The influence of different conditions on the sulfur-capture efficiency during fluidized-bed desulfurization was studied using both experimental and modeling methods. The effects of the temperature (∼1120 or ∼1200 K) and gas atmosphere (90% N2 or 90% CO2) were studied using one limestone type. The CO2 atmosphere increased the degree of conversion compared to traditional air combustion conditions using both calcination–sulfation and direct sulfation methods. The scanning electron microscopy–energy-dispersive spectrometry analysis of spent sorbent particles revealed different sulfation patterns in different conditions. The N2 atmosphere produced a network sulfation or core–shell sulfation structure depending upon the temperature. Direct sulfation produced a core–shell structure with a thicker sulfate layer. A uniform pattern was observed for many particles in the CO2 atmosphere using indirect sulfation. The experimental results were analyzed using a time-dependent one-dimensional particle model that can accommodate simultaneous reactions. The model was used to interpret the test results and to determine the magnitude of reactions and diffusion rates as a function of the radius and time. The development of a Thiele number, conversion curve, and conversion profile during the reactions was used to explain the observed results.