The important combustion property of coal for our purposes is the amount of heat produces on burning, or the calorific value of the coal (see Lesson 2, Section 3).
In earlier lessons we learned that the ash content of coal affects the calorific value in two ways. Firstly, the ash will not burn; secondly, the ash absorbs heat from the burning coal. For these two reasons, the higher the ash content, the lower the calorific value.
Further, the higher the relative density of the coal, the higher its ash content. Therefore, the higher the relative density of the coal the lower its calorific value. As the price of coal is calculated mainly from its calorific value, the higher its relative density, the lower will be its market value.
Hence, as in coal preparation we can separate coal from shale using the difference in relative density; by the same means we control the ash content and so the calorific value of the products. As the quality of the product determines the market price, this is therefore controlled by the efficiency of the separation process.