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Coal as a Fuel

Coal

Coal is a non-renewable fossil solid fuel formed by a series of natural geo-chemical process from the plant remains accumulated together with other sediments. It is a combustible brown to black sedimentary rock, composed of heterogeneous components.
Two theories, Drift & In Situ explain the formation of Coal seams.


DRIFT THEORY

As per theory, coal seams are believed to be formed out of plants and trees which grew millions of years ago and fell down to earth quake and tectonic activates, ground subsided and the plants material drifted to lakes, river valleys etc, by flow of water covered by sediment of sand and earth. The process of deposition continued for millions of years in layers as undergone geo-chemical changes such as heat, bacterial decay, pressure etc. to form coal seams.

IN SITU THEORY

The plants; where it grew, subsided under the earth; submerged in the water at the same place.The process of deposition continued in layers and undergone geo-chemical changes as explained above to form coal seams. It is believed that in India coal seams are formed as per drift theory.

Stage of coal formation
  1. Peat:It is the first stage of coal formation process. It is light, porous and fibrous substance and light to dark in colour. It contains approximately 35% moisture about 10% volatile matter, 4.5% fixed carbon and having calorific value of about 3500 B.Th. U.(2000K.Cals/Kg.)
  2. Lignite:It is brown in colour and contains 25-45% moisture. Its calorific value ranges between 6000-7000 B.Th.U (3300-3900K.Cals/kg.)
  3. Sub Bituminous Coal: This coal is intermediate between lignite and bituminous coal. Bright in colour it contains about 10-25% moisture and its calorific value ranges from 8000 to 10000 B.Th.U. (4450-5500 K.Cals/Kg.)
  4. Bituminous Coal:It is pitch black to dark in colour. It is harder and denser and able to stand exposures It is considered as all purpose coal because of its excellent heating quality. Its calorific value ranges between 11000 to 15000B.Th.U (6100-8300 K.Cals /Kg.)
  5. Semi Bituminous Coal: This is superior grade of bituminous coal containing 1 to 18%volatile matter and calorific value ranges between 12000-15400B.Th.U (6600-8550 K.Cals /Kg.)
  6. Anthracite: It is hard coal with iron black colour and sub metallic to brilliant luster. Its calorific value ranges from 14000 to 15000B.Th.U.(7700-8300K.Cals/Kg.)
Constituents of coal & type

    Coal type is related to the type of plant material in the peat and the extent of its biochemical and chemical alteration. Type can assessed in term of variety of petro graphics analysis, meceral and  microlithotype. Megascopically distinguishable ingredients in coal are Victorian, Clarian, Durian & Fusains, analogous to the “Rock Type” Called litho types. Microscopically, the litho types can be split into a series of “minerals” The organic equivalents of minerals in the rocks. These are vitrinite, Micrinite, Oxinite, Intertinite & fusinite.

There are four types of macro components in coal, these are:
  1. Vitrain-Bright glossy & homogeneous components of coal, having a massive texture & showing characteristics vitreous conchoidal fracture.
  2. Clarain-Bright component of coal, but less brighter than vitrain, it is heterogeneous material with a banded structure & has a definite and smooth surface when fractured at right angles to the bedding plane.
  3. Durain-Dull hard component of coal with a greyish black colour. It is heterogeneous and has a firm granular texture.
  4. Fusain-Fusain occurs as patches or wedges of soft, fibrous material resembling charcoal. It is highly fibrous & can be readily powdered by fingers.
    The Micro-component called macerals are as under:

  • Vitrinite: Major component of vitrain & one of two principal components of clarain.
  • Fusinite: Major component of fusain.
  • Micrinite:  One of the two principal components of durain
  • Liptinite or simply Exinite: The other principal components of clarain & durain. It is  essentially mixture of the fossilized remains of the spores, cuticles & resins etc.
On the basis of physical & chemical changes that have taken place in the transformation of the  vegetation matter  into coal, the varieties like peat, Lignite, bituminous, Semi Bituminous, Semi Anthracite & Anthracite are formed depending on the nature and composition of the organic matter, its volatile content, caloric value and moisture content.

