Decomposition of SF6 in an RF 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.

Advances in Continuous Flow Fluorination Reactions

Fluorination reactions are important in constructing organofluorine motifs, which contribute to favorable biological properties in pharmaceuticals and agrochemicals. However, fluorination reagents and reactions are associated with various problems, such as their hazardous nature, high exothermicity, and poor selectivity and scalability. Continuous flow has emerged as a transformative technology to provide many advantages relative to batch syntheses. This review article summarizes recent continuous flow techniques that address the limitations and challenges of fluorination reactions. Approaches based on different flow techniques are discussed, including gas-liquid reactions, packed-bed reactors, in-line purifications, streamlined multistep synthesis, large-scale reactions well as flow photoredox- and electrocatalysis.

Simulation and experimental study on the degradation of the greenhouse gas SF6 by thermal plasma

SF₆ gas is widely used on many occasions especially in the power equipment, but it has been restricted since Kyoto Protocol as the strongest greenhouse gas. To reduce the SF₆ emission, several methods are now used such the recycling & purification and the SF₆ degradation. Considering the huge market of SF₆ and the recent demand in the field of power equipment, it is necessary to explore new ways to thoroughly destroy SF₆. This work brought out the idea to degrade retired SF₆ by thermal plasma. A simplified kinetic model was established to predict the feasibility of this idea as well as the degradation products of SF₆, and then the prototype of SF₆ degradation by thermal plasma was built and tested. In thermal plasma, SF₆ gradually decomposed into atoms, and then H₂ was added to capture the released F atoms to generate HF and also prevent the association reactions of SF₆. In order to achieve the desired degradation effect, the reaction temperature and the mixing ratio of H₂ should be sufficiently high. However, excessive H₂ could generate the H₂S, and excessive discharge power could decrease the energy yield. When the flow rate of SF₆/H₂ was set as 8/30 L/min and the discharge current was set as 100A, the destruction removal efficiency (DRE) of SF₆ was 99.0% and the energy yield was 206 g/kWh. This work also discusses how to further treat the by-products such as HF and S from this prototype effectively.

Repurposing of F-gases: challenges and opportunities in fluorine chemistry

Fluorinated gases (F-gases) are routinely employed as refrigerants, blowing agents, and electrical insulators. These volatile compounds are potent greenhouse gases and consequently their release to the environment creates a significant contribution to global warming. This review article seeks to summarise: (i) the current applications of F-gases, (ii) the environmental issues caused by F-gases, (iii) current methods of destruction of F-gases and (iv) recent work in the field towards the chemical repurposing of F-gases. There is a great opportunity to tackle the environmental and sustainability issues created by F-gases by developing reactions that repurpose these molecules.

Disposal methods, health effects and emission regulations for sulfur hexafluoride and its by-products

Sulfur hexafluoride (SF₆) is the most potent greenhouse gas contributed by the power and semiconductor industries. The global emissions of gas in the past 10 years have increased tremendously due to lack of disposal routes. This was brought to 190 nations' attention in the Kyoto Protocol for the need of emission control measures to reduce its impacts of climate change and global warming. Various novel techniques have surfaced to tackle this issue, such as non-thermal plasma (NTP) which includes radio frequency plasma, microwave plasma, dielectric barrier discharge, and electron beam. The main by-products resulting from the decomposition of SF₆ by these techniques are sulfur oxyfluorides, sulfur dioxide, hydrofluoric acid, and fluorine gas. This environmental and health effects as well as global emission of SF₆ gas are considered a threat to humans and the climate, where modern disposal methods of contaminated SF₆ gas and its by-products should replace the conventional approaches. Relevant government policies on the safety and disposal concern of SF₆ gas are reviewed and challenges and further research directions for the disposal of SF₆ gas are highlighted in this review article.

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.

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.

