Abstract: A fuel gas generator for lean gas generation by gasification of organic, inorganic, or fossil fuel substances. The triple shell structure of the generator includes a frame casing, a shaft jacket, and a reactor shaft. A fuel feeder including a fuel feed container is located substantially at the top of the reactor shaft and has a gas-tight entry lock. Reaction gas feed and ash discharge are located substantially at the base of the reactor shaft. Preheating, degassing, oxidation, and reduction zones and lean gas take-off orifices having at least one lean gas removal pipe connected thereto are arranged one after another in the shaft. The reactor shaft is gas-tight except for the lean gas take-off orifices. A firebox defined by a conical constriction is located in a middle region of the reactor shaft substantially below the preheating and degassing zones.