A fluidized-bed reactor system with at least two fluidized-bed reactors, each being a circulating fluidized bed, a particle line with a particle separator for transferring fluidized-bed particles from the first to the second reactor, and a particle line exiting at the lower half of the second reactor for transferring fluidized-bed particles back to the first reactor, wherein, at least in the second reactor, two or more reaction zones separated by one or more flow controllers and that the particle line opens into the second reactor above at least one flow controller.