ELECTROSTATIC PRECIPITATORS (ESP)

Reproduced from Machinery Lloyd and Electrical Engineering (Vol. 37, No. 5 27th February 1965) with kind permission of the publishers – The Certificated Engineer.

Since national and local authorities tightened legislation controlling the emission of gases and vapours to the atmosphere, there has been a demand for efficient equipment to clean exhaust gases from power stations, the iron and steel industries, chemical plants and so on. Of the equipment available, electrostatic precipitators have long been regarded as the best means of providing highly efficient removal of dust and liquid droplets from air, gas or vapour streams, but in the past, relatively high capital costs have precluded their use in many cases.

More recently, the difference in the price of the electrostatic precipitator, compared with other types of cleaning equipment, has been reduced. This has come about by the development of new components and it is claimed that the running and replacement costs of an electrostatic precipitator are now lower than those obtained with other types of a collector of comparable efficiency. This efficiency of the collection can be varied to suit the users' requirements and the operating conditions, but it can exceed 99 per cent. The gas volume is not limited and the pressure drop across an electrostatic precipitator is by far the lowest of any high-efficiency dust collector and is generally less than 0.5 in wg. Solid and liquid particles, including those in the sub-micron range, are readily collected by electrostatic precipitation. With dry gases, the dust is removed from the collecting electrodes using mechanical shock which is imparted to the plates. In the case of wet gases, continuous irrigation or intermittent spray washing may be employed.

Principles of Electrostatic Precipitation

The principle involved in the precipitation of dust or mist in an electrostatic precipitator is that of imparting an electrical charge to the solid or liquid particles entrained in the gas stream, and causing them to be attracted to, and to settle on, the surfaces of collecting electrodes under the influence of a strong electrostatic field.

Between the rows of collecting electrodes (Fig. 1) are situated the discharge electrodes to which is applied a high voltage electric potential, usually of negative polarity, to produce an electric field between the electrodes and promote a corona discharge from the discharge electrodes. This has the effect of ionizing or dissociating gas molecules into vast numbers of positive and negative ions, which are immediately attracted to electrodes of opposite polarity.

As the ions predominantly originate in the 710se proximity of the discharge electrode. those bearing a positive charge have a very short distance to travel and consequently impart a charge to only a small proportion of the particles in the dust stream.

In the gas flow channels between the electrodes, the intense flow of negative ions travels to the collecting electrodes. Particulate matter in the gas stream becomes electrically charged by collision with, and adhesion to, negative ions, and is attracted to the collecting electrodes. When the charged particles are deposited on the electrodes they lose their charge and are held by natural inter-molecular forces of adhesion until these are disrupted by mechanical rapping or washing down.

It can be shown that the efficiency of the precipitator is related to the specific collecting area and the particle migration velocity (Fig. 2).

Design

To calculate the collecting area, the projected area of the electrodes is considered without taking into account the actual contours of their surface area.

The migration velocity is derived from experience and it takes into consideration the results obtained on similar applications or pilot plants with allowances made for gas distribution, dust resistivity, gas condition and other factors which have bearing on the efficiency of precipitation.

The principle of electrostatic precipitation gives freedom of design which allows the precipitators to be made in either horizontal or vertical gas flow types and the number of casings and electrostatic fields or zones can be arranged to meet particular requirements. There are, however, certain limitations as each precipitator must have a certain collecting electrode area to suit its operating conditions. Whether this area should comprise one or several mechanically or electrically separate electrostatic fields or zones depends on various factors.

For precipitators with a vertical gas flow, it is impractical to provide more than one zone in the direction of the gas flow. It is too complicated and expensive, as regards suspension from the insulators and providing rapping mechanisms and dust hoppers, to arrange several separate zones in series in the direction of the gas flow. Despite this, vertical precipitators are sometimes recommended when the amount of gas to be cleaned is relatively small, the dust can be easily precipitated and the available ground area is restricted.

For precipitators with horizontal gas flow, one, two or more separate casings can be arranged in parallel, each with one or more zones in series.

