Coal
Coal has been formed in nature as a result of the slow decomposition of vegetable matter. Plants get converted into coal, deep below the surface of the Earth, under the influence of heat, pressure and limited supply of air.
Coal is found in India in large quantities at Ranigang (West Bengal) and at Bokaro (Bihar).
The process of conversion of vegetable matter to coal is called coalification.
Classification of coal
Vegetable matters are converted in to coal. Various stages of conversion are given below :
| Wood → Peat → Lignite → Bituminous coal → Anthracite coal |
(a) Peat:
(i) This is the first stage of the conversion of vegetable matter into coal.
(ii) It contains a large percentage of organic matter.
(iii) It’s carbon content is about 60%
(iv) It is a brown jelly-like mass.
(v) Its calorific value is about 5400k cal/kg on air dry basis.
(vi) It is not a good fuel.
(b) Lignite (brown coal)
(i) It is the second stage of coal formation (coalification)
(ii) It is a fairly hard dark-brown substance.
(iii) It is compact in structure and its carbon content is about 60% – 70%.
(iv) The calorific value is about 6500 7100°K.cal/kg on air drybasis.
(v) It is not a good fuel because of its low carbon content
(c) Bituminous coal (common coal)
(i) It is the third stage of coal formation (coalification).
(ii) It is a black, hard stony substance.
| Property | Sub-bituminous coal | Bituminous | Semi-bituminous |
| Carbon percentage(%) | 75 – 83 | 78 -90 | 90 – 95 |
| Moisture and volatile content | High | Low | Very low |
| Calorific value (kcal/kg) | 7000 | 8000 – 9000 | 8500 – 8600 |
(d) Anthracite :
(i) It is a class of highest rank coal containing 92% 98% of carbon and is a poorest volatile matter.
(ii) It’s calorific value is about 8650°kcal/kg.
(iii) It is the hardest of all kinds of coal.
(iv) It has a high ignition temperature and produces very high temperature.
Analysis of coal
The quality of coal is assessed by the following types of analyses.
(a) Proximate analysis and
(b) Ultimate analysis
This is a functional or empirical analysis. It gives information regarding the practical utility of the fuel. This involves the following analysis.
(i) Moisture content
(ii) Volatile matter content
(iii) Ash content
(iv) Fixed carbon
- The water content present in coal is expressed as the moisture content in chemical analysis.
- One gram of pure dry powdered coal is taken in a dry and accurately weighed silica crucible.
- It is heated for 1h in a small muffle furnace at 105°C
- The crucible is cooled in a desiccator and weighed.
- The percentage loss in weight of the sample is found out.
Percentage of moisture =-\frac{\text { Loss in weight of coal }}{- \text { Weight of coal taken }} \times 100
Significance
(i) Moisture reduces the effective calorific value of the coal.
(ii) Lesser the moisture content better is the quality.
(ii) Volatile matter
- It is also known as volatile carbonaceous matter (VCM).
- The dried sample of coal is taken is a crucible, covered with a lid and weighed accurately.
- This crusible is heated for exactly 7° min in an electric furnace at 950°C.
- The coal is heated in such a way that no air is allowed to come in contact.
- The complete removal of volatile matter is judged by bubbling the gas through a water sea.
- The volatile matter consists of combustible gases like H2, CO, CH4, other hydrocarbons, tar vapours and some incombustible gases like CO2 and water vapour.
- The decrease in weight in this process is reported of the volatile matter on percentage basis.
Significance
(i) A high volatile matter containing coal burns with a long flame, produces high smoke and has low calorific value.
(ii) Hence, lesser the volatile matter better is the quality of coal.
(iii) Ash content
- The residual coal in the crucible is then heated without the lid in a muffle furnace at 700-750°C for half-an hour. Stir the ash with a platinum wire carefully.
- The crucible is now cooled in a desiccator.
- The process of cooling and weighing is continued till a constant weight is obtained.
- The weight of ash is calculated-by subtracting the weight of the crusible.
Significance
(i) Ash is a noncombustible matter and so reduces the calorific value of the fuel.
(ii) Ash also causes hindrance to the flow of air and heat. Hence lower the ash content, better is the grade of coal.
(iv) Fixed carbon
Fixed carbon is determined by subtracting the sum of percentage of moisture, volatile and ash contents from 100 ,
| Percentage of fixed carbon = 100 – (Percentage of moisture content + Percentage of volatile matter + Percentage of ash content) |
Significance
- Greater the percentage of fixed carbon, greater is its calorific value and better is the quality of coal.
- As the percentage of fixed carbon increases in the fuel, the percentage of volatile matter be comes less.
- The percentage of fixed carbon helps in designing the furnace.
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