Kushal

i think that if the process producing the fuel liberates more energy, then on oxidation of the fuel, less energy will be liberated.

and, i think it is easier to separate the methane from CO2 (maybe cooling the mixture to make the CO2 solidify?!) than taking the ethanol out of the broth.

i had in mind that it was that energy released from the above reactions that were used, and not the energy from the further oxidation of methane or ethanol. :-S

but what you say makes much more sense. thnks