Couple quickies --
(1) the (substantial) energy stored in the hydrogen bonds of short-chain hydrocarbon fuels is not available to use as fuel energy; and
(2) instead of denuding the earth to produce biofuels, (which they would) AND consuming ABSURD amounts of 'fresh' water, we should look at aquatic chlorophyta (green plants that grow in the oceans-- plankton, kelp, Ulva lactuca, usw) as a means to capture carbon.
Just to really scare the greenies ... We could use NUCLEAR POWER plants to 'sterilize' municipal sewage; use the sewage as the nitrogen and phosphorous source the phytoplankton would love; use the waste heat from the reactors to drive the fermentation (or other process), and use the biomass of the phytoplankton to produce methane and fertilizer. Oh, the phytoplankton are also naturally pretty good heavy metal sinks as well, ( not as good as water hyacinths ) so we could clean up the sewage as we turn it (and sunlight and CO2) into fuel.
OK, kevlar now on ...
Perhaps I'm misunderstanding - are you saying that the formation of H2O in the combustion of hydrocarbons is not exothermic?