Ash deposition mechanisms in Waste-to-Energy plants

Some ash particles in municipal and industrial waste adhere to heat exchanger tube surfaces, which causes problems such as heat-transfer inhibition, high temperature corrosion and low utilization in Waste-to-Energy (WtE) plants. The objective of this study is to develop new surface treatment materials and techniques which can decrease the ash deposition and the corrosion, and to provide further understanding of ash deposition mechanisms for various metal surfaces in WtE plants. First, the adhesion force between an ash pellet, which was made of ash sampled from a WtE plant, and an alloy specimen was measured to investigate the mechanisms that increase ash deposition. Second, the adhesion interfaces of the specimens were analyzed after the adhesion force measurement. The result was that the adhesion forces of all specimens increased with the interface temperature, and there was a clear temperature dependency on the force. The adhesion force of the ash pellet to stainless steel AISI 430 and 304 were larger than to 310S or to a surface-modified AISI 304 due to a lower Cr content. In particular, there was a correlation between Ni + Cr content in the surface of the alloy specimens and the adhesion force. Moreover, the analysis of AISI 304 with SEM-EDS have shown that the active oxidation involved in interface reactions. In addition, analysis of the adhesion interface and the thermodynamic equilibrium calculation supported the results of the adhesion measurements. Specifically, results of the calculations were that partial pressure of Fe-chloride gases are higher than that of Ni or Cr-chloride gases.