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UBC Theses and Dissertations

NO[sub x] and N2O emissions from circulating fluidized bed combustion Sung, Lawrence Yu Jen


Parametric studies on NO[sub x] and N₂O emissions from circulating fluidized bed combustion were conducted by analyzing NO[sub x] and N₂O emission data from the UBC pilot plant facility. The O₂, NO[sub x] and N₂O concentration profiles in the combustor were also investigated. A comprehensive NO/ N₂O model was developed to predict the NO and N₂O emissions from CFB under various operating conditions and the axial NO and N₂O concentration profiles. The parametric studies were carried out by combusting four different fuels: Conoco coke, Poplar River lignite, CANMET pitch and Mt. Klappan anthracite, with the volatile content ranging from 6.5% (Mt. Klappan anthracite) to 67.7% (CANMET pitch) and the fuel nitrogen content varying from 0.9 wt % (Mt. Klappan anthracite and Poplar River lignite) to 1.9% (Conoco coke). The effects of fuel properties, temperature, excess air (0₂%) and limestone addition on the N[sub x]O and N₂O emissions were investigated with an emphasis on reducing both species simultaneously. Both axial and lateral O₂, NO[sub x] and N₂O concentration gradients existed in a CFBC riser, shedding some light on the formation and reduction mechanism of N[sub x]O and N₂O and indicating that a core-annulus structure existed in a CFB riser. Fifteen NO and N₂O formation and reduction reactions were included in the NO and N₂O model developed, with eight homogeneous and seven heterogeneous reactions. The reaction rate of each reaction was expressed in terms of global kinetics. Global kinetics for the gas phase reactions involved were derived from Hulgaard's (1991) experiments with HCN, NH₃ and N₂O in a plug flow reactor. The NO/ N₂O model, based on a modified Senior and Brereton's model, could predict N[sub x]O and N₂O emissions as functions of operating conditions. The NO/ N₂O model was applied to the Conoco coke combustion in the UBC pilot plant. With three unknown reaction rate constants in the NO and N₂O reaction scheme serving as fitting parameters, the predicted NO and N₂O concentration profiles showed fair agreement with the experimental one. Sensitivity analysis was performed on the reaction rate constants involved in the proposed reaction scheme, showing the relative importance of these reactions in the NO and N₂O reaction scheme. The effects of excess air, limestone addition and the height of secondary air port on the NO and N₂O emissions predicted by the NO/ N₂O model qualitatively agree with what have been observed from the UBC pilot plant. The NO/ N₂O model predicted that there was an optimized P/S ratio for the NO .and N₂O emission control for Conoco coke combustion and that the CFB riser with an abrupt exit would result in lower NO and N₂O emissions.

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