Abstract: The machinery and methods disclosed herein are based on the use of a specialized extruder configured to continuously convey and plasticize/moltenize selected lignocellulosic biomass and/or waste plastic materials into a novel variable volume tubular reactor, wherein the plasticized/moltenized material undergoes reaction with circumferentially injected supercritical water—thereby yielding valuable simple sugar solutions and/or liquid hydrocarbon mixtures (e.g., “neodiesel”), both of which are key chemical commodity products. The reaction time may be adjusted by changing the reactor volume. The machinery includes four zones: (1) a feedstock conveyance and plasticization/moltenization zone; (2) a steam generation and manifold distribution zone; (3) a central supercritical water reaction zone; and (4) a pressure let-down and reaction product separation zone.