Apparatus for cleaning high temperature, high pressure gases includes a pressure vessel, and a filtration unit within the pressure vessel. The filtration unit is adapted to communicate with a fluidized bed reactor gas outlet, and includes at least one dirty gas chamber housing a plurality of porous, ceramic filter tubes, and at least one clean gas chamber for receiving clean gas from the ceramic filter tubes. The dirty gas chamber includes a particle outlet for removing particles separated from the gas by the filter tubes. The porous ceramic filter tubes are horizontally oriented within the filtration unit or housing and mounted within water cooled walls of the housing. The fluidized bed reactor and associated cyclone separator may be located within the pressure vessel with the filtration unit, or located outside the pressure vessel and in communication with the filtration unit inside the pressure vessel.