Effect of the Concentrations of Different Flue Gas Components on Mercury Speciation

Abstract

This paper presents and establishes a reaction kinetic model of Hg/Cl/C/H/O/N/S to investigate the reaction characteristics of mercury during coal combustion and elucidate its migration and transformation mechanisms in flue gas. Using CHEMKIN software, the influence of HCl, Cl₂, and other flue gas components on the mercury oxidation reaction rate is examined. Building on this, the mechanism of Hg homogeneous oxidation under the influence of multi-component and multi-reaction interactions is revealed. The results indicated that as Hg concentration increased, the transformation rate of mercury also increased. As the reaction temperature increases, the reaction rate of HCl and elemental mercury also increases, leading to a higher transformation rate of mercury at elevated temperatures. Additionally, an increase in Cl₂ concentration leads to a higher amount of HgCl₂ produced. When the Cl₂ concentration was 4 × 10⁻⁵ mol/L, the amount of mercury chloride produced was highest, increasing by 40% compared to the absence of Cl₂. As chlorine concentration increases, more Hg²⁺ is converted from Hg⁰, enhancing its capture and removal by existing technologies, which significantly contributes to environmental sustainability and mercury emission control in coal-fired power plants. It is also shown that the rate of change of HgCl₂ varies with different Cl₂ concentrations, with higher Cl₂ concentrations inhibiting mercury oxidation beyond a certain threshold. The reaction was most intense when the mercury concentration was 5 × 10⁻⁵ mol/L. At this concentration, the largest amount of HgCl₂ is produced. The mercury conversion rate curve remained consistent after adding NO and SO₂, with a HgCl₂ amount increasing as NO and SO₂ concentrations rose. This indicates that the addition of NO and SO₂ converts Hg⁰ to Hg²⁺, thereby improving mercury removal efficiency and contributing to sustainability.

1. Introduction

Mercury is a highly toxic pollutant with significant volatility, bioaccumulation, and environmental persistence; even small amounts of mercury can cause substantial harm to human health and the ecological environment. Therefore, mercury elimination in the environment has been recognized as one of the world’s highest priorities, necessitating strict control measures in relevant production and usage areas. According to statistics, global anthropogenic mercury emissions in 2015 amounted to 2220 tons, with coal combustion accounting for 21%. As a major coal-consuming country [1], mercury emission from coal is the largest source of mercury pollution in China. Given that China’s current energy consumption structure is difficult to change in the short term, coal-fired power plants continue to dominate the power industry. To achieve sustainable development, it is imperative to address mercury pollution control.

In coal flue gas, mercury primarily exists in the forms of elemental mercury (Hg⁰), gaseous mercury oxide (Hg²⁺), and particulate mercury (Hg^p) [2]. Hg⁰ is characterized by high volatility and low water solubility [3] and can exist in a relatively stable form in coal flue gas, making it difficult to remove with existing pollution control equipment [4]. Therefore, the conversion of Hg⁰ is crucial for improving the efficiency of mercury removal from flue gas in coal-fired power plants. Numerous studies have shown that chlorine in coal flue gas plays a key role in influencing the distribution and removal efficiency of mercury. Chlorine in flue gas originates from coal, with most coal in China being low-chlorine or ultra-low-chlorine coal [5]. Adding chloride to coal to increase its chlorine content is considered an effective method for controlling mercury emissions. Therefore, in this study, CHEMKIN 17.0 software was used to simulate the effects of coal flue gas components (HCl, Cl₂, NO, Hg, SO₂) on the transformation of Hg forms.

