Abstract: A cyclone separator for a solids-laden process gas from a reactor also provides a pressure seal for the reactor. The cyclone includes a main housing with a longitudinal axis. The housing is made of a membrane wall construction having a plurality of tubes arranged around the axis and encased within membrane wall panels. A portion of the tubes are bent outwardly to form an inlet which communicates with the main housing for receiving the solids-laden process gas from the boiler. Solids are separated from the solids-laden process gas as they swirl together in the main housing of the cyclone separator. A partition wall is disposed at the lower section of the main housing around the longitudinal axis for defining an outer chamber and an inner chamber adjacent to the outer chamber. Gas is provided to the outer and inner chambers for creating fluidized beds of the solids in the outer and inner chambers. Solids are passed from the inner chamber to the outer chamber through an underflow port in the partition wall.