NOVELTY - The reactor has a set of primary char gasification chambers (5) in fluid communication with a particle separator (4). A set of secondary char gasification chambers (9) collectively comprises an inlet (9a) for particles from the primary chambers and an outlet (9b) that is in fluid communication with a lower part of a pyrolysis reaction chamber (1) to provide fluidizing gas to the pyrolysis reaction chamber, where the secondary chambers are collectively defined with internal vertical reaction volume of 2% and less than 30% of collective volume of the primary chambers.    USE - Circulating fluidized bed reactor for performing thermal processing of added carbonaceous material e.g. combustible product gas carbonaceous material such as organic material e.g. cereal straw, rice straw and related grain cleaning waste stream, crops residue e.g. sugar cane residue, sorghum residue, beet residue, maize residue, potato residue, nut residue, tea residue, cotton residue, wine residue, olive residue and oil palm residue, algae e.g. sea weed, energy crop e.g. grass such as Miscantus, short rotation forest crop residue e.g. willow residue and poplar residue, crop with elevated content of salt due to growth in proximity with salty water or contact with salty water, meat production industry residue e.g. meat residue and bone meal residue, carbonaceous waste product e.g. household rubbish, dried animal remains or animal manure such as dewatered manure slurry, municipal or industrial organic waste such as organic fractions derived from streams and sewage sludge or dried wastewater sludge, and energy containing residue e.g. fiber or lignin products from wood and raw organic product processing (all claimed), used as fuel.    ADVANTAGE - The reactor allows a bottom surface of the primary chambers with surface elements to collectively slope downward against an outlet of the primary chambers to collect and effectively drain particles, so that fluid connection with the secondary chambers can enhance bed height difference and vertical reaction volume within the secondary chamber can provide possibility for increased retention time of the particles, while facilitating comparatively slow and productive temperature moderation based on endothermic char conversion, thus improving char conversion efficiency and decreasing load of fine recirculating char particles experienced by the particle separator by addition of air and steam in the secondary chambers.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a process for manufacturing product gas with a desirable heating value from carbonaceous material.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a circulating fluidized bed reactor in a relative position of units and conduits through which gas and particles flow.    Pyrolysis reaction chamber (1)    Particle separator (4)    Primary char gasification chambers (5)    Secondary char gasification chambers (9)    Inlet (9a)    Outlet (9b)