Thermal Conversion of Sodium Phytate Using the Oxygen Carrier Ilmenite Interaction with Na-Phosphate and Its Effect on Reactivity

Chemical looping combustion (CLC) can be used to convert biomass for heat and/or power production while efficiently capturing the produced CO₂. This is possible because the biomass is oxidized by an oxygen carrier instead of directly by air. However, the ash species in biomass can interact with the oxygen carrier causing agglomeration and/or reducing its reactivity. One of the ash elements previously reported to cause problems is phosphorus and especially in combination with alkali. In this work, the interaction between a benchmark oxygen carrier, ilmenite, and a phosphorus model compound, sodium phytate, was studied up to a temperature of 1100 °C in N₂ using a fixed bed setup. Activated carbon and NaH₂PO₄ (thermally decomposing to NaPO₃) were also used to study the individual effect of carbon and inorganic Na-phosphate. The CO and CO₂ concentration in the flue gas was measured to monitor the oxidation of the samples, which showed that ilmenite participated in the conversion of Na-phytate starting from about 600 °C. Scanning electron microscopy coupled with energy dispersive X-ray spectroscopy analysis of cross sections of the ilmenite residues revealed that Na-phosphate (forming from Na-phytate) penetrates porous ilmenite particles to a greater extent compared to denser particles, which may reduce the agglomeration tendencies since a lower amount of sticky Na-phosphate melt will coat the particle surface. The effect of Na-phytate on the reactivity of ilmenite was quantitatively determined in a fluidized bed using 50% syngas or CO in N₂. For a loading of 1.5 wt % Na-phytate, the reactivity toward CO decreased to only 20% of the reference sample. The reason was partly attributed to a decreased surface area but is likely also due to the formation of less reactive Na–Fe-phosphates. A compilation of thermodynamic data relevant for the NaPO₃–FeO_x (x = 1 or 1.5) system shows that NaPO₃ can form a melt containing dissolved iron starting from around 600 °C and that sodium and phosphorus are present solely in this form above approximately 930 °C at equilibrium.