NOVELTY - The apparatus comprises a reactor vessel having a top, an opposing base, a cylindrical side wall (18)between them, a hood (78) and an opening in the hood for venting flue gas, a heat source disposed in a radiant heat zone (72) adjacent to the base (54) for creating convective and radiant heat zones, a helical reactor coil (42) disposed vertically in the reactor vessel traversing the heat zones, and a support means disposed inside the reactor vessel, where the support means comprises floating frame and clamps securing the coil to the frame such that the coil is slidably engaged with the frame.    USE - The apparatus is useful for producing synthesis gas from cellulosic biomass materials by pyrolysis (claimed), where the biomass includes granulated wood, rice hulls or chopped cane and the gas obtained is rich in hydrogen and carbon-containing components such as carbon monoxide, carbon dioxide and methane that are converted to fuel such as methanol or ethanol and used as fuel gas for industrial power plants.    ADVANTAGE - The apparatus ensures easy, efficient and economical production of synthesis gas with improved quality.    DETAILED DESCRIPTION - The apparatus comprises a reactor vessel having a top, an opposing base, a cylindrical side wall (18) between them, a hood (78) affixed to the cylindrical side wall adjacent to the top of the reactor vessel, and an opening in the hood for venting flue gases, where the vessel contains the heat required to chemically transform a biomass feed to synthesis gas and the reactor vessel has a convective heat zone in an upper portion of the vessel and a radiant heat zone (72) disposed in a lower portion of the vessel, a heat source disposed in the radiant heat zone adjacent to the base (54) for creating the convective and radiant heat zones, a helical reactor coil (42) disposed vertically in the reactor vessel traversing the convective heat zone and the radiant heat zone, where the reactor coil in spaced relation to the side wall has an input end for receiving biomass feed and an opposing output end for discharging synthesis gas, the input end of the reactor coil is disposed in the convective heat zone and the output end of the reactor coil is disposed in the radiant heat zone, and a support means disposed substantially inside the reactor vessel, where the support means comprises a floating frame and clamps securing the coil to the frame such that the coil is slidably engaged with the frame, and the expansion and contraction of the coil upon heating and cooling is accommodated. The reactor coil comprises a first portion having a first internal diameter disposed generally in the convective heat zone and a second portion having a second internal diameter disposed generally in the radiant heat zone, where the second internal diameter is greater than the first internal diameter. The second portion of the reactor coil is disposed between the first portion of the reactor coil and the base of the reactor vessel. The reactor coil comprises individual interconnected loops forming a helix and the number of individual loops of the first portion of the reactor coil is greater than the number of individual loops of the second portion of the reactor coil. The apparatus further comprises a heat shield disposed generally axially within an upper portion of the reactor coil above the radiant heat zone. The heat shield reflects heat from the heat source back into the convective heat zone thus providing additional heat to the reactor coil. The heat shield has a substantially cylindrical upper portion and a bottom portion exhibiting a truncated cone shape with a closed base, the bottom portion has a varying diameter that progressively increases as the distance to the radiant heat zone increases thus maximizing the transfer of convective heat to the reactor coil as the flue gases rise through the reactor vessel. The apparatus further comprises a pressurized mixing vessel external to the reactor vessel for fluidizing the biomass with a transport gas having a feed inlet, a transport gas inlet, and a biomass feed/transport gas mixture outlet, which is fluidly interconnected with the input end of the reactor coil. The mixing vessel has a conical bottom portion to enhance the mixing of the biomass and the transport gas. The transport gas inlet is tangentially affixed to a side wall of the conical bottom portion, where the transport gas causes a swirling motion about the side wall as it enters the mixing vessel effectively fluidizing the biomass feed in the transport gas.    DESCRIPTION OF DRAWING(S) - The diagram shows a front elevation view, partially in section, of a mixing vessel and biomass gasifier.    Sidewall (18)    Reactor coil (42)    Base (54)    Radiant heat zone (72)    Hood. (78)