Thread Content
For a considerable length of time, coal will remain one of the main conventional energy sources. During coal combustion, various pollutants are generated, including particulate matter (PM), SO2, NOx, As, Se, Hg, Cr, and others. Since mercury and its compounds possess considerable toxicity even at very low concentrations, they can cause severe pollution to ecological environments such as water, air, and soil, and have a direct impact on the health of humans, animals, and plants. For these reasons, they are classified as among the most toxic trace heavy metal pollutants. Internationally, research on controlling the emission of mercury, this harmful element from coal-fired power plants, has become a focal point. In terms of mercury removal, current research focuses mainly on the dry injection removal method based on the direct injection of solid powder adsorbents into the flue gas, and the adsorbent-based mercury removal method using fixed beds. Although the adsorbent method boasts a high mercury removal efficiency, it is also costly and presents significant challenges for industrial application. In contrast, mercury removal methods based on existing pollution control devices have relative advantages in this regard; the most typical example is the simultaneous mercury removal using wet flue gas desulfurization systems (WFGD). 1 Properties and morphological transformations of mercury in flue gas 1.1 Content and morphological distribution of mercury in flue gas The mass concentration of mercury in the atmosphere is 2–5 ng/m3, whereas the emission mass concentration of mercury from coal-fired power plants ranges from 800–4800 ng/m3. Mercury emission forms include three types: gaseous elemental mercury (Hg0), gaseous divalent ion mercury (Hg2+), and particulate mercury (HgP) ； The ratio of elemental mercury to divalent mercury is uncertain, depending mainly on factors such as the type of coal, combustion conditions, temperature, and flue gas composition. Since divalent mercury is soluble in water, it can be removed using conventional wet flue gas desulfurization systems. In contrast, elemental mercury is insoluble in water and highly volatile; it cannot be removed by the desulfurization solution and is emitted along with the flue gas, making it a relatively difficult substance to remove ； Therefore, the removal efficiency of total mercury in flue gas by WFGD mainly depends on the morphological distribution of mercury in the flue gas. 1.2 Influence of various components in flue gas on the form of mercury Various components in flue gas, such as chlorides, SO2, and NOx, have a significant impact on the distribution of mercury in its different forms within the flue gas. Schager and Hall, along with others, studied the chemical reaction properties of various gas components in coal-fired flue gases (CO2, HCl, Cl2, SO2, NO2, N2O, NO, NH3, H2S) with mercury within the temperature range of 20–900 °C. They found that Hg does not react with NH3, N2O, SO2, and H2S; it reacts slowly with NO2, while it reacts rapidly with HCl, Cl2, and O2. The respective reaction equations are as follows: 2Hg0(g) + O2(g) → 2HgO(s, g) (1) Hg0(g) + Cl2(g) → 2HgCl2(s, g) (2) 2Hg0(g) + Cl2(g) → Hg2Cl2(s) (3) Hg0(s) + 2HCl(g) → HgCl2(s, g) + H2 (4) 2Hg0(g) + 4HCl(g) + O2(g) → 2HgCl2(s, g) + 2H2O(g) (5) 4Hg0(g) + 4HCl(g) + O2(g) → 2HgCl2(s) + 2H2O(g) (6) Hg0(g) + NO2(g) → HgO(s, g) + NO(g) (7) Hall and others also found that although HCl can oxidize Hg0(g), Cl2(g) is more reactive in the mercury-chlorine system. During coal combustion, chlorine is primarily evaporated in the form of HCl(g). When the temperature drops to 430–475 °C, Cl2(g) can be formed through the following reaction: http://www.nmtech.com.cn/jishuwang/upload1/080513949397540.jpg Sliger et al. believe that, in the formation of HgCl2 in coal combustion flue gas, chlorine atoms act as an important intermediate in the oxidation reaction between Hg and chlorine; these chlorine atoms can rapidly oxidize Hg0 at any temperature present in the flue gas. The behavior of chlorine atoms during coal combustion determines the extent of the oxidation reaction between Hg0 and these chlorine atoms. According to data from the U.S. Environmental Protection Agency, when the chlorine content in coal is higher than 1×10‑3, the proportion of elemental mercury in the flue gas relative to the total mercury content is less than 20%. Furthermore, Hg0 does not react directly with SO2, but the presence of SO2 in flue gas inhibits the reaction between Hg0 and HCl, mainly by consuming Cl2(g), thereby preventing the formation of HgCl2(g) or reducing the catalytic activity of the fly ash. The reaction is as follows: Cl2(g) + SO2(g) + H2O → 2HCl(g) + SO3(g) (9). However, when SO2 is present in the flue gas, at temperatures ≤ 500 K, mercury exists mostly in the solid form of HgSO4; in other words, the presence of SO2 facilitates the oxidation of mercury, especially at low temperatures, as SO2 is more reactive than O2. The higher the SO2 content, the higher the temperature at which mercury will undergo conversion. At the same time, the presence of water vapor in the flue gas also reduces the tendency of Hg0 to be oxidized to Hg2+ by other gases. 