HCl dry removal with modified Ca-based sorbents at moderate to high temperatures

Modified Ca-based sorbents were obtained by adding sodium alkali into Ca(OH)2 and CaCO3. Reactive properties of modified Ca-based sorbents with acidic gases were investigated through reacting with gaseous HCl at 450–760°C, and SEM and XRD technologies were adopted to get information on the reaction mechanism. Experimental data showed that HCl dry removal efficiencies increased with temperature before 700°C for all of the investigated sorbents, and there existed improved sorbents that corresponded to the highest removal efficiencies under the similar conditions. SEM photographs exhibited morphology difference between original and improved sorbents both before and after the reaction; and displayed that improved sorbents formed more porous product layers than original sorbents especially at higher temperature when product sintering became heavier, which is favorable to HCl dry removal. XRD analysis showed that (1) improved Ca(OH)2 and CaCO3 were less crystalline than original lime and limestone; (2) the reaction product species of improved Ca(OH)2 changed with reaction temperature, while for original Ca(OH)2 the same product species appeared for all of the tested temperatures; and (3) for improved CaCO3, the only product at lower temperatures was CaCl2.2H2O and more product species were produced when temperature was higher than 650°C, but no CaCl2.Ca(OH)2.H2O formed at 700°C, while for the case of original CaCO3, the undesired CaCl2.Ca(OH)2.H2O appeared at 700°C. Presently, reaction temperature interval of 650–700°C is recommended for improved Ca(OH)2 to get the highest efficiency, for improved CaCO3 reaction at higher temperature deserves further investigation to make a good choice.