Mercury Adsorption/Oxidation Mechanisms on Fly Ash Under N₂ Atmosphere

Abstract

Mercury adsorption/oxidation plays a crucial role in mercury transformation during coal combustion. To gain an intuitive understanding of the adsorption/oxidation mechanisms between mercury and fly ash, changes in mercury speciation of fly ash before and after Hg adsorption were investigated using temperature-programmed decomposition–atomic fluorescence spectroscopy (TPD-AFS). The results directly reveal that the primary adsorption/oxidation mechanism between mercury and fly ash is the heterogeneous oxidation reaction of Hg⁰ to HgCl₂. The mercury adsorption capacity exhibits a strong positive correlation with both the unburned carbon (UBC) content and the specific surface area (SSA) of the fly ash, whereas the presence of metal oxides has a negligible effect on mercury adsorption. Higher inlet concentrations of Hg⁰ enhance mercury adsorption, while flue gas components such as N₂, O₂, and CO₂ have minimal influence on mercury adsorption.

1. Introduction

Mercury (Hg) has raised significant environmental concerns owing to its toxicity, persistence, and bioaccumulation. Coal combustion, especially in coal-fired power plants (CFPPs), is recognized as the major anthropogenic source of mercury emissions [1]. Over recent decades, a range of effective adsorbents has been designed to mitigate mercury pollution [2]. Among these, fly ash has garnered considerable attention due to its abundance and low cost [3]. Importantly, during the mercury transformation process in coal combustion, a fraction of Hg²⁺ adsorbs onto fly ash, forming particle-bound mercury (Hg^p), which can be efficiently removed by dust collectors [4]. Furthermore, fly ash injection technology has emerged as a promising approach for mercury removal in CFPPs [5].

Numerous studies have investigated mercury adsorption/oxidation by fly ash, focusing on the influence of fly ash composition (e.g., UBC content, particle size, surface area and metal oxides), and flue gas constituents [6,7]. Generally, both the content and type of UBC in fly ash significantly influence mercury adsorption [8,9,10], while larger particle sizes and higher surface areas enhance the mercury adsorption capacity [11,12]. The presence of metal oxides (e.g., Fe₂O₃, CuO, Al₂O₃, TiO₂) in fly ash facilitates mercury adsorption [13]. Additionally, flue gas components such as NO, HCl, and O₂ improve mercury capture [14,15,16,17], whereas SO₂ tends to inhibit mercury adsorption [18]. However, the mercury adsorption/oxidation mechanism is still unclear due to its complexity and requires further investigation.

Based on published studies, the effects of flue gas components on mercury adsorption mainly focus on hydrogen chloride, sulfur oxide, nitrogen oxide and oxygen [13,14,15,16,17]. However, the influence of N₂ has rarely been investigated, despite its importance in comprehensively understanding mercury adsorption/oxidation. As an inert gas, N₂ does not directly participate in mercury adsorption/oxidation. Therefore, mercury adsorption/oxidation on fly ash under N₂ atmosphere can be attributed solely to the interactions between mercury and fly ash. Based on the adsorption/oxidation mechanisms between mercury and fly ash, we can gain deeper insights into the overall adsorption/oxidation mechanisms involving mercury, fly ash, and flue gas components. Furthermore, understanding these mechanisms under N₂ atmosphere could facilitate the development of innovative mercury control technologies that utilize inert gas environments. Wang et al. [19] conducted a study on mercury adsorption and oxidation on fly ash in an N₂ atmosphere. Their results confirm that mercury adsorption occurred simultaneously with oxidation. However, under N₂ atmosphere, the mercury adsorption/oxidation on fly ash has not been directly revealed, and its influences remain unclear.

In this study, mercury adsorption/oxidation on fly ash under N₂ atmosphere was investigated. Changes in mercury speciation of fly ash before and after Hg adsorption were analyzed using TPD-AFS technology, which directly revealed the mercury adsorption/oxidation mechanisms. Based on these mechanistic insights, we comprehensively analyzed the influence of fly ash components, inlet mercury concentration, and atmospheric composition on the adsorption/oxidation processes.

