Experimental and Adsorption Kinetics Study of Hg⁰ Removal from Flue Gas by Silver-Loaded Rice Husk Gasification Char

Abstract

Coal holds a significant position in China’s energy consumption structure. However, the release of Hg⁰ during coal combustion poses a serious threat to human health. Traditional activated carbon for Hg⁰ removal is expensive; finding efficient, inexpensive and renewable adsorbents for Hg⁰ removal has become a top priority. Rice husk gasification char (RHGC) is a solid waste generated by biomass gasification power generation, which, loaded with silver to remove Hg⁰, could achieve the purpose of waste treatment. This paper examines the Hg⁰ removal performance of silver-loaded rice husk gasification char (SRHGC) under different operating conditions through experimental analysis. This study employed quasi-first-order, quasi-second-order, and internal diffusion kinetics adsorption equations to model the amount of Hg⁰ removed by SRHGC at different temperatures, thereby inferring the reaction mechanism. The results indicate that Hg⁰ removal efficiency of SRHGC increased by about 80%. The Hg⁰ removal ability was directly related to silver load, and the amount of Hg⁰ removed by SRHGC did not a exhibit a simple inverse relationship with particle size. Additionally, the Hg⁰ removal efficiency of SRHGC declined with increasing adsorption temperature. The removal of Hg⁰ by SRHGC conformed to the quasi-second-order kinetic equation, with the adsorption rate constant decreasing as the temperature rose, consistent with experimental observations. This paper provides both experimental and theoretical references for future modification and optimization of RHGC for coal-fired flue gas treatment, and also offers valuable insights into Hg⁰ removal by carbon-based adsorbents.

1. Introduction

With the development of the economy and the improvement of living standards, the demand for energy and electricity has grown rapidly [1]. In 2022, global energy consumption reached an all-time high, with primary energy consumption totaling 20.6 billion tons of standard coal, marking a 2.2% year-on-year increase and essentially returning to the average growth level prior to the pandemic [2]. Coal would occupy a long-term position in China’s energy structure. In 2023, China consumed 5.72 billion tons of standard coal, accounting for 55.3% of its total energy consumption. The flue gas released by coal burning contains CO₂, NO_X, SO₂, and the heavy metal mercury; these pollutants can cause serious environmental pollution. CO₂ causes the greenhouse effect, SO₂ and NO_X cause acid rain, and mercury causes mental illness [3,4]. The control technologies for SO₂, NO_X, and CO₂ are relatively mature and could effectively remove these pollutants using adsorbents or devices [5,6]. However, Hg⁰ removal technology is still in the development stage. Hg⁰ is highly toxic, volatile, lipophilic, and bio-accumulative, and would seriously harm human health [7,8,9,10]. Furthermore, Hg⁰ is insoluble in water, making it difficult to remove using traditional separation technologies [11].

Activated carbon (AC) is effective in removing Hg⁰ from coal-fired flue gas, but the cost is high [12,13,14]. Therefore, finding an efficient and cost-effective adsorbent is imperative. Rice husk gasification char (RHGC), a solid waste from biomass gasification power generation, has gained increasing attention for recycling due to the “zero-waste city” concept [15]. RHGC possesses highly active surface functional groups and a developed microporous structure, making it an effective adsorbent for Hg⁰ in flue gas. Loading RHGC with silver (SRHGC) enhances its Hg⁰ removal capabilities and supports waste treatment objectives. Additionally, SRHGC can be regenerated after heating, making it a potential long-term adsorbent [16,17]. There are limited studies on Hg⁰ removal using RHGC, especially after silver loading. This article provides valuable insights and a reference for future researchers studying Hg⁰ removal with RHGC.

Removing Hg⁰ with adsorbents is a complex process involving both surface adsorption and internal diffusion. Domestic and international scholars have predicted the performance of carbon-based adsorbents for Hg⁰ removal by establishing various models [18]. This study investigated the Hg⁰ removal performance of SRHGC under different conditions through experimental studies. Additionally, the quasi-first-order kinetic equation, the quasi-second-order kinetic equation, and the intraparticle diffusion equation were used to simulate the Hg⁰ removal behavior of SRHGC at different adsorption temperatures, providing further insights into the Hg⁰ removal mechanism. In the context of carbon neutrality and carbon peak goals, utilizing inexpensive and renewable SRHGC for Hg⁰ removal is particularly relevant. Applying simulated kinetics methods to predict experimental results enhances the efficiency, economy, and predictability of the experiments, offering significant economic and practical benefits.

