The effect of limestone feed during bubbling fluidized bed combustion of coal on emissions of SO 2 , NO x and N 2 O was investigated. Two types of bubbling fluidized bed combustors (BFBCs) were employed; One was a conventional single-stage BFBC in which limestone was fed to the bed where coal was burnt, and the other was a two-stage BFBC in which coal was burnt in the lower bed and the desulfurization of the flue gas from the lower bed was conducted in the upper bed. For the single-stage BFBC, limestone feed into the bed decreased SO 2 and N 2 O emissions but it increased NO x emission. The two-stage BFBC was found to decrease SO 2 and N 2 O emissions without increasing NO x emission. The results of SO 2 capture and N 2 O reduction in the upper bed of two-stage BFBC were analyzed using Kunii-Levenspiel model. The results of the calculation agreed fairly well with the experimental results.
The effect of water vapor concentration on the reaction rates of NH3 oxidation and N2O
decomposition catalyzed by calcined limestone was investigated by use of a fixed bed reactor
under atmospheric fluidized bed combustion conditions. Both reactions were inhibited by the
presence of water vapor. Inhibition of NH3 oxidation by H2O could be expressed as a decrease in
the number of available active sites according to Langmuir-type adsorption of H2O on one kind
of active site. Inhibition of N2O decomposition by H2O could be approximated by assuming two
kinds of active sites, both of which adsorb H2O according to Langmuir-type adsorption equilibrium,
but the adsorption equilibrium constant was different between two types of active sites. The
equilibrium constant of H2O adsorption on active sites for NH3 oxidation was found to be different
from those of H2O adsorption on active sites for N2O decomposition.
In this work, decomposition of NH3 to N2 over limestone was investigated as a mean to reduce NO, emission from fluidized bed combustors. A fxed bed study was conducted and the effect of C02 and H2O on the products of NH3 decomposition over limestone was studied. In the absence of C02 and HzO, NH3 was decomposed to N2. From a NH3-CO2 mixture, (NH2)zCO was formed through NH3 decomposition over both calcined limestone at a COz concentration of 15% and uncalcined limestone at a C02 concentration of 75%. However, from the NH3-CO2-HzO mixture, (NH&CO was not formed. The presence of H2O is necessary to evaluate the rate and the products of NH3 decomposition over limestone under commercial fluidized bed combustion conditions.
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