2. Desulfurization performance of wet flue gas desulfurization systems. Due to its advantages such as stable operation and high desulfurization efficiency, the wet flue gas desulfurization technology is widely used in coal-fired power plants both domestically and internationally. At the same time, since the divalent mercury present in flue gas is soluble in water and remains in the desulfurization solution where it can be removed, the Argonne Laboratory in the United States believes that WFGD holds great potential for the comprehensive treatment of flue gas (removal of SO2, NOx, and Hg). The mechanism involves the following: during coal combustion, due to the high temperatures in the furnace, most of the mercury contained in the coal vaporizes and exists in gaseous form. After the flue gas leaves the furnace, the temperature drops, and the gaseous mercury reacts with other components in the flue gas; part of it is oxidized to divalent mercury (mainly HgCl2 and a small amount of HgO). This oxidized form of mercury is soluble in water and remains in the desulfurization solution, thereby being removed, while elemental mercury, which is insoluble in water, is emitted along with the flue gas. 2.1 Decontamination performance of WFGD Under normal conditions, the temperature of flue gas after desulfurization drops from 80–150 °C to 40–60 °C, which facilitates the oxidation of elemental mercury to mercuric ions. Moreover, desulfurization devices can remove 80%–95% of Hg2+ from the flue gas, thereby helping to improve the overall efficiency of mercury removal. According to field tests conducted at power plants by the U.S. Department of Energy and EPRI, the removal efficiency of WFGD for total mercury in flue gas ranges from 10% to 80%, with this value depending primarily on the ratio of elemental mercury to divalent mercury. 2.2 Factors affecting the mercury removal efficiency of WFGD Since divalent mercury in flue gas is soluble in water, it can be captured by the desulfurization solution and removed, whereas elemental mercury is insoluble in water; therefore, the mercury removal efficiency depends primarily on the proportion of divalent mercury in the total mercury content, that is, the degree of oxidation of elemental mercury in the flue gas. The factors affecting the oxidation rate of elemental mercury in flue gas mainly include the type of coal used, flue gas temperature, flue gas humidity, flue gas flow rate, flue gas composition, liquid-to-gas ratio, desulfurization solution concentration, and the pH of the desulfurization solution. 2.2.1 Type of coal According to the National Energy Technology Laboratory (NETL), the form of mercury in flue gas varies depending on the type of coal used. In the flue gases generated by bituminous coal combustion, elemental mercury, divalent mercury, and particulate mercury account for 20%, 35%, and 45% of the total mercury content, respectively ； In the flue gases from bituminous coal combustion, about 65% of the total mercury exists in its elemental form, 20% is in oxidized forms, and 15% is in particulate form ； In the flue gases from lignite combustion, elemental mercury, divalent mercury, and particulate mercury account for 85%, 10%, and 5% of the total mercury, respectively. It can be seen that the content of divalent mercury in the flue gas generated by bituminous coal combustion is relatively high, as the high chlorine content in bituminous coal facilitates the oxidation of elemental mercury to divalent mercury in the flue gas, resulting in a relatively high mercury removal efficiency through WFGD ； The flue gas generated by the combustion of anthracite and lignite contains higher levels of elemental mercury, as these two types of coal have lower chlorine content, which hinders the oxidation of elemental mercury and results in lower removal efficiency. The main type of coal used for combustion in our country is lignite, which makes control efforts relatively more difficult. 