2. Materials and Methods

2.1. Sample Preparation

Fly ash was collected from three low-calorific-value (LCV) CFPPs, QX (Shanxi Qinxin Coal Industry Co., Ltd. Coal Gangue Power Plant, Changzhi City, China), YW (Shanxi Lu’an Yuwu Thermal Power Co., Ltd., Changzhi City, China) and YH (Shanxi Yonghao Coal Gangue Power Generation Co., Ltd, Shuozhou City, China). All plants employed circulating fluidized bed (CFB) (Taiyuan Boiler Group Co., Ltd., Taiyuan City, China; Wuhan Boiler Group Co., Ltd., Wuhan City, China; Dongfang Boiler Co., Ltd., Zigong City, China) technology, with generating capacities ranging from 10 to 150 MW. Fly ash was captured using electrostatic precipitators or fabric filters. The samples were sieved through a 100-mesh screen before use.

Standard mercury compounds (HgCl₂ or HgO) in the matrix of fly ash were prepared using the successive dry dilution method. To obtain mercury-free fly ash, YW fly ash was heated at 650 °C for 2 h. Subsequently, 0.1 g of standard mercury compound was thoroughly homogenized with 10 g of mercury-free fly ash. This mixture was further diluted with mercury-free fly ash using the same procedure until the mercury concentration reached 10⁻⁹ g/g.

2.2. Thermal Decomposition and Mercury Adsorption Experiments

0.1 g of sample was loaded into a quartz boat and positioned at the center of TPD. Under a constant N₂ purge (300 mL/min), the sample underwent heating (20 °C/min) from room temperature up to 1200 °C. The gaseous products were directly swept to AFS. The continuous dynamic release intensity of Hg⁰ is recorded by the AFS [20].

The experimental platform for mercury adsorption (Figure 1) consists of a mercury vapor generation device, a mercury adsorption device, and an AFS. Mercury vapor was produced using a mercury permeation tube, with the temperature controlled by a thermostatic bath. The generated mercury vapor was transported into the mercury adsorption device using a mixed carrier gas of Ar (350 mL/min) and N₂ (650 mL/min). The fixed-bed adsorption device comprises a double-layer (inner/outer) quartz tube. A glass fiber carrier was uniformly placed on the quartz microporous plate, and 1 g of fly ash was evenly spread on the glass fiber carrier, resulting in a fly ash bed thickness of approximately 5 mm. The outer tube was filled with water from a thermostatic water bath. The adsorbed mercury vapor was analyzed and measured online using AFS.

The specific surface area (SSA) was measured using BET analysis (TriStar II 3020, Version 3.02) based on nitrogen adsorption/desorption isotherm, and the mineral composition was analyzed using X-ray fluorescence (XRF) (S8 Tiger, Bruker Corporation, Karlsruhe, Germany). The unburned carbon content (UBC) in the fly ashes was determined by a thermogravimetric analyzer (TG209F1, NETZSCH Group, Bavaria, Germany).

2.3. Adsorption Analysis

The Hg adsorption performance was evaluated based on the penetration rate at specific time intervals. The penetration rate (Ψ) is expressed as Equation (1):

$$\mathrm{\Psi }={\displaystyle \frac{\mathrm{C}}{{\mathrm{C}}_0}}\times 100\%$$

(1)

C: outlet Hg concentration; C₀: inlet Hg concentration.

