Decomposition of SF6and H2S Mixture in Radio Frequency Plasma Environment

Recent progresses, challenges and proposals on SF6 emission reduction approaches

The increasing utilization and emission of sulfur hexafluoride (SF6) pose severe threats to the climate and the environment, owing to its potent greenhouse gas properties. In this paper, we comprehensively review the recent progresses of SF6 emission reduction approaches. Currently, the use and emission of SF6 are still on the rise, and mainly concentrated in the power industry. Restrictive use and emission reduction policies are fundamental step in guiding SF6 emission, but they are poor promoted in developing economies. More specific policies and regulations are needed in conjunction with timely and accurate assessments of SF6 atmospheric properties and emissions. SF6 recovery is the direct emission reduction approach, but defects in recovery methods and equipment limit its applications. The development of SF6 purification technologies and optimizations in recovery devices and processes are needed for its treatment of different regions and SF6 volumes. SF6 degradation is the final step of waste gas treatment, and its development needs to better balance the degradation rate and product selectivity, as well as to improve their multi-scenario responsiveness. SF6 substitution is a necessity for future large-scale SF6 emission reduction. Improvements in SF6-free applications and its long-term stability are critical via new gas design, gas mixture optimization and equipment updates. Finally, all the emission reduction approaches are closely related, and promoting their synergistic development and complementarity is the ultimate way to realize SF6 lifecycle management.

SF6 abatement in a packed bed plasma reactor: study towards the effect of O2 concentration

SF₆ is a greenhouse gas with extremely high global warming potential value (GWP). In this paper, oxygen and a packed bed plasma reactor (PBR) were applied to remove it. The synergistic effect between oxygen and PBRs was evaluated by the destruction and removal efficiency (DRE) and energy yield (EY) at different oxygen concentrations. The results show that excessive oxygen weakened the micro-discharge in a PBR to suppress SF₆ degradation while the addition of a proper amount of oxygen (1–4%) can improve the DRE and EY. 2% O₂ in the system had the best promoting effect on the destruction of 6–10% SF₆, which made the maximum energy yield (EY) increase by 50.99% to 37.99 g kW⁻¹ h⁻¹ (SF₆ concentration was 10%, flow rate was 150 mL min⁻¹). Moreover, in the flow rate range of 100 mL min⁻¹ to 250 mL min⁻¹, the DRE decreased and the EY increased with the flow rate. In addition, the selectivity of different products were affected by the oxygen concentration. For 6% SF₆, SO₂F₂ selectivity was always the highest while SO₂ was always the lowest; when the oxygen concentration did not exceed 2%, SOF₂ selectivity was higher than SOF₄, otherwise, SOF₄ selectivity was higher than SOF₂. This paper provided experimental support for better understanding of the effect of additional gas concentration on SF₆ decomposition in a PBR.

SF₆ is a greenhouse gas with extremely high global warming potential (GWP).

Abatement of SF6 in the presence of NH3 by dielectric barrier discharge plasma

In this paper, the degradation rate, energy yield and the degradation by-products of SF₆ was studied when different concentrations of NH₃ were added. When NH₃ concentration increased from 0 to 2%, the degradation rate efficiency(DRE) of SF₆ increased from 60% to 97.23% under the flow rate of 50ml/min and 94W input power. The energy yield(EY) reached 4.16g/kWh. In addition, we found that increasing the flow rate to 250ml/min, the DRE decreased to 58.71%, but the EY increased to 12.55g/kWh. The main gas by-products are SOF₂, SO₂F₂, SO₂, OF₂, HF and NF₃. When the concentration of initial NH₃ increased, the SO₂ concentration increased while the concentrations of SOF₂, SO₂F₂, SOF₄ decreased. In addition, we found that a pale yellow film formed on the surface of the reactor wall. XPS analysis showed that the solid products were mainly S, NH₃HF and NH₄HF₂. The emission spectra show that NH₃ addition can effectively promote the formation of active particles and increase plasma density.The addition of NH₃ can convert some of the sulfur and fluorine into solid products and reduce the production of toxic gases.

