A method for transforming selected plant or plant-derived materials, and optionally selected waste plastics, into a plurality of phenolic reaction products having a lower sulphur content than the original feedstock, via supercritical water is disclosed. The method comprises: conveying the selected plant or plant-derived materials, and optionally waste plastic material, through an extruder, wherein the extruder is configured to continuously convey the selected feedstock to a supercritical fluid reaction zone; injecting hot compressed water into the supercritical fluid reaction zone, while the extruder is conveying the selected plant and/or plant-derived mixture and optionally waste plastic material into the supercritical fluid reaction zone so as to yield a water-containing mixture; retaining the mixture within the reaction zone for a period of time sufficient to yield the plurality of phenolic reaction products having a lower sulphur content than the original feedstock. The reaction zone may be characterized by a tubular reactor having an adjustably positionable inner tubular spear, wherein the tubular reactor and the inner tubular spear further define an annular space within the reaction zone, and wherein the mixture flows through the annular space and into a reaction products chamber for separation into three phases.