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In this article we will discuss about:- 1. Introduction to Combustion Equipment for Steam Boilers 2. Requirements of Combustion Equipment of Steam Boilers 3. Classification.
Introduction to Combustion Equipment for Steam Boilers:
The combustion equipment is a component of the steam generator. Since the source of heat is the combustion of a fuel, a working unit must have, whatever, equipment is necessary to receive the fuel and air, proportioned to each other and to the boiler steam demand, mix, ignite, and perform and other special combustion duties, such as distillation of volatile from coal prior to ignition.
i. Fluid fuels are handled by burners; solid lump fuels by stokers.
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ii. In boiler plants hand firing on grates is practically unheard of nowadays in new plants, although there are many small industrial plants still in service with hand firing.
iii. The fuels are mainly bituminous coal, fuel oil and natural gas mentioned in order of importance. All are composed of hydrocarbons, and coal has, as well, much fixed carbon and little sulphur.
To burn these fuels to the desired end products, CO2 and H2O, requires:
(i) Air in sufficient proportions,
(ii) A good mixing of the fuel and air,
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(iii) A turbulence or relative motion between fuel and air. The combustion equipment must fulfill these requirements and in addition, be capable of close regulation of rate of firing the fuel, for boilers which ordinarily operate on variable load. Coal-firing equipment must also have means for holding and discharging the ash residue.
Requirements of Combustion Equipment of Steam Boilers:
1. Thorough mixing of fuel and air.
2. Optimum fuel-air ratios, leading to most complete combustion possible, maintained over full load range.
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3. Ready and accurate response of rate of fuel feed to load demand (usually as reflected in boiler steam pressure).
4. Continuous and reliable ignition of fuel.
5. Practical distillation of volatile components of coal.
6. Adequate control over point of formation and accumulation of ash, when coal is the fuel.
Natural gas is used as a boiler fuel in gas well regions where fuel is relatively cheap and coal sources comparatively distant. The transportation of natural gas over land to supply cities with domestic and industrial heat has made the gas in the well more valuable and the gas-fired steam generator more difficult to justify in comparison with coal, or fuel cost alone. Cleanliness and convenience in use are other criteria of selection, but more decisive in small plants in central power stations.
Transportation costs add less to the delivery price of oil than gas; also fuel oil may be stored in tanks at a reasonable cost, whereas, gas cannot. Hence although fuel oil is usually more costly than coal per kg of steam generated, many operators select fuel oil burners rather than stokers because of the simplicity and cleanliness of storing and transporting the fuel from storage to burner.
Classification of Combustion Equipment of Steam Boilers:
Depending on the type of combustion equipment, boilers may be classified as follows:
1. Solid Fuels Fired:
(a) Hand fired
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(b) Stoker Fired:
(i) Overfeed stokers
(ii) Underfeed stokers.
(c) Pulverised Fuel Fired:
(i) Unit system
(ii) Central system
(iii) Combination of (i) and (ii).
2. Liquid Fuel Fired:
(a) Injection system
(b) Evaporation system
(c) Combination of (a) and (b).
3. Gaseous Fuel Fired:
(a) Atmospheric pressure system
(b) High pressure system.
Combustion Equipment for Solid Fuels Selection Considerations:
While selecting combustion equipment for solid fuels the following considerations should be taken into account:
1. Initial cost of the equipment.
2. Sufficient combustion space and its ability to withstand high flame temperature.
3. Area of the grate (over which fuel burns).
4. Operating cost.
5. Minimum smoke nuisance.
6. Flexibility of operation.
7. Arrangements for thorough mixing of air with fuel for efficient combustion.
Burning of Coal:
The two most commonly used methods for the burning of coal are:
1. Stroker firing
2. Pulverised fuel firing.
The selection of one of the above methods depends upon the following factors:
(i) Characteristics of the coal available.
(ii) Capacity of the boiler unit.
(iii) Load fluctuations.
(iv) Station load factor.
(y) Reliability and efficiency of the various types of combustion equipment available.
1. Stoker Firing:
A “stoker” is a power operated fuel feeding mechanism and grate.