Coal Rank

    This implies the degree of coalification. It is a measure of the maturity of coal & is assessed in terms of some property of vitrinite meceral such as moisture, carbon, volatile matter and Vitrinite reflectivity. While Lignite are essentially low rank coal. Anthracite are in the last  stage of coalification and form the highest rank of coal.
    
In India, the coal rank parameters have been moisture, Carbon, hydrogen and Volatile content etc. but lately vitrinite reflectance has been used for determination of coal Rank.
  
Based on the rank studies carried out in the past, Indian Coals traditionally have classified as high rank prime, Medium & Semi to weakly coking coals and low rank sub bituminous / bituminous non coking coals.


COAL CLASSIFICATION

    The term class is retained for association with the actual usage of the coal for example, coking or Stream raising or gas making.
    Classification is done to provide as much scientific and technical information as possible for its proper and various industrial uses. This is based on four basic and two supplementary parameters.

a.    Basic Parameters:
  • Gross calorific value.
  • Volatile matter.
  • Caking index.
  • L.T.G.K./Coke Type.
b.   Supplementary Parameters:
  • Maximum thickness of plastic layer (MTPL)
  • Equilibrated moisture.
COKING COALS:

    The coking coal are those types which when heated without contact with air, result in formation of a carbonaceous, nearly volatile free, strong and porous mass. These are classified on the quality of coke produced by them individually.The laboratory Corbonisation test broadly indicate the suitability of coal for blast furnace use but fail to give precisely the strength  of the coke to be produced. Coal petrographic studies on the other hand not only provide information about  maceral composition but also the rank of the various vitrinite present in the coal on the basis of the reflection of Vitrinite which is the principal reactive constituent in coking coals during carbonization. For Indian Coals, reflectance percentage of Vitrinite of various ranks is ranging from 0.67 in sub bituminous coat to 2.81 in the anthracites coal. The best prime coking coals have a reflectance from 0.95 to 1.4% with a reactive content of 35 to 60%

NON COKING COALS:

All coal which are either poor or feebly coking or non coking, be they of high or low volatile types, have been grouped under this category.

COAL GRADE

The term grade in India generally refers to the commercial degree of purity of coal, as assessed from the amount of chief impurities present that is ash and moisture.This classification of coal is not concerned with such grading.

GROSS CALORIFIC VALUE:

The number of heat units liberated when unit weight of the fuel is burnt at constant volume in Oxygen saturated with water vapour; the original material fit final product being at 25oC. The residual materials are taken as Carbon dioxide, Sulphur dioxide, Nitrogen and water, and the residual water other than that originally present as vapour, is taken as being in the liquid state.The gross calorific value at constant volume is the one usually used in coal technology. It assume that all heat produces is available, including the heat of condensation if any, steam(resulting from the combustion of the hydrogen of the fuel) to water at room temperature.

NET CALORIFIC VALUE:

The gross calorific value less the heat of condensation or otherwise of any steam below 100oC is termed as net Calorific value.

ASH:

    It is the inorganic residue left when the powdered laboratory sample of coal has been incinerated in air determined in air in an open dish until it no longer changes in weight at 815+/-10oC

MOSTURE:


    Water expelled in its various form when coal is tested under various specified conditions. It is determined by heating the coal simple to 108+/-2oC under different specified conditions.

VOLATILE MATTER:

    It is the loss in weight minus the air dried moisture when the air dried coal is heated, out of contact with air, under specified conditions at 900+/-10oC.

ANALYSIS OF COAL


   For determining of quality for commercial purpose, it is analyzed in the laboratory for its different constituents and characteristics. Basically the coal analysis is divided into two types.

Proximate Analysis

This involves the determination of moisture, ash, volatile matter. This gives the information for practical utility of coal and its commercial grading. The proximate analysis of coal may be reported on any of the following conditions:
  • As received sample basis.
  • On air dried sample basis.
  • On equilibrated condition basis.
  • On unit or dry mineral matter free basis.

