Apparatus for producing synthesis gas from a biomass feed in a closed, helical coil reactor utilizing a heat source such as a natural gas burner. The reactor includes the helical coil disposed in a reactor vessel with the heat source positioned toward a bottom of the reactor vessel. A heat shield is preferably disposed within at least an upper section of the helical coil, and the coil is placed adjacent to, but spaced from, a sidewall of the vessel so that convective heat may flow upwardly and around the individual coils of the helical coil. A lower section of the helical coil is preferably exposed to direct heat from the heat source. This lower section may exhibit a coil diameter that is greater than a coil diameter of the upper section. The biomass feed may be supplied to the helical coil from a mixing chamber including a spiraling screw valve.