Advantages of Stoker Firing:
1. A cheaper grade of fuel can be used.
2. A higher efficiency attained.
3. A greater flexibility of operations assured.
4. Less smoke produced.
5. Generally less building space is necessary.
6. Can be used for small or large boiler units.
7. Very reliable, maintenance charges are reasonably low.
8. Practically immune from explosions.
9. Reduction in auxiliary plant.
10. Capital investment as compared to pulverised fuel system is less.
11. Some reserve is gained by the large amount of coal stored on the grate in the event of coal handling plant failure.
Disadvantages of Stoker Firing:
1. Construction is complicated.
2. In case of very large units the initial cost may be rather higher than with pulverised fuel.
3. There is always a certain amount of loss of coal in the form of riddling through the grates.
4. Sudden variations in the steam demand cannot be met to the same degree.
5. Troubles due to slagging and clinkering of combustion chamber walls are experienced.
6. Banking and standby losses are always present.
7. Structural arrangements are not so simple and surrounding floors have to be designed for heavy loadings.
8. There is excessive wear of moving parts due to abrasive action of coal.
Classification of stoker firing:
Automatic stokers are classified as follows:
1. Overfeed stokers.
2. Underfeed stokers.
In case of overfeed stokers, the coal is fed into the grate above the point of air admission and in case of underfeed stokers, the coal is admitted into the furnace below the point of air admission.
1. Overfeed Stokers:
Principle of Operations:
Refer to Fig. 12.7.
The principle of an overfeed stoker is discussed below:
The fuel bed section receives fresh coal on top surface. The ignition plane lies between green coal and incandescent coke.
The air (with its water vapour content from atmosphere) enters the bottom of the grate under pressure. In flowing through the grate opening the air is heated while it cools the grate. The warm air then passes through a layer of hot ashes and picks up the heat energy.
The region immediately above the ashes contains a mixture of incandescent coke and ash, coke content increasing in upward direction. As the air comes in contact with incandescent coke, the oxygen reacts with carbon to form carbon-dioxide.
Water vapour entering with the air reacts with coke to form CO2, CO and free H2. Upon further travel through the incandescent region some of the CO2 reacts with coke to form CO. Hence no free O2 will be present in the gases leaving the incandescent region.
Fresh fuel undergoing distillation of its volatile matter forms the top-most layer of the fuel bed.
Heat for distillation and eventually ignition comes from the following four sources:
(i) By conduction from the incandescent coke below.
(ii) From high temperature gases diffusing through the surface of the bed.
(iii) By radiation from flames and hot gases in the furnace.
(iv) From the hot furnace walls.
The ignition zone lies directly below the raw fuel undergoing distillation.
To burn the combustible gases, additional ‘secondary air’ must be fed into the furnace to supply the needed oxygen. The secondary air must be injected at considerable speed to create turbulence and to penetrate to all parts of the area above the fuel bed. The combustible gases then completely burn in the furnace.
Fuel, coke and ash in the fuel bed move in direction opposite to that of air and gases. Raw fuel continually drops on the surface of the bed. The rising air cools the ash until it finally rests in a plane immediately adjacent to the grate.
Types of Overfeed Stokers:
The “overfeed stokers” are used for large capacity boiler installation where the coal is burnt with pulverisation.
These stokers are mainly of following two types:
(i) Travelling grate stoker
(a) Chain grate type
(b) Bar grate type
(ii) Spreader stoker.
(i) Travelling Grate Stoker:
These stokers may be chain grate type or bar grate type. These two differ only in the details of grate construction.
Fig. 12.8 shows a “Chain grate stoker”.
A chain grate stoker consists of flexible 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 front sprocket is connected to a variable speed drive mechanism. The speed of the stroker is 15 cm to 50 cm per minute.
The coal bed thickness is shown for all times by an index plate. This can be regulated either by adjusting the opening of fuel grate or by the speed control of the stoker driving motor.
The air is admitted from the underside of the grate which is divided into several compartments each connected to an air duct. The grate should be saved from being overheated. For this, coal should have sufficient ash content which will form a layer on the grate.
Since there is practically no agitation of the fuel bed, ‘non-coking coals’ are best suited for chain grate stokers.
The rate of burning with this stoker is 200 to 300 kg per m2 per hour when forced draught is used.
