In this article we will discuss about:- 1. Introduction to Coal 2. Origin of Coal 3. Ranking of Coal 4. Bended Constituents of Coal 5. Coal Analysis and Commercial Grades 6. Coking Coal 7. Coal Washing 8. Production of Petrol from Coal 9. Uses of Coal.
Contents:
- Introduction to Coal
- Origin of Coal
- Ranking of Coal
- Bended Constituents of Coal
- Coal Analysis and Commercial Grades
- Coking Coal
- Coal Washing
- Production of Petrol from Coal
- Uses of Coal
1. Introduction to Coal:
Energy is available to man from fuels (listed below), biogas, hydropower, the sun, the wind, hot water springs, sea-waves, and internal heat of volcanoes. Attempts to harness energy from the sea waves and the volcanoes are in preliminary stage.
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Fuels may be divided into 4 classes:
i. Solid – Coke, coal, wood, charcoal, bagasse, lignite, cow dung, etc.
ii. Liquid – Diesel, petroleum and its derivatives, coal tar and its derivatives, shale oils, alcohol, etc.
iii. Gaseous – Lighting gas, coke oven gas, producer gas, blast furnace gas, water gas, natural gas, etc.
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iv. Nuclear – Uranium based energy, etc.
About half the actual energy consumption in India is commercial, consisting of coal, oil nuclear and hydel. The other half is non-commercial like firewood, agricultural waste and animal excreta. The share of coal is about 60 per cent of all commercial primary energy, in terms of heat content as coal equivalent.
Coal- 59.5 percent
Oil- 27 percent
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Hydro- 12.5 percent
Nuclear- 1 percent
Wind-power, a source of non-conventional renewable energy, makes very little contribution to the overall generation in India.
2.
Origin of Coal:
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Coal is stratified carbonised remains of plant material. Two theories, known as the In Situ Theory and the Drift Theory, have been advanced by the scientists to explain the heavy accumulations of vegetable and organic matter which formed the coal seams.
Drift Theory and In-Situ Theory:
In India, the sequence of the strata containing the Coal seams is in most cases suggestive of continuous deposition under water. There is hardly any record of erect stems with attached roots occurring in the coal seams; on the Other hand indications of prostrate or inclined trunks are numerous.
Most of the coal seams are invariably inter-bedded with shale and sand stone of definitely sedimentary origin (fresh water or marine). These are other indications have led to the acceptance of the Drift Theory to explain the heavy accumulation of vegetable matter.
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It is believed that the coal seams, as we see them today, were formed out of plants and trees which grew on this earth in abundance millions of years ago under climatic conditions which were far different than today.
Due to earthquakes, tornadoes and other tectonic activities these trees fell down, and the plant material and trees, were drifted to lakes, river valleys, estuaries and seas by water. Large sediments of sand and earth also were carried by water and deposited over the vegetable matter.
The process continued over millions of years during which the lakes, sea-beds, lagoons and rivers subsided further and were filled again by fresh plant material and sediments of earth and sand. The organic matter, under its own weight and the weight of the sediments under which it got buried, was compressed and the heat generated by such compression, the bacterial action, and absence of oxygen resulted into transformation of the plant material to hard semi burnt solid known as coal.
This natural process of carbonisation accompanied by bacterial action, resulted in evolution of gas that got entrapped into the coal. It is marsh and clay deposits were changed into shales and the sand, into sandstone.
In-Situ Theory:
The heavy accumulation of plant material which formed coal in Britain is explained by the in-situ theory. According to this theory, the areas where the vegetation grew, subsided and the trees were submerged in water which flew into the subsided area. They were buried beneath the earth and sand brought by the water. Fresh vegetation grew over the debris in the area and the process repeated over thousands of years.
The plants and animals which were buried in the large accumulation of plant, clay and earth sediments are preserved as fossils in many cases and can be seen in coal associated rocks.
3.
Ranking of Coal:
Wood, peat, lignite, bituminous coal and anthracite are the main stages of transformation from plant material to coal, anthracite being the final stage or rank. Higher rank indicates coal with high carbon content. Anthracite is the purest coal but does not necessarily have the maximum heating value which depends upon the percentage of carbon and hydrogen.
The transformation from wood to anthracite is accompanied by progressive elimination of hydrogen and oxygen and gradual increase of carbon content. The inorganic matter of the original plant material and whatever got mixed with it during the buildup of the heavy plant deposits constitutes what is known as ash in coal.
The sp. gr. of coal is highest in anthracite (about 1.5), and lowest in lignite (about 1.2) whereas sp. gr. of bituminous coal varies between 1.28 and 1.4., the calorific value of lignite is nearly 2400 kcal/kg and that of bituminous coals varies from 2500 to 6600 kcal/kg.
The thickness of coal seams varies from a few centimeters to nearly 140 metres in this country but thickness below 1.2 m is considered uneconomic for coal extraction in our country though in Britain and Germany a coal seam 1 m or 1.2 m thick is considered economically workable. The thickest seam in India, and perhaps the second thickest in the world, is Jhingurda Top seam (140 m).
