Method Of Fuel Firing
The solid fuels are fired into the furnace by the following methods :
1. Hand firing. 2. Mechanical firing.
Hand Firing
This is a simple method of firing coal into the furnace. It requires no capital investment. It is used for smaller plants. This method of fuel firing is discontinuous process, and there is a limit to the size of furnace which can be efficiently fired by this method. Adjustments are to be made every time for the supply of air when fresh coal is fed into furnace.
Hand Fired Grates.
A hand fired grate is used to support the fuel bed and admit air for combustion. While burning coal the total area of air openings varies from 30 to 50% of the total grate area. The grate area required for an installation depends upon various factors such as its heating surface, the rating at which it is to be operated and the type of fuel burnt by it. The width of air openings varies from 3 to 12 mm.
The construction of the grate should be such that it is kept uniformly cool by incoming air. It should allow ash to pass freely. Hand fired grates are made up of cast iron. The various types of hand fired grates are shown in Fig. 4.10. In large furnaces vertical shaking grates of circular type are used.
The main characteristic of a grate fired furnaces are the heat liberation per unit of grate area and per unit of volume. The heat liberation per unit area of fire grate area is calculated as follows:
H = (W × C)/A
where
H = Heat liberation per unit of fire grate area
W = Rate of fuel consumption (kg/sec)
C = Lower heating value of fuel (kcal/kg)
A = Fire grate area (m2 )
The heat liberation per unit of furnace volume is given by the following expression:
H = (W × C)/V
where
H = Heat liberation per unit volume
W = Rate of fuel consumption (kg/sec)
C = Lower heating value of fuel (kcal/kg)
V = Volume of furnace (m3 ).
These two characteristics depend on the following factors :
(i) Grade of fuel
(ii) Design of furnace
(iii) Method of combustion.
Hand Fire Grate Furnace.
Fig. shows a hand fire grate furnace with a stationary fuel bed. The grate divides it into the furnace space in which the fuel is fired and an ash pit through which the necessary air required for combustion is supplied. The grate is arranged horizontally and supports a stationary bed of burning fuel.
The fuel is charged by hand through the fire door. The total space in the grate used for the passage of air is called its useful section.
In a hand fired furnace the fuel is periodically shovelled on to the fuel bed burning on the grate, and is heated up by the burning fuel and hot masonry of the furnace. The fuel dries, and then evolves gaseous matter (volatiles combustibles) which rise into the furnace space and mix with air and burn forming a flame. The fuel left on the grate gradually transforms into coke and burns-up. Ash remains on the grate which drops through it into ash pit from which it is removed at regular intervals. Hand fired furnaces are simple in design and can burn the fuel successfully but they have some disadvantages also mentioned below:
(i) The efficiency of a hand fired furnace is low.
(ii) Attending to furnace requires hard manual labour.
(iii) Study process of fuel feed is not maintained.
Cleaning of hand fired furnaces may be mechanized by use of rocking grate bars as shown in Fig. The grate bars loosen the slag and cause some of it to drop together with the ash into the bunker without disturbing the process of combustion.
Rocking Grate Bars.
Mechanical Firing (Stokers)
Mechanical stokers are commonly used to feed solid fuels into the furnace in medium and large size power plants.
The various advantages of stoker firing are as follows :
(i) Large quantities of fuel can be fed into the furnace. Thus greater combustion capacity is achieved.
(ii) Poorer grades of fuel can be burnt easily.
(iii) Stoker save labour of handling ash and are self-cleaning.
(iv) By using stokers better furnace conditions can be maintained by feeding coal at a uniform rate.
(v) Stokers save coal and increase the efficiency of coal firing. The main disadvantages of stokers are their more costs of operation and repairing resulting from high furnace temperatures.
Principles of Stokers. The working of various types of stokers is based on the following two principles:
1. Overfeed Principle. According to this principle (Fig. 4.13) the primary air enters the grate from the bottom. The air while moving through the grate openings gets heated up and air while moving through the grate openings gets heated up and the grate is cooled.
