Study On Conversion Of Nitrogen To NO During Char Combustion

At present,the NOx emission from coal-fired boilers in China has caused serious environmental pollution.The technologies of air classification and low NOx burner has been employed to to control the NOx produced in the combustion process.These techniques can reduce NO to N2 by changing the air-fuel ratio to form a fuel-rich zone.However,although these techniques can successfully reduce most of the volatile NOx to N2,it is very difficult to remove NOx from coal char,which is mainly due to the difference.of that the conversion mechanism of char nitrogen to NO and the volatile nitrogen.In order to achieve ultra-low emissions,the existing low-NOx combustion technology must be improved according to the NO release rule during the combustion of coal char.Therefore,in order to solve the problem of NOx removal,it is necessary to study the NO release during the combustion of char.In this paper,some key parameters influencing the NO conversion during the char combustion are analyzed in detail by the mechanism test reactor and numerical simulation,which provide theoretical basis and data support for the combustion technology of reducing NOx in coal-fired boilers.In this thesis,the distribution and migration of nitrogen,carbon and oxygen functional group in coal char prepared at high temperature were firstly studied by X-ray photoelectron spectroscopy(XPS)and the effect of nitrogen content and nitrogen-containing functionalities on the final NO release was further studied.It is found that after high temperature pyrolysis,pyrrolic-N(N-5)and pyridinic-N(N-6)are most abundant forms of organically bound nitrogen in chars.With the pyrolysis temperature increasing,the content of protonated pyridinic-N(N-Q)decreases continuously and the overall trend of the N-6 content increases whereas that of N-5 reduces slightly.It indicates that N-6 has much higher heat stability.The increase of pyrolysis temperature will lead to the increase of the degree of graphitization and the decrease of char reactivity.There is a good correlation between N-6 content and N/NO conversion,indicating that more coal char N involved in NO reduction.The random pore model(RPM,Random pore model)was used to study the variation of pore structure in char oxidation and reduction.It is found that the pore structure of char-O2 reaction experiences a greater development compared to the char-NO reaction.The mass transfer process of O2 and NO in the char pore is also studied using a kinetics reaction model.It is found that the more developed coal pore structure can enhance the diffusion of O2 and NO in the particles.For the char-O2 reaction process,the effective factor increases first and then decreases slightly,which is mainly attributed to the change of pore structure during the char combustion.The effect of ambient oxygen concentration on NO release during coal char combustion was studied by HTF(Horizontal tube furnace)reactor at combustion temperature of 700 °C~1100 °C.The results show that addition of quartz sand in the packed-layer mixture can effectively inhibit the secondary reaction of char and NO among the char particles.During the char combustion tests,char–N/NO conversion decreases with increasing ambient oxygen concentration at low temperature(700 °C-900 °C).The oppsite increasing trend is observed that the N/NO conversion increases slightly when the oxygen concentration increases at high temperature(~1100 °C).This indicates that the effect mechanism of oxygen concentration on the N/NO conversion is different when a change of the oxidization from kinetic to transition or diffusion control occurs.In the range of 900 °C to 1400 °C,the effect of particle size on the N/NO conversion of char was further studied.It is found that the variation of particle size does not significantly affect the intrinsic reactivity of char but the diffusion of O2 in the particles.At different oxygen concentrations and temperatures,increasing the particle size,N/NO conversion shows an overall upward trend.The kinetic model was used to calculate the effective reaction area and the diffusion depth of O2 in the particles under different oxygen concentration and particle size.It is found that the decrease of the effective reaction area and the diffusion depth will directly lead to the decrease of N/NO conversion which can successfully explain the experimental results.The ratio of O/C in chars with different particle size changing with the burn-off was analyzed.It is found that the smaller the particle size,the larger the O/C ratio,which means that more O2 diffused to the particle pore surface.This further validates the model calculation.The effects of different pyrolysis conditions on the conversion of N/ NO were investigated by a drop tube furnace and a fixed bed reactor.It is found that secondary pyrolysis char has a larger pore area than primary pyrolysis char,but the reactivity deteriorates due to the increased graphitization degree attributed to the influence of heat inactivation.The higher the intrinsic reactivity of char is,the lower the N/NO conversion is,suggesting that intrinsic reactivity is an important determination on N/NO conversion.In the late stage of combustion,the significant increase of NO/(CO + CO2)is mainly attributed to the decrease of reaction area and the enrichment of N element.Finally,the numerical simulation of N/NO conversion in char combustion process was carried out by Matlab.It is found that the reactivity of coal char under the different temperatures and oxygen concentrations simulated by the simulation is in good agreement with the experimental results.And the simulation results show that increasing the oxygen concentration,the N/NO conversion increases.However,the opposite trend is found when increasing the reaction temperature and the background NO concentration.This indicates that the oxygen concentration,the temperature and the background NO concentration can affect the diffusion of O2 in the particles.Considering the effects of temperature and oxygen concentration on the N/NO conversion,the simulated values are higher than the experimental values,mainly due to the fact that the model does not consider the interaction between particles and the effect of oxygen concentration on the effective reaction area.