COAL

PART - I

Coal is a combustible solid. It usually stratified and formed by the burial of the partially decomposed vegetation in the past geological years, which is for more than about 300 million years. After the pressure and temperature, former due to standing mass of the crust above and later due to the heat of the Earth’s interior, continued to rest in the absence of air, changing the physical as well as chemical property of the substances buried under.
Variation in the proportions of various plant components causes the different type of Coal. Chemically, Coal is highly complex organic matter with varying amount of water, with the presence of nitrogen and sulphur in the form of organic and in-organic matter. The contents of impurities in the Coal make different grades of Coal. On the basis of origin, Coal may be Humic or Banded Coal (derived from forests on the land) and Sapropelic Coal (derived from the aquatic plants).

There are different types of Coal according to the time period, pressure and composition:

i) Peat

Peat is the product of anaerobic decomposition of various vegetable remains under moist conditions. Peat is the first stage of the Coal formation from cellulose. It is neither Coal nor Wood in the proper manner but something intermediate between the two, which is formed by the gradual decaying of the vegetable matter in the moist places through an incomplete transformation of plant to Coal. The composition of the organic matter in Peat depends upon the Degree of Decomposition. Greater the decomposition greater will be the Carbon and lesser the Oxygen. The ash content of the Peat is not so high and it is not an economical fuel due to the cost involved in the drying and handling. Destructive distillation of Peat is also done which results in coke, hydrocarbon oils, fuel gases, wax and tar etc.

ii) Lignite

Lignite or Brown Coal is an immature Coal and has composition intermediate between the Peat and Bituminous Coals. Lignite may be amorphous, fibrous or woody in texture. They are usually brown in colour but become dark on exposure to air. Lignite absorbs Oxygen when exposed to air and gets ignited spontaneously.

iii) Pulverised Coal

Pulverised Coal is obtained by grinding fossil Coal or Oil shale. Pulverised Coal burns quickly and completely considering the adequate mixture of pulverised Coal and air for the combustion. The pulverised ore can easily be handled like a liquid fuel and can be transformed through pipes. The fineness of the pulverised coal depends upon the concentration of combustible volatile and ash, density of the coal and the volume of combustion chamber available. Fuels with the low concentration of volatiles should be ground more finely. The Process of Pulverisation consists of Crushing, Drying and Grinding.

iv) Sub-Bituminous

It is a Dull Black coal. Sub- Bituminous coal is denser and harder than Lignite also has much lesser moisture content than Lignite. The calorific value is less than that of the Bituminous Coal. It ignites very easily.

v) Bituminous

It is a commonly used commercial rank Coal. It is harder than Lignite and Sub-Bituminous coal. A good bituminous coal is composed of alternate dull and bright bands. It is brittle in nature and easily breaks into thin blocks when strike, it also has a good concentration of Carbon in its composition. This type of coal is also used for the production of coal but not all Bituminous coal is used for the Coking. The coal used for the coking must be highly, Bituminous and contain very small proportion of sulphur and ash. For coking the coal must be washed in a special type of mechanical washery, so that to reduce the sulphur as well as ash content from the coal. Bituminous coals are easy to handle and have excellent heating qualities, this type of coal is the mostly been used throughout the World. There are two types of bituminous coals the Low Volatile coals (fuel ratio less than 2) and the High Volatile coals (fuel ratio more than 2). High volatile coal burns with long flames and used for gas industry, coal tar distillation and for the manufacturing glass etc.

vi) Anthracite

Anthracite is the final stage of the coal formation from the vegetable matter. It is very hard, compact with a semi-metallic lusture. Anthracite is the fossil coal with the highest percentage of fixed carbon. It burns with a very small flame which is not very strongly luminous. It contains the minimum volatile content among all form of coals. This type of coal is the most prised and don’t have coking properties. The ash content of such a coal is high making it not to be used in various industries.



S.No. TYPE CALORIFIC VALUE CARBON CONTENT
i Anthracite 14000-15000 *BTU 86-88 %
ii Bituminous 11000-15000 BTU
a) Low volatile 78-86 %
b) Medium volatile 69-78 %
c) High volatile < 69 %
iii Sub-Bituminous 7000-15000 BTU
iv Lignite 4000-6000 BTU


*BTU = 252 Calories


Coal is divided into different Grades depending upon the ash content, moisture content, calorific value, presence of volatile matter, coking property, carbon content and various other factors. These grades are:


For Non-Coking Coal

Grade K Cal/kg
A Exceed 6200
B Between 5600-6200
C Between 4940-5600
D Between 4200-4940
E Between 3360-4200
F Between 2400-3360
G Between 1300-2400

For Coking Coal
Grade % Ash
W-I Between 18-21
W-II Between 21-24
W-III Between 24-28
W-IV Between 28-35



Coking And Non-Coking Coal

A coal is said to be Coking if it has the tendency to soften, swell and stick together during combustion. Bituminous coals have the coking properties where as Anthracite, sub-Bituminous and Lignite coals are non-coking coals. Coking coals are generally of two types:

a) Firm Non-Swelling coking coals and
b) Excessively Swelling coking coals

The Firm Non-swelling coals are those which yield compact hard coke without any increase in the volume originally associated with the coal there fore are good to use in blast furnaces, where as excessively swelling type of coking coal swells excessively when coke is formed. All those type of coal are said to be a good coking coal which have a ratio of H:O above 58.

Roles of Sulphur and Ash in Coal

Sulphur in Coal is present as Iron pyrites, Calcium sulphate or gypsum and as Organic sulphate. Carbonisation is the process which expels sulphur as hydrogen sulphide, carbon sulphide and thiophene but still remains in coke. The sulphur present in the coal can be removed to some extent by washing the coal, resulting in the improvement of the grade and adding the economic value to the price of the coal.
Ash content effects the quality of the coal. The ash content depends upon the type of vegetation the coal is formend from. Thickness of ash decides the grade of coal, coal with thick ash considered to be a good quality of coal. The ash present in the coal can be removed to certain extent by washing the coal.