Unravelling the pore templating effect on CO2 adsorption performance of alkali metal nitrates promoted MgO pellets

Abstract

Alkali metal salts (AMS) promoted MgO with high CO2 uptakes represents a promising adsorbent candidate for CO2 capture. The AMS-MgO composite adsorbents are facing the challenges of increased pressure drop or elutriation when employed in dual fixed-bed reactor or twin circulating fluidized-bed configurations. Granulation by the extrusion-spheronization technique represents an effective problem-solving strategy. However, textural properties and CO2 uptakes of MgO-based adsorbent pellets might be adversely affected by the granulation process. In this work, AMS-MgO composite adsorbent pellets were prepared by the extrusion-spheronization method. Citric acid (CA), ammonium bicarbonate (AB), urea (UA) and microcrystalline cellulose (MC) were employed as pore-forming templates to improve the porous structures and CO2 uptakes of the AMS-MgO pellets. CO2 uptakes and adsorption kinetics of the adsorbent pellets were studied using a fixed-bed reactor. Effects of granulation and pore templating on the structure-performance relationships of the AMS-MgO pellets were unraveled. Granulation had resulted in the remarkable decrease in CO2 uptake from 10.87 to 4.24 mmol CO2/g due to the impaired textural parameters. The employed sacrificial templates endowed the AMS-MgO pellets with enhanced CO2 uptakes and improved carbonation kinetics. This was associated with extended surface area and expanded pore structures due to gas liberation in the pyrolysis of templates, which had facilitated the kinetics of CO2 bulk diffusion. The desired AMS-MgO-MC pellet modified with MC template showed a high CO2 uptake of 7.54 mmol CO2/g in 50%CO2 at 340 °C, and its CO2 uptake was stabilized around 5.0 mmol CO2/g within 20 cycles. Besides, the AMS-MgO-MC pellet also exhibited good mechanical properties. The results will guide the rational design of highly efficient and robust MgO-based adsorbent pellets for CO2 capture applications.