Here is a compilation of essays on ‘Coal‘ for class 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Coal‘ especially written for school and college students.
1. Essay on the Meaning and Classification of Coal:
Coal occurs naturally in seams and is the result of decomposition of vegetation accumulated in the earth, millions of years ago having got transformed by the action of intense heat and pressure. The conversion of vegetation into coal requires ages of time. Coal contains moisture, carbon, hydrogen, sulphur, nitrogen, oxygen and ash.
The coals can be classified in increasing order of heat or calorific value, namely peat, lignite, sub-bituminous, bituminous, semi-bituminous, semi-anthracite, anthracite and super anthracite coals. Peat is the first stage in the process of transformation of buried vegetation into coal. Its enthalpy of combustion is about 3,000 kJ/kg. Owing to a high moisture content (60 to 90%) and low carbon content (5 to 20%), peat is not suitable for use in power plants.
Lignite is the next stage in the development of coal. It has about 30 to 50% moisture and 20 to 40% carbon and has an enthalpy of combustion of 13,800- 17,600 kJ/kg. Lignite can be used for power generation but only locally owing to difficulties encountered in transportation. Sub-bituminous coal, also called the black lignite, has moisture content 17-20% and the volatile matter 35-45% and has the enthalpy of combustion of 18,800-23,000 kJ/kg.
Bituminous coal has low moisture content and non-disintegrating properties and is the most common variety of coal and is widely used for almost all purposes. Bituminous coals may be classified as caking or cooking and free burning. It has 60 to 80% carbon and 6 to 10% ash and enthalpy of combustion of 23,000-34,000 kJ/kg. It is widely used in power plants. Semi-bituminous coals are the highest grade of bituminous coals having high fixed carbon content and the highest heating values.
These have volatile matter between 14 and 22% and enthalpy of combustion between 27,000 and 35,000 kJ/kg. These coals are one of the best for power generation. Semi-anthracite coals are harder than the bituminous coals, found in small quantities and are, therefore, costlier for power generation.
These coals have enthalpy of combustion between 33,500 and 34,750 kJ/ kg. Anthracite coals have the highest carbon content but do not have the highest calorific value. Because of its high cost and scarcity, it is used mainly for domestic purposes. Super-anthracite coals are very hard with a shiny black surface. It is non-coking and is very difficult to ignite and so has little importance in the power generation.
2. Essay on the Selection of Coal for Power Plants:
The selection of coal for a power plant depends on a number of factors such as calorific value, weatherability, sulphur content, ash content, particle size, grindability index and caking characteristics.
Calorific value represents the amount of energy in a given mass and coal with a higher calorific value is preferred. Weatherability is a measure of the ability of coal to withstand exposure to environment without excessive crumbling. From the point of view of storage, the coal with higher weatherability is, obviously preferable. Sulphur is one of the combustible elements in the coal and produces energy but its primary combustion product, sulphur dioxide, is a health hazard.
The sulphur content of the coal should, therefore, be low (preferably below 1%). Ash is an impurity, produces no heat and is to be removed from the furnace and disposed off. However, a minimum of 4 to 6 per cent ash is required in the coal burnt on hand fire grates and some stokers for protection of grates against overheating. The size requirements for different equipments vary widely. The grindability index is inversely proportional to the power required to grind the coal to certain fineness. Higher the grindability index, better the coal is.
India has large deposits of coals (around 170 billion tonnes) in Bengal, Bihar, MP, Orissa etc. In general, Indian coals have high ash content (as high as 40%). The high ash content of coal reduces the boiler thermal efficiency because heat loss through unburnt carbon, excessive clinker formation and heat in ashes is considerably more. It also adds to the difficulty in the disposal of hot ash.
Thus the use of coal with high ash content increases the plant size and transportation cost and reduces the thermal efficiency. In addition, it makes the control difficult because of irregular combustion. High ash content coals can be used more economically in pulverized form because in that form the thermal efficiency may be as high as 90% and controls could be made simpler just by adjustment of position of burners in pulverized fuel boilers. All new thermal power plants use pulverized coal.
Use of coal for power generation causes immense environmental pollution because sulphur dioxide having detrimental effect on our respiratory system is produced due to sulphur content in the coal. Carbon dioxide, carbon monoxide and particulates emissions from coal based thermal power plants are other important environmental pollutants.
