Review on Mercury Control during Co-Firing Coal and Biomass under O₂/CO₂ Atmosphere

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 CO₂ and H₂O in oxy-fuel flue gas inhibit mercury oxidation, while the effects of NO_x and SO₂ are dual-sided. Higher concentrations of fly ash in oxy-fuel flue gas are conducive to the removal of Hg⁰. Additionally, under oxy-fuel conditions, CO₂ and metal ions such as Fe²⁺ 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 CO₂ in oxy-fuel biomass co-firing flue gas will be the future focus of deep mercury removal from BECCS systems.

1. Introduction

Since the Industrial Revolution, the CO₂ released from fossil fuel combustion has caused the global greenhouse effect and serious damage to the ecological environment. Carbon reduction is a critical issue in the field of energy and environment for countries around the world. As electricity production still heavily relies on the combustion of fossil fuels, the carbon emissions from the power industry account for the largest proportion of total carbon emissions. The low-carbon transformation of the power system is of great significance for achieving global carbon reduction goals [1].

There are three main methods used to control carbon emissions from the power system. The first involves improving the efficiency of the system to reduce energy consumption and thus decrease the input of fuels. The second involves using zero-carbon fuels to replace traditional fossil fuels in combustion reactions. The third involves capturing CO₂ from the 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 μg/m³ [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. In LNG plants, to prevent similar amalgamation reactions that threaten the safety of LNG equipment, the mercury concentration needs to be controlled below 0.01 μg/m³ [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 (Hg⁰), with low solubility and high volatility, is the most difficult to remove, while oxidized mercury (Hg²⁺) and particulate mercury (Hg^p) are relatively easier to remove [12,13]. Therefore, promoting the transformation of Hg⁰ to Hg²⁺ 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 (Cl) 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 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_g) during combustion, while others remain in the ash as solid-phase alkali metal salts (AMS_s) or alkali metal oxides (AMO_s). AMS_g can interact with other flue gas components, affecting the distribution of mercury species. The surfaces of AMS_s or AMO_s provide active sites for the heterogeneous reactions of mercury. Additionally, in O₂/CO₂ combustion flue gas, high concentrations of CO₂ and H₂O can also impact the oxidation of elemental mercury [16]. It can be seen that, when co-firing biomass under O₂/CO₂ 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 Hg⁰ to Hg²⁺ and Hg^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.

2. Mercury in BECCS System

2.1. BECCS System

Biomass is a promising zero-carbon fuel [17]. Unlike other zero-carbon fuels, biomass molecules contain carbon atoms, and the combustion of biomass does produce CO₂. However, the carbon atoms in biomass are absorbed from the atmosphere during its growth and converted through photosynthesis. Therefore, from a lifecycle perspective, biomass can be considered as a zero-carbon fuel, and the CO₂ generated from its combustion should not be included in carbon emission calculations. Biomass and coal are both solid fuels with similar combustion processes. Co-firing biomass in traditional coal-fired boilers is feasible and can reduce carbon emissions from power plants.

Oxy-fuel combustion is an important method for carbon capture. In an oxy-fuel combustion system, air is no longer introduced into the furnace. Instead, oxygen is mixed with recycled flue gas (primarily CO₂ and H₂O) and fed into the furnace to react with the fuel [18]. As a result, N₂, which makes up the majority of air, is no longer present in the flue gas. The oxy-fuel combustion flue gas mainly consists of CO₂ and H₂O. After removing H₂O through condensation, a high concentration of CO₂ is left, with some other impurities. Therefore, compared to carbon capture under traditional air combustion conditions, the cost of separating and purifying CO₂ from flue gas in oxy-fuel combustion is significantly reduced [19]. Furthermore, the absence of N₂ in the flue gas of oxy-fuel combustion inhibits the formation of thermal NO_x [20]. In summary, oxy-fuel combustion can drive the widespread application of carbon capture technology.

Biomass co-firing and oxy-fuel combustion both contribute to reducing CO₂ emissions. Combining these two technologies can enhance carbon reduction dramatically. With a high proportion of biomass blending, negative carbon emissions can even be achieved. Additionally, retrofitting traditional coal-fired units with biomass co-firing and oxy-fuel combustion has a relatively low difficulty and cost. Therefore, biomass co-firing under O₂/CO₂ atmosphere plays a significant role in achieving global carbon reduction goals.

