CO₂ Uptake Potential of Ca-Based Air Pollution Control Residues over Repeated Carbonation–Calcination Cycles

The operation of dry processes for acid gas removal from flue gas in waste-to-energy plants based on the use of calcium hydroxide as a solid sorbent generates a solid waste stream containing fly ash, unreacted calcium hydroxide, and the products of its reaction with acid pollutants in the flue gas (HCl and SO₂). To date, the fate of the solid waste stream is to be put into a landfill in the absence of commercially viable recycling approaches. The present study investigates the potential of these residues as CO₂ sorbents in the calcium looping process. Samples collected in different waste-to-energy plants were tested over multiple carbonation–calcination cycles, comparing their performance to that of limestone. Although inferior, the CO₂ sorption capacity of the residues resulted in values comparable to that of limestone and that steadily increased for a significant number of cycles. This peculiar behavior was attributed to the presence of a chlorinated phase, which enhances the CO₂ uptake in the diffusion-controlled stage of carbonation by reducing the product layer resistance to CO₂ diffusion. No significant release of acid gases was observed at the characteristic temperatures of calcium looping carbonation