Fossil fuels such as petroleum oil, coal, and natural gas have been accumulated in the earth over a fairy long period of time.
To burn these fossil resources and release carbon dioxides into the air means that the carbons, which have been restored deep in the earth, are exhausted to the atmosphere all of a sudden, which leads to a big increase of carbon dioxides content in the atmosphere.
Meanwhile, the resource of biogas plant is organic matter, the production of plant’s photosynthesis. Plants produce oxygen and carbohydrates from carbon dioxides by photosynthesis.
So even if the organic matter produces carbon dioxides in the course of biogas formation, there is a good balance between the exhaustion of carbon dioxides into the atmosphere and absorption of carbon dioxides from the atmosphere.
And what is important is that this process of giving out and taking in should occur in a short span of time.
This idea of carbon circulation is “carbon neutral”.