The effect of sulphur dioxide and steam on the CO2 capture in calcium looping: comparison between two limestones

Calcium Looping (CaL), performed in dual interconnected Fluidised Bed (FB) reactors, provides a feasible and robust path to accomplish carbon capture from CO2-bearing exhaust gas. The performance of the CaL technology is affected by sorbent deactivation (due to thermal sintering) upon iterated looping and by particle attrition/fragmentation. An issue that has been comparatively less scrutinised is represented by the possible concurrent effect of steam and sulphur dioxide in terms of sorbent availability and selective uptake. Detailed mechanistic studies indicate that exposure to steam can enhance CO2 diffusion across the carbonated product layer improving the accessibility of unconverted CaO, hence CO2 uptake is improved. On the other hand, SO2 may irreversibly overtake CO2 in the competitive absorption on CaO with detrimental effects on sorbent performance. This study aims at performing a deeper scrutiny of the performance of Ca-based sorbents in the ternary CO2-SO2-H2O system. The experimental campaign consisted of the characterisation of the CO2 uptake by two different limestones during tests performed in a lab-scale twin FB reactor operated with different fluidising reaction environments. The effect of the presence of steam and sulphur dioxide, considered either alone or together, is discussed in terms of CO2 capture capacity of the sorbent, its tendency to undergo attrition and its microstructural properties. Each CaL test consisted of ten calcination/carbonation cycles, plus an 11th calcination stage. The calciner was operated at 940°C with a fluidising atmosphere containing 70% CO2 (in air). The carbonator was operated at 650°C. Six different operating conditions were investigated by changing the fluidising atmosphere during carbonation, with a fixed concentration of 15% CO2 kept in each case. Analysis of the flue gas composition at the exhaust of the carbonator enabled quantitative assessment of the CO2 capture capacity of the sorbent. Moreover, sorbent attrition and fragmentation were investigated by analysing the fines elutriation rate, and the particle size distribution of the bed sorbent inventory. Finally, porosimetric analyses enabled to shed light on features of the raw and processed sorbents, which could be correlated with sorbent uptake, attrition and fragmentation.