## 4 Mercury Migration under O\({}_{2}\)/Co\({}_{2}\) Atmosphere
### Hg Re-Emission from WFGD  
Due to the high solubility of oxidized mercury, Hg\({}^{2+}\) is easily captured by desulfurization slurries and desulfurization wastewater in Wet Flue Gas Desulfurization (WFGD) systems, achieving mercury removal from flue gas. However, in WFGD systems, Hg\({}^{2+}\) can be reduced under certain conditions and released back into the flue gas in the form of Hg\({}^{0}\), adversely affecting the mercury removal efficiency [64; 65]. In oxy-fuel combustion systems in particular, stricter limitations are imposed on mercury concentrations. The re-emission of Hg\({}^{0}\) may lead to a failure to meet the requirements for deep mercury removal, threatening the safe operation of the system. Therefore, it is necessary to conduct research on the re-emission of Hg\({}^{0}\) in WFGD systems under oxy-fuel conditions.  
[PERSON] et al. [66] studied the influence of the main components (CO\({}_{2}\), O\({}_{2}\), and H\({}_{2}\)O) in oxy-fuel combustion flue gas on the re-emission of Hg\({}^{0}\) in the WFGD system. The concentrations of O\({}_{2}\) and CO\({}_{2}\) have a significant impact on the re-emission of mercury, while the influence of H\({}_{2}\)O can be negligible. Due to the reaction between O\({}_{2}\) and metals in a low oxidation state, high concentrations of O\({}_{2}\) inhibit the re-emission of Hg\({}^{0}\). When metal impurities in limestone are the main factors causing the Hg\({}^{0}\) re-emission, the stabilizing effect of O\({}_{2}\) on mercury in the slurry is more obvious. High concentrations of CO\({}_{2}\) lead to a decrease in the pH of the slurry and affect the redox potential, thereby inhibiting Hg\({}^{0}\) re-emission.  
[PERSON] et al. [49; 67] studied the effects of O\({}_{2}\), CO\({}_{2}\), and metal elements such as Fe, Mn, and Ni on the re-emission of Hg\({}^{0}\). The results show that adding O\({}_{2}\) in a pure N\({}_{2}\) atmosphere significantly enhances the re-emission of Hg\({}^{0}\) due to the decrease in pH. However, adding 75% CO\({}_{2}\) in a N\({}_{2}\) atmosphere limits the re-emission of Hg\({}^{0}\) due to a slight decrease in pH. Under oxy-fuel conditions, increasing the O\({}_{2}\) concentration from 3% to 6% leads to a slight decrease in the re-emission of Hg\({}^{0}\). Further increasing the O\({}_{2}\) concentration to 12% does not have a significant influence on the Hg\({}^{0}\) re-emission. Higher CO\({}_{2}\) concentrations are unfavorable for Hg\({}^{0}\) re-emission, as more mercury remains in the solid phase. In both air-fuel and oxy-fuel atmospheres, Hg\({}^{0}\) re-emission is inhibited by the metal ions. Fe\({}^{2+}\) could transform to the Fe(OH)\({}_{2}\) precipitate and the Fe(OH)\({}_{3}\) precipitate. Both Fe(OH)\({}_{2}\) and Fe(OH)\({}_{3}\) precipitates can adsorb mercury species. Hence, the re-emission of mercury from WFGD systems is inhibited. The negative effect of Fe\({}^{2+}\) increases with a higher CO\({}_{2}\) concentration (Figure 7).  
Figure 7: Hg re–emission and migration under air and oxy-fuel conditions [67]. Copyright 2020 American Chemical Society, Washington, DC, USA.  
In general, under oxy-fuel conditions, the transformation of mercury species and the re-emission of Hg\({}^{0}\) in WFGD systems differ from those under air conditions. For oxy-fuel biomass co-firing systems, the impacts of both biomass blending and O\({}_{2}\)/CO\({}_{2}\) atmosphere need to be considered simultaneously.