Fuel Handling Systems in Power Plants
Coal needs to be stored at various stages of the preparation process, conveyed around the plant facilities. Coal handling is part of the larger field of bulk. Coal delivery equipment is one of the major components of plant cost. The various steps involved in coal handling are as follows:
Various step Involved in coal handling
1. Coal delivery
2. Unloading
3. Preparation
4. Transfer
5. Outdoor storage
6. Covered storage
7. Implant handing
8. Weighing and measuring
9. Feeding the coal into furnace
1. Coal delivery:
The transportation of coal from supply points to power stations situated near to sea or river is delivered by ships while it is supplied by rail or trucks which are situated away from sea or river. The transportation of coal by trucks is used if the railway facilities are not available.
2. Unloading :
For unloading the coal at the power station depends on equipment. If coal delivered by trucks, there is no need of unloading device as the trucks may dump the coal to the outdoor storage. Coal is easily handled if the lift trucks with scoop are used.
In case the coal is delivered by railways wagons, ships, the unloading may be done by car shakes, rotary car dumpers. cranes, grab buckets and coal accelerators. Rotary car dumpers although costly are quite efficient for unloading closed wagons.
3. Coal Preparation :
If delivery of the coal is in the form of big lumps and improper size, the coal preparation or sizing of coal is done by crushers, breakers, pulverizing mill, driers and magnetic separators.
4. Coal Transfer:
After preparation coal is transferred to the dead storage by means of the following systems.
(i) Belt conveyors
(ii) Screw conveyors
(iii) Bucket elevators
(iv) Grab bucket elevators
(v) Skip hoists
(vi) Flight conveyor
(i) Belt Conveyor:
● It consists of an endless belt moving over a pair of rotating drums called as rollers. The roller is provided at the center supporting on shaft at certain distance. The belt is made up of rubber or canvas which have sufficient strength and stiffness.
● Belt conveyor is suitable for the transfer of coal for long distances. It is used in medium and large power plants for a distance of 400 meters. The initial cost of system is not high and less power consumption. The slope or an inclination provided is about 20 with the horizontal at which coal can be successfully elevated by belt conveyor. Average speed preferred is about 60 to 100 m/min and load carrying capacity of 50 to 100 tons/hour
Advantages:
1. Rate of coal transfer easily vary by controlling belt speed.
2. Power require is minimum.
3. Repair and maintenance cost is low
Disadvantage:
1. Use is limited to belt inclination up to 20
2. Not suitable for short distance.
(ii) Screw Conveyor:
● It consists of an endless helicoid screw fitted to a shaft or containing either a spiral blade coiled around a shaft enclosed in a cylindrical housing. The housing is having inlet to feed the coal and discharge port at other end.The one end of shaft is connected to the driving motor while other end is supported in bearing in a housing or casing. The diameter of screw is 15 to 50 cm. The screw while rotating in a trough transfers the coal from feeding end to the discharge end.
● Screw conveyors can be operated with the flow of material inclined upward. As the angle of inclination increases, the capacity of a given unit rapidly decreases. The rate of volume transfer is proportional to the rotation rate of the shaft.This system is suitable, where coal is to be transfered over shorter distance and space limitations exist. The initial cost of the consumption is high and there is considerable wear o screw. Rotation of screw varies between 75-125 r.p.m
Advantages:
1. It is compact require minimum space.
2. It can be used for dust transfer also.
Disadvantages:
1. It has limit of length of it should be upto 30 meter which develop torsional stress.
2. Life is less due to wear and tear.
(iii) Bucket elevator :
● It consists of buckets fixed to a chain or belt. The chain/belt moves over two wheels/roller. The buckets can be also triangular in cross section and set close to on the belt or chain with little or no clearance between them. This is a continuous bucket elevator. Its main use is to carry difficult materials at slow speed. The coal is carried by the bucket from bottom and discharge from the top. Its maximum height is 30 meter and used up to inclination of 60 with horizontal. Its capacity up to 60 ton/hour
(iv) Grab bucket elevator :
● It performs two functions simultaneously that it lifts and transfers coal on a single rail or track from one point to the other. The coal lifted by grab buckets is transferred to overhead bunker or storage. This system requires less power for operation and requires minimum maintenance.
● The grab bucket conveyor can be used with crane or tower. Although the initial cost of this system is high but operating cost is less.
(v) Flight conveyor:
● It consists of one or two strands of chain to which steel scraper or flights are attached, which scrap the coal through a trough having identical shape. This coal is discharged in the bottom of trough. It is low in first cost but has large energy consumption. There is considerable wear. Skip hoist and bucket elevators lift the coal vertically while Belts and flight conveyors move the coal horizontally or on inclines.
Advantages:
(i) They can be used to transfer coal as well as ash.
(ii) The speed of conveyor can be regulated easily.
(iii) They have a rugged construction.
(iv) They need little operational care.
Disadvantages :
The Various disadvantages of flight conveyors are as follows:
(i) There is more wear due to dragging action.
(ii) Power consumption is more
(iii) Maintenance cost is high.
Pulverized Fuel Handling System :
● Two methods are in general use to feed the pulverized fuel to the combustion chamber of the power plant.
