Development of Characterizing System for Surface Structure and Interaction between Fine Particles at High Temperature Conditions with Special Atmosphere

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To characterize the increasing mechanism of cohesive behavior of ash powders at elevated and high-temperature conditions, two type of adhesion force testing system at high temperature were developed. The modified ash powders were prepared from fine pure amorphous silica powder coated and heat-treated on the surface with 0.5 wt% sodium and/or potassium and the cohesive behavior and deformation properties were determined. In the high-temperature range above 800 ℃, since a rapid increase of tensile strength and plastic rupture deformation occurred in both the natural ash samples and the model ash powder samples prepared from alkali metal and pure silica powder, it is suggested that the coating alkali metal reacted with the amorphous silica phase and formed a small amount of low-melting-point eutectic materials, such as K_2O・4SiO_2. Since these eutectic materials formed a small amount of liquid phase at inter-particles at about 800 ℃. To reduce the liquid bridge force at high temperature conditions, two methods were developed in this research. One is the high temperature chemical reaction by the addition of fine CaO powder with fine coal powder during coal combustion process to reduce such liquid bridge formation at high temperature condition. Another approach is the coating of ultra-fine carbon powder or the addition of coarse kaoline particle with ash particle to increase the surface distance and to decrease the wettability of liquid phase on ash particles. Both approaches is useful to reduce the adhesion behavior of ash particle at high temperature conditions.