The fly ash leads to slagging and fouling in the furnace and flue,and the ash particle sticking behavior will be affected by Alkali and Alkaline Earth Metals (AAEM) rich in coal. In order to reduce the threat of molten,sticky ash particles adhering to heat exchanging surfaces,the occurrence form and transformation characteristics of AAEM in the combustion process were analyzed,the influence of AAEM steam condensation on the characteristics of fly ash was discussed,and the deposition mechanism of fly ash sticking on the heat exchange surface was analyzed. The results show that the difference of combustion temperature and occurrence form of AAEM leads to different released processes and amount. The increase of temperature will promote the release of AAEM. With the decrease of the solubility of AAEM in the extract,it will be more difficult to release AAEM during combustion.The AAEM released into the gas phase then heterogeneous condenses on the heat exchange surface or ash particles surface to form a sticky layer,which increases the capture efficiency of fly ash on the heat exchange surface,homogeneous condenses in the gas phase to form aerosols,and aerosols as precursors form sub-micron particles in the subsequent coalescence process. The deposition of fly ash is affected by many factors,such as molecular diffusion,Brownian motion,eddy impaction,thermophoresis,gravity sedimentation,inertial impaction and so on. Gravity sedimentation and inertial impaction mainly affect the trajectories of large ash particles,while other factors mainly affect the trajectories of small particles.The deposition layer have a layered structure. The formation of the inner layer is mainly caused by inorganic steam condensation and small particle impaction,and the outer deposition layer is mainly formed by large particle impaction. The fly ash adhered to the heat exchange surface affects the thermal efficiency of the boiler,corrodes metal at high temperature,and buries hidden dangers for the safe and stable operation of the boiler. Therefore,many scholars have proposed pulverized coal washing or mixed combustion,soot-blowing,coating of heat exchange surface and structural improvement of furnace to slow down the deposit formation. However,due to the changeable components in coal,it is difficult to predict the slagging and fouling behavior perfectly. It is necessary to strengthen the research on the elements with different occurrence forms,deposition of different forms of fly ash and the complex physical-chemistry reactions in the formation process of fly ash,so as to provide theoretical support for the prevention and control of slagging and contamination.