A downflow, gravity type gasifier includes a vertically oriented chamber having a loading zone at the top, followed by a drying zone, followed by a pyrolytic zone for pyrolytic decomposition, followed by a cracking zone for cracking the tars, followed by a reduction zone and an oxidation zone wherein the gas is generated and the combustible material is burned. The cracking zone has a divider, e.g. a cruciform divider, in it, and each of the resulting sections of the cracking zone is individually supplied with air for more uniform distribution of air and more uniform cracking than heretofore available. In addition to the air supplied to the periphery at the cracking zone, air outlets are arranged in respective radial arrays of the outlets for more uniform distribution of air into the reduction and oxidation zone. Steam is generated in the cooling water jacket surrounding the oxidation zone and that steam is delivered into and passed through the interior of a cruciform-shaped radiator disposed in the drying zone for heating the drying zone. The air from the arrays of air outlets creates an additional oxidation zone performing additional oxidation, so as to deliver substantially tar-free gas from the gas outlet of the gasifier.