Emission Characteristics Of Nitrogen Oxides In Superfine Pulverized Coal Combustion And Quantum Chemistry Study On NO-Char Reaction Heterogeneous Reaction

Energy and environment are two eternal topics in this world.The utilization of energy will cause pollutions to the environment,which has been a main concern worldwide.During the process of coal combustion,pollutant gases such carbon dioxide?CO₂?,nitrogen oxides?NO_x?and sulfide oxides?SO_x?enter the atmospheric layer,which triggers a series of physical and chemical reactions.For example,the effect of carbon dioxide results in the climate change as well as the rise of global temperature that is responsible for greenhouse effect.Hence,the survival of human species is at risk.In Paris Climate Change Summit at the end of 2015,the conference aimed to further control the emissions of carbon dioxide and fulfill the goal that was formulated at the Copenhagen Conference.At the basis of United Nations Framework Convention on Climate Change?UNFCCC?and Kyoto Protocol,Paris Conference also targeted to draw up multilateral agreement which makes efforts to restrict the global warming.Nitrogen oxides and sulfide oxides are important precursors of acid rain,and for nitrogen oxides,they will also cause respiratory deceases and photochemical pollution.Nowadays atmospheric fog and haze are of a great concern to the society.The formation course of haze is that superfine particle,generally from coal combustion,transforms into the aerosol particle which eventually results in the formation of dust particles through weak diffusion environment.This thesis mainly focus on the removal of nitrogen oxides by various methods,combined with the latest technology in CO₂ atmosphere pulverized coal combustion,this thesis also aims to simultaneously remove NO_x and CO₂ during coal combustion.In Chapter Two,it mainly describes about the selection of coal sample and the methods of producing pulverized coal.The definition of superfine pulverized coal and the particle size distribution is discussed as well.Meanwhile,this thesis summarizes the modern testing facility?TG-FTIR and SAXS mearsuring instrument?which includes bascis,principles and parameter settings etc.Finally,the test rig used in the following chapters,called the multi-function on dimensional drop-tube furnace,is emphasized to depict the experimental systems such as air-preheating system,feeding system and temperature contolling system.Last but not least,the debugging process and the methods,processes,instructions of the test rig are also introduced.In Chapter Three,TG-FTIR measuring technique and multiple-function one dimensional drop-tube furnace are both applied to study on reaction kinetics,evolution of gaseous substances,and functional groups during coal pyrolysis.The result shows that NMG bituminous coal establishes the phase of devolatilization and char formation earlier,while SH bituminous coal involves itself more in the process of carbonization and polycondensation.From the analysis of reaction kinetics,the activation energy of high temperature zone is much higher than that of low medium temperature zone,which is probably due to the fact that the energy barrier required by char polycondensation is higher than the devolatization stage.For different gaseous substances and functional groups,the evolution of SH bituminous coal is lower because of its decreased carboxyl groups.The CO yield of NMG bituminous coal is more due to increased side chains and phenolic hydroxyl.As for SH bituminous coal with the decrease of particle size,the CH₄ adsorption peak falls off,but the impact of particle size on NMG coal is not obvious.The evolution trend of aromatic compound C=C is similar to C=O carbonyl compound.As the particle size decreases,the CH4yield increases.While the case of SH₄4.26 is a little different,it is probably caused by the particle agglomeration.The nitrogen-containing gases have a close relationship in the process of pyrolysis,the conversion of HCN is mainly through secondary reaction and oxidation mechanism to HCN and NO,respectively.In Chapter Four,multi-functional one dimensional drop tube furnace is applied to investigate the NO_x emission characteristics of air-staged combustion for bituminous and anthracite superfine pulverized coal.Meanwhile,with the combination of Small Anger X-ray Scattering method in Shanghai Synchrotron Radiation Facility,microscopic reaction mechanism is introduced to interpret the interaction between NO-char and some phenomena of NO_x emission trends.In