Numerical Study on the Homogeneous Reactions of Mercury in a 600 MW Coal-Fired Utility Boiler

The homogeneous oxidation of elemental mercury (Hg0) can promote Hg pollution control in coal-fired power plants, while the mechanisms and quantitative contributions of homogeneous reactions in Hg0 oxidation, especially the reactions between Hg and chlorine (Cl), are still unclear. Here, a numerical study on the homogeneous reactions of Hg was conducted within a 600 MW tangentially fired boiler for the first time. A novel Hg sub-model was established by coupling the thermodynamics, reaction kinetics and fluid dynamics. The results showed that the higher Cl content in coal was beneficial to the oxidation of Hg0. The homogeneous reactions of Hg mainly occurred in the vertical flue pass at low temperature. Hg0 was still the dominant Hg-containing species at the boiler exit, and the concentration of mercury chloride (HgCl2) was the highest among the oxidized mercury. When low-Cl coal was fired, the addition of a small amount of chlorine species into the boiler at the burnout area increased the ratio of HgCl2 by over 16 times without causing serious chlorine corrosion problems.


Introduction
Coal-fired utility boilers are the predominant anthropogenic mercury (Hg) sources in many regions around the world [1,2]. Mercury is released from coal during the combustion process, with only about 2% of mercury remaining in the bottom slag. Mercury in flue gas mainly exists in three forms: gaseous elemental mercury (Hg 0 ), gaseous oxidized mercury (Hg 2+ ) and particulate mercury (Hg p ) [3][4][5]. Among them, gaseous oxidized mercury has high solubility and can be removed in wet flue gas desulfurization devices. Particulate mercury can be collected using particulate matter control devices, such as electrostatic precipitators (ESPs) and fabric filters (FFs). As a result, most mercury emitted to the atmosphere from coal-fired power plants is in the form of gaseous elemental mercury, which is difficult to remove. Gaseous elemental mercury is the only stable form of mercury in the temperature range of furnaces (1050~1400 • C). During the combustion process, the flue gas temperature decreases gradually as the flue gas flows. Part of Hg 0 will then be oxidized into Hg 2+ by other flue gas components, while some of the Hg 0 will be captured by fly ash particles [6]. Some scholars have carried out thermodynamic equilibrium analyses on mercury species and have reported that only Hg 0 can exist stably when the temperature is higher than 750 • C, while all Hg 0 will be oxidized into Hg 2+ if the temperature is below 450 • C [7,8]. However, the measured data show that Hg 0 still occupies a large proportion of Hg emitted from the flue gas because the oxidation process of Hg 0 is limited by the kinetic factors [9,10]. For example, Chen et al. reported that the fractions of gaseous elemental mercury, gaseous oxidized mercury and particulate mercury before ESP/FF are 10.4%~93.2%, 3.7%~44.3% and 1.3%~59.5%, respectively [9]. The migration of mercury species in coal-fired boilers is affected by the coal type and boiler operating conditions [7,[11][12][13]. Additionally, the concentrations of HCl, NO x , SO 2 and other components in flue gas also greatly influence the oxidation reactions of Hg 0 [12,14].
The oxidation processes of Hg 0 in flue gas include homogeneous oxidation and heterogeneous oxidation. Homogeneous oxidation refers to the redox reactions between Hg 0 and flue gas components within the boiler. Heterogeneous oxidation occurs on the surfaces of adsorbents or catalysts, for which the reaction temperature is even lower. The influence of HCl on the homogeneous oxidation of Hg 0 was studied by Sliger et al. [15]. The results showed that increasing the concentration of HCl could increase the temperature zone of homogeneous Hg reactions significantly, which meant that Hg 0 could be oxidized earlier in the boiler. Galbreath et al. conducted a bench-scale study on a 7 kW coal combustion system and found that the HCl addition could enhance the oxidation of Hg 0 significantly [16]. Wang et al. focused on the heterogeneous oxidation of Hg 0 and compared the influences of NaCl, CaCl 2 and HCl additions [17]. However, compared with the extensive investigations on heterogeneous mercury reactions, the research on the homogeneous reactions of mercury in a full-scale utility boiler is still insufficient [18,19]. The distributions of the mercury-related species inside the furnace and flue pass are yet to be revealed. Considering the decisive role of the forms of mercury species in choosing the mercury removal approach, it is necessary to investigate the homogeneous reactions between mercury and other flue gas components for mercury emission control from coal-fired power plants.
Because of the extremely low concentrations of Hg and its compounds, it is difficult to carry out experimental studies on the homogeneous reaction of mercury. The computational fluid dynamics (CFD) method is an alternative way to investigate the homogeneous reactions of Hg in coal-fired utility boilers. Taking into account the vital influences of Hg-Cl reactions on the homogeneous oxidation of Hg 0 , we conducted this numerical study with the following aims: (1) revealing the influence of the chlorine content in coal on the homogeneous reactions of Hg; (2) evaluating the quantitative impacts of the addition of Cl-containing species on the homogeneous oxidation of Hg 0 inside a full-scale boiler.
For the first time, a novel mercury sub-model was established in a full-scale tangentially coal-fired boiler to achieve the coupling of thermodynamics, reaction kinetics and fluid dynamics. The thermodynamics data for mercury species and the kinetics data for the elemental reactions are needed to conduct the simulation of the homogeneous mercury reactions. In addition to the homogeneous reactions of Hg, the flow field, the temperature field and the flue gas component distribution within the boiler were also analyzed. The results of this work are of significance in understanding the mechanisms of homogeneous Hg reactions and will lay a foundation for efficient mercury pollution control in coal-fired utility boilers.

