Non-Thermal Plasma Technology for Further Purification of Flue Gas in the Resource Utilization Process of Waste Mercury Catalyst: A Case Study in Xinjiang, China

Abstract

This study aims to advance the recycling of mercury-containing waste and promote sustainable development within the polyvinyl chloride (PVC) industry. Our innovative system integrates pre-treatment technology (spraying potassium permanganate and demisting and dust removal) with efficient non-thermal plasma oxidation, resulting in excellent treatment efficiency, low cost, and simple operation. With a processing capacity of 3000 m³/h, the concentration of mercury emissions in flue gas can achieve the target of <0.01 mg/m³, boasting a removal efficiency exceeding 98%, which satisfies the standard “Emission standard of air pollutants for industrial kiln and furnace” (GB 9078-1996). Our results can provide technical support for the comprehensive purification of mercury-containing flue gas during the resource recovery process from mercury-containing waste. The application of our system can contribute to reducing mercury emissions in the PVC industry, lowering occupational exposure risks for workers, and promoting China’s better compliance with “the Minamata Convention on Mercury”.

1. Introduction

Due to China’s energy structure, which is characterized by a high coal content, low oil content, and low natural gas content, the predominant method for polyvinyl chloride (PVC) production is the mercury-based calcium carbide process [1,2]. This process will continue to exist for a long time until technology that utilizes mercury-free catalysts becomes viable. Notably, each ton of PVC production in China typically consumes between 1.2 kg and 1.6 kg of mercury catalyst (mercury chloride, HgCl₂), resulting in an annual consumption of HgCl₂ ranging from 700 to 1200 tons [3]. In the PVC production process, the main mercury-containing waste consists of waste mercury catalyst and waste activated carbon. The commonly used HgCl₂ catalyst is susceptible to carbon poisoning and deactivation, which requires frequent disposal of the spent mercury catalyst. The waste typically contains a mass fraction of HgCl₂ of approximately 2% to 3% [4]. Similarly, waste activated carbon used to treat mercury-containing waste gas in PVC production by adsorbing HgCl₂ has a similar composition to waste mercury catalysts [5,6]. Based on statistics, the calcium carbide PVC industry in China generates an estimated 10,000 to 15,000 tons of mercury-containing waste, which requires transportation to certified facilities for resource recovery [5,6]. Within the waste stream, waste mercury catalysts and mercury-containing activated carbon account for 36% and 51% of the total HgCl₂ used, respectively [7].

The mercury in these waste materials has the potential for resource recovery. Mercury (Hg) is persistent, transported long-range and bioconcentrated, and is one of the most toxic heavy metal pollutants in ecosystems [8]. Improper treatment or disposal by resource recovery enterprises can lead to mercury pollution, posing significant threats to the environment and human health [9]. Exposure to HgCl₂ has been linked to various adverse health effects, such as neurotoxicity leading to cognitive impairment and motor dysfunction, dysfunction of the central auditory system, as well as nephrotoxicity, hepatotoxicity, and increased mortality rates [10]. Before “the Minamata Convention on Mercury” came into effect, China had relatively weak management of mercury pollution prevention and control, and specialized mercury reduction technologies for the PVC industry were immature. “The Minamata Convention on Mercury” sets clear requirements for the chlor-alkali industry, promoting various measures to reduce reliance on mercury originating from primary mercury mining and implementing measures to minimize mercury emissions and releases into the environment. In 2015, the Ministry of Ecology and Environment of China issued the “Technical Policy for Mercury Pollution Prevention and Control”, which is a landmark document in China’s efforts to prevent and control mercury pollution, providing comprehensive support for mercury emission reduction in the PVC industry.