The period during which coal was formed extends over millions of years. From astronomical considerations, the evidence available from radio-activity of minerals, and other data scientists consider the age of the earth as a planet over 4000 million years but the period during which sedimentary strata have been laid down is of more important to geologists. Archaean or Precambrian is the oldest period, and is longer than all the others put together.
The Indian coalfields were formed during two main geological epochs:
1. Permian period, and
2. Tertiary period.
A sub period of Permian is known a Gondwana period and the series of rocks formed during that period are known as rocks of Gondwana system.
The Gondwana Sone, Koel, Mahanadi, Pranhita Godavari, Narmada, Wardha, Pench, etc. and over the regions of Bengal, Bihar, Orissa, Uttar Pradesh, Madhya Pradesh, Andhra Pradesh and Maharashtra.
They possess bituminous coals and produce nearly 97% of the total production in the country. The tertiary period coals produced during the periods Eocene to Pliocene are available in Assam, Nagaland, Meghalaya, Arunachal, Sikkim, Kashmir, Kutch, Rajasthan, and Neyveli (Tamil Nadu) as deposits of lignite or a stage between lignite and bituminous, as in the case of Assam and other areas of North East India.
The tertiary coals of North East India are altogether of different origin compared to the Gondawana coals and are very low is ash but high in sulphur content; some of them have very high caking index and can be considered medium coking. Their total production accounts for only 2% of the coal output in the country.
Semi- anthracite coal is available only in a few small pockets near Darjeeling and in Jammu Kashmir where the heat of intense mountain building movements of the sub-Himalayan zone converted patches of semi bituminous coal into semi anthracite.
At Rajhara Colliery (Palamau district, Bihar) the coal that is extracted is sometimes called semi anthracite because of its high calorific value, low v.m. content, (3 to 10%), and appearance. Its ash percentage varies from 13 to 15 and moisture is about 2%. The coal is difficult to ignite but once ignited, continues to burn for long without smoke (smokeless fuel).
India holds position of being the fifth largest coal producing country in the world. Its coal reserves have been officially put at 1% of the world’s total coal reserves. In the world’s coal reserves share of CIS countries (including Russia) is 53%, USA 27% and China 9.4%.
4.
Bended Constituents of Coal:
The bituminous coal contains four different visible bands, vitrains, clarian, durain and fusain which are rock types and not chemical entities. Thus different vitrains have different chemical compositions. There are subdivisions of these main rock types into macerals and petro logical units, e.g., vitrinite, fusinite, exinite, resinite, micrinite, etc.
All the four bands are not present in the same sample of coal and very often only two bands exist-
1. Vitrain – It forms the bright layers, vitreous in texture in a coal sample and is to be seen in high quality coals. It is friable, breaks in small cubical or rectangular pieces and is clean to touch.
2. Clarain – It also available in coal as bright layers, parallel to the bedding planes. It does not soil the fingers, resembles vitrain and is present in high quality coals.
3. Durain – This constituent band has dull appearance and is lenticular in shape. Compared to vitrain and clarain it is hard.
4. Fusain – It is dull appearance, friable, soils the fingers, and is the most impure band in coal. The layers resembles charcoal. The band is present in all types of coals.
5. Coal Analysis and Commercial Grades
:
Size gradation of coal
Coal is classified generally into the following four sizes for purposes of marketing –
Steam coal – 50 mm to 200 mm and above.
Rubbed coal – 25 mm to 50 mm.
Slack coal – 0 mm to 25 mm.
(Sometimes coal of 0 mm to 50 mm is also considered as slack coal by some coal-users).
Run-of-mine (R.O.M.) coal-unscreened coal of all sizes.
The quality of coal depends upon-
(1) The chemical composition of the original plant material which formed the coal,
(2) The mode of accumulation and burial of the plant debris,
(3) The extraneous inorganic matter than got inter-mixed with the plant material,
(4) The extent of bacterial decay,
(5) The manner and duration of carbonisation,
(6) Age of the deposits, and
(7) Subsequence geological disturbances.
Analysis of coal is expressed in two ways:
1. Ultimate analysis.
2. Proximate analysis.
1. Ultimate Analysis:
Ultimate analysis gives the percentage of elements presents (oxygen, hydrogen, carbon, nitrogen, sulphur and phosphorus) and is useful to consider the suitability of coal for certain purposes, particularly in the chemical industry.
The proximate analysis gives the percentage (by weight) of ash, moisture, volatile matter and fixed carbon. Ash is the inorganic residue left when coal or coke is incinerated in air to constant weight under specified conditions. Moisture is the water expelled in various forms when coal is heated under specified conditions.
Volatile matter which consists of various gases in coal is equal to the total loss in weight minus the moisture when coal or coke is heated under specified conditions. Fixed carbon is obtained by subtracting from 100 the sum of percentage of moisture, ash and volatile matter.
