Abstract: Coal is transported through a hot carbonizing furnace chamber successfully on two endless traveling chain grates, the first of which is higher than the second and the second being run at a slower speed than the first so as to form a thicker bed than the coal on the first grate. Air is fed through the coal on the first grate in sub-stoichiometric amounts at low velocity so as to drive off the volatiles while air and steam are fed to the coal in the second grate in amounts sufficient to burn the coal. Gaseous by-products are exhausted from the furnace chamber and ashes are discharged through a conventional take-off.