Boiler Description and Mesh Generation
Supercritical tangentially boilers are widely used in coal-fired power plants because of their good flame fullness and high combustion efficiency [20][21][22][23]. Therefore, a 600 MW supercritical tangentially fired boiler was chosen in this study, as shown in Figure 1a. The width, depth and elevation of the furnace of this boiler were 18,816 mm, 17,696 mm and 72,350 mm, respectively. The geometric model of the solution area was built using SOLIDWORKS according to the structure of the boiler. Appropriate simplifications were employed during the model construction. For example, the thicknesses of the water wall and the heat exchanger was considered as zero. Each corner of the furnace was equipped with 6 layers of primary air (PA) nozzles, including one layer for standby. Additionally, the specific layouts of auxiliary air (AUX) nozzles, close-coupled over fire air (CCOFA) nozzles, underfire air (UFA) nozzles and separated over fire air (SOFA) nozzles are shown in Figure 1b. and the heat exchanger was considered as zero. Each corner of the furnace was equipped with 6 layers of primary air (PA) nozzles, including one layer for standby. Additionally, the specific layouts of auxiliary air (AUX) nozzles, close-coupled over fire air (CCOFA) nozzles, underfire air (UFA) nozzles and separated over fire air (SOFA) nozzles are shown in Figure 1b. ICEM software was used to generate the hexahedral structured mesh system of the boiler. The mesh near the burner area was refined to improve the accuracy of the simulation results. Three sets of mesh systems were built, with the numbers of cells being 1,259,490, 1,695,378 and 2,147,981 respectively. The grid-independence test showed that the difference between the two mesh systems with 1,695,378 and 2,147,981 cells was negligible. Therefore, considering the calculation accuracy and the calculation cost, the mesh system with 1,695,378 cells was selected in this study. Figure 2 shows the mesh of the whole furnace and the mesh of the cross-section near the burner area. ICEM software was used to generate the hexahedral structured mesh system of the boiler. The mesh near the burner area was refined to improve the accuracy of the simulation results. Three sets of mesh systems were built, with the numbers of cells being 1,259,490, 1,695,378 and 2,147,981 respectively. The grid-independence test showed that the difference between the two mesh systems with 1,695,378 and 2,147,981 cells was negligible. Therefore, considering the calculation accuracy and the calculation cost, the mesh system with 1,695,378 cells was selected in this study. Figure 2 shows the mesh of the whole furnace and the mesh of the cross-section near the burner area.

