What is the nature of coke during the extraction of metals?

Coke is a byproduct of coal that is primarily used as a reducing agent in the extraction of metals. (like iron, zinc etc.) 

For example- ZnO is reduced to Zn after heating with carbon(coke). Coke is used as a reducing agent in this chemical equation.

$ZnO + C \left(coke\right) \stackrel{∆}{\to } Zn + CO$