2.2. 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 re-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.

3. Mercury Migration under Coal/Biomass Co-Firing Conditions

3.1. Impacts of Chlorine

Biomass has the advantages of being an abundant resource, generating zero carbon emissions, and being renewable. Research has shown that biomass has a high volatile content, and when co-fired with coal, it can significantly improve the ignition and burnout characteristics. Therefore, co-firing biomass with coal is an effective method to reduce carbon emissions, enhance the energy utilization efficiency, and realize sustainable energy development [30]. However, biomass also contains mercury elements, and in some cases, the mercury content in biomass is higher than that in coal [31]. Biomass combustion has become a significant source of atmospheric mercury, with global annual mercury emissions exceeding 500 tons [32].

Chlorine is the key element involved in mercury oxidation, and it is typically found in much higher concentrations in most biomass than in coal [33]. It is generally believed that the content of chlorine in flue gas plays a decisive role in the oxidation of mercury. Sliger et al. studied the impact of chlorine on the homogeneous oxidation of Hg⁰ by altering the HCl concentration in the flue gas. It was found that increasing the HCl concentration could raise the temperature at which the gaseous Hg⁰ oxidation reaction begins, indicating that Hg⁰ can be oxidized to Hg²⁺ earlier [34]. The relevant reactions are shown in Equations (1)–(4), with the reaction temperature ranging between 400 and 700 °C.

Hg + Cl = HgCl

(1)

HgCl + HCl = HgCl₂ + H

(2)

HgCl + Cl₂ = HgCl₂ + Cl

(3)

HgCl + Cl = HgCl₂

(4)

In addition to homogeneous oxidation, Cl can also facilitate the oxidation of gaseous Hg⁰ through heterogeneous reactions, as shown in Figure 3 [35]. Heterogeneous oxidation refers to the Hg⁰ oxidation reactions that occur on the surface of adsorbents or catalysts, and the temperature range for these reactions is lower than that of homogeneous oxidation reactions. Currently, three mechanisms can explain the heterogeneous oxidation process of mercury with chlorine. The first mechanism is the Deacon reaction, which involves the catalytic redox reaction of gaseous Hg⁰ with chlorine species within 300–400 °C, with the assistance of a catalyst. In this mechanism, HCl is oxidized to Cl₂, and Hg⁰ reacts with Cl₂ to form HgCl and Cl, which further reacts to produce HgCl₂ [36]. The second mechanism is the Langmuir–Hinshelwood mechanism, where gaseous Hg⁰ and chlorine adsorb on the surface of the materials and undergo redox reactions in an adsorbed state, leading to the formation of adsorbed Hg²⁺, which then desorbs back to gaseous Hg²⁺ [37]. The third mechanism is the Eley–Rideal mechanism, where gaseous Hg⁰ adsorbs on the materials’ surface and reacts with gaseous chlorine species in the flue gas. Some researchers suggest that chlorine species are initially adsorbed on the materials’ surface before reacting with gaseous Hg⁰ [38]. Contreras et al. [39] found that when sub-bituminous coal is burned under air conditions, the majority of the mercury species are released in the form of Hg⁰ to the gas phase, with Hg²⁺ accounting for only about 2%; however, when co-firing sub-bituminous coal and thistle, the proportion of Hg²⁺ increases to over 50%, exceeding Hg⁰. Under oxy-fuel conditions (O₂ concentration is 30%), after co-firing with thistle, the proportion of Hg²⁺ exceeds 80%. This is mainly attributed to the higher concentration of Cl elements in biomass, leading to an increase in the HgCl₂ content.

In general, compared to coal combustion conditions, especially when using low-chlorine coal, co-firing biomass is likely to increase the concentration of chlorine species in the flue gas. Considering the strong promoting effect of chlorine on the oxidation and removal of Hg⁰, biomass blending is advantageous for achieving the goal of deep mercury removal.

