Experiments were carried out in a CFB test unit to investigate how fuel conversion and pollutant formation during coal combustion is affected by air staging. Staging of combustion air, in particular the variation of its most important parameters, the primary stoichiometric ratio and the residence time in the zone before secondary air addition were of prime importance. An anthracite, a bitunninous coal, and a lignite were used in the experiments. The exhaust concentrations and axial concentration profiles of flue gas constituents were measured. The fuel conversion and pollutant formation occur primarily in the oxygen-rich zone directly above the bottom nozzle plate. If the residence time in the air-lean operated primary zone is sufficiently long, the main combustion phase will be followed, after the consumption of oxygen, by a phase dominated by further devolatilization of the still nonconverted fuel proportion, which strongly affects formation and decomposition of pollutants in both the primary and secondary zones. In the stoichiometric ratio range between 0.7 and 0.9, the efficiency of desulphurization is reduced because of the thermodynamic instability of the sulphur capture phases CaS and CaSO 4 . In CFB combustion, primary stoichiometric ratio and primary residence time can be varied within a large range, in contrast to pulverized coal combustion, without significant change in the exhaust NO concentration. The reason for this is seen especially in the retardation of the H 2 /O 2 mechanism and the residual char combustion, due to the low combustion temperature. The exhaust N 2 O concentration is largely determined by the homogeneous N 2 O reduction through H radicals and the heterogeneous N 2 O formation from NO reduction on the surface of residual char. The suppression of the heterogeneous N 2 O formation in the secondary zone is an important key to the reduction of the exhaust N 2 O concentration.