## 3 Mercury Migration under Coal/Biomass Co-Firing Conditions
### Impacts of Particulate Matters (PM)  
The heterogeneous oxidation of mercury plays a significant role in mercury speciation transformation. Particulate matter in the flue gas, such as fly ash and unburned carbon, provides sufficient reaction sites for the conversion of Hg\({}^{0}\) to Hg\({}^{p}\) and Hg\({}^{2+}\). Metal oxides in the fly ash may act as catalysts in the oxidation reaction of Hg\({}^{0}\). Unburned carbon itself acts as a carbon-based adsorbent, and its abundant surface functional groups provide adsorption sites for Hg\({}^{0}\). However, the impact of the particulate matter in the flue gas on the transformation of mercury speciation under oxy-fuel biomass co-firing conditions is still controversial. In an experimental study by [PERSON] et al. [39], it was found that under coal combustion conditions, over 80% of the mercury in the flue gas is adsorbed onto fine particles, with the remaining mercury primarily captured by fly ash, and less than 5% of the mercury exists in the gaseous phase. However, when 25% biomass is co-fired, the proportion of mercury adsorbed onto fine particles decreases, especially under oxy-fuel conditions. The proportion of gaseous mercury notably increases to over 30% under O\({}_{2}\)/CO\({}_{2}\) atmosphere, and can even reach up to 60% under a 30% oxygen concentration. The proportion of mercury in fly ash is the lowest, showing a slight decrease compared to air conditions. This is attributed to the improved combustion conditions caused by the mixing of biomass, leading to reduced fly ash and unburned carbon, and a decreased concentration of fine particles in the flue gas. As a result, more mercury in the flue gas exists in the gaseous phase. The study conducted by [PERSON] et al. [40] arrived at different conclusions. They investigated the co-firing of dried sawdust sludge with anthracite coal and found that it resulted in the formation of more fine particles in the flue gas (as shown in Figure 4). These  
Figure 3: The heterogeneous oxidation process of the Hg\({}^{0}\) reacted with chlorinated materials [35]. Copyright 2021 Elsevier B.V., Amsterdam, The Netherlands. particles exhibited an ideal porous structure and a sufficient surface area, which had a good adsorption effect on gaseous mercury, leading to more elemental mercury being captured and converted into particulate-phase mercury. The increased fine particles in the fine gas mainly originated from the synergistic effect of biomass and coal co-combustion. The specific process is as follows: The biomass had a higher volatile content, making it easier to ignite. After ignition, the combustion of biomass produced ash gradually. The residual porous biomass particles bonded to the surface of anthracic coal, while the anthracic coal combusted. As the local temperature increased, the combustion of the fixed carbon in the anthracic coal accelerated. Subsequently, due to the release of a large amount of heat from the fixed carbon combustion, the structure of the bonded porous biomass particles was destroyed, forming fine particles. Finally, after the combustion of fixed carbon was completed, the generated fine particles floated into the fine gas. Based on the above research, the more PM, the more favorable the transformation from Hg\({}^{0}\) to Hg\({}^{\text{P}}\), which is beneficial for mercury removal from fine gas. However, there is still controversy over the trend in the PM concentration in blue gas after co-firing biomass. It has been reported that the ash formation mechanisms during coal combustion can be roughly categorized into processes such as the fragmentation of coke and external minerals, the gasification and condensation of internal minerals, and aggregation and coalescence between particles [41]. The generation of ash from biomass combustion also involves these mechanisms, but the main difference compared to coal is the fact that most of the ash formation during biomass combustion occurs through gasification and condensation. Different fuel characteristics and ash compositions are the main reasons.