3.2. 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 Hg⁰ to Hg^p and Hg²⁺. Metal oxides in the fly ash may act as catalysts in the oxidation reaction of Hg⁰. Unburned carbon itself acts as a carbon-based adsorbent, and its abundant surface functional groups provide adsorption sites for Hg⁰. 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 Contreras 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 O₂/CO₂ 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 Hao 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 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 flue 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 anthracite coal, while the anthracite coal combusted. As the local temperature increased, the combustion of the fixed carbon in the anthracite 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 flue gas.

Based on the above research, the more PM, the more favorable the transformation from Hg⁰ to Hg^p, which is beneficial for mercury removal from flue gas. However, there is still controversy over the trend in the PM concentration in flue 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. During biomass combustion, a large amount of alkali metals and chlorides volatilize to form gaseous components such as HCl(g), KCl(g), KOH(g), and NaCl(g) [42]. When this part of the flue gas cools down, these gaseous intermediates are highly prone to condensation, promoting the formation of fine particles [43]. During the co-firing of coal and biomass, in addition to focusing on the different ash formation mechanisms of the two fuels, the synergistic effects that may occur during the co-combustion of blended fuels should not be overlooked, as they may also lead to the formation of a large amount of PM. Overall, finer particles have a larger surface area, explaining why the smaller particles formed after biomass combustion contain higher levels of heavy metals and exhibit greater toxicity [44,45].

3.3. 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 °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 SO₂ 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 Hg⁰ converts to HgCl₂ becomes higher. For fuel ratios of 0:1, 1:3, and 1:1, the equilibrium temperatures at which n(Hg)/n(HgCl₂) = 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.

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.

4. Mercury Migration under O₂/CO₂ Atmosphere

4.1. Impacts of CO₂

In oxy-fuel combustion technology, oxygen is mixed with recirculated flue gas in a certain proportion to replace air as the combustion oxidizer [49]. Due to the change in the combustion atmosphere, the distribution of mercury species inside the furnace under oxy-fuel conditions differs from that in traditional air combustion. One of the most significant characteristics of the oxy-fuel combustion atmosphere is the high concentration of CO₂ in the flue gas. Therefore, it is essential to understand the impact of CO₂ on mercury oxidation.

Research has shown that there are differences in the release characteristics of Hg⁰ at different temperatures when the combustion atmosphere switches from air to O₂/CO₂ [50]. Between 250 and 400 °C, the concentration of Hg⁰ released in the air atmosphere is higher than in the O₂/CO₂ atmosphere. This may be attributed to the higher specific heat capacity of CO₂, causing a delay in the release of volatiles, volatile combustion and char combustion in the O₂/CO₂ atmosphere compared to the air atmosphere. Certain gas components that precipitate earlier in the air atmosphere may inhibit the oxidation of Hg⁰ [51]. In the temperature range of 400–600 °C, the concentration of released Hg⁰ in the O₂/CO₂ atmosphere is slightly higher than in the air atmosphere. This might be due to the high concentration of CO₂ in the flue gas. Under O₂/CO₂ combustion conditions, the high concentration of CO₂ leads to the production of a significant amount of CO through reduction reactions with carbon during combustion initiation. This results in the much higher generation of CO compared to that in the air atmosphere, creating a stronger reducing atmosphere that reduces the proportion of oxidized Hg⁰, leading to a slightly higher release of Hg⁰ compared to the air atmosphere.

For the O₂/CO₂ atmosphere, different concentrations of CO₂ can also impact the oxidation of Hg⁰. Experimental results have shown that when the CO₂ concentration is below 30%, the influence of CO₂ on the species distribution of mercury is negligible. However, when the CO₂ concentration increases from 30% to 50%, there is an increase in the release concentration of gaseous mercury, indicating that high concentrations of CO₂ can inhibit the oxidation of mercury. This could be explained by several factors. Firstly, the high concentration of CO₂ can reduce the conversion rate of N in the fuel to NO and promote the reduction of NO, leading to a decrease in the generation of NO₂, which in turn reduces the probability of Hg⁰ being oxidized by NO₂ (Figure 6) [52]. Secondly, after CO₂ begins to combust, CO is produced, creating a strong reducing atmosphere, leading to the significant inhibition of Hg⁰ oxidation. In addition, CO₂ can react with chlorine groups, thus inhibiting the oxidation of mercury by chlorine [53]. Thermodynamic equilibrium calculations have shown that CO₂ primarily reacts with chlorine and forms four substances: CCl₃, CCl₄, COCl, and COCl₂. The molar concentrations of CCl₃ and CCl₄ are at least 10¹³ times lower than those of COCl and COCl₂ throughout the entire temperature range (150–1000 °C). When the temperature is below 650 °C, the molar concentration of COCl₂ is much lower than that of COCl. Therefore, CO₂ mainly reacts with chlorine to produce COCl and COCl₂, depleting the Cl₂ and Cl in the flue gas, which are crucial for oxidizing elemental mercury. The reduction in the concentration of Cl₂ and Cl leads to a decrease in HgCl₂, which enhances the inhibitory effect of CO₂ on mercury oxidation.

