This is a term for the gases evolved from decomposition of organic material, as in landfills or sewage treatment.  Dominant products are CO2 and methane, accompanied by H2S. Greenhouse gases.  This term applies to all the gases that potentially can modify the upper atmosphere, tending to retain the heat from the earth that is normally lost by radiation.  The gases that are of most concern are CO2 and methane, because of the high volumes that are released into the atmosphere every day.  Hydrocarbon vapors as released from gasoline and from oil refineries are even more potent sources. Green fuels is a term I am not familiar with, unless it refers to hydrogen which produces no greenhouse gases by combustion.