Representative recovery processes mainly include distillation and controlled oxygen dry distillation [6,11]. The distillation method primarily recovers elemental mercury (Hg⁰), while the controlled oxygen dry distillation method primarily recovers HgCl₂. The distillation–condensation method is commonly used, but it has low efficiency in recovering materials and poses difficulties with regard to pollution control. In recent years, there have been reports of new processes for treating waste mercury catalysts. Qiu et al. [12]. and Yu et al. [13]. invented a technology for treating waste mercury catalysts using hydrometallurgy. Ren et al. [14] utilized ball mills to immobilize mercury in waste mercury catalysts by grinding it with sulfur, but their widespread application has not been observed. Distillation is the central component of the entire resource utilization process, and the distillation furnace is a sealed structure. During the heating process, it is essential to maximize the thermal volatilization of mercury, as it is crucial for improving the mercury recovery rate [15]. The condensation system is a process that involves the recovery of crude mercury from mercury-containing vapor through condensation. The recovered mercury is then processed and purified to obtain the final product.

Currently, the treatment of waste gas containing mercury after distillation and condensation typically involves methods such as dust removal, potassium permanganate oxidation, and activated carbon adsorption [16]. Mercury in flue gas typically exists in three forms: elemental mercury (Hg⁰), divalent mercury ions (Hg (II)), and particulate-bound mercury (Hg^p) [17]. Among them, Hg (II) is highly soluble in water and can be effectively treated through spraying or adsorption technology. Hg^p can be removed through dust removal processes. However, a significant portion of Hg⁰ in flue gas is difficult to remove. Conventional flue gas purification technologies are unable to effectively remove Hg⁰ [7,11,12,13]. From an industry-wide perspective, the overall processing technology is outdated and the process routes represented by spray oxidation and activated carbon adsorption make it difficult to achieve stable and standardized emissions. Furthermore, the remaining concentration of mercury in waste activated carbon is high at 1000 mg/kg, classifying it as hazardous waste and requiring special treatment. If an efficient technology can be used to oxidize relatively insoluble elemental mercury, it is possible to achieve a comprehensive treatment of mercury-containing waste gas. This signifies a significant advancement in technological innovation for preventing and controlling mercury pollution.

The non-thermal plasma reactor generates a large number of high-energy electrons that collide with mercury atoms, oxidizing them into divalent mercury. This process leads to the conversion of mercury from insoluble to soluble, or from uncharged to charged forms. Through the process of salt formation, Hg⁰ is removed from the gas phase and transformed into other products that are easily removable [18]. Non-thermal plasma technology has a wide range of applications in the field of environmental protection, including the disposal of waste fluorescent lamps, non-ferrous metal smelting, co-disposal of fly ash in cement kilns, medical waste incineration, and other fields, and is efficient and low-cost [19,20]. This technology can also be used as a calibration approach for gaseous oxidized mercury by nonthermal plasma oxidation of elemental mercury due to its quantitative oxidation capabilities [21].

Environmental Technology Verification (ETV), developed by the United States Environmental Protection Agency, encompasses a set of procedures and methods that involve the authorization of technology developers (owners), users, governments, or other relevant parties in accordance with applicable national laws, regulations, and standards [22,23]. Since the mid-1990s, environmental protection departments in countries and regions such as Canada, Japan, South Korea, and the European Union have successively established ETV evaluation systems [24,25]. As of May 2013, a total of 1368 validation and evaluation projects have been completed in various countries worldwide. In 2015, over 30 countries carried out ETV evaluations, which had a positive impact on technological innovation and transfer [26]. However, ETV evaluations also have some shortcomings, such as requiring significant time, manpower, and financial investment, particularly in data collection, testing, and validation.

The goal of this technology verification is to confirm the efficacy of non-thermal plasma coupling treatment technology in treating mercury-containing flue gas produced during the disposal of waste mercury catalysts. Xinjiang, China, is an important base for the production and sale of HgCl₂ catalysts for PVC as well as the recycling of mercury-containing waste resources. Thus, in our research, we selected Xinjiang, China, as our experimental site.

Our research team conducted a study on the efficiency of Hg⁰ oxidation using a self-developed direct current (DC) high-voltage narrow-pulse non-thermal plasma reaction device. The study focused on the effects of power supply parameters (voltage, frequency) and simulated concentrations of O₂, H₂O, HCl, NO, and SO₂ in flue gas. The study investigated the synergistic control mechanism of non-thermal plasma for reducing NOx, SO₂, and Hg⁰, and determined the feasibility of this technology. This technology has been partially applied in related engineering.

