Effect of SO2 on HCl removal over ethanol-hydrated CaO adsorbent: Mechanism of competitive adsorption and product layer shielding

Abstract

The elimination of hydrogen chloride (HCl) is essential for wastewater sequestration and safe operation of equipment in coal-fired power plants. A new adsorbent of ethanol-hydrated CaO has been developed, which possesses a 100% HCl removal efficiency lasting 30 min in a fixed bed reactor being much superior to traditional dechlorinators. Nevertheless, SO2 in flue gas has instinctive and strong competition with HCl on the alkaline adsorbent that results in detrimental effects and makes the removal mechanisms confused and unclear. To solve this issue, a series of experiments were designed to evaluate how SO2 influences the performance of HCl adsorption over the adsorbent. Comprehensive characterizations and density functional theory (DFT) calculations further studied the mechanisms of the competition mechanism between SO2 and HCl. The results indicate that SO2 has a detrimental impact on HCl removal by consuming the active Ca site, diminishing the alkalinity and worsening the surface activity due to the product layer formation. DFT calculations reveal that the high adsorption energy of SO2 suppresses the active sites for HCl removal. The faded surface electrostatic potentials (ESP) around the sulfurized Ca active sites lead to apparent activity shielding on the absorbent surface. Besides, the frontier molecular orbital (FMO) theory unveils a narrow electronic transfer energy gap for SO2 adsorption. These findings contribute to elucidation of SO2 competition to HCl on the calcium-based adsorbent in coal-fired flue gas.