NOVELTY - The biogenic feedstock (1) is pyrolyzed, producing a gas of high calorific value. This is then is heated to 1000 degrees C to 1200 degrees C, by sub-stoichiometric combustion with a controlled supply of air. Solid carbon is then added and converted to fuel gas components. The biogenic material is initially carbonized at low temperatures of 350 degrees C to 550 degrees C, in an indirectly-heated rotary reactor (3). Air supplied to the high calorific value gas, burns carbon monoxide and hydrocarbons exothermically (4), to reach the higher temperatures.    USE - To produce synthesis gas from diverse biogenic solids.    ADVANTAGE - Earlier plants gasified wood or sewage sludge. The new plant is less restricted in its feedstock, examples being straw, waste and other products from which synthesis gas could not previously be recovered.    DETAILED DESCRIPTION - The gas at 1000 degrees C to 1200 degrees C, contains long-chain hydrocarbons which are broken down into short-chain compounds. This forms a fuel gas suitable for motive power applications, e.g. for burning in a gas engine. The fuel gas is then brought into contact with solid carbon in a post-gasification reactor (5). An endothermic heterogenic water-gas reaction ensues, at about 800 degrees C. This converts the solid carbon into gaseous carbon monoxide and hydrogen, i.e. a combustible synthesis gas (2). This gas is cooled in stages and purified by dust removal. The reactor (5) takes one of various alternative forms, as follows. It is a cyclone which swirls the solid carbon particles in the gas. It is a rotating tube keeping the particles in motion against its inner wall. It is a fluidized bed suspending the particles above a gas-permeable grate. A gas scrubber and/or a fine particle filter, follow the post-gasification reactor. A synthesis gas burner follows the reactor, to recover thermal energy. AN INDEPENDENT CLAIM IS INCLUDED FOR corresponding plant.    DESCRIPTION OF DRAWING(S) - The drawing shows a block schematic diagram of key stages in the process.(Drawing includes non-English language text)    biogenic feedstock (1)    combustible synthesis gas (2)    indirectly-heated rotary reactor (3)    exothermic partial-oxidation stage (4)    post-gasification reactor (5)