In this blog post, we will explore how the pretreatment process of coal increases combustion efficiency and reduces environmental pollution.
The coal we commonly know is called briquette coal, which is coal that has been refined. Basically, coal consists of moisture, carbon, ash (the part that becomes ash after combustion), and volatile matter (volatile components). The only part we use directly is carbon, and the rest is unnecessary and reduces efficiency, so it is best to remove it. Pre-treatment of coal is the process of removing these unnecessary parts to increase efficiency. Coal that has undergone this process is used as the charcoal we commonly know. Pre-treatment not only improves quality, but also prevents pollution by removing sulfur compounds that affect air pollution, and reduces transportation costs by reducing the volume of impurities. Therefore, the pre-treatment process of coal is essential and useful.
The horizontal axis of the photo represents the methods used in the pretreatment process, and the vertical axis represents the size of the coal particles. Mined coal is processed in various ways, and the method selected is determined by factors such as cost, time, and efficiency. The solid lines represent the movement of coal, and the dotted lines represent the movement of impurities. The mined coal is crushed to control the maximum particle size, and then foreign matter such as iron and wood is removed before being supplied.
Pre-treatment methods are broadly divided into sizing, cleaning, and dewatering. Sizing is the process of dividing crushed coal into different particle sizes so that the appropriate screening method can be selected. A typical method for dividing coal into different particle sizes is the screen method.
This method uses a sieve to classify coal into various sizes. It is also possible to separate coal by size using a cyclone. This method uses centrifugal force to separate light materials to the top and heavy materials to the bottom. After the sizing process, the coal undergoes cleaning. Cleaning is the most important part of coal pretreatment, and is the process of separating pure coal from rock and minerals.
There are several methods depending on the particle size, as the efficiency of the machines varies depending on the particle size. For coarse particles (+10 mm), the dense medium vessel method is typically used. This method refines coal using a fluid with a density between that of ash and coal. The main purpose of refining coarse coal is to reduce ash, and it is relatively easy to separate.
Medium and small particles refer to particles with a particle size of 0.5 mm to 10 mm. Cyclones and spirals are mainly used for this particle size. A cyclone is shaped like an inverted cone, and centrifugal force is used to collect heavy particles toward the wall and light particles toward the center.
Spirals also use centrifugal force, and when minerals are poured into a spiral tube with water, heavy particles flow toward the center axis and light particles flow toward the outside. Medium particle coal purification methods are used when it is necessary to reduce ash and sulfur. Fine refers to particles with a particle size of 0.5 mm or less.
The particle size of fine particles is too small for methods using density or centrifugal force to be effective. For this particle size, the flotation method is used. This method is a sorting method that utilizes the surface properties of minerals, namely hydrophobicity and hydrophilicity. When coal is mixed with water and air bubbles are introduced, the hydrophobic coal attaches to the air bubbles and floats, while the hydrophilic impurities are dispersed in the water. At this point, the air bubbles on the surface are removed, separating the coal from the impurities.
After the overall purification of the coal is complete, it must undergo a dewatering process. Since most purification methods use water, moisture is a major factor in reducing the efficiency of coal. Centrifuges are mainly used in the dewatering process. Centrifuges are easy to understand if you think of them as washing machine spin cycles.
Coal that has undergone all pretreatment processes is formed into briquettes or other uniform shapes and distributed. Impurities that remain after the pretreatment process are collected in a thickener and simply precipitated to increase their concentration, and then discharged in a highly viscous form. As such, coal is not used as it is mined, but undergoes a pretreatment process to improve its quality and efficiency.
