Ash Disposal in Thermal (Steam) Power Plants

The ash generated after burning the coal in the boiler is disposed to discharged site or ash pond in thermal power plant. The various methods are used for disposing the ash collected hopper to the discharged site or discharged hopper which are located far away from the thermal power plant

Vacuum extraction plant:

This system of disposal  is used on both stoker and pulverized fuel installations. The crushers are not required if it used with pulverized coal fired boilers

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Water Ejection System:

This system also can be used with stoker as well as with pulverized fuel fired boilers with equal efficiency. Its adoption is more economical if the higher pressure water is used in the system.

The dust is periodically extracted from the dust hoppers by the water ejector and discharged into the sluiceway in the form of slurry. It capacity lies between 60 to 80 tonns per hour

Steam Ejector System:

This system is also used to carry the dust to the disposal site. 

Mechanical Conveyors:

The dust or ash is carried with the help of mechanical conveyors like screw and belt provided it is wetted carrying

The ash and dust is transferred either using wet system or dry system. 

Wet System:

In the wet system, ash is transported to ash ponds in the form of slurry. As ash settles in the pond, the part of the water evaporates and the remainder is either recylced or impounded. The system requires installations of pipe-lines and construction of embarkments.

Dry System:

In the dry system, ash is transported to the disposal site in a relatively dry state (in the form of paste). Water is added only to compact the ash. It does not require embankment to hold the ash. Compacted ash surface are covered with soil and seeded.

Combination of these two basic types also exist. Ash slurry may be pumped to a pond and after dewatering, it may be evacuated and transported to a dry site for final disposal. For boiler bottom ash, combination systems are commonly used. The bottom ash slurry is often dewatered and transported by the trucks to a dry disposal site.

Some of the advantages and disadvantages of wet and dry ash disposal systems are listed below:

Advantages of Wet Ash Disposal System:

• Transportation of ash by pipelines eliminates the noise and dust.
• In wet ash disposal system use of manned equipment is eliminated

Disadvantages of Wet Steam Ash Disposal System:

• Large quantities of leachate under a positive pressure head in the pond pose a constant threat to the ground water quality. This is eliminated by providing surface preparation and artificial lining which are very costly
• The transport water is normally recycled. This requires additional pipelines, pumping equipment, treatment facilities and substantial operational and maintenance cost
• Large are is required in the case of wet ash disposal system compared to dry ash disposal system (almost twice the area is required)
• Scaling and cementation within the pipelines, particularly when the slurry contains calcium. magnesium and sulphate ashes, may render this wet ash disposal unsuitable in certain cases
• Plenty of water resources are required in this process
• It is not flexible to relocate the other discharge site. 

Advantages of Dry Ash Disposal System:

• Leachate quantities are significantly reduced. Linear to disposal area can be eliminated by fixation of the ash. Ash piles can be designed to provide drainage at different levels.
• Water and power requirements are considerably less in dry ash discharge system
• Compacted ash is a structural material which can be sold (used in manufacture of bricks)
• Required storage area and volume is considerably less than wet type discharge system
• The ash disposal site has a wider choice of the land after closure

Disadvantages of Dry Ash Disposal System:

• Wetting of ash containing calcium and magnesium forms lumps which may stick to the conveyor belts. Larger lumps must be broken before transportation

Dy ash disposal system is often used as it has the advantage of reduced water demand, reduced risk of ground water contamination, reduced energy requirement and reduced storage volumes.