Coal is a combustible, black or brownish-black sedimentary rock primarily composed of carbon and hydrocarbons. It forms over millions of years from the remains of plants that accumulated in swampy environments, subjected to intense pressure and heat deep within the Earth’s crust. This geological process transforms the organic matter into a dense, energy-rich material.
Coal serves as a significant global energy source. It is predominantly used to generate electricity in power plants, where its combustion heats water to produce steam, driving turbines to create power. Beyond electricity generation, coal also plays a role in industrial processes, such as steel production and cement manufacturing. Coal extraction is essential for meeting global energy and industrial demands.
Surface Mining Methods
When coal deposits are located relatively close to the Earth’s surface, surface mining methods are employed. These techniques involve removing the overlying soil and rock, known as overburden, to access the coal seam. These operations utilize large machinery to move material.
Strip mining is a common surface method, particularly effective for horizontal coal seams. In area strip mining, parallel cuts are made into a flat or gently rolling terrain, with overburden from one strip being placed into the previously mined one. Contour strip mining, used in hilly or mountainous regions, follows the contours of the land, removing overburden from the side of a hill to expose the coal.
Open-pit mining, another surface technique, is used for large, deep, and often circular or elliptical deposits. This method creates a large, terraced pit that deepens over time as the coal is extracted. Mountaintop removal blasts and removes a mountain summit to expose deeply buried coal seams, with displaced rock and soil deposited into adjacent valleys.
Large-scale equipment is characteristic of surface mining operations. Draglines, massive excavators, are used to remove overburden. Power shovels and front-end loaders then load the exposed coal onto haul trucks for transport to processing facilities.
Underground Mining Methods
When coal seams lie deep beneath the Earth’s surface, underground mining methods become necessary. These techniques involve creating a network of tunnels and shafts to reach the coal. Access to these deep seams is achieved through vertical shafts, inclined slopes, or horizontal drifts, depending on the topography and depth of the deposit.
Room and pillar mining is an underground method where a continuous mining machine cuts coal from the seam, creating rooms or roadways. Pillars of coal are deliberately left in place to support the mine roof, preventing collapses. This method recovers between 40% and 60% of the coal, leaving the remaining coal as support structures.
Longwall mining is a mechanized underground method that allows for a higher recovery rate, often exceeding 75%. A shearer machine, equipped with rotating cutting drums, moves back and forth across a long coal face, shearing off coal. Hydraulic roof supports, called shields, temporarily hold up the roof while the coal is extracted, and then advance, allowing the roof behind to collapse in a controlled manner. This continuous process creates a long, advancing panel of extracted coal.
Specialized equipment is used for underground operations. Continuous miners are track-mounted machines with a rotating drum equipped with carbide bits that rip coal from the seam. Shearers are specific to longwall mining, designed to cut coal along the length of the panel. Powered roof supports, or shields, provide temporary roof control in longwall sections, advancing with the shearer to protect workers and equipment. Shuttle cars or conveyor belts transport the mined coal from the working face to the main mine transportation system.
Post-Extraction Processing
After coal is extracted, it undergoes processing steps before transport and utilization. This preparation, called coal preparation or beneficiation, enhances its quality and suitability for various applications. The initial step involves crushing and sizing the raw coal.
Crushing breaks down larger chunks of coal into more manageable sizes, making it easier to handle and process further. Sizing then sorts the coal into specific particle ranges, which is important for optimizing its combustion in power plants or its use in industrial processes. This mechanical reduction ensures a more uniform product.
Following crushing and sizing, washing or cleaning processes remove impurities from the coal. These impurities include rock, ash-forming minerals, and sulfur, which can reduce the coal’s energy content and contribute to emissions when burned. Processes like dense-medium separation, froth flotation, or spirals use differences in density to separate the coal from unwanted materials, improving its purity and energy value.
Finally, drying may be necessary for higher moisture coals. Reducing the moisture content increases the coal’s heating value per unit weight and reduces transportation costs, as less water is being shipped. These post-extraction steps deliver a consistent, higher-quality product that meets end-user requirements.