Sintering and reactivity of CaC o₃ -based sorbents for in situ c o₂ capture in fluidized beds under realistic calcination conditions

Sintering during calcination/carbonation may introduce substantial economic penalties for a C O₂ looping cycle using limestone/dolomite-derived sorbents. Here, cyclic carbonation and calcination reactions were investigated for C O₂ capture under fluidized bed combustion (FBC) conditions. The cyclic carbonation characteristics of CaC O₃ -derived sorbents were compared at various calcination temperatures (700-925°C) and different gas stream compositions: pure N₂ and a realistic calciner environment where high concentrations of C O₂ >80-90% (and the presence of S O₂) are expected. The conditions during carbonation employed here were 700°C and 15% C O₂ in N₂ and 0.18% or 0.50% S O₂ in selected tests, i.e., typically expected for a carbonator. Up to 20 calcination/carbonation cycles were conducted using a thermogravimetric analyzer (TGA) apparatus. Three Canadian limestones were tested: Kelly Rock, Havelock, and Cadomin, using a prescreened particle size range of 400-650 μm. In addition, calcined Kelly Rock and Cadomin samples were hydrated by steam and examined. Sorbent reactivity was reduced whenever S O₂ was introduced to either the calcining or carbonation streams. The multicyclic capture capacity of CaO for C O₂ was substantially reduced at high concentrations of C O₂ during the sorbent regeneration process and carbonation conversion of the Kelly Rock sample obtained after 20 cycles was only 10.5%. Hydrated sorbents performed better for C O₂ capture, but also showed significant deterioration following calcination in high C O ₂ gas streams. This indicates that high C O₂ and S O ₂ levels in the gas stream lead to lower CaO conversion because of enhanced sintering and irreversible formation of CaS O₄. Such effects can be reduced by separating sulfation and carbonation and by introducing steam to avoid extremely high C O₂ atmospheres, albeit at a higher cost and/or increased engineering complexity.