Development of CFB Technology to Provide Flexible Air/Oxy Operation for a Power Plant with CCS

Fuel flexibility is one of the most important benefits of circulating fluidized bed (CFB) combustion, but it sets challenges for boiler design to reach high efficiency, low emissions and excellent controllability with different kinds of fuels and their blends. Oxygen-firing in CFB conditions requires additionally a substantial number of process and design variations. Foster Wheeler is developing its CFB boiler technology toward a Flexi-BurnTM CFB application, which features the additional capability to operate in normal air mode as well as oxy-fuel mode with carbon capture and storage (CCS).
VTT Technical Research Centre of Finland has unique small scale (30-100 kW) circulating fluidized bed combustor (CFBC) applicable for both oxygen and air combustion research. The combustor is equipped with O2 and CO2 feed from gas tanks and also with flue gas recirculation for demonstrating real oxygen-firing conditions. In the pilot scale CFBC combustion process and its sub-processes can be studied very effectively at well controlled and measurable conditions. The process knowledge and development of models combined with high quality data are the key elements when improving the design of the CFB boiler.
VTT and Foster Wheeler have a long history of co-operation in research and development of CFB combustion technology, and development of oxygen-firing CFB since 2004. Several projects containing experimental test campaigns combined with development of models and design tools has been carried out. The latest co-operation project in November 2008 was an extensive CFB-pilot test campaign with Polish bituminous coal, which will be also used in the 460 MWe supercritical CFB power plant at Łagisza. Combustion profiles and performance, emission levels and sulfur capture by limestone were measured at different process conditions for air and oxygen combustion modes. A one dimensional design model owned by Foster Wheeler has been validated for the VTT’s pilot scale CFBC, and is used as basis for further development of the model. The developed model based on small scale experimental results can be utilized after validation in designing of commercial scale CFB boilers.
In this paper the results of the CFB-pilot test campaign with Polish bituminous coal (Łagisza’s coal) at air and oxygen combustion conditions are presented. In addition to high concentrations of CO2 and H2O in gas, also differences in emissions formation and capture were obtained. Test balances were calculated with the current version of the one dimensional design model. Modeled gas profiles (O2, CO2 and CO), temperature profiles, and unburned carbon in fly and bottom ashes were compared to measured values. Experiments done at test facilities of different scales provide information about the differences between air and oxygen combustion, and the acquired knowledge is being incorporated in boiler design tools. Preliminary development and design of an integrated Flexi-Burn CFB power plant has already been conducted in both air and oxy-fuel mode, and the continuing work aims at more extensive and detailed knowledge and designs.