## 4 Mercury Migration under O\({}_{2}\)/Co\({}_{2}\) Atmosphere  
### Impacts of CO\({}_{2}\)  
In oxy-fuel combustion technology, oxygen is mixed with recirculated flue gas in a certain proportion to replace air as the combustion oxidizer [49]. Due to the change in the combustion atmosphere, the distribution of mercury species inside the furnace under oxy-fuel conditions differs from that in traditional air combustion. One of the most significant characteristics of the oxy-fuel combustion atmosphere is the high concentration of CO\({}_{2}\) in the flue gas. Therefore, it is essential to understand the impact of CO\({}_{2}\) on mercury oxidation. Research has shown that there are differences in the release characteristics of Hg\({}^{0}\) at different temperatures when the combustion atmosphere switches from air to O\({}_{2}\)/CO\({}_{2}\)[50]. Between 250 and 400 \({}^{\circ}\)C, the concentration of Hg\({}^{0}\) released in the air atmosphere is higher than in the O\({}_{2}\)/CO\({}_{2}\) atmosphere. This may be attributed to the higher specific heat capacity of CO\({}_{2}\), causing a delay in the release of volatiles, volatile combustion and char combustion in the O\({}_{2}\)/CO\({}_{2}\) atmosphere compared to the air atmosphere. Certain gas components that precipitate earlier in the air atmosphere may inhibit the oxidation of Hg\({}^{0}\)[51]. In the temperature range of 400-600 \({}^{\circ}\)C, the concentration of released Hg\({}^{0}\) in the O\({}_{2}\)/CO\({}_{2}\) atmosphere is slightly higher than in the air atmosphere. This might be due to the high concentration of CO\({}_{2}\) in the flue gas. Under O\({}_{2}\)/CO\({}_{2}\) combustion conditions, the high concentration of CO\({}_{2}\) leads to the production of a significant amount of CO through reduction reactions with carbon during combustion initiation. This results in the much higher generation of CO compared to that in the air atmosphere, creating a stronger reducing atmosphere that reduces the proportion of oxidized Hg\({}^{0}\), leading to a slightly higher release of Hg\({}^{0}\) compared to the air atmosphere. For the O\({}_{2}\)/CO\({}_{2}\) atmosphere, different concentrations of CO\({}_{2}\) can also impact the oxidation of Hg\({}^{0}\). Experimental results have shown that when the CO\({}_{2}\) concentration is below 30%, the influence of CO\({}_{2}\) on the species distribution of mercury is negligible. However, when the CO\({}_{2}\) concentration increases from 30% to 50%, there is an increase in the release concentration of gaseous mercury, indicating that high concentrations of CO\({}_{2}\) can inhibit the oxidation of mercury. This could be explained by several factors. Firstly, the high concentration of CO\({}_{2}\) can reduce the conversion rate of N in the fuel to NO and promote the reduction of NO, leading to a decrease in the generation of NO\({}_{2}\), which in turn reduces the probability of Hg\({}^{0}\) being oxidized by NO\({}_{2}\) (Figure 6) [52]. Secondly, after CO\({}_{2}\) begins to combust, CO is produced, creating a strong reducing atmosphere, leading to the significant inhibition of Hg\({}^{0}\) oxidation. In addition, CO\({}_{2}\) can react with chlorine groups, thus inhibiting the oxidation of mercury by chlorine [53]. Thermodynamic equilibrium calculations have shown that CO\({}_{2}\) primarily reacts with chlorine and forms four substances: CCl\({}_{3}\), CCl\({}_{4}\), COCl, and COCl\({}_{2}\). The molar concentrations of CCl\({}_{3}\) and CCl\({}_{4}\) are at least 10\({}^{13}\) times lower than those of COCl and COCl\({}_{2}\) throughout the entire temperature range (150-1000 \({}^{\circ}\)C). When the temperature is below 650 \({}^{\circ}\)C, the molar concentration of COCl\({}_{2}\) is much lower than that of COCl. Therefore, CO\({}_{2}\) mainly reacts with chlorine to produce COCl and COCl\({}_{2}\), depleting the Cl\({}_{2}\) and Cl in the flue gas, which are crucial for oxidizing elemental mercury.