Design Considerations of Oxy-Fuel Fired Supercritical Pressure Circulating Fluidized Bed Boilers

Researchers around the globe are focussing on the capture and storage of carbon dioxide generated from the combustion of coal in boilers for power generation. Oxygen fired boilers have the advantage of creating CO₂ rich flue gas which enables the CO₂ to be captured in a more cost effective manner compared to post combustion capture of CO₂ from an air fired boiler. This paper discusses design considerations for oxy-fuel fired supercritical circulating fluidized bed boilers. A 420 MW_e supercritical CFB boiler firing coal with oxygen has been considered for the study. An analytical/semi-empirical model has been developed to model fuel combustion and heat transfer in the furnace and convection pass. The fuel burns with oxygen supplied at the bottom of the bed and the fluidization velocity and bed temperature is controlled by flue gas recirculation. The model is used to design the boiler and determine its performance characteristics. The heating surfaces, recirculation ratios and other parameters have been designed to achieve the required boiler capacity. The impact of the CO₂ rich flue gas on CFB boiler design is compared with conventional air blown CFB boilers. The study reveals that oxy-fuel combustion in a CFB combustor does not alter the heat transfer characteristics when compared with combustion with air. In the convection bank, oxyfuel combustion increases both convective and radiative components of heat transfer, thereby reducing the size of the heat transfer banks required.