Welcome to Engineers Rail‘s another blog article, namely- Classification of Coal, from your host and dost Abhishek.
If you want to learn about the kinds, definitions, and typical explanations of Coal, Then you’ve come to the correct spot. If you stick with me and scroll slowly, this may be your last stop.
So, I won’t waste much of your time and will direct your attention to the topic of today’s stream. Don’t forget to collect the classification of coal pdf at the end of the article.
Before diving into the mainstream, I will start with some basics first to give you an overview of coal.
So, let’s get started…
What is Coal?
It is the solid mineral material found under the earth. It is black in colour. Its formation under the earth takes place in millions of years from the buried biomass, by the combined action of moisture, temperature, bacteria etc.
Theories Behind the formation of coal-
There are two theories popular among all and they are-
According to the first theory, Where ever vegetation grew and collected in bulk, the formation of cool took place. The first theory gives us an idea about the purity of coal.
According to the second theory: Trees and vegetation floating in rivers collected in lakes and formed coal.
The second theory confirms the deposition of coal in thick layers found in coal mines. In due course of time, the first peat was formed from trees by the bacteria reaction.
Thereafter heat and pressure acted upon it and in stepwise changes, good-quality coal was formed. Later on, the quantity of moisture reduced slowly and pure solid carbon was deposited.
Steps of coal Formation-
The formation of coal in nature took place in the following steps:
- Plant debris
- Sub-bituminous coal
- Semi-bituminous coal
- Brown coal
- Bituminous coal
- Semi-anthracite coal
Important Elements in the Composition of Coal-
The main elements of coal are carbon, hydrogen, sulphur and oxygen. Their percentage depends upon the place where coal deposits are found and the conditions of temperature and pressure at that place.
Thus the percentage of elements of coal and its quality depends upon the place where it is found.
The proximate analysis of coal shows the percentage by weight as shown below:
|Elements||Composition in Percentage|
|Fixed Carbon||16 to 92%|
|Volatile Matter||5 to 50%|
|Moisture||3 to 40%|
|Ash||20 to 30%|
Now, Let’s move to the mainstream of the Article, That is-
Types of Coal-
Peoples generally ask that, On what basis coal is classified? So, Here is the answer-
Coal is classified On the basis of formation and Coal can be classified into 7 types of coal with carbon percentage, which is-
- Lignite or Brown Coal
- Sub-Bituminous Coal
- Bituminous Coal
- Semi-bituminous coal
- Anthracite coal
Let’s deal with these coal types one by one-
It is a spongy and humid material of brown colour. It is found in boggy land te contains a large amount of water (about 90%) and the rest of it is solid material.
It is dried for 1 to 2 months, After drying its water content reduces to about 30%. It is compressed to form blocks so that it can be transported for use.
The average calorific value of peat is 23 MJ/kg. Its combustion takes place with a specific odour and burns with a smoky flame.
Dry peat has the following elements by percentage:
Uses of Peat Coal-
The use of peat is negligible in India. In European countries, it is used as domestic fuel. It is also used in gas-producing plants, power-production plants and rail engine boilers.
2. Lignite or Brown Coal-
It is the second stage of the formation of coal after peat. Its properties are found in between peat and bituminous coal.
It is deep red-brown in colour, therefore it is known as grey coal. When broken its section appears like wood. It is found near earth’s crest in about 30 metres thick layers.
When it is received from mines, it contains about 40% moisture and 60% carbon. After drying it breaks, so it is difficult to be stored.
It is brittle coal, So it can be used as pulverised fuel or it may be dried and mixed with binding materials to be moulded into briquette coal which is used as fuel for domestic and industrial use.
Its average calorific value is 25 MJ/kg.
The percentage of elements by weight is approximately given below:
|Carbon||60 to 75%|
|Oxygen||20 to 25%|
|Moisture||Up to 20%|
Uses of Lignite or Brown Coal-
In India and Britain, there are big stocks of lignite. In well-equipped furnaces, it burns with long flame without forming clinkers. It is used in fertilizer plants, thermal plants, and industrial and domestic supplies.
3. Sub-Bituminous Coal-
This coal is of better quality than lignite coal, having higher calorific value. It is black in colour, with a cross-section of a wood-like structure and contains moisture less than lignite.
It is also called black lignite. It is broken into pieces after drying in the air. So it is used in pulverized condition or in block form. Its calorific value is 27.5 MJ/kg.
The composition of Sub-Bituminous Coal by weight is:
4. Bituminous Coal-
It is the next stage of the formation of coal. Its colour is in between deep to jet black. It is also called soft coal. It is found all over the world.
It contains little quantity of moisture and carbon. It is weather-resistant and burns with a yellow flame. Bituminous coal has an average calorific value of 33.5 MJ/Kg.
It is mainly used for the production of steam.
According to the quantity of volatile material, it has three varieties-
- Low volatile: It is most appropriate for metallurgy.
- Medium volatile: It is suitable for the production of steam.
- Highly volatile: It is suitable for the production of coal gas.
