In this article we will discuss about:- 1. Meaning of Electrostatic Precipitators 2. Principles of Electrical Precipitation 3. Process 4. Merits and Demerits.
Meaning of Electrostatic Precipitator:
Electrostatic precipitator (ESP) remove the gas borne particulate matter and the mist (liquid droplets) from the flue gas stream due to the action of electrostatic forces. When the gas stream passes through an electric field, the particles become charged and are migrated to the oppositely charged electrode-commonly known as Collection electrodes on the surfaces of which a porous layer of dust builds up with time.
The collection electrodes are earthed. The other electrodes employed to generate electric field-commonly called discharge electrode has negative voltage in the range of 15000 to 100000 volts. The discharge electrode is normally held midway between the collection electrodes (in plate precipitator). The collected particles are removed mechanically by periodic vibration, rapping or rinsing.
For, more than six decades, the ESP’s have been in the use to control particulate emissions in several industries ranging from cement plants, thermal power houses, coke-oven batteries, recovery furnace exhausts of pulp and paper mills, oil refineries, nonferrous metal refining furnace exhaust etc. Table 9.1 presents a comprehensive list of industries where ESP’s are in use.
Principles of Electrical Precipitation:
The electrostatic precipitation has the following steps:
(1) Generation of an electric field
(2) Generation of electric charge
(3) Transfer of electric charge to a dust particle
ADVERTISEMENTS:
(4) Migration of charged particle to collection electrode
(5) Adhesion of the charged dust particle to the surface of the collection electrode.
(6) Removal of the dust layer from the collection electrode (Rapping).
(7) Collection of particle in a hopper.
ADVERTISEMENTS:
(8) Ultimate removal of dust precipitation process must work efficiently.
Process of Electrostatic Precipitation:
Fig. 9.7 shows the schematic of the electrostatic precipitation process once voltage V is applied across the electrodes.
The voltage drop, ∆Vd is across the dust particles layer formed on the collection is given by:
where pd is the specific resistivity of the dust layer, Id, is the current density in the layer (A/m2) and p is the thickness of the dust particle layer, m.
ADVERTISEMENTS:
The specific dust resistivity is given as,
where R is the ohmic resistance of the dust layer, ohm, S is the area of the electrode applied to sample, m2.
ADVERTISEMENTS:
Field intensity in the dust layer is given by:
Once the charged particles move under electric field, attaining certain drift velocity, Uα, and get separated from the gas stream and deposited on the electrode surface, a layer of dust starts forming on the electrode surface which further affects the separation process and further built-up of the dust layer at not a uniform rate.
This process will create a voltage drop ∆Vd across the dust layer increasing with the build-up therefore with time, so diminishing the available voltage drop across the electrodes for ionization, (V- ∆vd), until a new current-voltage equilibrium is established.
Fig. 9.8 shows the typical successive equilibrium states formed between the voltage level and the current as the deposit layers on the electrodes grow gradually thicker.
Since the growth of the layer and decline in the net available voltage will reduce the collection efficiency, the layers are removed periodically by rapping, in the form of clusters or agglomerates, cohesive enough to resist excess commination and heavy and compact enough to drop into hoppers without any substantial re-entrainment.
The deposition of particles and the formation and nature of dust layer are governed by external and internal factors.
The formation of a layer, and the processes that subsequently take place in it, are characterized by several constants, all expressed in terms of time:
(i) The relaxation time, τc, that governs the separating process
(ii) The time constant, τu, that describes the formation and growth of the layer
(iii) The time constant, τc, of the electrical processes in the layer, especially of the discharging process.
The growth of the layer thickness d is expressed as,
where τH = time constant for the process of layer growth. Pmd is the density of the dust layer, kg/m3 is the distance F is the electrode configuration factor,
(plate precipitators)
n = number of discharge electrodes in the plate precipitators
a = s/Rc
s = distance between successive discharge electrodes, m
µg = viscosity of the gas at the temperature and pressure of operation, NS/m2.
Q max.s = maximum charge that can be built-up after a very exposure of the particle to the charging process by ion bombardment.
β = relative particle charge, Q/Q max.s
SSE = shape factor defined as, (V- ∆vd,) SE, = B Vmax SSmax
C = number density of particles, number/m3.
Merits and Demerits of Electrostatic Precipitations:
The electrostatic precipitators have the following merits and demerits:
1. Ease of Operation:
A wide range of carrier gas flow rate and dust loading have but little effect on efficiency.
2. Capacity:
High gas capacity upto 106 m3/hr are common.
3. Low Energy Consumption:
About 0.1 to 0.8 kwh/1000 m3 of gas, this reduces the noise level as well.
4. Operating Condition:
The temperature as high as 425°C may be used. The SO2 or corrosive gases concentration docs not affect much its operation.
5. Recovery of Valuable Products:
Valuable product in dry form as in spray-drying/ recovery furnace of pulp mills can be recovered.
6. Efficiency:
Any efficiency from low to high can be achieved with any flow regardless of particle size. As high as 99+% efficiency can be achieved.
7. Collection Mode:
Dry as well as wet collection of the particulate matter may be achieved. Dry collection saves on water consumption (in wet scrubbers) and disposal problems.
8. Maintenance and Operation:
It requires very low maintenance and operating costs.
9. It is ineffective in gaseous pollutants.
10. Ii is unsafe for operation with explosive gases.
11. Capital cost: It has very high initial investment.
12. Electrical Resistivity:
It is not good where the particles have high electrical resistivity.
13. At low Temperatures, fly ash collection from flue gases is adversely affected unless a certain amount of gaseous pollutants like SO2 is present, or the fuel gas is heated. Therefore, the type of dust plays an important role in the efficacy of ESP’S.