2. Materials and Methods

In the experiment, RHGC was obtained from the Jiangsu Gaoyou biomass gasification power plant. The carbon content was 43.58%, determined using a German Elementar Vario ELIII element analyzer (analytical precision C ≤ 0.1 abs). After drying, grinding, and sieving, particle sizes of 97–125 µm, 125–200 µm, and 200–450 µm were selected for use. RHGC (particle size 125–200 µm) was impregnated with 20% hydrochloric acid for 30 min, placed in a vacuum oven at 50 °C for 2 h, cooled to room temperature, washed to neutrality with deionized water, and dried for later use. A total of 6 mL of silver nitrate solution with a mass concentration of 2 mg/mL was placed in a beaker, and the pH was adjusted to 9.7 with ammonia water. A total of 300 mg of dried RHGC was added and the mixture was shaken at a constant temperature of 298 K for 24 h. RHGC was then filtered and placed in an electric furnace tube, where Ag(NH₃)²⁺ was reduced to elemental silver under a N₂ atmosphere at 120 °C for 4 h. After the reduction process was complete, heating was stopped and the sample, which was silver-loaded rice husk gasification char (labeled as SRHGC, mass ratio of AgNO₃ to RHGC of approximately 40 mg/g), was allowed to cool to room temperature before being removed and stored in a desiccator for later use. The same method was used to prepare SRHGC-20 (mass ratio of AgNO₃ to RHGC was about 20 mg/g) and SRHGC-60 (mass ratio of AgNO₃ to RHGC was about 60 mg/g), which were stored in a desiccator for later use. The experiment was conducted in a fixed-bed experimental setup [19].

3. Results

3.1. X-ray Diffraction

Qualitative analysis of the phase and composition of SRHGC was conducted using a Rigaku D/max-2550 PC XRD analyzer from Japan. The XRD pattern of SRHGC is shown in Figure 1. A relatively sharp diffraction peak appeared near the diffraction angle 2θ = 24°, indicating that SRHGC was amorphous with a certain degree of crystallinity. A very sharp diffraction peak at 2θ = 38° was observed, which was a typical characteristic diffraction peak of elemental silver, confirming the presence of elemental silver in RHGC. The height and sharpness of this peak reflected the silver content and crystallinity in the sample. Additionally, a sharp diffraction peak at 2θ = 78° was noted, caused by silver entering RHGC through ion exchange, further confirming the successful loading of silver onto RHGC.

3.2. Hg⁰ Removal Performance of SRHGC under Different Conditions

3.2.1. Hg⁰ Removal Performance of Different Adsorbents

For the experiment, 300 mg each of RHGC, SRHGC, and AC were selected, along with an additional 450 mg of SRHGC (labeled as 2-SRHGC). The experimental conditions were set as follows: the temperature was maintained at 160 °C, the equilibrium gas was N₂ with a flow rate of 3 L/min, the inlet concentration of Hg⁰ was 38.6 μg/m³, and the adsorption time was 120 min. The experimental results are shown in Figure 2.

Figure 2 shows that the Hg⁰ removal efficiency stabilized after 20 min. The Hg⁰ removal efficiencies of the four adsorbents followed this order: RHGC < SRHGC < AC < 2-SRHGC. The Hg⁰ removal efficiency of RHGC increased by nearly 80%. Although the Hg⁰ removal efficiency of an equivalent amount of SRHGC was lower than that of AC, increasing the quantity of SRHGC to 450 mg resulted in a higher Hg⁰ removal efficiency compared to AC. This improvement was attributed to the increased mass, which provided a longer contact time between the adsorbent and mercury vapor, thereby enhancing Hg⁰ removal efficacy. As a cost-effective industrial solid waste, SRHGC has the potential to replace AC for Hg⁰ removal.

