A pilot-scale dual fluidized bed combustion system was used for CO2. capture using limestone sorbent with CaO-CaCO3 looping. The sorbent was regenerated at high temperature using an air- or oxygen-fired fluidized bed calciner with flue gas recycle firing hardwood pellets, Two limestones were evaluated for CaO-CaCo3 looping. Changes in the sorbent morphology during the tests were identified by scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy(EDX). Changes in pore size distribution and sorbent surface area that occurred during reaction were determined by N-2 BET porosimetry. Thermogravimetric analysis (TGA) was used to determine the activity of the sorbent after processing in the dual fluidized bed combustion system. It was found that oxygen-fired calcination with high CO2 partial pressure reduced the effectiveness of the two limestone sorbents for CO2 capture when compared to material calcined under oxygen-enhanced air combustion. A shell 1-2 mu m thick, with reduced porosity, was formed around the sorbent (particle) and is believed to be responsible for reduced conversion of CaO to CaCO3. It is believed that ash deposition contributes to the formation of the shell.