The conditions for CO2 reduction in a circulating fluidized bed (CFB) oxy-boiler are studied, that is, operation with pure oxygen, diluted by recirculated flue gases to moderate the combustion process. Two cases are analyzed: the ready-to-convert case, a normal air-fired CFB boiler, only slightly modified to be operated with oxygen instead of air for CO2 capture, and a more general option, an entirely new design, employing high oxygen concentration in the input to the oxy-fuel CFB boiler. It is found that at a given fuel load, the relevant parameters for maintaining the CFB performance (bed temperature and fluidization velocity) in the ready-to-convert case cannot be kept entirely equal to those in the air-fired case, and some compromise has to be found. The new-design case results in a smaller boiler than that of the comparable air-fired case, depending on the oxygen concentration and the corresponding flue-gas recirculation. This case is expected to contribute favorably to reduction of the cost of CO2 removal. (C) 2014 Elsevier Ltd. All rights reserved.