Mechanisms Of Nitrogen Transfer In Chemical Looping Combustion Of Biomass

CO₂ during fossil fuel combustion is one of the major greenhouse gases. Chemical looping combustion （CLC） is a novel and non-flaming combustion process which features inherent separation of CO₂.Compared with traditional combustion, oxygen atom comes from an oxygen carrier in CLC instead of air or oxygen. CO₂ and vapour are the primary products in CLC, which is conducive to CO₂ capture.CLC is also a high efficiency combustion with oxygen carrier cycled between fuel reactor and air reactor.As is known, there is plenty of biomass in China. Biomass energy is a kind of clean and renewable energy, and it is significant to combine biomass with CLC. The environmental problems with nitrogen oxide （NOx） emission are attaching more and more attention.However, there are few researches about the release of NOx during CLC process of biomass.The mechanism of nitrogen transfer in CLC process of biomass has not been carried out yet.Therefore, the purpose of this thesis was to experimentally investigate the release mechanism of NOx formation and nitrogen transfer during CLC process of biomass.Based on the synthesis of the highly reactivity,the characteristics of nitrogen transfer using nickel-based oxygen carrier in CLC with rice husk as fuel has been investigated experimentally at 850℃. The results indicated that N₂O and NO are the prime products, and the concentration of N₂O is more than that of NO. On the basis of the chemical thermodynamic analysis and experimental results, NOx was mainly produced by the volatile-N during volatile combustion process. The nitrogen intermediate of HCN could be oxidized by NiO to N₂O.The another nitrogen intermediate of NH3 hardly reacted with the NiO.Moreover, the K, Ca in biomass was detected in the reduced nickel-based oxygen carrier by complex physical and chemical reactions.Chemical looping with oxygen uncoupling （CLOU） is the extension and development of the CLC. The copper-based oxygen carrier was produced by dry roll compacting granulator. A series of tests were conducted to investigate the mechansims of NOx formation and evolution in a fixed bed reactor. Experimental results showed that N₂O generation was 30.9% and NO generation was 21.9% at 850℃.At the same time, NO₂ was detected in the outlet gases. The volatile-N of biomass probably contributed most to the produce of NO and NO₂, while the char-N of biomass played an equal role in the produce of N₂O as the volatile-N did. After reacting with rice husk, the copper-based oxygen carrier particles presented still porous, favoring the rapid regeneration of oxygen carrier with oxygen.The emission features of nitrogen was investigated during CLC of rice husk based on natural hematite in a batch fixed bed reactor. Different variables affecting the process were tested, such as reduction temperature, steam content and cyclic number, which used as gasification agent. Results showed that the higher reduction temperature resulted in a higher carbon conversion.However, carbon conversion decreased with the multiple cycles. Additionally, the carbon conversion reached the highest value when the steam flow selected was 1 g/min. Furthermore, there were no NO₂ detected at all experiments tested. The rise of reduction temperature from 750℃ to 900℃ could contribute to the rise of NO, whereas the concentration of N₂O reached the highest value at 850℃. Both N₂O and NO increased remarkably with the increase of steam flow. Nevertheless, the increasing rate of NO was faster than that of N₂O. Moreover, with the increase of cyclic reduction number, the concentration of NO decreased gradually,while the concentration of N₂O exhibited a first increasing and then decreasing trend.Besides,some sintering was observed at particle surface of the hematite oxygen carrier after multiple redox reactions.Furthermore, using N-containing model compound in researching helps to exclude the interference of secondary factors, which is one of the important ways for the study of the nitrogen transfer in CLC of biomass.The experinments were conducted using Glutamic acid, Glycine and Phenylalanine as N-containing model compounds.During the reduction in CLC, the effects of the temperature and potassium addition on the nitrogen transfer from the three amino acids were studied. The tests suggested that the volatile-N released quickly during the whole process,and the yields of NO and NO₂ increased with the increase of temperature, whereas N₂O concentration showed no apparent pattem.The potassium addition had no notable influence on NOx release of the three amino acids during CLC process except that NO emission from Phenylalanine.