Microwave thermal pre-treatment and calcination of biomineralised sorbents for calcium looping

Waste eggshells have been investigated as biomineralised CO₂ sorbents for the calcium looping cycle due to their high Ca content, low cost, availability, and high CO₂ uptake. This work investigates a novel heat-pre-treatment using eggshells as raw material. The eggshells were simultaneously calcined and thermally pre-treated via microwave heating using different powers and times whilst maintaining the same input energy in a single mode cavity reactor. The calcined eggshells were characterised using standard characterisation techniques such as TGA and isotherms of N₂ adsorption at 77 K. The dielectric properties were measured using the cavity perturbation technique at 2.45 GHz. Numerical electromagnetic simulations were performed using COMSOL Multiphysics and results were used to optimise the experimental setup. The treated materials were subjected to carbonation/calcination cycles in order to assess their suitability as a calcium-looping sorbent. The sorbent that exhibited a more stable CO₂ uptake was the one treated at the highest power (800 W) 4.5 mmol CO₂/g sorbent after 20 cycles under mild calcination conditions. Adsorbents prepared at 400, 600 and 800 W displayed similar CO₂ uptake after 20 cycles when the calcination conditions were under more realistic conditions.