The material handling system in a power plant is more exhaustive and should be suitable to handle large quantities of coal and ash generated. In a typical power plant the present generation boilers used are of fluidised bed type, which has got higher efficiency compared to the technology available yester years.
The FBC boilers require coal sized to -6 mm from ROM coal and hence it is of prime importance that the coal crushing and screening plant is highly efficient and reliable. Indian coal that has about 40% ash, generates huge quantities of ash on daily basis and hence requires reliable and comprehensive ash handling system in a power plant can lead to accumulation of large quantity of ash and the unit may have to be shut down. A typical power plant will have an independent coal / lignite crushing and screening plant and an ash handling plant. The coal/lignite handling plant is also connected to a dust extraction system, which maintains the stringent pollution control norms by sucking fugitive dust from the crusher/screen houses. A modern generation power plant hence handles large quantities of bulk material - like coal and ash.
A typical power plant is highly capital intensive and requires investment to the tune of Rs 3 crores per MW of installed capacity. Hence a reliable, efficient and cost effective bulk material handling system is required. Apart from the above, in present days the hot three gases generated due to exothermic reduction process of iron ore in sponge iron plant is also used for generation of power. Waste heat recovery boilers use the temperature for generation of superheated steam for the turbine and normally a typical 100 TPD kiln produces enough heat to generate 2.2 MW power. Those boilers also generate ash / particulate matters, which need to be handled continuously.
Coal Handling System
Basically coal / lignite handling system consists of a hopper for receiving coal from front-end loaders or dumpers. A grating is provided above the hopper, which limits the maximum size of coal / lignite going to the conveyor. A vibro-feeder that controls the flow from the hopper to belt conveyor and carries it to a primary crusher or a vibrating screens depending on the size of ROM coal / lignite. From the double deck-vibrating screen -6 mm present in the raw material directly goes to the bunker. Any material above +6 mm gets crushed for the desired sizing and is recirculated to the screen.
The type of vibrating screen and crusher is decided on the type of fuel, moisture present, HGI, index and the size of ROM material. Normally, in India, the vibrating screens used for this application are either circular motion type or linear motion type. Crushers used are hammer mills, impactors or roll crusher. It should also be ensured that no metallic particles get mixed with coal and gas to the crusher who would reduce the crusher life to a great extent. Now-a-days metal detectors, electro- magnetic separators can be used for removing foreign metallic items from the ROM coal / lignite. The crusher should also ensure that only correct size particles go to boiler bunker. Higher size particle would not get fluidised and fines won escape with out getting burnt. The crusher and screen is thus the heart of the plant and should be chosen with utmost care with due consultation with specialists. In India, Bevcon Wayors Pvt Ltd specialises in coal/lignite handling plants.
Ash Handling Plant
We can divide Ash Handling of modern day boilers into two major types i.e. Mechanical type and Pneumatic Type.
Mechanical type ash handling again can be broadly divided into two types. They are:
1.Use of Screw/Belt Conveyors and Bucket Elevators.
Use of screw/belt conveyors and bucket elevators is mostly done in small capacity plants with capacities not exceeding 2-3 TPH. Limitation of screw conveyors and bucket elevators is that it cannot handle huge capacities and is maintenance prone.
2. Use of Chain Conveyors.
Use of chain conveyors is highly successful in higher capacity plants up to 7-8 TPH. The chain conveyors use forged link chains and can operate extreme conditions with high degree of reliability and very less power consumption. Chain conveyors with high temperature of the bank ash as the bottom of chain conveyors can be lined with cast basalt.
·Pneumatic type ash handling is the most popular method used in medium level power plants. It uses dense phase conveying system for conveying ash is totally enclosed without any leakage.
Conveying material moves through the pipeline slowly as a coherent slug at a low velocity, which ensures minimum wear and tear. The system can convey materials up to a distance of around 200-250 mts. The general principle of operation for dense phase pneumatic conveying is as below.
General Principal Operation for Dense Phase Pneumatic Conveying
1. Conveying material is collected in hopper and is sensed by the permissive level probe provided in hopper and initiates conveying cycle. The system has dual operability either through level probe or through an auto timer. In timer mode of operation the system goes on continuous cycle after a preset time interval. Moreover the system has manual override facility.
2. The inlet valve on the top of vessel opens and allows material to gravitate into the vessel. The valve closes after the preset adjustable time delay.
3. Once the valve closing is ensured through a limit switch valve its circumferential inflatable seal inflates and ensures complete leak proof.
4. Now, convey air is injected into the vessel after ensuring seal pressure. Thus vessel is pressurised and material resistance leads to pressure build up which conveys material to destination point.
5. When conveying is complete, the pressure drops down to near atmospheric pressure and is sensed by control system. The conveying air supply to the system is then stopped.
6. Now the system is ready to accept next charge of material on command from level probe/auto timer.
7. Normally the destination side is also provided with high-level switch, which gives an alarm when filled up and prevents any further transfer of material into it.
8. The system is provided with various accessories like bag filters, ash conditioner to make the system environmental friendly. In a typical ash handling plant, the ash storage silo is designed for 1-day storage. The ash in silo is discharged on a lorry through an ash conditioner, which sprays water in mist form and prevents the ash from escaping to the atmosphere.
Bevcon Wayors has been working as a solution provider to the material-handling sector for now almost a decade and a half through technology transfers and constant expansion, adding value to the material handling sector. The company manufactures and deals with various types of crushers. Working principle of a few types is mentioned below.
A gyratory cone crusher is one of the main types of primary crushers in a mine or ore processing plant. Gyratory cone crushers are designated in size either by the gap and mantle diameter or by the size of the receiving opening. Gyratory cone crushers can be used for primary or secondary crushing. The crushing action is caused by the closing of the gap between the mantle line (movable) mounted on the central vertical spindle and the concave liners (fixed) mounted on the main frame of the crusher. The gap is opened and closed by an eccentric on the bottom of the spindle that causes the central vertical spindle to gyrate. The vertical spindle is free to rotate around its own axis. The crusher is a short - shaft suspended spindle type, meaning that the main shaft is suspended at the top and that the eccentric is mounted above the gear. The short-shaft design has suspended the long-shaft design in which the eccentric is mounted below the gear.
Roll crushers are compression type crushers, and were once widely used in mining. They have, within the last ten or so years, fallen into disfavor among roll crushers are compression type crushers, and were once widely used in mining. They have, within the last ten or so years, fallen into disfavour among mining and processing companies. The probable reason is because the large mines require very large crushed product output with minimal cost, makes the roll crusher uncompetitive. The role crushers are not nearly as productive as cone crushers, with respect to volume, and they do have a little higher maintenance associated with them. Roll crushers do, however, give a very close product size distribution, and if the ore is not too abrasive, they do not have high maintenance costs.
Roll crushers have a theoretical maximum reduction ratio of 4:1. If a 2-inch particle is fed to the roll crusher the absolute smallest size one could expect from the crusher is 1/2 inch.