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.