Explore coal energy
Explore the world of coal, from its extraction to its role in energy production. Gain valuable knowledge about its benefits and challenges in today’s market.
Definition of Coal
Coal is a type of rock composed of modified plant remains and various minerals. According to Swapaksha’s definition from 1965, coal is a class of easily combustible rocks composed of more than 700 carbonaceous substances, formed by the compaction and alteration of plant matter with the help of underground pressure and heat.
How is a coal formed?
Coal is formed from forest plants millions of years ago. All these plants were stored in suitable locations under layers of sand and mud, where microbial action occurred, and through solidification and transformation aided by the earth’s pressure and heat, the porous organic aggregate became peat. In most cases, the fossils of tree roots and saplings in the mudstones below the coal seams bear witness to the extinct forest lands of days gone by.
The transition from plant matter to coal generally occurs in two phases. Biochemical transformation is active in the first phase. During this time, various microbes control the chemical changes of stored plant matter. Major plant components such as lignin, cellulose, protein, wax, and resins undergo extensive chemical changes. At the end of this phase, the organic material formed is usually referred to as peat.
After this, the process of coal formation continues as the material moves deeper underground, where higher pressure and temperature reduce the amount of oxygen, hydrogen, and other volatile materials in the peat, while the amount of carbon proportionally increases. As the carbon content increases, the heating value of the coal also increases.
In accordance with the pressure and temperature of the pit, higher-rank coal is gradually transformed into lignite, then bituminous, and finally anthracite coal. It is now almost universally accepted that coal is formed by the natural process of plant material being buried underground, with slow decomposition under underground pressure and heat.
Classification of Coal
Coal can be classified in a number of ways, taking into account several important aspects.
- According to its source, coal can be divided into two categories:
(a) Humic or banded coals
(b) Sapropelic coals
(a) Humic coals: Coals formed from terrestrial plants are called humic or banded coals. Humic or banded coal consists of alternating bright and dull coal layers. These coal layers or bands can be divided into four groups:
- Vitrain: These are bright, black, and brittle coals. They break into conchoidal pieces. Under the microscope, each line or band appears to originate from a separate piece of wood or bark, as primary cell attachments are visible.
- Clarain: These are brilliant black but less brilliant than vitrain. They break irregularly and are less brittle than vitrain. Some parts look like vitrain, but some parts are transparent.
- Durain: These coals are dull like clay, with no visible layers. They are hard, fine-grained, and break irregularly.
- Fusain: Also known as mineral charcoal, fusain is a soft, black, powder-like substance, with thin layers found within coal. It shows attachment of root cells in wood.
(b) Sapropelic Coal: Coal produced from organic material other than wood is called sapropelic coal. This type is usually formed by decay in the absence of air, from aquatic plants and animals.
- Geological Classification of Coal:
Coal can also be divided into two main categories from a geological perspective:
(a) Gondwana Coal: Coal produced in the Gondwana rock strata during the Permian geological period (230 to 270 million years ago) is called Gondwana coal.
Along with Australia, Africa, and South America, the Indian subcontinent was part of a massive landmass called Gondwana, located at the southern end of the southern hemisphere. Coal produced during that time is known as Gondwana coal.
Tertiary Coal: Tertiary coal formed 12 to 65 million years ago during the Tertiary period. Coal from this era, found in rock layers, is called Tertiary coal.
Below are the descriptions of different ranks of coal
✓ (a) Peat: Peat is practically the first stage in coal formation from plant organic matter. Though it is not called coal, all coal originates from peat in its initial state. Aquatic or terrestrial plant matter accumulates in favorable conditions and undergoes partial decomposition and solidification at shallow depths underground, forming a porous organic aggregate known as peat. Peat is available in three states.
