An apparatus is provided for compressing a loose solid feedstock. The apparatus includes a two stage compressor. The first compression stage is a screw compressor. The second compressor stage is a reciprocating compressor. The reciprocating compressor operates co-axially with, and receives its feed from, the screw compressor. The compressor establishes feedstock plug in a discharge pipe. A choke cone maintains pressure in the plug. The choke cone is actively controlled in response to operation of the compressor. The discharge from the compressor has a cooling jacket, and includes an expanding taper. The reciprocating compressor, the screw compressor, and the choke cone are all adjustable in real time to control the compression of the feedstock according to a pre-programmed schedule that need not have equal compression and retraction strokes. The operation of the screw compressor may be advanced or eased off depending on the motion of the reciprocating compressor.