Coal that has been exposed to contact with water in the seam, in a washery, during storage or transit in rainy season in open wagons or trucks, may carry free or visible water adhered to the surface or in cleavages and cracks. Such moisture is known as free moisture.
Some moisture is always present in the coal and formed part and parcel of it during its natural formation; such moisture is called inherent moisture. Total moisture in coal is the sum of free moisture and inherent moisture.
Proximate analysis gives the percentage of inherent moisture. The external moisture i.e. free moisture content is determined by drying in air samples of the as-received fuel on trays until their weight is constant.
There are four different ways in which the proximate analysis of coal can be reported:
1. Analysis on as-received sample basis
2. Analysis on air-dried sample basis.
3. Analysis on equilibrated-conditions basis.
4. Analysis on unit-coal basis or dry mineral-matter-free (dmmf) basis, or pure coal basis.
Reporting analysis on as-received sample basis is not a common practice. The coal samples have to be collected in sealed containers so that the moisture adhering to the coal is not dried up during transit or storage pending analysis. Some power houses attach importance to such analysis to know the real coal and the external moisture adhering to it so that payment is made only for the coal to the suppliers.
Reporting analysis on air-dried sample basis is commonly adopted practice. An air dried sample is that which is spread in thin layers in a laboratory and exposed to its atmosphere- for a few days till its weight is constant. Generally 2-3 days suffice, depending upon relative humidity.
A sample which is spread in thin layers in a laboratory for 6-10 hours and air allowed passing over it a laboratory temperature and humidity, also attains constant weight and is treated as an air-dried sample. Where a large number of samples are to be analyzed, a quicker method of getting an air-dried sample is to heat a laboratory sample in an oven to nearly 50°C and keep it at that temperature for 1½ to 3 hours, (as per British Standard Specifications).
In an air-dried sample the external moisture adhering to it dries up, the time depending upon relative humidity. Analysis of air-dried sampled can be carried out in most of the laboratories which are not equipped with special chamber having conditions of equilibrated atmosphere.
Reporting analysis on equilibrated conditions basis is a more scientific method. The analysis is done on coal samples passing through IS 20 sieve after equilibrating under the conditions given below for 48 hours.
Atmospheric temperature = 40° ± 2°C
Relative humidity = 60% ± 2%
Only some laboratories are equipped with chambers having such conditions of equilibration. This analysis provides a uniform basis for comparison of coals from different parts of the country is local temperature and relative humidity do not affect the moisture percentage in coal analysis. Commercial grades fixed by the Govt. of India are based on analysis carried out under equilibrated conditions. Coal containing less than 2% moisture are known as low moisture coals; if moisture is more than 2%, it is high moisture coal.
Reporting analysis on unit-coal basis (dmm basis) provides a better way of comparing ranks of different coals. The analysis indicates the nature of the coal substance, that is, the pure combustible or organic part of coal, viz. carbon and volatile matter. Moisture and ash do not contribute to the heating capacity of coal and in dmmf analysis only the v.m. and fixed carbon are reported as percentages of the coal substance.
Laboratory Methods for Proximate Analysis:
Samples of coal are collected from wagons, trucks or conveyor belts in a manner laid down in the Indian Standard Specifications or in agreements between the suppliers and consumers. To get a representative sample for analysis or other tests in the laboratory, the collected sample in subjected to the processes of crushing, coning and quartering.
These operations are repeated two or three times to get a laboratory sample, usually of 50 grams, which can pass through IS 20 sieve (corresponding to BS 72 sieve). For the proximate analysis only 1-5 grammes of the sample prepared in the aforesaid manner is taken for each test.
Moisture percentage (by weight) of coal is determined by heating an air-dried sample of coal in a shallow glass dish covered with a ground-glass cover plate at 105° to 110°C until the weight is constant. Loss of weight gives moisture content (inherent moisture). Volatile matter is determined by heating a laboratory sample of coal in a muffle furnace out of contact with air for about 7 minutes at a temperature of 925°C. Loss of weight gives the weight of moisture and volatile matter.
Percentage of V.M. is calculated after determining moisture percentage separately as stated here. Ash percentage is determined after heating a sample of coal in an open silica dish in a well-ventilated muffle furnace to nearly 815°C till the weight remains constant. The inorganic residue left on the dish is ash. Percentage of fixed carbon is obtained by subtracting from 100 the total percentage of ash, moisture and V.M. The mineral matter equals 1.1 x ash percentage.
Grades of Coking and Non-Coking Coals:
The existing grades of coals were formulated by the Govt. of India in July 1979, for non-coking they are based on useful heat value calculated from the results of proximate analysis. The calorific value of coal or any fuel can be determined in a laboratory in a bomb calorimeter but in the case of coal it can also be calculated if the ultimate analysis or proximate analysis results are known.
Ultimate analysis should give the percentage of carbon, hydrogen, oxygen, nitrogen, sulphur, ash, and moisture. The substances which produce heat when a fuel burns are chiefly carbon and hydrogen, and sulphur to a slight extent.
