In precise terms, a precipitator can be regarded as a big box. The particulate loaded gases are drawn into a single side of the box the application of diffusers and perforated plate to uniformly dispense the gas. Within, huge voltage electrodes divulge a negative charge to the particulates that are entrained within the gas. It is important to note that these particles which are negatively charged are then attracted to a collecting surface (normally grounded and also positively charged). In the end the gas leaves the box subsequently cleaning up to 99%.
Within the box, the particulates from the ongoing flow of dust build up on the collection tubes or plates. At episodic ranges, the plates in a Dry Electrostatic precipitator are tapped and the tubes within a Wet Electrostatic precipitator are blushed, making the particulates to drop in the assemblage hopper (Abbas & Hossein, 2009). Within a Dry Electrostatic Precipitator, it is significant note that the particles are normally discharged from the hopper through a rotary screw procedure. Consequently, within the WET ESP, it should be noted that design of the hoppers are in a way that enhances gravity drain or even pump the flush water to the specific disposal/treatment.
Electrostatic precipitators (ESPs) are extensively useful in the control of particulate emissions from both industrial process sources and boilers. To enhance constant operation of the ESPs at the levels considered to be maximum emission regulation efficiency levels, preventive O&M and source specific procedures have to be acknowledged and thereafter instigated.
Basic Principles of Electrostatic Precipitators
Electrostatic precipitators take into account the usage of electrical forces to attract particles within a gas that is incoming. These particles are thereafter supplied with electrical charges through compelling them to pass by a region whereby ions, which are gaseous in nature, pass through. The passage is referred to as the corona (Eskom Holdings SOC Ltd, 2011). The field, which is electrical in nature and compels the charged particulates to the fortifications, emerges from electrodes, which in this case are maintained at a higher voltage within the center of the flow passage.
Within full sized electrostatic precipitators, numerous support systems and auxiliaries are necessary so as to enhance effective implementation of the electrostatic precipitation principle. Electrical devices for yielding big voltage supply are a necessity for the ESP. A rectifier (to convert DC voltage from AC) and superior voltage transformer (to boost the line voltage) are a necessity to the precipitation procedure.
The collected particulate matter within the plates ought to be removed through application of a rapping technique or even through flushing of water suppose it is a wet ESP as indicated above. The particulate loaded stream of gas must pass through the electrostatic precipitator unit at velocities, which are acceptable, so as to allow for effective assemblage of particulate matter. Both fan and duct system must be designed and subsequently configured in a way that ensures constant movement of gas through the system.
Numerous types of electrostatic precipitator designs are useful for an extensive range of particulate application. The main types of ESP useful in the industrial emission regulation include:
- Wire plate precipitator
- Flat plate precipitator
- Tubular precipitator
- Wet precipitator
Wire Plate Precipitator
The main distinction of this type of precipitator is that gas flows between electrodes and sheets of metals. Such electrodes are extensive wires that are weighted and hang between the plates and in some case even supported there by structures which are mast like in nature. They are referred to as the rigid frames on some occasions. Inside every flow passage, gas flow ought to pass every wire in series as it passes through the unit. It is essential to note hereby that the plate wire electrostatic precipitator permits numerous flow streets to function in parallel and every lane can be somehow extensive. As a consequence, this kind of ESP is well adapted to handle huge capacity of gas. The necessity to rap the plates so as to remove the material collected has made the plate to be separated to distinct parts, normally three or four in sequence with one another, which can, therefore, be independently rapped (Praechter, 2013). The supplies of power are normally separated into sections in a similar way to get higher voltages of operation. It should be noted that the sectionalization process can also be done to achieve increased reliability. Similarly to the collector wire, dust must be removed from the discharge electrode wires where they are deposited during the procedures of electrostatic precipitation.
Flat Plate Precipitator
A noteworthy amount of smaller precipitators, normally ranging from 100,000-200,000 acfm, use plates that are flat in nature instead of using wires for the electrodes of high voltage. Such flat plate enhances/intensifies the typical electric field, which can be applied in the gathering of the particulates, and they offer an increased surface area for the process of particles collection. Significantly, it is important to note that the generation of corona is not a possibility on flat plates in solitary. For this reason corona generating electrodes are positioned behind and sometimes ahead of the flat plate collecting areas. Such electrodes can be needles that are sharp pointed and subsequently attached to the edges of the plates or sometimes sovereign corona wires.