However, some researchers have found different results. Studies have shown that the oxidation rate of mercury is slightly higher in simulated flue gas containing CO₂ compared to conditions without CO₂. This could be related to the ability of CO₂ to promote the generation of more free O atoms, C-O and C=O functional groups [54].

In general, the impact of CO₂ on mercury oxidation has dual-sided effects. The reducing atmosphere created by high concentrations of CO₂ may hinder the oxidation of mercury, and CO₂ may inhibit the oxidation of elemental mercury by consuming Cl and reducing NO₂. However, CO₂ may also increase the active sites on adsorbents or catalyst surfaces, promoting the oxidation of Hg⁰.

4.2. Impacts of Other Flue Gas Components

In addition to CO₂, the impact of other flue gas components in oxy-fuel combustion on Hg oxidation is also worth considering. Due to the introduction of recycled flue gas, flue gas in the oxy-fuel system contains a higher concentration of H₂O, and its effect on Hg oxidation has not been conclusively determined. Some literature suggests that even in the presence of chlorine species, a high concentration of H₂O can inhibit the oxidation of mercury [55]. However, de las Obras-Loscertales et al. [56] have indicated that the impact of water vapor in oxy-fuel flue gas on the species distribution of mercury is slight.

In simulated flue gas containing NO, CO₂, and O₂, NO can promote the oxidation of Hg⁰ [52]. This promoting effect can largely be attributed to the formation of NO₂ because NO₂ has strong oxidizing properties. NO₂ can react with the adsorbed elemental mercury on the adsorbent surface, as well as react with gaseous elemental mercury in the simulated flue gas. The reaction processes belong to the Langmuir–Hinshelwood mechanism and the Eley–Rideal mechanism, respectively [13]. According to the literature, NO may compete for adsorption sites with gaseous Hg⁰, resulting in an inhibitory effect on the removal of Hg⁰ from the flue gas [57]. However, this phenomenon only occurs when there is no O₂. For oxy-fuel combustion, the competitive adsorption between NO and Hg can be neglected.

In terms of SO₂, studies have indicated that SO₂ could limit the oxidation of mercury. On the one hand, SO₂ can consume the O and OH radicals that are involved in the Hg⁰ oxidation reactions [58]. On the other hand, the competitive adsorption between SO₂ and Hg⁰ can inhibit the oxidation and removal of Hg⁰ [59]. Furthermore, the introduction of SO₂ can alter the composition of the surface functional groups of the Hg⁰ oxidation catalyst, leading to the irreversible inhibition of mercury removal by the catalyst [60]. The mercury removal efficiency of the catalyst will not recover with a decreased SO₂ concentration. Although some studies suggest that part of SO₂ can react with O₂ to form SO₃, which then undergoes a redox reaction with Hg0 to produce HgSO₄ and HgO [52,61], the oxidation rate of Hg⁰ in an O₂-SO₂ atmosphere is very limited [58]. The mercury removal efficiency of the catalyst will not recover with a decreased SO₂ concentration.

Studies have indicated that the concentration of fly ash is higher under oxy-fuel conditions, which is favorable for the formation of particulate mercury [62]. Similarly, Stam et al. found that trace elements tend to accumulate in oxy-fuel combustion flue gas due to flue gas recirculation [63].

