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Coal stands as a crucial global energy source, having played a pivotal role in the Industrial Revolution and socio-economic development. It is currently the most widely utilized fossil fuel, supplying up to 25% of the world's primary energy needs. Energy derived from coal serves various aspects of daily life and plays a critical role in numerous industrial sectors. So, what exactly is coal? How is it formed? And in which areas is it applied? Let's delve into these questions in the following article.

1. What is Coal?

Coal is a type of sedimentary rock, typically dark brown or black, found in multi-layered rock strata or veins. It is discovered worldwide, primarily originating from prehistoric forests and swamps, which were buried over millions of years.

2. The Formation Process of Coal

Coal is derived from the remains of ancient plants buried underground. Over millions of years, heat and pressure have altered its chemical properties. This transformation involves several stages, beginning with the decomposition of plant matter into peat under anaerobic conditions. Subsequently, it evolves into brown coal (lignite), semi-bituminous coal, and eventually, into bituminous coal, ultimately becoming anthracite. This process encompasses complex biological and geological renewal.

3. Characteristics and Properties of Coal

- Characteristics of Coal

• Coal primarily consists of carbon, along with other elements like hydrogen, oxygen, nitrogen, and sulfur.
• Coal's color is typically black, although it can exhibit variations depending on the source, appearing as brown, silver, yellow, or dull.
• It has a high calorific value, ranging from approximately 5500 to 7500 kcal/kg.
• Coal's appearance varies based on age, mining methods, and refining equipment.
• Coal can absorb toxins and retain gases, substances dissolved in solutions, or vapor on its surface.

- Properties of Coal

• Coal is characterized by high hardness and weight, with carbon content ranging from 75% to 95%, varying based on its origin. Lower impurity content typically results in higher hardness.
• Moisture content in coal is typically around 5-6%, and higher moisture levels lead to reduced heat generation.
• Coal ash, a product of mineral components when burned, greatly impacts its combustion properties. A higher ash content diminishes the heat output, causing fouling on heat-absorbing surfaces and decreasing efficiency.
• Volatile matter (Vk%) represents the gases produced during coal's thermal decomposition in the absence of oxygen, including hydrogen, carbon dioxide, hydrogen carbide, and carbon monoxide. The younger the coal, the higher the volatile matter percentage.
• The calorific value Q (Cal/g, Kj/kg) indicates the heat released when 1kg of coal completely burns. Higher calorific values denote more efficient combustion.

Learn more: Coal and the properties of coal

4. Classification of Coal

- Classifying coal by evolutionary stages: The classification of coal depends on the carbon content and the amount of heat energy that coal can generate, determined by the pressure and heat impact over time during formation.

• Peat: Peat is an organic layer on the surface of the earth, formed by the incomplete decomposition of vegetative residues buried over time under continuous anaerobic conditions such as swamps, volcanic areas, deserts, forests, etc. It mainly consists of vegetation from families such as sedges, pines, willows, grasses, rushes, reeds, etc., common in tropical regions. Peat has a high moisture content of 42.1% and an average resolution level of 35.3%, and the deeper the mines, the higher the resolution capacity of peat.
• Lignite (brown coal): Lignite usually occurs in small fragments, being the youngest fossil fuel but with the lowest heat energy content, with a carbon content ranging from 25-35%, moisture content around 66%, and ash content ranging from 6-19%. Lignite contains 60-80% combustible matter and is commonly used in pulverized form in boilers and small thermal power plants, steam power plants. It is also the first type of coal to be gasified.
• Sub-bituminous coal: From a geological perspective, sub-bituminous coal is a younger type of coal formed over 200 million years ago. In its dry state without ash, it contains 35% - 45% carbon. This type of coal often appears as woody debris rather than having a solid shiny structure, with colors ranging from dark brown to black. Sub-bituminous coal has a lower moisture content than lignite, making it slightly harder, and easier to transport, and store. Many power plants have shifted to using sub-bituminous coal due to its low sulfur content of less than 1%, reducing environmental pollution.
• Bituminous coal: Bituminous coal is a coal type rich in volatile matter, with a long flame length and the ability to self-bind when heated in anaerobic conditions. Bituminous coal has a moisture content of about 17%, nitrogen content ranging from 0.5-2%, fixed carbon content accounting for 85%, and ash content up to 12% by weight. There are two types of bituminous coal: thermal coal and coking coal.
• Anthracite coal: Anthracite coal has the highest heat value among all coal grades, containing 86% - 97% carbon, minimal impurities, and a metallic luster. Anthracite coal is used in the metallurgical industry for power generation. Additionally, due to its high hardness, strong structure, and excellent chemical resistance even in acidic and alkaline environments, anthracite coal is widely used in large-scale water treatment plants, industrial and domestic water purification plants for heavy metal removal, decolorization, odor removal, dissolved organic matter removal, and trace pollutant removal.
• Graphite: Graphite is one of the forms of carbon existing in nature, rarely occurs in its pure form but is usually found within natural mineral deposits such as quartz, calcite, mica, iron-containing meteorites, and turmaline. Under normal conditions, graphite is a solid crystalline substance with a gray-black color and a layered structure. In each layer, each carbon atom covalently bonds with three neighboring carbon atoms forming the vertices of an equilateral triangle. Adjacent layers are weakly bonded to each other, allowing easy separation and sliding over each other, giving graphite a soft structure. It is used as the core of pencils, leaving black marks on paper due to the multiple layers of crystals. Despite being composed of a non-metallic element, graphite exhibits excellent electrical conductivity, making it suitable for use as electrodes.

