CFB reactor to CFB combustor - The R&D of CFB combustion in China

The development of Circulating fluidized bed (CFB)combustion was based on the CFB chemical reactor. Before CFB boiler, the theory of fluidization, heterogeneous reaction and heat transfer in two phase flow was already well developed in chemical engineering. While, the history of R&D of CFB combustion showed us there were still so much uncertainties in the CFB combustion phenomenon. In present paper, the difference in the circulating system for CFB reactor and CFB combustor is introduced, followed by the concept of mass balance in an open circulation loop of the CFB boiler. Based on the fundamentals of fluidization, the fluidization regimes in CFB boiler is distinguished and the possibility of different fast bed status at upper furnace is analyzed. The further understanding on the combustion in CFB boiler is also introduced with the concept of fractional heat releasing along the height of furnace and the finding about the reduce combustion condition in dense bed zone in CFB furnace. The above new understanding leads to the principle of fluidization status design for CFB boiler. Further, the fluidization status design guide line is given. In the past 30 years, Chinese boiler works first verified the licensed CFB technologies according to the fluidization status guide line, then re-specified the flue gas speed in furnace to reduce the serious erosion on water wall for Chinese coal. By such improvement, Chines boiler works developed series capacity CFB boiler form 75T/h until 600MW supercritical CFB successfully. Then, Chinese researchers suggested re-specify the bed inventory to save draft fan power. The idea was practiced in China and forming advanced energy saving CFB boilers instead of traditional one. The update progress is the R&D on the ultra-low emission CFB process. The idea is further improving the bed quality in CFB boiler in order to form finer particle circulating. According to the observation, the NOx emission from such furnace could be less than 50mg/Nm3. Ca/S ratio was also decreased for same desulfurizing efficiency