This paper presents fundamentals of heat transfer in fluidized beds with extreme polydisperse materials. The practical application here is the drying of lignite in a pressurized steam fluidized bed. However, the results presented are universal. This paper begins with a short review of the literature regarding the state of the art of heat transfer in fluidized beds, followed by an explanation of the test setup and experimental procedure. The dependency of the detected heat transfer coefficients upon the band width of the grain-size distribution and superficial velocity is analyzed. This research shows that the heat transfer coefficient can be increased by up to 60% while minimizing the fluidization velocity. Because lignite can exhibit a water content of up to 60% in its unprocessed state, this factor is taken into account as well. This can affect the heat transfer coefficient by up to 100%. Existing models to predict the heat transfer coefficient are then upgraded to take into account water content and heating tube diameter, which is also an important factor. It will be shown in the experiment findings that the heat transfer coefficient can vary by more than 30% when using industrial-sized heating tube diameters. This is an important addition, as the effect of the tube diameter is left out of most models completely. Finally, a new mean diameter for characterizing polydisperse bulk solids for heat transfer predictions in fluidized beds is defined. (C) 2012 Elsevier B.V. All rights reserved.