The HTW-pilot plant (circulating fluidized bed) of the institut of Ferrous Metallurgy of the Technical University of Aachen is used for lean gas production with carbon containing fuels. Theoretical considerations about increasing residence time of solid matter and decreasing unwanted discharge of finest particles by changing the free-boards geometry have been investigated in a 1:1 modell at room temperature. The discharge could be decreased by up to 34%. The diameter of the discharged particles decreased. The residence time of solid matter increased significantly by building up two fluidized beds one upon another