As opposed to the plate wire or even the tubular EPSs, flat plate precipitators design functions equally well with either positive polarity or negative polarity. This design of precipitator functions with less or even no current of corona flowing through the dust collected; apart from directly below the corona wires or needles (Sutherland, 2008). Due to this, two consequences emerge. The first one being the unit in some way becomes less vulnerable to back corona than conventional units are due to the fact that no back corona is yielded in the dust collected. The second consequence is the fact that there is lack of current in the layer collected, and consequently this condition causes an electric force that in a way has a habit of removing the layer from the collecting surface; subsequently, this can cause high rapping losses.
The earlier electrostatic precipitators turned out to be tubular, similar to the smoke stacks on which they were positioned, with the electrode (high voltage) running through the tube axis. Precipitators that are tubular in design possess characteristic applications in iron and steel sinter plants, tar removal (coke oven by-product gas cleaning) and sulphuric acid plants. The tubular units are still useful for some applications, with numerous tubes functioning parallel so as to keep up with intensified flow of gas.
A tubular electrostatic precipitator is primarily a single stage unit and is distinct in having the entire passage of gas through the region of electrode. The great voltage electrode functions at single voltage for the whole extension of the tube, and the current fluctuates throughout the extension as the particles are discharged from the system (Abbas & Hossein, 2009). Around the collecting region, there exist no sneakage parts; however, the non-uniformities of the corona may permit some particulates to escape charging for a significant portion of the tube extension.
These EPSs, which are tubular in design, form up just a little portion of the electrostatic population and subsequently are most frequently applied where the particulate is either sticky or wet. Such, normally cleaned with water, possess entrainment losses of an inferior degree than do the dry particulate precipitators.
Wet Electrostatic Precipitator
The chief distinctions in the kinds of wet electrostatic precipitator nowadays are as follows: the collector’s shape, whether treatment of the gas flow is horizontal or vertical, whether inbound gas is pre conditioned with sprays of water, and whether the whole ESP is operated wet.
Electrostatic System Components
The discharge control performance of an electrostatic precipitator is dependent on the design and functionality status of its system apparatuses. The most important system areas that are well known to diverse ESPs types include:
- Sectionalization and energization
- Rapping techniques
- High voltage frame insulators
- Hoppers and solids discharge equipment
- Gas distribution
Sectionalization and Energization
In order to achieve high efficiency in the use of precipitators, it is necessary to use more than one electrical field. Precipitators, which are highly efficient, possess more than a single electrical field. Two or more fields are usually delivered in the gas flow direction. For huge furnaces, the flow of gas can be divided into two or more compartments each of which has numerous fields in sequences. The process of sectionalization of the precipitators enhances both the reliability and the performance of the precipitator. One of the fundamental reasons behind sectionalization is the noteworthy particle concentration gradient, as well as dust layer thickness gradients between the outlet and inlet of the precipitator (Eskom Holdings SOC Ltd, 2011). Within the inlet of the EPS, the layer of dust accrues quickly because 60-80 percent of the mass is rapidly collected. Due to this, the field is made to be more susceptible to the electric sparking as a result of the non-uniformities within the dust layer electric field. Subsequently, the fine particles that are firstly charged in the inlet field however not gathered make a space charge in the inter electrode region.
For dry variety of electrostatic precipitators, there exist two major methods to rapping:
- External roof mounted rappers
- Internal rotating hammer rappers
The external rappers are linked to classes of collection plates or a distinct high voltage frame through way of shaft seals, insulators, and rapper shafts. The benefit of such roof mounted rappers is connected to the fact that the magnitudes can be attuned/adjusted for fluctuations in resistivity in dust layer and subsequently there exist access to the rapper during operation. The main disadvantage is that the huge amount of rapper shaft constituents has the capacity to offset rapping dynamism and become bound to the cold or hot surfaces.
The internal rotating rappers contain distinct rappers for every plate of collection. Because of the huger rapping forces imminent, such can be applied temperately high resistivity dusts. The limitations of these rappers turn out to be the incapacity to adjust the intensity and frequency in different parts of the precipitator and the consequent remoteness for upkeep. In addition, the internal rotating hammer rappers has the possibility of being susceptible to conservation complications like distortion of the hammer anvils, failure of the linkages, bowing of the support shafts in addition to the shearing of the hammer bolts.
High Voltage Frame Insulators
When dry and wet SSPs are used, two main types of insulators are used i.e. the bus line post insulator and the high voltage frame support insulator. It is necessary to label the top and ensure the frames are supported by means of posts or cylindrical insulators.