2.2.2 Flue gas temperature and humidity: Mercury in coal volatilizes in the high-temperature areas of the furnace to form thermodynamically stable elemental mercury, which exists in gaseous form. As the flue gas enters the ducts, it passes through dust and desulfurization devices before being discharged. As the temperature of the flue gas drops, part of the elemental mercury reacts homogeneously or heterogeneously with other components in the flue gas to form divalent mercury (mainly HgCl2 and a small amount of HgO) as well as particulate mercury. Therefore, flue gas temperature has a certain impact on the form of mercury; the lower the temperature, the more favorable it is for the oxidation of gaseous elemental mercury, thereby enhancing the mercury removal efficiency of the subsequent WFGD process. The presence of water vapor in flue gas reduces the oxidation of elemental mercury; therefore, an increase in flue gas humidity is unfavorable for mercury removal via WFGD. However, the mechanism behind this effect remains unclear and requires further investigation. 2.2.3 Flue gas composition: The flue gas generated by coal combustion contains various components such as particulate matter, SO2, NOx, HCl, Cl2, CO2, O2, N2, and Hg. All of these components have an impact on the mercury removal efficiency of WFGD. (1) Effect of Cl2: Since elemental mercury reacts rapidly with Cl2, about 70% of the reaction occurs within 1.1 seconds at 500 °C. Therefore, the presence of Cl2 in the flue gas facilitates the oxidation of elemental mercury, thereby improving the mercury removal efficiency of WFGD. (2) Effect of HCl: Within the range of 500–900 °C, Hg0 in flue gas reacts rapidly with HCl (4). In the presence of O2, Hg0 can also react with HCl and O2 (5) and (6), being oxidized to Hg2+, which is then captured and removed by WFGD, thereby improving the mercury removal efficiency of WFGD. The higher the HCl content in the flue gas, the more favorable it is for the oxidation of mercury in the flue gas, allowing oxidized mercury to constitute a larger proportion of the total mercury amount, and thereby improving the efficiency of WFGD in removing mercury. Furthermore, the higher the HCl content, the higher the onset and end temperatures for the conversion of mercury (Hg0→Hg2+); the easier this conversion is, the more favorable it is for mercury to be captured in WFGD. (3) Effect of O2: SO2 in the flue gas is absorbed in WFGD; the resulting sulfites and bisulfites have reducing properties, which can reduce Hg2+ absorbed by the desulfurization solution to Hg0, thereby causing the re-emission of elemental mercury. However, O2 in the flue gas can oxidize sulfites and bisulfites to sulfates, which then react with Hg2+ to form mercuric sulfate, thereby helping to improve the efficiency of mercury removal. Furthermore, in the boiler combustion area or during flue gas transport, elemental mercury in areas with lower temperatures can rapidly undergo an oxidation reaction with O2 to be converted into mercuric oxide in the divalent state. The higher the volume fraction of O2, the wider the temperature range within which mercury conversion (Hg0→Hg2+) occurs; when the volume fraction of O2 is low, the temperature at which mercury begins to convert is also relatively lower, and the conversion rate is lower as well ； Under high-temperature combustion conditions, changes in the volume fraction of O2 have almost no effect on the mercury conversion rate. (4) Effect of SO2: In WFGD, the mercury removal efficiency decreases as the SO2 concentration in the flue gas increases. This is because the sulfites and bisulfites formed upon the absorption of SO2 can reduce Hg2+, which has been absorbed by the desulfurization solution. An increase in SO2 content in the flue gas leads to an increase in the concentrations of SO32– and HSO3– in the solution, thereby promoting the reduction of Hg2+ and resulting in the re-emission of Hg0, which in turn reduces the mercury removal efficiency of WFGD. Furthermore, the presence of SO2 in the flue gas does not affect the onset temperature for mercury conversion, but it increases the cutoff temperature; as a result, the temperature range for mercury conversion is reduced, which is unfavorable for the oxidation and removal of elemental mercury. (5) Effect of NOx: The presence of NOx in the flue gas has almost no effect on the morphological transformation of mercury. However, the sequential batch reactor (SCR) commonly used for removing NOx can promote the conversion of elemental mercury to oxidized mercury, thereby helping to improve the mercury removal efficiency of WFGD. (6) Effect of water vapor: The presence of water vapor in flue gas reduces the oxidation of elemental mercury; therefore, an increase in flue gas humidity is unfavorable for mercury removal via WFGD. 2.2.4 Properties of the desulfurization solution (1) pH of the desulfurization solution: The oxidized mercury that is absorbed, along with the sulfites or bisulfites formed by the absorption of SO2, undergoes a series of reactions to be reduced to elemental