Potassium interactions with copper slag and magnetite fines in chemical-looping processes

Using oxygen-carrying bed materials is a promising alternative to conventional fluidized bed combustion. Biomass-derived fuels contain ash rich in K and P, which might react with the oxygen carrier, leading to agglomeration and other problems. This study investigates the performance of copper slag (Järnsand, Fe-Si-oxide) and magnetite fines (MAF, Fe₃O₄) as oxygen carriers in a lab-scale fluidized bed reactor with subsequent material analysis. The conversion of methane and the fluidization was monitored, as K₂CO₃ or KH₂PO₄ was added as ash model compound. The fuel conversion was mainly unaffected by K-salt addition, apart from when K₂CO₃ was added to MAF at 950 °C and the conversion increased, along with increased porosity. Järnsand captured K from K₂CO₃. Mg and Al inherent to Järnsand participated in the interaction, contributing to increasing the melting point of the formed K-silicates. In MAF, the uptake of K was low: thermodynamic calculations suggested the formation of KFe₁₁O₁₇ and small amounts of slag. KH₂PO₄ always caused agglomeration by a melt-induced mechanism. The K-P-melt absorbed Fe in MAF or Ca in Järnsand. In conclusion, Järnsand seems like a promising oxygen carrier for biomass-derived fuels, while MAF might suffer from poor particle integrity in the presence of K-rich ash species.