The adsorption efficiency at a certain time (η) can be expressed as Equation (2):

$$\mathrm{\eta }=1-\mathrm{\Psi }$$

(2)

By plotting adsorption efficiency as a function of time, the Hg adsorption efficiency curve can be obtained. The cumulative Hg adsorption capacity from the beginning of adsorption to time t is calculated as Equation (3):

$$\mathrm{q}={\displaystyle \frac{\mathrm{Q}\times {\mathrm{C}}_0\times {\displaystyle {\int }_{\mathrm{0}}^{\mathrm{t}}\mathrm{\eta }\mathrm{d}\mathrm{t}}}{\mathrm{m}}}={\displaystyle \frac{{\mathrm{A}}_{\mathrm{a}}\times {\mathrm{C}}_{\mathrm{0}}}{\mathrm{m}}}$$

(3)

q: cumulative Hg adsorption capacity at a specific time (ng/g); t: adsorption time (min); Q: carrier gas flow rate through the adsorbent layer (m³/min); m: mass of adsorbent (g); A_a: the area under the adsorption efficiency curve, as shown in Figure 2.

3. Results and Discussion

3.1. Physicochemical Properties of Fly Ash

The chemical composition and BET of fly ash are summarized in

Table 1. Chemical composition and BET of fly ash.

wt %	YH	YW	QX
SiO2	48.69	49.74	31.07
Al2O3	39.15	33.35	17.24
Fe2O3	3.46	3.14	5.09
CaO	4.5	6.61	32.02
MgO	0.36	0.82	1.36
TiO2	1.13	1.23	0.59
SO3	0.69	2.11	11.35
K2O	0.5	0.95	0.6
Na2O	0.06	0.68	0.14
P2O5	0.2	0.22	0.17
UBC %	10.59	25.66	14.7
SSA m2/g	8.48	26.27	8.63

.

As shown in Table 1, both YH and YW fly ashes are primarily composed of SiO₂ and Al₂O₃, with secondary amounts of CaO and Fe₂O₃. Other components, including MgO, TiO₂, SO₃, K₂O, Na₂O, and P₂O₅, are present at much lower concentrations. In contrast, QX fly ash is characterized by high levels of CaO, SiO₂, Al₂O₃, SO₃, and Fe₂O₃, with only trace amounts of MgO, TiO₂, K₂O, Na₂O, and P₂O₅ detected. YH and YW fly ashes possess higher concentrations of SiO₂ and Al₂O₃ compared to QX fly ash. Conversely, the CaO content is significantly elevated in QX fly ash, a characteristic that may be a direct consequence of the desulfurization by co-fired calcium oxide.

The SSAs of the fly ash decrease in the following order: YW (26.27 m²/g) > QX (8.63 m²/g) > YH (8.48 m²/g). Previous studies have indicated that the SSA of fly ash is generally less than 10 m²/g [21]. Consistent with this trend, the SSAs of QX and YH fly ash are below 10 m²/g, while the SSA of YW fly ash is significantly higher than the typical range. The larger specific surface area (SSA) of YW fly ash is likely associated with its relatively large average pore size and well-developed pore structure [21].

The UBC content follows the order: YW fly ash (25.66%) > QX fly ash (14.7%) > YH fly ash (10.5%). The measured UBC contents are greater than those reported in other studies [19,22], primarily due to the poor combustion characteristics of LCV coal. When LCV coals of different qualities are blended and combusted, the combustion characteristic index rises rapidly, resulting in a decrease in the combustion rate. This decline in combustion rate is particularly pronounced when the properties of the blended coals differ significantly, which may result in reduced burnout efficiency.

3.2. Adsorption/Oxidation Mechanisms Between Mercury and Fly Ash

With an inlet Hg concentration of 40 ng/min and N₂ as the balance gas at 35 °C, the Hg adsorption breakthrough curves for QX, YH and YW fly ash are presented in Figure 3.

As illustrated in Figure 3, the extent of mercury concentration reduction varied among the three fly ashes. YW fly ash exhibited the greatest reduction, with the Hg concentration declining to approximately 55% of the initial value, whereas the concentrations for QX and YH fly ashes decreased to about 70% of the initial value. In terms of mercury adsorption efficiency, YH fly ash showed the lowest performance, reaching saturation within 30 min. In contrast, YW fly ash demonstrated the highest adsorption efficiency, requiring 75 min to approach saturation. Figure 3 shows that the equilibrium adsorption times for the three fly ashes follow the order: YW > QX > YH. Consequently, the Hg⁰ adsorption efficiency decreases in the sequence of YW, QX, and YH.