Any oxygen which may be present causes a loss of heat, because it uses up its equivalent amount of hydrogen, leaving only the excess as a heat producer. Moisture in the coal, together with that produced by the combination of any oxygen present, takes up heat in its conversion into steam, and of course the mineral matter or ash is a non-producer of heat. Calculated heating values do not, as a rule, agree very closely with practical tests or even with calorimetric results.
The formula circulated by the Govt. of India for calculation of useful heat value for non-coking coals only is:
Useful heat value = 8900 – 138 (A + M)
Where the U.H.V. is in kilocalories per kg of coal, A is ash % and M is moisture %. Both relative humidity and 40°C temperature as per ISS: 1350 – 1959.
If the non-coking coal contains less than 2% moisture and less than 19% volatile matter, from the value calculated by the above formula, deduct 150 kcal/kg for every 1% V.M. below 19% and that gives the actual U.H.V. for the coal.
Ungraded coals are coals whose ash, or ash + moisture content exceed the above specifications but such ash or ash + moisture does not exceed 50% of coal.
Rejects are coals whose ash or ash + moisture are in excess of 50% of the coal.
Ash% of coking shall be determined after air drying as per ISS : 1350 – 1959. If the moisture so determined is more than 2%, the determination shall be after equilibrating at 60% relative humidity and 40°C temperature. Coking coals gradation is on ash% only.
Non-coking coal is one which does not belong to the category of coking, semi-coking or weakly coking.
No grades are fixed for coals of Andhra Pradesh (Singareni group of mines).
The proximate analysis of some coal seams is as follows:
Kargali top seam (Sawang Colliery) excluding bands, air-dried basis, percent; Moisture 0.7-0.8; Ash 18.4 – 19.7; Volatile matter 28.4 – 30.4; Fixed carbon 62.5 – 49.1.
Purewa top seam (Singrauli Coalfield), on equilibrated basis i.e. 60% R.H. & 40°C excluding dirt bands, percent; Moisture 5.3 to 8.5; Ash 22.6 to 40.0 ; volatile matter 24.0 to 30.9; Fixed Carbon 48 to 21; Calorific value kcal/kg. 3300 to 4200.
The general characters of coal of the Barakar and Raniganj Stages are shown below:
6. Coking Coal:
Some types of coal, when heated in the absence of air to a temperature above 600°C, form a solid, porous residue called coke and the coal so heated is said to be carbonised. Coal which is capable of carbonisation is said to possess coking property.
There are two processes of coal carbonisation:
1. Low temperature carbonisation.
2. High temperature carbonisation.
1. Low Temperature Carbonisation:
In the low temperature carbonisation the coal is heated in the absence of air to a temperature of about 650°C. The coke formed is not strong enough for metallurgical purpose but is soft and can be used as domestic fuel, nearly free from smoke.
If the process is carried on in a plant which recovers the gases of combustion, such gases yield a few by-products and chemicals e.g., at Neyveli Lignite Complex. In the collieries, however, low temperature carbonisation is carried on in a crude way, only to form soft coke without recovering the gases which escape into the atmosphere.
The simplest way is to stack steam coal, set it on fire and cover it up with slack coal so that the steam coal burns without contact with air. After burning for a period of 36 to 48 hours when the burning stack practically ceases to produce smoke, it is quenched with water and soft coke is produced.
Two low-temperature carbonisation plants for bituminous coals have been set up in the country- (i) Ramkrishnapur in Andhara Pradesh with coal input of 900 te/day, and (ii) Dhankuni in West Bengal with coal input of 1500 te/day, to produce nearly 0.35 million te of soft coke per year and nearly half a million m3 of standard gas per day.
2. High Temperature Carbonisation:
In the high temperature carbonisation coal is heated in absence of air to a temperature of 1000°C 1100°C in specially constructed ovens. The gases of combustion are allowed to escape into the atmosphere in the beehive coke oven plants, (e.g., at Ena colliery and many other collieries in Jharia field) but in the by-product recovery plant they are recovered to obtain a number of chemicals, e.g. at Bararee, Loyabad, Lodna, Bhowra in Jharia coalfield.
By product recovery plants are installed at all the steel plants, Giridih colliery and at Durgapur Projects Ltd. at Durgapur. The coke formed after allowing the coal to burn for 30 hours and then quenching it with water is hard and suitable for metallurgical purposes. The coke oven gas formed during H.T. carbonisation is used as a gaseous fuel in the steel plants and in some other industries.
Hard Coke Manufacture in Bee Hive Coke Ovens:
There are two types of Bee Hive coke oven plants (or batteries).
