Study On The Conversion Mechanism Of Volatile Nitrogen During Fuel Combustion In Circulating Fluidized Bed Boilers

Circulating fluidized bed?CFB?boiler is characterized as wide fuel adaptability,high combustion efficiency,and wide load adjustment range.The relatively low furnace temperature?around 900??is within the range of optimal de SO_x reactions,which is suitbale for in-furnace de SO_x prcess and beneficial for reducing thermal NO_x formation;however,the amount of nitrous oxide?N₂O?emission is at least one magnitude higher than that of conventional pulverized coal-fired?PC?boielrs.N₂O is a hazardous pollutant causing global warming and ozone depletion.Therefore,the control of NO_x and N₂O in CFB boilers is of equal importantce.Given the characteristics of nitrogen oxide emissions from CFB boiler,this work aims to elucidate the conversion law of volatile nitrogen during fuel combustion in CFB boilers.Quantum chemistry calculations based on density function theory?DFT?were used to explore the conversion from main contents of volatile nitrogen including HCN,NH₃,and HNCO to nitrogen oxides.Emphasis was put on the effect of in-furnace de SO_x process on the nitrogen conversion from volatie nitrogen to nitogen oxides.With the help of CaO?1 0 0?surface model,the conversion of HCN,NH₃,HNCO to nitrogen oxides on CaO?1 0 0?surface was studied to eludicate the effect of in-furnace de SO_x process on the conversion of HCN,NH₃,HNCO.Theoretical results found that the CaO?1 0 0?surface could promote the H abstraction reaction from HCN to CN,and promote the reaction between CN and O₂ forming NCO.The NCO reaction with O₂ forming NO is greatly catalyzed with the energy barrier decreasing from 0.349e V to 0.026 e V,but reactions between NCO and NO forming N₂O or N₂ were inhibited.CaO?1 0 0?surface could also promote the H abstraction from NH₃ and catalyze the reaction between NH₃ and H but inhibit the reaction between NH₃ and OH.More importantly,CaO?1 0 0?surfae was able to significantly catalyze the reaction between NH and O forming NO,thus promoting the production of NO from NH₃ oxidation.Beside,CaO?1 0 0?surface could also promote the H-N bond cleavage and catalyze the formation of NCO from HNCO.Laboratory-scale flow reactor was used to study the catalytic effect of CaO for N₂O decompositoin in comparison with CaSO₄ and main components of circulating ashes in CFB boilers.Experimental resutls showed that CaO was strong catalyst for N₂O decomposition,while CaSO₄ was almost inert for N₂O decomposition.CaO could also decrease the selectivity of NO from N₂O decomposition.The decomposition from N₂O to N₂ and NO catalyzed by CaO was studied by investigating the N₂O decomposition routes on CaO?1 0 0?and CaO?1 1 0?surfaces.The energy barrier of O atom transfer on CaO?1 0 0?and?1 1 0?surfaces was 0.989 e V and 0.861 e V respectively.N₂O decomposition on CaO?1 0 0?surface was likely to yield N₂ but hardly to yield NO by Gibss free energy analysis.The rate-determining step of N₂O decomposition on CaO?1 0 0?surface was the sruface recovery process,while the O atom transfer step was the rate-determining step for CaO?1 1 0?surface.To investigate the effect of CaO sulfurization on the catalytic activity of CaO for N₂O decomposition in the reducing atmosphere,a perfect CaO?1 0 0?surface model doped by two S atoms was used to elucidate the effect of partial sulfurization on CaO surface activity for N₂O decomposition.When CaO was locally sulfruized into CaS,its catalytic ability for N₂O decomposition deteriorated.If CaO was completely sulfurized into CaS,the catalytic ability of perfect CaS further decerased but still maintianed certain degree of catalytic ability for N₂O decomposition in comparison to N₂O homogeneous decomposition case?The energy barrier of O atom transfer step is1.228 e V?.In addition,CaS?1 0 0?surface was able to catalyze the reduction of N₂O by CO.To reveal the effect of in-furnance de SO_x process on the catalytic decpomsition of N₂O on CaO surface in the oxidizing atmosphere.The local sulfation of CaO?1 00?surface was first studied.Results showed that CaO?1 0 0?surface partial sulfation was more likely to proceed via CaSO₃ as sulfation intermediate.The formation of local CaSO₃ species weakened the surface activity for N₂O decomposition in comparison with perfect CaO?1 0 0?surface with the energy barrier increasing to 1.340 e V.When local CaSO₃ was further sulfated into CaSO₄,its catalytic ability for N₂O decomposition disappeared and the energy barrier of N₂O decomposition on the local CaSO₄ is 2.967 e V.The electronic interaction of CaO-CaSO₄ composite surface layer happened at the interface of CaO and CaSO₄,and the outer CaSO₄ was not influenced by the inner CaO.A further study on the catalytic ability of CaSO₄?0 0 1?and CaSO₄?0 1 0?surface showed that there was negligible catalytic ability of CaSO₄ for N₂O decompositoin.