To investigate the mercury adsorption/oxidation mechanisms under N₂ atmosphere, changes in mercury chemical species in the fly ash before and after Hg adsorption were identified using TPD-AFS. The original fly ash samples were labeled QX, YH, and YW. After reaching adsorption equilibrium, the corresponding samples were labeled QXXF, YHXF, and YWXF, respectively. The results are presented in Figure 4.

As shown in Figure 4, the release temperature range and main peak temperature of mercury for all three fly ashes remained essentially unchanged before and after adsorption, indicating that the mercury compounds formed after Hg⁰ adsorption under an N₂ atmosphere are consistent with those in the original fly ash [19,23]. Under an N₂ atmosphere, besides undergoing adsorption, fly ash also promotes an oxidation reaction with Hg⁰. Notably, a change in mercury speciation before and after Hg adsorption was not distinctly observable. To better characterize the adsorption and oxidation behavior, the fly ash was pre-heated to remove existing mercury. Higher pretreatment temperatures resulted in greater losses of UBC, consequently reducing the Hg⁰ adsorption/oxidation capacities. The maximum peak temperature was approximately 295 °C to balance mercury removal with carbon retention. The dried fly ash samples were pretreated by heating at 350 °C for 2 h. The pretreated fly ash samples were labeled QXH, YHH, and YWH, and the pretreated fly ash after the sample reached adsorption equilibrium was labeled QXHXF, YHHXF and YWHXF. The occurrence forms of Hg in pretreated fly ash are presented in Figure 5.

As shown in Figure 5, both QXH and YWH fly ash samples exhibited minimal mercury release, indicating effective mercury removal. In contrast, YHH fly ash displayed a pronounced release peak at approximately 500 °C, suggesting the retention of residual mercury due to its inherently higher mercury content. After mercury adsorption, all three fly ash samples began to release mercury at around 100 °C with a primary peak at approximately 260 °C. Notably, the intensity of the 500 °C peak in YHH fly ash remained virtually unchanged after adsorption, indicating that this peak corresponds to stable mercury species that are not involved in the adsorption process.

To identify Hg speciation in fly ash, numerous studies have investigated the pyrolysis behavior of standard mercury compounds using fly ash as reference materials [19,23,24,25]. In this study, TPD-AFS experiments were conducted on standard mercury compounds (HgCl₂ and HgO) using YWH fly ash as reference materials, representing the typical Hg speciation in fly ash. The obtained results were compared with literature-reported pyrolysis temperature ranges and peak temperatures for these standard compounds. The Hg release characteristics of HgCl₂ and HgO based on YWH fly ash are presented in Figure 6.

As shown in Figure 6, the initial mercury release temperature of HgCl₂ is approximately 150 °C with the main release peak at about 255 °C. For HgO, the initial mercury release temperature is around 120 °C with the main peak at about 315 °C, which is consistent with the reported peak temperatures for HgCl₂/fly ash (240 °C) and HgO/fly ash (312 °C) [19].

As shown in Figure 5, the initial Hg release temperature for all three fly ashes after Hg adsorption is about 100 °C and the main peak temperature was about 260 °C, which are closest to the initial mercury release temperature (120 °C) and peak temperature (255 °C) of HgCl₂. Therefore, the main mercury chemical species in three fly ashes is HgCl₂. This indicates that the mercury adsorption/oxidation mechanism on fly ash is the heterogeneous oxidation reaction of Hg⁰ into HgCl₂. As N₂ is an inactive gas and does not participate in mercury oxidation, the mercury oxidation on fly ash under N₂ atmosphere can be attributed to residual chlorine in the fly ash. Earlier investigations indicate that chlorine can persist in fly ash even after heating at 650 °C for 4 h [19]. The heterogeneous oxidation reaction is facilitated by the interaction of Hg⁰ with chlorine-containing functional groups, where active chlorine species promote the conversion of Hg⁰ to HgCl₂. The reactions are shown in Equations (4) and (5) [26].