Advantages of Chain Grate Stoker:
1. Simple in construction.
2. Initial cost low.
3. Maintenance charges low.
4. Self-cleaning stoker.
5. Gives high release rates per unit volume of the furnace.
6. Heat release rates can be controlled just by controlling the speed of chain.
Disadvantages of Chain Grate Stoker:
1. Preheated air temperatures are limited to 180°C maximum.
2. The clinker troubles are very common.
3. There is always some loss of coal in the form of fine particles through riddlings.
4. Ignition arches are required (to suit specific furnace conditions).
5. This cannot be used for high capacity boilers (200 tonnes/hour or more).
(ii) Spreader Stoker:
Refer to Fig. 12.9.
a. In this type of stoker the coal is not fed into furnace by means of grate. The function of the grate is only to support a bed of ash and move it out of the furnace.
b. From the coal hopper, coal is fed into the path of a rotor by means of a conveyer, and is thrown into the furnace by the rotor and is burnt in suspension. The air for combustion is supplied through the holes in the grate.
c. The secondary air (or overfire air) to create turbulence and supply oxygen for thorough combustion of coal is supplied through nozzles located directly above the ignition arch.
d. Unburnt coal and ash are deposited on the grate which can be moved periodically to remove ash out of the furnace.
i. Spreader stokers can burn any type of coal.
ii. This type of stoker can be used for boiler capacities from 70000 kg to 140000 kg of steam per hour. The heat release rate of 10 x 106 kcal/m2-hr is possible with stationary grate and of 20 x 106 kcal/m2-hr is possible with travelling grate.
Advantages of Spreader Stoker:
1. A wide variety of coal can be burnt.
2. This stoker is simple to operate, easy to light up and bring into commission.
3. The use of high temperature preheated air is possible.
4. Operation cost is considerably low.
5. The clinkering difficulties are reduced even with coals which have high clinkering tendencies.
6. Volatile matter is easily burnt.
7. Fire arches etc. are generally not required with this type of stokers.
8. As the depth of coal bed on the grate is usually limited to 10 to 15 cm only, fluctuating loads can be easily met with.
Disadvantages of Spreader Stoker:
1. It is difficult to operate spreader with varying sizes of coal with varying moisture content.
2. Fly-ash is much more.
3. No remedy for clinkder troubles.
4. There is a possibility of some fuel loss in the cinders up the stack because of the thin fuel bed and suspension burning.
2. Underfeed Feeders:
Principle of Operation:
Refer to Fig. 12.10 (a).
i. The underfeed principle is suitable for burning the semi-bituminous and bituminous coals.
ii. Air entering through the holes in the grate comes in contact with the raw coal (green coal). Then it passes through the incandescent coke where reactions similar to overfeed system take place. The gases produced then pass through a layer of ash. The secondary air is supplied to burn the combustible gases.
The underfeed stokers fall into two main groups, the single retort and multi-retort stokers.
Multi-Retort Underfeed Stokers:
Refer to Fig. 12.10 (b)
i. The stoker consists of a series of sloping parallel troughs formed by tuyere stacks. These troughs are called retorts. Under the coal hopper at the head end of the retorts, feeding rams reciprocate back and forth. With the ram in the outer position, coal from the hopper falls into space vacated by the ram. On the inward stroke the ram forces the coal into the retort.
ii. The height and profile of the fuel bed is controlled by secondary, or distributing rams. These rams oscillate parallel to the retort axes, the length of their stokes can be varied as needed. They slowly move the entire fuel bed down the length of the stoker.
iii. At the rear of the stoker the partly burned fuel bed moves onto an extension grate arranged in sections. These sections also oscillate parallel to the fuel-bed movement. The sharp slope of the stoker aids in moving the fuel bed. Fuel- bed movement keeps it slightly agitated to break up clinker formation. From extension grate the ash moves onto ash dump plate. Tilting the dump plate at long intervals deposits the ash in the ash pit below.
iv. Primary air from the wind box underneath the stoker enters the fuel bed through holes in the vertical sides of the tuyeres. The extension grate carries a much thinner fuel bed and so must have a lower air pressure under it. The air entering from the main wind box into the extension-grate wind box is regulated by a controlling air damper.
a. In this stoker the number of retorts may vary from 2 to 20 with coal burning capacity ranging from 300 kg to 2000 kg per hour per retort.
b. Underfeed strokers are suitable for non-clinkering, high voltatile coals having caking properties and low ash contents.
Advantages of Multi-Retort Underfeed Stokers:
1. High thermal efficiency (as compared to chain grate stokers).
2. Combustion rate is considerably higher.
3. The grate is self-cleaning.
4. Part load efficiency is high particularly with multi-retort type.
5. Different varieties of coals can be used.
6. Much higher steaming rates are possible with this type of stoker.
7. Grate bars, tuyeres and retorts are not subjected to high temperature as they remain always in contact with fresh coal.
