| China has suppressed the United States to become the world's largest CO2 emitter, while the coal combustion is responsible for more than 60% of all CO2 emission. Oxy-fuel combustion which recirculates the flue gas into the furnace together with the pure O2 for combustion, effectively enriches CO2 concentration and also lowers the pollutants exhausts, especially the NO emission. The previous study reported that the NO emission of oxy-fuel combustion is much lower than that of traditional air combustion. Moreover, the wet-recycle method used for oxy-fuel combustion emits even lower NO than dry-recycled method. The NO formation mechanism in oxy-fuel combustion is different from that in air combustion process. However, the NO formation and reduction mechanism in oxy-fuel combustion is not studied sufficiently. Especially the impact of water vapor on NO formation in wet-recycle method is unclear. Therefore, it is very important to understand and study of the NO formation mechanism in oxy-fuel combustion in order to guide lower-NO emission method.From the perspective of the differences between air and oxy-fuel combustion, the influences of high concentration CO2, recycled NO and water vapor on N chemistry during oxy-fuel combustion were studied in this thesis. The key scientific issues involved are list as below:(1) The homogeneous and heterogeneous effect of high concentration CO2 and recycled NO on NO exhausts in oxy-fuel combustion and their specific contribution; (2) The influences of water vapor on NO formation process from coal-N (nitrogen in coal) and on recycled NO reduction process respectively; (3) The detailed impact of water vapor on volatile-N (volatile nitrogen) and char-N (char nitrogen) oxidation process during wet-recycle method. With regard to the above issues, in-depth researches including experiments and simulations were carried out.Based on the combustions of three typical Chinese coals (a lignite, a bituminous coal and an anthracite) in air and oxy-fuel combustion, the amount of NO emitted in oxy-fuel combustion has been found to be 40% of that in air combustion. The ratio of NO exhausts in oxy-fuel combustion to that in air combustion increased with the coal rank increasing. High concentration CO2 contributed 20%?40% of the NO emission decrement from air combustion. The reduction reactions between recycled NO and volatile materials or char contributed the major NO emission decrement (approximately 60%). The main recycled NO reduction route differed for the three coals. For the lignite and the high content volatile bituminous coal, the homogeneous recycled NO reduction with volatile matters was the main route. While major part of recycled NO was reduced into N2 by char through heterogeneous reactions during the anthracite combustion. The amount and composition of volatile or non-volatile N were considered as a decisive factor.When compared to the NO exhausts in air combustion, approximately 70% of NO was reduced during wet oxy-fuel combustion. While water vapor contributed 10%?20% for this NO decrement and the reduction reactions of recycled NO contributed about 50%. The NO formation from coal-N in both air and oxy-fuel combustion was suppressed by water vapor, while the suppression in air combustion was larger than that in oxy-fuel combustion. The influence of water vapor on recycled NO reductions were different for the three coals since the major recycled NO removal route differed among them. The experiment also found that, although if the water vapor accelerated the reduction of recycled NO, the suppression effect of water vapor on NO formation from coal-N was more important.The coal-N oxidation process includes volatile-N and char-N transformation to NO, which occurs at different combustion stage. Therefore, the NO formation form volatile-N or char-N could be quantified through the measurement of NO concentration in gas sampled at different residence time. The results showed that the NO formation from both volatile-N and char-N were suppressed by water vapor in wet-recycle method. The suppression was larger for NO formation from volatile-N than that from char-N, which increased with the O2 fraction increasing. The N 1s spectra of raw coal and the chars in CO2/H2O and O2/CO2/H2O atmosphere could be fitted as five peaks at 398.8 eV,400.3 eV,401.4 eV and the region of 402-405 eV. They were inferred as N-6 (pyridine-N), N-5 (pyrrolic-N), N-Q (quaternary nitrogen), N-X1 and N-X2 (unclear nitrogen). The presence of water vapor in both CO2/H2O and O2/CO2/H2O atmosphere accelerated the transformation of N-6 into N-Q.The homogeneous mechanism Mendiara 09 was carried out to investigate the influence of water vapor on volatile-N transformation. The simulative result showed that the NO emission in CH4/NH3 and CH4/HCN/NH3 wet oxy-fuel combustion was lower than that in dry oxy-fuel combustion. However, the reduction of NO by CH4 was enhanced when water vapor was added. The change of NO emission before and after water vapor addition from simulative results agreed well with that from experimental results. Therefore, Mendiara 09 mechanism which was used for dry-recycle condition can also be also adapted for high water vapor condition (20%). Both physical and chemical effects of water vapor could impact the transformation of volatile-N, whereas the specific contribution of each effect was determined by the composition of volatile matter. The main transformation routes of HCN and NH3 showed little change with ant without water vapor. However, the decomposition of reactant through each route differed due to the compositions of O/H/OH radical pool were changed by the indirect effect of water vapor. A simplified mechanism for N chemistry was proposed based on the productive and sensitive analysis of Mendiara 09. The formation of NO and N2 from the simplified mechanism agreed well with those from the detailed mechanism. |
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