Numerous researchers have conducted extensive studies on mercury speciation in coal-fired flue gas. Sliger et al. [6] established a homogeneous reaction kinetic model for mercury oxidation in coal-fired flue gas. The model comprised 23 homogeneous elementary reactions, with the Hg/Cl sub-mechanism including five elementary reactions. Quantum chemical calculations were used to report a constant reaction rate. Furthermore, the predictions of the developed kinetic model were consistent with the experimental results. In this context, an increase in HCl concentration led to a gradual decrease in Hg concentration. The analysis indicated that the homogeneous oxidation of mercury primarily proceeded through two elementary reactions: Hg + Cl = HgCl and HgCl + Cl = HgCl₂. Mercury oxidation predominantly occurred under cooled ambient conditions within the probe, within a temperature range of 400–700 °C. In another work, Mamani-Paco et al. [7] conducted experiments to investigate the impact of HCl and Cl₂ on mercury oxidation. In the experiment, they employed a mercury concentration of 50 μg/m³, a Cl₂ concentration of 50 ppm, and an HCl concentration of 100 ppm. The results indicated that with 50 ppm of chlorine, the mercury oxidation rate was 7–10%, whereas it increased to 36–45% with 100 ppm of chlorine. It is noteworthy that the addition of HCl increased the total chloride ion content. In this scenario, the oxidation rate was not significantly different from that observed with only 50 ppm Cl. Schager and Hall [8] studied the chemical reaction properties of various gas components (CO₂, HCl, Cl₂, SO₂, NO₂, NO, NO, NH₃, and H₂S) in coal-fired flue gas at a temperature in the range of 20 °C~900 °C. They found that Hg(g) could react slowly with NO₂(g), showed no reaction with NH₃(g), NO₂(g), SO₂(g), and H₂S(g), and exhibited rapid reactions with HCl(g) and Cl₂(g). The aforementioned studies demonstrate that several scholars have established kinetic models for mercury oxidation and investigated the effects of varying chlorine concentrations on the mercury oxidation rate. However, the impact of other flue gas components on mercury has not been extensively studied.

Furthermore, Minghou et al. [9] incorporated six elementary reactions related to HgO into Widmer’s [10] model, indicating that the reactions were closely related to oxygen. The kinetic parameters were derived from the work of Yang [11], Niksa [12,13], Edwards [14], and Liu et al. [15]. They reported that ClO in the flue gas reacts with elemental Hg to form HgO. At 700 °C, HCl and HOCl in the flue gas could transform HgO into HgCl, which was eventually oxidized into HgCl₂. Although the effect of O on Hg oxidation was weak, its presence in the reaction system should not be overlooked. In this context, O₂ has a direct effect on mercury oxidation. While Cl₂ and HCl play a major role in the mercury oxidation process and are highly oxidizing, their concentrations in the flue gas are much lower than that of O₂. Overall, approximately 10% of mercury is oxidized. Regarding simulation tools, CHEMKIN software [16,17] can accurately and efficiently simulate complex kinetic reactions and model idealized flow reactions. It also enables effective analysis of the results. Based on the existing mercury speciation transformation model, Fei et al. [18] used CHEMKIN software to analyze the relationship between Hg and Cl in the homogeneous oxidation kinetic models of mercury in six types of coal-fired flue gas. The dynamic parameters of the C/H/O/N/S/Cl submechanism are derived from the NIST database [19] and Roesler [20], Qiu [21], and Dryer et al. [22]. Additionally, they investigated the key elementary reaction processes in the homogeneous oxidation of elemental mercury in each model, along with the main oxidized substances. Although some studies have investigated the key elementary reaction processes and main oxidizing substances in the homogeneous oxidation of elemental mercury in each model, there has been no in-depth study on the effect of various flue gas components on mercury in coal-burning flue gas. Therefore, building on the previous studies, this paper further investigates the effects of varying flue gas component concentrations on mercury forms.

In this study, the mercury kinetic model is employed to investigate the key elementary reaction processes and primary oxidation species in the homogeneous oxidation of elemental mercury, using CHEMKIN software. The impact of major flue gas components on changes in mercury speciation is examined through scientific evaluation and control. Furthermore, the study explores and characterizes the homogeneous oxidation reaction mechanism of Hg under the influence of multi-component and multi-reaction interactions. The effect of flue gas component concentrations on mercury speciation is emphasized. The findings provide a theoretical foundation and valuable insights to support the effective control of mercury emissions in coal-fired flue gas.

2. Reaction Mechanism of Mercury During Coal Combustion

2.1. Thermochemical Properties

Chemical thermodynamics is closely tied to the characterization of equilibrium states in multi-component reaction systems. Thermodynamic calculations enable the determination of thermal and compositional properties of the products at equilibrium. Additionally, thermodynamics plays a crucial role in elucidating reaction kinetics, as thermodynamic data are used to calculate the equilibrium constant. More importantly, thermochemical properties are essential for benchmark theoretical calculations. Thermodynamic data for these substances are sourced from the CHEMKIN database. The thermochemical properties of various mercury species, including the enthalpy of formation, standard entropy, and specific heat capacity, are presented in

Table 1. Thermochemical properties of different species of mercury: standard enthalpy of formation, standard molar entropy, and specific heat capacity.

