Ok What is the advantage of using Sponge Iron vs regular iron ore? I don't see any advantage. Same input vs same output. Maybe I am missing something. Looks to me like it just uses up more coal to get the final output of molten steel. I do see that it allows the use of the open hearth furnace to process iron ore (after use of rotary kiln) but, that does not look like an advantage.

I see the advantage of using Coke vs Charcoal. Coke does not use up my trees.

For each more complex addition to processing an ore should give either a faster or more out of the raw ore or it becomes just a waste of materials.

Going via sponge iron has a few differences, what is an advantage and what a disadvantage depends on how your playthrough is progressing.

Main points:

• It can be produced with raw coal (no charcoal or coke production) or fuel gas (later on).
• Slightly less space and pop demand.
• Until Blast Furnace 2, it is the only route that only uses 2 ore per steel plate (unless you rush recycling for the oxy furnace).
• Sponge iron via fuel gas + AF2 uses slightly less primary energy versus H2 AF2 (which is only a point if you do not consider FBR power to be "free").

But yes, in general sponge iron is probably not the main route (as also indicated by its position in the research tree) and somewhat for flavour and roleplay. And maybe if you want to minmax to the extrem to replace iron scrap in some oxygen furnaces.

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