why is coke put into blast furnaces along with the iron ores?

why is coke put into blast furnaces along with the iron ores?i understand that limestone is put in for the removal of impurities but i dont understand why coke ismixing with oxygen it produces CO2.but this is bad for the environment?????please help! gt;gt;gt;gt;gt;gt;gt;gt;10 pts to best answer!!!!!!!!!!!!!!lt;lt;lt;lt;lt;lt;lt;lt;lt;lt;lt;lt;lt;lt;lt;