N₂O Formation Mechanism Research Of Oil Shale And Shale Char In Fluidized Bed Combustion

With the growing demand for energy and the gradual decrease in conventional energy resources, oil shale would be a very important alternative energy resource, which could demonstrate potential value for utilization as well as shale oil that it contains. How to efficiently exploit and utilize oil shale rich and widespread throughout the world, and to carry out thermal conversion and engineering application, will be of great practical significance for balancing energy supply and demand, and promoting social development. Oil shale has high volatile content and low fixed carbon content, which determines that its pyrolysis and combustion characteristics are so different from conventional coal that volatile is the major content of oil shale combustion, for example, the releasing heat of volatile is 2.57 times that of fixed carbon for Huadian oil shale. The special characteristics of oil shale make it necessary to carry out thorough research. In addition, it is also an important research content that realizing the resource utilization of oil refining residue - shale char. As a utilization method, fluidized bed combustion of oil shale would produce higher nitrous oxide (N₂O) emission concentration than the general pulverized coal. N₂O is an ozone-layer depleting greenhouse gas, and is ranked third largest of greenhouse gases following the carbon dioxide, methane. With the increasing requirements of environmental protection, how to control the emissions of N₂O effectively in fluidized bed combustion is an urgent issues needing to resolve.For the purpose of utilizing domestic oil shale resources cleanly and efficient, and on the basis of analyzing the above practical problem, the paper carried out the concrete research through the path of thermal conversion process of oil shale, which included pyrolysis and combustion reaction mechanism of oil shale. In order to model N₂O formation mechanism, the combustion experiment of oil shale and its shale char in small-scale and semi-industrial fluidized bed were carride out, and it was monitored and recorded that the pollutant emission of oil shale fluidized bed combustion according to the operation characteristics. Based on all these theoretical model and experimental results, the N₂O formation in fluidized bed was modeled in order to find the key component and reaction affecting N₂O formation.During the research of oil shale pyrolysis, the structural composition of oil shale was analyzed, and in the further study, the process that oil shale converts to non-condensible gas, shale oil and shale char at pyrolysis conditions, and how the functional groups in oil shale evolve were investigated. By the improving thermogravimetric test units, gas chromatography methods was used to measure the concentration of non-condensible gas in oil shale pyrolysis reaction. Based on total balance equation of each element C, H, O, N, S in ultimate analysis, gross equilibrium model was built up by assuming group-composition distribution of shale oil. The contents of ten major gas outputs in pyrolysis were calculated, and the results were validated by experimental data.During the detailed process research of oil shale combustion, the detailed combustion mechanism of combustible component in non-condensible gas was established, and the fine elementary reaction mechanism of nitrogen-containing materials HCN and NH₃ was built too in a hierarchical manner. The nitrogen-containing intermediates were classified and the conversion path of them was analyzed, and then the conversion map in the combustion process was deduced. Using the above model about nitrogen-containing precursors HCN and NH₃ fine reaction mechanism, the N₂O emission characteristics of the plug flow reactor were studied at five different HCN: NH₃ ratios. The results shows that HCN: NH₃ ratio is higher, N₂O formation concentration is significantly higher, and HCN tends to produce N₂O.During the experimental research of fluidized bed combustion characteristics, the small-scale fluidized bed experimental rig were used to carry out the combustion experiment of oil shale and shale char, which can provide some guidance for designing large-scale fluidized bed. In the small-scale fluidized bed, the experiments were proceeded which consisted of the effect of seven operating factors on N₂O emission, such as particle size, Ca/S ratio, bed material height, bed temperature, secondary air ratio, secondary air rate circulation ratio, and adopted samples of five mixture ratio including 10%, 20%, 30%, 50%, 75% and 100% of shale char content to apply tests about combustion characteristics. The grey relational analysis was used to treat the experiments data of seven operating factors and made order for them using obtained grey relational degree. The calculation results indicate that bed temperature and excess air have marked influence on N₂O emission.Based on the small-scale fluidized bed experimental results, the large-scale fluidized bed experimental device adopting XDC800 control system were designed and built which could provide the foundation for industrial fluidized bed boiler, the continuous combustion experiments were done for five mixture ratio including 25%, 50%, 75% and 100%, recording the operation course relating to air flow, pressure and temperature distribution in furnace height in real-time, and using zirconia oxygen analyzer and GASMET portable infrared analyzer to on-line monitor flue gas emissions in the process of combustion. Combined with all these data, pollutant emission characteristics of oil shale and shale char were analyzed under the large-scale fluidized bed. In absence of auxiliary heat source, bed temperature would be gradually increased to a certain balance temperature by formation heat of the fuel samples including oil shale and different mixture ratio with shale char obtained at pyrolysis conditions of 520℃, which reflected good characteristics of fluidized bed combustion.Combined with element gross equilibrium models and combustion reaction detailed mechanism model of oil shale, the zone model of fluidized bed combustion was established, and then was used to calculate simulation cases under the conditions corresponding to the same operating parameters of the experiment cases. The calculation results were in good agreement with thermal experimental data. The results showed N₂O formation in the dense phase region is far higher than that in the dilute phase region. N₂O formation in the dense phase region and dilute phase region of fluidized bed was discussed separately, and importance factor of N₂O major formation reaction was studied in the dense phase and dilute phase region. After detailed analysis of N₂O formation mechanism from the calculation results, the main N₂O formation map was obtained as well as the key component and reaction affecting N₂O formation. NH is the key component, and NH+NO=N₂O+H is the key reaction of N₂O formation. These results will provide a theoretical basis for taking effective measures to control N₂O emissions in the future. Finally, the detailed elementary reaction mechanism was simplified to obtain the simplified mechanism with high accuracy, which would help to identify and analyze a critical step affecting N₂O formation.