Ash Handling System

 Effect of Ash:

1.      It lowers the GCV of coal.

2.      Ash with low fusion point forms a deposit on the tube wall, affecting the generation of steam.

Low fusible ash forms clinker that contribute to clogging as well as cause severe corrosion of bar grates of the stoker furnace.

Bonded ash deposit:

It is a denser layer of slag firmly bonded to heat transfer surfaces of the boiler. At Higher flue gas temperature most of the ash particles suspended in the flue gas remains in the plastic stage and may form a sticky mass on the outer surface of the water wall and super heater tube. It quickly grows in thickness by arresting fly ash particles from flue gas and subsequently such physicochemical process as cracking, crystallization sulphation etc. may occur to strengthen the bond.

Factor responsible for the formation of bonded as deposit:

1.      The mineral matter present in flue gas.

2.      Length of exposure of mineral matter of flue gas to high temperature.

3.      Furnace temperature.

4.      Heating rate.

5.      Flue gas composition.

6.      Temperature of flue gas, fly ash and heating surface in those zone where formation of ash deposit is more likely.

7.      Physicochemical process taking place inside the deposited mass of ash.

Ash  fusion temperature:

The temperature at which the ash starting to melt and forms clinker is called ash fusion temperature.

1.      Initial deformation temperature – At this temperature ash starts its primary decomposition and deformation (solid-solid phase).

2.      Hemispherical temperature – At this temperature liquid solid phase reaction occurs. A part of solid mass on melting dissolves in it and the rest part of the solid forming a homogeneous melt. The homogeneous high viscos melt gives a hemispherical shape to the cylindrical ash specimen.

3.      Flow temperature – On further heating the viscosity of the homogeneous melt falls to such a degree, so that the melt tends to flow. This temperature is termed as flow temperature.

Ash fusion behavior:

Based on fusion behavior, coal is classified as-

1.      Refractory – hemispherical temperature above 1400 °C.

2.      Medium refractory - hemispherical temperature 1300 - 1400 °C.

3.      Fusible - hemispherical temperature below 1350 °C.

Based on chemical composition-

1.      Ash fusion index (R) = Refracting components / Fluxing components

R = (SiO2 + Al2O3) / (Fe2O3 + CaO + MgO + Alkalies)

R => Low => coal is clinker in nature.

R => High => coal is refractory in nature.

 

2.      Hemispherical temperature

H.T. (°C) = 1030 + 73.3 x Rm

 

Molar Index (Rm) = (3.33SiO2 + 1.97Al2O3) / (2. 5Fe2O3 + 3.57CaO + 5MgO + 3.22(Na2O + K2O))

 

3.      Slagging – Deposition formed on heat exchanger surface in radiation zone.

Slagging Index (Rs) = ((Fe2O3 + CaO + MgO + Na2O + K2O) / (SiO2 + Al2O3 + TiO2)) x S

 

Slagging Index	Slagging category
Less than 0.6	Low
0.6 – 2.0	Medium
2.0 – 2.6	High
Above 2.6	Very high

 

4.      Fouling – Deposition formed in convection zone.

Fouling Index (Rf) = ((Fe2O3 + CaO + MgO + Na2O + K2O) / (SiO2 + Al2O3 + TiO2)) x Na2O

 

Fouling Index	Fouling category
Less than 0.2	Low
0.2 – 0.5	Medium
0.5 – 1.0	High
Above 1.0	Severe

 

Ash handling system:

1.      Mechanical (Conveyor belt through a water seal)

2.      Hydraulic

3.      Pneumatic

4.      Steam jet

Types of fly ash collection system:

Mechanical		Electrical
Dry Type	Wet Type	ESP
Gravitational Separator	Spray type scrubber	Rod Type
Cyclone Separator	Packed bed scrubber	Plate Type
	Impingement type Separator

 

Electrostatic precipitator - It is a device to precipitate suspended fly ash and dust particles from the flue gas by ionizing the particles in a electric field and collecting them subsequently on opposite charge electric plate. (F.G. cottrell in the year 1960)

Electrostatic-precipitator

Cyclone separator - The best laden flue gas is allowed to enter a conical shell tangentially setting up a swirling motion in the body of the gas which cost off heavier particulate solid by imparting to them a centrifugal force. Dust particles collected at the bottom are separated out.

Advantage:

·       Low maintenance cost.

·       Higher efficiency for bigger size particles.

·       Higher efficiency at Higher load.

Disadvantage:

·       Fine dust particles escape separation.

·       Non flexible in terms of volume handled.

·       Efficiency declines with increase of fineness.

·       Considerable loss of pressure.

·       More power required to produce very high vertex velocity of the flue gas.

 

Multi Cone Dust collector.

Bag house filter – Dust laden flue gas is allowed to pass through the bag of clothes. These arrest the suspended dust particles and allowed the clean gas to pass out. When the pores of the fabric get clogged, it is regenerated by a gentle reverse flow of air.

Merits:

·       High efficiency 99.9%.

·       Efficiency is independent of the amount of dust in flue gas.

·       Particulate solids of size 1 micron and above.

Demerits:

·       Used for low sulphur coal (less than 1%)

Bag Filter

Cyclone scrubber - It is a kind of wet type mechanical separator in which the flue gas enters tangentially through a cyclone near the bottom of the scrubber and swirls upward. It comes in contact with a heavy dose of water droplet sprayed through centrally located spray nozzle. Dust particles absorbed by water droplets get sticky conglomerate and dropout. Which  is collected in that dust settling tank at the bottom.

Pneumatic Ash Handling system - A high velocity air stream is directed to collect ash and dust from all discharge points. Ash from boiler units falls through the hopper, passes through crusher and drops into the stream of air. Air borne ash and dust particles are separated in cyclone type separator. Finally the ash is allowed to pass through a filter and vented to atmosphere by an exhauster fan. The ash collected in the ash separator is trucked away for disposal.

Advantage:

·       Dust free operation, all fly ash and dust is eliminated.

·       No spillage problem or rehandling trouble.

·       No chance of ash freezing, picking or sticking in storage bin.

·       Flexibility in application to suit various plant condition.

·       Less installation cost per ton of ash handled.

Disadvantage:

·       Noisy operation.

·       High maintenance charge due to abrasive ash.

Hydraulic Ash Handling System:

1.      Low velocity system -- Ash from furnace grate falls into a channel of water passing low velocity and is carried out to sump tank. Where it settled, reclaimed and transported, while the separated water is recycled.

Velocity of water - 3 to 5 m/sec.

Capacity - 50 TPH through a distance 500 m.

2.      High velocity system - High pressure water jetting out from the nozzle fitted at top and side strike the hot and molten ash as it rolls off the chain grate. Water from tap nozzle quenches the ash while the side nozzles feeds the necessary driving force to transport the ash down the channel provided at bottom. The ash is allowed to settle in settling pit and the separated water is recycled.

Capacity – 120 TPH distance up to 1 km.