As Received sample Basis:

Reporting analysis on as received sample basis is not very common. The coal samples are collected in sealed containers, so that the moisture adhering to the coal is not dried up during transit or storage pending analysis. This is important generally for determining external moisture(total moisture).

On Air Dried Sample Basis:

This is a commonly adopted practice. The coal samples are allowed to expose in the laboratory atmosphere for a few days when the external moisture is dried up depending upon relative humidity and analysis can be carried out.


On Equilibrated condition Basis.


Analysis of coal samples on equilibrated condition basis is a standard practice. This is carried out on coal samples passing through 72 Mesh sieve and after equilibrating under the conditions of 60% R.H. & 40oC for almost 72 Hrs till the constant weight prevail.

On Unit Coal Or Dry Mineral Matter Free Basis.

Reporting analysis on unit coal basis provides a better way of comparing different coals. The analysis indicates the nature of the pure coal substance i.e. combustible of the organic part of coal i.e. Carbon and volatile matters. Moisture and Ash do not contribute to the heating capacity of coal.

Analysis of coal on Equilibrated Basis:
(i.e. at 40oC & 60% RH)
The procedure involved is as follows:
Sample received at the laboratory should pass through 212 micron IS sieve and air dried for 24 Hrs. At room temperatur
e.

1.Determination of moisture on air dried basis:

Take about 1.5 gms. Of air dried sample in a uniformly thin layer. Now put the weighing vessel in an air-oven and heat at a temperature of 108+/-2oC until there is no further loss in mass. This normally takes about 1 to 1.5 hours. Express the loss in mass as a % age of the total mass of the sample.
 
M%(Air dried moisture)    =100(M2-m3)/(M2-M1)    
Where,
M1= Mass in grams of empty weighing vessel plus lid.
M2=Mass in gram of weighing vessel with lid plus coal.
M3=mass in grams of weighing vessel with lid plus dry coal
.

2.Determination of ash on air dried basis:

Weigh a clean dry empty crucible and lid. Weigh accurately about 1.0gm. of laboratory sample and spread the coal uniformly in the crucible. Insert the uncovered crucible into the muffle furnace at room temperature. Raise the temperature to 500oC in 30 Minutes and 815+/-10oC in a further 30 to 60 minutes and maintain at this temperature for 60 minutes. Cover the crucible with its lid and remove from the muffle furnace and allow it to cool for 10 minutes and finally take in a desiccator. Weigh the mass of Ash by difference.

3.Determination of Equilibrate Moisture:


(i.e. at 40oC & 60% RH)
Take about 5 gms. Of Laboratory Sample in a 100ml beaker and pour 20 ml. of water and gently boil the contents for about 15 minutes and remove the visible water through filtration and pressing the contents in between filter papers.Take about 1.5gms. of wet coal in glass dish and spread the coal in a uniformly thin layer place the uncovered capsule in a desiccators which is half filed with a solution of H2SO4. of specific gravity 1.28 to give a relative humidity of 60% at 40oC and leave the whole thing for 24 hours. Withdraw and break the lumps and spread the coal uniformly and leave it for another 48 hours or till the change in mass does not exceed 0.001 gm. Now put the weighted dish in an air oven maintained at 108+/-2oC for about an hour and determine the moisture by the formula as given in procedure (1) above.


4.Determination of Equilibrate Ash:

This is calculated by following formula:
Equilibrated Ash%=
((100-Equilibrated moisture)/(100-Air dried moisture))*Ash% on Air Dried basis


ASH FUSION TEMPERATURE


It is a range of temperature over which the ash fuses. This is signified by three characteristics temperature known as initial deformation temperature, hemispherical temperature and flow temperature. The range starts from initial deformation and ends flow temperature.
The average chemical composition of ash and fusion ranges in Indian coal is as under:

Si02-50-60%          CaO-2-5%
Al203-22-30%         Mg0-2-5%
Fe203-1-10%          Alkalies-1-3%
S03-0.4-3%            P2A5-0.1-3%

Some Rare Earth / Trace elements also present. Ash fusion Range-1160-1450oC.


