The two methods used to feed the pulverized fuel to the combustion chamber of the power plant: 1. Unit System 2. Central or Bin System.
In unit system, each burner of the plant in fired by one or more unit pulverizers connected to the burners, while in central system, the fuel is pulverized in the central plant and then distributed to each furnace with the help of high pressurized air currents. Each type of 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 widely in size. It is necessary to make the coal of 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 (30mm). The crushed coal is further passed over magnetic separator which removes pyrites and trapped iron.
1. Unit System:
In this system, each burner or a group of burners and pulverizer constitute a unit. Crushed coal is fed to the pulverizer through feeder at a variable rate governed by the pulverizer through feeder at a variable rate governed by the combustion requirements of furnace and steam generating rates required in the boiler.
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Hot air 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 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) It is simple in layout and cheaper than central system.
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(2) The coal transportation system is simple.
(3) It allows direct control of combustion from the pulverizer.
(4) The maintenance charges are less as spares required are less.
(5) In the replacement of stokers, the old conveyor system can be used.
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(6) Coal which requires drying for satisfactory function of the central system is generally supplied without drying in the unit system.
(7) It affords better control of fuel feed into the furnace.
Disadvantages of Unitized System:
(1) The mill operates at variable load as per load on the power
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(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 fault in preparation unit may put the entire steam generator out of use.
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(6) Strict maintenance is desired as the operation of the plant directly depends upon the pulverizing mill.
(7) There is excessive wear and tear of the fan blades as it handles air and coal particles.
2. Central or Bin System:
The crushed coal is fed to the drier from the raw coal bunker by gravity. The drying of coal is effected 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 (bin) with the help of conveyor. The central system uses practically all the equipments used in the unit system with higher capacity of each part. Storage bins are also used in addition to other equipments. This bin may contain from 12 to 24 hrs. Supply of pulverized coal. The energy consumption of this system lies between 15 to 25 KWHr per ton of coal pulverized.
Advantages of Central System:
(1) The central system is flexible and changes can be made to accommodate quick changes in demand. There is always a supply of fuel available in reserve in the boiler, bunkers since any mill can be used to supply any boiler, the outage of parts of the mills or even a short outage of entire pulverizing plant will not cause a boiler plant outage.
(2) There is greater degree of flexibility as the quantity of fuel and air can be separately controlled.
(3) The pulverizer always runs at its rated load irrespective of the load on the plant, therefore its power consumption per ton of coal crushed per hour is less.
(4) Burners can be operated independently of the operation of coal preparation.
(5) The pulverizer can be shut down when sufficient reserve capacity has been achieved. The same can be used during peak load periods.
(6) The fan handles only air therefore there is no problem of excessive wear of fan blades.
(7) It offers good control over the fineness of coal.
(8) More latitude is provided in the arrangement of the bunkers.
(9) The boiler accessories are unobstructed.
(10) The labour required is less.
(11) The mill capacity is low as it always works at rated load. But additional equipment needed for separating air from pulverized coal before storing in the bin, and the conveying system itself, offset the economy of lower mill capacity. Presently, the central system has been discarded in favor of unit system owing to building requirements, high initial cost and complications in most of the parts.
Disadvantages of Central System:
(1) The initial setup cost is high and occupies a large space.
(2) The power consumption of auxiliaries is high. Therefore, the overall power consumption per ton to coal handled is higher than unit system.
(3) There is possibility of fire hazards due to the stored pulverized coal.
(4) The coal transportation system becomes more complex.
(5) Dryers are essential.
(6) The operation and maintenance charges 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.