is designed a rotating arm 3 makes it possible to equalize the biomass over the available surface fig2 is a simplified view of the reactor 14 of fig1 and diagrammatically indicates the approximate positions of the three reaction areas namely the pyrolysis area 16 the oxidation area 17 and the reduction area 18 and the drying area 25 in the reactor 14 according to the invention the oxidation area 17 is located approximately at the height of the reactive gas inlet 4 the pyrolysis area 16 is located higher above the air diffuser cone 13 and the reduction area 18 lower above the reactive gas lower inlet which is optional 6 the ash passes through a grid 8 located in the lower portion 23 of the reactor body and accumulates below the said grid 8 it is periodically or continuously eliminated by a rotating arm 9 typically a double arm through an ash evacuation duct 10 such as is shown diagrammatically in fig3 the main gasification agent reactive gas inlet 4 is located at mid height of the bed 15 it is comprised of a set of ducts or pipes that are terminated with means for injecting 19 29 which are typically injection buses distributed over the periphery of the body 22 of the reactor 14 and supplied by the ducts 4 in the gasifier 14 according to the invention another reactive gas inlet is provided in the upper portion of the bed 15 by the upper duct 5 terminating by a cone for diffusing reactive gas 13 the cone for diffusing 13 allows for a better supply with reactive gas of the oxidation area 17 and an increase.