Multi-zone precipitators are mostly given preference because of their adaptability to varying operating requirements and the facility to adjust each zone to collect a specific grading of dust particles. A further advantage is that each zone can be provided with its high voltage unit to ensure the highest possible efficiency of collection of each dust particle size grading, which results in a better overall collection of dust than when a single unit is provided.

In a multi-zone precipitator, the first zone receives the heaviest dust loading and as a result, the voltage can be applied without excessive flashover and is usually lower than in the subsequent zones. When voltages are independently applied they can be easily adjusted to suit the conditions in each zone.

A sub-division of the precipitators into several zones has the further advantage of restricting to a small portion of the plant, the effect of any short circuit in the precipitator which would otherwise result in substantially lower performance. Therefore, the outlet dust burden is not suddenly increased if a short circuit fault occurs.

Rapping off the collecting electrodes can also be sequenced in each separate zone and related to the amount of dust to be removed. Intervals from a few minutes to a few hours between rapping cycles for removal of collected dust in each zone can be adjusted to ensure uniform removal of dust from the electrodes.

Discharge Electrodes

The purpose of the discharge electrodes is to promote the corona discharge and the profiles of these electrodes are selected to suit each particular application. Exhaustive research has demonstrated in almost every case that a high corona discharge intensity can be achieved by selecting a type of electrode specially designed for the gas condition being handled.

In the design of discharge electrodes, special attention must be given to the effects of temperature, corrosion, metal fatigue and mechanical strain when they are subjected to rapping.

Some typical electrode profiles are shown in Fig. 3. In certain special instances, plate type discharge electrodes may be used.

The suspended discharge electrode assembly is fully insulated from earth potential and is accurately positioned between adjacent rows of collecting electrodes.

The discharge electrodes in the dry type precipitators are usually mounted in tubular frames which transmit the vibrations caused by rapping to the full length of each electrode and this arrangement with improved design of suspension from the insulators, provides rigid location and overcomes some difficulties often experienced with other types.

Collecting Electrodes

A special feature of the dry type precipitators is the availability of a range of collecting electrodes with various surface profiles. The profile requirements for collecting electrodes for heavily dust-laden gases are first that the surface area should be free from irregularities and sharp edges, to ensure uniform distribution of the electrostatic field and an even distribution of the discharge potential. Secondly, the electrode should have as high as possible natural vibration frequency to ensure that even the finest dust particles are dislodged by a single impact during rapping.

The profile of the collecting electrodes contributes to the stability of the dust when it builds upon the electrodes between rapping sequences. The main feature comprises the catch pockets formed on each side of the individual electrodes, thereby reducing to a minimum the re-enactment into the gas stream of the collected dust.

Each electrode is pivoted at its top-end between supporting beams; whilst the bottom end is allowed to swing freely, within limits, between guide beams, when rapping takes place.

Usually, the first zone which receives the heavy dust burden is cleaned by rapping more frequently than the following stages, through which a lighter dust burden is passing. The intervals between rapping sequences depend on the properties of the dust. Each zone is provided with a rapping assembly which in turn is controlled by timers. This arrangement, from its flexibility in arranging the operation of the rapping mechanism; should avoid re-entrainment of the dust into the gas stream and also ensure that the rapping mechanism is not operated more frequently than necessary.

Rapping blows are transmitted to the electrodes through cams and compression springs in the majority of dry dust precipitators. The advantage of this system lies in its flexibility. For example, compression springs are employed to suit the force required and the throw of the cam is determined tv establish the effective movement of the individual rows of collecting electrodes. The individual cams are disposed on the camshaft throughout 360 degrees so that the individual rows or pairs of collecting electrodes are rapped in sequence, thereby preventing heavy deposits of dust being sheared off the electrodes at any one time and producing momentarily an unsightly emission of dust-laden gas from the chimney stack.

Alternatively, flail hammer rapping may be used.

With this system, a flail hammer with its anvil is mounted at one end of each row of collecting electrodes. The hammers for each zone are carried on a shaft which passes through a seal in the sidewall of the casing and is driven from a geared motor unit mounted externally to the casing.

With flail hammer rapping, the individual collecting electrodes in each row are so suspended that the effect of the rapping blow is transmitted throughout the row of electrodes.

