CO2 and H2O chemisorption mechanism on different potassium-promoted sorbents for SEWGS processes

Abstract

The sorption kinetics and capacities of CO2 and H2O were investigated for two different potassium-promoted hydrotalcite sorbents and potassium-promoted alumina. Thermogravimetric analysis (TGA) and packed-bed reactor (PBR) breakthrough experiments were performed using sequences of adsorption and desorption steps in different gas mixtures containing CO2 and H2O. Experiments were carried out at an operating temperature of 400 °C with different partial pressures ranging from 0.025 bar to 0.3 bar for CO2 and 0.1 to 0.3 bar for H2O respectively. It was found that a sorption mechanism with different adsorption sites, developed for one of the sorbents, also applies for the other sorbents where capacities are different and depending on the sorbent. From experimental results it was deduced that K2CO3 promotion is mainly responsible for a reactive CO2 adsorption site, which can only be regenerated with steam. The adsorption capacity for this site is enhanced for K2CO3 promoted alumina compared to K2CO3 promoted hydrotalcite. A second adsorption site for CO2, which can be regenerated with N2 is dominant on the K2CO3 promoted hydrotalcite with a high MgO content. This indicates that MgO is probably responsible for the formation of basic sites on the surface of the sorbent, which are relatively easily regenerated at the investigated experimental conditions. The results also show that the sorbent with the highest MgO loading has the highest cyclic working capacity under dry adsorption conditions, whereas the hydrotalcite-based adsorbent with a lower MgO content has the highest cyclic working capacity for CO2 at wet conditions and is therefore the preferred sorbent for sorption-enhanced water-gas shift applications.