(b) Lignite: This is the next phase of coal after peat. It is also called brown coal due to its thick brown or dark brown color. This coal is harder than peat, layered, and sometimes wood particles are visible within it. Due to its high moisture content (15% to 18%), it splits upon drying. It contains low levels of fixed carbon (34% to 60%) and has an average heating value of 25,000 KJ/Kg. This coal burns with a bright yellow flame and smoke. Lignite coal is used to prepare producer gas. When dried, it contains about 6% moisture, making it easy to use as fuel in powder form.
(c) Sub-bituminous coal: This coal is intermediate between lignite and bituminous coal. It is dull, black, and stratified. It breaks along the horizontal plane but does not break along the vertical plane like bituminous coal. It has 50% less moisture than lignite and lower ash content but lacks coking power. This coal is considered medium-quality fuel, with an average heating value of 27,000 KJ/Kg. It is used in briquette or pulverized form.
(d) Bituminous: This is the next stage of coal after lignite. Bituminous coal is dense, hard, brittle, and dark black. It breaks upon impact, and one of its characteristics is that it can fracture on both flat and steep surfaces, resulting in cubic fragments. The moisture content of this coal…
Contains low levels (4% to 6%) of volatile matter and high levels of fixed carbon (75% to 90%). It burns with a long yellow flame and smoke. Bituminous coal can be divided into three sub-categories—high volatile, medium volatile, and low volatile—depending on the amount of volatile matter present. Bituminous coal can be either caking or non-caking. Coals that melt and behave plastically during ignition are called caking coals; these are used in the production of bituminous gas. Non-caking coal, which burns freely without melting upon ignition, is used to generate steam in steam plants. Bituminous coal is widely used across various fields and is considered one of the best types of coal. The average heating value of bituminous coal is 33,500 KJ/Kg. The average calorific value of caking bituminous coal is 35,000 KJ/Kg, and for non-caking bituminous coal, it is 33,000 KJ/Kg.
(6) Semi-bituminous: This is an intermediate stage between anthracite and bituminous coal and has caking properties. It contains 1% to 4% moisture, 10% to 12% volatile matter, 80% to 85% fixed carbon, and 1% to 4% ash. It is used in thermal power plants, primarily in pulverized form on moving grates.
(f) Anthracite coal: This coal is extremely hard and bright dark black in color. It is brittle and fractures with a conchoidal pattern when broken. Anthracite coal has the highest fixed carbon content (typically 80% to 90%) and the lowest volatile matter (below 4%). Its calorific value is about 36,000 KJ/Kg.
Anthracite coal is used as a high-quality fuel for steam generation and general power generation. It has zero caking power. It is difficult and expensive to pulverize, so it is used on grates in forced draught systems.
Properties of Coal
- Color of Coal: By looking at the color of coal, one can get an idea about its rank. Lignite coal color is brown or brownish black sub-bituminous coal color is black. So it is seen that the color of high rank coal is black.
- Coal Texture: Lignite coal grains are irregular (amorphous), fibrous, and woody, but higher-grade coal is tough, hard, and brittle.
- Hardness: From lignite to anthracite, coal becomes progressively harder. Lignite is relatively soft, while anthracite is the hardest coal, with a hardness value of three on the Mohs scale (3 Mohs).
- Friability: The tendency of coal to break during loading, unloading, and shipping is called friability. Splint and channel coal are less prone to crushing than common coal. A shatter test is performed to check crushing capacity: a 59-pound sample of coal is dropped onto a steel plate twice from a height of approximately two meters (6′). As a result, the coal breaks into pieces. The degree to which the coal is crushed depends on its crushing capacity. Grinding ability, or friability, can be determined by sifting the pieces through a sieve. The value obtained in the shatter test is recorded in the size stability index. An index value of 100 indicates unpulverized coal.
- Specific Gravity of Coal: The specific gravity of coal depends on the mineral and ash content. High-quality coal has a higher specific gravity than lower-quality coal. For example, anthracite has a specific gravity of 1.4 to 1.7, while lignite’s specific gravity is 1.20 to 1.30. The specific gravity of coking coal is calculated using the following formula:
Relative gravity G=1.27+a
Where a = is the volume of coal ash per unit weight.