3.2.2. Effect of Different Silver Loads on Hg⁰ Removal Performance of SRHGC

For the experiment, 300 mg each of SRHGC-20, SRHGC, and SRHGC-60 were selected. The experimental conditions were as follows: the temperature was maintained at 160 °C, the equilibrium gas was N₂ with a flow rate of 3 L/min, the inlet concentration of Hg⁰ was 38.6 μg/m³, and the adsorption time was 120 min. The experimental results are shown in Figure 3.

As seen in Figure 3, SRHGC-60 exhibited the highest Hg⁰ removal efficiency, followed by SRHGC and then SRHGC-20. The Hg⁰ removal ability was directly related to the silver load. After adsorption stabilization, the Hg⁰ removal efficiency of SRHGC-60 reached 100%, SRHGC achieved about 90%, and SRHGC-20 attained 70%. Considering both silver load and efficiency, SRHGC demonstrated the best overall performance in removing Hg⁰.

3.2.3. Effect of Different Particle Sizes on Hg⁰ Removal Performance of SRHGC

For the experiment, 300 mg of SRHGC with particle sizes of 97–125 µm, 125–200 µm, and 200–450 µm were selected. The experimental conditions were as follows: the temperature was maintained at 160 °C, the equilibrium gas was N₂ with a flow rate of 3 L/min, the inlet concentration of Hg⁰ was 38.6 μg/m³, and the adsorption time was 120 min. The experimental results are shown in Figure 4.

As shown in Figure 4, the Hg⁰ adsorption capacity was 41.79 μg/g for particles sized 200–450 µm, increased to 43.54 μg/g for particles sized 125–200 µm, and was 42.87 μg/g for particles sized 97–125 µm. This variation was due to the relationship between particle size and factors such as mass transfer resistance, specific surface area, and the internal diffusion coefficient of SRHGC. When particle size decreased, the specific surface area increased, and the internal diffusion coefficient also rose, facilitating the diffusion of Hg⁰ to the surface of the adsorption layer and enhancing the adsorption of Hg⁰ by SRHGC. At this stage, the positive effects dominated. However, further reduction in particle size increased the mass transfer resistance of the adsorption layer, leading to a rise in penetration pressure drop. Beyond a certain range, the specific surface area of SRHGC no longer increased with decreasing particle size, and the pressure drop continued to rise, weakening the effect of physical adsorption. Consequently, the negative effects became more significant. Therefore, to improve the Hg⁰ adsorption performance of the adsorbent, the positive effects should outweigh the negative effects. Based on these findings, a particle size of 125–200 µm was selected for this experiment [20,21].

3.2.4. Effect of Different Temperatures on Hg⁰ Removal Performance of SRHGC

For this experiment, 300 mg of SRHGC with a particle size of 125–200 µm was selected. The experimental temperatures were set at 120 °C, 160 °C, and 200 °C. The equilibrium gas was N² with a flow rate of 3 L/min, the inlet concentration of Hg⁰ was 38.6 μg/m³, and the adsorption time was 120 min. The results are shown in Figure 5. The Hg⁰ adsorption capacities of SRHGC at 120 °C, 160 °C, and 200 °C were approximately 44.09 μg/g, 43.54 μg/g, and 41.15 μg/g, respectively. As the temperature increased, the Hg⁰ adsorption capacity consistently decreased. This trend could be attributed to the nature of adsorption pores on the surface of SRHGC, where physical adsorption was predominant, and gaseous mercury was preferentially adsorbed at lower temperatures. As the temperature rose, the chemisorption rate surpassed the physical adsorption rate, potentially destroying the chemical bonds of oxygen-containing functional groups. Moreover, Hg⁰ might degrade on the surface of SRHGC due to high temperatures, leading to a decrease in chemisorption capacity. Consequently, the adsorption capacity of SRHGC diminished at higher temperatures, with the rate of decline accelerating with temperature. Based on these findings, 160 °C was selected for this experiment.

