Introduction
Coal is a fossil fuel that originates from the remnants of ancient plants and trees that existed millions of years ago. It is primarily consists of carbon along with other elements like hydrogen, sulfur, oxygen and nitrogen.
Origin theories of coal :
1. In situ Theory : The in situ theory suggests that coal formed in place, meaning it was formed from plant material that grew and accumulated in swamps or marshy environments where it eventually transformed into coal. According to this theory, the plants grew and died in the same location, with their remains accumulating and undergoing a process of coalification. gradually these layers of sediment buried the plant material, subjecting it to heat and pressure, ultimately forming coal.
2. Drift Theory: The drift theory proposes that coal originated from plant material that was transported and deposited by water or wind. This theory suggests that the plant material accumulated in different areas, such as river deltas, lake beds, or along the coastlines, and was later buried by sediment. The transported plant material then underwent coalification due to the heat and pressure associated with burial.
Both theories have supporting evidence and observations.
Note :
The coal formation are influenced by various factors - the type of vegetation, environmental conditions, burial processes and geological history. The formation of coal results from both in situ and drift mechanisms, with their significance varying based on the specific coal deposit.
Composition:
Types of coal :
Fig.: Classification of coal
Depending upon carbon content, energy potential and properties :
1. Lignite : Also known as brown coal, lignite is the lowest rank of coal. It has a carbon content of around 25-35% and a high moisture content of up to 50%. Lignite is soft and crumbly, with a low energy content. Primarily utilized for generating electricity in power plants.
2. Sub-bituminous coal : This type of coal has a carbon content ranges from 35% to 45% and a lower moisture content compared to lignite, typically around 20-30%. It has a higher energy content than lignite and is commonly used for electricity generation and industrial applications.
3. Bituminous coal: Bituminous coal is the most widely used type of coal. It has a carbon content ranging from 45% to 86% and a moisture content of around 8-15%. Bituminous coal is relatively hard and dense, with a high energy content. Used for electricity generation, steel production, cement manufacturing and other industrial processes.
4. Anthracite: Anthracite coal has the highest carbon content among coal types, ranging from 86% to 98%. It has a low moisture content of about 2-6%. Anthracite is hard, glossy, and has a high energy content. It burns cleanly and efficiently, making it valuable for residential heating, as well as for industrial purposes such as smelting and metal production.
The energy content and quality of coal increase as you move from lignite to sub-bituminous, bituminous and finally anthracite.
Each type of coal has different properties and applications, with higher-rank coals (anthracite and bituminous) generally having greater energy value and lower-rank coals (sub-bituminous and lignite) having lower energy value.
Others types of coal :
i. Microscopic classification
ii. Macroscopic classification
i. Microscopic classification :Microscopic classification of coal involves examining coal samples under a microscope to assess their composition, structure and petrographic characteristics. This classification provides detailed information about the coal's organic components, mineral matter, and degree of coalification.
Main microscopic classifications of coal :
a. Vitrinite: It is the most common maceral in coal, forms from woody plant material. It appears shiny and reflective under the microscope and is rich in carbon, adding to coal’s fuel value.
b. Liptinite : Liptinite macerals are derived from spores, resins and waxy materials. They appear as translucent to opaque particles under a microscope. Liptinite macerals include exinite, sporinite and resinite. They have relatively low carbon content and contribute to the volatile matter in coal.
c. Inertinite : Inertinite macerals are derived from plant material that has been highly altered by heat and pressure. They appear as dull, opaque particles under a microscope. Inertinite macerals include fusinite, semifusinite, and micrinite. They have relatively high carbon content and are often associated with higher-rank coals.
2. Microlithotypes : On the basis of no. of macerals , coal can be didived into three groups :
a. Monomaceral Coal
b. Bimaceral Coal
c. Trimaceral Coal
a. Monomaceral Coal : Monomaceral coal refers to coal samples that predominantly contain a single maceral type. For example, if a coal sample is composed almost entirely of vitrinite macerals with only trace amounts of inertinite and liptinite, it would be classified as monomaceral coal. Monomaceral coal samples are relatively rare, as most coals contain a mixture of maceral types.
b. Bimaceral Coal : Bimaceral coal describes coal samples that predominantly consist of two distinct maceral types. For example a coal sample that contains significant amounts of both vitrinite and inertinite macerals, with only minor amounts of liptinite, would be classified as bimaceral coal. Bimaceral coals are more common than monomaceral coals but still relatively less prevalent compared to coals with a broader maceral composition.
c. Trimaceral Coal : Trimaceral coal indicates coal samples that are composed of three main maceral types in significant proportions. For example - a coal sample with substantial amounts of vitrinite, inertinite, and liptinite macerals would be considered trimaceral coal. Trimaceral coals are more representative of typical coal compositions and are often found in various coal deposits worldwide.
ii. Macroscopic classification :
The macroscopic classification of coal involves categorizing coal based on its visible physical characteristics and properties.
Macroscopic classifications of coal are -
a. humic coal
b. sapropelic coal
a. Humic coal: Humic coal, also known as huminite, is a type of coal that is derived from the accumulation and transformation of terrestrial plant material. It is the most common type of coal found worldwide and gives a banded look.
Lithotypes constituents of humic coal :
Vitrain, clarain, durain, and fusain are lithotypes within humic coal that represent different components and characteristics of the coal. These lithotypes can be identified based on their visual appearance and petrographic composition.