1. Ordinary or conventional type (called Sabji Bhatta in Hindi colloquially).
2. English type (sometimes called Tata Bhatta or CFRI Bhatta colloquially).
In external appearance both ovens look alike but the difference lies mainly in the manner of circulation inside the oven of the smoke and hot gases resulting from burning of coal. In the English type, there are flues i.e. passages for hot gases below the refractory brick floor of the coal bed (but not on the sides) and the coal is better heated within a short period compared to the ordinary oven, in which the hot gases and smoke do not so circulate before escaping into the atmosphere.
The burning period of raw coal for conversion into hard coke, called coking cycle, is nearly 36 hours in English ovens and it can be reduced to even 24 hrs. with better quality of coal. In the ordinary ovens the normal burning period is 72 hours.
The conversion factor for the two types is : In English ovens 4.0 te raw coal yield 3 te hard coke; in the ordinary type 4.5 te raw coal yield 3 te hard coke. The capacity of each oven varies from 3.5 to 5 te of raw coal.
In both types of batteries, the ovens are constructed back to back and the two ovens have a common chimney. For economical operation, a battery should consist of minimum 12 ovens. Raw coal has to be charged in a crushed form (-10 mm size) to the ovens.
In a by-product coke oven plant the primary derivatives from coal, by weight, are:
1. Coal tar, 3% yield on coal, approximately i.e. 30 kg/te of raw coal.
2. Crude benzole, approx. 0.8% yield on coal.
3. Ammonium sulphate, approx. 1.1% yield on coal. Crude pyridine production is 0.006% yield on coal.
4. Hard coke, 0.70 to 0.74 te per te of raw coal.
The secondary derivatives from the above by-products are naphthalene, phenols, creosote oil, benzene, pyridine, etc.
The tests for coking coals cover those properties of coal that usually enters into consideration of its suitability for carbonisation, etc.
These are:
1. Caking Index.
2. Proximate analysis.
3. Chemical composition of coal ash.
4. Fusion range of coal ash.
5. Swelling test.
6. Plastically test.
7. Determination of undesirable salts.
Caking Index:
The caking property of coal is represented by a numerical called caking index (also called agglutinating index). The figure represents the maximum ratio of sand to coal in a mixture which, after carbonisation, gives a coherent mass capable of supporting a 500 g weight.
The sand should be of a specified quality. The maximum ratio of sand and coal is determined in the laboratory by a process of elimination of mixtures of different ratios. For this test the weight of coal and sand mixture, as prescribed by the Indian Standard Specifications is 25 g, but the procedure adopted by Tata Iron and Steel Co. uses only 5 g of mixture.
It is essential that the loose powder produced by the weight should not exceed 5% of the weight of the sand- coal mixture. The caking index gives only a relative idea of the capacity of coal to yield coke. For production of hard coke suitable for metallurgical purposes, the coal should possess caking index of 22 and above as determined by British Standard procedure.
Coal, which by itself gives, on carbonisation, coke suitable for metallurgical purposes, is called prime coking coal and has a caking index of 22 and above. A medium coking coal is that which gives coke slightly inferior to metallurgical coke.
Such coal has caking index between 17 and 22. Semi coking coal (also called blendable coal) falls much short of the requirements of prime coking coal and has a caking index of 10 to 17; for example, coal in Magma coalfield. Semi coking coal produces reasonably good soft coke. Though caking index is a major criterion to decide the coking quality of coal, there are other qualities, as given below, which a coking coal should possess.
i. It should not swell on carbonisation; otherwise walls of coke oven will be damaged.
ii. It should have low phosphorous (less than 0.15%) and low sulphur (less that 0.6% content).
iii. It should have carbon content high enough (above 58%) to give coke with minimum 75% carbon.
iv. It should have low ash content nearly 17 to 18% or below. The maximum ash that can be tolerated in the coke is 22.5%.
v. V.M. content should not exceed 26%.
vi. It should be able to yield coke of certain physical characteristics given below (I.S.S. – 439: 1965) –
(a) Shatter index – over 85
(b) Haven stability – over 40%
(c) Porosity – 35 to 48% and above.
(d) Micum index, total on 40 mm, % by wt. (minimum) – 75
(e) Breslau hardness – 80 or over (indicating hard coke)
Plasticity Test and the Plastometer:
When heated in the absence of air coking coals first become plastic and then solidify again on further heating. On an average the coals soften between 320°C and 350°C, attain maximum fluidity at temperatures between 350°C and 400°C, and solidify again between 400°C and 425°C the temperature of softening and solidifying and the degree of plasticity vary from coal to coal.
Non coking coals do not show any plastic behaviour. Property of coking coal to become plastic on heating in absence of air is known as its Plasticity and it is used as an index of its coking capacity. Although caking index, free swelling index, etc. indicate the coking property of coal, they do not reveal small changes in the coking property.
However, they are adopted for routine testing because of their speed. Plasticity tests are generally time-taking and hence they are used mostly for periodical checking of individual coals which go into the oven for carbonization, and in a steel plant, for the routine testing of one or two coal-mix samples in a day.
It may be mentioned that there is not optimum value of plasticity index for obtaining a good coke.
