## Mercury Removal Methods  
### Adsorption  
In order to achieve deep mercury removal in BECCS systems, relying on existing air pollution control devices for mercury removal is insufficient. Specialized mercury removal methods, such as adsorption and catalytic oxidation, must be employed to reduce the mercury concentration in the flue gas. Mercury removal by adsorption is a method that promotes the transformation of gaseous mercury into particulate mercury, which can then be removed by particulate matter control devices. The key to the large-scale application of this method lies in obtaining high-performance mercury adsorbents. Due to its well-developed pore structure, large specific surface area, and abundant surface functional groups, activated carbon has attracted widespread attention in the field of mercury removal. Currently, the most promising technology for mercury removal via adsorption is activated carbon injection (ACI). This technology can achieve an ideal mercury removal efficiency and has been applied in practical boilers. However, according to the U.S. Environmental Protection Agency (EPA) and the Department of Energy (DOE), the operational costs of ACI are very high, costing tens of thousands or even hundreds of thousands of dollars to remove 1 kg of mercury, which is deemed unacceptable for traditional coal-fired power plants. Many researchers have studied the modification of activated carbon to enhance the mercury removal efficiency and reduce the amount of activated carbon used. [PERSON] et al. [68] employed the gas-phase deposition method to modify activated carbon. They found that the activated carbon treated with iodine vapor had a much higher capacity to adsorb elemental mercury compared to activated carbon modified with bromine vapor. [PERSON] et al. [69] used KI for the modification of activated carbon, significantly improving the efficiency of mercury removal. [PERSON] et al. [70] found that although the specific surface area of activated carbon decreased after modification with NH\({}_{4}\)Br, its mercury adsorption capacity significantly increased. [PERSON] et al. [71] studied the stability of mercury adsorbed on FeCl\({}_{3}\)-modified activated carbon and found that the mercury content in the filtrate was far below the safety limits. Considering the high reactivity of sulfur with mercury at room temperature, sulfur-modified activated carbon has also received much attention. Scholars carried out research on the mercury removal mechanisms, modification methods, and influence of flue gas components on the mercury removal efficiency [72; 73; 74; 75; 76; 77; 78; 79; 80; 81]. [PERSON] et al. [82] pretreated activated carbon with nitric acid, enhancing the mercury adsorption capacity. In addition, other modification methods for activated carbon, including alkali metal modification, transition metal oxide modification, and plasma modification, have also been reported [83; 84; 85; 86; 87]. In addition to activated carbon, some researchers have studied the mercury removal efficiency of ore-based adsorbents. However, the mercury removal efficiency of these sorbents is relatively low and thus requires modification. This includes, for example, natural zeolites doped with Ag or CuCl\({}_{2}\)[88; 89; 90]. Furthermore, bentonite or vermiculite can also achieve good mercury removal effects after modification [91]. In coal-fired flue gas, a significant amount of fly ash exists in flue gas. Some scholars have pointed out that unburned carbon and oxygen-containing functional groups in fly ash also have certain mercury removal capabilities [92; 93]. Due to its relatively low cost, fly ash has the potential to be a promising mercury adsorbent. The efficiency of mercury removal using fly ash is influenced by many factors. Its lower temperature, higher unburned carbon content, larger specific surface area, and higher chlorine content lead to an increase in the mercury removal efficiency.