Miscanthus ash transformation and interaction with bed materials at high temperature

• We describe the Miscanthus ash behavior and the interaction with bed materials at high temperature.
• The harvest time significantly influences the ash transformation at high temperature.
• The main difference in reducing atmosphere is the nature of sulfur.
• In contact with silica sand only physical adhesion occurs.
• Olivine plays an active role in agglomeration via the diffusion of Fe into the molten ash.

This work focused on the transformation of inorganics in Miscanthus ash and the interaction of ashes with silica sand, olivine and calcined olivine. The samples were heat-treated in a laboratory furnace without the fluidization of the bed particles. The transformation of Miscanthus ash was studied with experimental methods (chemical fractionation analysis, ICP-MS, XRD, SEM-EDX) and with thermodynamic calculations (FactSage) between 400 °C and 1400 °C. The ash-bed material interface was analyzed by scanning electron microscope (SEM-EDX). The effect of the harvest time and the atmosphere were investigated. The characterization of the ashes (winter harvest) revealed the presence of alkali salts, carbonates and silica at low temperature and the formation of Ca and Mg-silicates, Ca-phosphate and molten K-silicate above 750 °C. In oxidizing atmosphere the S formed K2SO4, while in reducing atmosphere the S left in the gaseous phase. The ash of the summer harvest Miscanthus contains more K and less Ca and Si, therefore carbonates and salts are formed both at low and high temperature. The interaction tests revealed only physical adhesion between the molten ash and the silica sand, while the olivine played an active role in the agglomerate formation via the diffusion of Fe into the molten ash.

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