Hoppers and Solids Discharge Equipment
For the case of a dry type EPS, appropriate design when considering hopper and solids discharge apparatuses is specifically significant. Such units sustain huge mass loading and various reaction products always tend to be prone to bridging at hygroscopic. Thermal insulation, as well as Hopper heaters, are significant so as to deter the conditions of hopper overflow which could consequently lead to under voltage trip of a field thus in the long run bringing about serious complications related to plate-to-discharge alignment of electrode.
One of the most significant phases in guaranteeing gas distribution which is considered adequate is to permit for enough space for the purposes of steady outlet and inlet transition sections (Praechter, 2013). Units possessing sharp duct turns after and before the transition also tend to be susceptible to problems of gas distribution. Adequate distribution of gas can be attained by the application of one or more perforated gas distribution screens which are positioned both at the outlet and the inlet of the precipitator.
Advantages of Electrostatic Precipitator
- The first benefit of an electrostatic precipitator is linked to the fact on the capacity it has to effectively remove or discharge very tiny particles like mist, fly ash, and smoke. The approximate range of removal of dust is satisfactorily huge, that is 0.01 micron to 1.00 micron. The tiny particles of dust which are below 10 microns do not have the capacity to be removed by the aid of mechanical separators and subsequently wet scrubbers are not positioned to be applied/ used if adequate water is now available. Under such conditions, this kind of device is much more effective.
- ESPs are also most effective when it comes to dealing with gas which is highly loaded with dust. That is as much as 100grams per cu. Meter)
- It is important to note that the draught loss of an electrostatic precipitator system is the lowest when comparing all other forms of systems. Example: 1 cm of water.
- Electrostatic precipitators provide ease of operation. This is to imply that, such systems are user friendly despite their mechanical complexities thus just need a little bit of tutorial.
- Last benefit among the ones considered is that, there is collection of dust in dry form and can subsequently be discharge either wet or dry.
Disadvantages of Electrostatic Precipitator
Despite all the benefits which come hand in hand with EPSs, it is something regrettable that there are some limitations which come as a result of usage. As much as there exits such limitations it is noteworthy to indicate thatsome of the disadvantages can be averted.
Some of the disadvantages include:
- Due to the close proximity between the charged plates and the high potential used, it is a necessity to safeguard the whole collector from sparking through provision of a fine mesh prior to the procedures of chamber ionization (Sutherland, 2008). It is a significant notation because studies and research simulations indicate that even a tiny piece of paper might cause sparking in case it is carried across nearby plates of wires.
- The second limitation is in regard to the direct current which is not available within the modern plants. Due to this, significant electrical apparatuses is a necessity so as to enhance conversion of low voltage such as 400v A.C to high voltage such as 6000v D.C. Consequentially, this in turn heightens the capital cost of the whole device as up as 40 to even 60 cents per 1000 kilograms of rated fixed steam yielding capacity.
- The effectiveness and the efficiency of the collector are not maintained suppose the gas velocity surpasses that for which the plant is intended. In addition, the dust carried with the gases heightens with a rise of gas velocity. Lastly, it is important to note that the running charges are also substantially great as the volume of power needed for charging is significantly huge.
Conclusion: Generally, all electrostatic precipitators, despite the fact that they are widely categorized, still possess similar features in terms of distinct components and operate through charging liquid aerosols or particles, collecting them and at the end removing them from the electrostatic precipitator before final disposal within a landfill/reuse in the industrial procedure.
ESPs are sometimes known as the cold side, tubular, or even some different descriptor can be used as long as the definition is met. The designs of ESPs most of the time incorporate a number of ESP characteristics into a single unit. For instance, a typical electrostatic precipitator that is useful in the discharge of particulate matter from within a boiler which is coal fired is expected to be a cold side, single stage, plate ESP.
All in all, it is very critical to recall that an ESP is particularly designed to collect liquids or particulate matter for a discrete industrial application. Vendors are ought to use those features, i.e. plates, tubes, etc., that most promptly enrich the removal process of the pollutants from the flue gas.
As needed, supplies of power for the ESP transform the pulsating dc voltage within the range of 20,000-100,000 from the industrial ac voltage ranging within 220-480 volts. This kind of supply involves filter capacitors, high voltage rectifiers and a set-up transformer. The unit has the capacity of supplying either full wave or half wave rectified voltage of dc. There exist auxiliary controls and components to permit the voltage to be attuned to the utmost level possible devoid of extreme sparking and subsequently safeguard the electrodes and the supply in case of a short circuit or even a heavy arc.