Analysis Of Uncoupled Effects Of CO₂ Dilution On Methane Flame Combustion Characteristics

As a clean fossil energy,natural gas is of great significance to alleviate environmental problems such as smog and realize clean heating.The main problem of natural gas combustion is the emission of nitrogen oxides.Flue gas recirculation and pure oxy-fuel combustion technologies are expected to become a new technical route to achieve"zero nitrogen"emissions and reduce carbon capture costs.Compared with combustion in conventional air atmosphere,high concentration of CO₂ has a significant impact on combustion flame structure,flame stability,flame extinguishing limit and chemical reaction kinetic mechanism,etc.Scholars have also conducted extensive research.However,how to decouple the synergistic effect of CO₂ on natural gas flames and determine the uncoupled effects of CO₂ characteristics（chemical,thermal,transport and radiative characteristics）on flame characteristics has always been a hot and difficult point in basic natural gas combustion theory research.Determining the uncoupled effects of CO₂ provides an important theoretical basis and technical support for the application and development of flue gas recycling technology.This paper establishes a flame combustion model and proposes solutions and strategies for CO₂ uncoupled effects,which are applied to one-dimensional opposed diffusion flames and two-dimensional coaxial jet diffusion flames.Combined with experimental measurement methods,the basic characteristics of CO₂ addition and dilution on CH₄ combustion flames are deeply explored.influences.The main research contents and conclusions are as follows:（1）Using sensitivity analysis method（SA）and directed relation graph with error propagation（DRGEP）to simplify the detailed mechanism of CO₂ diluting CH₄/O₂/N₂premixed flame under high pressure environment,which through the comparison of experimental data,the existing published literature data and the numerical results of a variety of detailed chemical mechanisms has been verified,and confirming the accuracy of the simplified mechanism in this paper.（2）The effect of CO₂ dilution on the flame speed and flame extinction limit of CH₄ premixed flame is explored by using the built premixed flame test bench and numerical calculation program and the simplified mechanism obtained in this paper.The results show that CO₂ dilution reduces the concentration of intermediate free radicals（especially OH free radicals）,inhibits the propagation of chain reactions and releases heat from combustion,thereby hindering the propagation of flames.Moreover,the addition of CO₂ diluent seriously damages the stability of the flame and reduces the limit flow rate of flame blowing.Through the definition of"Change Rate of the Sensitivity Coefficient"of the flame speed,the effect of CO₂ on the flame speed sensitivity coefficient is accurately quantified,and the elementary response that CO₂ dilution has the greatest impact on the flame velocity sensitivity coefficient is not consistent with the elementary response that is most important to the flame speed.（3）A one-dimensional counterflow flame mathematical model including detailed chemical reaction mechanism and radiation heat transfer is established,and a calculation program is written in FORTRAN language;a solution strategy for analyzing the uncoupled effect of CO₂ on the flame is proposed.The effects of CO₂ dilution on the fuel side and the oxidizer side,and uncoupled effects on basic combustion characteristics such as flame extinguishing limit,flame front position,free radical distribution and flame temperature are explored.The study found that the main effects of CO₂ dilution are different,and the main effects that affect the flame temperature are also different.The fuel side CO₂ dilution radiation dominates,and the oxidation side CO₂ dilution thermodynamics dominates;regardless of the fuel side or the oxidizer side CO₂ dilution.The thermal dominates the position of the flame front and the flame extension limit.（4）Using the established one-dimensional hedging diffusion flame model and the written program,the uncoupled effect of pressure on the CO₂ dilution of the flame extinguishing limit is studied.Under different combustion conditions,the effects of the uncoupled effects of CO₂ on the flame extinguishment limit were quantitatively compared;the“The pressure factor of the extinction limit”was defined to realize the quantitative comparison of the influence of CO₂dilution on the change of the flame extinguishment limit.The results show that CO₂ has a positive effect on the flame extinguishment limit.Although the degree of influence of flame extinguishing characteristics will increase with the increase of pressure,the degree of increase is gradually reduced.This conclusion is of great significance to enrich the flameout limit theory.In addition,the chemical kinetics process under the uncoupled effect of CO₂dilution and addition is clarified,and the change law of CH₄ consumption path,OH radical generation and consumption rate under each uncoupled effect,and the heat release rate of important element reactions are obtained.（5）A two-dimensional numerical model of coaxial jet diffusion flame was constructed,and adopting the WSGGM radiation model which takes into account the radiation and absorption of radiant gas,and more accurately explore and reveal the influence of CO₂ dilution uncoupled effect on flame temperature.At the same time,it is pointed out that CO₂ addition can reduce the formation and emission of thermal NO_Xand promote NO_X.The effect of CO₂ dilution on thermal NO_X generation is mainly achieved by lowering the temperature through its thermal properties（high specific heat and low diffusion characteristics）;while the promote reduction of NO_X is mainly due to its chemical effect by participating in CO₂+CH=CO+HCO reaction which consumes more CH groups,resulting in the reduction of HCN which is the precursor of rapid NO_Xformation in the CH+N₂=HCN+H reaction.In addition,the diffusion flame experiment platform built in this paper is used to investigate the effect of CO₂ dilution and coaxial jet flame on the macro flame structure size.