To avoid extreme temperatures and reducing conditions along combustion grate surfaces reducing the risk of grate material deterioration-the configuration and out-placing of air passages through a grate have been investigated by mathematical modeling. In the two-dimensional CFD-simulations of wood-char combustion On top of the grate, the number and width of the air passages through the grate were varied. The different grate configurations were further investigated for different air flux rates and for the recirculation of flue gases to the combustion air. The results of the simulation; show that the maximal temperature in the grate is sensitive to the amount of air fed to the bed and,,at constant air flow rate, to the entering velocity of the combustion air to the fuel bed. The possibility of using the air flow, as a control parameter in modern grate :Furnaces is limited, due to a general wish to reduce the air flow through the grate to reduce nitric oxide emissions. Extreme temperatures in the grate should, therefore, according to the modelin;, be avoided by making sure that the entering velocity of the air to the Fuel bed is sufficiently high. Practically, this can be achieved by reducing the number of passages in the grate, or by introducing flue-gas recycling. However, to avoid areas of reducing conditio as along the grate, the number of passages should not be reduced further than resulting in a maximal distance of 4-5 cm between them, when working with pure air,