:Ash This is measured as a percentage of the coal's weight not burned.Typically, desired ash is between 6 and 8 percent of the coal's weight for all types of coal. This number is frequently changed based upon where the coal is mined. For example,Ohio coal mines typically average around 11 percent ash, which is relatively high. On the other hand, Wisconsin averages around 5 percent.The amount of ash required depends on the operations of the plant. as the ash can be reused and sold in a secondary market.