1. Vitrain : Vitrain is a lithotype of humic coal that primarily consists of the maceral known as vitrinite. Vitrinite is a shiny, reflective maceral that represents the remnants of woody tissues from plants. Vitrain has a glassy or vitreous appearance and is often characterized by its high reflectance under a microscope. It has a high carbon content and is associated with high-quality, high-rank coals.
2. Clarain : Clarain is a lithotype of humic coal that predominantly contains vitrinite macerals. Clarain has a lower reflectance and may show more variability in reflectance values than the vitrain. It can exhibit various textures and levels of reflectance, indicating differences in the degree of coalification.
3. Durain : Durain is rich in inertinite macerals-organic matter that has been heavily altered, often by oxidation or heat. Under the microscope, it appears dull and opaque, a contrast to the shinier forms like vitrain. The high inertinite content lowers its reflectance, making it distinct from other coal bands such as vitrain and clarain.
4. Fusain : Fusain is consists of charcoal-like material originates from the incomplete combustion of plant material. It is characterized by its porous structure and blackened appearance. Fusain often appears as fragmented or partially burned plant remains within the coal matrix. It has a low reflectance and is associated with lower-rank coals.
b. sapropelic coal : Sapropelic coal, specifically non-banded sapropelic coal, refers to a type of coal that originates from the accumulation and transformation of organic-rich aquatic plant and animal matter in stagnant water. It is different from humic coal that originates from the terrestrial plant material.
Non-banded sapropelic coal shows homogeneous appearance without distinct banding or layering. It forms mainly from organic matter such as algae, spores and microscopic aquatic life rather than visible plant parts.
Types of sapropelic coal :
1. Boghead coal
2. Cannel coal
1. Boghead Coal: Boghead coal, also known as torbanite, is a type of sapropelic coal that was historically mined in Scotland. It is named after the Boghead area in Scotland, where it was first discovered. Boghead coal is notable for its exceptionally high oil content and was valued for its ability to produce oil when heated. It has a waxy appearance and can be easily cut or carved.
Boghead coal developed in freshwater environment such as ancient lakes or lagoons during the Carboniferous and Permian periods. It is rich in algal remains and spores, its high oil content comes from the lipid-rich algae, this unique characteristic is apart from other coal types.
2. Cannel Coal: Cannel coal is another type of sapropelic coal that is characterized by its distinct physical properties. It has a relatively homogeneous and fine-grained texture, often resembling shale or compacted clay. Cannel coal has a high carbon content and a relatively low ash content, making it a high-quality coal for fuel purposes.
Cannel coal originated in shallow marine or lacustrine environments such as coastal swamps or deltaic regions, during the Carboniferous and Permian periods. It is primarily composed of the remains of algae, particularly freshwater or brackish water algae. It has a high hydrogen content, contributes to its superior combustion characteristics and relatively low smoke emission.
Coal formation :
Coal is formed through a process called coalification, which occurs over millions of years. The formation of coal involves the transformation of organic plant material into a carbon-rich substance under specific conditions.
1. Accumulation of Plant Material : Coal formation begins with the accumulation of large quantities of organic plant material, such as ferns, trees, and other vegetation. This typically occurs in swampy or marshy environments where these material can accumulate and be protected from decay by oxygen.
2. Peat Formation : The plant material undergoes partial decomposition in the swampy environment due to bacterial action and limited oxygen. This partially decomposed plant matter is known as peat,which consists of about 90% water and 10% organic material.
3. Burial and Compaction : Over time, layers of sediment including mud and sand accumulates on top of the peat. The weight of this overlying sediment exerts pressure on the peat, leading to compaction. As a result water is gradually squeezed out and the volume of the peat decreases.
4. Heat and Pressure : As more layers of sediment accumulate, the buried peat is subjected to increasing heat and pressure from the overlying rocks and sediments. These heat and pressure cause physical and chemical changes in the peat, leading to the formation of coal.
5. Coalification : The heat and pressure during burial cause the chemical composition of the peat to change. Volatile components, such as water, methane, and carbon dioxide, are gradually expelled, leaving behind a carbon-rich solid material. This process of chemical and physical transformation is known as coalification.
6. Types of Coal : The degree of coalification and the resulting type of coal depend on factors such as temperature, pressure, and the duration of coalification. The stages of coalification or coal ranks include lignite, sub bituminous coal, bituminous coal and finally anthracite. The carbon content and energy content increase as coal progresses from lignite to anthracite.
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| Fig.: Grade of coal |
Conclusion :
coal is a valuable and versatile sedimentary rock that are most important in global energy production and various industrial processes. It is primarily composed of organic material that accumulated and underwent coalification over millions of years in swampy environments. It is a major source of electricity generation and provides heat and energy for residential, commercial and industrial applications.
The classification of coal is based on its physical properties, chemical composition, and petrographic characteristics. The main types of coal include lignite, sub bituminous coal, bituminous coal, and anthracite, with each type having distinct properties and energy content.
The extraction and use of coal have a strong environmental footprint, leading to air pollution, habitat disturbance and increased greenhouse gas emissions.
Coal is still widely used for power generation in different countries. While its use raises environmental concerns, advancements in cleaner technologies and carbon capture methods offer solutions to mitigate its negative impacts.