the working condition design of anthracite coal,multi-staged combustion is adopted to further realize the abatement of NO_x emission.The result shows that the coal rank brings about a good impact on the NO_x reduction,and that the NO_x emission of NMG bituminous coal is less,which may be due to its abundant reactive sites in coal char.According to different coal ranks,there exists a value of particle size where the maximum NO_x yield takes place.The NO_x reduction performance of NMG coal is much better than that of SH coal.For YQ anthracite coal,the conventional particle is more suitable for shallow staged combustion,and its burnout ratio is not therefore affected.The NO_x reduction performance of multi-staged combustion is more superior than that of single-staged combustion,without the sacrifice of burnout ratio.The NO_x reduction efficiency of superfine pulverized coal is better than that of conventional coal.In Chapter Five,in order to simultaneously remove NO_x and CO₂,reburning of superfine pulverized coal is carried out in two atmospheres?N₂/CO₂?,using drop tube furnace.Meanwhile,monitoring of both nitrogen oxide precursors and other gaseous species is launched to provide more data helpful for the explanation of various NO_x trends.The results show that the reburning fuel fraction and oxygen concentration are suggested to be controlled at proper parameters.The advantage of superfine pulverized coal reburning is not apparent when it is operated under low reburning fuel fraction.While in the working condition of high reburning fuel fraction,the advantage of NO_x reduction appears.Low oxygen concentration is preferable to assure the high NOx reduction performance for superfine pulverized coal.In the ruburning process of CO₂ atmosphere,SH₂1.30 performs the best in NO_x removal compared to other particle sizes when operated under low oxygen concentration and high reburning fuel fraction.When it comes to medium reburning fuel fraction?15%²0%?,for particle size of SH₁7.44 and SH₄4.26,there exists a platform where the increase of reburning fuel fraction isn't able to efficiently enhance the NO_x reduction performance.The increase of temperature isn't expected to make the NO_x redcution trend line in monotonicity.The enhancement of inlet NO concentration in reburning zone can't improve the NO_x reduction efficiency.In Chapter Six,this thesis applies the method of quantum chemistry calculation provided by the Gaussian Company,to simulate the procees of superfine pulverized coal combustion,more importantly,to find some specific law both in air-staged combustion and reburning combustion in N₂/CO₂ atmosphere.In the meanwhile,the outcome of the simulation can also interpret some phenomena happens in the experiment,making it possible to validate the results between experiment and simulation.The result of the investigation shows that when the adsorption of CO takes place on the surface of char,NO and CO go through several changes,such as rotation,extension,recombination,to eventually form the stucuture which is prone to the desorption process of CO₂ and N₂ molecules.The whole reaction parthway turns NO and CO molecules into CO₂ and N₂.In another pathway where O₂ exists in the atmosphere,the coal char is easily to go through the re-opening reaction after the adsorption of O₂ on the surface,then molecular structure is shaped to prepare for the NO adsorption.Following this elementary reaction,the coal char consequently undergoes the closure reaction and CO₂ desorption,which compels NO to form the interior nitrogen on the char surface,thus stopping nitrogen oxides from being released into the environment.Combined with air-staged combustion,O₂/CO₂ staged combustion,reburning combustion in N₂ and CO₂ atmosphere,the technology of superfine pulverized coal combustion has the advantage to overcome many drawbacks for the converntional coal combustion,such as low comprehensive combustion efficiency,poor stable combustion,slagging etc.In addition,these techiniques are also able to achieve the goal of simultaneous removal of multiple gases such as CO₂,NO_x,SO₂ etc.In the existing application of industry,through the reconstruction and relocation of the burners in the furnace,this technology can achieve the abatement of nitrogen oxides.In the end,compared with other nitrogen oxides removal techniques,the superfine pulverized coal combustion also owns the advantages such as low cost and flexiable operation,which makes it promising to be competitive in the application of market and industrial area.