Biogas can be generated by fermenting sewage. All buses in Stockholm run on it, it burns extremely clean and is carbon-neutral since it utilizes carbon already in the present carbon cycle.
Sorry, bad choice words. The work is not done by a yeast so perhaps it’s not technically fermenting. The work is done by a type of bacteria called archea. As far as I know the only byproduct Upon burning biogas is CO2 and water vapor.
You can barely call that biogas "burns extremely clean" with hydrogen sulfide in them, can you?
[1]: https://beerandbrewing.com/off-flavor-of-the-week-sulfur/