The End Of Fossil Fuels.

[Alex K]

Isn't water or steam already combusted/oxidized hydrogen? How would a hot furnace make water or steam combust more?

It doesn't, any well-designed furnace will burn all the Hydrogen, and all you can get out is the difference of energy levels of separate H2 and O2 versus H20, which is a fixed number.

The only thing you can do to get the maximum performance is to cool (not heat) the exhaust H2O + Nitrogen mixture which you get from burning the hydrogen, down far enough to let the water vapor in it condense. That gives you an additional 18% of heat. I literally just taught a class on that 1 hour ago

That being said, if you're driving a heat engine, the hotter your primary heat source, the better the efficiency because the theoretical maximum efficiency is = 1-T1/T2 where T1,2 are Kelvin Temperatures. This tends towards 1 for high temperatures though, and it can't ever give you more than the above-mentioned difference of binding energies, usually called the "higher heating value" of Hydrogen, which is 142 Megajoules per kg of Hydrogen burned.