It is also observed that:
1. Plastic property of coal improves on Washing.
2. No relationship can be established between the maximum fluidity of coals and their V.M. and F.C. content.
3. Caking index does seem to be related to the fluidity of coal.
Hard coke containing ash 20% or less by wt. is classified as Gr. I coke; more than 20% but less than 24% ash is Gr. II coke.
The following coal seams produce prime coking coals:
i. Jharia Coalfield – IX, X, XI and upper seams into XVIII seam.
ii. Giridih Coalfield – Upper Karharbari, Lower Karharbari and Bhadua seam.
iii. Raniganj Coalfield – Chanch, Begunia, Laikdih, Shyampur-5, Ramnagar, Khudia.
Medium coking coal is available from the following coal seams after washing-
i. Jharia Coalfield – V to VIII, Mohuda Top and Mohuda Bottom.
ii. Raniganj Coalfield – Dishergarh and Sanctoria seams in some areas only.
iii. Bokaro and Ramgarh Coalfield – Kargali, Kathara, Sawang, Jarangdih, Bermo, Karo, Uchitdih, Kedla VII and VIII of Ramgarh block one.
iv. Kanhan Valley coalfield – Damua – Rakhikol. It is more on the verge of semi coking coal.
Assam coals have very low moisture and low ash (3-7%). They produce excellent hard coke but as they are high in sulphur, their use is not permitted in blast furnaces. Sulphur is present in Assam coals in organic form (3-7%) and therefore difficult to separate; (V.M. 35- 40%).
Reserves of prime coking coal, so essential for the manufacture of iron and steel, are limited in the country, but that of non-coking coal are plentiful. If the limited reserves of prime coking coal in the country get exhausted, the medium coking coal by itself will not be able to produce metallurgical coke and prime coking coal will have to be imported. Keeping this in mind the Steel Plants are manufacturing hard coke in their coke ovens from a blend of nearly following composition.
Prime coking coal – 60%
Medium coking coal – 32%
Blendable coal – 8%
The blendable coal is generally selected on its gas-evolution quality as sufficient gas has to be produced in coke ovens for various heating operations in steel plants. For this reason Bhilai Steel Plant uses blendable coal of Dishergarh, Poniaty and other coal seams of distant Raniganj Coal Field in preference to relatively nearer Churcha seam (Chirimiri Coal Field) which is of low ash content, nearly 15% as the latter has comparatively low V.M. content.
The trend in the production of prime coking coal and medium coking coal (after washing) shows that it will not keep pace with the demand of planned increased production in steel plants. The Government is therefore importing prime coking coal at a very high price.
7. Coal Washing
:
The ash percentage in coal can be reduced by a process known as coal washing in coal washeries which are established mostly in Jharia, Raniganj, Bokaro and Karanpura coalfields for treating prime and medium coking coals. Impurities in coal are plenty and they are collectively known as ash.
When coal is extracted in underground or opencast mines, external impurities get mixed up with it e.g. rock, clay shale and in the case of mines having sad-stowing, sand also gets mixed with coal during mine operations. The impurities have higher specific gravities than good burnable coal.
The operation of washing coal therefore aims at floating the coal and sinking the impurities in water. As a matter of fact, coal won’t float on water. Coal has specific gravity of 1.28 to 1.3 against 1.0 for water. The fact that coal with a specific gravity of 1.28 to 1.3 sinks in water at one speed and its impurities with higher specific gravities sink faster, provides the basic principle for coal washing.
The difference in settling rates is used in a number of commercial washing devices or washers. A fluid mixture of sand (Sp. gr. 2.6) and water in which the sand particles are kept in an agitated form without permitting them to settle, has a specific gravity of nearly 1.60, so that coaly matter floats over such mixture and the impurities settle down.
This is the principle adopted in Chance coal washer, the first coal washer in India. In this process water is pumped into the cone through its sloping sides to produce a rapidly rotating, rising water flow into which an over-dense sand suspension is added in quantities to achieve the desired sp. gr. in the mixed flows. Raw coal is added to the top of the cone. The clean is removed by a weir in the surface of the suspensions and sinks are removed by a lock hopper in the base of the cone.
In another method of washing, the process of Jigging is employed to separate good quality coal from its impurities. Jigging is a simple process of particles-stratification in which the particle rearrangement results from an alternate expansion and compaction of a bed of particles by a pulsating fluid flow.
The bed of particles is carried on a performed deck through which water passes upward and downward in repeated pulsion and suction stroke (30-50 time per minute) and at the same time the bed is moved forward through the jig chamber in a lateral direction by the horizontal flow of water. The stratified bed is cut at various heights by subsurface-gates to remove shales and middlings.
The raw coal fed to the Chance washer and the jig washer is + 6 mm and – 100 mm in size.
Ultrafine coal particles of -28 mesh size are cleaned by a process known as froth flotation. The process is generally inapplicable to low-rank coals. In principle, froth flotation consists of bubbling air through a suspension of fine coal and water to which various chemicals are added with a view to produce stable froth.
