Nitrous oxide formation and destruction in fluidized bed combustion. A literature review of kinetics

摘要

Recent literature on the subject of nitrous oxide emission has been reviewed with emphasis on the kinetics necessasry to model the N(sub 2)O formation and destruction at conditions relevant for fluidized bed combustion. An increase in combustion temperature, air staging and increased volatile content in the fuel decreased the emission of N(sub 2)O, NH(sub 3) injection and increased excess air has the opposite effect. The addition of limestone was found to either decrease the N(sub 2)O emission or have no influence. The conclusion of this work is that the disagreement may be due to different catalytic activity for different types of limestone for the catalytic decomposition of N(sub 2)O. The formation and destruction of N(sub 2)O by homogeneous gas phase reactions was investigated experimentally and by modelling using comprehensive chemical kinetic models. The conclusion is that HNC is an important precursour for the N(sub 2)O formation. The decomposition of N(sub 2)O via reactions with H and OH is fast. The most important routes for N(sub 2)O formation by heterogeneous reactions are oxidation of char-nitrogen, catalytic reduction of NO and catalytic oxidation of NH(sub 3). The fraction of char-N conversion to N(sub 2)O has been found to vary between 1 and 10% during char combustion. Calcined limestone and char are the most important catalysts for heterogeneous decomposition of N(sub 2)O, but the activity depends on the type of limestone and char. For both materials there are two orders of magnetude between the rate constants for the most and the least active types. A reaction scheme is set up. A simplified calculation example for N(sub 2)O decompostion in a circulating fluidized bed combustor indicates that both homogeneous and heterogeneous reactions may be important. (AB) (36 refs.).