Total toxicity equivalents emissions of SF6, CHF3, and CCl2F2 decomposed in a RF plasma environment

Sulfur hexafluorine compound (SF6), trifluoromethane (CHF3) and diclorodifluoromethane (CCl2F2) are extensively used in the semiconductor industry. They are global warming gases. Most studies have addressed the effective decomposition of fluorine compounds, rather than the toxicity of decomposed by-products. Hence, the concepts of toxicity equivalents (TEQs) were applied in this work. The results indicated that HF and SiF4 were the two greatest contributors of TEQ to the SF6/H2/Ar plasma system, while F2 and SiF4 were the two greatest contributors to the SF6/O2/Ar system. Additionally, SiF4 and HF were the two greatest contributors of TEQ to both the CHF3/H2/Ar and CHF3/O2/Ar plasma systems. HF and HCl were the two greatest contributors of TEQ to the CCl2F2/H2/Ar plasma system, and Cl2 and COCl2 were the two greatest contributors to the CCl2F2/O2/Ar system. HCl and HF can be recovered using wet scrubbing, which reduces the toxicity of these emission gases. Consequently, the hydrogen-based plasma system was a better alternative for treating gases that contained SF6, CHF3 and CCl2F2 from the TEQs point of view.

Decomposition of benzene in the RF plasma environment. Part II. Formation of polycyclic aromatic hydrocarbons

This study investigated the characteristics of polycyclic aromatic hydrocarbons (PAHs) formed during the decomposition of benzene (C(6)H(6)) in radio-frequency (RF) plasma environments. The identification and quantification were accomplished by using a GC/MS for PAHs and an on-line Fourier transform infrared (FT-IR) spectrometer for the reactants and gaseous products. The analytical results show that PAHs were formed in both C(6)H(6)/Ar and C(6)H(6)/H(2)/Ar systems. In terms of individual PAHs, naphthalene (C(10)H(8)) was the predominant species found among the 21 PAHs under all operational conditions, phenanthrene and chrysene are the next. High-ring PAHs did not form easily in the C(6)H(6)/Ar and C(6)H(6)/H(2)/Ar system, especially at high input power and high C(6)H(6) feed concentration (C(C(6)H(6))) for the former system. Yields of PAHs with different ring numbers decreased with increasing ring number. At low input power, increasing C(C(6)H(6)) would promote yields of PAHs, while adding hydrogen as the auxiliary gas suppressed PAHs formation. Higher input power or addition of oxygen not only effectively suppresses PAHs formation but also completely destroys C(6)H(6). Owing to the absence of the principal intermediate species, phenol (C(6)H(5)OH), from the gas products of C(6)H(6)/O(2)/Ar system, H-abstraction-C(2)H(2)-addition (HACA) pathway is proposed as the primary mechanism for PAHs formation in the present study. Gas phase distribution of total-PAHs accounts for 20-95.3% at 2% of C(C(6)H(6)) among C(6)H(6)/Ar, C(6)H(6)/H(2)/Ar and C(6)H(6)/O(2)/Ar systems. This study suggests that gas-phase PAHs should not be ignored, particularly in C(6)H(6)/Ar systems under high input power and high C(C(6)H(6)) , or in C(6)H(6)/O(2)/Ar systems.

Abatement of sulfur hexafluoride emissions from the semiconductor manufacturing process by atmospheric-pressure plasmas

Sulfur hexafluoride (SF6) is an important gas for plasma etching processes in the semiconductor industry. SF6 intensely absorbs infrared radiation and, consequently, aggravates global warming. This study investigates SF6 abatement by nonthermal plasma technologies under atmospheric pressure. Two kinds of nonthermal plasma processes--dielectric barrier discharge (DBD) and combined plasma catalysis (CPC)--were employed and evaluated. Experimental results indicated that as much as 91% of SF6 was removed with DBDs at 20 kV of applied voltage and 150 Hz of discharge frequency for the gas stream containing 300 ppm SF6, 12% oxygen (O2), and 40% argon (Ar), with nitrogen (N2) as the carrier gas. Four additives, including Ar, O2, ethylene (C2H4), and H2O(g), are effective in enhancing SF6 abatement in the range of conditions studied. DBD achieves a higher SF6 removal efficiency than does CPC at the same operation condition. But CPC achieves a higher electrical energy utilization compared with DBD. However, poisoning of catalysts by sulfur (S)-containing species needs further investigation. SF6 is mainly converted to SOF2, SO2F4, sulfur dioxide (SO2), oxygen difluoride (OF2), and fluoride (F2). They do not cause global warming and can be captured by either wet scrubbing or adsorption. This study indicates that DBD and CPC are feasible control technologies for reducing SF6 emissions.