###### Abstract  
Combining biomass co-firing with oxy-fuel combustion is a promising Bioenergy with Carbon Capture and Storage (BECCS) technology. It has the potential to achieve a large-scale reduction in carbon emissions from traditional power plants, making it a powerful tool for addressing global climate change. However, mercury in the fuel can be released into the flue gas during combustion, posing a significant threat to the environment and human health. More importantly, mercury can also cause the fracture of metal equipment via amalgamation, which is a major risk for the system. Therefore, compared to conventional coal-fired power plants, the requirements for the mercury concentration in BECCS systems are much stricter. This article reviews the latest progress in mercury control under oxy-fuel biomass co-firing conditions, clarifies the impact of biomass co-firing on mercury species transformation, reveals the influence mechanisms of various flue gas components on elemental mercury oxidation under oxy-fuel combustion conditions, evaluates the advantages and disadvantages of various mercury removal methods, and finally provides an outlook for mercury control in BECCS systems. Research shows that after biomass co-firing, the concentrations of chlorine and alkali metals in the flue gas increase, which is beneficial for homogeneous and heterogeneous mercury oxidation. The changes in the particulate matter content could affect the transformation of gaseous mercury to particulate mercury. The high concentrations of CO\({}_{2}\) and H\({}_{2}\)O in oxy-fuel flue gas inhibit mercury oxidation, while the effects of NO\({}_{x}\) and SO\({}_{2}\) are dual-sided. Higher concentrations of fly ash in oxy-fuel flue gas are conducive to the removal of Hg\({}^{0}\). Additionally, under oxy-fuel conditions, CO\({}_{2}\) and metal ions such as Fe\({}^{2+}\) can inhibit the re-emission of mercury in WFGD systems. The development of efficient adsorbents and catalysts is the key to achieving deep mercury removal. Fully utilizing the advantages of chlorine, alkali metals, and CO\({}_{2}\) in oxy-fuel biomass co-firing flue gas will be the future focus of deep mercury removal from BECCS systems. mercury; biomass; oxy-fuel combustion; chlorine; alkali metal +
Footnote †: journal: Applied Sciences  
flue gas, through carbon capture and storage (CCS) technologies, to prevent the release of CO2 into the atmosphere. Considering the limited potential for improving efficiency, it is difficult to realize large-scale carbon reduction using the first method. By combining the second and the third methods, theoretically, negative carbon combustion can be achieved, which is attractive for countries that have set carbon neutrality goals. Among the zero-carbon fuels, biomass offers advantages such as a low cost, wide availability, and safety compared to hydrogen or ammonia, making it a promising option for application. O2/CO2 combustion can increase the concentration of CO2 in flue gas, leading to a notable reduction in carbon capture costs. The Intergovernmental Panel on Climate Change (IPCC) has suggested that the large-scale deployment of Bioenergy with Carbon Capture and Storage (BECCS) systems is a crucial means of achieving carbon reduction [2]. The combination of biomass co-firing and O2/CO2 combustion is one of the most promising BECCS technologies (seen in Figure 1) [3]. For BECCS systems, mercury not only poses threats to the ecological environment and human health, but also can lead to metal amalgamation, causing serious safety accidents [4]. Mercury is well known to have harmful effects on human health and the environment [5; 6; 7]. Globally, the current mercury emission standards for coal-fired power plants are mostly in the range of 5-30 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)[8; 9; 10]. Currently, although there are no specific standards regarding the mercury concentration in flue gas for BECCS systems, the mercury concentration requirements in lng plants can serve as a reference.