- Classification According to Intended Use

• Metallurgical coal: Primarily used in steel production.
• Thermal coal: Utilized for electricity generation when burned or in the cement production industry.
• Organic mud coal: Used in fertilizer production.
• Activated carbon: A filtration material used in wastewater treatment, water purification, and the production of distilled water, among other applications.

- Classification Based on Morphological Characteristics

• Peat: that has been screened and ground, meeting the standards for producing organic microbial fertilizers, is divided into three types:
  

  	Type 1	Type 2	Type 3
  Color	Black	Black and brown	Umber
  Sieve pass fineness	3.5mm	3.5mm	5.5mm
  Humidity	20-30%	20-30%	20-35%
  Organic content	30-35%	17-25%	<16%

• Coal dust (particle size <15mm): coal dust 2a, coal dust 2b, coal dust 32, coal dust 3c, coal dust 4a, coal dust 4b,...
• Cabcoal (particle size ranging from 6-100mm): coal bucket, two bucket coal, three bucket coal, four bucket coal, five bucket coal, six bucket coal, seven bucket coal, eight bucket coal

- Categorization of coal by country of origin

• Indonesian Coal: Indonesia is one of the world's largest coal exporters. Indonesian coal varies in rank, including lignite, sub-bituminous, and a smaller amount of bituminous coal. Its sulfur content is generally low to medium, providing a competitive price advantage. However, ash and moisture content can vary depending on the specific mine, impacting transportation and utilization efficiency.
• Australian Coal: Australia is a leading global coal supplier, renowned for its high quality. Australian coal is primarily bituminous and sub-bituminous, with high calorific value and low sulfur content, making it suitable for both metallurgy (coking coal) and power generation. Australia also produces anthracite, the highest rank of coal. The low sulfur content contributes to reduced air pollution; however, coal combustion still generates greenhouse gas emissions.
• Russian Coal: Russia possesses vast and diverse coal reserves, ranging from lignite to anthracite. Russia exports both thermal coal (primarily bituminous and sub-bituminous) and metallurgical coal (bituminous). Sulfur and ash content vary depending on the mining region. Due to the complex geopolitical situation, the Russian coal market can be highly volatile, impacting prices and supply.
• South African Coal: South Africa is a significant coal producer and exporter, primarily of high-quality bituminous coal, suitable for metallurgy, power generation, and industrial applications. South African coal has a high calorific value, low ash content, and medium sulfur content. South Africa also holds substantial anthracite reserves. Compared to Indonesian coal, South African coal is higher quality but also comes at a higher price.

5. Applications of Coal

- Electricity production: Coal is crucial for generating electricity, accounting for 37% of global electricity production.

- Metallurgy: Coal, when processed into coke, produces steel, aluminum, and copper.

- Construction materials: Coal is the primary energy source for cement production, and by-products from coal combustion play a vital role in the cement and concrete industry.

- Energy industry: Used as a solid fuel for electricity production and combustion.

- Gasification technology: Coal gasification produces syngas, used for electricity generation, fuel production, and chemical products.

- Chemistry: Coal is employed in pharmaceuticals, plastics, and artificial fibers.

- Other industries: Coal serves industries like paper, textiles, glass, and water purification, owing to its toxin-absorbing properties.

6. Benefits and Limitations of Using Coal

- Benefits

• Safety: Coal is safer to transport and store compared to other fossil fuels.
• Flexibility: Coal is versatile, and used in a wide range of industrial and technological processes.
• Convenience: Coal is readily usable upon extraction, unlike other energy sources that require extensive processing.
• Low cost: Abundant reserves and low transportation expenses make coal a relatively inexpensive energy source.

- Limitations

• Depletion of fossil energy: Fossil fuel resources, including coal, are being rapidly depleted.
• Environmental pollution: Burning fossil fuels, especially coal, contributes to environmental pollution and climate change.
• Health impacts: Coal mining and combustion result in pollution that can harm human health.