## 2 Mercury in BECCS System
### Hazards of Mercury  
Mercury in the atmosphere originates from the release of anthropogenic mercury sources and natural mercury sources. Regarding anthropogenic mercury sources, the Global Mercury Assessment report released by the United Nations Environment Programme (UNEP) in 2019 indicated that approximately 2220 tons of mercury is emitted into the atmosphere from anthropogenic sources annually, accounting for over half of the total mercury emissions [21]. These data show an increase of around 20% compared to the emissions in 2010. In terms of industry categories, the primary anthropogenic sources of mercury are the metallurgical industry and the combustion of fossil fuels. Mercury emissions from coal combustion make up nearly 88% of all mercury emissions from fossil fuel combustion. Therefore, reducing mercury emissions from the combustion of fossil fuels, especially those from coal combustion, is of significant importance for controlling global mercury pollution. Natural mercury emissions are mainly caused by volcanic activities, oceans, vegetation fires, and geological events [22; 23]. Notably, the mercury emissions generated by vegetation fires indicate that biomass also contains mercury elements, and the emission of mercury from the large-scale burning of biomass should not be underestimated.  
Elemental mercury is sparingly soluble in water, volatile, and chemically stable. It can persist in the environment for extended periods, being deposited in soil or water through global atmospheric circulation [24; 25; 26]. Mercury deposited in soil and water can be released into the atmosphere in the next decades or even centuries, re-entering the mercury cycle and undergoing a series of chemical reactions to form different types of mercury compounds, leading to global mercury contamination. Mercury and its compounds can enter the human body through the skin, respiratory system, and digestive system. Due to its ability to disrupt the structure of enzymes or proteins in the body, mercury can cause varying degrees of harm to human health. Elemental mercury can damage organs such as the liver, kidneys, and brain tissues; inorganic mercury compounds can harm the digestive tract and kidneys; and organic mercury compounds, particularly methylmercury, exhibit the highest toxicity [27]. Consuming fish grown in mercury-contaminated water is the primary route of human exposure to methylmercury. Methylmercury is a neurotoxin that can harm the physiological, neural, and behavioral functions of fish and other wildlife. Methylmercury bioaccumulates, with concentrations increasing up the food chain. As a result, predators at the top of the food chain may have methylmercury concentrations in their tissues that are more than ten million times higher than the levels in the contaminated water bodies.  
It is worth noting that in oxy-fuel biomass co-firing systems, mercury can also form an amalgam with aluminum within the system, leading to corrosion and fractures in the aluminum-made equipment, which is a significant risk factor for the system. As shown in Figure 2, the accumulation of mercury on metal surfaces and the formation of amalgams can result in serious accidents involving the fracture of metal equipment in LNG plants [28; 29]. Due to the higher mercury content in solid fuels such as coal compared to natural gas, the threat posed by mercury to the safe operation of oxy-fuel biomass co-firing systems cannot be ignored. To ensure the long-term safe operation of the system, a more thorough deep mercury removal process must be conducted before the flue gas enters the tail equipment. The limitations on the mercury concentration should be much stricter than those in traditional combustion systems.