Controlled MoS₂ layer etching using CF₄ plasma

A few-layered molybdenum disulfide (MoS2) thin film grown by plasma enhanced chemical vapor deposition was etched using a CF4 inductively coupled plasma, and the possibility of controlling the MoS2 layer thickness to a monolayer of MoS2 over a large area substrate was investigated. In addition, damage and contamination of the remaining MoS2 layer surface after etching and a possible method for film recovery was also investigated. The results from Raman spectroscopy and atomic force microscopy showed that one monolayer of MoS2 was etched by exposure to a CF4 plasma for 20 s after an initial incubation time of 20 s, i.e., the number of MoS2 layers could be controlled by exposure to the CF4 plasma for a certain processing time. However, XPS data showed that exposure to CF4 plasma induced a certain amount of damage and contamination by fluorine of the remaining MoS2 surface. After exposure to a H2S plasma for more than 10 min, the damage and fluorine contamination of the etched MoS2 surface could be effectively removed.

Efficient removal of sulfur hexafluoride (SF6) through reacting with recycled electroplating sludge

This paper reports that recycled electroplating sludge is able to efficiently remove greenhouse gas sulfur hexafluoride (SF6). The removal process involves various reactions of SF6 with the recycled sludge. Remarkably, the sludge completely removed SF6 at a capacity of 1.10 mmol/g (SF6/sludge) at 600 °C. More importantly, the evolved gases were SO2, SiF4, and a limited amount of HF, with no toxic SOF4, SO2F2, or SF4 being detected. These generated gases can be readily captured and removed by NaOH solution. The reacted solids were further found to be various metal fluorides, thus revealing that SF6 removal takes place by reacting with various metal oxides and silicate in the sludge. Moreover, the kinetic investigation revealed that the SF6 reaction with the sludge is a first-order chemically controlled process. This research thus demonstrates that the waste electroplating sludge can be potentially used as an effective removal agent for one of the notorious greenhouse gases, SF6.

Removal of gaseous polycyclic aromatic hydrocarbons from cooking fumes using an atmospheric plasma reactor

Plasma technology is becoming increasingly important for treating various environmental pollutants. Treatment of polycyclic aromatic hydrocarbons (PAHs), such as those emitted from electric ovens while roasting pork, using an atmospheric plasma reactor has seldom been studied. This study investigated the characteristics of five PAH species (acenaphthalene (AcPy), acenaphthene (Acp), anthracene (Ant), benzo[a]anthracene (BaA), and benzo(ghi)perylene (BghiP)) in fumes emitted while roasting pork. The removal efficiency at different plasma output powers (0.112, 0.138, and 0.156 kJ/m(3)) of the reactor was also investigated. In the experiments, cooking fumes were generated by a small electrical oven, with pork being roasted at 200 °C. After a steady state was reached, samples were collected at the inlet and outlet of the atmospheric plasma reactor. The PAHs were analyzed using gas chromatography-mass spectrophotometry. The experimental results indicated that the removal efficiency for each PAH was highest with the highest plasma reactor output power. This was also true of the total PAH concentration, but the total toxic equivalence, BaP(eq), was lowest at the medium power output. This demonstrates that the total toxicity and the removal of PAHs were not directly proportional, and careful consideration must be made by engineers when setting the treatment conditions.