Imported coal has higher calorific value and lower ash content (less than 10%). From detailed cost calculations, it is concluded that if the calorific value, ash content, transportation costs and all other costs are considered, the cost of Indian coal and imported coal per unit of energy generated is almost the same for power plants situated near the coastal areas.
As such imported coal is likely to play a vital role in power generation in the near future. BSES’s Dahanu power plant in Maharashtra meets about 30% of its coal consumption through imports. Many proposed thermal power plants of IPPs (independent power producers) are likely to be based on imported coal. However, the imported coal has higher sulphur content (up to 1.2%) as compared to Indian coals (less than 1%).
3. Essay on Fuel Handling of Coal:
Coal is the most commonly used fuel in thermal power plants. As enormous amount of coal is burnt in a thermal power plant (for example, a 200 MW power plant may require about 2,000 tonnes of coal per day) and the cost of coal may be as high as 60-70 per cent of the total operation cost of the plant, therefore, problems of coal handling in a thermal power plant require special consideration.
Coal can be handled manually or mechanically. Mechanical handling is usually adopted as it is reliable, expeditious and economical. Owing to large quantity of coal required to be handled every-day, mechanical handling has become absolutely necessary.
The main requirements of a coal handling plant are reliability, soundness and simplicity requiring a minimum of operatives and minimum of maintenance. Besides, the plant should be able to deliver the required quantity of coal at destination during peak hours.
Transportation or Delivery of Coal:
The steam power plant should be located near the coal fields as far as possible in order to reduce the transportation charges. In case the plant is located near a coal pit, transportation cost by rail, road, sea or river is avoided. If a coal mine is very near, sometimes aerial ropeways are employed to carry coal to the site.
There are three ways of transporting coal from coal mines to the site of power plant i.e., by sea or river, by road and by rail. If the power plant is situated on the bank of a river or near the seashore, it is often economical to transport coal in boats or barges, unload mechanically by cranes or grab buckets and place in the storage yard or directly to the conveyor system to be carried to the power plant. Transportation by road is possible for small and medium size plants only.
The chief advantage of this system is the possibility of carrying coal directly into the power house up to the point of consumption. Moreover due to less traffic restrictions it is considered better system in comparison to rail transport. Transportation of coal by rail, particularly for stations located interior, is still the most important means of transportation in common use.
A railway siding line is taken into the power station and the coal is delivered either in the storage yard or close to the point of utility. It can be unloaded by means of cranes, grab buckets or wagon tipplers. The capacity of the railway siding should be equal to three days requirement of the plant.
Methods of Coal Handling:
Irrespective of the method of transportation of coal adopted, the coal has to be carried to the boiler stokers or the coal preparation plant in the case of pulverised fuel firing.
The various stages in coal handling are:
(ii) Transfer to dead storage or preparation plant
(iii) Dead storage
(v) Live storage
(vi) Implant handling, and
(vii) Coal weighing.
The coal is unloaded from the point of delivery by means of:
(i) Coal shakers or coal accelerators,
(ii) Rotary car dumpers or wagon tipplers and
(iii) Grab buckets.
The choice of equipment will depend upon the method of transportation adopted.
The main equipment employed for taking the coal from the unloading site to the dead storage is:
(i) Belt conveyors
(ii) Screw conveyors
(iii) Bucket elevators
(iv) Skip hoist
(v) Grab bucket conveyors, and
(vi) Flight conveyors.
Storage of coal is essential to insure against complete shutdown of a power plant which may arise from failure of normal coal supply (owing to eventualities like strikes in coal mines, failure of transport system, shortage of coal etc.). Storage also permits some choice of the date of purchase, allowing the purchasers to take advantage of seasonal market conditions.
However, there are a number of factors, such as risk of spontaneous combustion, possible deterioration of coal during storage, interest on capital blocked, cost of insurance etc., which make storage of coal undesirable. The dead storage can be open space in one corner of the power station. Amount of coal to be stored depends on the available space for storage, transportation facilities and nearness of the power plant to coal mines. The usual practice is to store coal for one or two month requirements depending upon the distance of the power plant from the coal mines.
