[devy]

All yeast strains produce some amount of hydrogen sulfide during fermentation as a by-product of sulfate processing. [1]

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/

[fastbeef]

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.