In an attempt to investigate the dynamic process of ash deposition build-up on heating surfaces of biomass furnaces,a modified sampling system was set up at a 75t/hbiomass-fired circulating fluidized bed boiler in east china.Two stainless steel flakes tightly encircling the probe were fixed at different positions of the probe at every turn to collect the deposits formed at different tube temperature.Deposition sampling intervals from 1 hour to 1 month were accomplished.The micromorphology characteristic and elementary composition of the deposits were compared using scanning electron microscopy(SEM)and energy dispersive spectrometer(EDS).X-ray diffraction(XRD)analysis was also conducted to identify the chemical components of the deposits.Results suggest that the difference in temperature has influence on the earlier stage of deposit formation which is dominant by the condensation of KCl and capture of small particles(<5μm)consist mainly of Ca and s,while the influence weakens as the deposit grows when the inertial impaction of bed material particle(SiO2)larger than 20μm becomes the major factor of deposition build-up.The main components of the mature deposits after 1 month are K2Ca(SO4)2·H2O,SiO2,CaCO3 and CaSO4,it is unexpected that little KCl is found,which has much to do with the chlorine content in the fuel and the operating condition of the boiler,whose influence on the deposition build-up is also discussed.