Mercury Species	Enthalpy of Formation	Standard Entropy	Molar Heat Capacity J/(mol K)
	(KJ/mol)	(J mol−1 k−1)
			300	400	500	600	800	1000	1500
Hg0	61.38	174.97	20.80	20.78	20.77	20.77	20.78	20.79	20.80
HgCl	78.45	260.00	36.43	37.03	37.36	37.58	37.87	38.1.	38.58
HgCl2	−146.29	294.78	58.11	59.89	60.72	61.19	61.66	61.89	62.15
HgBr	104.18	271.54	37.22	37.56	37.78	37.96	38.24	38.50	39.09
HgBr2	−85.45	320.22	60.29	61.18	61.59	61.81	62.04	62.15	62.27
HgI	133.47	280.75	37.93	38.28	38.59	38.86	39.39	39.91	41.18
HgI2	−16.13	336.21	61.13	61.65	61.90	62.04	62.18	62.24	62.31
HgF	2.93	248.36	34.63	35.93	36.58	36.58	37.46	37.78	38.34
HgF2	−293.65	271.74	54.04	56.96	58.65	59.68	60.79	61.33	61.89
HgO	16.72	239.13	32.78	34.65	35.57	36.11	36.75	37.14	37.74
HgH	238.49	219.71	30.06	31.42	32.81	34.05	35.94	37.23	39.31
CH3Hg	188.10	131.00	23.23	26.72	29.91	32.68	36.89	40.25	45.66
HgOH	39.21	258.30	42.85	43.91	44.91	45.84	47.53	49.00	51.87
HgNO	183.63	298.33	41.47	41.76	42.26	42.97	44.48	44.48	47.57
HgNO2	124.10	340.29	53.17	56.10	58.98	61.53	65.50	68.09	71.39
HgONO	157.59	302.51	52.92	57.22	60.65	63.33	67.05	69.30	72.06
HgSO	153.57	296.36	42.01	43.51	44.81	45.90	47.28	48.07	48.99

[19].

2.2. Basic Kinetic Reaction Parameters

Mercury concentrations in coal-fired flue gas typically fall below 10 ppb. Therefore, mercury does not substantially influence the concentration of other flue gas components; however, these components participate in mercury oxidation due to their higher concentrations in coal-fired flue gas. Most dynamic mechanisms were derived from the National Institute of Standards and Technology (NIST) database [19].

The Arrhenius formula is an empirical formula for the relationship between the rate constant of chemical reaction and temperature, which was created by Arrhenius of Sweden. Arrhenius’ formula is as follows: k is the rate constant, R is the molar gas constant, T is the thermodynamic temperature, E_a is the apparent activation energy, and A is the prefactor (also known as the frequency factor).

$$k=A{e}^{-E_a/RT}$$

(1)

The activation energy of a reaction is the energy required for a molecule to transition from its ground state to an activated state conducive to chemical reactions. For elementary reactions, the activation energy is the activation energy of the elementary reaction itself. For complex non-elementary reactions, the activation energy corresponds to the apparent activation energy of the overall reaction, which is the algebraic sum of the activation energies of the individual elementary reactions.

Table 2. Basic kinetic reactions of mercury oxidation and their respective parameters.

Elementary Reaction	Pre-Exponential Factor A	Temperature Exponent β	Activation Energy E (cal/mol)
Hg + Cl + M = HgCl + M	2.40 × 108	1.4	−14,400
Hg + Cl2 = HgCl + Cl	1.39 × 1014	0.0	34,000
HgCl + Cl2 = HgCl2 + Cl	1.39 × 1014	0.0	1000
Hg + HOCl = HgCl + OH	4.27 × 1013	0.0	19,000
Hg + HCl = HgCl + H	4.94 × 1014	0.0	79,300
HgCl + HCl = HgCl2 + H	4.94 × 1014	0.0	21,500
HgCl + HOCl = HgCl2 + OH	4.27 × 1013	0.0	1000
Cl + Cl + M = Cl2 + M	1.44 × 10	0.0	−1800
H + Cl + M = HCl + M	1.70 × 10	0.0	0
HCl + H = H2 + Cl	1.34 × 10	0.0	3500
H + Cl2 = HCl + Cl	1.39 × 10	0.0	1200
O + HCl = OH + Cl	3.53	2.9	3510
O + Cl2 = ClO + Cl	1.28 × 10	0.0	3585
O + ClO = Cl + O2	1.32 × 10	0.0	−193
Cl + HO2 = HCl + O2	1.30 × 10	0.0	894
Cl + HO2 = OH + ClO	1.34 × 10	0.0	−388
ClO + H2 = HOCl + H	1.18 × 10	0.0	14,100
H + HOCl = HCl + OH	1.40 × 10	0.0	7620
Cl + HOCl = HCl + ClO	1.23 × 10	0.0	258
Cl2 + OH = Cl + HOCl	1.21 × 10	0.0	1810
O + HOCl = OH + ClO	1.28 × 10	0.0	4372
HOCl + M = OH + Cl + M	1.02 × 10	−3.0	56,720
Hg + ClO2 = HgO + ClO	1.87 × 10	0.0	51,270
Hg + O3 = HgO + O2	7.02 × 10	0.0	42,190
Hg + N2O = HgO + N2	5.08 × 10	0.0	59,810
HgO + HCl = HgCl + OH	9.63 × 10	0.0	8920
HgO + HOCl = HgCl + HO2	4.11 × 10	0.0	60,470
Hg + ClO = HgO + Cl	1.38 × 10	0.0	8320
2O + M<=>O2 + M	1.20 × 10	−1.0	0