2. Materials and Methods

2.1. Small-Scale Experiment

By simulating the composition of flue gas generated during the heat treatment process of typical mercury-containing waste, we conducted a study at a small-scale experimental level to investigate the factors influencing non-thermal plasma (NTP) treatment for mercury-containing flue gas.

2.1.1. Equipment

The experimental setup, as illustrated in Supplementary Figure S1, includes a gas distribution system, a high-voltage power supply, a plasma reactor, a water vapor generator, a U-shaped mixer, a VM-3000 Mercury Vapor Monitor (Karlsfeld, Germany), a flue gas treatment system, and a ventilation system. The gas distribution system consists of N₂, O₂, SO₂, and NO. The main gas used to generate plasma is N₂. The concentration of Hg used in our experiments is about 200 mg/m³.

The experiment uses a non-thermal plasma power supply with a pulse width and rising edge in the range of hundreds of nanoseconds. The parameters for the non-thermal plasma power supply for the small-scale experiment can be found in Supplementary Table S1. The non-thermal plasma reactor used employs a wire-tube configuration, consisting of a 320 mm long discharge tube with an inner diameter of 32 mm. The actual volume is 257 cm³. The discharge tube is arranged in parallel. The positive terminal of the power supply is connected to the high-voltage insulator of the reactor, while the negative terminal and ground wire are connected to the low-voltage insulator of the reactor.

2.1.2. Experimental Content and Measurement

The following is the main content and testing methods of this experiment.

(1) Effect of power supply parameters of Hg⁰ removal by non-thermal plasma. This study investigates the impact of voltage, frequency, pulse width, and pulse rising edge of the non-thermal plasma power supply on the rate of Hg⁰ oxidation in flue gas. The voltage and current were measured using an oscilloscope (MDO3024, Tektronix, Beaverton, OR, USA). The power of the plasma discharge was calculated according to Equation (1).

$$\mathrm{P}=\mathrm{f}{\int }_0^{\mathrm{T}}\mathrm{U}\left(\mathrm{t}\right)\mathrm{I}\left(\mathrm{t}\right)\mathrm{dt}$$

(1)

where P is the output power (Watt, W), f is pulse frequency of pulse power supply (Hz), U is Instantaneous voltage (V), and I is Instantaneous current (A).

(2) The influence of atmospheric conditions on the removal of mercury from flue gas using non-thermal plasma. This study examines the impact of atmospheric conditions on the efficiency of Hg⁰ oxidation in the presence of pollutants like NO and SO₂. The study aims to clarify the distinct effects of each pollutant on Hg⁰ oxidation efficiency. The concentration of SO₂ and NO was measured using the TESTO Flue Gas Analyzer (TESTO 350, Titisee-Neustadt, Germany).

2.2. Environmental Technology Verification (ETV Process)

2.2.1. Site Selection

The technology demonstration and verification took place at a company in Xinjiang. The company specializes in recycling and safely disposing of waste materials containing mercury, such as discarded mercury catalysts. During the operation of a fluidized bed furnace to treat waste mercury catalysts, flue gas containing mercury is generated and emitted, with a mercury content ranging from 15 to 20 mg/m³. Despite the implementation of a multi-stage mercury absorption process, it is not possible to achieve stable and standardized emissions. The flue gas mercury emissions do not meet the mercury emission concentration limit of 0.01 mg/m³ as specified in the “Emission standard of air pollutants for industrial kiln and furnace” (GB 9078-1996) [27].

2.2.2. On-Site Process

A non-thermal plasma coupling treatment device is utilized to effectively control mercury, SO₂, NOx, particulate matter, and other pollutants in the flue gas [28,29]. The flue gas treatment process first involves a four-stage washing pretreatment to remove dust particles from the flue gas and reduce the concentration of mercury vapor. During the washing treatment, the generation of a certain amount of saturated water vapor and the entrainment of liquid droplets significantly increase the moisture content of the flue gas. Then, the water vapor content in the flue gas is reduced using the mist removal system to prevent any potential impact on the subsequent system. Finally, the treated flue gas enters the non-thermal plasma integrated system, which combines non-thermal plasma efficient oxidation with environmentally functional material (modified silica) adsorption, to achieve effective degradation and thorough purification, ensuring compliance with the emission standard. The parameters for the non-thermal plasma power supply for on-site experiments are provided in Supplementary Table S2.