1. "Unit System"
2. 'Centralor Bin System'
● In unit system, every burner of the plant in fired by one or more unit pulverizers connected to the burners, whereas in central or Bin system, the coal is pulverized in the plant which is located at centrally and then it distributed to each furnace with the help of high pressurized air currents. Every fuel handling system consists of crusher magnetic separators. dryers, pulverizing mills, storage bins, conveyors and feeders.
● The coal received by the plant from the mine varies in size and shape. Hence it is the need of plant combustion furnace to available the coal in uniform size before passing the pulverizer for efficient grinding. The coal received from the mine is passed through a preliminary crusher to reduce its size to allowable limit (30 mm) The crushed coal is further passed over magnetic separator which removes pyrites and trapped iron.
1. Unit System :
● This system consists of a single burner or a group of burners and a pulverizer unit. The crushed coal is supply to the pulverizer through feeder at a variable rate according to the pulverizer equirement through feeder at a variable rate control by the combustion requirements of furnace and steam generating rates required in the boiler.
● The preheated air or flue gases from or fire gases are passed through the feeder to dry the coal before feeding to the pulverizer. The pulverized coal is carried from the mill with the help of induced draft, primary air fan. This further carries the coal through short delivery pipe to the burner. The secondary air is supplied to the burner before entering the fuel into the combustion chamber.
Advantages of Unit System :
(1) The layout is simple for coal transportation and cheaper than central system.
(2) It permits the direct control of combustion from the pulverizer.
(3) The repair and maintenance cost is less due to spares components requirement less.
(4) The old conveyor system can be used during the replacement of stokers.
(5) It keep better control on the fuel feed into the furnace.
(6) Coal which requires drying for satisfactory function of the central system is generally supplied without drying in the unit system.
Disadvantages of Unit System:
(1) The mill operates at variable load as per load on the power
(2) The total capacity of all mills must be higher than for the central system with the load factors common in practice.
(3) The degree of flexibility is less than that of central system.
(4) In event of failure of the auxiliaries of one of the burner, the burner has to put off as there is no reserve capacity
(5) The any fault in preparation unit may stop the working of the entire steam generator.
(6) As the working of the plant directly depends upon the pulverizing mill compalsary maintenance is required.
(7) There is excessive wear and tear of the fan blades as it handles air and coal particles.
2. Central or Bin System:
● The systematic layout of Central or Bin System. The crushed coal is supply to the drier by gravity from the raw coal bunker. The drying of coal is done either by using hot gases, preheated air or bled steam. The dried coal is fed to the pulverizer with the help of feeder. The pulverized coal is carried from the pulverizer mill with the help of air and it is separated in the cyclone separator.
● The separated pulverized coal is transferred to the central bunker with the help of belt or chain conveyor. All the equipment used in the centralbin system are of higher capacity. Storage bins are also used as an addition equipment whichcontain supply of pulverized coal for 12 to 24 hrs. The energy consumption of this system lies between 15 to 25 kW-hr per ton of coal pulverized
Advantages of Central System :
(1) It is more flexible and quick adjusted according to changes in demand.
(2) The quantity of fuel and air more flexible as they are separately controlled.
(3) Power consumption per ton of coal crushed per hour is less as pulverizer runs at its design load irrespective of the load on the plant.
(4) Operations of burners are independent of coal preparation.
(5) The labour cost is less as required less manpower.
(6) It offers good control over the fineness of coal.
(7) The fan handles only air therefore there is no problembof excessive wear of fan blades.
(8) The pulverizer can be shut down when sufficient reserve capacity has been achieved. The same can be used during peak load periods.
Disadvantages of Central System :
(1) Large space is require for plant setup so cost is high.
(2) High power consumption due to use of auxiliaries hence the overall power consumption per ton to coal handled is higher than unit system.
(3) Due to pulverized coal stored there is possibility of fire hazards.
(4) The coal transportation system becomes more complex.
(5) The requirement of dryers are essential.
(6) The operation and maintenance cost are higher than the unit system of same capacity.
The bin system is not favored in modern power stations, on account of dangers from explosion of coal air mixtures and tendency of the stored pulverized fuel to cake. In addition to this maintenance costs, losses through vents, capital costs, and operating costs are all against system consequently the unit system is adopted universally for all high capacity installations.
Pulverizing Mills :
There are various types of pulverizing mills.
1. Ball Mill :
A line diagram of ball mill using two classifiers is shown in Fig. 3.2.9. It consists of a slowly rotating drum which is partly filled with steel balls. Raw coal from feeders is supplied to the classifiers from where it moves to the drum by means of a screw conveyor. As the drum rotates the coal get pulverized due to the combine impact between coal and steel balls. Hot air is introduced into the drum. The powdered coal is picked up by the air and the coal air mixture enters the classifiers, where sharp changes in the direction of the mixture throw out the oversized coal particles. The over-sized particles are returned to the drum. The coal air mixture from the classifier moves to the exhauster fan and then it is supplied to the burners.
2. Ball and Race Mills:
In this mill the coal passes between the rotating elements again and again until it has been pulverized to desired degree of fineness. The coal is crushed between two moving surfaces, namely, balls and races. The upper stationary race and lower rotating race driven by a worm and gear hold the balls between them. The raw coal supplied falls on the inner side of the races, The moving balls and races catch coal between them to crush it to a powder. The necessary force needed for crushing is applied with the help of springs. The hot air supplied picks up the coal dust as it flows between the balls and races and then enters the classifier. Where oversized coal particles are returned for further grinding.