The composition of Bituminous Coal by weight is:
|Carbon||75 to 90%|
|Moisture||4 to 6%|
5. Semi-Bituminous Coal-
It is more hard and bright than the general bituminous coal. It is between bituminous and anthracite coal. It has less volatile material and traces of moisture, ash and sulphur.
It burns with short flame and produces less smoke at the time of combustion.
This coal is mostly used for the production of steam. It is burnt on the grate or used in pulverised condition.
The composition of Semi-Bituminous Coal by weight is:
6. Anthracite coal-
This is the last form of coal in the formation process of coal and is known as hard coal. This is pure, bright, moistureless, high carbon and low volatile material coal.
This is mostly found in slate layers and dark black in colour. When broken it gives semi-metallic lustre.
It burns slowly with a short blue flame at high temperatures and gives less smoke while burning. It neither softens nor swells.
It contains carbon up to 98%. The volatile materials maybe 8% or less. Its specific gravity is 1.3 to 1.8 and the average calorific value is 36 MJ/kg.
This type of coal is rarely found in the world. It is used for steam generation and power generation.
The composition of Anthracite Coal by weight is:
There is also a sub-type of anthracite coal, Which is-
Types of Anthracite coal-
There are two types of this coal and they are-
- Semi-Anthracite Coal
- Meta-Anthracite Coal
I. Semi-Anthracite Coal-
It burns with a short flame with ease. It swells to some extent when it is soft but it does not form clinker.
The percentage composition of semi-Anthracite by weight is-
The composition of Semi-Anthracite Coal by weight is:
II. Meta-Anthracite Coal-
It is the best quality coal under this category. It has maximum quantity solid coal and meager quantity of moisture and volatile materials.
Its percentage composition by weight is:
The composition of Meta-Anthracite Coal by weight is:
This is the last member of the deposition of coal. It is the coal of the highest and purest quality. Its colour is black. It contains 98% carbon.
Its formation takes place after anthracite. It burns at high temperatures and is generally mixed with another type of coal.
Apart from fuel, graphite is used as a refractory material, lubricant, coating and electrode making.
Very high-quality graphite is used in Rocket and Nuclear techniques. The high-quality graphite is prepared from petroleum coke and tar mixed together and heating the mixture under pressure up to 2600°C to 3000°C for several hours and cooling it slowly.
The Percentage of Carbon in Graphite is:
Other Kinds of Coal-
Some other kinds of commercial coal are described below-
- Steam Coal
- Cooking Coal
- Canel Coal
1. Steam Coal-
This is a bituminous type of coal, in which volatile materials are present up to 12% to 35%. It burns with a yellow flame and emits smoke when hot.
It does not form clinker and burns on the grate with the case, thus ash falls down the grate itself. Steam coal is specially used in boiler furnaces to produce steam so-called steam coal.
Its percentage composition by weight is given below:
The composition of steam Coal by weight is:
2. Cooking Coal-
This is semi-bituminous and semi-anthracite in nature which swells while burning. It burns with a light yellow and blue flame.
It can burn at comparatively low temperatures and does not emit smoke and soot. After burning it does not form clinker or scales.
Its percentage composition by weight is given below;
The composition of Cooking Coal by weight is:
3. Canel Coal-
Its qualities lie in between bituminous coal and sub-bituminous coal with some special qualities. This coal is hard and brittle with an ugly look.
It contains a high percentage quantity of hydrogen. When hot it gives a large quantity of gas, so it is used for gas production.
This is valuable coal and burns like a candle.
There are big deposits of coal in India which are in the fourth position in the World just after the U.S.A.. Russia and China. There is multiple classification of coal in India but, majority of coal used by Indians is Bituminous Coal.
According to an estimate, deposits of coal in India can last for 900 years with the present rate of consumption. In India, there are 60% of deposits of coal in Madhya Pradesh, Bihar and West Bengal.
The total known deposit of coal in India is about 6 x 104 kg while the annual consumption of coal is about 10.3 x 10 kg.
Coal is used in thermal power plants, steel and fertilizer plants, railways, domestic use etc. The quality of coal found in India is not good enough in comparison to the quality of coal in other countries.
The main causes of the inferior quality of coal in India are as under:
- About 35% coal of the total coal deposit in India is of caking type and the rest of the coal deposit is average caking coal of bituminous type.
- In Indian coal, the quantity of ash is very high and combustible carbon is comparatively less. Thus the calorific value of Indian coal is less. Separation of ash material from coal is a difficult and costly job.
- The percentage of sulphur in Indian coal is very less (Approx- 0.5%). The quantity of Hydrogen is also very less. The self-combustion of coal takes place spontaneously.
- In India, the deposits of high-grade coal are almost negligible. Only low-grade coal is available. The formation period of coal in India is about 150 to 200 lac per year. The thickness of coal layers in India varies from 4 m to 30 m at different places.
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Abhishek Tiwary is a blogger by passion and a Quality Engineer by profession. He completed his B.Tech degree in the year 2017. Now working in a reputed firm. He loves to share his knowledge with others.