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Suppression of nitrogen oxides emission by flue gas recycling was experimentally examined for coal combustion in an atmospheric bubbling fluidized bed. An Australian bituminous coal crushed to sizes smaller than 5 mm was burnt at 1120 K and superficial gas velocity of 1.0 m/s in a 0.158 m i.d. and 3 m high combustor. The combustion was performed in two different modes, namely, an exit gas recycling mode (ERM) where CO 2 -rich flue gas was recycled and fed to the combustor with pure O 2 and a once-through mode (OTM) where gas with various compositions as well as air was fed to the combustor without the recycling. In ERM with the inlet concentration of O 2 being kept at 21 vol %, the overall fractional conversions of fuel nitrogen into NO x and N 2 O were 0.0083 and 0.012, respectively, which were respectively equivalent to about 1/9 and 1/6 of those in OTM with air. In OTM, the effect of the inlet gas composition on NO x and N 2 O emissions was examined at various inlet N 2 , CO 2 , and H 2 O concentrations (C N2 (i), C CO2 (i), and C H2O (i), respectively). The results showed that the combustion efficiency and the in-bed char concentration are both independent of the inlet gas composition. The fractional conversion of fuel nitrogen into NO x was 0.077 at C N2 (i) ) 79 vol % and decreased linearly with increasing C CO2 (i) and C H2O -(i) down to 0.044 with the respective values of C CO2 (i) and C H2O (i) being 79 vol % on a dry basis and 5 vol % on a wet basis. On the other hand, the fuel nitrogen conversion into N 2 O was independent of C CO2 (i) and slightly increased with C H2O (i). The in-bed reduction of NO or N 2 O added into the inlet gas was also evaluated to estimate the reduction extent of the nitrogen oxides recycled in ERM. The reduction ratios of NO to N 2 and/or N 2 O, NO to N 2 O, and N 2 O to N 2 , which were determined by assuming no interaction between added NO and N 2 O, were 0.81, 0.04, and 0.80, respectively, regardless of C CO2 (i) and C N2 (i) at C H2O (i) ) 0. Both conversions of NO to N 2 and N 2 O to N 2 increased with C H2O (i). On the basis of the results for OTM, the overall fuel nitrogen conversions in ERM were estimated as 0.76% for NO x and 1.4% for N 2 O, again assuming that reduction of the nitrogen oxides occurred independent of their formation from the coal. The considerably lower NO x and N 2 O emissions in ERM than those in OTM with air was reasonably well explained by much higher C CO2 (i) and C H2O (i) and the extensive reduction of NO x and N 2 O recycled in the former combustion mode.

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