## 6 Conclusions and Outlooks  
Biomass co-firing under O\({}_{2}\)/CO\({}_{2}\) atmosphere is a promising BECCS technology and represents a crucial breakthrough point for the low-carbon transformation of the power system. However, the presence of mercury in the flue gas not only threatens ecology and human health, but also can lead to serious safety incidents. Therefore, conventional mercury removal methods are inadequate to address the urgent need for deep mercury removal technologies tailored to BECCS systems. Transforming elemental mercury into oxidized mercury and particulate mercury is the core strategy for controlling mercury emissions. However, compared to traditional coal combustion under air conditions, BECCS systems have distinct flue gas compositions. Understanding the influence of each component on the mercury species distribution is essential in advanced mercury removal technologies. Chlorine is an important reactant in mercury oxidation reactions. Biomass contains high levels of chlorine. Chlorine species in the flue gas can undergo both homogeneous and heterogeneous reactions to convert Hg\({}^{0}\) into HgCl\({}_{2}\). This is beneficial for achieving the goal of deep mercury removal, especially for the boilers burning low-chlorine coal. After introducing biomass, the concentrations of ash and unburned carbon in the flue gas decrease, while the concentration of fine particulate matter may increase due to processes such as fracturing and gasification-condensation. These particulate matters have a large specific surface area, on which mercury may accumulate. Biomass contains abundant alkali metals. Thermodynamic calculations in homogeneous systems suggest that elements such as K/Na can promote the oxidation of mercury. Furthermore, alkaline components adsorbed onto fly ash can affect the mercury species distribution through heterogeneous reactions. In oxy-fuel flue gas, high concentrations of CO\({}_{2}\) can create a reducing atmosphere, inhibit the formation of NO\({}_{2}\), consume Cl groups, and have a negative effect on mercury oxidation. However, some scholars also point out that CO\({}_{2}\) may slightly promote the generation of free oxygen atoms. In addition, H\({}_{2}\)O in oxy-fuel flue gas may also inhibit the oxidation of mercury. The concentrations of NO\({}_{2}\) and SO\({}_{2}\) in oxy-fuel biomass co-firing flue gas are lower than those in traditional coal combustion flue gas. Nevertheless, considering the complex effects of NO\({}_{x}\) and SO\({}_{2}\) on mercury oxidation, their impact on the mercury species distribution should not be overlooked. The concentration of fly ash in oxy-fuel flue gas is higher than that under air conditions, which is advantageous for the removal of elemental mercury. Under the oxy-fuel atmosphere, attention should be paid to the re-emission of mercury in the WFGD system. High concentrations of CO\({}_{2}\) can reduce the pH and redox potential of gypsum slurry, thereby inhibiting the re-emission of Hg\({}^{0}\). Metal ions such as Fe\({}^{2+}\) can also inhibit the re-emission of mercury, and the inhibitory effect increases with the CO\({}_{2}\) concentration. In comparison, the effect of Fe\({}^{2+}\) on the mercury re-emission under oxy-fuel conditions is more pronounced than that under air conditions. Relying on traditional air pollution control devices, such as SCR, WFGD and PMCD, for simultaneous mercury removal makes it almost impossible to meet the mercury concentration requirements for BECCS systems. Specialized deep mercury removal methods such as adsorption and catalytic oxidation must be employed. The adsorption method has the advantages of simple equipment and convenient operation. However, issues relating to the generation of solid waste pollution from the used adsorbent, materials recycling and regeneration are worth further study. The loading of various metals can achieve the protection of catalytic active sites and prevent the influence of competing components.