###### Abstract
lng plants, to prevent similar amalgamation reactions that threaten the safety of lng equipment, the mercury concentration needs to be controlled below 0.01 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)[11]. Therefore, for BECCS systems, in order to prevent the long-term accumulation of mercury on metals, deep mercury removal needs to be achieved before the flue gas enters the decarbonization equipment. The existing mercury emission standards for traditional power plants are inapplicable when aiming to meet the requirements of BECCS systems. The removal difficulty varies for different forms of mercury. Elemental mercury (Hg\({}^{0}\)), with low solubility and high volatility, is the most difficult to remove, while oxidized mercury (Hg\({}^{2+}\)) and particulate mercury (Hg\({}^{p}\)) are relatively easier to remove [12; 13]. Therefore, promoting the transformation of Hg\({}^{0}\) to Hg\({}^{2+}\) and Hg\({}^{p}\) is the key to realizing deep mercury removal. Unlike the conditions in traditional power plants where coal is burned in air, biomass co-firing in O2/CO2 atmosphere introduces a series of new changes in mercury release and transformation in the furnace. The high content of chlorine (CI) and alkali metals (AM) in biomass can impact the distribution of mercury species [14; 15]. Chlorine has a great impact on oxidizing elemental mercury, and the chlorine content in  
Figure 1: Life cycle CO2 emissions of coal/biomass co-firing power plant with and without CCS [3]. Copyright 2018 Elsevier B.V., Amsterdam, The Netherlands. biomass is often a magnitude higher than coal. Biomass also contains a high concentration of alkali metal elements, some of which are released into the flue gas as gas-phase alkali metal salts (AMS\({}_{\text{g}}\)) during combustion, while others remain in the ash as solid-phase alkali metal salts (AMS\({}_{\text{s}}\)) or alkali metal oxides (AMO\({}_{\text{s}}\)). AMS\({}_{\text{g}}\) can interact with other flue gas components, affecting the distribution of mercury species. The surfaces of AMS\({}_{\text{s}}\) or AMO\({}_{\text{s}}\) provide active sites for the heterogeneous reactions of mercury. Additionally, in O\({}_{2}\)/CO\({}_{2}\) combustion flue gas, high concentrations of CO\({}_{2}\) and H\({}_{2}\)O can also impact the oxidation of elemental mercury [16]. It can be seen that, when co-firing biomass under O\({}_{2}\)/CO\({}_{2}\) atmosphere, various components can influence the transformation of mercury species directly or indirectly. Both homogeneous and heterogeneous reactions exist, making it challenging to analyze specific factors quantitatively. While there have been many studies on the reactions between mercury and flue gas components in traditional coal combustion flue gas, the mechanisms of mercury-Cl-AM interactions under oxy-fuel biomass co-firing conditions require further investigation. To meet the requirements for deep mercury removal under oxy-fuel biomass co-firing conditions, in addition to elucidating the mechanisms of the transformation from Hg\({}^{0}\) to Hg\({}^{2+}\) and Hg\({}^{\text{p}}\), it is essential to clarify the kinetic laws of mercury-related reactions and develop high-efficiency mercury removal materials. The main mercury removal methods include adsorption and catalytic oxidation. In the BECCS system, the targeted design of mercury adsorbents and catalysts after considering the effect of various flue gas components on the mercury removal efficiency is the key to obtaining high-performance mercury removal materials. Currently, there is no comprehensive review on the reaction mechanisms of mercury in BECCS systems. The aim of this paper is to clarify the impact mechanisms of chlorine, alkali metals, and other components on the homogeneous and heterogeneous oxidations of mercury under oxy-fuel biomass co-firing conditions. The findings will help ensure the safe operation of BECCS systems, accelerating the low-carbon transformation of power systems, and ultimately promoting the achievement of global carbon reduction goals.