The Sabatier Process
The Sabatier process was first described by Paul Sabatier, who published the first paper on it in 1913.
Under a somewhat elevated pressure and temperature (300°-400° C.), and in the presence of an appropriate catalyst, hydrogen combines with carbon dioxide to produce methane. The reaction is:
CO2 + 4H2 → CH4 + 2H2O
The catalyst can be any of several, but nickel and aluminum oxide both work.
The implication is that, given some extra electrical power and hydrogen, the exhaust from a gas fired power plants can be captured and the carbon dioxide converted to methane. The methane can be used to fuel the power plant.
This requires more energy than it produces, but it has the advantage of making it possible to store excess power generated by solar or wind power plant in the form of methane, for use when wind and solar power are not available in sufficient quantity. In effect, this makes the gas power plant a battery for the grid, to even out supply and demand requirements.