Effect of Heterogeneous Reactions of Coal Char on Nitrous Oxide Formation and Reduction in a Circulating Fluidized Bed

Abstract

Fluidized-bed combustion has been recognized as a clean coal combustion technology. However, it has been discovered that the nitrous oxide (N2O) emission from fluidized-bed coal combustion is much higher than that from pulverized coal combustion. N2O emission from circulating fluidized-bed coal combustion is even higher. Heterogeneous reactions of char play an important role in N2O formation and reduction in circulating fluidized-bed coal combustion but there exist many unknowns. In this work the effects of heterogeneous reactions of char on N2O formation and reduction were examined in a bench-scale circulating fluidized bed. The experiments revealed that during circulating fluidized-bed coal combustion, N2O formation from oxidization of char-N has the same order as N2O formation from NO reduction by char, and neither of them is negligible (inlet O2 concentration: 21%, temperature: 1135−1225 K). Absorption of oxygen on the surface of char is not an indispensable condition for N2O formation from NO reduction, i.e., NO can also be reduced into N2O on the surface of char in absence of O2. N2O formation from NO reduction in the presence of O2 contributes about 70% and is the main path of N2O formation from NO reduction. N2O formation from NO reduction in the absence of O2 contributes about 30%, and the relative importance of these two mechanisms of N2O formation does not change significantly with NO concentration. Reduction of N2O by char is a first-order reaction with an Arrhenius dependence on temperature. The kinetics of N2O reduction by various chars were obtained (974−1223 K). Different chars have different effects on N2O reduction, suggesting that N2O reduction ability of char has a dependence on the property of the char itself. These results may partially account for the diversities in N2O emissions during combustion of different coals. On the basis of the results of this work, char-related reaction paths of N2O formation and reduction were proposed.