## 3 Mercury Migration under Coal/Biomass Co-Firing Conditions  
### Impacts of Chlorine  
Biomass has the advantages of being an abundant resource, generating zero carbon emissions, and being renewable. Research has shown that biomass has a high volatile content, and when co-fired with coal, it can significantly improve the ignition and burnout characteristics. Therefore, co-firing biomass with coal is an effective method to reduce carbon emissions, enhance the energy utilization efficiency, and realize sustainable energy development [30]. However, biomass also contains mercury elements, and in some cases, the mercury content in biomass is higher than that in coal [31]. Biomass combustion has become a significant source of atmospheric mercury, with global annual mercury emissions exceeding 500 tons [32]. Chlorine is the key element involved in mercury oxidation, and it is typically found in much higher concentrations in most biomass than in coal [33]. It is generally believed that the content of chlorine in flue gas plays a decisive role in the oxidation of mercury. [PERSON] et al. studied the impact of chlorine on the homogeneous oxidation of Hg\({}^{0}\) by altering the HCl concentration in the flue gas. It was found that increasing the HCl concentration could raise the temperature at which the gaseous Hg\({}^{0}\) oxidation reaction begins, indicating that Hg\({}^{0}\) can be oxidized to Hg\({}^{2+}\) earlier [34]. The relevant reactions are shown in Equations (1)-(4), with the reaction temperature ranging between 400 and 700 \({}^{\circ}\)C.  
Hg + Cl = HgCl (1) HgCl + HCl = HgCl\({}_{2}\) + H (2) HgCl + Cl = HgCl\({}_{2}\) + Cl (3) HgCl + Cl = HgCl\({}_{2}\) (4)  
In addition to homogeneous oxidation, Cl can also facilitate the oxidation of gaseous Hg\({}^{0}\) through heterogeneous reactions, as shown in Figure 3[35]. Heterogeneous oxidation refers to the Hg\({}^{0}\) oxidation reactions that occur on the surface of adsorbents or catalysts, and the temperature range for these reactions is lower than that of homogeneous oxidation reactions. Currently, three mechanisms can explain the heterogeneous oxidation process of mercury with chlorine. The first mechanism is the Deacon reaction, which involves the catalytic redox reaction of gaseous Hg\({}^{0}\) with chlorine species within 300-400 \({}^{\circ}\)C, with the assistance of a catalyst. In this mechanism, HCl is oxidized to Cl\({}_{2}\), and Hg\({}^{0}\) reacts with Cl\({}_{2}\) to form HgCl and Cl, which further reacts to produce HgCl\({}_{2}\)[36]. The second mechanism is the Langmuir-Hinshelwood mechanism, where gaseous Hg\({}^{0}\) and chlorine adsorb on the surface of the materials and undergo redox reactions in an adsorbed state, leading to the formation of adsorbed Hg\({}^{2+}\), which then desorbs back to gaseous Hg\({}^{2+}\)[37]. The third mechanism is the Eley-Rideal mechanism, where gaseous Hg\({}^{0}\) adsorbs on the materials' surface and reacts with gaseous chlorine species in the flue gas. Some researchers suggest that chlorine species are initially adsorbed on the materials' surface before reacting with  
Figure 2: Fracture of metal equipment caused by Hg accumulation. **(a)** Macroscopic view of a ruptured 5083-0 Al-Mg alloy inlet nozzle of a heat exchanger; **(b)** LME failure on a brazed aluminum heat exchanger [28]. Copyright 2020 Elsevier B.V., Amsterdam, The Netherlands. gaseous Hg\({}^{0}\)[38]. [PERSON] et al. [39] found that when sub-bituminous coal is burned under air conditions, the majority of the mercury species are released in the form of Hg\({}^{0}\) to the gas phase, with Hg\({}^{2+}\) accounting for only about 2%; however, when co-firing sub-bituminous coal and thistle, the proportion of Hg\({}^{2+}\) increases to over 50%, exceeding Hg\({}^{0}\). Under oxy-fuel conditions (O\({}_{2}\) concentration is 30%), after co-firing with thistle, the proportion of Hg\({}^{2+}\) exceeds 80%. This is mainly attributed to the higher concentration of Cl elements in biomass, leading to an increase in the HgCl\({}_{2}\) content.