The hot air that moves through a layer of ash and picks up additional energy. The air then passes through a layer of incandescent coke where oxygen reacts with coke to form-C02 and water vapours accompanying the air react with incandescent coke to form CO2, CO and free H2. The gases leaving the surface of fuel bed contain volatile matter of raw fuel and gases like CO2, CO, H2, N2 and H2O. Then additional air known as secondary air is supplied to burn the combustible gases. The combustion gases entering the boiler consist of N2, CO2, O2 and H2O and also CO if the combustion is not complete.
2. Underfeed Principle. Fig shows underfeed principle. In underfeed principle air entering through the holes in the grate comes in contact with the raw coal (green coal).
Underfeed Principle. Then it passes through the incandescent coke where reactions similar to overfeed system take place. The gases produced then passes through a layer of ash. The secondary air is supplied to burn the combustible gases. Underfeed principle is suitable for burning the semi-bituminous and bituminous coals.
Types of Stokers. The various types of stokers are as follows:
Various Tyles of Stokers. Charging of fuel into the furnace is mechanized by means of stokers of various types. They are installed above the fire doors underneath the bunkers which supply the fuel. The bunkers receive the fuel from a conveyor.
(i) Chain Grate Stoker. Chain grate stoker and traveling grate stoker differ only in grate construction. A chain grate stoker (Fig) consists of an endless chain which forms a support for the fuel bed.
The chain travels over two sprocket wheels, one at the front and one at the rear of furnace. The traveling chain receives coal at its front end through a hopper and carries it into the furnace. The ash is tipped from the rear end of chain. The speed of grate (chain) can be adjusted to suit the firing condition. The air required for combustion enters through the air inlets situated below the grate. Stokers are used for burning non-coking free burning high volatile high ash coals. Although initial cost of this stoker is high but operation and maintenance cost is low.
The traveling grate stoker also uses an endless chain but differs in that it carries small grate bars which actually support the fuel fed. It is used to burn lignite, very small sizes of anthracites coke breeze etc.
The stokers are suitable for low ratings because the fuel must be burnt before it reaches the rear of the furnace. With forced draught, rate of combustion is nearly 30 to 50 lb of coal per square foot of grate area per hour, for bituminous 20 to 35 pounds per square foot per hour for anthracite.
Spreader Stoker.
(ii) Spreader Stoker. A spreader stoker is shown in Fig. 4.17. In this stoker the coal from the hopper is fed on to a feeder which measures the coal in accordance to the requirements. Feeder is a rotating drum fitted with blades. Feeders can be reciprocating rams, endless belts, spiral worms etc. From the feeder the coal drops on to spreader distributor which spread the coal over the furnace. The spreader system should distribute the coal evenly over the entire grate area. The spreader speed depends on the size of coal.
Advantages
The various advantages of spreader stoker are as follows :
1. Its operation cost is low.
2. A wide variety of coal can be burnt easily by this stoker.
3. A thin fuel bed on the grate is helpful in meeting the fluctuating loads.
4. Ash under the fire is cooled by the incoming air and this minimizes clinkering.
5. The fuel burns rapidly and there is little coking with coking fuels.
Disadvantages
1. The spreader does not work satisfactorily with varying size of coal.
2. In this stoker the coal burns in suspension and due to this fly ash is discharged with flue gases which requires an efficient dust collecting equipment.
Multi-retort Stoker.
(iii) Multi-retort Stoker. A multi-retort stoker is shown in Fig. 4.18. The coal falling from the hopper is pushed forward during the inward stroke of stoker ram. The distributing rams (pushers) then slowly move the entire coal bed down the length of stoker. The length of stroke of pushers can be varied as desired. The slope of stroke helps in moving the fuel bed and this fuel bed movement keeps it slightly agitated to break up clinker formation. The primary air enters the fuel bed from main wind box situated below the stoker. Partly burnt coal moves on to the extension grate. A thinner fuel bed on the extension grate requires lower air pressure under it. The air entering from the main wind box into the extension grate wind box is regulated by an air damper.
As sufficient amount of coal always remains on the grate, this stoker can be used under large boilers (upto 500,000 lb per hr capacity) to obtain high rates of combustion. Due to thick fuel bed the air supplied from the main wind box should be at higher pressure.