NO_x Formation and Reduction With NH_3 in Fluidized Bed Combustion - The Influence of the O_2 Concentration on the Kinetics

摘要

In (circulating) fluidized bed combustors, NO_x and N_2O emissions originate mainly from nitrogen in the fuel. The formation of NO_x and reduction to N_2 follow complicated reaction paths involving both homogeneous and heterogeneous reactions. In these reaction paths oxygen plays an important role. When air staging is used in a (circulating) fluidized bed combustor, the O_2 concentration in the combustor varies over a broad range. Limestone, used as a sorbent for sulfur retention, is a good catalyst for the formation of NO_x. In this study the influence of the O_2 concentration is investigated on homogenous reactions and on reactions catalysed by limestone. In a fixed bed reactor the NO formation and reduction reactions are studied at 850°C. With a kinetic model, rate expressions for the NO formation and reduction reactions are obtained from experimental data. The results show, that the O_2 concentration has an influence on the homogenous formation and reduction of NO, which differs from most literature that assumes a zero dependency. For the reactions catalysed by CaO, no influence of the O_2 concentration was found. Rate expressions for the homogeneous reduction and oxidation reactions that are O_2 dependent, give a good description of the O_2 influence on the homogenous NO formation and reduction. The rate of the reduction reaction was similar to the one reported by Jensen et al. (1995). The rate of the oxidation reaction is found to be 50 percent lower. For the heterogenous oxidations, O_2 independent rate expressions are obtained that give a reasonably good description of the NO formation. The O_2 dependency found in the heterogenous experiments is caused by the homogeneous reactions that are O_2 dependent. The rate constants of the heterogenous reaction found in this work are significantly higher than the ones found by Johnsson (1990). With the model developed in this work, it is possible t o predict the influence of O_2 concentration on the emissions of NO and NH_3.