Multiple sorption cycles evaluation of cadmium oxide-alkali metal halide mixtures for pre-combustion CO2 capture

Cadmium oxide–alkali metal halide mixtures for pre-combustion CO2 capture were made using a wet mixing approach. Some of the samples were pelletised in as-synthesised state as well as with SBA-15 silica addition. In a multiple CO2 sorption cycle test via thermogravimetric analysis, the best performing powder material by capacity and kinetics (a cadmium oxide made from carbonate doped with 17.5 wt% sodium iodide) exhibited a sorption capacity loss from 17 to 2 wt% after 25 cycles of partial pressure swing sorption at atmospheric pressure and temperatures of 285 and 305 °C. When the initial decomposition of the carbonate took place in inert gas (Ar or N2 instead of air), the cyclic stability was improved. Water addition (1 vol%) to the sorption gas further improved the cyclic CO2 sorption stability and capacity. Elemental analysis of the samples after cyclic exposure to CO2 revealed that the capacity loss is associated with loss of iodine, whereas the sodium remains. Water addition, however, had no significant effect on this iodine loss. Pellets made from carbonate performed with a working capacity of 10 wt%, but lost their mechanical integrity during multicyclic sorption. If made in the oxide state, pellets remained sturdy, but showed almost no working capacity. The addition of 13.7 wt% SBA-15 improved the working capacity of the oxide pellet to a stable value of 5.2 wt% over 25 cycles. In situ powder X-ray diffraction showed the reversible isothermal phase transformation of CdO to CdCO3 during three cycles of sorption and also revealed the presence of a crystalline sodium iodide phase, which appeared to be lost with increasing number of sorption cycles.