To demonstrate process feasibility of in situ CO 2 capture from combustion of fossil fuels using Ca-based sorbent looping technology, a flexible atmospheric dual fluidized bed combustion system has been constructed. Both reactors have an ID of 100 mm and can be operated at up to 1000 °C at atmospheric pressure. This paper presents preliminary results for a variety of operating conditions, including sorbent looping rate, flue gas stream volume, CaO/CO 2 ratio and combustion mode for supplying heat to the sorbent regenerator, including oxy-fuel combustion of biomass and coal with flue gas recirculation to achieve high-concentration CO 2 in the off-gas. It is the authors' belief that this study is the first demonstration of this technology using a pilot-scale dual fluidized bed system, with continuous sorbent looping for in situ CO 2 capture, albeit at atmospheric pressure. A multi-cycle test was conducted and a high CO 2 capture efficiency (> 90%) was achieved for the first several cycles, which decreased to a still acceptable level (> 75%) even after more than 25 cycles. The cyclic sorbent was sampled on-line and showed general agreement with the features observed using a lab-scale thermogravimetric analysis (TGA) apparatus. CO 2 capture efficiency decreased with increasing number of sorbent looping cycles as expected, and sorbent attrition was found to be another significant factor to be limiting sorbent performance.