Coal processing, often termed coal preparation, is a series of mechanical and chemical steps that convert raw, run-of-mine material into a usable energy product. Coal extracted from the earth is an impure mixture that must be treated to meet the strict quality specifications of power plants and industrial consumers. This preparation maximizes the coal’s energy efficiency, improves combustion performance, and significantly reduces the environmental impact from burning excess mineral matter. The process is designed to upgrade the fuel value before it is transported.
Initial Preparation and Sizing
The initial stage of processing focuses on physically reducing the size of the raw coal and classifying it for subsequent treatment. Run-of-Mine (ROM) coal, which contains large lumps and extraneous rock, first enters primary crushers or breakers. These machines mechanically reduce the material to a more uniform size, making it manageable for the preparation plant equipment.
Following crushing, the coal passes over vibrating screens for sizing or classification. These screens separate the crushed material into distinct size fractions, which is necessary because different cleaning technologies are optimized for different particle diameters. Larger coal pieces may be sent for further size reduction, while the correctly sized fractions are directed to the appropriate cleaning circuits.
Coal Cleaning and Beneficiation
The core of the preparation process is beneficiation, a procedure that physically separates the combustible coal from non-combustible mineral impurities like rock, shale, and clay. This separation relies on the principle of density difference, where the organic coal component is significantly lighter than the associated mineral matter. Cleaning is performed using wet processes to improve the heating value of the final product and reduce the concentration of ash and sulfur.
A dominant method for coarse and intermediate-sized coal is dense medium separation. This uses a liquid suspension, often a mixture of finely ground magnetite and water, to create a fluid with a specific, intermediate density. When the coal is introduced, the lighter coal particles float and are collected, while the heavier impurities sink and are rejected. For finer coal particles, other methods are employed, such as jigging, which uses pulsating water currents, or froth flotation.
Froth Flotation
Froth flotation is the primary method for cleaning very fine coal, utilizing the different surface properties of the materials. The fine coal is mixed into a water slurry with chemical reagents that make the coal particles water-repellent, or hydrophobic. Air is then bubbled through the mixture, causing the hydrophobic coal to attach to the bubbles and rise to the surface as a froth concentrate. This technique is effective for recovering valuable energy content from ultrafine particles that density-based methods cannot effectively treat.
Final Preparation and Transportation
After the cleaning process, the cleaned coal contains significant water that must be managed to meet consumer specifications and reduce transportation costs. The first step is mechanical dewatering, which uses equipment like centrifuges and vacuum filters to remove a large portion of the residual moisture. Centrifuges use rotational force to spin the water out of the coal.
If a lower moisture content is required, the coal may undergo thermal drying in specialized units, such as flash or fluidized-bed dryers, which use hot gases to evaporate the remaining water. The final clean coal product is conveyed to large stockpiles for temporary storage, providing a buffer between the plant’s production rate and shipping. From these stockpiles, the coal is loaded onto trains, barges, or trucks for efficient bulk transport to the consuming facility.
Advanced Coal Transformation
Beyond physical cleaning, coal can undergo advanced transformation, fundamentally altering its chemical structure to create different energy carriers or industrial feedstocks. One such method is coal gasification, which converts coal into synthesis gas (syngas), a mixture primarily composed of hydrogen and carbon monoxide. This occurs when coal reacts at high temperatures and pressures with a controlled amount of oxygen and steam.
Coal liquefaction is another chemical transformation that converts solid coal into liquid fuels, similar to petroleum products. This can be achieved through indirect liquefaction, where the syngas produced from gasification is converted into liquids using the Fischer-Tropsch process. Alternatively, direct liquefaction processes involve reacting coal directly with hydrogen at elevated temperatures and high pressures to break down the coal’s molecular structure. A third process is coking, where coal is heated in the absence of air to remove volatile matter, producing coke, a high-carbon material used as a fuel and reducing agent in steel manufacturing.