Valorization of waste eggshell for CO₂ sorbents production by sol-gel citric acid treatment in a fixed-bed reactor

This study assesses the potential of the eggshells as a promising and alternative CO₂ adsorbent. With the aim of enhancing the CO₂ capture performance of raw eggshell, it was modified by sol-gel citric acid treatment. Both adsorbents were characterized by N₂ adsorption-desorption, XRF, XRD and SEM techniques. Preliminary adsorption tests in TGA revealed that the acid-modified eggshell achieved significantly higher carbonation conversion than the raw eggshell adsorbent in the 650–750 ºC adsorption temperature range, reaching a carbonation conversion of 69% at 700 ºC. The cyclic performance of the modified eggshell, tested in TGA at 700 ºC under 15 vol% CO₂, confirmed that sol-gel acid treatment led to a very stable adsorbent, with a slight improvement in CO₂ adsorption capacity during the first four cycles (from 0.54 to 0.58 g_CO2 g_ads⁻¹), followed by stable performance due to self-reactivation phenomena. Moreover, additional experiments were carried out in a fixed bed reactor under more realistic conditions to study the influence of temperature (600–750 ºC) and CO₂ flow rate (50–125 mL min⁻¹) using the modified eggshell. An increase in temperature under 15 vol% CO₂ stream led to a decrease in the carbonation conversion of the adsorbent, with the maximum CO₂ uptake at 600 ºC (0.64 g_CO2 g_ads⁻¹). Regarding the role of the CO₂ flow rate at 700 ºC, increasing from 75 to 125 mL min⁻¹ CO₂ had minimal effect, maintaining a capture amount of approximately 0.49 g_CO2 g_ads⁻¹. These results confirmed the potential of the acid-modified eggshell as a low-cost and environmental friendly adsorbent for CO₂ capture, which indeed may significantly improve the economic feasibility of the CCUS processes.