No abstract

No abstract

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.

No abstract

No abstract

No abstract

No abstract

No abstract