Sulfur Hexafluoride: The Nightmare Greenhouse Gas That’s Just Too Useful To Stop Using

Sulfur hexafluoride (SF₆) is not nearly as infamous as CO₂, with the latter getting most of the blame for anthropogenic climate change. Yet while measures are being implemented to curb the release of CO₂, for SF₆ the same does not appear to be the case, despite the potentially much greater impact that SF₆ has. This is because when released into the atmosphere, CO₂ only has a global warming potential (GWP) of 1, whereas that of methane is about 28 over 100 years, and SF₆ has a GWP of well over 22,000 over that same time period.

Also of note here is that while methane will last only about 12.4 years in the atmosphere, SF₆ is so stable that it lasts thousands of years, currently estimated at roughly 3,200 years. When we touched upon sulfur hexafluoride back in 2019 in the context of greenhouse gases, it was noted that most SF₆ is used for — and leaks from — high-voltage switchgear (mechanical switches), transformers and related, where the gas’ inert and stable nature makes it ideal for preventing and quenching electrical arcing.

With the rapid growth of highly distributed energy production in the form of mostly (offshore) wind turbines and PV solar parks, this also means that each of these is equipped with its own (gas-filled) switchgear. With SF₆ still highly prevalent in this market, this seems like an excellent opportunity to look into how far SF₆ usage has dropped, and whether we may be able to manage to avert a potential disaster.

Best at Not Doing Anything

Sulfur hexafluoride (SF6) skeletal formula with dimensions.

What makes SF₆ such an excellent, one-stop shop choice for quelling electrical arcs and insulating high-voltage electrical system is because of its stability. Generally, it does not readily interact with other substances, which leads to its properties of being colorless, non-flammable and non-toxic. Unfortunately, this lack of chemical reactivity also means that it can hang around in e.g. the Earth’s atmosphere for a very long time.

Although SF₆ occurs naturally, the overwhelming majority is produced by humans, for use in industrial processes and medicine, but primarily in high-voltage electrical systems as a dielectric gas. The main purpose of a dielectric gas here is to increase the breakdown voltage so that higher voltages can be used in less space, generally relative to air.

For when some arcing does occur, the purpose of the gas should also be to quench the arcing, which is where SF₆ shines. Although a small part of the gas may be broken down into the toxic S₂F₁₀ (disulfur decafluoride), most breakdown products will quickly reform into SF₆, which makes it a low-maintenance choice for switchgear. Especially for gear that ends up being installed somewhere remote and relatively inaccessible, this is a very helpful property.

Because SF₆ is non-toxic and has a high molecular weight, it has also found use as an inverse party gag to helium: where helium’s low molecular density makes for an increase in perceived pitch when speaking through a helium-filled medium, breathing in SF₆ will significantly lower the pitch of one’s voice until the gas has been expelled from the person’s airways.

Gases Want to Be Free

The growth of sulfur hexafluoride (SF6) in Earth’s atmosphere during years 2000 – 2020. (credit: AGAGE)

An unfortunate side-effect of our planet’s gaseous atmosphere is that any gases which escape from containment, or which are released through human activity end up joining said atmosphere. How concerned we should be about this depends on the gas in question. When CFCs were found to be rapidly eroding the Earth’s ozone layer, this made it crucial to immediately eliminate any significant release of this gas. This was accomplished via the Montreal Protocol, which saw a rapid cessation of most uses of CFCs.

In the case of SF₆, it would seem fair to ask just what the scope of the threat is. To assess this we can look at AGAGE’s data. This is the Advanced Global Atmospheric Gases Experiment, which keeps track of a wide range of gases in the atmosphere. Their findings are that the amount of SF₆ has significantly increased since 2000, increasing from about 4 ppt (parts per trillion) to around 10 ppt by 2020, with a linear increase becoming noticeable around 1970. Pre-industrial troposphere levels were roughly around 54 ppq (parts per quadrillion).

As over 80% of the SF₆ that is produced is used in the electrical power industry, this is also not surprisingly the biggest source of leaks. Much of this is due to the distributed nature, instead of the gas being used in a closely monitored industrial process, items like switchgear are located literally around the world, in deserts, at the top of wind turbines and in the middle of fields. When being installed, repaired or decommissioned, switchgear can also be damaged, with SF₆ gas escaping into the atmosphere.

