Study On The Generation Of Carbonaceous Dust Formed Via Chemical Vapor Deposition In HTGR

During the operation of high temperature gas-cooled reactors?HTGRs?,massive carbonaceous dust can be generated,which will bring serious threats to the safe operation of HTGRs.Past researches have been mainly focused on the physical generation mechanism.However,discrepancies have been observed between the predicted dust production which is mainly based on physical friction model and on-site observation.Thus,to explain the observed discrepancies and to help us better optimize the operation and design of HTGR,it is necessary to analyze the chemical generation mechanism.During the operation of HTGRs,under certain conditions,some chemical reactions will be caused due to the existence of some gaseous impurities,and then carbonaceous dust can be produced via chemical vapor deposition?CVD?.Based on CVD tube furnace and chemical kinetics model,the generation of carbonaceous dust formed via CVD in HTGRs has been studied in this thesis.In terms of experiments,the generation of carbonaceous dust formed via CVD from H₂/CO/Ar gas mixture on Inconel 617 Alloy has been mainly studied.As the study shows,after deposition,the content of C element over the surface of the substrate increases significantly and it can be verified that the solide carbon can be generated via CVD.With the increase of H₂ flow rate,the production of carbonaceous dust will increase first and then decrease,the increase of which can be attributed to CO disproportionation reaction?2CO?C+CO₂?,hydrogenation reaction?CO+H₂?C+H₂O?and H₂ reducibility,and the decrease of which can be explained by H₂ etching effect?C+2H₂?CH₄?.In addition,when reaction temperature rises,the production of carbonaceous dust will demonstrate an exponential increasing trend.As for Ar,with the increase of Ar flow rate,the dust generation will decrease gradually,but the decreasing trend slows down.When reaction time is prolonged,more dust can be generation,but the increasing trend slows down as well.Additionally,two kind of particles,large agglomerate particles and small isolated particles,can be found over the surface of the substrate.The generation mechanisms of them can be explained by carburization and precipitation mechanism,and surface catalysis mechanism.We also further analyze the particle size distribution of the small isolated particles.It can be found that the sizes of these particles formed via CVD are much smaller than the sizes of the particles formed via physical friction.Generally speaking,the sizes of the particles formed via CVD are around tens to hundreds nm,and the variation trends of the particle sizes is grossly consistent with carbonaceous dust production.In terms of numerical calculation,based on chemical kinetics,three reactions,2CO?C+CO₂,CO+H₂?C+H₂O and C+2H₂?CH₄,are mainly analyzed.A chemical kinetics model has been developed,and the chemical kinetic parameters have been obtained by fitting the experimental data.Based on our model,the generation of carbonaceous dust over steam generator in high temperature gas-cooled reactor pebble-bed module has been estimated,the production of which is around 0.5176 kg.This value is only one magnitude less than the dust generation based on physical generation model.It cannot be ignored.Also,it need to be suggested that for the control of gaseous impurities in HTGRs,it is unnecessary to reduce the contents of all impurities.A much more advisable and practical method is to control a few impurities contents,e.g.H₂,which will inhibit the generation of carbonaceous dust to some extent.In addition,researchers should pay high attention to the generation of carbonaceous dust under high temperature.This thesis analyzes the generation of carbonaceous dust formed via CVD in HTGRs both by experiments and numerical calculation,the results of which are helpful to better evaluate the production of carbonaceous dust in HTGRs and are also instructive to the design of HTGRs.