NO_x and N₂O emission in bubbling fluidized-bed coal combustion with oxygen and recycled flue gas: Macroscopic characteristics of their formation and reduction

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 CO₂-rich flue gas was recycled and fed to the combustor with pure O₂ 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 O₂ being kept at 21 vol %, the overall fractional conversions of fuel nitrogen into NO_x and N₂O 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 N₂O emissions was examined at various inlet N₂, CO₂, and H₂O 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 N₂O was independent of C_CO2(i) and slightly increased with C_H2O(i). The in-bed reduction of NO or N₂O 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 N₂ and/or N₂O, NO to N₂O, and N₂O to N₂, which were determined by assuming no interaction between added NO and N₂O, 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 N₂ and N₂O to N₂ 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 N₂O, again assuming that reduction of the nitrogen oxides occurred independent of their formation from the coal. The considerably lower NO_x and N₂O 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 N₂O recycled in the former combustion mode.