4.3. Hg Re-Emission from WFGD

Due to the high solubility of oxidized mercury, Hg²⁺ is easily captured by desulfurization slurries and desulfurization wastewater in Wet Flue Gas Desulfurization (WFGD) systems, achieving mercury removal from flue gas. However, in WFGD systems, Hg²⁺ can be reduced under certain conditions and released back into the flue gas in the form of Hg⁰, adversely affecting the mercury removal efficiency [64,65]. In oxy-fuel combustion systems in particular, stricter limitations are imposed on mercury concentrations. The re-emission of Hg⁰ may lead to a failure to meet the requirements for deep mercury removal, threatening the safe operation of the system. Therefore, it is necessary to conduct research on the re-emission of Hg⁰ in WFGD systems under oxy-fuel conditions.

Ochoa-González et al. [66] studied the influence of the main components (CO₂, O₂, and H₂O) in oxy-fuel combustion flue gas on the re-emission of Hg⁰ in the WFGD system. The concentrations of O₂ and CO₂ have a significant impact on the re-emission of mercury, while the influence of H₂O can be negligible. Due to the reaction between O₂ and metals in a low oxidation state, high concentrations of O₂ inhibit the re-emission of Hg⁰. When metal impurities in limestone are the main factors causing the Hg⁰ re-emission, the stabilizing effect of O₂ on mercury in the slurry is more obvious. High concentrations of CO₂ lead to a decrease in the pH of the slurry and affect the redox potential, thereby inhibiting Hg⁰ re-emission.

Xu et al. [49,67] studied the effects of O₂, CO₂, and metal elements such as Fe, Mn, and Ni on the re-emission of Hg⁰. The results show that adding O₂ in a pure N₂ atmosphere significantly enhances the re-emission of Hg⁰ due to the decrease in pH. However, adding 75% CO₂ in a N₂ atmosphere limits the re-emission of Hg⁰ due to a slight decrease in pH. Under oxy-fuel conditions, increasing the O₂ concentration from 3% to 6% leads to a slight decrease in the re-emission of Hg⁰. Further increasing the O₂ concentration to 12% does not have a significant influence on the Hg⁰ re-emission. Higher CO₂ concentrations are unfavorable for Hg⁰ re-emission, as more mercury remains in the solid phase. In both air-fuel and oxy-fuel atmospheres, Hg⁰ re-emission is inhibited by the metal ions. Fe²⁺ could transform to the Fe(OH)₂ precipitate and the Fe(OH)₃ precipitate. Both Fe(OH)₂ and Fe(OH)₃ precipitates can adsorb mercury species. Hence, the re-emission of mercury from WFGD systems is inhibited. The negative effect of Fe²⁺ increases with a higher CO₂ concentration (Figure 7).

In general, under oxy-fuel conditions, the transformation of mercury species and the re-emission of Hg⁰ in WFGD systems differ from those under air conditions. For oxy-fuel biomass co-firing systems, the impacts of both biomass blending and O₂/CO₂ atmosphere need to be considered simultaneously.

5. Mercury Removal Methods

5.1. Adsorption

In order to achieve deep mercury removal in BECCS systems, relying on existing air pollution control devices for mercury removal is insufficient. Specialized mercury removal methods, such as adsorption and catalytic oxidation, must be employed to reduce the mercury concentration in the flue gas.

Mercury removal by adsorption is a method that promotes the transformation of gaseous mercury into particulate mercury, which can then be removed by particulate matter control devices. The key to the large-scale application of this method lies in obtaining high-performance mercury adsorbents. Due to its well-developed pore structure, large specific surface area, and abundant surface functional groups, activated carbon has attracted widespread attention in the field of mercury removal. Currently, the most promising technology for mercury removal via adsorption is activated carbon injection (ACI). This technology can achieve an ideal mercury removal efficiency and has been applied in practical boilers. However, according to the U.S. Environmental Protection Agency (EPA) and the Department of Energy (DOE), the operational costs of ACI are very high, costing tens of thousands or even hundreds of thousands of dollars to remove 1 kg of mercury, which is deemed unacceptable for traditional coal-fired power plants. Many researchers have studied the modification of activated carbon to enhance the mercury removal efficiency and reduce the amount of activated carbon used. Cai et al. [68] employed the gas-phase deposition method to modify activated carbon. They found that the activated carbon treated with iodine vapor had a much higher capacity to adsorb elemental mercury compared to activated carbon modified with bromine vapor. Tong et al. [69] used KI for the modification of activated carbon, significantly improving the efficiency of mercury removal. Li et al. [70] found that although the specific surface area of activated carbon decreased after modification with NH₄Br, its mercury adsorption capacity significantly increased. Luo et al. [71] studied the stability of mercury adsorbed on FeCl₃-modified activated carbon and found that the mercury content in the filtrate was far below the safety limits. Considering the high reactivity of sulfur with mercury at room temperature, sulfur-modified activated carbon has also received much attention. Scholars carried out research on the mercury removal mechanisms, modification methods, and influence of flue gas components on the mercury removal efficiency [72,73,74,75,76,77,78,79,80,81]. Sun et al. [82] pretreated activated carbon with nitric acid, enhancing the mercury adsorption capacity. In addition, other modification methods for activated carbon, including alkali metal modification, transition metal oxide modification, and plasma modification, have also been reported [83,84,85,86,87].

