Experimental and Modeling Study of Sulfur Capture by Limestone in Selected Conditions of Air-Fired and Oxy-fuel Circulating Fluidized-Bed Boilers

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.