Hydrogen, alternative gases and decarbonising cities:

Biomethane still emits CO2 at combustion stage and potential methane through production and transportation.
It is thus only net zero if the production method leads to additional capturing of CO2 or avoidance of waste.

Hydrogen does not emit CO2 at the point of combustion, but there may still be (significant) lifecycle emissions include from:
1) Fossil gas production and transportation through methane leakage (for so called blue or turquoise hydrogen)
2) The limitations of the carbon capture method (the maximum achieved is so far 95%)
3) The electricity used to produce electrolysis hydrogen.

As a result, sustainable gases are a scarce premium product and likely to be expensive. Between 1/10 th and ¼ of current fossil gas use by 2050 at EU level.