shows the basic kinetic reactions and parameters of mercury oxidation. In elementary reactions, (M) represents a protective or carrier gas. Data are adapted from Yang [11], Niksa [12,13], Edwards [14], and Liu et al. [15].

2.3. Flue Gas Component Parameters

CHEMKIN software is a gas phase chemical reaction kinetics software developed by Sandia National Laboratory for simulating gas phase chemical reaction and surface chemical reaction kinetics; it can solve complex flow problems with chemical reactions.

In the study of mercury oxidation kinetics, the reaction process involving mercury and the associated reaction conditions are simplified. The combustion simulation is performed using the continuous stirred-tank reactor model [23]. This model assumes that the calculation intensity is reduced, and the combustion process in the reactor can be described by a detailed chemical reaction mechanism, which is the reaction model under the ideal state of a completely mixed reaction in a certain volume. According to the literature [15,18], the pressure is set to an initial value of 1.0 atm, the temperature range is set from 1000 to 2000 K, and the selected mechanisms are Hg-Cl-H-O-C and Hg-Cl-H-O-C-N. The adopted flue gas composition is based on the simulated flue gas from Mamani-Paco et al. [7], which consists of approximately 25% H₂O, 11% CO₂, and 56% N₂ by volume. The initial component concentration value is set as shown in

Table 3. Initial concentrations of flue gas components.

Component	N2	H2O	CO2	O2	NO	HCl	Cl2	Hg
mol/L	5.6 × 10−1	2.5 × 10−1	1.1 × 10−1	7 × 10−2	1 × 10−5	1 × 10−5	1 × 10−5	1 × 10−5

. By changing the mole fraction of HCl, O₂, H₂O, CO₂, etc., CHEMKIN software is applied to draw the diagram. The effect of each component on mercury oxidation was illustrated by analyzing temperature–time–HgCl₂ curves and the curves of important chemical reactions with time and temperature.

3. Results and Discussion

3.1. Effect of HCl Concentration on Mercury Speciation

Considering the elementary reactions involving mercury, it is clear that Cl plays a leading role in the mercury oxidation process. In this context, chlorine atoms and elemental mercury form an intermediate HgCl component. This intermediate then undergoes a series of reactions with Cl, Cl₂, and HCl to produce mercury chloride as the final product. The mechanism of interaction between chlorine-containing compounds and elemental mercury has been a focus of recent studies, with the chlorine element in flue gas primarily being hydrochloric acid. Therefore, to study the oxidation effect of hydrochloric acid on mercury, the concentrations of all other components in the flue gas, as listed in

Table 4. Simulation of the concentration value of each component of flue gas.

Component	N2	H2O	CO2	O2	NO	Cl2	Hg	HCl
mol/L	5.6 × 10−1	2.5 × 10−1	1.1 × 10−1	7 × 10−2	1 × 10−5	1 × 10−5	1 × 10−5	1 × 10−5~5 × 10−5

, we kept constant while varying the concentration of HCl between 1 × 10⁻⁵ mol/L and 5 × 10⁻⁵ mol/L. Using the Hg-Cl-H-O-C mechanism, the model was studied over a temperature range of 1000–2000 K. The variations in the amount of HgCl₂ with temperature and time under different HCl concentrations are presented in Figure 1.