In addition to activated carbon, some researchers have studied the mercury removal efficiency of ore-based adsorbents. However, the mercury removal efficiency of these sorbents is relatively low and thus requires modification. This includes, for example, natural zeolites doped with Ag or CuCl₂ [88,89,90]. Furthermore, bentonite or vermiculite can also achieve good mercury removal effects after modification [91]. In coal-fired flue gas, a significant amount of fly ash exists in flue gas. Some scholars have pointed out that unburned carbon and oxygen-containing functional groups in fly ash also have certain mercury removal capabilities [92,93]. Due to its relatively low cost, fly ash has the potential to be a promising mercury adsorbent. The efficiency of mercury removal using fly ash is influenced by many factors. Its lower temperature, higher unburned carbon content, larger specific surface area, and higher chlorine content lead to an increase in the mercury removal efficiency. In addition, acidic gases in flue gas, such as HCl, H₂SO₄, NO₂, have a promoting effect on the mercury removal using fly ash [94]. To further improve the mercury removal efficiency, researchers have conducted studies on the modification of fly ash. Xu et al. [95] achieved a high mercury removal efficiency by treating fly ash with CuBr₂, CuCl₂, and FeCl₃. Diao et al. [96] modified fly ash with calcium oxide and found that when the mass ratio of calcium oxide was 33%, the modified fly ash achieved the highest mercury removal efficiency.

Noble metals such as palladium (Pd), rhodium (Rh), iridium (Ir), gold (Au), and silver (Ag) have the potential to remove mercury. Hou et al. [97] prepared Pb-Al₂O₃ adsorbents via the impregnation method and tested their Hg⁰ removal efficiency at high temperatures. This mercury removal ability should be mostly attributed to the formation of a Pb-Hg amalgam on its surface. Gold is highly stable and an Au-Hg amalgam is also difficult to decompose at high temperatures. Researchers added Au and Cl to activated carbon to prepare high-performance mercury adsorbents [98]. Shirkhanloo et al. [99] loaded nano-Ag particles on small glass beads for the removal of elemental mercury (Figure 8). The results showed that this material had the highest mercury removal efficiency at 60 °C, while mercury desorbed from the adsorbent at 245 °C.

In fact, the key to mercury removal via adsorption is to convert gaseous mercury into particulate mercury. The development of high-efficiency and low-cost mercury adsorbents is essential to ensure the widespread application of this method. The ultimate goal of all the related research on mercury adsorbents is to reduce the operational costs of the adsorbents. However, the high cost of modified activated carbon and modified noble metal materials limits their effectiveness in reducing the operational costs. Additionally, the efficiency of ore-based adsorbents and fly ash for mercury removal is relatively low and susceptible to various factors, especially the chlorine content, which directly determines the actual mercury removal efficiency of these adsorbents. Therefore, the search for high-efficiency, stable, and relatively low-cost mercury adsorbents remains the main bottleneck restricting the application of adsorption-based mercury removal technology.

