The role of external heat exchangers in oxy-fuel circulating fluidized bed

Recently, oxy-fuel combustion has received increasing attention as one of the short-term solutions for capturing CO2 from power plants. Despite their quick development, there are several issues that need further research. Combustion characteristics, heat transfer, emissions levels, optimum oxygen concentration and flue gas recycle are the main concern of the literature. All these issues may introduce several changes in power plant and boiler configuration that causes uncertainty in the final design of future installations. In this respect, oxy-fuel combustion in fluidized bed seems to present advantages in terms of flexibility, operational adaptability and simplicity of design modifications respect to air combustion. These issues have been though scarcely analysed in open literature. Oxy-fuel combustion in fluidized beds would require the use of additional heat transfer surfaces in order to control the bed temperature and the pollutant formation, minimising the CO emission and taking advantage of the fuel energy in the steam cycle. Moreover, the adaptability to different operational conditions (flue gas recycle, oxygen concentration, boiler load, etc.) inherent to fluidized bed characteristic would require an accurate location of heat transfer surfaces. To achieve this aim, this paper describes and compares the simulation results of a circulating fluidized bed boiler with an external heat exchanger, working under air and oxy-firing conditions. Calculation of fluidization conditions and comparison with different oxy-fuel combustion characteristics are presented. Results show the suitable location of heat transfer equipment in circulating fluidized bed to obtain similar temperature profiles along the boiler. A broad range of conditions have been run for different boiler designs, and several operational limitations have been set for selected boiler geometries. (c) 2012 Elsevier Ltd. All rights reserved.