Ultimate Analysis:

The analysis of coal / Coke expressed in terms of Carbon, hydrogen, Nitrogen, Sulphur and Oxygen Contents come under the category of ultimate analysis.It involves the estimation of proportion of Ash, Carbon, Hydrogen, Nitrogen, Sulphur , Oxygen and Phosphorous. The amount of Carbon , Hydrogen and Nitrogen are determined directly and Oxygen by difference. The Sulphur present in coal may be in the form of Organic Sulphur or compound as mineral Sulphides like Pyrite or as Sulphates mostly or Calcium & Iron. Phosphorous is generally present in a small amount as organic calcium phosphates derived from the intensive rocks.

Useful heat Value:

This can be termed as net Calorific Value and is calculated by the empirical formula as below:

UHV=8900-138(Ash%+Moist %) in K. Cal/KG

    Ash and Moisture percent shall be determined in accordance with the relevant clause of ISI on equilibrated condition at 60% relative humidity and 40oC.


Long Flame:

Long Flame is a characteristic of coal which basically depends upon the volatile matter present in coal. Range of gross CV and dried moisture are two other factors which determine the long flame characteristics of coal. Following parameters  are as laid down in IS 770-1964, general classification of coal(revised).

Group
Volatile%(On unit coal)
Gross C.V. in K.Cal/Kg. (On unit Coal)
Air dried moisture % at 60% RH & 40oC On Mineral free Coal Basis)
B4
Over 32
8060 to 8440
3 TO 7
B5
Over 32
7500 to 8060
7 TO 14

Unit Coal:

 It is the organic substance of the coal that is free from moisture and mineral matter.

Caking Index:

The caking power of coal is the property of coal to form a coherent mass when the finally powdered coal is heated under specified condition in the laboratory. The power of forming a satisfactory coke as distinct from chart, is associated with the phenomenon of particle coherence, softening and pyrolytics swelling and shrinkage and is influenced be the rate of heating and other factors.

L.T.G. K. ( Low Temperature Gray king Assay)

 It is to assess the caking properties of coal or a blend of coal by carbonizing in a Laboratory assay under standard conditions at maximum temperature of 600oC. The cake residue from the carbonization of finely ground coal at 600oC is classified in comparison with series of described coke types. For strongly swelling coals, the coal is blended with electrodes carbon in a proportion which forms on carbonization, a strong, hard coke of the same volume original coal and electrode carbon mixture.

F.S.I (Free Swelling Index):

It is a number  given to the coal when 1 gm of coal is carbonized at 900oC. The number is giving from 0 to 9.

Coke:


Solid residue left after destructive distillation of coal in absence of air.

Micum Index:


The portion of coke retained on a 43mm test sieve (M.40) or passing a 10mm test sieve after being subjected to micum test.

Shatter Index:

The %age of coke retained on a sieve of stated aperture after subjectd to the shatter test.
  • Stream-Fraction of Run of Mine or raw coal passing through 200mm screen but retained on 50mm screen.
  • Slack-Coal with a specified top size, usually below 50 mm with no tower size limit.
  • ROM-Coal as produced in mining operations, without any preparation.
  • P.ROM- Where Coal handling Plants or mechanical screening / crushing facilities are available & top size of coal is limited within the maximum range of 200mm-250mm size(Processed ROM)

Density

The Density of coal is important property used for its reserves estimation, coal washing, handling and commercial use. Four types of densities are generally used.

A.Specific Gravity:
Specific Gravity or “Gravity true” which gives specific density without voids of the coal and is used for scientific purposes, it is also called particle density. Specific Gravity of Bituminous coal normally varies between 1.27 to 1.45

B.Gravity Natural: Gravity natural is the volume of alone of coal in situ (before mining) in cubic meters. This is used for reserve estimation and mine planning and is also called tonnage factor (Cub. Mtr./ton or cubic ft/ton)

C.Gravity Dry:Gravity dry is the density without water content.

D.Bulk Density:Bulk density is the volume of mined coal in cubic meter which makes a ton of coal or in (Cubic feet per tonne of coal).  This is used generally for marketing purpose and estimation of mined coal stocks etc.


