How is coal used in steel production, and what happens to the carbon?
1. The coal is first converted to coke, a porous material made up of the fixed carbon from the coal, and the ash. Coking involves heating coal in a controlled oxygen environment where the volatile components of coal are driven off, not burned off, as water, coal gas, and coal tar. Two of these are hydrocarbon products with economic value, though sometimes they are used as a fuel within the coking process itself. Coal which is used for coking contains volatile components which comprise 26-29% of the coal. Of these volatile by-products, coal gas and coal tar, most end up being burned somewhere. (Coal tar was once used for making tar and asphalt, but has been all but replaced by by-products of petroleum distillation.) At this stage, some of the carbon in the original coal is now in by-products, but most of it is in the coke. A very small proportion of the original coal will be burned to maintain the heat necessary for coking - and all of that carbon will be emitted to the atmosphere as carbon dioxide.
2. Iron ore is smelted down to pig iron using the coke from step 1.
Into a continous hot mixture of iron ore, coke, and limestone (in approximate proportions of 4:2:1) oxygen or air is introduced. The carbon in the coke provides the combustible fuel which is kept at a temperature of about 1600 degrees Celsius. These high temperatures are necessary for smelting and steel making, and coke burns at more than 1000°C higher than coal. Most of the carbon from the combustion of coke in smelting iron ore is emitted as carbon dioxide, though the pig iron itself ends up with up to 5% carbon content. 3. Pig iron, and recycled steel are then further heated in a blast furnace, with some of the carbon from the pig iron, and some additional coke used as fuel. Again, virtually all of the carbon ends up as carbon dioxide, even in the production of "carbon steel".
Virtually all of the carbon in coal used for steel production ends up in the atmosphere, as carbon dioxide.