It is evident from the comparison of various lines in Figure 1 that, with the increase in the mass fraction of chlorine, the proportion of elemental mercury in flue gas decreases, while the conversion rate of divalent mercury increases. However, when HCl increases beyond a certain threshold, it reduces the content of Hg in the oxidized state, becoming an inhibitory factor in Hg oxidation. As the amount of hydrochloric acid increases within a certain range, the amount of HgCl₂ also increases significantly. The rate of change of HgCl₂ varies under different HCl concentrations, and the temperature range of HgCl₂ widens with increasing Cl content, ultimately leading to an increase in mercury content. As the chlorine content increases, more Hg²⁺ is converted from Hg⁰, allowing for greater capture and removal by existing technologies, thereby contributing to environmental sustainability and the control of mercury emissions from coal-fired power plants. Considering the variation trends at different temperatures, it is observed that the greater the amount of HCl, the more significant its impact on the reaction. Specifically, the reaction is most rapid at HCl concentrations of 3 × 10⁻⁵ mol/L and 4 × 10⁻⁵ mol/L, while the rate of formation of HgCl₂ is slightly slower at an HCl concentration of 5 × 10⁻⁵ mol/L. Conversely, the reaction is slowest at a concentration of 2 × 10⁻⁵ mol/L. Under stable conditions, the temperature range for HgCl₂ is approximately 1100–1400 K in all considered cases. Considering the variation in the amount of HgCl₂ over time, as shown in Figure 1, it is observed that, within a certain period, the rate of change of HgCl₂ intensifies as the amount of HCl increases. Thus, the more HCl added, the faster the reaction. However, when HCl exceeds a certain threshold, the content of Hg in the oxidized state decreases, becoming an inhibitory factor in Hg oxidation. Other researchers [24] suggest that Hg follows the same morphological distribution pattern under varying Cl content, existing as a relatively stable hydride at low temperatures and as Hg⁰ at high temperatures. The higher the Cl content in the flue gas, the wider the temperature range over which HgCl₂ remains stable. The Cl content directly influences the morphological distribution of Hg, which aligns with the simulation results presented in this study.

The primary reaction pathway for homogeneous mercury oxidation is a two-step process: Hg → HgCl → HgCl₂ [9]. Therefore, the homogeneous oxidation process occurs in two stages: Hg + Cl + M → HgCl + M and HgCl + Cl₂ → HgCl₂ + Cl [9]. In this process, Hg reacts with Cl atoms to form the intermediate HgCl. Subsequently, HgCl is further oxidized to form HgCl₂ via various elementary reactions. Overall, the homogeneous mercury oxidation process in the isothermal stage is primarily governed by the reaction rate of HgCl + Cl₂ → HgCl₂ + Cl. During the cooling stage, the mercury oxidation process is governed by the reaction rate of Hg + Cl + M → HgCl + M.

During coal combustion, a significant portion of the chlorine in the coal exists as hydrogen chloride [25]. The following formula represents the reaction between Hg and HCl:

Hg⁰(g) + 2HCl(g)→HgCl₂(g)+ H₂(g)

(2)

When coal is burned, if the corresponding chlorine content in coal is high, Hg will mainly exist in the form of HgCl₂ in the resulting flue gas. As the flue gas moves toward the chimney outlet, it passes over multiple surfaces, gradually cooling. Consequently, the mercury in the flue gas undergoes both physical and chemical reactions. More than one-third of the mercury reacts with other components in the flue gas to form various mercury compounds [25]. Among these, mercury chloride is the predominant product of the reaction. Generally, mercury chloride has a high vapor pressure [25], which significantly promotes the volatilization of mercury from coal and delays its condensation. As a result, mercury is released into the atmosphere as a gas, causing significant environmental harm. However, because mercury chloride is easily adsorbed by adsorbents and is soluble in water, its release into the environment can be effectively controlled by employing flue gas dust removal and wet desulfurization technologies.

Furthermore, the kinetic simulation results [26] indicated that, within the combustion zone’s temperature range, chlorine content was highest. As the flue gas temperature gradually decreases, most chlorine and hydrogen atoms combine to form hydrogen chloride, while a smaller portion of chlorine atoms form Cl₂. Additionally, as the temperature increases, the concentration of mercury ions also rises. Therefore, with increasing reaction temperature, it is observed that at relatively low temperatures, the flue gas cools more rapidly toward the outlet. Furthermore, the concentration of chlorine atoms increases with rising reaction temperature, leading to a greater extent of Hg⁰(g) reacting with chlorine atoms. As the reaction temperature rises, the amount of chlorine atoms precipitating from coal gradually increases. This is accompanied by a substantial amount of chlorine atoms being converted into HCl and Cl₂. As a result, the oxidation of Hg⁰(g) by hydrogen chloride and chlorine gases increases. This leads to an increase in Hg²⁺ content and enhances the conversion rate of elemental mercury to oxidized mercury.

