[flumpmaster]

Renewable natural gas / bio gas is alive and growing in the US. The economics are supported by California Low Carbon Fuel Standard credits.

Sources include captured landfill gas and bio digesters processing animal manure.

Captured bio gas is injected into adjacent natural gas transmission pipe lines and commingled with chemically identical fossil natural gas.

[pfdietz]

The CO2 has to be scrubbed out. This is not necessarily all negative, since if the CO2 is injected underground for sequestration it makes biogas carbon negative.