8. Overload capacity of the boiler is high as large amount of coal is carried on the grate.
9. Smokeless operation is possible even at very light load.
10. With the use of clinker grinder, more heat can be liberated out of fuel.
11. Substantial amount of coal always remains on the grate so that the boiler may remain in service in the event of temporary breakdown of the coal supply system.
12. It can be used with all refractory furnaces because of non-exposure of stoker mechanism to the furnace.
Disadvantages of Multi-Retort Underfeed Stokers:
1. High initial cost.
2. Require large building space.
3. The clinker troubles are usually present.
4. Low grade fuels with high ash content cannot be burnt economically.
2. Pulverised Fuel Firing:
In pulverised fuel firing system the coal is reduced to a fine powder with the help of grinding mill and then projected into the combustion chamber with the help of hot air current. The amount of air required (known as secondary air) to complete the combustion is supplied separately to the combustion chamber.
The resulting turbulence in the combustion chamber helps for uniform mixing of fuel and air and thorough combustion. The amount of air which is used to carry the coal and to dry it before entering into the combustion chamber is known as Primary air and the amount of air which is supplied separately for completing the combustion is known as Secondary air.
The efficiency of the pulverised fuel firing system mostly depends upon the size of the powder. The fineness of the coal should be such as 70% of it would pass through a 200 mesh sieve and 90% through 50 mesh sieve.
Fig. 12.11. shows elements of pulverised coal system.
Advantages of Pulverised Fuel Firing:
1. Any grade of coal can be used since coal is powdered before use.
2. The rate of feed of the fuel can be regulated properly resulting in fuel economy.
3. Since there is almost complete combustion of the fuel there is increased rate of evaporation and higher boiler efficiency.
4. Greater capacity to meet peak loads.
5. The system is practically free from sagging and clinkering troubles.
6. No standby losses due to banked fires.
7. Practically no ash handling problems.
8. No moving part in the furnace is subjected to high temperatures.
9. This system works successfully with or in combination with gas and oil.
10. Much smaller quantity of air is required as compared to that of stoker firing.
11. The external heating surfaces are free from corrosion.
12. It is possible to use highly preheated secondary air (350°C) which helps for rapid flame propagation.
13. The furnace volume required is considerably less.
Disadvantages of Pulverised Fuel Firing:
1. High capital cost.
2. Lot of fly-ash in the exhaust, which makes the removing of fine dust uneconomical.
3. The possibility of explosion is more as coal burns like gas.
4. The maintenance of furnace brickwork is costly.
5. Special equipment is needed to start this system.
6. The skilled operators are required.
7. A separate coal preparation plant is necessary.
8. High furnace temperatures cause rapid deterioration of the refractory surfaces of the furnace.
9. Nuisance is created by the emission of very fine particles of grit and dust.
10. Fine regular grinding of fuel and proper distribution to burners is usually difficult to achieve.
Pulverised Fuel Handling:
Basically, pulverised fuel plants may be divided into the following two systems:
1. Unit system.
2. Central system.
1. Unit System:
A unit system is shown in Fig. 12.12.
Most pulverised coal plants are now being installed with unit pulveriser.
The unit system is so called from the fact that each burner or burner group and the pulveriser constitute a unit. Crushed coal is fed to the pulverising mill at a variable rate governed by the combustion requirements of the boiler and furnace. ‘Primary air’ is admitted to the mill and becomes the transport air which carries the coal through the short delivery pipe to the burner. This air may be preheated if mill drying is desirable.
Advantages of Unit System:
1. The layout is simple and permits easy operation.
2. It is cheaper than central system.
3. Less spaces are required.
4. It allows direct control of combustion from the pulveriser.
5. Maintenance charges are less.
6. There is no complex transportation system.
7. In a replacement of stoker, the old conveyor and bunker equipment may be used.
8. Coal which would require drying in order to function satisfactorily in the central system may usually be employed without drying in the unit system.
Disadvantages of Unit System:
1. Firing aisle is obstructed with pulverising equipment, unless the latter is relegated to a basement.
2. The mills operate at variable load, a condition not especially conducive to best results.
3. With load factors in common practice, total mill capacity must be higher than for the central system.
4. Flexibility is less than central system.
2. Central System:
This system is illustrated in Fig. 12.13.
