Sodium Ferrite/Carbon Dioxide Reactivity for High Temperature Thermochemical Energy Storage

Abstract

The reaction of sodium ferrite (NaFeO₂) with carbon dioxide shows great promise for use in new high temperature thermochemical energy storage (TcES) systems. Therefore, the chemical reaction between NaFeO₂ and CO₂ was investigated via thermogravimetric experiments. Analysis of x-ray diffraction patterns confirmed that a mixture of Na₂CO₃ and Fe₂O₃ was completely converted to NaFeO₂ after heating at 900°C. Under a CO₂ pressure of 100 kPa, decarbonation and carbonation of the sample proceeded at temperatures over 850°C and under 750°C, respectively. The starting temperatures of decarbonation and carbonation decreased with decreasing CO₂ pressure. A cyclic experiment was conducted using the pressure swing absorption method at temperatures of 700–900°C. The change in reacted mole fraction kept increasing gradually after the second cycle, and it reached 0.835 during the 15th cycle. According to surface observations, a porous structure formed after the 15th cycle. This improved CO₂ diffusivity through the sample and it appears to be the cause of the enhanced reactivity observed during the cyclic experiment with increasing cycle number. The volumetric and the gravimetric thermal energy densities of NaFeO₂ were estimated as 760 kJ L⁻¹ and 450 kJ kg⁻¹, respectively. These results indicate that NaFeO₂ has potential to be used as a TcES material at temperatures around 700–900°C for utilization of surplus heat in iron-making and other high-temperature processes.