Influence of steam addition during carbonation or calcination on the CO2 capture performance of Ca9Al6O18CaO sorbent

Calcium looping process, on the basis of the reversible reaction between CaO and CO2, is a promising technology for capture of CO2 from different gas streams. The performance of CO2 sorbents could be easily influenced by the gas stream composition. As the exhaust gas stream generally contains a considerable amount of steam, a good understanding of its side effects on the performance of CaO-based sorbents is essential. However, there is not consensus on the mechanism with which the presence of steam during carbonation influences the CO2 capture performance and contradictory results have been reported regarding the effect of steam addition during calcination. This work aims to investigate the effect of steam addition during either carbonation or calcination on the reactivity of a home-made synthetic CaO sorbent containing 78 wt% CaO and 22 wt% Ca9Al6O18. Various concentrations of steam up to 9.5 vol% were provided during either carbonation or calcination for 15 carbonation-calcination cycles (carbonation: 650 or 550 °C; calcination: 800 °C). The morphology changes of the sorbent after cyclic carbonation-calcination experiments were studied in detail at different operating conditions. It was concluded that the sorbent reactivity was significantly increased for all concentrations of steam injected during carbonation step, due to the accelerated solid-state diffusion and steam catalysis. In case of steam addition during calcination, the carbonation performance was affected negatively or positively depending on the concentration of steam in the gas stream. For 2.3 vol% steam injection, the sorbent reactivity was worsened, while the presence of 9.5 vol% steam increased the CO2 capture capacity during 9 initial cycles. Such behavior was attributed to the intensified material sintering in the presence of steam during calcination step, which results in the formation of large pores and the decrease of specific surface area.