A central pulverising system employs a limited number of large capacity pulverisers at a central point to prepare coal for all the burners. Driers, if required, are conveniently installed at this point. From the pulverisers the coal is transported to a central storage bin where it is deposited and its transporting air vented from the bin through a “cyclone”.
This bin may contain from 12 to 24 hours supply of pulverised coal. From the bin the coal is metered to the burners by motor-driven feeders of varied design. Primary air, added at the feeders, floats the coal to the burners.
Advantages of Central System:
1. Offers good control of coal fineness.
2. The pulverising mill may work at constant load because of the storage capacity between it and the burners.
3. The boiler aisels are unobstructed.
4. More latitude in the arrangement and number of burners is allowed to the designers.
5. The large storage is protection against interruption of fuel supply to the burners.
6. Less labour is required.
7. Power consumption per tonne of coal handled is low.
8. Burners can be operated independent of the operation of coal preparation plant.
9. Fans handle only air, as such, there is no problem of excessive wear as in case of unit system, where air and coal both are handled by the fan.
Disadvantages of Central System:
1. Driers are usually necessary.
2. Fire hazard of quantities of stored pulverised coal.
3. Central preparation may require a separate building.
4. Additional cost and complexity of coal transportation system.
5. Power consumption of auxiliaries is high.
Pulveriser:
Coal is pulverised in order to increase its surface exposure, thus promoting rapid combustion without using large quantities of excess air. A pulveriser is the most important part of a pulverised coal system.
Pulverisers (sometimes called mills) are classified as follows:
1. Attrition Mills:
(i) Bowl mills
(ii) Ball and race mills.
2. Impact Mills:
(i) Ball mills
(ii) Hammer mills
Pulverisers are driven by electric motors with the feeders either actuated by the main drive or by a small d.c. motor, depending upon the control used.
Burners:
Primary air that carries the powdered coal from the mill to the furnace is only about 20% of the total air needed for combustion. Before the coal enters the furnace, it must be mixed with additional air, known as secondary air, in burners mounted in the furnace wall. In addition to the prime function of mixing, burners must also maintain stable ignition of fuel-air mix and control the flame shape and travel in the furnace.
Ignition depends on the rate of flame propagation. To prevent flash back into the burner, the coal-air mixture must move away from the burner at a rate equal to flame front travel. Too much secondary air can cool the mixture and prevent its heating to ignition temperature.
The requirements of a burner can be summarised as follows:
(i) The coal and air should be so handled that there is stability of ignition.
(ii) The combustion is complete.
(iii) In the flame the heat is uniformly developed avoiding any superheat spots.
(iv) Adequate protection against overheating, internal fires and excessive abrasive wear.
Burners:
Pulverised fuel burners may be classified as follows:
1. Long flame burners.
2. Turbulent burners.
3. Tangential burners.
4. Cyclone burners.
Oil Burners:
Principle of Oil Firing:
The functions of an oil burner are to mix the fuel and air in proper proportion and to prepare the fuel for combustion. Fig. 12.14 shows the principle of oil firing.
Classification of Oil Burners:
The oil burners may be classified as:
1. Vapourising Oil Burnres:
(a) Atmospheric pressure atomising burner.
(b) Rotating cup burner
(c) Recirculation burner
(d) Wick type burner
2. Atomising Fuel Burners:
(a) Mechanical or oil pressure atomising burner
(b) Steam or high pressure air atomising burner
(c) Low pressure air atomising burner.
Gas Burners:
Gas burning claims the following advantages:
(i) It is much simpler as the fuel is ready for combustion and requires no preparation.
(ii) Furnace temperature can be easily controlled.
(iii) A long slow burning flame with uniform and gradual heat liberation can be produced.
(iv) Cleanliness.
(v) High chimney is not required.
(vi) No ash removal is required.
For generation of steam, natural gas is invariably used in the following cases:
(i) Gas producing areas.
(ii) Areas served by gas transmission lines.
(iii) Where coal is costlier.
Typical gas burners used are shown in Figs. 12.15, 12.16 and 12.17.
Refer to Fig. 12.15. In this burner the mixing is poor and a fairly long flame results.
Refer to Fig. 12.16. This is a ring type burner in which a short flame is obtained.
Refer to Fig. 12.17. This arrangement is used when both gas and air are under pressure.
In order to prevent the flame from turning back the velocity of the gas should be more than the “rate of flame propagation”.