Reclaimation is the process of taking coal from dead storage for preparation or further feeding to hoppers or live storage. This may be accomplished by scrapers, belt conveyors, and bull dozers. In case of vertical movement it can be done by skip hoists or buckets elevators.
Live storage consists of about one day requirement of coal of power plant and is usually a covered storage in the power station near the boiler furnace. It can be provided with bunkers and coal bins.
Input handling refers to handling of coal between the live storage and firing equipment. In case of simple stoker firing only chutes may be required to feed the coal from storage bunkers to the firing units. However, gates and valves are included in the system for controlling the flow of coal according to the load on the power plant. In case of pulverised fuel firing system equipments such as chutes, pulverising mills, feeders, weighing equipment and many others for implant handling would be required.
Coal weighing enables one to have an idea of total quantity of coal delivered at the site and also whether or not proper quantity has been burnt as per load on the plant.
It can be accomplished by:
(i) Weighing bridge,
(ii) Belt scale, and
(iii) Automatic recording system.
Coal is delivered to the power plants in trucks or railway wagons. It is weighed on balance and then unloaded into underground hoppers or bunkers. The wagon can be unloaded either manually or by using rotary wagon tipplers. In the later case, the wagon is moved into a rotor and is secured in it. Then the rotor is turned by electric drive and coal is unloaded from the open wagon.
This considerably reduces the unloading time. From the bunkers, the coal is lifted by conveyor to the transfer tower from where it can be delivered either to the coal storage or by the conveyor to a crusher. After passing through magnetic separators and screens, coal is crushed in crushers into pieces 25-30 mm in size for stoker firing and 10-20 mm when pulverized fuel is fired in boiler furnaces. The crushed coal is then milled to a fine powder in the later case and then the fuel is carried through automatic weighed to a transfer tower where fuel is lifted to the level of the bunker gallery and distributed between boiler hoppers by a conveyor.
4. Essay on Fuel Combustion and Combustion Equipment:
Fuel is burnt in a confined space called the furnace. An efficient combustion of fuel is essential for economical operation of a power plant.
The necessary requirements for efficient combustion of fuels are as follows:
(i) The proper quantity of primary or secondary air required for complete combustion.
(ii) Adequate stoker or grate area required for burning particular quantity of fuel.
(iii) Minimum operating and maintenance cost.
(iv) Attainment of proper designed temperature.
(v) There should be no formation of caking during burning of fuel.
(vi) The system should be easy to handle and dependable.
Although there are various designs and methods of firing but mainly there are two ways of firing namely solid fuel firing, and pulverised fuel firing.
In case of solid fuel firing two general methods, namely hand firing and stoker or mechanical firing, are available.
In hand firing the coal is put into the furnace frequently by shovels. The primary and secondary air needed for combustion is regulated by dampers. Generally the grate of such boilers consists of bars over which the coal is put. This method of firing is simple and does not require any capital investment. This method can however be used only in small installations as uniformity of combustion is difficult to control in this type of firing. Further, adjustments for the supply of air are to be made every time coal is fed to the furnace.
Stokers give mechanical feeding of coal. Mechanical stokers receive fuel by gravity, carry it to the furnace for combustion and after combustion discharge the ash at the appropriate point.
The advantages of stoker or mechanical firing over hand firing are given below:
(i) Uniform feeding of fuel into furnace.
(ii) Easy control of firing resulting in better regulation and efficient combustion.
(iii) Saving in labour cost.
(iv) Possibility of use of poor grades of coal due to better control.
(v) Greater combustion capacity.
(vi) Fluctuation of load demands can be met because of control of combustion.
(vii) Practically immune from explosions.
(viii) Very reliable, and maintenance charges are reasonably low.
However, the stoker or mechanical firings have the following drawbacks:
(i) Complicated construction.
(ii) Trouble due to slagging and clinkering of combustion chamber walls.
(iii) Loss of fuel in the form of riddlings through the grates.
(iv) Banking and standby losses are always present.
(v) Sudden variations in the steam demand cannot be met to the same degree.
(vi) With very large units the initial cost may be rather high than with pulverized fuel.
