Nitrogen chemistry in FBC with limestone addition

Abstract

A laboratory study of NO and N2O formation from limestone-catalyzed HCN oxidation under fluidizedbed combustion (FBC) conditions is reported. The experiments include the influence of NO and O2 on HCN oxidation over calcined limestone, formation of CaCN2 by reaction between HCN and calcined limestone, and oxidation of CaCN2 and simultaneous HCN oxidation and sulfation of the limestone in the presence of NO. For oxidation of HCN over calcined limestone at O2 concentrations of about 5000 ppmv or more, NO is formed with a selectivity of 50–60%. At a temperature of 1025 K, the selectivity for N2O formation exhibits a maximum at an O2 concentration of 1000–2000 ppmv. The presence of NO in the inlet gas enhances the formation of N2O. The selectivity for NO and N2O decreases and increases, respectively, with increasing degree of sulfation of the limestone. The results from the present work and from the literature are used to discuss how limestone addition influences the nitrogen chemistry in FBC. The results indicate that (1) the NO emission decreases for low volatile fuels such as pet coke because NO is reduced by CO over the calcinel limestone; and (2) for medium- to high-volatile fuels, the NO and N2O emission increases and decreases, respectively, because calcined limestone catalyzes the oxidation of volatile nitrogen (HCN and NH3) to NO and the decomposition of N2O. For large CFBCs, limestone has only a small effect on N2O. This is partly because a major part of the N2O emitted is formed in the upper part of the riser where the solids concentration is low and little N2O is destroyed heterogeneously.