Coke, a material made from heating but not combusting coal, is a common fuel source for use in forges and in blacksmithing. Coke is made in the same way as charcoal, in that it is heated to high temperatures, but starved of oxygen to prevent it from burning. This produces a light, porous rock that burns cleaner than coal, with less tar residue. Coke requires a substantial amount of heat to ignite; this is usually accomplished with a wood or gas fire.
Cut the top and bottom sides off of your metal drum using an angle grinder. Leave about 2 inches of drum attached to one end when you cut it off; one side can be discarded, the other will be used as a cover to seal air in the drum.
Hammer the edge of the cover piece (the end with 2 inches of drum) from the inside outward to create a slight flare to allow it to be placed over top of the remaining drum cylinder.
Cut four squares from the other edge of the cylinder, along its edge, to create four feet that that will allow your cylinder to be stood up, while also being able to access and light a fire whose heat will travel up into the cylinder. These squares should be no more than 12 inches wide and tall.
Place a round grill inside the barrel so that it will sit directly above the fire. Depending on the type of drum you use, there may be indentations that will hold the grill in place; otherwise, strike the drum from the outside in to create indentations that will support the round grill.
Place your coal on the grill with the barrel standing upright on its feet. Place the cut off end with the flar over top of the barrel and secure in place by tapping it snug with your hammer.
Light a small fire directly under the barrel and grill. The heat from the fire will rise and cook the coal, but the lack of airflow through the cylinder will prevent it from igniting. Let the fire burn down after about 30 minutes and collect your coke.
Check with your local authorities regarding the legal status of lighting such fires on your property before doing so.
Always wear appropriate safety equipment when operating power tools and working with metal and fire.