3.2. Effect of Cl₂ Concentration on Mercury Speciation

This section evaluates the effect of Cl₂ content, varied between 1 × 10⁻⁵ mol/L and 5 × 10⁻⁵ mol/L, on the mercury oxidation process while keeping the concentrations of other components constant. The simulation in this study was conducted at temperatures ranging from 1000 to 2000 K. The resulting variation curves of mercury chloride amount with temperature and time are presented in Figure 2.

Table 5. Simulation of the concentration value of each component of flue gas.

Component	N2	H2O	CO2	O2	NO	HCl	Hg	Cl2
mol/L	5.6 × 10−1	2.5 × 10−1	1.1 × 10−1	7.1 × 10−2	1 × 10−5	1 × 10−5	1 × 10−5	1 × 10−5~5 × 10−5

shows the concentration values of each component of the simulated selected flue gas.

It is well established that chlorine is an essential component in the oxidation of Hg⁰. During coal combustion, most of the chlorine exists as chlorine atoms. After cooling, the flue gas produces hydrogen chloride and chlorine gas [27]. The reactions involved can be described by the following equations:

Cl + H↔HCl

(3)

2Cl↔Cl₂

(4)

4Cl + 2H₂O↔4HCl + O₂

(5)

4HCl + O₂↔2Cl₂ + 2H₂O

(6)

The results shown in Figure 2 indicate that the amount of HgCl increases with increasing Cl₂ concentration. The intensity of the change in HgCl₂ varies with different Cl₂ concentrations. As the mass fraction of chlorine increases, the proportion of elemental mercury in the flue gas decreases, and the conversion rate of divalent mercury increases. The greater the amount of divalent mercury produced, the more effective mercury removal measures become in reducing mercury emissions from power plants, thereby mitigating the environmental and health risks. However, when Cl₂ increases beyond a certain threshold, the amount of Hg in the oxidized state decreases, inhibiting mercury oxidation. Additionally, the variation of HgCl₂ amount with temperature shows that greater Cl₂ concentration has a more significant impact on the reaction. The reaction proceeds most rapidly when the Cl₂ concentration is 4 × 10⁻⁵ mol/L, resulting in the largest amount of HgCl₂ produced. During the stable phase, the temperature range for HgCl₂ in all cases is 1100–1400 K. The variation of HgCl₂ with time shows that, over a certain period, the rate of change intensifies as Cl₂ concentration increases. Thus, the greater the increase in Cl₂ concentration, the larger the amount of HgCl₂ produced. However, the reaction reaches a near-stable phase within a short initial period.

Furthermore, research [28] results have shown that Hg⁰ reacts more favorably with HCl at higher temperatures, while it reacts with Cl₂ at lower temperatures. In a related study, Agarwal et al. [29] found that 95% of Hg⁰ oxidation occurred when Cl₂ concentration exceeded 50 ppmv. From an electronic structure perspective, Cl₂ is a much stronger oxidizer than HCl.

3.3. Effect of Hg Concentration on Mercury Speciation

This section examines the effect of varying Hg content on mercury oxidation. The Hg content ranges from 1 × 10⁻⁵ mol/L to 5 × 10⁻⁵ mol/L, while the concentrations of the other components in

Table 6. Simulation of the concentration value of each component of flue gas.

Component	N2	H2O	CO2	O2	HCl	Cl2	NO	Hg
mol/L	5.6 × 10−1	2.5 × 10−1	1.1 × 10−1	7 × 10−2	1 × 10−5	1 × 10−5	1 × 10−5	1 × 10−5~5 × 10−5

remain constant. The simulation and analysis were performed within a temperature range of 1000 to 2000 K under these conditions. The resulting variation curves of mercury chloride with temperature and time at different Hg concentrations are shown in Figure 3.

As shown in Figure 3, an increase in Hg content leads to a corresponding increase in HgCl₂, with varying rates of change depending on Hg concentration. Upon examining the change in HgCl₂ with temperature, it is observed that a higher Hg content has a stronger impact on the reaction. In this regard, the reaction becomes more intense as Hg content increases. The reaction is most intense when the Hg concentration reaches 5 × 10⁻⁵ mol/L. At this concentration, the largest amount of HgCl₂ is generated in the reaction. Additionally, higher Hg concentrations lead to more pronounced variations in the amount of HgCl₂. Upon reaching a stable phase, the temperature range of HgCl₂ is consistently between 1100 and 1400 K. Considering the impact of time on the variation trend, it is observed that the change in HgCl₂ intensifies as Hg content increases over time. It is evident that the greater the increase in Hg, the faster the reaction rate. However, when Hg reaches a critical threshold, the amount of HgCl₂ produced decreases, inhibiting mercury chlorination.

