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 NOx 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 NOx and N2O emissions was examined at various inlet N2, CO2, and H2O concentrations (CN2(i), CCO2(i), and CH2O(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 NOx was 0.077 at CN2(i) = 79 vol % and decreased linearly with increasing CCO2(i) and CH2O-(i) down to 0.044 with the respective values of CCO2(i) and CH2O(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 CCO2(i) and slightly increased with CH2O(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 CCO2(i) and CN2(i) at CH2O(i) = 0. Both conversions of NO to N2 and N2O to N2 increased with CH2O(i)- On the basis of the results for OTM, the overall fuel nitrogen conversions in ERM were estimated as 0.76% for NOx and 1.4% for N2O, again assuming that reduction of the nitrogen oxides occurred independent of their formation from the coal. The considerably lower NOx and N2O emissions in ERM than those in OTM with air was reasonably well explained by much higher CCO2(i) and CH2O(i) and the extensive reduction of NOx and N2O recycled in the former combustion mode.
All Science Journal Classification (ASJC) codes
- Chemical Engineering(all)
- Fuel Technology
- Energy Engineering and Power Technology