Coal particles attach themselves to the froth but not the particles of rejects because of the preferential physical attachment of air bubbles to coal-particles. The froth, with the attached coal particles, floats to the surface and is removed.
The material that forms ash in coal is finely disseminated over the entire coal mass in Indian coals unlike in the coals of other foreign countries like Britain, Germany, U.S.S.R., etc. The reason for this’ is the manner in which our coals have been formed as explained in the Drift theory.
Indian coals, therefore, cannot be washed as easily as foreign coals in commercial washers. To separate the ash from the heat-producing coaly material, the coal is crushed to a small size along with attached extraneous impurities, before feeding to the washer.
After washing, the end products are washed (also called clean) coal, middlings, and rejects in a 3-products washery and only washed coal and rejects if it is a two-product washery. The clean coal has much less ash percentage compared to that of the raw coal and it is dispatched to steel plants for use in the captive coke oven plants or to other consumers interested in low ash coal.
The middlings, though higher in ash content, have sufficient heating value and can be used it steam boilers designed for high ash coal. The rejects are discarded near the washery. The yield of clean in a washery may be 60 to 75% of the raw coal depending upon the quality of input.
8. Production of
Petrol from Coal:
The industrially advanced countries of the world have developed technology of producing petroleum and gas from the traditional fuel, coal. Chemically, petroleum oil has a higher hydrogen-carbon ratio than coal; this is responsible for its lightness, mobility and versatility as fuel and use in chemical processes.
To convert coal into petroleum the main requirement is to increase the proportion of hydrogen by synthesis. This hydrogenation was first achieved by the German scientist Bergius in 1914.
The processes which can be considered suitable for preparing petrol and allied chemicals from coal are:
1. Hydrogenation of coal, e.g. Bergius Process.
2. Solvent extraction of coal, e.g. Pott Broche Process.
3. Hydrogenation of tar obtained from pyrolysis/carbonisation of coal, e.g. Fischer Tropsch Process.
Of these processes hydrogenation of coal was adopted on a large scale in Germany during the II world war. It was named after its inventor, Bergius, as Bergius process. During the war years Germany was producing by this process nearly 5.0 million tonnes of synthetic petrol to cater to its war machinery as the outside natural petroleum supply was cut off.
The process involved in mixing powdered coal with vehicle oil (which is derived from the process) and subjected to hydrogen pressure based on iron bad poor activity and plant costs were very high because of the high pressure of operation. About 2.5 te of coal would produce one te of petroleum products. Due to the unfavourable economics the plants were all shut down after the war.
Another low-cost synthesis of oil is now finding popularity in Japan, the U.K. and some other countries. In this process powdered coil is carbonised to produce large quantities of tar and gas. The tar distillates are then hydrogenated to give petroleum products like gasoline, kerosene and diesel oil as well as other chemical byproducts.
In India the C.F.R.I, has conducted experiments on Assam coals which are easily amenable to hydrogenation for conversion into petrol. But the initial experiments have proved the economics of such conversion not favourable.
Besides the production of petroleum from coal there are now two more revolutionary trends in the synthetic fuel sector using coal as the base, namely:
(1) Hydrogasification of coal, and
(2) In-situ gasification of coal.
In hydrogasification the coal is gasified with oxygen and steam at higher pressure producing, when purified, a high energy gas consisting of methane and hydrogen.
Under in-situ gasification the underground seam of coal is totally gasified while oxygen or air is supplied through the shafts and large dia. boreholds or galleries. The ash remains underground while the low-grade gas is used in gas turbines generating electricity. Several power stations are being operated in the U.S.S.R. by this process from thick seams of brown coal (lignite).
9. Uses of Coal
:
Chief use of coal in industry is as a fuel for steam raising in the boilers in power houses and locomotives, gas producers, brick kilns, furnaces and for household purposes in the form of soft coke.
Boilers of railway locomotives and most of the Lancashire boilers used for steam raising are fitted with fire grates which are suitable for coal of large size. Hence coal over 50 mm in size is called steam coal; coal of 25 mm to 50 mm in size is called rubble coal.
Slack coal is that which is smaller than 25 mm in size though sizes upto 50 mm are also sometimes called slack coal. Run-of-mine coal is that which is not screened or sized and therefore contains steam, rubble and slack sizes.
The boilers for steam raising in large power houses are designed for inferior (high ash) coal in pulverised form. The coal that can be easily ground or pulverised has a high Hardgrove grindability index and hard coals have low H.G.I. For boilers using pulverised fuel, coal should have H.G.I, of 50 and above and low caking index.
Moreover the ash should have initial deformation temperature (I.D.T.) above 1100°C, the hemispherical point over 1100°C, the hemispherical point over 1100°C and flow point over 1200°C. Lower figures result in clinker formation which restricts air flow to the coal burning on fire gates and adversely affects coal combustion.