Coal Parameters and Boundary Conditions
The coal used in this boiler was bituminous coal with a relatively high volatile content. The coal properties coal ash analysis can be found in Tables S1 and S2,

Coal Parameters and Boundary Conditions
The coal used in this boiler was bituminous coal with a relatively high volatile content. The coal properties coal ash analysis can be found in Tables S1 and S2, respectively, in the Supporting Information. The size of coal particles followed the Rosin-Rammler distribution [24]. The minimum size, the maximum size and the average size of the coal particles were 1 µm, 200 µm and 61 µm, respectively.
This study can be divided into two parts. The first part focuses on the influence of the overall Cl concentration in the furnace; that is, the level of Cl in the coal. The second part focuses on the impacts of introducing a small amount of Cl-containing compounds from SOFA nozzles on mercury species distributions. For all cases studied in this paper, the total excess air ratio and the excess air ratio in the primary zone were set as 1.2 and 0.8, respectively. The coal feeding rate of the boiler was 60.08 kg·s −1 , and the total air flow rate was 550.85 kg·s −1 . The temperatures of primary air and secondary air (SA) were kept at 353 K and 612 K, respectively. A mass flow inlet boundary was adopted for air nozzles during the calculation. More details about the operating conditions and the nozzle sizes are listed in Table 1.

Coal Combustion and NO x Formation
The combustion of coal is a complicated process, including preheating, drying, volatile release, ignition, combustion of volatiles, combustion of char and burnout, along with other physical and chemical processes, such as multiphase flow, heat radiation and heat convection. In this paper, FLUENT was used to simulate the flow, heat transfer and combustion processes in the furnace. The fluid flow was described using the standard k-ε model, which has been widely used in the calculation of flow fields in boiler furnaces [25]. The two-competing-rate model was adopted to calculate the release of volatiles while the kinetic-diffusion-limited rate model was selected to calculate the char combustion process [26]. Additionally, the turbulent dispersion of coal particles was predicted using a stochastic tracking approach. The radiative transfer equation inside the furnace was solved using the discrete ordinate (DO) radiation model. The non-premixed combustion process was simplified and solved by applying the assumed-shape probability density function (PDF) approach [27]. As for the pressure-velocity coupling, the phase-coupled SIMPLE (PC-SIMPLE) algorithm was used to solve the equations [28]. After obtaining the velocity field, temperature field and concentration field of flue gas components in the furnace, the distributions of nitrogen oxides species were then calculated. The main species of nitrogen oxides generated during coal combustion are NO and NO 2 , among which NO accounts for more than 90%. In addition, a small amount of N 2 O is formed. According to the formation path, NO x can be divided into thermal NO x , fuel NO x and prompt NO x . In terms of coalfired boilers, thermal NO x and fuel NO x are dominant while prompt NO x is negligible [29]. In this study, the formation of thermal NO x was described using the Zeldovich mechanism. The fuel NO x was generated from volatile nitrogen and char nitrogen, with HCN and NH 3 as the important intermediate products during the reaction.

