Field of the invention

The present invention relates to a method and an apparatus for effecting electrostatic precipitation of dust from a stream of gas, especially effluent gas from an industrial process. 5 Background to the invention

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 0 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 5 use a device capable of a higher dust-collection efficiency The 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 0 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 5 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 volume flowrates of

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-effective, economic means of de-dusting 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 350°C) 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 comprises, in essence, one or more electrical discharge electrodes, hich 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 betv/een 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 earthed 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.

The rectifier 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 kilovolts, until excessive sparking or arcing is detected, then it withdraws or reduces the high-tension voltage by pre-set increments, until cessation cf 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 type of control .system has the characteristic 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 5 operating conditions, hence the electrostatic 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 of the rectifier set is, for the majority of the operating time, significantly 10 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. 15 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. 20 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- 5 electrode damage because of the difficulty of adjusting voltage by the small amounts necessary to achieve the required current changes.

Thus, there is still a need for an Improved method of reducing both the usage of electric pov/er and the 0 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 of _. the actual power output of a rectifier set associated with an electrostatic 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 fiel (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 stream, 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

having an associated rectifier set; dust-monitoring means positioned downstream of the electrostatic precipitator(s) and adapted to generate a data signal indicative of the concentration of dust in the gas; and control means associated with at least one of the said rectifier sets and responsive to the data signal; characterised in that the control means is adapted to compare the concentration indicated by the data signal with a predetermined target 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 Figure 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- fiel 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 v/ith 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 optical density meter 6 ^' is installed In the gas out- 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 electrical 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 preferably chosen to be significantly below the maximum emission level permitted by the environmental control authority. For example, the maximum dust concentration

3 - -- typically allowed by a control authority Is lOOmg/m - and, in such a case, the target Value for the emission could be set at 80mg/m . (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

3 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 (conveniently 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

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 the collection efficiency of the precipitators is reduced. In other words, the control system continually 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 individual calibration curve .of the part- icular optical density meter 6 is stored within the memory associated with the microprocessor ±n 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 of 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 collection 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 control system 7. Generally, the rectifier-set current is altered in a stepwise manner from one level to another. The number of steps in the range of current of the high- voltage rectifier sets is determined b ^• 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 betw'een the voltage (in kV) and the current (in mA) of a typical precipitator-electrode rectifier set. 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 x _Q .

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 x , x _^ , ₃ » 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 betv/een successive levels of current will decrease as the current is lowered from the rectifier limit x.. In other words, (x _Q - ₁ ) (x^x.) (x - ₃ ) and so on. For example, if x _Q is 100%, x., could be 85%, x ₂ 75%, x 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

output is again increased automatically. The control systems conventionally employed comprise a thyristor regulator with appropriate spark- and arc-sensing circuits ha ing outputs 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 output 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- 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 next 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 electrostatic 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 generally preferable that the control means should act to alter the limit 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 x- 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 set 9 in turn to lower its current 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 dust concentrat- ion in pipe 2 and the precipitator characteristics, it will be necessary to lower some or all of the rectifier sets to x_, and it may even be necessary to embark upon a sequence to lower the operating levels of one or more rectifier sets yet further, to x_ or belov. 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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each rectifier set; if, for any reason, one becomes inoperative and its output voltage falls below a pre¬ determined level, that high-voltage rectifier set is automatically excluded from- the adjustment sequence. It is necessary to remove the dust from the collector plates in the precipitator. 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 effect 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 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 100% level during rapping. 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 fields operating at maximum power. On completion of the rapping operation, the microprocessor operation is resumed initially with the same current limit settings as those which 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

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 system will comprise means to obviate 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 react 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 concentration in the outlet gas, as measured by the optical density meter,

and the preselected target concentration (the former being updated every time the control system 7 effects a comparison on which an adjustment decision is based, for example at one-second intervals), 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 exerted 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 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, the 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 implied 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

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 precipitators; on the contrary, the present invention, as indicated above, can cope with dust burdens that vary continuously and widely.