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

Journal article, 2025

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.