Allotropes Of Carbon-Coal

There are many allotropes of carbon which means it has identical chemical properties but different physical properties. The allotropes of carbon exist in two forms which are crystalline and amorphous. Coal is an amorphous allotropic form of carbon. It is a non-renewable fossil fuel that is combustible and consists of a variable amount of hydrogen, sulphur, nitrogen, oxygen, and carbon. One of the primary coal uses is in the generation of electricity. This article is a comprehensive overview of coal, its uses, formation, properties, and types.

How is coal formed? 

It is formed in a million years from the dead and decayed plant matter buried under the surface of the Earth. The formation of coal is known as coalification, and a geological process forms it. The dead plants under the surface of Earth were prevented from decomposition due to the presence of acidic water and mud. This led to peat formation, which trapped carbon in large amounts under the deeply buried sediments. Over a million years, under pressure and temperature, peat converted into coal.

Structure of Coal 

Coal structure predominantly consists of carbon atoms. The basic structure consists of a three-dimensional polymeric network of hypothetic monomeric “basic units” with a mean molecular weight of 400. The basic units are connected by weak linkage. The coalification process increases the aromatic part of the coal, and the expansion of the network forms a pore structure and is known to trap supercritical gases.

Types of Coal

There are four types of coal, and each of them differs in terms of its carbon content. Here are the four types:

• Anthracite – It has the highest concentration of carbon and is of the highest rank. It is hard, brittle, and black lustrous. It consists of 92 – 98% carbon and a low percentage of volatile matter.
• Bituminous – Coal consists of 70 – 90% of carbon content and is majorly used in steam-electric power generation. It has a high melting point, blocky, shiny and smooth with alternating shiny and dull layers.
• Sub Bituminous – The Coal is black in colour and dull in appearance. It consists of 35 – 45% of carbon and is primarily used in electricity generation.
• Lignite – It consists of carbon content of 25 – 35% and is considered the most inferior amongst all the types. It has a high moisture content, low heating value, and primary coal use is generating electricity.

Properties of Coal

It is combustible and is black or brownish-black in colour. Depending on the pore size, density, particle surface area etc., the coal use is defined.

• Coal density – The density of coal is denser than water but less dense than most mineral and rock matter. The purity of coal determines the relative density for the specific gravity of coal. The energy density of coal is around 24 megajoules per kilogram.  
• Porosity – The porosity of the coal is controlled by the density of coal. The size of the pores varies in three size ranges which are macropores (diameter greater than 50 nm), mesopores (diameter 2 to 50 nanometers), and micropores (diameter less than 2 nm). The pore size is important for processes like gasification, liquefaction, and surface area for purifying.
• Moisture content is an important factor, and various coals trap moisture. The moisture content in coal occurs in four forms which are

– Surface moisture is held on the surface of the coal.

– Hygroscopic moisture is held within microfractures of coal and held by microcapillary.

– Decomposition moisture is held within decomposed organic matter in coal.

– Mineral moisture is majorly part of hydrous silicate crystal structure.

Uses of Coal 

Coal is a non-renewable fossil, and hence it is used across various industries for the generation of energy. Here are some of the coal uses:

• One of the prominent coal uses is the generation of electricity. Thermal Coal is combustible, and hence it is used in the power stations for burning for producing electricity. Through the combustion process, it powers the thermal plants for electricity.
• It is used in the steel manufacturing process where coal is baked in a furnace. This process produces coal coke, and it is used for the iron ore smelting process, which converts it into iron and eventually produces steel.
• Coal gasification is a process used to produce syngas, a mixture of carbon monoxide and hydrogen gas. The syngas fire the gas turbines which eventually lead to electricity production. Syngas even helps in converting into fuel like gasoline and diesel.
• It is used in the production of a range of chemicals and fertilisers. Many chemicals like ammonia, olefins, acetic acid, formaldehyde etc., are chemical derivatives of primary chemicals produced from coal.
• It is largely used in powering the operations of various industries like cement, power, aluminium, chemical, pharmaceutical industries etc. Coal is used as a source of energy in the manufacturing of products.

Conclusion

Coal is a form of non-renewable energy and is one of the most important allotropes of carbon. It is combustible, and hence it is used for electricity generation and provides energy for various industries involved in a range of manufacturing products. Carbon is the backbone of coal, and hence with the strong bonds, it makes coal a sturdy and hard element with a high level of combustion. The topic will help gain knowledge on coal and why it is one of the important allotropes of carbon.