for control purposes. The
target value of the emission level must, of course, be
within the range determined by the characteristics o~ the
individual electrostatic precipitators 1.
Each of the discharge electrodes has an associated
~- high-voltage rectifier set 9. In accordance with the
present invention, the output of each rectifier set may
be adjusted (thereby adjusting the collectlon efficiency
of -the precipitator as a whole, since the efficiency is
dependent upon the electric-field strength or intensity
which, in turn, is determined by the output of the
rectifier set) in accordance with signals from the
contro~ system 7. Generally, the rectifier-set current
is altered in a stepwise manner from one l~vel to another.
The number of steps in the range of current of the high-
voltage rectifier sets is determined by -the microprocessor
program. The steps do not have to be uniform throughout
the range of current and it is desirable for them to be
so spaced that control of the dust emission is approx-
imately linear.
The foregoing aspect of the invention may be con-
sidered with reference to the accompanying Figure 2.
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This Figure represents (in a purely diagrammatic fashion)
the relationship between the voltage (in kV) and the
current (in mA) of a typical precipitator-electrode
rectifier se-t.
As shown in Figure 2, the voltage tends to increase
steeply with current at low current values but the rate
of increase in voltage lessens considerably as the
current approaches the maximum level at which the rec-
tifier set can be operated (the maximum possible oper-
ating level or "rectifier limit"), represented by xO.
Levels of current of the rectifier set may be established
by imposing limits on the operating current under the
control of the microprocessor-based system 7~ These
imposed limits are designated x1, x2, X3, etc. in
Figure 2. It will be clear from the shape of the curve
in Figure 2 that a given change in current will have a
smaller effect on the power out-put at higher levels of
current than will a corresponding change in current at
lower levels. Accordingly, in order to obtain a uniform
variation in rectifier output, the difference between
successive levels of current will decrease as the
current is lowered from the rectifier limit xO. In
other words, (xO-x1) ~ (xl-x2) 7 (x2-x3) and so on.
For example, if xO is 100%, x1 could be 85%, x2 75%,
x3 70% and so on. It is preferred that these current-
limit levels be so selected that the emission level will
be adjustable in substantially equal steps.
The pre-set levels of current can be imposed as
follows. High-voltage rectifier sets for electrostatic
precipitators are usually provided with a means by
which the voltage output is automatically increased
until sparking or arcing occurs within the precipitator.
At this point the voltage is reduced to a level below
which the sparking or arcing is maintained and the
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output is again increased automatically. The control
systems conventionally employed comprise a thyristor
regulator with appropriate spark- and arc-sensing
circuits having outpu-ts that are connected to and control
the conduction angle of the thyristor regulator. In the
absence of sparking or arcing, the voltage output of the
- high-voltage rectifier is controlled by a current-sensing
circuit such that the current is maintained either at
the rated current outpu-t of the high-voltage rectifier
set,or, by manual adjustment, at some preselected level
of current lower than the rated~current. In accordance
with the present invention, the microprocessor control
system is connected to the current-controlling circuit
in such a way as to determine the maximum current that
the high-voltage rectifier set is permitted to produce
at any given time, this maximum being adjusted as
necessary in response to the data signal from the dust-
I monitoring means.
In general, the comparison between the indicated
(measured) concentration of dust and the target con-
centration will be repeated at predetermined intervals
of time. Preferably, after any adjustment in the
operating conditions, sufficient time is allowed for the
effect to be registered by the optical density meter 6
before the néxt adjustment (if such is required) is
made. To that end, the control system 7 may incorporate
a clock or timing mechanism or circuit of known design.
The invention is applicable to any desired arrange-
ment of elec-trostatic precipitator fields in parallel
and/or in series.
The adjustment of the level of current of each
high-voltage rectifier set is effected according to a
control strategy appropriate to the number and the
configuration of the electric-field systems. For
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example, if the system comprises two or more electric
fields it is possible to adjust the limit of the current
of all of the high-voltage'rectifier sets simultaneously.
However, it is ~ene-,ally pref`erable -that the con-trol
means should act -to alter the limi-t of the current of
the rectifier sets in sequence. Thus, for example, if
the three fields A, B and C in Figure 1 are operating
at a level of current xl and the optical density meter
6 detects an emission level that is below the target
level, then the control system 7 may address each
rectifier se-t 9 in turn to lower its curren-t until the
overall efficiency of the precipitator 1 is such that
the actual dust concentration in pipe 5 rises to the
target level. Depending on the initial ~ust concentrat-
ion in pipe 2 and the precipitator characteristics, itwill be necessàry to lower some or all of the rectifier
sets to x2, and it may even be necessary to embark upon
l a sequence to lower the operating levels of one or more
rectifier sets yet further, to X3 or below. Conversely,
when the detected emission is above the target, the
current of one or more rectifier sets must be raised.
The sequence in which the rectifier sets are
adjusted may differ, depending upon the direction of
deviation from the target dust concentration, i.e. on
whether the system is being altered in order to decrease
or increase the overall collection efficiency, in order
to keep all of the high-voltage rectifier sets in step.