Homogeneous Oxidation of Hg 0
Based on the simulation results of coal combustion and NO x formation, the homogeneous oxidation process of Hg 0 was calculated using the eddy dissipation concept (EDC) model. The EDC model takes into account the specific chemical reaction mechanism in turbulence, which assumes that all chemical reactions take place in a very small turbulent structure [30,31]. It should be mentioned that tabulated chemistry modeling has been used by several researchers as an alternative method to EDC in some cases for better combination with the detailed mechanisms [32][33][34]. However, the accuracy of these gaseous flameletbased tabulated chemistry models would be reduced for coal combustion cases because the large coal particles may pass through the flame surface during the combustion process. Additionally, in order to consider the slow chemical processes, such as the generation of pollutants (Hg, NO x , SO x , etc.), the tabulated chemistry models need to be further extended to the unsteady form. Hence, we use the EDC model to carry out the numerical study for Hg reactions within the coal-fired utility boiler.
The homogeneous oxidation of Hg 0 in coal-fired boilers is very complicated and affected by many factors, such as the temperature distribution and concentration of flue gas components. Moreover, as there is no sub-model for Hg reaction in FLUENT, a new Hg model must be built and imported for calculation. Some scholars believe that the reactions between mercury and chlorine are the most important for the homogeneous reaction of mercury. Based on this, a set of kinetic model containing 8 reversible elementary reactions was proposed [35]. Sliger et al. proposed that the most important factors affecting the homogeneous reaction of Hg were the HCl concentration, flue gas cooling rate and background gas composition, and found that the reaction between Hg and Cl was the main pathway for the oxidation of Hg 0 [15]. The findings of Xu et al. showed that oxygen also plays a key role in the homogeneous reaction of mercury in addition to chlorine. Therefore, a kinetic model for homogeneous reactions of Hg was proposed, including HgO-related elemental reactions [36]. According to the results of the sensitivity analysis, the reaction of Hg + ClO = HgO + Cl is as important as the widely used reactions of Hg + Cl 2 = HgCl + Cl and Hg + HCl = HgCl + H.
Based on these previous studies, a novel Hg sub-model was proposed to achieve the coupling of thermodynamics, reaction kinetics and fluid dynamics for the homogeneous reactions of Hg in a full-scale boiler. Here, 32 elementary reactions were selected to describe the homogeneous oxidation process of Hg 0 in a coal-fired utility boiler. The kinetic parameters of the reactions were obtained from the literature and have been verified in experimental studies [15]. The thermodynamic data for Hg and HgCl were obtained from the NIST database and imported into CHEMKIN together with elemental reactions. Then, the che file (shown in Table S2 in the Supporting Information), which can be used as the source file for the Hg sub-model in FLUENT, was generated by FitDat.

Model Validation
In order to validate the model used in this study, the simulation results were compared with the practical measured data from the coal-fired power plant. As shown in Figure 3, the flue gas temperature and the O 2 concentration at the furnace outlet, the wall heat flux, the carbon content in fly ash and the NO x emission were selected for comparison and validation. The largest error occurred in the carbon content in fly ash, with a relative error Energies 2022, 15, 446 6 of 16 of 7.56%. The relative errors of the temperature of the furnace outlet, O 2 concentration at the furnace outlet, the wall heat flux and the NO x emission were 2.70%, 3.62%, 3.35% and 1.41%, respectively, which are acceptable results for the practical application of coal-fired utility boilers [37]. Hence, the model adopted in this study can simulate the flow, combustion and heat transfer processes well. In terms of the Hg distribution, the model was validated by comparing the simulation results with the findings from the literature, which can be found in Section 4.3, together with the discussion on the homogeneous reactions of Hg.

Model Validation
In order to validate the model used in this study, the simulation results were compared with the practical measured data from the coal-fired power plant. As shown in Figure 3, the flue gas temperature and the O2 concentration at the furnace outlet, the wall heat flux, the carbon content in fly ash and the NOx emission were selected for comparison and validation. The largest error occurred in the carbon content in fly ash, with a relative error of 7.56%. The relative errors of the temperature of the furnace outlet, O2 concentration at the furnace outlet, the wall heat flux and the NOx emission were 2.70%, 3.62%, 3.35% and 1.41%, respectively, which are acceptable results for the practical application of coal-fired utility boilers [37]. Hence, the model adopted in this study can simulate the flow, combustion and heat transfer processes well. In terms of the Hg distribution, the model was validated by comparing the simulation results with the findings from the literature, which can be found in Section 4.3, together with the discussion on the homogeneous reactions of Hg.

