(71c) Analysis and Control of Adhesion Behavior of Sewage Sludge Combustion Ashes at High Temperature

In recent energy recycle system of sewage sludge combustion, fine ash particle deposition and growth on the surface of heat exchanger, heater and wall decrease the energy recycle efficiency and hindered stable operation of combustion plants. These phenomena were generated by the adhesiveness and stickiness increase of ash particles at high temperature. Since the sewage sludge combustion ash included higher contents of alkali metals, phosphate and chloride, the stickiness of ashes promoted at high temperature compared with coal combustion ashes. It seems that eutectics phase with relatively low melting points are contained in ash, and generated liquid phase at high temperature. Since phosphate concentration in sewage sludge combustion ash is generally higher than that in other biomass and coal combustion ash. However, the action mechanism of phosphate on adhesion phenomenon has not been discussed.

In this work, 4 kinds of sewage sludge combustion ash from commercial plant, of which two samples were collected during stable operation in 2 different plants, and other 2 samples were collected after ash adhesion and growth trouble in different plants, were studied. By using 4 samples, after particles characterization, such as particle size distribution and chemical component analysis, tensile strengths of each ash powder bed at high temperature condition ranging from 673 to 1073 K were measured by split type tensile strength tester. In order to analyze the increasing mechanism of adhesion behavior of ash particles, thermodynamic equilibrium calculations applied for liquid phase formation behavior. The results suggest that the concentration of phosphate in ash would influence the liquid phase formation at high temperature, the adhesion phenomenon would be influenced consequently. Ashes with higher contents of phosphate tend to generate K₂O-SiO₂ series eutectic compounds, of which the melting points are relative low. On the contrary, ashes with lower contents of phosphate, the formation of low melting point compound was restrained, thus the adhesiveness of ashes decrease. Based on the calculation results, we estimated the useful element, alumina, to hinder liquid phase formation, and the effect of alumina addition on tensile strength of ash layer at high temperature were examined.