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Experimental Study On Mechanism Of NO Conversion During Coal Oxy-fuel Circulating Fluidized Bed Combustion

Posted on:2018-06-03Degree:DoctorType:Dissertation
Country:ChinaCandidate:M X XuFull Text:PDF
GTID:1312330512992929Subject:Thermal Engineering
Abstract/Summary:PDF Full Text Request
Coal is one of the most important fossil fuel in China.However,the emission of carbon dioxide(CO2),which is the main source of greenhouse gases,during coal combustion can damage the ecology.The technology for CO2 capture and storage is in urgent demand.Oxy-fuel circulating fluidized bed(oxy-fuel CFB),which combines the advantages of oxy-fuel combustion with those of CFB technology,is one of the most promising technologies to capture CO2.During oxy-fuel CFB combustion,NO,which is also a significant impurity in the flue gas,can heavily affect the processes of CO2 compression,transport and storage.Therefore,it is important and useful to study the characteristics of NO formation and emission during oxy-fuel CFB combustion.The provision of detailed relevant knowledge regarding to the nitrogen chemistry will facilitate the development of strategies of lower NO during oxy-fuel combustion.The present research focuses on the process of NO formation and transition during coal oxy CFB combustion.The effect of CO2 on the fuel nitrogen conversion,the emission of NO during coal O2/CO2 combustion with high oxygen concentration in CFB,and the characteristics of recycled NO reduction over char during oxy-fuel fluidized bed combustion were investigated respectively.Besides,the emission of NO during coal combustion at high oxygen concentration in a 1 MWth pilot-scale oxy-fuel CFB was also studied.The major conclusions can be summarized as follows:To investigate the fuel nitrogen conversion during coal rapid pyrolysis,tests with bituminous coal and petroleum coke were accomplished under N2 and CO2 atmosphere via a fixed bed reactor.It was found that the effect of CO2 on fuel nitrogen conversion during coal rapid pyrolysis depended heavily on the temperature of pyrolysis.At lower temperature,CO2 could suppress the nitrogen release from fuel nitrogen to NO precursors while it could enhance the conversion at higher temperature,because of the CO2 gasification with char.Besides,there were more quaternary nitrogen consumed during rapid pyrolysis in CO2 atmosphere than that in N2 atmosphere.To investigate the NO emission during O2/CO2 CFB combustion,tests were carried out in a 50 kWth CFB combustor.The behaviors of NO formation and transition in the combustor were studied via analyzing the NO axial profiles along the combustor height.The results shown that NO could be reduced along the combustor after it formed in the dense zone.Lifting the height of the secondary gas injection,increasing the ratio and the oxygen concentration of secondary gas could significantly suppress the formation of NO in the dense zone.To explain the reduction process of recycled NO over char during oxy-fuel fluidized bed combustion,tests associated with the reduction of recycled NO were accomplished via a bubbling fluidized bed.It was determined that the apparent activation energy of recycled NO reduction over char was much lower when char combustion occurred at the same time.The recycled NO and CO with low concentration had little effects on the reduction of recycled NO while the reduction ratios of recycled NO increased as the recycled SO2 concentration increased.Besides,the reduction of recycled NO over char was promoted by the increase of oxygen concentration in the feeding gas and reaction temperature.Experiments about bituminous coal combustion during oxy-fuel CFB combustion were also accomplished in a 1 MWth pilot-scale oxy-fuel CFB system.The influence of common operating parameters on the emission of NO in the flue gas was investigated.It was found that the emission of NO in oxy-fuel combustion with high oxygen concentration was much lower than that in air-firing condition(in mg/MJ).The increasing operating temperature could improve the conversion of fuel nitrogen to NO.Because of the catalysis of CaO,the desulfurization had a negative effect on the emission of NO.Besides,it was also found that oxygen staging was a promising choice for oxy-fuel CFB combustion with high oxygen concentration to control the emission of NO.
Keywords/Search Tags:Circulating fluidized bed, Oxy-fuel, coal, Fuel nitrogen, Recycled flue gas, Oxygen staging
PDF Full Text Request
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