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Chemical formula of Coal that is available naturally is estimated as 
         C₁₃₅ H₉₆ O₉ N S      and   coal also has some moisture (H₂O).

During the destructive distillation process most of other elements go out and Carbon content increases to  85% to 95%.  Coke has a little moisture absorbed through its pores.

  It can be like   Cx Hy Oz N S   or  Cx Hy Oz with high  value of x and small values of y and z.