Abstract: As a pivotal technological pathway, the coal-biomass co-firing power generation technology is instrumental in advancing the green and low-carbon transformation of the coal power industry and in achieving clean, low-carbon, and efficient utilization of coal resources. However, the issue of viscous ash deposition and low-temperature corrosion in heat exchange equipment stands as a critical bottleneck hindering the safe and efficient operation of the tail flue, accounting for approximately 75% of downtime and up to 54% of maintenance costs. The high chlorine, high alkali, and high moisture characteristics of biomass lead to significant changes in the content of water vapor, hydrogen chloride, sulfur dioxide, and other components in the flue gas during co-combustion with coal. The deposition of ash particles on low-temperature heating surfaces is influenced by the crystallization - condensation - deliquescence process, making the evolution of ash particle viscosity more complex.The latest research progress on the evolution of ash particle viscosity is reviewed, and the impact of acid condensation is delved into, ammonium bisulfate crystallization, and hygroscopic salt deliquescence on the formation mechanism of viscous ash deposition. The evolution of ash particle viscosity depends on the coupled effects of condensation, crystallization, and deliquescence. Current research on ash particle viscous evolution primarily focuses on qualitative analysis, lacking process monitoring methods for deformation, agglomeration, and viscous phenomena of ash particles, making it impossible to accurately determine the viscous characteristics of ash particles on low-temperature heating surfaces. Current prediction models related to ash deposition only consider the collision, elasticity, and capture criteria between ash particles, without accounting for the influence of viscous ash particles on the deposition process. Therefore, existing ash deposition models urgently require quantitative studies on factors such as the condensation of deposited ash particles with acidic gases, ammonium bisulfate crystallization, and the deliquescence of hygroscopic salts, in order to develop new models capable of predicting the issue of adhesive ash deposition under co-combustion conditions. Regarding the low-temperature corrosion phenomenon caused by viscous ash deposition, current research has only qualitatively indicated that the viscous ash formed by the coupling effect of deposited ash particles and condensed acid liquid on low-temperature heating surfaces has a certain influence on corrosion characteristics. However, the detailed mechanism of how viscous ash affects corrosion products has not been thoroughly explored. Meanwhile, the deposition process of ash particles on low-temperature heating surfaces is influenced by the coupling effects of crystallization - condensation - deliquescence, resulting in varying adhesion characteristics of the ash particles. The viscous ash deposition forms a dense corrosive ash layer on low-temperature heating surfaces, resulting in heterogeneous distribution characteristics of acid radical ions and alkali metal ion concentrations on the wall surface. The multi-component mass transfer, ion migration behavior, and corrosion mechanisms during the process are complex, leading to corrosion characteristics of viscous heating surfaces that differ from dew point corrosion. Therefore, it is essential to consider the heterogeneous distribution characteristics of viscous ash deposition caused by the environmental flow-heat transfer-mass transfer in the flue gas of coal-biomass co-firing power generation units, and to thoroughly investigate the influence mechanisms of these heterogeneous distribution characteristics on low-temperature corrosion. Finally, the current research focus areas are summarized, and future research directions are suggested, which should be concentrated on constructing adhesive ash deposition growth models and investigating the impact mechanism of its heterogeneous distribution on low-temperature corrosion. These studies will offer vital theoretical support and practical guidance for the long-term efficient and stable operation of coal and biomass coupled power generation units.