## 3 Mercury Migration under Coal/Biomass Co-Firing Conditions
### Impacts of Alkali Metals  
Biomass contains abundant alkali metal elements, which can impact both the homogeneous and heterogeneous oxidation reactions of mercury. Studies have shown that with increasing temperature, the alkali metals in biomass continuously release from the solid phase to the gas phase. Alkali metals in the gas phase mainly exist as chlorides and hydroxides. During biomass combustion, the chlorides remaining in the solid phase gradually transform into silicates at high temperatures and undergo severe melting above 900 \({}^{\circ}\)C, leading to pore blockage and reduced volatilization rates. The addition of coal effectively alleviates slagging, with a significant decrease in the volatilization rate of potassium (K) due to the formation of high-melting-point stable silicoaluminate salts. Compared to the biomass combustion conditions, the volatilization rate of alkali metals during co-combustion is significantly reduced, with alkali metals mainly existing in sulfates at lower temperatures and transforming into silicoaluminate salts as temperatures rise [46].  
In terms of homogeneous reactions, through thermodynamic equilibrium calculations, scholars have found that in atmospheres containing C/I/O/N/S/Cl/K/Na, within the temperature range in which mercury oxidation is initiated (below 1100 K), the presence of alkali metals reduces HCl and decreases the SO\({}_{2}\) content, which is beneficial for the oxidation of mercury. This trend becomes more pronounced with the addition of biomass. By building the chemical and gas-phase equilibrium models, it was discovered that a higher proportion of biomass co-firing is more favorable for the oxidation of mercury (Figure 5) [47]. Figure 5a shows the results for the Hg/C/I/O/N/S/Cl system, only considering the impact of Cl elements in biomass, while Figure 5b shows the results for the Hg/C/I/O/N/S/Cl/K/Na system, taking into account the influence of both Cl and alkali metals. The results indicate that with a higher proportion of biomass blending and a higher Cl content, the temperature at which Hg\({}^{0}\) converts to HgCl\({}_{2}\) becomes higher. For fuel ratios of 0:1, 1:3, and 1:1, the equilibrium temperatures at which n(Hg)/n(HgCl\({}_{2}\)) = 1:1 are 725, 785, and 831 K, respectively, suggesting a higher starting temperature for Hg oxidation. The introduction of K/Na further strengthens this trend, with equilibrium temperatures of 739, 802, and 840 K for fuel ratios of 0:1, 1:3, and 1:1, respectively, promoting the oxidation of mercury. However, these conclusions are based on theoretical calculations, and experimental research is needed to validate the above results.  
Regarding heterogeneous reactions, as mentioned earlier, the alkali metals in biomass can form fine particles through gasification-condensation, which can adsorb heavy metal elements and facilitate the removal of elemental mercury from the gas phase. Additionally, studies have shown that alkaline components such as iron oxide and calcium oxide in fly ash can react with chlorine-containing components in the flue gas, affecting the oxidation process of elemental mercury indirectly [48]. However, the impact of alkali metal components in fly ash on the efficiency of elemental mercury oxidation is still yet to be revealed.  
Figure 5: Effects of alkali metals on Hg species distributions. (**a**) Mercury oxidation in different flue gases in the Hg/C/H/O/N/S/Cl model; (**b**) Mercury oxidation in different flue gases in the Hg/C/H/O/N/S/Cl/K/Na model [47].  
Overall, there is controversy regarding the mercury species distribution during biomass and coal co-combustion. The influence mechanisms of the high contents of Cl and AM in biomass on the homogeneous and heterogeneous reactions of mercury need to be clarified. Moreover, former studies have been conducted under traditional air combustion conditions, highlighting the need for more comprehensive research on the mercury species transformation mechanisms in biomass co-firing flue gas under oxy-fuel conditions.