Electrostatic Precipitator:
Electrostatic precipitator is based on Corona discharge that provide a simple and stable means of charging and collecting suspended dust particles in a moving stream of gas.
Step I - Ionization | Gas molecules in the vicinity of high voltage discharge electrode (negative) break down to form a train of plasma- a stream of positively charged ions plus free electrons. This occurs when applied voltage reaches a critical level. |
Step II – Corona generation | The free electrons being repulsed by the discharge electrode move towards the positive (ground) surface and collide with gas molecule, forming negative ions in the process. The negatively charged ions having lower mobility form a negatively charges space cloud, which is called corona. The space charged tends to stabilize the corona by reducing the further emission of high energy electrons. |
Step III – Charging and collecting dust particles | Following the establishment of corona, the dust particles entering it become negatively charged by ions present. Thereafter these ionized particles are attracted by positively charged electrode called collecting electrode, where their charges are neutralized and they settle on the electrode surface. |
Step IV – Particle removal | The collected dust particles are removed, in the dry process, by rapping the electrode surface electromagnetically, pneumatically or mechanically to slough away the particles into the hopper. In the wet process, the deposited material is rinse with an irrigating liquid. |
Fig: Dust collection mechanism in ESP
| Advantage | Disadvantage |
1 | It is the most effective method to remove very fine particulates, as fine as 0.001µ. | High capital cost of equipment. |
2 | Very useful for high dust loaded gas. Flue gas containing dust particles as high as 100 g/Nm3 can be effectively cleaned. | Power requirement is considerably high. |
3 | High dust removal efficiency. (99 to 99.5 %) | Removal efficiency drop with the increase of gas velocity. |
4 | Easy and smooth operation. | A good mount of floor space is occupied. |
5 | Minimum drought loss. |
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6 | Least maintenance cost. |
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7 | Both wet and dry dust can be removed. |
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Collection efficiency of an ESP:
µ = 1 – exp (-A Vm/V)
Where –
A | = | Effective collecting area | |
V | = | Gas flow through the precipitator | |
Vm | = | Migration velocity of particles | r = radius of particles ϕ1 = strength of the particle charging field ϕ2 = strength of the particle collecting field θ = viscosity or fractional resistant coefficient of the gas |
Factor effecting collection efficiency of an ESP: | ||
1 | Particle size | Collection efficiency will increase with increase of particle size and decease with the fineness of particle. |
2 | Flue gas velocity | The efficiency of collector increases with the decrease of gas velocity and decreases with its increase. |
3 | Area of collecting surface | Collection efficiency can be raised by increasing the collecting area and migration velocity. |
4 | Corona characteristics |
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5 | Field strength |
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6 | Particle resistivity |
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7 | Rapping |
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Fire/explosion in ESP
Possible causes=>
1. Unburnt in fuel- unburnt exceeding 10 % could be hazardous for ESP.
2. Formation of clunkers in ash hopper – If ash remains unclear, it get mixed with unburnt fuel and forms clinker. The temperature of this clinker is very high, which damages the ESP internals.
3. Air ingress – Air ingress in ESP forms clinker.
4. Explosive mixture – If there is accumulation of too much CO, due to improper combustion in boiler, comes in contact with O2 any small spark may result explosion. This happens probably during boiler start up and shut down.
Precaution :
1. Vibrator in ash hopper should be in running condition.
2. CO monitoring.
3. ESP should be kept air tight.
4. Periodically check unburnt in ash.
5. Ash removal system should be reliable and efficient.
6. Ash level sensor should be working properly.
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