NOX 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 expe rimentally examined for coal combustion in an atmospheric bubbling flu idized bed. An Australian bituminous coal crushed to sizes smaller tha n 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 C O2-rich flue gas was recycled and fed to the combustor with pure O-2 a nd a once-through mode (OTM) where gas with various compositions as we ll 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 fra ctional conversions of fuel nitrogen into NOx and N2O were 0.0083 and 0.012, respectively, which were respectively equivalent to about 1/9 a nd 1/6 of those in OTM with air. In OTM, the effect of the inlet gas c omposition on NOx and N2O emissions was examined at various inlet N-2, CO2, and H2O concentrations (C-N2(i), C-CO2(i) and C-H2O(i), respecti vely). The results showed that the combustion efficiency and the in-be d char concentration are both independent of the inlet gas composition The fractional conversion of fuel nitrogen into NOx was 0.077 at C-N2 (i) = 79 vol % and decreased linearly with increasing C-CO2(i) and C-H 2O(i) (i) down to 0.044 with the respective values of C-CO2(i) and C-H 2O(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 reductio n ratios of NO to N-2 and/or N2O, NO to N2O, and N2O to N-2, which wer e 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 N-2 and N2O to N-2 increa sed with C-H2O(i). On the basis of the results for OTM, the overall fu el nitrogen conversions in ERM were estimated as 0.76% for NOx and 1.4 % for N2O, again assuming that reduction of the nitrogen oxides occurr ed independent of their formation from the coal, The considerably lowe r NOx and N2O emissions in ERM than those in OTM with air was reasonab ly well explained by much higher C-CO2(i) and C-H2O(i) and the extensi ve reduction of NOx and N2O recycled in the former combustion mode.