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Coal blends are now widely used by the power generation industry and the general characteristics are well known. Attention is still directed to the emission of NO_x, which is subject to more stringent regulation, and to the amount of carbon in ash. The latter is increased when low NO_x burners are employed, which is the norm now. It is also increased as a result of additional air staging when over-fire air is added in furnaces, especially tangential fired systems. Such a furnace is studied here. Two approaches can be employed for prediction of NO_x and unburned carbon. The first approach uses global models such as the ‘slice’ model which requires the combustor reaction conditions as an input but which has a detailed coal combustion mechanism. The second involves a computational fluid dynamic model that in principle can give detailed information about all aspects of combustion, but usually is restricted in the detail …