UNIT-I ==> Dust Collectors

 SMOKE AND DUST REMOVAL

            In coal fed furnaces the products of combustion contain particles of solid matter floating in suspension. This may be smoke or dust. The production of smoke indicates that combustion conditions are faulty and amount of smoke produced can be reduced by improving the furnace design.

            Dust particles are mainly ash particles called fly ash intermixed with some quantity of carbon ash material called cinders. Gas borne particles larger than 1μ in diameter are called dust and when such particles become greater in size than 100p they are called cinders. Smoke is produced due to the incomplete combustion of fuels, smoke particles are less than 10p in size.

            The disposal smoke to the atmosphere is not desirable due to the following reasons:

1. A smoky atmosphere is less healthful than smoke free air.

2. Smoke is produced due to incomplete combustion of coal. This will create a big economic loss due to loss of heating value of coal.

3. In a smoky atmosphere lower standards of cleanliness are prevalent. Buildings, clothings, furniture etc. becomes dirty due to smoke. Smoke corrodes the metals and darkens the paints.

    To avoid smoke nuisance the coal should be completely burnt in the furnace. The presence of dense smoke indicates poor furnace conditions and a loss in efficiency and capacity of a boiler plant. A small amount of smoke leaving chimney shows good furnace conditions whereas smokeless chimney does not necessarily mean a better efficiency in the boiler room.

To avoid the atmospheric pollution the fly ash must be removed from the gaseous products of combustion before they leaves the chimney.

 

Dust Collectors:

            Are classified into 2 types

1.     Mechanical Dust Collectors

2.     Electrical Dust Collectors

 

Mechanical Dust Collectors

           

Mechanical dust collectors are a category of particulate matter control devices that use mechanical methods to capture and remove dust and other solid particles from industrial processes, including those in thermal power plants. These devices typically rely on mechanisms such as gravity, centrifugal force, or impact to separate particles from the gas stream.

           

[Fig. a] by increasing the cross-sectional area of duct through which dust laden gases are passing, the velocity of gases is reduced and causes heavier dust particles to fall down. Changing the direction of flow [Fig. b] of flue gases causes the heavier particles of settle out. Sometime baffles are provided as shown in [Fig. c] to separate the heavier particles.

Subdivided into 2 categories wet & Dry Types.

Wet type dust collectors called scrubbers make use a liquid (usually water) to capture and remove pollutants through a combination of physical and chemical processes. Wet scrubbers are effective in removing both particulate matter and certain gases, making them versatile solutions for emissions control.

1. Venturi Scrubbers: Venturi scrubbers operate based on the Venturi effect, which involves a constriction in a duct that increases gas velocity. In a venturi scrubber, the gas stream is passed through a narrow throat, creating a region of high gas velocity. As the gas accelerates, water droplets are introduced into the gas stream. The high velocity and collision with water droplets cause particles to be captured and absorbed. Venturi scrubbers are particularly effective for removing fine particulate matter and certain gases.

2. Spray Towers: Spray tower scrubbers, also known as spray chambers, involve introducing water droplets into the gas stream using spray nozzles or atomizers. The gas passes through a chamber filled with water droplets, and the particles are captured as they collide with the droplets. Spray towers are relatively simple and are effective for removing larger particles from the gas stream.

3. Packed Bed Scrubbers: Packed bed scrubbers consist of a tower filled with packing material, which increases the contact area between the gas and the liquid. The gas stream is passed through the packed bed, and the liquid (usually water) is introduced from the top. Particles and gases are absorbed or captured by the liquid as they pass through the packed bed.

4. Wet Electrostatic Precipitators (WESP): Wet electrostatic precipitators are a combination of traditional electrostatic precipitators and wet scrubbers. In a WESP, an electrical charge is applied to particles in the gas stream, causing them to be attracted to charged plates or tubes. Simultaneously, a liquid mist is introduced into the gas stream, capturing the charged particles and removing them from the gas stream.

 Dry type dust collectors include gravitational, cyclone, louvred and baffle dust collectors.

 Dry type dust collectors, also known as dry scrubbers or dry particulate control devices, are air pollution control systems used to remove particulate matter from industrial processes without the use of a liquid medium. These devices capture and collect solid particles from exhaust gases through various mechanical and electrostatic methods. Dry type dust collectors are commonly used in thermal power plants to control emissions and ensure compliance with environmental regulations. Here are some common types of dry type dust collectors:

1. Electrostatic Precipitators (ESPs): Electrostatic precipitators use electrostatic forces to capture and remove particulate matter from the gas stream. The gas passes through a series of charged electrodes, where particles become charged. These charged particles are then attracted to oppositely charged collection plates or electrodes. Periodically, the collected particles are dislodged from the plates and collected in hoppers.

2. Fabric Filters (Baghouses): Fabric filters, also known as baghouses, use fabric filter bags to capture and collect particulate matter. The gas stream passes through the bags, and the particles are captured on the surface of the bags. Pulse-jet cleaning or reverse air methods are used to periodically dislodge and remove the accumulated particles from the bags. 

3. Cyclone Separators: Cyclone separators, while often associated with wet scrubbers, can also be used in dry applications. In dry cyclones, the gas stream passes through a cyclonic chamber, creating centrifugal forces that separate larger particles from the gas. The collected particles are then directed to a hopper for removal.

4. Mechanical Shakers and Vibrators: Some dry-type dust collectors use mechanical means to shake or vibrate the collection surfaces, dislodging the accumulated particles. These devices are often used in conjunction with fabric filters or cartridge collectors to aid in particle removal.

5. Hybrid Systems: Hybrid systems combine different types of dry collectors to enhance efficiency. For example, an ESP can be followed by a fabric filter to capture smaller particles that the ESP might miss.