Enhancing mercury removal across air pollution control devices for coal-fired power plants by desulfurization wastewater evaporation

Experimental and Mechanistic Study of Synergistic Removal of Hg by Evaporation from Desulfurization Wastewater

The flue evaporation of desulfurization wastewater can solve the problem that it is difficult to remove some heavy metal ions and chloride ions by conventional methods. A large amount of chloride ions in desulfurization wastewater can also promote the catalytic oxidation removal of Hg in the flue gas. The migration character of chloride ions in the flue evaporation process of desulfurization wastewater was studied by using the coal-fired thermal state experimental platform. The concentrations of Hg0 and Hg2+ in the flue gas at the inlet and outlet of selective catalytic reduction denitration (SCR), electrostatic precipitator (ESP), and wet desulfurization (WFGD) devices were tested, and the synergistic removal of traditional pollutant removal equipment by flue evaporation of desulfurization wastewater was analyzed. The influence of Hg and the effect of the evaporation of desulfurization wastewater at different positions on the removal of Hg in the flue gas were compared and analyzed, and the catalytic mechanism of Hg on the SCR surface was further revealed. The results show that 10% chloride ions enter the flue gas after the desulfurization wastewater evaporates. The content of chlorine elements and evaporation temperature influence the evaporation of desulfurization wastewater. The mechanism of SCR catalytic oxidation of Hg0 was explored; oxygen atoms have catalytic oxidation effects on Hg0 at different positions in the V2O5 molecule in SCR; and chloride ions can enhance the catalytic oxidation of Hg0 by V2O5. The intermediate product HgCl is generated, which is finally converted into HgCl2. The oxidation efficiency of Hg0 in electrostatic precipitation (ESP) is increased from 3% to 18%, and the removal efficiency of Hg is increased from 5% to 10%. The removal efficiency of Hg2+ in WFGD is basically maintained at approximately 85%. In addition, a small amount of Hg2+ was restored to Hg0 in WFGD. The removal efficiency of Hg0 in the flue gas of evaporative desulfurization wastewater before SCR is 65%, and the removal efficiency of gaseous Hg is 62%. When the evaporative desulfurization wastewater before ESP, the synergistic removal efficiency of Hg0 is 39%, and the gaseous Hg removal efficiency is 39%, and the removal efficiency of Hg is 40%. Evaporation of the desulfurization wastewater before SCR was more conducive to the coordinated removal of Hg by the device.

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

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.