Mechanical stokers are of two types namely:
i. Underfeed Stokers, and
ii. Overfeed Stokers.
i. Underfeed Stokers:
The coal is fed into the furnace below the point of admission of air. Coal from the hopper is pushed into the retort by means of reciprocating plunger. When the coal gets heated, all the volatiles in it are distilled and when coal reaches the zone of active combustion, it is in the form of coke and ash. The ash discharge plates are at the back of the furnace and by the time coal is pushed down on these plates; all the combustion has been completed.
Air is admitted into the furnace through holes in the retort sides. The coal is continuously agitated by the plunger and also by three pusher plates along the retort bottom. As such the fuel bed remains porous and free from clinkers. Underfeed stokers are stable for non-clinkering, high- volatile coals having caking properties and low ash content.
ii. Overfeed Stokers:
Overfeed stokers can be further classified as chain grate stokers, travelling grate stokers a spreader stokers.
Chain grate and travelling grate stokers are similar in appearance but differ in the grate construction. Chain grate consist of flexible endless chain as in crawlers of heavy machines and travelling grates consist of grate bars carried by steel chains. The speed of the stoker is 127 mm to 508 mm per minute. An index plate with pointer indicates the thickness of coal bed at all times. This can be regulated either by adjusting the opening of the fuel gate 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 is to be protected against overheating and it is achieved by using coal having sufficient ash content. Since there is practically no agitation of the fuel bed, non-caking coals are best suited for chain grate stokers. Such stokers can burn about 150 kg of coal/m2/ hour with natural draught and 200-300 kg of coal/m2/ hour with forced draught.
In the spreader stoker, the coal is not fed into the 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. From the coal hopper, coal is fed into the path of a rotor by means of a conveyor, 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.
Over fire air or secondary air to create turbulence and supply oxygen for the thorough combustion of coal is supplied through nozzles located directly above the ignition arch. Un-burnt coal and ash are deposited on the grate which can be moved periodically to remove ash out of the furnace. Spreader stokers can burn any type of coal from lignite to semi-anthracite, whether they are free burning or coking. Their disadvantage is that fly ash is much more. Such stokers can be employed for boiler capacities from 70,000 kg per hour of steam to about 140,000 kg per hour.
Pulverised Fuel Firing:
The use of pulverised fuel firing has revolutionized the design of boilers as regards their capacity, pressure of steam and its temperature. Pulverization is a means of exposing a large surface area to the action of oxygen and consequently helping the combustion.
Solid fuels can be used in a powdered form and burn like oil and gases. The coal is first dried usually by the gases, ground to a fine powder in a pulverised mill and then projected into the combustion chambers by means of a current of hot air. A further volume of preheated air to make up the necessary amount for combustion is blown in separately and the resulting turbulence in the high pressure combustion chamber helps in thorough combustion of the fuel.
The air used to dry the coal and convey the powdered fuel to the furnace is called the primary air and the air blown in separately to complete the combustion is called the secondary air. The amount of primary air may range from as little as 10% to almost the full combustion air requirements depending on the type of the pulveriser and the load.
The pulverised fuel firing has the following advantages:
(i) In pulverised fuel firing better combustion is achieved due to use of hot air at temperature ranging from about 260°C to 370°C.
(ii) Surface exposure is increased resulting in rapid combustion without the use of large amounts of excess air.
(iii) Low grade of coal can be used since it is used in powdered form. Even fine wet coal can be used provided the conveying equipment can carry it to the pulverizing mill.
(iv) The rate of feed of the fuel can be regulated properly resulting in fuel economy.
(v) The surface area is increased in almost the ratio 400:1, therefore, high rates of combustion are possible. Moreover, a smaller quantity of air is required than when the fuel is burnt in lump form.
(vi) The smaller quantity of excess air and thorough mixing of air and fuel produce a high furnace temperature with little smoke.
(vii) Increased rate of evaporation and higher boiler efficiency because of almost complete combustion of fuel.
(viii) The system is practically free from slagging and clinkering troubles.
(ix) Ash removing troubles are reduced.
(x) No standby losses due to banked fires.
(xi) Firing of boiler becomes easy. The boiler can be started from cold conditions very rapidly.
(xii) Fluctuations of loads can be easily met.
(xiii) Automatic combustion control can be employed.
(xiv) Preheated air can be used successfully.
