Postcombustion and Its Influences in 135 MWe CFB Boilers

In the cyclone of a circulating fluidized bed (CFB) boiler, a noticeable increment offlue gas temperature, caused by combustion of combustible gas and unburnt carbon content, is often found. Such phenomenon is defined as postcombustion, and it could introduce overheating of reheated and superheated steam and extra heat loss of exhaust flue gas. In this paper, mathematical modeling and field measurements on postcombustion in 135MWe commercial CFB boilers were conducted. A novel one-dimensional combustion model taking postcombustion into account was developed. With this model, the overall combustion performance, including size distribution of various ashes, temperature profile, and carbon content profiles along the furnace height, heat release fraction in the cyclone and furnace were predicted. Field measurements were conducted by sampling gas and solid at different positions in the boiler under different loads. The measured data and corresponding model-calculated results were compared. Both prediction and field measurements showed postcombustion introduced a temperature increment of flue gas in the cyclone of the 135MWe CFB boiler in the range of20-50 °C when a low-volatile bituminous coal was fired. Although it had little influence on ash size distribution, postcombustion had a remarkable influence on the carbon content profile and temperature profile in the furnace. Moreover, it introduced about 4-7% heat release in the cyclone over the total heat release in the boiler. This fraction slightly increased with total air flow rate and boiler load. Model calculations were also conducted on other two 135MWe CFB boilers burning lignite and anthracite coal, respectively. The results confirmed that postcombustion was sensitive to coal type and became more severe as the volatile content of the coal decreased.