2.2.3. Sample Collection

According to the characteristics of the technology being evaluated and the objectives of the evaluation, the samples collected for experimental testing include gas samples and solid residues. According to “The determination of particulates and sampling methods of gaseous pollutants from exhaust gas of stationary” (GB/T 16157-1996) [30] and the “Stationary source emission-Determination of mercury-Cold atomic absorption spectrophotometry” (HJ 543-2009) [31], sampling was conducted at the following locations: outlet of the tube cooler (Point 1#), outlet of the four-stage series spray tower (Point 2#), outlet of the dust and mist removal equipment (Point 3#), outlet of the primary oxidation catalyst bed (Point 4#), and outlet of the tertiary oxidation catalyst bed (Point 5#). The sampling locations are depicted in Supplementary Figure S2. To ensure accurate data, sampling must be done simultaneously at each sampling point.

2.2.4. Sample Measurement

The methods for determining mercury concentration, SO₂ concentration, NOx concentration, and particulate matter concentration are specified in the following standards: “Stationary source emission-Determination of mercury-Cold atomic absorption spectrophotometry” (HJ 543-2009) [31], “Stationary source emission-Determination of sulfur dioxide—Fixed potential by electrolysis method” (HJ/T 57-2017) [32], “Stationary source emission-Determination of nitrogen oxides-Fixed potential by electrolysis method” (HJ 693-2014) [33], and “Stationary source emission—Determination of mass concentration of particulate matter at low concentration—Manual gravimetric method” (HJ 836-2017) [34], respectively.

2.2.5. ETV Test Parameters

The testing parameters are divided into two categories: environmental effects and operating processes. In this assessment, suitable parameters were chosen according to the characteristics and validation objectives of non-thermal plasma technology, as depicted in

Table 1. List of Test Parameters.

Parameter Category	Object	Specific Parameters
Environmental effect parameters	inlet and outlet exhaust gas	temperature, pressure, flow rate
	air pollutant	mercury content, NOx, SO2, particulate matter, etc. in exhaust gas
Process operating parameters	non-thermal plasma system	stable operating time
		pulse frequency
		pulse voltage
		pulse current
	processing scale	unit time processing capacity

.

Security measures: Our research was conducted under sealed negative pressure experimental conditions, where the exhaust gas was treated with potassium permanganate solution.

Data analysis: The data for this paper was organized using Excel 2021, while the graphs were created using Origin 2022.

3. Results for Small-Scale Experiment

3.1. Effect of Power Supply Parameters of Hg⁰ Removal by Non-Thermal Plasma

3.1.1. Voltage

The effect of voltage change on the rate of Hg⁰ oxidation is illustrated in Figure 1. The experiment was conducted with a pulse frequency of 600 Hz. It can be seen that as the voltage reaches the threshold level, both the single pulse energy and injection energy increase with the rise in voltage. When the voltage reaches 13.1 kV, stable plasma is generated. As the voltage increases, the concentration of Hg⁰ decreases rapidly, eventually reaching an optimal level of approximately 88% oxidation rate when the voltage reaches 13.1 kV. Taking cost into consideration, we have selected 13.1 kV as the optimal input voltage.

3.1.2. Pulse Frequency

The impact of varying pulse frequency on the rate of Hg⁰ oxidation is illustrated in Figure 2. The experiment was conducted at an input voltage of 13.1 kV. The oxidation rate of Hg⁰ increases gradually as the power frequency increases. When the pulse frequency reaches approximately 200 Hz, the rate of mercury oxidation reaches 80%. Afterward, as the pulse frequency increased, the rate of mercury oxidation basically stabilized.

