The limestone behavior in an oxy-fuel CFB process was studied by means of modeling. Different modeling methods were combined: the calculation of particle trajectories in a three-dimensional steady state CFB furnace model was utilized to investigate the real process environment which the particles experience and a particle model was used to study the effects of the transient environment on limestone reactions. The studied cases included the air-fired and oxygen-fired combustion and conditions on both sides of the equilibrium curve for calcination, which enabled both indirect and direct sulfation mechanisms. Also the effect of the return leg system was studied; it was shown that depending on the conditions in the return leg, a complex behavior consisting of simultaneous reactions (calcination-carbonation-sulfation-direct sulfation) is conceivable. Moreover, the modeling approach was used to evaluate whether the transient environment has an effect on the limestone behavior compared to the averaged conditions and in which conditions the effect is the largest. The difference between the averaged and transient conditions was notable only in the conditions which were close to the equilibrium curve. (C) 2013 Elsevier Ltd. All rights reserved.