A three-dimensional model of a circulating fluidized bed gasifier was developed, which uses continuous radial profiles of velocities and solids hold-up with regard to the description of fluid mechanics. A complex reaction network of sewage sludge gasification is included in the model. In the simulation calculations the influence of the axial location and the number of feeding points was examined for gasifiers of different scales. It was found that due to the very fast decomposition of the volatiles and the high volatile content in the sewage sludge, lateral mixing of the gas around the feeding port is not complete, and plumes with high pyrolysis gas concentrations are formed. Better mixing was found to be achieved by a larger number of fuel feeding ports which are recommended in the case of high-volatile fuels and are shown to be necessary for larger bed widths. (c) 2005 Elsevier Ltd. All rights reserved.