3.1.3. Pulse Width

The effect of pulse width variation on Hg⁰ oxidation rate is illustrated in Figure 3. The experiment was conducted at an input voltage of 13.1 kV. The oxidation rate of Hg⁰ increases gradually with the pulse width in the range of 0–1000 ns. Upon reaching approximately 300 ns, the concentration of Hg⁰ at the output ceases to decrease. With further increases in pulse width (300–1000 ns), the oxidation rate of Hg⁰ remains relatively constant. This indicates that the device’s oxidizing capacity for Hg⁰ has reached its limit. Under these conditions, a variety of output frequencies (12 Hz, 25 Hz, 50 Hz, 100 Hz) were chosen for comparative experiments. It was observed that as the frequency increased, the oxidation rate of Hg⁰ also increased to some extent. Notably, at a frequency of 100 Hz, the curve demonstrates excellent performance in oxidizing Hg⁰, while at 12 Hz, the overall rate of Hg⁰ oxidation is lower. The oxidation rate change curves of Hg⁰ at different frequencies all exhibit a similar trend.

3.1.4. Pulse Rising Edge

The effect of pulse rising edge variation on the Hg⁰ oxidation rate is illustrated in Figure 4. It can be observed that the rising edge of the pulse has minimal influence on the oxidation of mercury. The variation in the rising edge of the pulse has little effect on both the energy of a single pulse and the rate of Hg⁰ oxidation.

3.2. Effect of Atmospheric Conditions of Hg⁰ Removal by Non-Thermal Plasma

3.2.1. NO Concentration

The effect of NO on non-thermal plasma oxidation of Hg⁰ is illustrated in Figure 5. It can be concluded that the rate of Hg⁰ oxidation gradually decreases as the NO concentration increases. This indicates that NO inhibits the oxidation of Hg⁰ [35]. There is a competitive relationship between NO and Hg⁰. Due to the faster reaction rate between NO and O₃, O₃ is consumed by NO, leading to a reduction in the number of active particles that react with Hg⁰. As a result, the rate of Hg⁰ oxidation decreases. Based on this, it can be inferred that if NO is present in the processed gas component, it is essential to elevate the pulse voltage or frequency to achieve a greater energy density and attain a specific Hg⁰ oxidation rate.

3.2.2. SO₂ Concentration

The effect of SO₂ concentration on non-thermal plasma oxidation of Hg is illustrated in Figure 6. It can be concluded that the presence of SO₂ inhibits the oxidation of Hg⁰, compared to the effect of NO on the Hg⁰ oxidation rate in non-thermal plasma. However, the effect of SO₂ on the rate of Hg⁰ oxidation is relatively small. This is primarily because the reaction rate between SO₂ and O₃ is relatively low, and there is also relatively little competition between SO₂ and Hg⁰ [35].

The process of non-thermal plasma synergistic control of NO_x, SO₂, and Hg⁰ can be delineated into three stages: ① Generation of DC high-voltage pulse discharge low-temperature plasma through the inelastic collision of high-energy electrons and gas molecules, leading to the excitation and dissociation of gas molecules, thereby producing reactive radicals such as ·N, ·O, etc. ② Reaction of pulse discharge with the excited state of gas molecules, resulting in the formation of ·O and O₃. ③ Utilization of ·O, O₃, and other reactive species to react with NO_x and Hg⁰ in the gas, thereby achieving oxidative removal. The specific reaction processes have been described in the Supplementary Materials SD1.

4. ETV Process of Non-Thermal Plasma

This project is being studied based on the system and methods of ETV. ETV refers to the authorization of technology developers (owners), users, governments, or other relevant parties in accordance with applicable national laws, regulations, and standards. It is conducted in accordance with the requirements of the “Environmental management—Environmental technology verification” (GB/T 24034-2019) [36] and “General Protocol for Environmental Technology Verification” (T/CSES 01-2015) [37].

