This is a fairly simple idea to give some depth to the game. I think that each fuel should burn at a different temperature. Fuel temperature would determine 2 things.
1: What percentage of the furnace time to use when smelting something. So a really hot fuel will melt gold a lot quicker than a cooler fuel.
2: Whether or not a certain fuel can actually melt something. So wood would not be able to melt Titanium.
This would add just a tiny bit of depth to crafting, And I think that bit of depth will improve things. Especially if it was made so that items with a furnace recipe could have a maximum fuel temperature for things like food. This way you can't just cook food with uranium, You'd have to cook it with wood or charcoal (Maybe add charcoal too?)
Currently, Uranium is too expensive to justify using it as a fuel source, And wood burns too fast to justify using it as a fuel source. This would give a reason to use these fuels if you needed to worry about which was the right temperature to smelt the thing you want to smelt.