Steam-enhanced calcium looping cycles with calcium aluminate pellets doped with bromides

This study explores the effect of calcium bromide (CaBr₂) doping of lime-based sorbents in the presence of steam during calcination/carbonation cycles. Two sorbents were tested: natural limestone (Cadomin, Canada) and a synthetic sorbent (pellets) prepared from Cadomin limestone with addition of calcium aluminate cement. The mixture of calcined limestone and cement was pelletized in a mechanical granulator that uses spray water as the part of the pelletization process. Both the original limestone and the prepared pellets were impregnated with a dilute CaBr₂ solution to achieve a Ca/Br mole ratio of 500:1. The CO₂ carrying activities of the sorbents were tested during calcination/carbonation cycles in a thermogravimetric analysis (TGA) apparatus. Realistic calcination conditions during the reaction cycles were employed: 900 C with a CO₂ sweep gas. Multicycle tests were carried out with steam 15% and without steam present in the carbonation gas stream (20% CO₂, 15% steam or 0% steam, N₂ balance in both cases). The results showed that doping with CaBr₂ has a beneficial effect on sorbent CO₂ capture activity, and in particular, the conversion rate during the diffusion-controlled stage of carbonation was found to exhibit a strong synergic enhancement in the presence of steam. The effects of doping and steam were more pronounced in the case of synthetic pellets, resulting in an uptake of 23.8 g of CO₂/100 g of sorbent after 31 cycles, which represents a conversion of 35.6%. This CO₂ capture uptake is very high compared with that of pellets with no CaBr₂ addition and no steam present during the reaction cycles, where only 15.0 g of CO₂/100 g of sorbent (22.5% conversion) was seen after 10 cycles. These results suggest that the preparation of synthetic sorbents for calcium looping using solutions containing small amount of bromides would be beneficial in practical applications, and steam will either be produced by firing almost any fuel or be found in flue gas suitable for processing by calcium looping.