5.2. Catalytic Oxidation

Some transition metals or transition metal oxides can serve as catalysts to promote the oxidation of elemental mercury. Studies indicate that iron (Fe)-based materials can achieve a good Hg⁰ oxidation efficiency [100,101,102]. However, some scholars have pointed out that when bituminous coal is used, Fe-based materials exhibit a lower Hg⁰ oxidation efficiency, and the concentrations of the flue gas components can also have an impact [103,104,105]. Kong et al. [106] prepared a novel Fe-based nanomaterial and tested its mercury removal performance in simulated flue gas. The results showed that the nano-Fe-Si material prepared by the hydrothermal method had an excellent mercury removal efficiency and adsorption capacity. The reaction occurring on the material surface was the rate-limiting step. NO and HCl were beneficial for the Hg⁰ removal, while SO₂ inhibited the mercury removal efficiency. Yang et al. [107] used hematite to prepare a series of magnetic Fe-based spinel materials, which could be easily recycled and have the potential to be used in industrial applications.

Manganese (Mn) exhibits various oxidation states, with a valence ranging from +2 to +6, meaning that manganese oxides possess active lattice oxygen [108]. Compared to other catalysts, Mn-based catalysts can achieve high Hg⁰ oxidation efficiency at lower temperatures [109,110,111]. Studies have shown that Mn-based catalysts exhibit good catalytic effects at temperatures below 170 °C [112]. Mixing MnO₂ with Ca(OH)₂ or Al₂O₃ can catalyze the Hg0 oxidation, but high concentrations of SO₂ could inhibit the mercury oxidation reactions [113,114]. To address this, some scholars have proposed the addition of Mo to prevent the sulfur poisoning of Mn-based catalysts [115]. Scala et al. [108] achieved a good adsorption performance by adding manganese oxides to alumina and vermiculite. Xu et al. [116] used a wet impregnation method to add Mn to commercial magnesite, creating a novel adsorbent with high Hg removal efficiency at low temperatures. Ji et al. [117] incorporated manganese oxides into TiO₂, achieving high mercury removal efficiency. The characterization results indicated that the Mn⁴⁺ and lattice oxygen in oxides play a crucial role in the oxidation of elemental mercury.

Copper (Cu) is a relatively low-cost transition metal element that can switch between the +1 and +2 oxidation states. Copper oxides possess oxygen storage capacity and exhibit good catalytic oxidation abilities [118]. Kim et al. [119] coated porous carbon materials with Cu using a chemical deposition method. The results showed that loading Cu onto the materials led to a significant improvement in the mercury removal efficiency, despite the decrease in the specific surface area and total pore volume (Figure 9). Du et al. [120] prepared CuO_x-Al₂O₃ materials, achieving good mercury removal effects. When mixed with activated carbon in a certain proportion, it can help reduce the operational costs of ACI. Zhao et al. [121] modified activated coke with copper oxides, demonstrating that the modification altered the surface functional groups of the activated coke and enhanced its mercury removal capacity. Cobalt (Co) is a common catalyst component with strong catalytic oxidation abilities. Scholars have combined cobalt oxides with TiO₂ or Al₂O₃, obtaining a favorable mercury removal performance [122,123]. To further enhance the mercury removal and sulfur resistance capabilities of catalysts, researchers have mixed various metal oxides, including Mn, Cu, Co and Al, and studied the synergistic effects on their catalytic oxidation performance [124].

It is worth noting that the Cl and AM in biomass can be utilized for the modification of adsorbents or catalysts to enhance the mercury removal performance. Additionally, the high concentration of CO₂ in oxy-fuel combustion may generate more active sites on the material surface, thereby improving the mercury removal efficiency [53]. Fully utilizing these characteristics is vital for removing the gaseous mercury from BECCS systems. The decoupling of mercury-related homogeneous and heterogeneous reactions under oxy-fuel biomass co-firing conditions, the elucidation of the influences of Cl, AM, and CO₂ on the distribution of mercury species, and the construction of a comprehensive mercury reaction kinetics model will provide theoretical guidance for deep mercury removal in BECCS systems.

6. Conclusions and Outlooks

Biomass co-firing under O₂/CO₂ atmosphere is a promising BECCS technology and represents a crucial breakthrough point for the low-carbon transformation of the power system. However, the presence of mercury in the flue gas not only threatens ecology and human health, but also can lead to serious safety incidents. Therefore, conventional mercury removal methods are inadequate to address the urgent need for deep mercury removal technologies tailored to BECCS systems.