(xv) Larger capacity to meet peak loads.
(xvi) The pulverizing equipment is outside the furnace; therefore, it can be repaired without cooling down the unit.
However, the pulverised fuel firings have the following disadvantages:
(i) Coal preparation plant is required which makes the installation expensive in initial cost.
(ii) Lot of fly ash in the exhaust makes the removal of fine dust uneconomical.
(iii) There is a risk of explosion as coal is burnt like a gas. So, skilled personnel are required.
(iv) Special equipment is required for starting this system.
(v) Higher combustion temperatures cause higher thermal losses in the flue gases.
(vi) The operating cost of pulverisation plant from energy utilisation point of view is also quite high.
(vii) The high furnace temperatures, unburnt fuel and the fluxing effect of ash etc. deteriorates the refractory material.
(viii) The plant requires electrostatic precipitator to reduce stack emissions to acceptable level.
The advantages of using pulverized fuel outweigh the disadvantages and all modern plants use pulverized fuel.
There are two systems of preparation of coal into a fine powder namely unit system and the bin or storage or central system. In unit system a separate pulverising unit is provided for each furnace and the fuel is fired directly into the furnace without being stored whereas in central system the fuel is pulverised in a central plant and stored in bins or bunkers.
From the bunkers, the pulverised fuel is distributed to various burners through separate feeders in accordance with the load demands. Each system consists of crushers, magnetic separators, driers, pulverising mills, storage bins, conveyors and feeders.
In unit system, the raw coal falls by gravity from the overhead bunker into a feeder where it is dried with the help of hot air. From the feeder, the coal passes on to the pulverising mill where it is crushed into a fine powder. The fine powder is extracted from the mill by means of an exhaust fan via a separator and blown into the furnace along with the hot air.
In the separator, the big particles of coal are separated from the fine dust and these again fall down into the mill. It is simple, cheaper in installation and operation and easy in regulation. The drawbacks of the system are requirement of a reserve plant for each unit making the system more expensive. Moreover, the pulverising unit has to work under fluctuating load demands and the flow of fuel through the burners is not uniform.
In central system the crushed coal passes from the pulveriser to a separator where oversize coal particles are separated from the fine powder. The coal laden air then flows into a cyclon where the air is separated from the coal powder and returned to the mill circuit by means of an exhaust fan. From the cyclon, the pulverised coal is conveyed to bunker by means of a screw conveyor. From the bunker, the fuel is fed to the various burners through separate feeders. This plant is more reliable. The quantities of fuel and air can be regulated accurately.
Moreover, the pulveriser can work under constant speed irrespective of load demand. Also, the exhaust fan handles comparatively clean air. The drawbacks are requirement of large capital and large building space and higher operation and maintenance charges. Due to this, the central system has been superseded by the unit system.
There are three principles, which are employed in pulverising of coal, namely:
(ii) Attrition or grinding and
All the three principles are usually employed in every pulverising mill; degrees of utilisation of different principles in various pulverising mills may vary depending upon their design.
In impact system, the coal is crushed to powder due to hammering action of paddles. The peripheral speed is of the order of 5,400 metres/minute. Output is about 5 tonnes/ hour and energy consumption is about 18-20 kWh/tonne. The system is compact, low in cost and simple in operation. However energy consumption and maintenance cost are high and it is difficult to maintain the finess of coal after pulverisation.
(ii) Attrition or Grinding:
In attrition or grinding of coal, coal is grounded by the movement of a revolving roller against a race. The peripheral speed is about 180-210 metres/minute. The output is about 10 tonnes/hour and energy consumption is roughly 10-15 kWh per tonne. The plant occupies less space and is quiet in action. However, the initial cost is higher.
In crushing system, steel balls are allowed to move in a steel cylinder lined with renewable cast iron liner plates on the inside. The drum is supported at each end on bearings. Owing to continuous tumbling action, the steel balls strike against coal pieces reducing them to powder by combined impact, attrition and crushing action. Peripheral speed is about 120-135 metres/minute. Output is 20 tonnes/hour and energy consumption is about 20-25 kWh/tonne. The system is simple in operation, cheaper in initial cost, built costly in operation cost. There is considerable quantity of coal in the mill which acts as a reservoir.
