Effects of heterogeneous reactions of coal char on nitrous oxide formationand reduction in a circulating fluidized bed

Fluidized-bed combustion has been recognized as a clean coal combustion tec hnology. However, it has been discovered that the nitrous oxide (N2O emissi on from fluidized-bed coal combustion is much higher than that from pulveri zed coal combustion. N2O emission from circulating fluidized-bed coal combu stion is even higher. Heterogeneous reactions of char play an important rol e in N2O formation and reduction in circulating fluidized-bed coal combusti on 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 circu lating fluidized-bed coal combustion, N2O formation from oxidization of cha r-N has the same order as N2O formation from NO reduction by char, and neit her of them is negligible (inlet O-2 concentration: 21%, temperature: 1135- 1225 K). Absorption of oxygen on the surface of char is not an indispensabl e condition for N2O formation from NO reduction, i.e., NO can also be reduc ed into N2O on the surface of char in absence of O-2. N2O formation from NO reduction in the presence of O-2 contributes about 70% and is the main pat h of N2O formation from NO reduction. N2O formation from NO reduction in th e; absence of O-2 contributes about 30%, and the relative importance of the se two mechanisms of N2O formation does not change significantly with NO co ncentration. Reduction of N2O by char is a first-order reaction with an Arr henius 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 depende nce 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 N 2O formation and reduction were proposed.