3.3. Adsorption Kinetics Study of Hg⁰ Removal by SRHGC

The experimental data for Hg⁰ removal by SRHGC at 120 °C (SRHCC-120), 160 °C (SRHCC-160), and 200 °C (SRHCC-200) were analyzed and fitted by using the quasi-first-order kinetic equation [22], the quasi-second-order kinetic equation [23] and the in-particle diffusion equation [24], the aim of which was to further study the mechanism of removing Hg⁰ from SRHGC at different temperatures. In the figure below, q_e is the adsorption amount of mercury on the adsorbent at adsorption equilibrium (μg/g); q_t is the adsorption amount of mercury on the adsorbent at time t (μg/g); t is the adsorption time (min); k₁ is the rate constant of the adsorption quasi-first-order model (min⁻¹); k₂ is the rate constant of the bi-media rate equation (g/μg·min); C is related to the thickness at the boundary (μg/g); h and R² are the fitting parameters. The results are shown below.

3.3.1. Quasi-First-Order Kinetic Equation Fitting

According to the kinetic fitting results in Figure 6, the relevant kinetic parameters and correlation coefficients were obtained, as shown in

Table 1. Related parameters of quasi-first-order dynamic equation.

Sample	qe	K1	R2
SRHGC-120	44.09	0.022	0.95199
SRHGC-160	43.54	0.0216	0.95164
SRHGC-200	41.15	0.02135	0.95307

.

3.3.2. Quasi-Second-Order Kinetic Equation Fitting

According to the kinetic fitting results in Figure 7, the relevant kinetic parameters and correlation coefficients were obtained, as shown in

Table 2. Related parameters of pseudo-second-order kinetic equation.

Sample	h	K2	R2
SRHGC-120	0.517	2.22 × 10−5	0.99348
SRHGC-160	0.504	2.15 × 10−5	0.99623
SRHGC-200	0.475	2.11 × 10−5	0.99713

.

3.3.3. In-Particle Diffusion Equation Fitting

According to the kinetic fitting results in Figure 8, the relevant kinetic parameters and correlation coefficients were obtained, as shown in

Table 3. Intraparticle diffusion equation-related parameters.

Sample	C	K1	R2
SRHGC-120	−15.85088	5.36538	0.99063
SRHGC-160	−14.69875	5.23916	0.99233
SRHGC-200	−14.05459	4.938	0.99103

.

Figure 6, Figure 7 and Figure 8 show the fitting of Hg⁰ adsorption capacity of SRHGC at different temperatures using the quasi-first-order, quasi-second-order, and internal diffusion kinetic equations, respectively. Table 1, Table 2 and Table 3 present the parameters obtained by fitting three kinetic equations at different temperatures. The fact that none of the lines in Figure 4 pass through the origin indicates that intraparticle diffusion is not the sole factor controlling the adsorption process. The adsorption of Hg⁰ on the adsorbent surface was divided into two stages: internal diffusion and surface adsorption. In the initial stage, the adsorption rate was faster, while the internal diffusion rate was slower. The R² of the quasi-second-order kinetic fitting parameters were all above 0.993. The fitting curves were also very consistent with the experimental data of SRHGC, indicating that the adsorption of Hg⁰ on SRHGC conforms to the quasi-second-order kinetic equation. It can be seen from Table 1, Table 2 and Table 3 that the adsorption rate constant of the quasi-first-order, quasi-second-order, and internal diffusion kinetic equations decreases with the increase in temperature, indicating that the adsorption performance of the adsorbent Hg⁰ decreases with the increase in temperature, hindering the adsorption of Hg⁰, which was consistent with the experimental results.

4. Conclusions

This paper investigated the Hg⁰ removal performance of silver-loaded rice husk gasification char (SRHGC) under various operating conditions through experimental studies. The adsorption kinetics were analyzed using quasi-first-order, quasi-second-order, and intraparticle diffusion equations to model the amount of Hg⁰ adsorbed by SRHGC at different temperatures, thereby inferring the reaction mechanism. The results show that the Hg⁰ removal efficiency of SRHGC improved by approximately 80%. The amount of Hg⁰ removed by SRHGC did not exhibit a simple inverse relationship with particle size, and the Hg⁰ removal efficiency decreased with increasing adsorption temperature. The process of removing Hg⁰ from SRHGC followed the quasi-second-order kinetic equation, with the adsorption rate constant decreasing as the temperature increased, consistent with the experimental findings. This study provides both experimental and theoretical references for the subsequent modification and selection of adsorbents. It also lays a solid foundation for the application of RHGC in the environmental protection field, particularly in the efficient removal of Hg⁰ from flue gas, offering significant economic and practical benefits.