The modern ash handling systems usually used in large steam power plants are: 1. Belt Conveyor System 2. Pneumatic System 3. Hydraulic System 4. Steam Jet System.
1. Belt Conveyor System:
In this system, the ash is made to fall through a water seal over the belt conveyor in order to cool it down and then carried to a dumping site over the belt. This is a continuous handling and the power consumption is low. It can deliver around 3 tonnes of ash per hour with a speed of 0.3 m/minute. Life of belt is about 5 years in normal working conditions. This system of ash handling is employed in small power plants.
2. Pneumatic System:
In this system, air is employed as the medium for driving the ash through a pipe over long distances. The ash is passed on to the crushers from the boilers and then into the conveying pipe. Air is sucked through the delivery end which makes the ash to flow into the separators where the ash is collected in ash hoppers. The dusty air is filtered and exhausted to atmosphere through the exhaust fan.
The system can handle 5-30 tonnes of ash per hour. The advantages of this system are that it can carry ash through long distances and dust menace is reduced. Disadvantages of this system are that large amount of wear and tear results in the conveying pipe, high labour and maintenance charges and noisy operation. This system is usually employed for disposal of fly ash.
3. Hydraulic System:
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In this system, a stream of water carries ash along with it in a closed channel and disposes it off to the proper site. This system can be used for large capacity power plants where the ash is to be disposed off over long distances. This is a healthy, clean, dustless and completely enclosed system. This system can also handle molten ash breaking it into small sizes.
Hydraulic systems are of two types namely:
(i) High pressure system, and
(ii) Low pressure system.
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High pressure system operates intermittently whereas low pressure system is continuous one. In a high pressure system the hoppers below the boilers are fitted with water nozzles both at the top and on sides. The top nozzles quench the ash while the side ones provide the driving force for the ash. The ash and water then flow along a trough to a receiving hopper or sump where the ash is separated from the water. The water is used again while the ash is sent out through carriages. High pressure system is more expensive to install and operate.
In a low pressure system a trough or drain is provided below the boilers and water is made to flow through the trough. The ash directly falls into the trough and is carried by the water to the sumps and tanks. In the sump, the water and ash are made to pass through a screen in order to separate them out. The water is pumped back to the trough and used again while the ash is removed through carrier units and sent out to the dumping site.
4. Steam Jet System:
This system employs jets of high pressure blowing in the direction of ash travel through a conveying pipe in which the ash from the boiler ash hoppers is fed. Chilled iron or nickel alloy are used in lining the pipe as ash is abrasive and travels at a high speed. This system is useful where installation of other ash handling plants and conveyors is not possible because of lack of space or where the path of travel of ash is not straight. This system is employed in small and medium size plants. Steam consumption is about 110 kg per tonne of material conveyed.