Simulation Results before Considering Hg Reactions
Before importing the mercury sub-model, the combustion process in the boiler was calculated using the non-premixed combustion model. After the temperature distribution, velocity distribution and flue gas components distribution were obtained, the NO x model was activated. An imaginary coordinate was established within the boiler, which started from the bottom of the hopper and ended at the outlet of the calculation zone. A crosssection perpendicular to the coordinate was built every 1 m along the axis. The average flue gas temperature on each cross-section was calculated, with the results shown in Figure 4. It can be seen that the coordinate axis passed through the hopper area, the primary zone, the burnout area, the horizontal flue pass and the vertical flue pass. The fuel was injected into the furnace at the primary zone and mixed with the PA and SA, resulting in the intense combustion reaction and the rapid rise in flue gas temperature. Figure 5 shows the temperature contour of the cross-section at 25 m, from which it can be seen that the ideal tangential combustion was formed in the primary zone. In the burnout zone, the introducing of SOFA caused further combustion of the unburned carbon, so the temperature of the flue gas increased again. Then, the temperature of the flue gas dropped gradually because of the heat transfer between the flue gas and the water wall or other heat exchangers. resulting in the intense combustion reaction and the rapid rise in flue gas temperature. Figure 5 shows the temperature contour of the cross-section at 25 m, from which it can be seen that the ideal tangential combustion was formed in the primary zone. In the burnout zone, the introducing of SOFA caused further combustion of the unburned carbon, so the temperature of the flue gas increased again. Then, the temperature of the flue gas dropped gradually because of the heat transfer between the flue gas and the water wall or other heat exchangers.   resulting in the intense combustion reaction and the rapid rise in flue gas temperature. Figure 5 shows the temperature contour of the cross-section at 25 m, from which it can be seen that the ideal tangential combustion was formed in the primary zone. In the burnout zone, the introducing of SOFA caused further combustion of the unburned carbon, so the temperature of the flue gas increased again. Then, the temperature of the flue gas dropped gradually because of the heat transfer between the flue gas and the water wall or other heat exchangers.    Figure 6 shows the velocity contour and distributions of the main flue gas components (including O 2 , CO 2 , H 2 O and NO) within the boiler. As can be seen from Figure 6a, the flow field in the furnace showed good symmetry. The maximum velocity appeared in the vicinity of the nozzles, especially near the SOFA nozzles, while the flue gas velocity in the furnace center was relatively lower. The O 2 carried by PA and SA was mainly consumed in the primary zone, leading to the lower O 2 mole concentration in the center of the furnace, as shown in Figure 6b. Part of the O 2 was added through the SOFA nozzles for the oxidation of unburned carbon and CO, after which the O 2 concentration decreased gradually. Figure 6c presents the distribution of CO 2 . As the main product of coal combustion, the distribution of CO 2 was almost opposite to that of O 2 . The H 2 O distribution is shown in Figure 6d  lower in the central area of the burnout zone. H2O can react with chlorine to form OH through the equation of Cl + H2O = OH + HCl, which would participate in the Hg-related reactions. Therefore, it is necessary to consider the distribution of H2O in the furnace before the calculation of the Hg model. Similarly, nitrogen oxides could also be involved in the homogeneous reactions of mercury. The distribution of NO is shown in Figure 6e. The highest concentration of NO can be observed in the vicinity of the nozzles in the primary zone.