4.1. Non-Thermal Plasma Equipment Integration

4.1.1. Pretreatment System

(1)

Facilities for spraying potassium permanganate

The washing process, which uses a four-stage spray tower and a potassium permanganate solution, has the ability to reduce the mercury concentration in flue gas to 1–2 mg/m³, or even achieve lower emission levels. This process achieves the goal of preliminary treatment for mercury in flue gas. The design parameters of the equipment are as follows: processing flue gas volume: 10,000 m³/h; gas flow rate: less than 0.7 m/s.

(2)

Demisting and dust removal facilities

By utilizing the electrons emitted from the cathode in a high-voltage electric field and the negative ions produced through electron collisions with air molecules, particulate matter and water mist particles are captured. The particles are then charged and adsorbed onto the anode, effectively achieving the goal of cleaning. The equipment requirements and technical specifications are as follows: the power supply should be high-voltage constant current DC, and the power supply power should not exceed 20 kW.

4.1.2. Non-Thermal Plasma Coupling Treatment System

(1)

Non-thermal plasma power supply

The non-thermal plasma power supply primarily comprises two components: a DC charging power supply and a MARX generator module. The pulse non-thermal plasma power supply is based on the MARX generator circuit, which rapidly generates high voltage from low voltage, eliminating the need for large low-frequency and high-voltage transformers. The use of modular circuits reduces the voltage resistance requirements and minimizes the power supply volume, thereby decreasing the inductance of the output circuit. By adjusting the pulse voltage, pulse current, pulse frequency, and other parameters, the core technology of the DC high-voltage narrow-pulse plasma power supply has been developed to meet the requirements of pulse rising edges in the range of hundreds of nanoseconds. This technology enables stable and controllable power pulse discharge corona.

(2)

Plasma reactor

A non-thermal plasma tube reactor is utilized, and the reactor structure is depicted in Figure S3 of the Supplementary Materials. The flue gas containing mercury is introduced into the non-thermal plasma reactor using an induced draft fan. In the reactor, Hg⁰ in the flue gas is converted into Hg²⁺, which then reacts with the negatively charged oxygen ions produced in the reactor to form electrically neutral mercury (II) oxide [38].

(3)

Environmental functional material adsorption system

Utilizing ceramic nano single-molecule porous materials for targeted adsorption. The specific surface area of this material ranges from 200 to 800 m²/g, and it demonstrates a strong adsorption effect on Hg²⁺, Hg⁺, and molecular mercury compounds [39]. The primary technical indicators of this material are as follows: ceramic nano adsorption capacity: 0.4 g/kg; ceramic nano regeneration cycle: 6 months; the flue gas flow rate inside the adsorption box/regeneration tower is 0.385 m/s. Ceramic nanomaterials, when utilized as adsorption materials for non-thermal plasma oxidation, can exhibit outstanding adsorption and capture capabilities. This provides the necessary conditions for the subsequent regeneration of ceramic nanomaterials.

4.1.3. Operating Process

The non-thermal plasma coupling treatment system ran continuously for 72 h. The spray tower’s flow rate is 80 m³/h. The concentration of potassium permanganate is 0.5%. During the testing period, the inlet mercury concentration of the system remained stable at around 4.37 mg/m³, while the outlet mercury concentration was below 0.012 mg/m³. The operating parameters remained stable within the frequency range of the dust removal equipment, which was between 2.88 and 4.2 Hz. The flow rate of the spray tower is 80 mg/m³. The electric pressure was maintained at 38 kV, and the frequency was set at 1000 Hz. Additionally, the flue gas was consistently discharged to meet the required standards. The main materials and energy consumed in this process include water, electricity, potassium permanganate powder, and the adsorbent materials. The flue gas treatment capacity is calculated based on the actual operation of 300 days per year and 24 h per day. The annual operating cost of the system and the detailed information are in the Supplementary Materials SD2.

4.2. Results

During the ETV period, the main results are as follows:

(1)

The on-site experiment utilized a cascaded oxidation treatment technique that combines washing with potassium permanganate solution and coupling with non-thermal plasma technology. Elemental mercury in the flue gas is oxidized to form oxidized mercury species, which are then adsorbed by the solution and environmentally friendly functional materials, resulting in optimal treatment efficiency. This technology enables the further purification of mercury, improves treatment efficiency, reduces material usage, and lowers operating costs.

