Electro-static Precipitator (ESP)

After the coal combustion in the Boiler, Ash and Flue gases are generated. These are nothing, but the Un-burnt coal particles. The heavier un-burnt particles fall down and collected as the bottom ash, while the lighter ash particles flow towards the atmosphere through the chimney due to the furnace draft. These particulate matters, SOX (Sulphur Oxides) & NOX (Nitrogen Oxides) emitted from the boiler through chimney to environment are called Flue gas, it contaminate atmosphere (Depletion of ozone layer) and affect health and vegetation.

Due to these effects there is a need of controlling the emissions to a level. In Indian coal the percentage of Sulphur is very less so we don’t need any system for De-sulphnisation, but to reduce the particulate matters from flue gas many methods are available. Some of them are

• Mechanical Cyclone Collectors
• Venturi Scrubber
• Fabric filters
• Electrostatic Precipitators
• Electrostatic Precipitator + Bag Filters)

Now-a-days Compact electro static bag precipitator (COEBP) is widely used by Indian power sector for the removal of fly ash from the flue gas.

Why COEBP (Compact electro static bag precipitator ?

The reason we are going for COEBP is that, Flue gas contains many particulate matters which are having some coarse particles and some fine particles. Coarse particles are big in size as compared to the fine particles. The disadvantage of ESP is that it separates only coarse particles (*later we will explain the function of ESP) in the flue gas, not the fine particles which move further and damage the environment. On the other hand fabric filters separate all the particles but the cost of fabric is very high, if we use it only the lifetime of the filter will reduce and emission system will not be economical. So due to these reasons, we use COEBP (Compact electro static bag precipitator), ESP removes coarse particles and bag filter removes fine ash particles from the flue gas.

(Schematic of ESP)

In COEBP, ESP and filters placed in series (one after other), First coarse particles are removed and then fine particles are removed.

Principle of ESP:

ESP works on the principle of electrostatic attraction. In this phenomenon a negatively charged particle or molecule is attracted to a positively charged molecule. In ESP, the flue gas passes into a chamber where the individual particles of fly ash are given an electrical charge (Negative Charge) by the absorption of a negative ion from a high voltage Negative DC ionizing field. So for that we have to provide a high voltage of DC in a chamber where a high voltage negative corona will generate and all the particles will be ionized.

• Ionization of gases and charging of dust particles with the help of corona generation.
• Movement of the particles to the collector plates & their Deposition
• Removing particles from the collecting surface with the help of hammer.

Construction of ESP:

The ESP consists of two set of electrodes,

• Thin wires called discharge or emitting electrodes
• Collecting electrodes in the form of plates

The emitting electrodes are placed in between two plates (Collecting plates) and the emitting electrodes are connected to the negative polarity of a high Voltage of DC source. The collecting electrodes are connected to the positive of the source and grounded.

Now two questions will arise from the above, the first question is that why does emitting electrodes are in the form of wire not plate or any other form and secondly why do collecting electrodes are in the form of plate not wire?

The answer is that, the amount of particulate matters in the flue gas is in very large quantity so if we are using collecting electrodes in the form of wires then the fly ash will not be deposited on the wire, similarly if we are using emitting electrodes in the form of plates then the field distribution will not be even and the ionization process will be affected.

Components of ESP:

Gas distribution screen: It is located at the inlet of the ESP. It is used to give an even distribution of gas over the precipitator’s entire cross sectional area.

Collecting System:

Above we already explained about collecting system, basically used for the deposition of all the charged particulate matters. The upper edges are provided with hooks, which are hung from support welded to the roof structure.

The Lower end of every plate has a shock receiving provision. Collecting system are connected to the positive of the source and grounded.

Emitting System:

Emitting system is used for emit the ion to charge the particulate matter. Mostly it is a wire in shape.

Rapping mechanism: Rapping mechanism is used in order to release/remove the deposited dust particles from the collecting plate. It is of two types:

• CERM: CERM stands for collecting electrode rapping mechanism. It consist tumbling hammer which is used to hammer the shock bar. Shock bar mounted at collecting plate lower end. When hammer hit the shock bar dust particles release from the collecting plate.
• EERM: EERM stands for emitting electrode rapping mechanism. During ESP operation a fraction of the dust will be collected on the emitting electrode. it is therefore necessary to rap the emitting electrodes. The rapping mechanism for EERM is located on the roof.

(Emitting Electrode Rapping Mechanism)

(Collecting Plate Rapping Mechanism)

Hopper:

Hopper is a large container used for dust or ash collection and lower portions of hoppers are provided with electrical heaters.They are pyramidal in shape.

Electrical system:

As we know that ESP action required corona generation, Due to this we required high voltage DC (25kV-80kV).So we use rectifier transformer, who steps-up and rectifies the voltage.

Bag filter:

Flue gas (fine particles) enters the bag house and passes through fabric bags, which act as filter. The bags are woven or felted cotton, synthetic or glass fiber material. Bags are supported by cages.

The separated fly ash is collected in a hopper from where this can be removed through mechanical screw conveyor or pneumatic conveying system.(Bag fabric = P84+ PTFE)