Experimental and numerical investigations of desulfurization wastewater evaporation in a lab-scale flue gas duct: evaporation and HCl release characteristics

ABSTRACT

The evaporation of desulfurization wastewater using flue gas has been considered as a promising technique. However, the release of HCl during the evaporation hinders the application of this technique. Herein, we investigated the evaporation in a laboratory-scale flue duct through experimental and numerical methods. The influences of operating parameters on characteristics of evaporation and HCl release were experimentally explored. The results show that when inlet gas temperature increases from 200°C to 350°C, gaseous HCl concentration significantly increases from 5.02 ppm to 70.96 ppm and HCl release ratio accordingly increases from 1.57% to 17.32%. However, when the wastewater only contains CaCl₂, the HCl release is effectively inhibited. Subsequently, the established CFD model was validated with the experiments. It was found that relative deviations between predicted and experimental outlet gas temperatures mostly locate within the range of −10%∼0%. The negative deviations are attributed to the neglect of the crust formation. Numerical studies also reveal that droplet evaporation temperature is close to the wet-bulb temperature of the inlet gas ranging from 321.47 K to 337.85 K. Moreover, thermodynamic analysis shows that when the wastewater contains equimolar MgCl₂ and CaCl₂, MgCl₂·6H₂O first crystalizes at about d/d₀ = 0.3. However, when the wastewater only contains CaCl₂, CaCl₂·2H₂O begins to crystallize at about d/d₀ = 0.25. Before crystallization, the equilibrium partial pressure of HCl is close to 0 Pa, indicating the HCl is mainly released at the particle drying stage. Further analysis suggests HCl should be mainly released from the decomposition of MgCl₂·6H₂O into MgOHCl.

GRAPHICAL ABSTRACT

KEYWORDS:

• Desulfurization wastewater
• flue gas evaporation
• HCl release
• evaporation simulation
• zero liquid discharge

Disclosure Statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by National Natural Science Foundation of China: [Grant Number No. 51806188 and No. 51276038]; Natural Science Foundation of Jiangsu Province of China: [Grant Number No. BK20180932]; Natural Science Foundation of the Jiangsu Higher Education Institutions of China: [Grant Number No. 18KJB470025].