Issues in deep peak regulation for circulating fluidized bed combustion: Variation of NO_x emissions with boiler load

In order to improve the consumption of renewable energies, certain amounts of circulating fluidized bed (CFB) boiler units should assume the task of deep peak regulation. However, the effect of CFB boiler load on pollutant emissions such as NO_x still needs to be clarified. In this paper, the NO_x emission characteristics of two industrial-scale CFB boilers within a wide load range (35%-100%) were further analyzed by using a comprehensive one-dimensional, two-phase CFB mathematical model. Simulation results reveal that, when the load ratio decreases, the NO_x emission decreases first and then increases. The non-monotonic variation trend is also confirmed by the operational data collected from the SC-350 boiler. However, for different boilers, the load ratio corresponding to the turning point of NO_x emission may be different, e.g., for the 135 MW_e super high steam pressure boiler, it is about 40%, while for the 350 MW_e supercritical boiler is 50%. On the one hand, the decrease in boiler load leads to a decline in the furnace temperature, which contributes to reducing NO_x emission due to the decrease of volatile yields, the lower conversion rate of Vol-N to NO_x, and the enhancement of the overall NO reduction on chars. On the other hand, at low loads, the excess air coefficient is generally set to high values, and air staging is weakened, resulting in adverse effects on the NO_x emission control. In addition, when the CFB boiler operates at low loads, the solid circulation loop performance usually worsens, and the heat loss caused by incomplete combustion may increase. This study points out that high NO_x emission is an unavoidable issue in the process of deep peak regulation for CFB boilers.