Pilot-scale study of CO ₂ capture by CaO-based sorbents in the presence of steam and SO ₂

Calcium looping cycles require an oxy-fired calciner burning coal for sorbent regeneration. Thus, in addition to O ₂ and CO ₂, the flue gases will include both steam and SO ₂, and similarly, carbonation of real flue gases will occur in the presence of steam. However, to date, most research has been done without either of these two gaseous components present. Here, batch combustion experiments were performed in a pilot-scale fluidized-bed reactor to study the effects of steam and SO ₂ addition on CO ₂ capture by limestone-based sorbents calcined under oxygen-enriched air and oxy-fuel conditions. The initial fast kinetically controlled CO ₂ capture stage was dramatically reduced when the sorbent was calcined at realistic temperatures in the presence of SO ₂. This is attributed to both greater sintering due to higher local calcination temperatures required by high CO ₂ concentrations and CaSO ₄ formation. By contrast, steam in the synthetic flue gas during carbonation extended the initial, high-efficiency CO ₂ capture period compared with that observed during carbonation with dry synthetic flue gases. A comparison between pilot-scale fluidized-bed combustion (FBC) and thermogravimetric analysis (TGA) results showed that sorbent reactivity was considerably lower during pilot-scale FBC testing, as anticipated given the higher calcination temperatures employed in the FBC reactor and the presence of the other feed gases. The enhanced CO ₂ capture efficiency in FBC reactors with steam present was also confirmed by TGA tests. These results are important because they demonstrate how sorbent deactivation effects seen in realistic FBC calcium-looping operation can be successfully reduced by the presence of steam in the carbonator.