Effects of Chlorine Content in Coal on Homogeneous Reactions of Hg
Based on the results in Section 4.2, the homogeneous reactions of mercury species in coal-fired boilers were studied by utilizing the mercury sub-model. The reason why the Hg-related homogeneous reactions can be calculated using the post-treatment approach is that the contents of mercury species are very low. The mole concentrations of mercury species are orders of magnitude lower than those of other main flue gas components, such as O2, CO2, H2O and NO, which means the influences of mercury-related reactions on the distributions of other flue gas components are negligible. In other word, the obtained simulation results in Section 4.2 are still applicable after considering the mercury submodel.
Since the reactions between Hg and Cl are vital in the homogeneous oxidation of Hg 0 , it is of great significance to study the effects of chlorine content in coal on the distributions of mercury species in coal-fired boilers [38]. Previous studies have shown that the Cl

Effects of Chlorine Content in Coal on Homogeneous Reactions of Hg
Based on the results in Section 4.2, the homogeneous reactions of mercury species in coal-fired boilers were studied by utilizing the mercury sub-model. The reason why the Hg-related homogeneous reactions can be calculated using the post-treatment approach is that the contents of mercury species are very low. The mole concentrations of mercury species are orders of magnitude lower than those of other main flue gas components, such as O 2 , CO 2 , H 2 O and NO, which means the influences of mercury-related reactions on the distributions of other flue gas components are negligible. In other word, the obtained simulation results in Section 4.2 are still applicable after considering the mercury sub-model.
Since the reactions between Hg and Cl are vital in the homogeneous oxidation of Hg 0 , it is of great significance to study the effects of chlorine content in coal on the distributions of mercury species in coal-fired boilers [38]. Previous studies have shown that the Cl element in coal would be released in the form of HCl after entering the furnace [39]. Additionally, Hg 0 is the dominant mercury species under the high-temperature conditions of the primary zone. Therefore, HCl and Hg 0 were introduced from the PA nozzles when setting boundary conditions. Two cases (case A and case B) were set up with various chlorine contents in coal. Specifically, low-chlorine coal was used in case A, while high-chlorine coal was used in case B. The concentration of HCl in PA was set as 100 µL·L −1 in case A, which meant the Cl content in the coal in case A was 270 mg·kg −1 . The average concentration of HCl in the boiler can be calculated using Equations (1)- (7): V H 2 O = 0.111H ar +0.0124M ar + 0.0161V 0 (5) where C A (HCl) represents the average concentration of HCl in the boiler for case A, C A1 (HCl) represents the concentration of HCl in PA for case A, V 1 represents the flow rate of PA, V represents the total air flow rate, V 0 represents the theoretical air flow rate and V y represents the flow rate of flue gas. Combined with the parameters in Table S1 and Table 1, it can be calculated that C A (HCl) was about 23.5 µL·L −1 . In case B, the concentration of HCl in PA was set as 400 µL·L −1 , which meant the Cl content in the high-Cl coal was 1080 mg·kg −1 . By replacing Equation (1) with the following equation, the average concentration of HCl in the boiler for case B can be obtained: where C B (HCl) represents the average concentration of HCl in the boiler for case B and C B1 (HCl) represents the concentration of HCl in PA for case B. The value of C B (HCl) was about 94.0 