Removal of Dust from Exhaust Gases | Electrical Engineering

In this article we will discuss about the process of removal of dust from exhaust gases by dust collectors and electrostatic precipitators.

The exhaust gases leaving the boiler contain particles of solid matter in suspension-smoke, dust, soot, fly ash or carbon as material called ‘cinder’. The quantity of these solid particles largely depends upon the method of fuel firing. Flue dust is greatest with pulverised fuel and spreader stoker firing systems and is much less with underfeed stoker systems.

In case of pulverized fuel firing, 60 to 80 per cent of the total ash produced in the furnace, escapes through the chimney as flue dust. Removal of dust from the exhaust gases is very important. The environment gets polluted and suspended matter can fall on the neighbour residential premises and create legal liabilities for the authorities. Cleaning of gas poses difficult problems because of large percentage of silica in the ash and fitness of typical fly ash.

Gas cleaning devices make use of certain physical/elec­trical properties of the particular matter of the gas stream. Basically gas cleaning devices called the dust collectors may be classified into mechanical and electrical ones (electro­static precipitators). Mechanical dust collectors have effi­ciency increasing with load while the efficiency of electro­static precipitators falls with the increase in load. Combina­tion of the two, giving constant efficiency characteristic, is often employed. Heavier dust particles are removed by me­chanical dust collectors and finer particles are eliminated by electrostatic precipitator.

Mechanical dust collectors can be further classified as wet and dry dust collectors. In wet type units, dust is washed away from the flue gases by spraying water on it. This sys­tem is usually not used because it needs large amounts of water. The principles employed in case of dry type mechani­cal dust collectors are shown in Fig. 3.34.

In Fig. 3.34 (a) the dust area is increased causing reduction in gas velocity and thus settles out heavier dust particles. In Fig. 3.34 (b) there is a sudden change in the direction of flow of the gas resulting in the settling out of the heavier particles which cannot flow along the gas. In Fig. 3.34 (c) the dust particles strike the baffles placed in the path of the exhaust gases and the dust particles will settle out.

Electrostatic Precipitator:

It essentially consists of two sets of electrodes which are completely insulated from each other and a high voltage elec­trostatic field is maintained across them. One set, called the emitting or discharge electrode, is in the form of thin wires and the other set is called the collecting electrode.

The emitting or discharge electrodes are placed in the centre of a pipe in case of tubular type precipitator (or midway between two plates in case of plate type precipitator) and are connected to negative polarity of HVDC source (25 to 100 kV) while the collecting electrodes are connected to the positive polarity of the source and are earthed.

High electrostatic field thus set up between the two sets of electrodes creates corona dis­charge and ionises the gas molecules as the flue gas flows through the tube or in between the plates. The dust particles in the gas acquire negative charge and are attracted to the electrodes connected to the positive polarity (collecting elec­trodes) and get deposited there. The deposited dust is made to fall off the electrodes when rapped mechanically.

The electrostatic precipitator may be:

(i) Plate or tubular type.

(ii) Horizontal flow or vertical flow type.

(iii) Dry or wet type.

In case of wet or irrigated precipitator, the deposited dust is removed by a water film flowing down on the inner side of the collecting electrodes. Usually, a dry type electro­static precipitator is employed.

The electrostatic precipitator has the advantages of high efficiency (up to 99.5%), low pressure drop, easy removal of collected particles and capacity of handling large volume of flue gases.

The limitations of this system are high capital and op­erating costs, and requirement of more space. The maximum collecting efficiency is maintained only for one value of gas velocity for which the system is designed.