mercury. The reaction mechanism is as follows: http://www.nmtech.com.cn/jishuwang/upload1/080513950419554.jpg A higher pH facilitates the oxidation of sulfites and bisulfites in the solution to sulfates, which then react further with mercury to form HgSO4, thereby improving the efficiency of mercury removal. (2) Desulfurization solution concentration: When flue gas passes through WFGD, Hg2+ is adsorbed by calcium-containing particles formed from the absorption of SO2; a heterogeneous reaction then occurs, reducing it to Hg0. An increase in the concentration of the absorption solution can weaken this adsorption, thereby reducing the reduction of mercury in its oxidized state and improving the efficiency of mercury removal. 2.2.5 Liquid-to-gas ratio In WFGD, the amount of desulfurization liquid used, that is, the liquid-to-gas ratio, also affects the mercury removal efficiency. As the liquid-to-gas ratio increases, the mixing of liquid and gas as well as the mass transfer rate increase accordingly, which raises the rate at which mercury in the divalent oxidation state is absorbed, thereby increasing the mercury removal efficiency of WFGD. 2.3 Re-emission and suppression of elemental mercury At the end of the 1990s, British company B&W, in its research on improving the mercury removal efficiency of enhanced wet FGD systems, discovered that under certain conditions, mercury in the divalent state, which had initially been captured by the FGD desulfurization solution, was released again in the form of elemental mercury. Since the sulfites and bisulfites formed by the absorption of SO2 in flue gas are reducing, they can reduce and absorb Hg2+ to form insoluble Hg0, resulting in the re-release of mercury; the reaction equation is given in (16). In addition, metal ions (such as iron, manganese, nickel, cobalt, tin, etc.) can also react with Hg2+ to reduce it, with the reaction as follows: 2M2+ + Hg2+ → Hg0 + 2M3+ (17). The reduced mercury removal efficiency of WFGD is caused by the re-emission of elemental mercury, and this can be mitigated through the following measures: ① Increasing the concentration of sulfides. ② Lowering the temperature (achieved by installing an energy recovery system) creates conditions favorable for the conversion of Hg0 to Hg2+. ③ Reducing the SO32‑/HSO3‑ ratio in the absorption solution, that is, increasing the HSO3‑ concentration or decreasing the SO32‑ concentration, can be achieved by lowering the pH of the desulfurization solution; however, this leads to a decrease in the SO2 absorption rate and also causes more severe corrosion of the equipment. Some studies also indicate that the mercury removal efficiency of WFGD increases as the pH of the desulfurization solution rises, because Hg2+ is reduced to Hg0 through a series of reactions with sulfites or bisulfites formed by the absorption of SO2, which in turn leads to a decrease in mercury removal efficiency. The results of the two studies are contradictory; therefore, further in-depth research is needed to determine how to control the concentrations of SO32‑ and HSO3‑ in the desulfurization solution. ④ Increase the total concentration of tetravalent sulfur (i.e., SO32‑, HSO3‑) in the absorption solution. This can be achieved by reducing the oxidation conditions or by adding small amounts of sodium salts such as NaHSO3 and NaHCO3. This method does not affect the absorption capacity of the absorbent solution for SO2, thereby reducing equipment corrosion caused by mercury removal. 3 Measures to improve the mercury removal efficiency of WFGD. WFGD removes mercury from flue gas primarily by absorbing mercuric ions in the divalent state; therefore, improving the mercury removal efficiency of WFGD involves increasing the proportion of divalent mercury within the total mercury content, that is, enhancing the oxidation rate of elemental mercury in the flue gas. There are various methods to increase the oxidation rate of elemental mercury, such as improving combustion conditions, altering the composition of the flue gas (increasing the levels of HCl, Cl2, and O2 in it), injecting activated carbon or catalysts into the flue gas, adding additives to the coal, and including oxidants in the absorption solution. This paper mainly discusses how to improve the mercury removal efficiency by adding oxidants. 