For these reasons the data required above coal by power house design engineers relate to the following points:
1. Proximate analysis.
2. Ultimate analysis.
3. Hardgrove grindability index of coal.
4. Composition of ash.
5. Initial ash deformation temperature.
6. Hemispherical temperature of ash
7. Ash fusion temperature.
Of all the constituents of ash the ones which matter most are iron and manganese oxide constituents as they lower the fusion temperature. Almost all ashes start softening above 1200°C and completely fuse at temperatures higher than 1300°C.
Coal is used for manufacture of hard coke which serves for heating purposes in smithy shops and in the furnaces. The hard coke also acts as a reducing agent when smelting oxidised iron or like Fe203 in the blast furnaces.
Gases produced from coal during carbonisation, if recovered in a by-product recovery plant, can be used for production of many chemicals, explosives, plastics, inks, paints, perfumes, fertilizers (e.g. at Talcher Fertilizer Plant) and also used for production of synthetic petrol.
The gaseous fuels available form coal/coke are:
1. Coke oven gas
2. Producer gas
3. Water gas
1. Coal Gas or Coke Oven Gas:
The process of manufacture involves complete gasification of coal subject to the limitations of the plant utilised for manufacture. The carbonisation is carried out by heating the cola in the absence of air in horizontal or vertical retorts if gas is the desired major product, or in the coke ovens if coke is the major product required.
Either method produces gases with similar compositions and a solid residue with an ash content that is higher than the original coal. The main differences in the gas making processes are in the quality of coke and in the quantity and character by-products. Typical analysis (% by volume) after the gas is washed is as follows- CO = 5.8; CO2 = 1.5; N2 = 1.0; H2 = 57.6; H2S = 0.7; CH4 = 29.6; C2H6 = 1.3; C2H4 = 2.5.
Heating Value of coal gas and coke oven gas is between 4000 and 5000 kcal/m3. Such high heating value makes its transport over long all its gas supply from the coke oven plant of Durgapur 30.0 mm dia. pipes. Coke oven gas is produced in large quantities at integrated steel plants making their own hard coke in coke oven batteries. The coke is used in the blast furnaces, and the coke oven gas along with blast furnace gas (heating value of B.F. gas nearly 850 kcal/m3), supplies heat energy for the plant functions.
2. Producer Gas:
It is formed by blowing a mixture of steam and air through a bed of hot coal and therefore its nitrogen content is very high and its heating value low. Because of its low heating value it is not transported any great distance and is used within or near the plant that produces it.
Gross heating value of producer gas manufactured from bituminous coal normally ranges from 1250 to 1600 kcal/m3. Range of gas composition is as follows (% by Vol.)- CO = 20 – 30; H2 = 8 – 20; CH4 – 0.5 – 3; CO2 = 3 – 9; N2 = 50 – 56; O2 = 0.1 – 0.3.
3. Water Gas:
Water gas is made in a manner similar to producer gas except that steam only is passed through the hot carbonaceous material. Hot air is first blown through the fuel bed to raise the fuel bed temperature to the desired level and steam is blown through until the temperature drops sufficiently to virtually stop the reaction.
Then air is blown through again to increase the bed temperature. Because water gas is made up mainly of H2 and CO, it burns with a characteristic blue flame. Heating value is approximately 2600 kcal/m3. Analysis % by vol- CO2 = 5.1; CO = 40.2; H2 = 50.0 : CH4 = 0.7 ; N2 = 4.0.
Coals suitable for gas productions are those having high V.M. and low caking index i.e. absence of the tendency to form clinkers. Coals from most of the seams in Raniganj coalifield are suitable for gas generation. Coals of Hingir-Rampur colliery and of Ghugus, Ballarpur and nearby collieries in the Wardha Valley coalfield are also suitable for gas plants. Coals from Kanhan valley coalfield in M.P. have a comparatively high caking index and as they form clinders, they are not considered desirable for gas production.
The norms of consumption of coal in some leading industries are as follows:
(a) Iron & Steel – Per tonne of pig iron produced, 1 te of hard coke equivalent to 1.3 te of washed coking coal (18% ash) or 2.2 to 2.8 te of raw coking coal; For every 1% additional ash in coal the consumption of coal for the blast furnace goes up by 7% to 8%.
(b) Power Plants – Inferior coal 3 tonnes per annum per kW of installed capacity; non-coking; in boilers with pulverised coal firing.
(c) Cement Manufacture – 1 te of coal for 3 tonnes of cement; non coking, slack.
(d) Paper – 2.25 te of coal per te of paper and 1.3 te of coal per te of pulp (in new plants); non-coking.
(e) Textiles – 0.38 te of coal per 1,000 m of cloth; non-coking.
(f) Rly. Locos – Non-coking steam coal.
(g) Sheet Glass – 1.46 te per te of glass manufactured.
(h) Brick Burning – For 1 lakh bricks 12-18 te; non-coking, slack.
(i) Tile Burning – For 1 lakh tiles, 20 te; slack; non-coking.