A downdraft gasifier (1) has an oxidant inlet (3), a biomass injector (2), a grate (9), a gas exit port (7), and an ash removal system (11). A sensor (10) maintains the height of the bed and a rotating paddle (5) maintains the top of the bed (4) at an even height. The grate arrangement (9) is preferably a sliding grate arrangement which actively moves ash material through the grate. An in-bed oxidant distributor (6) injects oxidant within the bed.