This has some real possibility of making Ballooners work.
https://www.ecowatch.com/floating-so...638980599.html
The Ballooners use solar power as both the 3rd and 4th edition state, generating methanol. Methanol can be converted to methane via both chemical and biological processes matching the 4th edition description. They not only have cities in the sky, but isolated solar islands to generate the lifting gases and other support they need in the sky, venturing out to scavenge or trade for other materials they need.
Note: If they used chemical processes to convert methanol to methane, they would also be generating hydrogen, a much better lifting gas than methane. No one ever said being a Ballooner was always safe.