Formation of fluorine for abating sulfur hexafluoride in an atmospheric-pressure plasma environment

In this study, a large amount of toxic and reactive fluorine (F(2)) was produced in the atmospheric-pressure microwave discharge environment by adding additives to abate sulfur hexafluoride (SF(6)). When H(2) was added, the selectivity of F(2) was as high as 89.7% at inlet H(2)/SF(6) molar ratio (R(H2)) = 1. Moreover, the conversion of SF(6) significantly increased from 33.7% (without additive) to 97.7% (R(H2) = 5) at [SF(6)]=1%, and 0.8 kW because the addition of H(2) inhibited the recombination of SF(6). With the addition of O(2), H(2)+O(2) or H(2)O, the selectivity of F(2) was still greater than 81.2%, though toxic byproducts, including SO(2)F(2), SOF(2), SOF(4), SO(2), NO, and HF, were detected. From optical emission spectra, SF(2) was identified, revealing the SF(6) dissociation process might be carried out rapidly through an electron impaction reaction: SF(6)-->SF(2)+4F. Subsequently, F(2) was formed via the recombination of F atoms.

A novel method to decompose two potent greenhouse gases: photoreduction of SF6 and SF5CF3 in the presence of propene

SF5CF3 and SF6 are the most effective greenhouse gases on a per molecule basis in the atmosphere. Original laboratory trial for photoreduction of them by use of propene as a reactant was performed to develop a novel technique to destroy them. The highly reductive radicals produced during the photolysis of propene at 184.9 nm, such as .CH3, .C2H3, and .C3H5, could efficiently decompose SF6 and SF5CF3 to CH4, elemental sulfur and trace amounts of fluorinated organic compounds. It was further demonstrated that the destruction and removal efficiency (DRE) of SF5X (X represented F or CF3) was highly dependent on the initial propene-to-SF5X ratio. The addition of certain amounts of oxygen and water vapor not only enhanced the DRE but avoided the generation of deposits. In both systems, employment nitrogen as dilution gas lessened the DRE slightly. Given the advantage of less toxic products, the technique might contribute to SF5X remediation.

Photoreductive degradation of sulfur hexafluoride in the presence of styrene

Sulfur hexafluoride (SF6) is known as one of the most powerful greenhouse gases in the atmosphere. Reductive photodegradation of SF6 by styrene has been studied with the purpose of developing a novel remediation for sulfur hexafluoride pollution. Effects of reaction conditions on the destruction and removal efficiency (DRE) of SF6 are examined in this study. Both initial styrene-to-SF6 ratio and initial oxygen concentration exert a significant influence on DRE. SF6 removal efficiency reaches a maximum value at the initial styrene-to-SF6 ratio of 0.2. It is found that DRE increases with oxygen concentration over the range of 0 to 0.09 mol/m3 and then decreases with increasing oxygen concentration. When water vapor is fed into the gas mixture, DRE is slightly enhanced over the whole studied time scale. The X-ray Photoelectron Spectroscopy (XPS) analysis, together with gas chromatography-mass spectrometry (GC-MS) and Fourier Transform Infrared spectroscopy (FT-IR) analysis, prove that nearly all the initial fluorine residing in the gas phase is in the form of SiF4, whereas, the initial sulfur is deposited in the form of elemental sulfur, after photodegradation. Free from toxic byproducts, photodegradation in the presence of styrene may serve as a promising technique for SF6 abatement.

Investigation of a new approach to decompose two potent greenhouse gases: photoreduction of SF(6) and SF(5)CF(3) in the presence of acetone

In this paper, we addressed the utilization of photochemical method as an innovative technology for the destruction and removal of two potent greenhouse gases, SF(6) and SF(5)CF(3). The destruction and removal efficiency (DRE) of the process was determined as a function of excitation wavelength, irradiation time, initial ratio of acetone to SF(5)X (X represented F or CF(3)), initial SF(5)X concentration, additive oxygen and water vapor concentration. A complete removal was achieved by a radiation period of 55min and 120min for SF(6)-CH(3)COCH(3) system and SF(5)CF(3)-CH(3)COCH(3) system respectively under 184.9nm irradiation. Extra addition of water vapor can enhance DRE by approximately 6% points in both systems. Further studies with GC/MS and FT-IR proved that no hazardous products such as S(2)F(10), SO(2)F(2), SOF(2), SOF(4) were generated in this process.