3.1 Addition of sulfides: Since trace amounts of H2S present in flue gas can react with Hg2+ at the gas-liquid interface within the WFGD system to form HgS precipitates, thereby inhibiting the reduction of Hg2+, the concentration of H2S in the flue gas can be increased to precipitate the absorbed divalent mercury ions and thus control the re-emission of elemental mercury. Increasing the H2S content in the flue gas can be achieved by injecting H2S into it, and this hypothesis has been confirmed by B&W Company through experiments in which H2S was injected into the flue gas before a wet FGD system; during these experiments, the injection of H2S completely eliminated mercury re-emission. The main reactions are as follows: H2S(g) —→ H+ + HS– (18) HS– + Hg2+ —→ HgS↓ + H+ (19) However, spraying H2S into flue gas requires a system for generating H2S as well as a delivery system, along with air distribution grids; this makes the process costly. To simplify things, B&W Company uses a liquid absorbent that can produce sulfides such as H2S (sodium hydrosulfide), which is sprayed into the wet FGD slurry to achieve the same result. Furthermore, Na2S and Na2S4 decompose at an appropriate temperature (around 121 °C) to produce S0 and S2–, which can react with Hg0 and HgCl2 to form HgS, thereby fixing the absorbed divalent mercury ions; thus, the addition of sulfides (Na2S, Na2S4) can also promote the oxidation of Hg0. 3.2 Fenton reagent: Adding Fenton reagent (Fe3+/H2O2) to the desulfurization solution can promote the oxidation of elemental mercury to mercuric ions, which are then absorbed by the desulfurization solution, thereby improving the mercury removal efficiency of WFGD. Lu and Tan, along with others, conducted laboratory-scale and pilot-scale experiments on the use of the Fenton reaction to promote the removal of mercury. These experiments showed that when Fenton reagents were added to the desulfurization solution, under conditions of a H2O2 concentration of around 0.02%, a Fe3+ concentration of about 0.01%, and a pH level of 1.0–3.0, the oxidation rate of elemental mercury in flue gas could reach 75% in laboratory tests ； And a mercury oxidation rate of 30% to 40% can also be achieved in pilot-scale tests. According to the research findings of Lu and Tan et al., the presence and concentration of SO2 have no significant effect on the oxidation of Hg0; therefore, the Fenton reagent can be used for mercury removal from flue gases generated by burning different coal types with varying sulfur contents. Furthermore, SO3 generated by the oxidation of SO2 has no negative effect on the solubility of Hg2+, which ensures the simultaneous removal of large amounts of Hg2+ from flue gas desulfurization solutions. Lu and Tan et al. also conducted experiments using a Fenton-like reagent (Cu2+ / H2O2) and found that the Fenton reagent (Fe3+ / H2O2) was more effective at catalyzing the oxidative degradation of Hg0 than Cu2+ / H2O2, as the reaction between Cu2+ and H2O2 does not yield ·OOH effectively. This result indicates that ·OOH plays a key role in the oxidation of Hg0. 3.3 Addition of KMnO4: Due to its strong oxidizing properties, KMnO4 can accelerate the oxidation of elemental mercury. Its main mechanism is that KMnO4 generates Mn2+ under acidic conditions, exhibiting autocatalytic behavior ； Under neutral and alkaline conditions, MnO2 is formed; MnO2 has an adsorptive effect on Hg2+, and under strongly alkaline conditions, ·OH free radicals are involved in the oxidation reaction. 4 Conclusion Utilizing the existing pollution control devices in coal-fired power plants to simultaneously remove mercury is a key research area for controlling mercury emissions in flue gas; the most typical approach is to use wet flue gas desulfurization systems for simultaneous mercury removal. Since only divalent mercury can dissolve in water and be removed by the desulfurization solution, while elemental mercury is emitted with the flue gas, how to promote the oxidation of elemental mercury in the flue gas to improve mercury removal efficiency has become a key focus of research. Based on an analysis of the changes in mercury forms in flue gas and the factors affecting the mercury removal efficiency of WFGD, this paper explores the enhancement of the oxidation of elemental mercury by adding several oxidants, thereby improving the mercury removal efficiency of WFGD. Since there is currently limited research in China on the use of wet flue gas desulfurization systems for mercury removal, to enable the application of this technology in industry, the main directions for future research include: (1) further investigating the impact of various components in flue gas on the transformation of mercury forms, and clarifying the reaction mechanisms between these components and mercury ； (2) Utilize the existing wet flue gas desulfurization equipment, and through experiments identify more economical and effective additives to promote the oxidation of elemental mercury in flue gas ； (3) Utilize the existing flue gas wet desulfurization equipment and improve operating conditions to suppress the re-emission of elemental mercury, thereby enhancing the mercury removal efficiency.