The trouble with forging with coal is finding it and affording it. A proper working fire used in earnest will eat up 20 lbs of coal in six hour session. I will let you do your own math , for me, coal cost is a is inhibiting to doing business.
I know of two commercial ( shippable ) quality coal sources 1, Centuar Forge - good medium quality coal, very consistent and reasonable cost ( even shipped ). 2, Kayne & Son, really high quality coal but very expensive.
My newest source is Morrel Metalsmiths in Colrain, MA . Very affordable ! We are visiting family in New Hampshire ( two hours away ) and drove over to get some. I will try it when I get home, they beat all other prices by 50% ! I hope it is good stuff.