Furthermore, increasing mercury content accelerates the transformation rate. Conversely, it reduces the point at which mercury oxidation stabilizes. Moreover, as temperature increases, the reaction rate between mercury and hydrogen chloride also rises. This is considered the dominant process for mercury oxidation at high temperatures. Therefore, it can be concluded that mercury undergoes significant transformation at high temperatures. In practical coal combustion applications, variations in mercury, chlorine, and sulfur content in coal arise due to differences in coal origin and type. This results in different forms of mercury in the flue gas following coal combustion. Additionally, factors such as combustion mode, flue design, and the presence of unconsidered components in the flue gas contribute to variations in mercury form in the flue gas during coal combustion. Consequently, appropriate mitigation measures must be implemented for varying environmental conditions.

3.4. Effect of NO Concentration on Mercury Speciation

The impact of varying NO content from 1 × 10⁻⁵ mol/L to 5 × 10⁻⁵ mol/L on the oxidation of mercury is analyzed and simulated, with other flue gas component concentrations held constant. This study was conducted over a temperature range of 1000 to 2000 K. The variation trends of mercury chloride with temperature and time under different NO concentrations are shown in Figure 4.

Table 7. Simulation of the concentration value of each component of flue gas.

Component	N2	H2O	CO2	O2	HCl	Cl2	Hg	NO
mol/L	5.6 × 10−1	2.5 × 10−1	1.1 × 10−1	7.1 × 10−2	1 × 10−5	1 × 10−5	1 × 10−5	1 × 10−5~5 × 10−5

shows the concentration values of each component of the simulated selected flue gas.

As shown in Figure 4, HgCl₂ increases with higher NO concentrations. This aligns with the experimental results reported by Gao Hongliang et al. [30] The rate of change in HgCl₂ varies with different NO concentrations. As the amount of NO increases, its effect on the reaction becomes more significant, with greater variations in HgCl₂ observed at higher temperatures. The variation curves overlap at NO concentrations of 5 × 10⁻⁵ mol/L and 4 × 10⁻⁵ mol/L. This suggests that the reaction intensity is similar at these concentrations. Furthermore, the reaction rate is slower at a NO concentration of 3 × 10⁻⁵ mol/L and slowest at 1 × 10⁻⁵ mol/L. During the stable phase, HgCl₂ temperatures in all cases range from approximately 1100 to 1400 K. Moreover, the figure highlights that the change in HgCl₂ over time intensifies as the NO content increases. In this regard, higher NO concentrations result in larger amounts of HgCl₂ being produced.

Furthermore, NO_X produced during coal combustion is predominantly in the form of NO, with a smaller amount of NO₂. As the temperature increases, NO₂ decomposes into NO, starting at 150 °C and ending at 600 °C. In their study, Rumayor et al. [31] reported that at a flue gas temperature of 150 °C, adding 200 ppmv and 400 ppmv of NO, the Hg²⁺ content in the atmosphere, where NO and N₂ are in equilibrium, can reach 5.0% and 7.5%, respectively. This is represented by reaction (7). Moreover, the percentage of Hg²⁺ can be increased to 14% by adding CO₂ and O₂ to the gas mixture. This occurs because NO reacts with O₂ to form NO₂ and free radical oxygen atoms (reaction 8), which possess strong oxidizing properties (reactions 9 and 10) [32]. Subsequently, NO and NO₂ react with Hg⁰ to form Hg(NO₂)₂ (reaction 11) and Hg(NO₃)₂.

2Hg⁰(g) + 2NO(g)→2HgO(g) + N₂(g)

(7)

NO(g) + O₂(g)↔NO₂(g) + O

(8)

Hg⁰(g) + O(g)→ HgO(g,s)

(9)

Hg⁰(g) + NO₂(g)→HgO(g) + NO(g)

(10)

Hg⁰(g) + NO₂(g)→Hg(NO₂)₂(g) + NO(g)

(11)

3.5. Effect of SO2 Concentration on Mercury Speciation

To analyze the impact of SO₂ concentration on mercury oxidation, the concentrations of all other components listed in Table 3 remain constant while varying the SO₂ content from 1 × 10⁻⁵ mol/L to 5 × 10⁻⁵ mol/L. Simulations of the variations in HgCl₂ with temperature and time under different SO₂ concentrations were conducted over a temperature range of 1000 to 2000 K, as shown in Figure 5.

