Potassium capture by ilmenite ore as the bed material during fluidized bed conversion

Abstract

Biomass is a complex fuel containing a heterogeneous mixture of organic matter and, to a lesser extent, inorganic matter. Inorganic ash-forming elements can cause serious ash-related problems in fluidized bed combustion systems, such as bed agglomeration, fouling and corrosion of heat transfer equipment. Potassium (K), a biomass ash component, is a major contributor to these phenomena. The aim of this work, in the context of high-potassium biomass fuel combustion, is to estimate the potential for the absorption of potassium by bed materials under different oxygen carrier aided combustion (OCAC) operating conditions, in terms of reaction gas composition, exposure duration, bed temperature, potassium salt type, and concentrations of potassium salt. Experiments were conducted with an electrically heated fluidized bed reactor (FBR). The potassium solution is injected into the dense bed region via a small tube. In the studied temperature range of 700–900 °C, the captured K amount increases significantly at temperatures lower than 850 °C. The gas atmosphere had a less notable effect on K capture, but the capture rate of potassium dropped to half after the K-solution was switched from KOH to K2CO3 in the CO2 gas atmosphere. The percentage of injected potassium that is captured is high (∼40–60 wt%) in the first 5 h, and it remains at a high level of about 35 wt% after 25 h. KTi8O16 is the only crystalline K-compound found in the bed solid samples when ilmenite ore was used as bed material. KAlSi3O8 and K0.43(NH4)0.53Al0.89Si2.11O6 were detected when olivine sand was used as bed material. Thermogravimetric analysis (TGA) indicated no significant effect of the spent ilmenite on its redox reactivity after its reaction with K-solutions in the fluidized bed; however, the O2-carrying capacity of spent ilmenite shows a 7 % reduction after 25 h of exposure to potassium in the fluidized bed.