Fouling of the convection section of a boiler has an effect on the efficiency of thermochemical biomass conversion. In this paper, a modelling for the heat transfer of a fouled fin is discussed. The temperature of the base of the fouled fin is assumed to be equal to the surface temperature of the wall and the difference of the heat transfer rate is 2% when compared to the results of finite element method calculations. A, calculating procedure for simulating the. thermal performance of the entire convection section is introduced. It was established that the calculations describe the effects of changes in the operation conditions with satisfactory accuracy. This was verified by measurements at three biomass boiler plants. (c) 2005 Elsevier Ltd. All rights reserved.