Table 8. Simulation of the concentration value of each component of flue gas.

Component	N2	H2O	CO2	O2	Cl2	NO	Hg	HCl	SO2
mol/L	5.6 × 10−1	2.5 × 10−1	1.1 × 10−1	7 × 10−2	1 × 10−5	1 × 10−5	1 × 10−5	1 × 10−5	1 × 10−5~5 × 10−5

shows the concentration values of each component of the simulated selected flue gas.

The results presented in Figure 5 show that the amount of HgCl₂ increases with an increase in SO₂ concentration. The rate of change of HgCl₂ varies with different SO₂ concentrations. As shown in the variation trend of HgCl₂ with temperature, a higher SO₂ concentration leads to a stronger effect on the reaction, making it more intense. Notably, the SO₂ concentration curves for 5 × 10⁻⁵ mol/L, 4 × 10⁻⁵ mol/L, and 2 × 10⁻⁵ mol/L show similar trends, while the other two curves also follow a similar pattern. Additionally, each curve shows an abrupt change in temperature at 1400 K, with the stable phase temperatures of HgCl₂ in all cases ranging from 1100 to 1400 K. Furthermore, within a certain period, the variation in HgCl₂ intensifies as the SO₂ concentration increases. At a specific point, the reactions undergo an abrupt change in the same manner. In this case, the reaction trends are consistent, resulting in similar final amounts of HgCl₂ produced.

During coal combustion, sulfur dioxide is the primary product of sulfur oxidation. Additionally, HgSO₄ is one of the primary oxidized mercury species in chlorine-free flue gas. It is noted that certain reaction intermediates, such as HgSO and HgSO₂, play a crucial role in the formation of HgSO₄. Furthermore, SO is an active free radical that reacts with Hg⁰ to form HgSO [33]. Subsequently, Hg⁰ can also react with SO₂ to form HgSO₂ species. The reaction enthalpies for Hg⁰ + SO and Hg⁰ + SO₂ are 20.9 kcal/mol and −3.0 kcal/mol, respectively. It is worth noting that SO₂ has an inhibitory effect on the homogeneous oxidation of mercury due to the irreversible reaction SO₂ + Cl₂ → SO₂Cl₂.

4. Conclusions

In this study, the CHEMKIN software was used to investigate the effect of HCl, Cl₂, and other flue gas components on mercury speciation. Using a scientific control approach, the mercury kinetic model was employed to examine the impact of varying concentrations of flue gas components on mercury oxidation. The main conclusions of this study are as follows.

(1) The reaction process involving mercury indicates that chlorine plays a dominant role in its oxidation. Chlorine atoms and elemental mercury combine to form the intermediate HgCl. Subsequently, HgCl reacts with Cl, Cl₂, and HCl to form the final product, mercury chloride. Within a specific concentration range, the amount of mercury chloride increases with higher hydrogen chloride concentrations. In the stable phase, the temperature range of HgCl₂ is between 1100 and 1400 K.

(2) As the Cl₂ concentration increases, the amount of HgCl₂ increases accordingly. The rate of change in HgCl₂ varies with Cl₂ concentrations. When the Cl₂ concentration was 4 × 10⁻⁵ mol/L, the amount of mercury chloride produced was highest, increasing by 40% compared to the absence of Cl₂. However, increasing the Cl₂ concentration beyond a certain level inhibits mercury oxidation. The reaction was most intense when the mercury concentration was 5 × 10⁻⁵mol/L. At this concentration, the largest amount of HgCl₂ is produced in the reaction. The mercury conversion rate curve remained consistent after adding NO and SO₂, with the amount of HgCl₂ increasing as the concentrations of NO and SO₂ rose.

(3) This study provides a theoretical basis and practical significance for the control of mercury in coal-fired flue gas, and effectively controlling mercury emission is conducive to achieving environmental protection goals and promoting sustainable development.

(4) The main reaction pathways for the homogeneous oxidation of Hg in coal-fired flue gas are controlled by the following three reactions: Hg + Cl + M = HgCl + M, Hg + HOCl = HgCl + OH, and Hg + HCl = HgCl + H.