In some strategies, it may be appropriate to operate
the rectifier sets 9 in a given precipitator 1 at levels
that are more than one step apart under normal operating
conditions. If adjustment is necessary, it may then be
necessary to use a sequence in which one of the rectifier
sets is altered more than once in succession before
another rectifier set is adjusted. It is desirable that
the control system should take account of the status of
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1~9~472
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each rectifier set; if, for any reason, one becomes
inoperative and its output voltage falls below a pre-
determined level, that nigh-voltage rectifier set is
automa-tically excluded lrom the ad~ustment sequence.
It is necessary to remo~e the dust from the
collector plates in the precipita-tor. The dust may be
dislodged from the plates - e.g. by vibrating them or
by striking them with hammer means - an operation known
as "rapping". Although it is possible to e~fect rapping
continuously, it is more common to rap a series of plates
in turn at predetermined intervals of time. However,
especially when the upstream plates are rapped (since
these bear the main burden of dust removal), rapping
can give rise to a cloud or "puff" of dust dislodged
from the electrode plates which can give rise to a
marked increase in the dust emission from the precipitat-
ors. The present invention can accommodate means to
i -overcome this problem as follows. In an electrostatic
precipitator containing a number of fields in series,
the high-voltage rectifier set connected to the outlet
field can be adjusted so that the current is at the lOO~o
level during rapping. I~here the electrostatic precipit-
ator contains parallel fields, all of the high-voltage
rectifier sets that are furthest downstream (e.g. the
two fields marked C in Figure 1) are adjusted to the
100% current level. By this means, the "puff" of dis-
lodged dust can be collected by the downstream field or
~ields operating at maximum power. On completion of the
rapping operation, the microprocessor operation is
resumed initially with the same current limit settings
as those ~hich obtained just prior to the commencement
of the rapping operation.
It is clearly desirable that the dust-monitoring
means should undergo automatic re-calibration, which
9~3472
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term includes any automatic self-checking, such as zero
and span checks. However, these could interfere with
the microprocessor control functions and normally, there-
fore, -the syste~ will co!nprlse rneans -to obvia-te such
interference. For example, for the duration of the
recalibration period, the current-limit settings which
obtained just prior to the re-calibration operation
could be maintained (the microprocessor control being
resumed at these settings initially, when the re~
calibration has been completed). Alternatively, the
current could be restored to the 100% level for all of
the rectifier sets for the duration of the re-calibration
period, -the microprocessor control being resumed, upon
completion of the re-calibration, at the current-limit
settings that obtained just prior to the re-calibration
period.
It is desirable to provide protection against
-excessive dust concentrations in the effluent gas due,
for example, to an abrupt change in the process con-
ditions giving rise to the dust-laden gas or to the
failure of a high-voltage rectifier set. In such cases, the
control system may be unable to reast quickly enough to
avoid an emission level higher than that permitted by
the environmental control authorities. It is preferred,
therefore, that the microprocessor-based control system
is also adapted to cause each rectifier set to operate
at the maximum possible operating current ~i.e. the 100%
limit) upon such an abrupt increase in measured dust
concentration. The control system then operates, as
appropriate, to reduce the limits on the current from
the 100% level to a new optimum.
In addition to a continuous display, usually in
digital form, of both the actual dust concen-tration in
; the outlet gas, as measured by the optical density meter,
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l~g8472
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and the preselected target concentration (the former
being updated every time the control system 7 effects a
comparison on which an adjustment decislon is based, for
example at one-second in~rvals~, the operator panel may
also give a continuous indication regarding -the status
of the microprocessor control system and of each of the
high-voltage rectifier sets, as well as an indicator
warning the operator when the dust concentration in the
outlet gas is higher than the limit set by the environ-
mental control authority.
The influence exer-ted by the microprocessor in the
control system is limited to the modification of the
level of current (usually by imposing a limit on the
level of current) of the high-voltage rectifier sets.
Preferably, the precipitators are equipped with the in-
built automatic voltage controllers customary in the art.
The operation of these controllers is not otherwise
t- -restricted, i.e. with the microprocessor control system
switched off, the operation of the rectifier sets would
be conventional. The controllers have means to sense
flashover from any of the electrode means and are adapted
to reduce the operating voltage of the rectifier set assoc-
iated with that electrode means until the flashover ceases.
Accordingly, t~e normal control of sparks and arcs
continues to be available, although such flashover will
be much less in evidence as a result of the reduced
operating voltages entailed in the practice of this invent-
ion.
As împlied above, the present invention offers a
fine control over the operation of an electrostatic
precipitator system, despite the fact that only one
parameter (the dust emission) need be monitored. This
can give significant savings in the consumption of
electrical energy, in addition to the savings due to
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the reduced wear of the electrode systems. Furthermore,
the practice of the present invention does not pre-
suppose a uniform dust burden in the gas entering the
precipita-tors; on -the contrary, the present inver!-ti3rl,
as indicated above, can cope with dust burdens -that
vary continuously and widely.
