What SF6's history? And why do gas handling processes around used SF6 matter?
In the energy industry, Sulphur Hexafluoride (SF6) is predominantly used as an insulating and arc quenching medium for medium voltage (MV) and high voltage (HV) gas insulated switchgear (GIS). GIS manufacturers began using SF6 as an insulating medium in the 1930s. Its large-scale application started in the 1960s when it was mostly used for HV applications. SF6’s use in the MV segment followed a few decades later. Today, SF6-based GIS can be found in both HV and MV electrical infrastructure around the world.
Its widespread adoption is due to its fantastic electrical properties. Additionally, it is non-flammable, non-toxic, thermally stable (i.e., it does not decompose until 500 °C), non-corrosive, and it naturally recombines after decomposition so that the permanent decomposition rate due to arcing is very low (SF6 and Alternatives Coalition, 2020). These characteristics give GIS an advantage over traditional competition – air insulated switchgear (AIS), solid insulated switchgear (SIS). These advantages take the form of GIS requiring minimal maintenance, having compact physical dimensions and high reliability.
However, despite all its strengths, SF6’s use in the energy industry’s days appear to be numbered. Due to its position as the world’s strongest greenhouse gas (GHG) under the Kyoto protocol (it is 23,500x stronger than CO2), environmental regulators around the world are increasingly focusing on it, with a phase-out highly likely in the coming years.
This, combined with the fact that the GIS installed in the 80s and 90s are
coming to the end of their operational lifetime, raises several questions about what generally happens with used SF6. And what will happen to global SF6 inventories once regulation phases out its main switchgear use-case? Below, we try to answer such questions in a Q&A format.
What are the options for used SF6?
Today, the switchgear users (utilities / industry) have three options when it comes to used SF6:
Off-site reconditioning
Off-site destruction
On-site recycling
Off-site reconditioning
What does it involve?
Reconditioning involves a third party extracting the SF6 gas and transporting it to a specialized facility where it is filtered and cleaned of contaminants such as oil, moisture, or other by-products, using a cryogenic process. The resulting SF6 has a higher level of purity and can be put back into circulation with virgin SF6.
Cryogenics is a process of liquifying the SF6 through a 3-step system:
The pressure of the sealed SF6-container to above 700 psi or 48.3 bar.
The temperature is then dropped from -20 °C to -30 °C to -40 °C. This liquifies the SF6, and other vapours remain suspended (nitrogen and oxygen).
Finally, separation takes place, allowing the other vapours to be released into the atmosphere.
What are the pros of reconditioning?
Reconditioning has less environmental impact than destruction due to the high SF6 emissions that take place during SF6 destruction. Also, SF6 owners don’t have to invest in their own equipment, and training of their personnel, as would be the case with on-site recycling. They also reduce their emissions liability.
What are the cons of reconditioning?
Users are reliant on third parties to remove the gas and conduct the cryogenic process, which can be expensive.
Off-site destruction
What does it involve? This involves the SF6 gas being destroyed in a specialized facility. The gas is generally reclaimed by a third party from the GIS.
What are the pros of destruction? It allows users to reduce their SF6-inventory and their own emissions.
What are the cons of destruction?
High levels of emissions take place during destruction. Unfortunately, they are difficult to quantify as there is no reliable research focusing on this. SF6 destruction is also expensive as it involves third parties, and requires extremely high temperatures (Zeng, F et al. 2014).
On-site recycling
What does it involve?
This has traditionally been the most popular option for users. It is similar to reconditioning insofar as it involves filtering and cleaning the SF6 of contaminants and by-products. However, this is done on-site by the users themselves using auxiliary filtration systems.
What are the pros of recycling?
The recycling is done in-house and takes place on-site, making it easy to control inventory.
What are the cons of recycling? Users need to invest in the right equipment and continuous training of their personnel, which increases operating costs. Also, the SF6’s purity levels cannot be improved.
What will happen to SF6 inventories once regulation phases out SF6 for new installations?
This question should be receiving much more attention by policymakers and industry, as currently there are no clear answers.
As mentioned, regulation likely will soon phase out SF6’s use in the energy sector, leaving no real viable option for what to do with the global inventory of this highly potent GHG.
Recycling and reconditioning of SF6 will have been made obsolete due to a lack of other real use-cases, and destruction is expensive, and - in its current form - results in a high (yet undefined) level of emissions.
Unless the destruction process is refined and made more economical, industry and policymakers will find themselves having to deal with SF6 without having an environmentally friendly and efficient disposal system.
Any more questions?
Why do users often prefer virgin SF6?
GIS users need high-grade SF6, which is more difficult to achieve with reconditioned or recycled SF6. Also, some users associate a stigma to reconditioned or recycled SF6 after having had bad experiences in the past, i.e., the SF6 was not reconditioned or recycled to a high enough standard.
Moreover, some switchgear OEMs don’t accept reconditioned SF6 in their products, and do not provide warranty if reconditioned SF6 is used.
How is the production of virgin SF6 responsible for more emissions?
SF6 is typically sent to switchgear manufacturers in a 45.5 kg bottle. If one compares emissions from the production of a virgin SF6 bottle with a reconditioned SF6 bottle - the production of a virgin bottle generally results in emissions of 1.5 kg to 3.6 kg of SF6, while the production of a reconditioned bottle would typically emit 0.29 kg of SF6.
Furthermore, the production of virgin SF6 can result in an SF6-surplus, as producers tend to send more virgin SF6 than is required for any one switchgear unit.
Summary
As regulation continues to restrict the usage of SF6, users should be aware of the different options available to them in terms of how to reuse or dispose of their SF6 inventory.
Depending on their needs and goals, users can choose between reconditioning, destruction and recycling.
Nevertheless, a long-term solution remains illusive with respect to sustainably and economically removing SF6 from circulation, as will likely soon be required by regulation.
Learn about how nuventura can help you go SF6-free here.
References
The content for this article was based on the podcast ‘What should I do with used SF6?’ by the ‘SF6 & Alternatives Coalition’.
Additional references
SF6 & Alternatives Coalition. 2020. "Alternative Insulation Technologies". Available here.
Zeng, F. et al. 2014. „Decomposition characteristics of SF6 under thermal fault for temperatures below 400°C”. IEEE Transactions on Dielectrics and Electrical Insulation 21(3):995-1004. Available here.