$${{\mathrm{H}}_{\mathrm{g}}}^0+{[\mathrm{C}\mathrm{l}]}^{-}\to {[{\mathrm{H}}_{\mathrm{g}}\mathrm{C}\mathrm{l}]}^{+}+2\mathrm{e}$$

(4)

$${{\mathrm{H}}_{\mathrm{g}}}^0+2{[\mathrm{Cl}]}^{-}\to {\left[{\mathrm{H}}_{\mathrm{g}}{\mathrm{Cl}}_2\right]}^{+}+2\mathrm{e}$$

(5)

3.3. Factors Affecting Mercury Adsorption/Oxidation on Fly Ash

3.3.1. Influence of Fly Ash Components

Fly ash components, such as fly ash UBC, SSA and inorganic chemical composition, have been identified as factors influencing mercury adsorption/oxidation on fly ash [27,28,29]. The mercury adsorption capacity under different conditions is summarized in

Table 2. The mercury adsorption capacity under different conditions.

Fly Ash	Atmosphere	Inlet Concentration (ng/min)	Adsorption Temperature (°C)	Adsorption Capacity (ng/g)
YW	N2	40	35	397.38
QX	N2	40	35	120.65
YH	N2	40	35	104.94
YW	N2	109.76	35	523.43
YW	N2	153.64	35	584.15
YW	O2	40	35	345.09
YW	CO2	40	35	331.49

. As shown in Table 2, under N₂ atmosphere at 35 °C with an inlet Hg⁰ concentration (40 ng/min), the Hg adsorption capacity follows the order: YW (397.38 ng/g) > QX (120.65 ng/g) > YH (104.94 ng/g). Since the adsorption conditions are identical, the observed differences in Hg adsorption capacities can be attributed to variations in fly ash components, such as UBC, SSA and metal oxide.

Previous studies have demonstrated that UBC facilitates mercury adsorption and oxidation [30,31]. As shown in Table 1, the UBC content follows the order: YW (25.66%) > QX (14.7%) > YH (10.5%). This ranking reveals a direct proportional relationship between UBC content and Hg⁰ adsorption capacity (Table 2), consistent with previous findings [32,33]. UBC significantly enhances mercury adsorption and oxidation capacities by promoting both physical adsorption and chemical oxidation processes.

SSA is another important parameter characterizing the adsorption properties of fly ash. Generally, fly ash with a higher SSA exhibits greater adsorption efficiency for Hg⁰ [34]. As shown in Table 1, the SSA values follow the order: YW (26.27 m²/g) > QX (8.63 m²/g) > YH (8.48 m²/g), indicating that the Hg⁰ adsorption capacity is proportional to the SSA of the three fly ash samples, which is consistent with previous studies [34,35].

The role of metal oxides in mercury adsorption/oxidation varies significantly under different experimental conditions. While some studies have suggested that iron oxides (e.g., Fe₂O₃) can promote mercury oxidation, other metal oxides, such as Al₂O₃, SiO₂, CaO, MgO, and TiO₂, generally exhibit negligible effects [36]. Wang et al. [19] reported that under N₂ atmosphere, Al₂O₃, Fe₂O₃, and TiO₂ display limited Hg⁰ adsorption capacity, while CaO and MgO show no observable activity. As shown in Table 1, the contents of metal oxides in the three fly ashes are as follows: Fe₂O₃: QX (5.09%) > YH (3.46%) > YW (3.14%); Al₂O₃: YH (39.15%) > YW (33.35%) > QX (17.24%); TiO₂: YW (1.23%) > YH (1.13%) > QX (0.59%). Notably, no clear correlation was observed between these metal oxide concentrations and the measured Hg⁰ adsorption capacities. This result implies that the contribution of metal oxides to Hg⁰ capture is substantially lower than that of UBC content and SSA, consistent with their limited oxidative or reactive potential under these experimental conditions.