Good gas distribution within a precipitator is essential to achieve the required collecting efficiency and various devices are employed to obtain this condition. The layout of the approach ducting can have a considerable bearing on the distribution of gas at the inlet to the precipitator. This factor is not easily predictable in the design stage. To obtain gas flow patterns, a scale model of the approach ductwork and precipitator sections can be used for tests in which a variety of devices may be employed to achieve good gas distribution through the electrical zones.

Visual effects are obtained by injecting balsa wood dust into the air stream (Fig. 4) and pilot surveys are undertaken at various positions to arrive at the flow pattern across the full section of the precipitator. The devices which may be employed to achieve optimum gas distribution, consist of splitter plates, guide vanes, louvres, perforated screen or a combination of any of these which are then related to the full-scale plant.

Electrical and Control Equipment

The high-tension units, comprising the rectifier, transformer, protective apparatus, and control gear are normally required to produce direct current at 30 to 75 kV. Depending on the size of the precipitator, its type, and the amounts and properties of the dust it is to remove, an ht unit of suitable capacity for each zone or section is chosen to deliver current in the range of 100 to 1 500 mA. Recently, selenium rectifier elements have been used extensively in the place of the rotary mechanical rectifiers which were originally used in most precipitator installations. Silicon rectifiers have also been employed in some transformer/rectifier units.

Normally the negative high voltage output terminal of the rectifier units is connected by an ht cable to the discharge electrodes in the precipitator; the positive terminal is connected to earth through low resistance in control circuits.

The a.c. supply is fed via the mains protective circuit breaker and transducers to the ht units where it is transformed and rectified. Voltage control is continuous, without contactors and is affected by changing the pre-magnetisation of the transductor. When the ht unit is instantaneously short-circuited by flashover, the transductor can carry almost the whole mains voltage, and at the same time limits the short circuit current. This prevents the development of an arc and, after a short delay, the normal voltage and current are restored.

To maintain the highest efficiency of dust collection in the precipitator, the voltage supplied should be controlled at a level slightly less than the flash-over value. Flash-overs are quite a common phenomenon during the operation of a precipitator, but must not develop into an arc of even short duration.

A certain number of flash-overs per unit of time can serve as the basis of control and hence become a reference for an automatic adjustment of the ht supplied to the precipitator. The automatic controls can be designed in various ways.

An electronic voltage regulator is used to sense the current surges generated by flash-overs and produces corresponding signal pulses. If the rate at which these pulses occur exceeds a pre-set maximum or falls below a preset minimum, the voltage regulator is operated, thus maintaining the ht voltage at a value just below that at which continuous flash-over would occur, i.e. at the optimum operating efficiency.

Should there be a tendency for a flash-over to develop into a sustained arc owing, for instance, to ~hanging gas conditions, a separate electronic device, independent of flash-over frequency and capable of discerning the difference between a flash-over and a heavy current discharge, passes a signal to the regulator causing an Instantaneous reduction of voltage? After a short delay period, the voltage automatically rises to the point at which it again is under control at the existing flash-over limit.

The Tilghman's control system differs from many types in that it operates without the use of mechanical contacts. It serves directly to control the precipitator current or voltage, through transductors having current controlling properties, without the need to operate rotating machineries such as pilot motors or pilot transformers.

It is also possible during operation to switch over from automatic to manual control when an alarm is used to indicate deviations from the pre-set values in the precipitator.

Dust Disposal

When the dry dust has been collected in a precipitator its removal often presents difficulties which can create further nuisances. For example, unless proper and regular attention is given to its discharge from the hoppers, the dust will accumulate to such an extent that the electrical zone will be reached and shorted out, thus causing failure of the precipitator.

As the majority of precipitators are operated under negative pressure conditions, care should be taken to ensure that cold air and moisture are not allowed to infiltrate into the hoppers otherwise difficulties due to caking. and bridging of the dust will occur, with subsequent removal difficulties. The two methods of dust disposal in use are intermittent removal, which usually necessitates manual operation and automatic continuous removal.

On dry dust disposal systems, it is sometimes advantageous to condition the dust before it is discharged. The conditioning is usually undertaken with the addition of water or binding agent so that the fine dust is agglomerated before disposal.