An introduction to the nature of coal

To comprehend the varied composition, properties and uses coal ash, it is important to begin with an understanding of the coal from which it is derived. Coal is essentially an assemblage of organic and inorganic material that transforms over geologic time into a relatively high energy density fuel through the process of coalification. Energy density, or rank, increases as coalification proceeds, which accounts for the wide ranges in coal quality that exist in the world today. Wherever practical, mined coal is cleaned using physical beneficiation techniques to reduce the amount of inorganic material present in order to increase energy density and reduce fuel impurities, a practice that is limited to higher rank coals. The most common practice for electricity generation is to pulverize coal prior to combustion. Combustion consumes organic phases, while remaining inorganic components, such as minerals and trace elements, transform into coal ash. The composition, properties and use of the residual ash is dependent upon both the inorganic material present in the fuel and the combustion conditions. Clean coal technologies (CCT) are practices developed to reduce emissions of SO_x and NO_x produced by coal combustion. Since CCTs are quite different from pulverized coal combustion, the composition and properties of the ash they produce are also quite different.