To investigate the scale-up problem of circulating fluidized bed combustors with particular respect to emissions, comparative combustion experiments have been performed in an industrial-size combustor (12 MWth, height 14 m, cross-sectional area 1.6 m x 1.6 m) and in a lab-scale facility (:height 16 m, inner diameter 100 mm). A comparison of the axial concentration profiles of oxygen, carbon monoxide, nitric oxide, nitrous oxide and ammonia along the riser height, obtained during the combustion of wood, pear and coal under conditions of equal fuel bed material, solid holdup and gas residence time, shows a basic similarity. This indicates that suitably sized and operated lab-scale combustors may indeed be valuable tools for the investigation of combustion phenomena. However, some significant deviations of the profiles can be recognized, too. These deviations are caused by three-dimensional effects in the large-scale combustor and indicate the limitations of small-scale experiments.