The aim of the study was to operate an existing kiln within acceptable range of CO and NOx emission as by the new emission limits. Clinker production requires stable conditions for the fuel and raw material supply in regard to chemistry, quality and supply. A stable relation between fuel and the required combustion air has to be ensured in order to realize a proper combustion. A mayor goal is an equal distribution of the single components within the fuel. Any start or stop in the fuel supply disturbs and destabilizes the system, hence leading to emission peaks. The preheater is suffering from a high CO level resulting from poor combustion and/or problems with the gas homogeneity in the calcining channel. Beside the residence time the gas homogeneity in the calciner area is a key factor for combustion performance. The plant experience and the observations made during the investigation showed that this point is critical for the existing design. The study is dealing with SNCR equipment, as an end of pipe solutions to reduce already formed NOx. The aim of reducing the NOx-levels below government emission limit is reached.