Transforming elemental mercury into oxidized mercury and particulate mercury is the core strategy for controlling mercury emissions. However, compared to traditional coal combustion under air conditions, BECCS systems have distinct flue gas compositions. Understanding the influence of each component on the mercury species distribution is essential in advanced mercury removal technologies.

Chlorine is an important reactant in mercury oxidation reactions. Biomass contains high levels of chlorine. Chlorine species in the flue gas can undergo both homogeneous and heterogeneous reactions to convert Hg⁰ into HgCl₂. This is beneficial for achieving the goal of deep mercury removal, especially for the boilers burning low-chlorine coal. After introducing biomass, the concentrations of ash and unburned carbon in the flue gas decrease, while the concentration of fine particulate matter may increase due to processes such as fracturing and gasification–condensation. These particulate matters have a large specific surface area, on which mercury may accumulate. Biomass contains abundant alkali metals. Thermodynamic calculations in homogeneous systems suggest that elements such as K/Na can promote the oxidation of mercury. Furthermore, alkaline components adsorbed onto fly ash can affect the mercury species distribution through heterogeneous reactions.

In oxy-fuel flue gas, high concentrations of CO₂ can create a reducing atmosphere, inhibit the formation of NO₂, consume Cl groups, and have a negative effect on mercury oxidation. However, some scholars also point out that CO₂ may slightly promote the generation of free oxygen atoms. In addition, H₂O in oxy-fuel flue gas may also inhibit the oxidation of mercury. The concentrations of NO_x and SO₂ in oxy-fuel biomass co-firing flue gas are lower than those in traditional coal combustion flue gas. Nevertheless, considering the complex effects of NO_x and SO₂ on mercury oxidation, their impact on the mercury species distribution should not be overlooked. The concentration of fly ash in oxy-fuel flue gas is higher than that under air conditions, which is advantageous for the removal of elemental mercury. Under the oxy-fuel atmosphere, attention should be paid to the re-emission of mercury in the WFGD system. High concentrations of CO₂ can reduce the pH and redox potential of gypsum slurry, thereby inhibiting the re-emission of Hg⁰. Metal ions such as Fe²⁺ can also inhibit the re-emission of mercury, and the inhibitory effect increases with the CO₂ concentration. In comparison, the effect of Fe²⁺ on the mercury re-emission under oxy-fuel conditions is more pronounced than that under air conditions.

Relying on traditional air pollution control devices, such as SCR, WFGD and PMCD, for simultaneous mercury removal makes it almost impossible to meet the mercury concentration requirements for BECCS systems. Specialized deep mercury removal methods such as adsorption and catalytic oxidation must be employed. The adsorption method has the advantages of simple equipment and convenient operation. However, issues relating to the generation of solid waste pollution from the used adsorbent, materials recycling and regeneration are worth further study. The loading of various metals can achieve the protection of catalytic active sites and prevent the influence of competing components. However, in a complex flue gas environment, broadening the temperature window of the catalyst and preventing catalyst poisoning should be the focus in future work. The key to the widespread application of deep mercury removal technologies lies in the development of highly efficient, stable, and cost-effective adsorbents and catalysts. The higher content of Cl and AM elements in biomass can enhance the mercury removal performance of the materials.

In terms of future work on deep mercury removal from oxy-fuel biomass co-firing flue gas, it is important to reveal the influences of Cl and AM species on mercury oxidation reactions, especially elucidating the synergistic effects between various components. Furthermore, building the kinetic models for homogeneous and heterogeneous mercury reactions and obtaining the relevant kinetic parameters could enable the accurate prediction of mercury species distribution. Finally, for the development of mercury adsorbents and catalysts, it is essential to fully consider the characteristics of the oxy-fuel biomass co-firing conditions, such as the high chlorine, alkali metal, and CO₂ concentrations in the flue gas, in order to further enhance the Hg⁰ removal performance. Furthermore, it is worth studying the changes in the microstructure of the adsorbents and catalysts after exposure to mercury, and attention should also be given to issues regarding the recycling and regeneration of the mercury removal materials. In general, mercury in flue gas poses a serious threat to the safe operation of BECCS systems. A thorough understanding of the mercury control mechanisms under oxy-fuel biomass co-firing conditions is essential for ensuring the safety of BECCS systems.