This formula gives accurate results when the coal ash content is 40%.
- Calorific Value of Coal: The heat production capacity of a unit mass of coal is called the calorific value of coal. The heat generation capacity of coal is one of its main quality determinants. High-flame, high-volatile coal has a low heating value, while low-flame, low-volatile coal has a high heat production value. The calorific value is found to be higher in coals whose volatile matter content is within 20%.
- Grindability of Coal: The Hard Groove method is a test used to determine how easily a coal will be ground into powder during processing. In this method, specially constructed mills pass through sieves with a 60 mesh size. (Passing through a No-16 mesh sieve) and retained on a No-30 mesh sieve, four types of 50 g air-dried coal are ground. After grinding, the extracted coal is weighed through a 200 mesh sieve. Grindability is determined using the following formula:
Grindability=6.93w+13text{Grindability} = 6.93w + 13Grindability=6.93w+13
Where W= is the difference between the weight of the initial 50 grams of coal and the weight of the coal remaining on the 200 mesh sieve. The grindability index is important in the case of pulverized coal. Coals that grind easily have a grindability index of 100.
- Weathering or Slaking Index of Coal: The tendency of coal left in open weather or coal kept in wet and dry conditions for a certain period of time to break down is called weathering. It is also referred to as the Slaking Index. To determine the slaking index, coal pieces are immersed in water for one hour, drained, and air-dried for twenty-four hours. The coal is then passed through a 0.263-inch square sieve. The percentage of coal passing through the sieve is calculated by weighing. The percentage of coal passing through the sieve will represent the weathering or slaking index of the coal.
- Agglutinating or Coking: There are some coals that behave plastically during combustion and tend to adhere to the fuel bed. Such coal is called agglutinating or coking coal. The plastic-like material or fused bed interferes with normal stoker operation. When determining the amount of volatile matter in coal, if a type of button is formed at the bottom of the crucible, this coal is referred to as having coking properties. Different coals have varying coking qualities. Some coals have no coking properties, while many coals exhibit very high coking properties. For example, anthracite has no coking properties, but bituminous coal does.
Role of Sulfur and Ash in Coal
(a) Sulfur: Sulfur compounds in coal are the most important harmful elements, so their presence is specifically measured and considered. Sulfur in coal can be found in the following states:
- Iron Pyrites
- Calcium Sulfate (Gypsum)
- Organic Sulfur
Carbonization results in organic sulfur compounds such as H₂S, CS₂, and thiophene, but also produces small amounts of sulfur with coke. Sulfur in the form of CaSO₄ is not easily removed, not even through the carbonization process. Sulfur in the form of iron pyrites can be removed to a certain extent by washing the coal. When coal is burned, sulfur is converted to sulfur oxides, which can corrode the air and pollute the environment. As a result, the quality and demand for coal with higher sulfur content are reduced. The presence of sulfur-containing slag in the ceramic industry also affects the quality of the final product. Therefore, it is necessary to remove sulfur from the coke as much as possible.
(b) Ash: The maximum ash content in coal may be 10% or more. Even very pure coal is found to contain 2-3% ash. Coal with a very high amount of ash is called bone coal, bituminous shale, or black slate. In brightly colored coal (anthraxylon or vitrain), the chemical composition of ash varies among different grades of coal. Ash contains silica, alumina, iron oxide, and small amounts of lime, magnesium, and other alkalis. Alumina (Al₂O₃) and silica (SiO₂) increase the fusion point of fly ash, while lime, magnesium, and other alkalis lower the fusion point. Based on this, the high fusion ash components are Al₂O₃ and 2SiO₂, while low fusion ash components include FeS₂ (iron pyrites), FeCO₃ (siderite), and CaCO₃ (calcite) or mixed carbonates.
A good quality coal should have as low an ash content as possible and a high fusion point of the ash.