how is coke formed?

Coke is formed by distructive distillation of coal. It is the process of heating coal in the absence of oxygen to obtain useful products. The immediate (product) black residue collected is coke along with this hot vapours which produces coal gas, liquor ammonia and coal tar on further destructive distillation process.
The process of heating coal in the absence of oxygen to obtain useful products is called destructive distillation of coal.