Melting furnaces should have a recipe that allows for the use of coke—either to improve process efficiency or to increase production output. For Level 2 melting furnaces, this could also be a mandatory requirement.
Producing coke would require a new facility—a coking plant ( https://en.wikipedia.org/wiki/Coking_factory)—in which raw coal (and possibly water) is processed to create coke.
As the wikipedia article suggest, it could also be possible to make the coking plant produce other product like natural gas, ammonia and other.