Experimental And Numerical Studies Of The Structure Of Laminar Coflow CO/H₂ Diffusion Flames In O₂/H₂O

Under the background of the increasingly serious global warming,CO₂ emission reduction has become an inevitable environmental problem.Zero-emission combustion technology is one of the most promising means to achieve high-efficiency and low-emission utilization of fossil fuels.The research group of the author proposed Oxy-coal combustion steam system of near-zero emissions?OCCSS?.In OCCSS,raw coal is gasificated or demineralizated o obtain syngas or ultra-clean coal,and these clean fuels combust with pure oxygen in the combustion chamber and steam is used to moderate the flame temperature,resulting in flue gas of high temperature and high pressure.After combustion,the flue gas is directly delivered to a advanced turbine to drive it and generate power.One of the merit of OCCSS is that CO₂ can be captured economically by simplely condensating the flue gas,and the application of the OCCSS system is expected to achieve high-efficiency utilization of coal and achieve zero emissions from pollution during power generation.The combustion system in OCCSS is characterized with high temperature,high pressure,high oxygen,high steam concentration.To better understand the combustion process under OCCSS related comditions,the structure of laminar coflow CO/H2 diffusion flames burning in O₂/H₂O was investigated in this study.Experimentally,the high-temperature coflow diffusion flame burner is designed and the experimental set-up is modified to acheieve the stable coflow diffusion flame.Luminescence images and OH*-chemiluminescence images and were captured using an intensified CCD and ICCD camera,respectively,to determine the flame shape.Temperature along the flame centerline were measured unsing a Type-B thermocouple.Numerically,NRC 2D flame code was improved using conjugate heat transfer theory to condsider the preaheating effect.Using this improved flame code,which employs incorporating detailed reaction mechanism,the preheating effect,non-grey radiation heat transfer,and complex thermal and transport properties,laminar coflow diffusion flames were modelled based on very fine grids.The influence of preheating effect and burner geometry on modelling laminar coflow CH4 diffusion flames were first investigated,compared with the results of carbonhydrogen/air diffusion flames in previous studies,the improved flame code in this study is validated to be more accurate.The characteristics of syngas and CH4 were studied and compared.Under the combustion conditions of this paper,placing the inlet boundary conditions 5.08 mm upstream above the burner exit is found enough to accurately simulate of the laminar coflow diffusion flames.The numerical method to simulate laminar coflow syngas diffusion flames in O₂/H₂O were determined.Based on the current experimental and numerical studies,effects of syngas compositions and oxidizer compositions on flame structure including flame attachment,OH formation and oxidation pathway were studied.The main findings can be summarized as follows:Under O₂/H₂O atmosphere,as the H2 content in syngas increases,the laminar coflow syngas diffusion flames attach to the fuel tube more deeply,namely,H2 promotes the attachment of syngas diffusion flames.Numerical analyses show that the enhancing effect of H2 on flame attachment is attributed the high reactivity of H2 through reaction OH + H2 = H + H2 O,the high diffusivity of H2,as well as chain-branching reactions involving H.OH formation decreases with increasing the H2 content in syngas when the flames burn in the oxidizer consisting of 25%O₂-75%H2O.This behavior is in contrast to the increasing OH mole fraction with increasing the H2 content when the syngas fuel burns in the oxidizer consisting of N2/O₂.Chemical reactions take place inside the fuel tube especially in the vicinity of the burner wall as a result of the preheating effect and the back-diffusion of active radicals.Syngas pyrolysis below the burner exit mainly includes CO oxidation reactions as well as HCO and HO₂ related reactions;however,H2 oxidation reactions hardly occur inside the fuel tube.When the O₂/H₂O ratio varies,the simultaneous changes of the O₂ concentration and the H2 O concentration make it very diffcult to study the influence of the oxidizer composition on the laminar coflow syngas diffusion flames.For this purpose,the effect of H2 O in the ozidizer on the structure of laminar coflow syngas diffusion flames is investigated by replacing N2 in air with H2O?CO₂?.The results show that the H2 O dilution mainly reduces the temperature below the flame tip,while the CO₂ dilution mainly reduces the temperature above the flame tip.The H2 O diluted ozidizer reduces the flame height while the CO₂ diluted ozidizer lengthens the laminar coflow syngas diffusion flame.With the increase of H2 O dilution,the OH production in the synthesis gas flame first increased and then decreased,and the peak OH mole fraction occurs with a 30% H2 O dilution ratio.With the increase of CO₂ dilution,the OH mole fraction is decreased monotonously.In the O₂/H₂O atmosphere,the effect of H2 O on OH formation in laminar coflow syngas diffusion flames is also non-monotonic,and the turning point occurs with a oxidizer composition of 10%H2O-90%O₂.The O₂ concentration in the O₂/H₂O oxidizer has little effect on the depth of the syngas flame attaches to the fuel tube,but it enhances the temperature of the burner tip significantly.In addition,the enhanced the flame temperature promotes the diffusion of H near the flame sheet into the fuel tube and accelerates the fuel pyrolsis.The mainchemical reactions for the fuel pyrolsis in the fuel tube is that the production and consumption process of HCO is promoted.Based on the above foundational research work,suggestions were made for industrial application research and technology development of the OCCSS system in the future..