3.3.2. Influence of Mercury Inlet Concentration

As demonstrated in the preceding analysis, YW fly ash exhibits a superior mercury adsorption capacity compared to the other tested fly ash samples. Therefore, YW fly ash was selected to systematically evaluate the influence of inlet Hg⁰ concentration and atmospheric conditions on adsorption efficiency.

Under N₂ atmosphere, three temperature and corresponding inlet Hg⁰ concentration conditions were investigated: 35 °C (40 ng/min), 50 °C (109.76 ng/min), and 55 °C (153.64 ng/min). The mercury breakthrough curves under different inlet Hg⁰ concentrations are presented in Figure 7.

As shown in Figure 7, during the initial 0–70 min, the mercury penetration rate increases significantly, and the adsorptive saturation time is reduced. This indicates that higher mercury concentrations accelerate the adsorption rate and adsorptive saturation time. This phenomenon can be attributed to the abundance of available active sites (both adsorptive and catalytic) on the surface of fresh fly ash, which facilitates efficient Hg adsorption/oxidation.

As shown in Table 2, the mercury adsorption capacity increases from 397.38 ng/g to 584.15 ng/g as the inlet Hg⁰ concentration is raised from 40 ng/min to 153.64 ng/min. These results demonstrate that higher inlet mercury concentrations promote greater adsorption on fly ash, which is consistent with previous findings [37]. Higher inlet concentrations of Hg⁰ markedly strengthen its diffusion from the gas phase to active sites inside particle pores. This process effectively overcomes interphase mass transfer resistance and increases the probability of surface collisions between gaseous mercury atoms and fly ash particles, thereby increasing the adsorption capacity [37].

3.3.3. Influence of Adsorption Atmosphere

Flue gas composition significantly influences mercury adsorption/oxidation processes. To systematically evaluate these effects, mercury adsorption on fly ash was investigated under N₂, O₂, and CO₂ atmospheres. At 35 °C and an inlet Hg⁰ concentration of 40 ng/min, the mercury breakthrough curves under different atmospheric conditions are presented in Figure 8.

As shown in Figure 8 and Table 2, the Hg adsorption capacity is lowest under CO₂ atmosphere (331.49 ng/g) and highest under N₂ atmosphere (397.38 ng/g). This range suggests that the three basic atmospheres—N₂, O₂, and CO₂—have little effect on mercury adsorption, which is consistent with previous studies [38]. Although Hg⁰ can be oxidized by O₂, its oxidation capacity is limited. Kinetic analyses reveal that the oxidative reactions occurring on fly ash particles primarily utilize chlorine derived from inherent chlorides present in the ash. Consequently, increasing the O₂ concentration exerts negligible influence on mercury adsorption and oxidation [39].

4. Conclusions

In this study, mercury adsorption/oxidation on fly ash under N₂ atmosphere was investigated using a fixed-bed test bench. Changes in mercury speciation of fly ash before and after Hg adsorption were analyzed using TPD-AFS technology, which directly revealed the mercury adsorption/oxidation mechanisms. The effects of fly ash components, inlet Hg⁰ concentration, and gas atmosphere on mercury adsorption and oxidation were systematically analyzed. The main conclusions are as follows:

(1) Mercury speciation analysis before and after Hg adsorption directly reveals that the primary adsorption/oxidation mechanism between mercury and fly ash is the heterogeneous oxidation reaction of Hg⁰ to HgCl₂.

(2) The Hg adsorption capacity exhibits a strong positive correlation with both the UBC content and the SSA of fly ash, whereas metal oxides in fly ash show negligible effects on mercury adsorption.

(3) With increasing inlet Hg⁰ concentration, both the mercury penetration rate and adsorption capacity increase. In contrast, flue gas components such as N₂, O₂, and CO₂ have little effect on mercury adsorption.