µL·L −1 . In addition, the total mercury concentration in the boiler was set at 60 µg·m −3 in both case A and case B. The Hg content in the coal can be calculated as 0.567 mg·kg −1 in this study. Figures 7-9 show the distributions of chlorine species in the boiler under various cases. It should be mentioned here based on case A that case C was set up with 200 µL·L −1 of HCl fed into the SOFA nozzles on the third layer (five layers of SOFA nozzles in total). The detailed results of case C will be discussed later on (Section 4.4). In this section, we only focus on the simulation results of case A and case B. Besides HCl, the concentrations of Cl and Cl 2 in case B were also obviously higher than those in case A. Comparing the concentrations of these species, HCl is the dominant form of chlorine species in the whole furnace. The distributions of mercury species in the furnace are presented in Figures 10-13. The contours show that the homogeneous oxidation of Hg 0 in the coal-fired boiler mainly occurred in the vertical flue pass, where the temperature was relatively lower. Because of the high temperature in the furnace zone, the concentrations of oxidized mercury species were at least two orders of magnitude lower than that of Hg 0 . At the exit of the boiler, Hg 0 was still the dominant Hg-containing species, while HgCl 2 had the highest concentration among the oxidized mercury species. These results are in good agreement with the experimental data from the literature, indicating that the Hg sub-model used in this paper can simulate the homogeneous reactions of mercury in the furnace accurately [15,39]. The concentration of Hg 0 was higher in the lower part of the furnace, which is consistent with the findings of Zhang et al. [39]. This can be attributed to the large relative atomic mass of Hg. As the flue gas flowed to the upper part of the furnace, the horizontal flue pass and the vertical flue pass, the temperature of the flue gas also gradually decreased. The homogeneous oxidation of Hg 0 then occurred, especially in the vertical flue pass.             In order to analyze the influence of chlorine content in coal on homogeneous reactions of Hg quantitatively, the ratios of Hg and its compounds at the boiler outlet were calculated, as listed in Table 2  The concentration of Hg 0 was higher in the lower part of the furnace, which is consistent with the findings of Zhang et al. [39]. This can be attributed to the large relative atomic mass of Hg. As the flue gas flowed to the upper part of the furnace, the horizontal flue pass and the vertical flue pass, the temperature of the flue gas also gradually decreased. The homogeneous oxidation of Hg 0 then occurred, especially in the vertical flue pass.
In order to analyze the influence of chlorine content in coal on homogeneous reactions of Hg quantitatively, the ratios of Hg and its compounds at the boiler outlet were calculated, as listed in Table 2. In spite of the intense oxidation reactions of Hg 0 , the concentration of Hg 0 remained above 90% in the vertical flue pass. Comparatively, the ratios of Hg 0 in case A and case B were 99.039% and 95.196%, respectively, which meant more Hg 0 underwent homogeneous oxidation in the case with higher Cl content in coal. The proportion of HgCl 2 in oxidized mercury is shown in Figure 14 to better analyze the composition of the oxidized mercury. It can be seen that HgCl 2 was the main component among the oxidized mercury species, which is consistent with the findings in the contours. The proportions of HgCl 2 were higher than 95.9% in both cases. The results confirm that chlorine plays an important role in homogeneous oxidation of Hg 0 in coal-fired boiler [40].

