Influence of material composition on the CO2 and H2O adsorption capacities and kinetics of potassium-promoted sorbents

Two different potassium-promoted hydrotalcite (HTC)-based adsorbents and a potassium-promoted alumina sorbent were investigated using thermogravimetric analysis (TGA) and different characterization methods in order to study CO2 and H2O adsorption capacity and kinetics. A higher Mg content improves the cyclic working capacity for CO2 due to the higher basicity of the material. The initial adsorption rate for CO2 is very fast for all sorbents, but for sorbents with higher MgO content, this fast-initial adsorption is followed by a slower CO2 uptake probably caused by the slow formation of bulk carbonates. A longer half-cycle time can therefore increase the CO2 cyclic working capacity for sorbents with a higher MgO content. Potassium-promoted alumina has a very stable CO2 cyclic working capacity at different operating temperatures compared to the potassium-promoted HTC's. Usually a higher operating temperature increases the desorption kinetics for a HTC-based adsorbent, but not for potassium-promoted alumina. HTC-based adsorbents show the highest cyclic working capacity for H2O. The adsorption kinetics for H2O are not influenced by the material composition, indicating that the mechanism behind the adsorption of H2O is different compared to CO2. Depending on the material composition, adsorption of steam at high operating temperatures (>500 degrees C) results in an irreversible decomposition of carbonate species. Steam can reduce the temperature where usually K2CO3 is irreversibly decomposed resulting in a significantly reduced cyclic working capacity, which is very important concerning the use of these sorbents for sorption-enhanced water-gas shift processes.