In-depth evaluation of a ZrO2 promoted CaO-based CO2 sorbent in fluidized bed reactor tests

Carbonate looping, based on the reversible gas-solid reaction of CaO with CO2, is considered as a promising alternative to amine scrubbing for post-combustion CO2 capture. Solid sorbents suffer however from degradation, mainly due to thermal sintering and elutriation of fine particles due to enhanced attrition rates in fluidizedbed reactors. In this work, a previously developed synthetic Zr-promoted CaO-based CO2 sorbent was tested in a fluidized bed reactor unit to determine its performance in cyclic CO2 capture over various operating conditions, relevant to industrial application. The material exhibited very high carbonation conversion (60-85%) during pre-breakthrough under all investigated conditions, with more than 75% CO2 removal. The addition of steam in both the carbonation and calcination steps resulted, not only in higher conversions, but also in significantly enhanced cyclic stability. Deactivation was less than 16% after 20 consecutive cycles. The performance of the sorbent was further tested under lower temperature difference between carbonation (680 degrees C) and calcination (750 degrees C), a scheme more favourable for utilizing the heat generated by the highly exothermic carbonation reaction for the thermal demands of the calciner in the actual process. The material displayed similar carbonation conversion, but inferior performance in terms of stability. Advanced post-reaction characterization with in-situ XRD revealed that even though the sintering effect was more limited due to the lower calcination temperature, calcination of CaCO3 was incomplete, rendering a small fraction of the sorbent inactive for CO2 capture. Under severe calcination conditions (920 degrees C and 80 vol% CO2 concentration) the sorbent maintained more than 70% of its initial sorption capacity (7.1 mol of CO2/kg of sorbent after 20 cycles), a value more than 5 times higher compared to natural limestone.