Effects of Adding HCl on Homogeneous Reactions of Hg
Considering the important role of Cl-containing compounds in homogeneous reactions of Hg, a small amount of HCl can be added into boilers to promote the oxidation of Hg 0 . Nevertheless, the quantitative influence of HCl addition on the homogeneous reactions of Hg in full-scale utility boilers remains unclear. In this section, the distributions of chlorine-containing and mercury-containing species in the boiler before and after adding HCl are studied. According to the above analyses, the homogeneous reactions of Hg hardly occurred in the primary zone. Hence, supplementary HCl was added through the SOFA nozzles in the burnout area (case C). Combining Equations (2)-(7) and Equation (9), it can be calculated that the average concentration of HCl in the boiler in case C was about 39.7 μL·L −1 , which was about 69% higher than that in case A, but still lower than in case B. The concentration of Hg was kept at 60 μg·m −3 in case C.
where CC(HCl) represents the average concentration of HCl in the boiler for case C, CSOFA-

Effects of Adding HCl on Homogeneous Reactions of Hg
Considering the important role of Cl-containing compounds in homogeneous reactions of Hg, a small amount of HCl can be added into boilers to promote the oxidation of Hg 0 . Nevertheless, the quantitative influence of HCl addition on the homogeneous reactions of Hg in full-scale utility boilers remains unclear. In this section, the distributions of chlorine-containing and mercury-containing species in the boiler before and after adding HCl are studied. According to the above analyses, the homogeneous reactions of Hg hardly occurred in the primary zone. Hence, supplementary HCl was added through the SOFA nozzles in the burnout area (case C). Combining Equations (2)-(7) and Equation (9), it can be calculated that the average concentration of HCl in the boiler in case C was about 39.7 µL·L −1 , which was about 69% higher than that in case A, but still lower than in case B. The concentration of Hg was kept at 60 µg·m −3 in case C.
where C C (HCl) represents the average concentration of HCl in the boiler for case C, C SOFA−3 (HCl) represents the concentration of HCl in the third layer of SOFA nozzles and V SOFA−3 represents the air flow rate from the third layer of SOFA nozzles. As shown in Figures 7-9, the concentrations of Cl-containing species in case C were significantly higher than those in case A, especially in the burnout area. HCl was still the dominant component, while the concentration of Cl 2 was the lowest. Figures 10-13 depict the contours of the Hg-containing species. The concentration of Hg 0 in the burnout area was reduced significantly after the addition of HCl. In the whole boiler, especially in the vertical flue pass, the concentrations of HgCl 2 and HgCl increased remarkably in case C, while the impact of HCl addition on HgO was relatively slight. As the removal of elemental mercury is much more difficult than that of oxidized mercury, HCl addition is beneficial for Hg pollution control from coal-fired power plants burning low-Cl coal [41].
According to Table 2, after the addition of HCl, the ratio of Hg 0 decreased from 99% to about 84%, while the ratio of HgCl 2 increased by over 16 times and the content of HgCl also increased significantly. It is worth noting that the ratio of Hg 0 in case C is even lower than that in case B, which indicates that the homogeneous oxidation of Hg 0 in case C was more intense than that in case B, although the average Cl concentration in the boiler was lower. Therefore, it can be concluded that the addition of a small amount of HCl from the burnout area had an obvious promotion effect on the homogeneous oxidation of Hg 0 .
Zhuang et al. have experimentally studied the influence of adding CaCl 2 on the oxidation of Hg 0 in boilers [41]. They found that adding 600~1000 µL·L −1 CaCl 2 would increase the concentration of chlorine in flue gas by 88.0~146.7 µL·L −1 . They reported that this level of chlorine concentration was common for boilers using high-Cl bituminous coal and would not cause serious corrosion problems. In this study, the average chlorine concentrations of flue gas in the three cases were 23.5 µL·L −1 , 94.0 µL·L −1 and 39.7 µL·L −1 , respectively. Compared with the results from Zhuang et al. [41], the Cl concentration in this paper was acceptable for practical engineering applications. Therefore, adding HCl into boilers fired with low-Cl coal can promote the homogeneous oxidation of Hg 0 significantly without causing serious corrosion problems.

Conclusions
A numerical study was conducted on the homogeneous reactions of mercury in a coalfired utility boiler. For the first time, a novel mercury sub-model was built in a full-scale tangentially coal-fired boiler to achieve the coupling of thermodynamics, reaction kinetics and fluid dynamics. Since Cl is the most important element in the homogeneous oxidation of Hg 0 , the influence of the Cl content in coal on the homogeneous reaction of Hg in a boiler was studied. In addition, the quantitative effect of adding HCl on the homogeneous reactions of Hg within a full-scale boiler fired with low-Cl coal was investigated, which has not been reported before. The following conclusions can be drawn: 1. Ideal tangential combustion was achieved in the primary zone, causing the flue gas temperature to rise rapidly to 1748 K. The velocity field showed good symmetry in the furnace. The concentrations of H 2 O (~0.06 in mole fraction) and NO (~7 × 10 −4 in mole fractions) were higher in the vicinity of the primary air nozzles; 2. More elemental mercury (~4.8%) was converted to oxidized mercury through homogeneous reactions when high-Cl coal was burned, which is conducive to mercury pollution control. HCl was the main component of chlorine species in the boiler, while the concentrations of Cl and Cl 2 were low. The homogeneous reactions of Hg mainly occurred in the vertical flue pass with lower temperature. At the boiler exit, Hg 0 remained