An experimental study of the decomposition and carbonation of magnesium carbonate for medium temperature thermochemical energy storage

To improve the energy efficiency of an industrial process thermochemical energy storage (TCES) can be used to store excess or typically wasted thermal energy for utilisation later. Magnesium carbonate (MgCO₃) has a turning temperature of 396 °C, a theoretical potential to store 1387 J/g and is low cost (~GBP 400/1000 kg). Research studies that assess MgCO₃ for use as a medium temperature TCES material are lacking, and, given its theoretical potential, research to address this is required. Decomposition (charging) tests and carbonation (discharging) tests at a range of different temperatures and pressures, with selected different gases used during the decomposition tests, were conducted to gain a better understanding of the real potential of MgCO₃ for medium temperature TCES. The thermal decomposition (charging) of MgCO₃ has been investigated using thermal analysis techniques including simultaneous thermogravimetric analysis and differential scanning calorimetry (TGA/DSC), TGA with attached residual gas analyser (RGA) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) (up to 650 °C). TGA, DSC and RGA data have been used to quantify the thermal decomposition enthalpy from each MgCO₃.xH₂O thermal decomposition step and separate the enthalpy from CO₂ decomposition and H₂O decomposition. Thermal analysis experiments were conducted at different temperatures and pressures (up to 40 bar) in a CO₂ atmosphere to investigate the carbonation (discharging) and reversibility of the decarbonation–carbonation reactions for MgCO₃. Experimental results have shown that MgCO₃.xH₂O has a three-step thermal decomposition, with a total decomposition enthalpy of ~1050 J/g under a nitrogen atmosphere. After normalisation the decomposition enthalpy due to CO₂ loss equates to 1030–1054 J/g. A CO₂ atmosphere is shown to change the thermal decomposition (charging) of MgCO₃.xH₂O, requiring a higher final temperature of ~630 °C to complete the decarbonation. The charging input power of MgCO₃.xH₂O was shown to vary from 4 to 8136 W/kg with different isothermal temperatures. The carbonation (discharging) of MgO was found to be problematic at pressures up to 40 bar in a pure CO₂ atmosphere. The experimental results presented show MgCO₃ has some characteristics that make it a candidate for thermochemical energy storage (high energy storage potential) and other characteristics that are problematic for its use (slow discharge) under the experimental test conditions. This study provides a comprehensive foundation for future research assessing the feasibility of using MgCO₃ as a medium temperature TCES material. Future research to determine conditions that improve the carbonation (discharging) process of MgO is required.