Does the combustion of ethanol and biogas contribute to the greenhouse effect?

No, one of the major advantages of biofuels is that the combustion of such fuel do not contribute to the greenhouse effect. This is because the amount of carbon dioxide emitted when such fuels are combusted is equal to the amount of carbon dioxide that the plant (which the fuel consists of) consumed before it was harvested.

When fossil fuels are combusted, on the other hand, a surplus of carbon dioxide is released, which contributes to the greenhouse effect.