How does the pretreatment of coal increase efficiency and quality?

Coal pretreatment is the process of removing moisture, ash, and volatiles to increase efficiency and reduce sulfur to improve quality. This process is accomplished through the steps of sizing, cleaning, and dewatering.

 

Coal as we know it is called briquettes, which is what coal looks like after it has been refined. Basically, coal is composed of moisture, carbon, ash, which is the part that becomes ash after combustion, and volatile matter, which is volatile. The carbon is the only part we directly use, and the rest is unnecessary and reduces efficiency, so it makes sense to remove it. This process of removing the carbon while increasing efficiency is called coal pretreatment. After this process, the coal is used as briquettes as we know it. Pretreatment not only improves quality, but also prevents pollution by removing sulfur, which contributes to air pollution, and reduces transportation costs by reducing the volume of impurities. Therefore, pretreatment of coal is an essential and useful process.

 

 

The horizontal axis of the photo shows the method used in the pretreatment process and the vertical axis shows the size of the coal particles. Mined coal is processed in several ways, and the choice of method is determined by cost, time, and efficiency. The solid lines show the path of the coal and the dashed lines show the path of the impurities. The mined coal is crushed for maximum particle size control, and foreign objects such as wire and wood are removed before it is delivered.
Pretreatment methods can be broadly categorized into sizing, cleaning, and dewatering. Sizing is the process of dividing the crushed coal into particle sizes so that the appropriate sorting method can be selected. The most common method for sizing coal is the screen method. This method uses sieves to classify coal into different particle sizes. Cyclones can also be used for particle size separation. This method uses centrifugal force to separate the lighter material upwards and the heavier material downwards.
After the sizing process, the coal is cleaned. Cleaning is the most important part of coal pretreatment, separating the pure coal from rocks and minerals. There are different methods depending on the particle size, as different machines are more or less efficient at different particle sizes. For the coarsest particles, Coarse (+10 mm), the Dense Medium Vessel method is typically used. This method uses a fluid with a density intermediate between ash and coal to refine the coal. Coarse-grained coal is characterized by a reduction in ash and is relatively easy to separate.
Medium and small particles are particles with a particle size of 0.5 mm to 10 mm. Cyclones and spirals are the most common types of particles in this size range. Cyclones are shaped like an inverted cone and use centrifugal force to collect heavier particles toward the walls and lighter particles toward the center. Spirals also utilize centrifugal force, but when minerals are flowed through a spiral with water, the heavier particles flow closer to the center and the lighter particles flow outward. Medium-grained coal refining methods are used when ash and sulfur reduction is required.
Fine refers to fines with a particle size of 0.5 mm or less. The particle size is so small that methods using density or centrifugal force are inefficient. At this particle size, the flotation method is used. This method utilizes the surface properties of minerals, hydrophobicity and hydrophilicity. When coal is mixed with water and air bubbles are added, the hydrophobic coal attaches to the air bubbles and floats, while the hydrophilic impurities are dispersed in the water. At this point, the coal and the impurities are separated by wiping the air bubbles off the surface.
After the overall purification of the coal, it needs to be dewatered. Since most refining methods use water, moisture is a big factor that reduces the efficiency of coal. The dewatering process is usually done using centrifuges. Centrifuges are easy to understand if you think of dehydrating a washing machine.
Once the coal has been through all the pretreatment processes, it is formed into briquettes or other bulk shapes for distribution. The impurities that come out of the pretreatment process are collected in a thickener, which simply settles them down to make them more concentrated and viscous. In this way, the coal is not used as it is mined, but is instead pretreated to improve its efficiency.