Physical Coal Cleaning

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Technology

After coal is mined it generally goes through a process known as preparation or coal cleaning. This is done for two main reasons. The first is to remove impurities in order to boost the heat content of the coal and thereby improve power plant capacity. The removal of impurities also will reduce maintenance costs at the power plant and extend plant life. The second reason for coal preparation is to reduce potential air pollutants, especially sulfur dioxide. The extent to which SO₂emissions can be reduced varies depending upon the amount of sulfur in the coal and the form of its occurrence. Sulfur in coal occurs in two forms: 1) organic sulfur that is chemically bonded with carbon; and, 2) inorganic sulfur. Pyrite is the most common form of inorganic sulfur. Physical coal cleaning works to remove only inorganic sulfur.

Physical coal cleaning techniques take advantage of the differences in specific gravity of the coal and its impurities. Hydrocyclones and gravity concentration devices are examples of such systems. When coal is finely ground, physical processes that take advantage of the surface properties of the coal materials can be used. For instance, froth flotation exploits the hydrophilic surface characteristics of mineral impurities and the hydrophobic nature of coal particles to achieve separation. Hydrocyclone, heavy media separation, and froth flotation technologies are illustrated below.

Performance
Physical coal cleaning systems have been shown to remove up to 90 percent of the pyritic sulfur in coal, although in some coals this amount can be as low as 20 percent.. However, pyritic sulfur generally accounts for only about one half of the total sulfur found in coal. For this reason physical coal cleaning is rarely thought of as a stand-alone SO₂ emission control strategy. In fact, physical coal cleaning ordinarily removes only 40-50 percent of the pyritic sulfur in typical llinois coal. This will lead to a 20 to 25 percent reduction in emissions of SO₂.
On the other hand, physical coal cleaning removes impurities other than sulfur, especially ash forming minerals. This has the effect of increasing the heating value of the product shipped from the mine and reducing the SO₂ emissions per unit of heat input to the power plant boiler. As a general rule the amount of sulfur and ash removed increases with decreasing coal particle size. This characteristic represents the principal drawback to the maximum use of physical coal cleaning because both wind and rain during shipping and storage adversely effect fine coal.
Costs
The costs of physical coal cleaning vary widely depending on the characteristics of the raw coal, the process used, the size of the preparation plant, and the intended end-use of the product. After initial washing, the extent to which the coal is crushed determined the resulting size fractions. The techniques for coal cleaning discussed above require finely ground coal. Clearly, the more complex the preparation processes are, the greater the number of steps required; hence, the greater the capital costs. The operating costs of coal cleaning facilities will vary depending upon the additives required, as in heavy medium and froth flotation. Sources of water and electricity needed for the preparation plant also are variable from one plant to another.
The costs of physical coal cleaning are usually reported in terms of the added cost of the cleaned product over the original run-of-mine coal. Capital costs (for construction of the coal cleaning facility) and operating costs can be separated, however, overall costs are often reported without identification of components. Capital costs reported range from $12 to $16 per ton of coal capacity with operating costs going from $3.17 per ton to $4.40 per ton for so called Level 4 cleaning featuring high Btu recovery, high levels of ash rejection (40-50%) and 30 to 50 percent sulfur removal. In terms of costs per ton of SO₂ removal the cost reported in the literature is $70 per ton with the highest at $563 per ton, with average costs well above $150 per ton. It is not difficult to see why the removal of SO₂ will never be the primary reason for physical coal cleaning.
Experience in Technology
Commercialization
Well over 70 percent of the coal mined in the United States goes trough some preparation process before it is consumed. These processes range from simple washing and sizing to elaborate fine coal cleaning system. The most recent (1995) census of coal preparation plants indicates that there are 345 plants in the United States, twenty of which are located in Illinois. The average capacity of preparation plants in the U.S. is 680 tons per hour. There are thirty-five design/construction firms with significant experience in building coal preparation plants.

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