Direct Measurement of Single-Particle Adhesion Behaviors on Metal Surfaces at High Temperatures Using Model and Modified Pulverized Coal Combustion Ash

Using a developed system that can measure the adhesion force between a spherical fine ash particle and a polished metal surface for superheaters under high-temperature conditions, the adhesion force of two kinds of spherical particles was determined at different experimental conditions. One sample was prepared from pure spherical silica particles and alkali metal, and another was modified pulverized coal ash samples with different Ca contents, prepared from pulverized coal by drop-tube furnace combustion. The adhesion force of each particle was dependent upon the maximum compressive force, temperature, and chemical composition. In combination with thermomechanical behavior measurements and calculations, it can be deduced that mechanical properties influenced by the temperature are among the factors that determine the adhesion behaviors of a particle. Because a particle maintains a solid or elastic deformation state during compression at relatively low temperatures, the adhesion force will not increase with increasing compression force. In contrast, as slag-phase formation or softening partially occurred at the contact point between particles at a high temperature, the adhesion force increases with increasing compressive force as a result of the expansion of the contact area via plastic deformation between a particle and a plate. Moreover, by adding CaO in pulverized coal for drop-tube furnace combustion, a complex change in the adhesion force of modified ash samples with different Ca concentrations was observed at different compression forces and temperatures. Overall, this system provides a new method to quantify single-particle adhesion force, and the experimental results contribute to the fundamental study of ash adhesion and deposition phenomena.