Study of the Reaction of Lime with HCl under Simulated Flue Gas Conditions Using X-ray Diffraction Characterization and Thermodynamic Prediction

Abstract

Hydrochloric acid (HCl) is commonly removed from flue gases using hydrated lime (Ca(OH)2) as a sorbent in the dry scrubbing process. Although this method is relatively easy to operate and the lime sorbent is cheap, this process is highly inefficient, as only ∼25% of the lime used is converted, due to the short contact time and the lack of understanding of the reaction chemistry of lime in a complex flue gas system. A comprehensive X-ray diffraction (XRD) characterization was carried out on the product of a hydrated lime reaction with HCl in a simulated flue gas system containing SO2, CO2, O2, and moisture. Together with thermodynamic calculations, the chemical nature of the multiple reactions that happened in this system was investigated. It was confirmed that the products of hydrated lime with HCl under the simulated flue gas conditions were chiefly Ca(OH)Cl, CaCl2, CaCO3, CaSO3·1/2H2O, CaSO4·2H2O, and anhydrous CaSO4. Previous results suggested that the presence of CO2 and SO2 in the system formed products with a high product diffusion resistance, leading to pore blockage and the premature termination of the reaction. However, in this study, experimental proofs showed that HCl was able to break down the CaCO3 and CaSO3·1/2H2O layer, allowing the reaction to continue. In the presence of excess O2 (and moisture), part of the CaSO3·1/2H2O was oxidized to CaSO4·2H2O and CaSO4. The presence of O2 is, therefore, counterproductive to the full utilization of hydrated lime, as sulfur in its S(VI) form is more stable and resistant to HCl attack, thereby preventing blocked pores from being cleared.