[holowoodman]

Well, actually, thermolysis for water occurs at 2200°C. Thermolysis of CO₂ starts at 1400°C, of CO at 3700°C. The melting point of iron is around 1500°C, similarly its oxides.

So water as a product is actually more stable than CO₂, and doesn't undergo thermolysis at the relevant temperatures for smelting iron. Whereas when going the CO₂ route, there is the risk of producing relevant amounts of CO, which is not as desirable and less efficient because it only absorbs half the oxygen.

Cost is a big question, but it will for sure be more expensive to use hydrogen. Back of the envelop calculation (250$/t coal price, need 1/3t of H_2 for the same effect, so H₂ may cost up to 750$/t, need 40kWh/kg for H₂ electrolysis at 100% efficiency) gives a breakeven electricity price of 1.875ct/kWh. While this happens from time to time due to overproduction, those prices will even out as soon as there is a market for that excess electricity through batteries, storage and electrolysis. Which means that cost-wise, the H₂ route will never be more effective than coal. To make it viable, coal use needs to be made more expensive through taxes and tariffs.

[Borg3]

Can you provide some citation about CO2 themolysis? I found just one paper from China....

[holowoodman]

That stuff is ages old, I doubt you will find current papers on it. Pick a chem textbook or table book, you should find it somewhere in there.