It is well known that since natural gas generates half the CO2 for the same energy produced as coal in current plants, that replacing old coal plants would reduce CO2 pollution by a half for the switched plants. Those calculations assumed the present efficiency of energy generation of both coal and natural gas plants at the same 33% as at present, for steam generating plants.
However, reading an article by Richard Muller, “Fugitive Methane and Greenhouse Gases” , has made me aware that new combined cycling natural gas plants can be up to 60% efficient. We redo that method comparing CO2 from replacing an old 33% efficient coal plant with a 60% efficient natural gas plant.
CO2 is proportional to fuel usage per molecule or atom, since coal is mostly burning carbon atoms C, and natural gas or methane, CH4, also contains only one carbon atom. The extra Hydrogens in CH4 oxidize with Oxygen to form water, but generate about the same amount of energy as oxidizing the Carbon atom in CH4 to CO2. Thus the factor of twice the energy from burning a methane molecule than a coal atom.
Taking a coal or carbon atom at 33% efficiency, means that you need 1 x 1/.33 = 3 carbon atoms to burn to generate the amount of electricity contained in burning the atom itself. For a natural gas molecule, you need only one half a molecule to get the same starting energy, but then for 60% efficiency, you need a factor of 1/0.60 more, giving the comparable fuel usage of 0.5 x 1/0.60 = 0.83 molecule. The ratio of CH4 to C fuel usage is then 0.83 / 3 = 0.28. That is also the ratio of CO2 pollution, since one C atom comes from each.
So instead of the ratio of CO2 from methane over coal being a half for present plants, the ratio is actually between a third and a quarter for a new combined cycle natural gas plant and an old coal plant.