Prediction and minimization of NO_x emission in a circulating fluidized bed combustor: A comprehensive mathematical model for CFB combustion

A comprehensive 1-dimensional/1.5-dimensional hybrid mathematical model is developed for predicting NO_x emission of a circulating fluidized bed (CFB) combustor under broader operating parameters. In this model, the local gas–solid fluidization state and gas/heat transfer conditions in different regions of a CFB combustor are specifically considered. Some two- or three-dimensional problems, such as bubble breakage over dense bed surface, secondary air injection, core-annular flow structure, and particle clusters in freeboard, are also taken into account in 1-D/1.5-D modeling. The detailed chemical kinetic mechanism is creatively used to describe the homogeneous reaction system towards CFB combustion simulation. In addition to operating parameters and fuel-specific inputs, no other model parameters can be trimmed from case to case. This integral CFB model is validated against the field test data obtained from three commercial CFB boilers with different capacities, some of which are first disclosed. Favorable comparisons are obtained between the predicted and measured results, involving particle size distributions, temperature and pressure profiles, and NO_x/SO₂ emissions. The final NO emission, as well as gas profiles, are somewhat different among the cases, which may be attributed to the discrepancy in boiler structure, fuel properties, and operating conditions. Further sensitivity analysis indicates that the proportion of volatile-N in total fuel-N, char combustion reactivity, and char-NO reactivity significantly impact the NO_x emission for CFB combustion. Meanwhile, the gas–solid fluidization state also plays an essential role in the NO_x emission and the in-furnace combustion efficiency, such as the gas flow distribution between phases, bubble size, secondary air penetration depth, etc. However, the NO_x emission seems insensitive to the particle external gas mass transfer coefficients.