Dust Collection System

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Dust Collection System

  • In coal fired boilers the flue gases have certain particles of solid matter in suspension this is called smoke or dust. In case of pulverized coal furnaces the fly ash remains in suspension with flue gases.
  • If the particles in suspension are of size from 1 to 100 microns it is called dust or smoke and for particles size more than 100 microns, it is called cinder.
  • Any dust particles leaving into chimney exhaust are objectionable and harmful for the health of human being and for plant life.
  • The production of smoke in chimney exhaust is also indicative of incomplete or improper combustion of fuel, in turn, it is indicative of low thermal efficiency of the plant.
  • Therefore, it is always necessary to clean the gas from dust, smoke or cinder particles before it is to be discharged from the chimney.

Types of Dust Collectors :

The dust collectors are of two types as follows :
  1. Mechanical Dust Collectors
  2. Electrostatics Dust Collectors
1. Mechanical Dust Collectors
Mechanical dust collectors are classified as :
  1. Dry type which are further classified as gravitational and cyclone separators.
  2. Wet type or scrubbers

Gravitational Dry Type Dust Collectors :

The basic principle of dry type dust collectors is shown in Figure A.
In these type of dust collectors the gas with dust is passed through a passage of increasing area in order to reduce its velocity. It causes the heavy dust particles to settle down and these are collected separately.

Gravitational Dry Type Mechanical Dust Collectors
Figure A

Wet Type Mechanical Dust Collectors (Scrubbers)  :
In wet type dust collectors , the dust is washed by using water sprays.
This method of dust collection is not suitable for large power plants since huge quantity of water is needed. Also the waste water will be a polluted water needing chemical neutralization before it is discharged into a water body.

Cyclone Type Mechanical Dust Collector :
  • A cyclone dust collector is shown in Figure B.
  • The laden gas is made to flow in a vortex along the sides of a cone, the cleaned gas leaves in the inner vortex upwards and the dust separated is collected at the bottom. 
  • The performance of cyclone dust collectors depend on the volume of gas handled, the inlet velocity and temperature, diameter to height ratio of cyclone chamber loading of dust particles in gas and the gas characteristics.
Cyclone Dust Collector
Figure B

Advantages of Cyclone Type Mechanical Dust Collector :
  1. It is simple, cheap and easy to operate.
  2. Maintenance cost is less. Efficiency increases with increased loading of dust particles.
  3. It can remove easily the dust and cinder particles.

Disadvantages of Cyclone Type Mechanical Dust Collector :

  1. Needs power to operate, thus operational cost is high compared to other type of dust collectors.
  2. Needs more space.
  3. The efficiency is low when it operates with very fine dust particles.

2. Electrostatic Dust Collectors (ESP) :

In 1905, Dr. F. G. Cottrell first introduced the concept of electrical equipment for collection of dust and the first commercial electrostatic precipitator was developed in 1937. It was used in many power plants used by the steel, paper mills and other industries. 
The principle of operation of electrostatic precipitator is shown in Figure C. 
The basic elements of the electrostatic precipitator are :
  1. Source of high voltage (440V, 50 Hz, 3 supply). 
  2. Two sets of electrodes insulated from each other called emitting and collecting electrodes. 
  3. A high voltage transfer. A rectifier to convert AC into DC.
  • It has two sets of electrodes insulated from each other. 
  • One set has the rows of electrically grounded vertical parallel plates called collection electrodes . The second set of electrodes consists of wires called the discharge or emirting electrodes. 
  • These electrodes are centrally located between each pair of collecting electrodes. The emitting electrodes carry the negatively charged high voltage (40 to 80 k) current from an external DC source. 
  • The dust laden gas is passed between these two electrodes which are oppositely charged. This gas becomes ionised as the high voltage of 40 to 80 kV is applied depending on the electrode spacing. 
  • Both positive and negative ions are formed. The positive ions travel to the negatively charged wire electrodes. 
  • The electrons follow the electrical field towards the grounded electrodes but their velocity decreases as they move away. 
  • Gas molecules capture the low velocity electrons and becomes negative ions. 
  • As these ions move to the collecting electrodes, they collide with fly ash particles in the gas stream and give them negative charge. 
  • The negatively charged fly ash are driven to the collecting plate by the electrostatic force and are separated from the gas. 
  • The use of these collectors is increasing due to strict pollution norms. Some times they are used in addition with cyclone collectors if the size of dust particles is high. It has a high collection efficiency of the order of 90% and above.
Principle of Electrostatic Precipitor
Figure C
Advantages of Electrostatic Dust Collectors (ESP) :
  1. It can remove fine dust particles efficiently.
  2. The maintenance charges are low.
  3. Easy to operate.
  4. It is most effective with high dust loaded gas (100 gm per m’).
  5. Dust is collected in dry for.
Disadvantages of Electrostatic Dust Collectors (ESP) :
  1. Capital cost is high.
  2. Operational charges are high.
  3. Space requirement is more compared to other dust collectors. If gas velocity above the designed speed increases, its collection efficiency is reduced.

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