(2)

The pulse voltage of the plasma power supply ranges from 10 to 35 kV, the pulse current ranges from 8 to 160 A, and the pulse frequency ranges from 100 to 1000 Hz. The facility’s operating parameters are normal and meet the requirements for continuous and stable operation for 72 h.

(3)

The test results indicate that this technology can meet the treatment requirements for flue gas containing mercury during the disposal of waste mercury catalysts. Furthermore, the environmental emission indicators meet the requirements of applicable national and local standards. When the system’s processing capacity is 3000 m³/h, the mercury removal efficiency can exceed 98%, and the mercury emission concentration in the flue gas can meet the mercury emission concentration limit of 0.01 mg/m³ as specified by the “Emission standard of air pollutants for industrial kiln and furnace” (GB 9078-1996) [27]. Currently, only Guizhou in China has established emission limit requirements for mercury and its compounds in the local standard “Emission Standards for mercury and its compound industrial pollutants” (DB52-1422-2019) [40], with a limit of 0.03 mg/m³. The concentration of mercury emissions also meets the emission standards. All other pollutants, such as particulate matter, SO₂, and NOx, also meet the standard requirements. The test results of flue gas samples are shown in

Table 2. Test results of flue gas samples.

Pollutant	Point 1# (mg/m3)	Point 2# (mg/m3)	Point 3# (mg/m3)	Point 4# (mg/m3)	Point 5# (mg/m3)	GB 9078-1996 (mg/m3)
Hg	3.3 ± 0.040	0.75 ± 0.051	0.63 ± 0.026	0.024 ±0.00030	0.010 ±0.00020	0.01
Particulate matter	11 ± 0.35	11 ± 0.55	9.3 ± 0.40	8.6 ± 0.15	7.6 ± 0.25	120
SO2	<3	<3	<3	<3	<3	550
NOx	<3	<3	<3	<3	<3	240

.

5. Conclusions and Perspective

The test results demonstrate the efficiency of this technology in meeting the requirements for treating mercury-containing exhaust gases during the disposal process of waste mercury catalysts. It serves as a viable alternative to traditional methods, achieving a remarkable mercury recovery rate exceeding 99%, with tail gas emissions maintained below 0.01 mg/m³. This addresses the persistent challenge of limited adsorption capacity observed in technologies such as activated carbon.

Non-thermal plasma technology operates at ambient temperature and pressure, presenting a straightforward process with high treatment efficiency and negligible secondary pollution. It effectively circumvents the constraints of conventional mercury pollution control methods, and has become a popular research topic for scholars globally.

The oxidation of elemental mercury by non-thermal plasma is influenced by several factors including voltage, pulse width, frequency of the power supply, and waste gas composition. Various industries and process technologies exhibit disparities in gas volumes and pollutant compositions, thereby necessitating divergent parameter designs for plasma power supplies and reactors. Thus, high frequency, high voltage, and appropriate pulse width considerations are pivotal in enhancing the power factor of the power supply for optimal application in plasma reactors.

This technology finds applicability in resource recovery from waste mercury catalysts, purification of mercury-containing flue gas from waste fluorescent tube disposal, and mitigation of mercury and dioxin emissions from diverse industries including chemical engineering, non-ferrous metal smelting, and waste incineration. Looking ahead, non-thermal plasma technology holds promise in supplanting activated carbon adsorption as an energy-efficient and eco-friendly method for simultaneous control of multiple pollutants. It stands poised to aid China in fulfilling its commitments under the Minamata Convention on Mercury by providing indispensable technical support. Moreover, the recovered mercury can help alleviate China’s mercury supply shortages and contribute to the sustainability of the PVC industry.

Nevertheless, it is imperative to acknowledge that the prevention and control of mercury pollution represent enduring challenges requiring sustained collaborative efforts among governmental bodies, enterprises, and research institutions. Continuous innovation aimed at meeting the evolving needs of industry development, regulatory compliance, and technological advancement remains paramount.