An apparatus is provided for treating a loose fibrous solid organic feedstock. The apparatus includes a reactor vessel and a compressor operable to introduce feedstock at elevated pressure into the reactor chamber. The apparatus includes means for controlling and modulating moisture level within the reactor. The apparatus includes an outflow or discharge assembly that has its own feeder or drive for packing feedstock into a discharge accumulator. The discharge accumulator feeds an outflow valve, in this case a uni-directional rotation ball valve operated by a servo motor. Discharge from the valve may be carried through a diffuser into a settling cyclone. In operation the ball valve is rotated to open and close the outflow passageway, the rate of rotation being variable in real time. The rate of rotation may be a function of the pressure or other parameters sensed in the reaction vessel, and, particularly, in the accumulator immediately upstream of the ball valve, and of proxy sensed values such as discharge feeder motor current.