The pulverising mills may also be classified according to the manner of air supply. In one type, the exhaust fan handles both air and powdered coal and is on the outlet side of the mill. In other type, the fan is on the inlet side of the mill and provides the necessary air current. In this system, the fan handles only clean air.
5. Essay on Coal Burner:
The coal burners are employed to fire the pulverised coal along with primary air into the furnace. The secondary air is admitted separately below the burner, around the burner or elsewhere in the furnace. The main requirement for a good coal burner is capability of producing uniform and stable flame with almost complete combustion of the fuel. Coal burners can be classified according to their design and by their arrangement in the furnace.
In one type of firing, say in opposed firing, the burners are placed on the opposite walls of the furnace and they fire directly against each other causing intimate mixing of the fuel and air. In second type of firing, say cross firing, the burner’s fire in the vertical direction and in horizontal direction and the fuel streams intersect with each other.
And in third type of firing, called the tangential firing, the burners are placed in the corners of the furnace and they send horizontal streams of air and fuel tangent to an imaginary circle in the centre of the furnace. Tangential firing results in intense turbulence and thorough mixing of the fuel and air. All the fuel and air nozzles can be titled 24° above and below the horizontal.
For firing crushed coal (not pulverised fuel) cyclon burners are employed. In such burners the crushed coal (max 6 mm size) from the feeder and the primary air enter with a vortex motion at the centre of cyclone. The secondary air is admitted separately in the vortex motion. The fuel is quickly burnt and ash in the form of molten slag drains down the inner wall of the cyclon. Hot flue gases with 10-20% of the ash in the coal in the form of fly ash enter the furnace.
Due to centrifugal action, most of this fly ash is thrown against the walls of the furnace and is drained away along with the molten film of slag. The flue gases leaving the furnace are quite clean to flow through the rest of heat exchanger passages. Thus better heat transfer and good combustion is obtained. Also frequent cleaning requirement of furnace and fly ash trouble are very much reduced.
6. Essay on Ash Handling:
Coal contains a considerable amount of ash. The percentage of ash in the coal varies from about 5% in good quality coals to about 40% in poor quality coals. Generally poor quality coal is used in steam power plants and, therefore, a steam power plant produces hundreds of tonnes of ash daily (a modern 2,000 MW steam power plant produces about 5,000 tonnes of ash daily). For removal of ash from the boilers and its disposal to the suitable site, it is quite difficult and quite elaborate equipment is required.
Ash handling comprises the following operations:
(i) Removal of ash from the furnace ash hoppers.
(ii) Transfer of this ash to a fill or storage and
(iii) Disposal of stored ash.
The ash can be disposed off in the following ways:
(i) Waste land sites may be reserved for the disposal of ash.
(ii) Building contractors may utilise it to fill the low lying areas.
(iii) Disused quarries within reasonable distance of the power plant may be employed for dumping the ash into the evacuated land.
(iv) Deep ponds may be made and the ash can be dumped into these ponds to fill them completely. When such ponds are completely filled, they may be covered with soil and seeded with grass.
(v) When seaborne coal is used, barges may take the ash to sea for disposal into a water grave.
The difficulties experienced in handling and disposal of ash are as follows:
(i) The ash coming out of the furnace is very hot.
(ii) The ash is dusty and so irritating and annoying in handling.
(iii) The ash is abrasive and wears out the containers.
(iv) It produces poisonous gases and corrosive acids when mixed with water.
(v) It forms clinkers by fusing together in lumps.
The chief requirements of a good ash handling plant are:
(i) Capability to operate with minimum personal attention and handle large clinkers as well as dust and soot.
(ii) Minimum operation and maintenance charges.
(iii) Adequate capacity to deal with the ultimate plant capacity.
(iv) Speedy disposal of ash.
(v) Ability of handling both dry and wet ash.
(vi) Operation with little noise and minimum dust menace.
Theoretically, whole of the ash from the furnace should get deposited in the ash hoppers, but actually 5 to 40% of it leaves the furnace with the outgoing gases. The small power stations use some conveyor arrangement to carry ash to dump sites directly or for carrying and loading it to trucks and wagons which transport it to the disposal site. Large power stations use more elaborate arrangements.