Top-down inversion emission estimate for western Europe (2013–2018). (Credit: Simmonds et al., 2020)

In a 2020 study based on the AGAGE findings titled The increasing atmospheric burden of the greenhouse gas sulfur hexafluoride (SF₆), Simmonds et al. cover the past 40 years of measurements. They note five main source of SF₆ leakage:

Electrical power industry
Magnesium industry
Aluminium industry
Electronics industry
SF₆ production itself

As for the major SF₆-emitting countries, these were deduced from measurements to be primarily China and South Korea in East-Asia, and Germany in Western Europe. In the case of Germany semiconductor producers are suspected of being major contributors.

As for high-voltage gas-insulated switchgear (GIS), these use as mentioned >80% of the annual production of SF₆, with medium-voltage GIS another 10%. These GIS tend to have a lifespan of 30-40 years, with new SF₆-based GIS being installed even today, each of which will suffer some level of leakage during normal operation due to the imperfect nature of seals. In the magnesium, aluminium, and semiconductor industries, leaks have been gradually reduced over time, but are still a significant source.

In 2018, global emissions of SF₆ were 9.0±0.4 Gg yr⁻¹, with 2018 CO₂ emissions being 33.1 Gt (33,100,000 Gg). Taking into account the much higher GWP (22800) of SF₆, this makes its 2018 emissions equivalent to about 205,200 Gg, or 0.6% of annual CO₂ emissions. While not an astounding number, we must take into account here that so far the emissions of SF6 are increasing year over year. Any SF₆-based GIS or similar installed today will be adding to this total for the next decades, while contributing to global warming for a longer period than the industrial era so far.

Alternatives

Clearly, replacing SF₆ and generally preventing it from leaking into the atmosphere is a good thing, then. Perhaps ironically, SF₆ previously replaced the use of oil in switchgear due to toxic and otherwise harmful substances, and some of the suggested replacements for SF₆ are themselves not as benign as this gas. Where possible, one of the best options is a vacuum, with a high vacuum providing very high dielectric insulation.

Maintaining a high vacuum is not easy, especially not over years, leading to alternatives ranging from plain air, CO₂, and various fluoride-based substances. Recently Owens et al. (2021) as researchers at 3M published a study on two SF6 alternatives which 3M sells commercially. Their commercial names are Novec 4710 ((CF₃)₂CFCN) and Novec 5110 ((CF₃)₂CFC(O)CF₃), both being fluoronitrile and fluoroketone mixes.

The idea is that such mixes are added to CO₂ or air inside the GIS, to improve the dielectric properties. In this configuration, Novec 5110 with air mixture looks pretty decent, with a (100-year) GWP of <1, but Novec 4710 with CO₂ mixture has a GWP of 398, which is better, but not great. SF6 also showed an overall better cold weather performance, down to -38 °C, compared to -27 °C for Novec 4710/CO₂, and 0 °C for Novec 5110/air.

US sulfur hexafluoride market. (Source: Grand View Research)

This highlights the complexity in replacing SF₆ in GIS applications, as each part of an electrical grid has different temperature ranges and other factors that would be a particular SF₆ alternative more attractive. With SF₆ being relatively cheap, universally applicable, and its use so far unencumbered within the electrical power industry — even within the EU’s F-gases regulations — it’s little wonder that the SF₆ market keeps growing year over year.

Not Just SF₆

The fluorinated gases have in common that they tend to be man-made, popular in industry and other applications, and have a high GWP. They include HFCs, PFCs, SF₆ and NF₃. Of these, HFCs are popular in refrigeration, where they replace the previously popular CFCs, along with a number of other gases. Through their production, use and eventual decommissioning, a significant amount of these gases end up in the atmosphere, where they contribute to the specter of anthropogenic global warming.

Looking at the popularity of these gases, the difficulty in finding replacements, and the push to produce more and ever cheaper refrigerators, wind turbines, and distributed power systems, it seems unlikely that we’ll be seeing a major change here. Meanwhile every day sees more of SF₆-based GIS and kin being installed in the world’s rush to decarbonize and expand the electrical grid, where they’ll continue to be a problem for decades to come.

Although this is a perhaps depressing perspective, some hope can be gained from the way the world came together to banish CFCs when it was clear that they formed an existential threat to all life on this Earth. Here is to hoping that we can do that a few more times.

This information was first published on https://hackaday.com/2021/11/10/sulfur-hexafluoride-the-nightmare-greenhouse-gas-thats-just-too-useful-to-stop-using/