Abstract: Emission of nitrogen oxides from a mineral-burning process, for example a cement-manufacturing process, is reduced by passing the exhaust gas from the rotary kiln in which the mineral raw materials are burnt to a combustion zone in which solid fuel elements, for example waste tyres, are maintained for a sufficient period and at a sufficient temperature for at least 30% by weight of the volatile combustible content of the solid fuel elements to be combusted, the combustion zone comprising at least one region in which the combustion of the solid fuel elements occurs under sub-stoichiometric conditions. The solid fuel elements may be introduced into the hot gas stream at at least one point between the mineral-inlet end of the rotary kiln and the lowermost cyclone of an associated preheater system, e.g. into the gas stream within a precalciner vessel or riser duct. Preferably, tertiary air from a clinker cooling apparatus is supplied downstream of the combustion zone (relative to the flow of kiln exhaust gas).