Apparatus for carrying out an essentially closed loop pyrolysis process includes a pyrolysis reactor, a first collection vessel in which the carbon containing solid residue of pyrolysis is separated from the products of the reactor and deposited, a conduit from the first collection vessel to a burner, a second collection vessel in which the particulate source of heat produced by the burner is deposited, and a return conduit from the second collection vessel to the pyrolysis reactor. A higher pressure is established at the outlet of each collection vessel than the pressure at the inlet to which it is coupled. Specifically, each collection vessel comprises a hopper, a standpipe extending downwardly from the hopper to the outlet of the collection vessel, an angle riser through which the particles at the outlet of the vessel are transported by a transport gas, and a vertical riser through which the particles are lifted to the inlet in a less dense state than in the corresponding vessel.