Grindability of Coal:

Grindability if coal is a measure of its hardness. There are two indices of grindability (a) hard grove grindability index (HGI) & (b) US Ball Mill Index which are taken into account while designing crusher / pulverisers. The average HGI for Indian Coals is 55 to 65(out of 100).

Coal mining Method:

Broadly there are two methods of coal mining viz. OPEN CAST AND UNDER GROUND method. The selection of the mining method depends on the depth of the coal availability and on the economic viability.


A.Under Ground Mining Method:


In underground mining, the quality of coal mined is generally maintained except in case of sudden roof fall and fires. The horizon is so selected that the shale/ Stone partings / Brands are left either in roof or in floor. In manual tub loading the extraneous material are segregated before loading for maintain better quality where as this is not possible in mechanical loaders i.e. LHD / SDLS. There are two methods of coal extraction namely.
  • Board and pillar method ( manual / Mechanized)
  • Long wall method.
A.Under Ground Mining Method:

In contrary to the UG mining, OC mining is always prone to quality determination unless proper precautions are taken to avoid it. For this maintenance of proper benching, drainage, systems, separate removal of shale / stone parting and effective manual picking is essential.
Sampling of coal
Sampling of coal is very important. The Physical and Chemical properties of coal are determined with the help of correct sampling and analysis techniques.


Method of sampling collection

Samples collection by various modes is done as per ISI: 436 (Part-I) 1964.
Sampling from Conveyor belt.
Increment of approximate 5kg. coal sample at regular interval from speed regulated belt is scooped with the help of a shovel across full section in one operatoin. Interval is fixed with Quantity moved so that a representative Gross sample could be drawn from each of the sub lot.
Sampling from loaded wagons / trucks

For the purpose of sampling, the loaded wagons in a lot shall be divided into a suitable number of sub-lots of approximately equal weights. A minimum of 25% of the wagons selected as per random table is to be sampled. At every selected points a sample shall be collected by taking the whole section of coal from top to Bottom over an area of 30 cm diameter. Minimum 350 kg. of sample from each sub lot shall be collected. Such sample shall be mixed to from a gross sample.

Sampling from stock

The quantity of coal in a stock pile shall be divided into a suitable number of sub lots of approximately equal weights by suitable marking the lines of demarcation on surface of the lot. The surface of each sub lot shall be leveled and one point for approximately every 250 MT of coal in the sub lot shall be chosen at random. A gross sample then be taken from each of the sub lot and then all the samples are mixed to form a Gross sample.
 
Sampling from coal seam

  • With band : The seam section is evenly exposed from top to bottom. The working thickness is measured and clean gunny begs are spread below to avoid contamination. Then a channel of 30 cm width and 10 cm depth is cut representing the entire cross section of the seam and the gross sample is collected.
  • Without band: For seam sampling without band alt shale / non coal bands over 10 cm is eliminated from the seam and sample are collected as above.
Procedure for sample preparation

Gross sample is crushed to 50 mm size by mechanical means and mixed thoroughly, quartered and coned. Two opposite quarters are retained, remaining rejected. The retained fraction is mixed and only one half portion retained. The material so obtained is crushed to 3.5 mm size. The crushed material is reduced by coning and quartering as per method specified in IS : 436 till about 2 kg sample is finally obtained which is passed through 212 micron IS sieve.

Infrastructure / Standard equipment required

For coal preparation
  • Primary Crusher – for crushing the sample to 50mm size,
  • Secondary Crusher – For crushing to 12.5mm to 3.35mm size.
  • Pulveriser – for pulverising 3.35 mm sample to mesh size.
Lab Equipments
  • Monopan balance / chemical balance
  • Humidity chamber / Desiccator for equilibration
  • Leb. Oven- for moisture determination
  • Hygrometer – to check R. H.
  • Muffle furnace – for determination of Ash and VM
  • Bomb Calorimeter – for determination of C. V.
Manpower
  • Chemist- One
  • Leb. Asstt.- Two
  • Sampling Asst.- Three
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