Research On The Mechanisms Of Fine Particulates Formation In The Early Stage Of Coal Combustion

Fine particulate matter is one of the major pollutants emitted from coal combustion.The early stage of coal combustion is of key importance in particulate matter formation.The transformation of evaporative minerals and the secondary reaction of heavy molecular volatile are on the same time scale.As a result,both the mineral and carbonaceous components have significant effect on the particulate matter formation in this early stage.Many scientific questions remain to be solved among this dynamic process of the multicomponent system.In this thesis,the formation and evolution of fine particulate matter in the early combustion stage were investigated by in-flame particle sampling analysis and population balance Monte Carlo simulation.Emphasis was given to the control mechanisms during the evaporation-condensation process of minerals,as well as the interaction between sodium and soot.The effect of the interaction between the components on particulate matter formation were elucidated.Firstly,the vaporization-condensation process and the elemental enrichment phenomena during mineral particle formation in the early combustion stage were studied using a flat-flame reactor and a particle sampling system.At low ambient temperature,gas-phase release was the control step of mineral particle formation,as increasing the ambient temperature led to a higher ultrafine particle yield.At above 1500 K,gas-toparticle conversion became the control step of mineral particle formation,as increasing the ambient temperature conversely reduced the ultrafine particle yield.The enrichment phenomena of sodium,magnesium and sulphur in particles smaller than 0.3 μm were controlled by homogeneous condensation,while the enrichment phenomena between 0.3 μm and 2 μm were controlled by surface reaction.Secondly,the effect of sodium-soot interaction on fine particle formation was investigated by sodium addition and particle sampling analysis.In the flame environment,vaporized sodium reacted with heavy molecular volatile to form sodium-doped soot particles.With the increase in the sodium concentration of coal,the doped particle yield decreased first and then increased.The sodium doping resulted in a disordered nanostructures of soot,as observed by both HRTEM and Raman spectroscopy.The sodium content showed a significant catalytic effect on soot oxidation.This catalytic effect saturated at a sodium concentration of 2.3%,while the activation energy of soot oxidation was lowered by 25%.The sodium-soot interaction reduced the ultrafine particle yield in the early combustion stage.Catalyzing soot oxidation and eliminating ultrafine mineral particles were the two main mechanisms by which the sodium-soot interaction affected the formation and evolution of particles.Finally,in the light of the interaction between the mineral and carbonaceous components,a grouped Monte-Carlo method for multicomponent particles was represented,which featured the computational domain discretization and the two-step stochastic process.Numerical simulation of fine particulate matter in the early combustion stage was conducted by considering the dynamics of sodium transformation,devolatilization,nucleation,coagulation,catalytic oxidation etc.The fine particulate matter formation characteristics with the sodium-soot interaction was obtained.The effect of the coal properties and the combustion condition on the fine particulate matter yield was clarified.The relative size of the coagulating objects was shown to play a key role in the evolution of particle size-composition distribution.The simulation results further revealed the interaction mechanisms between sodium and soot in the early stage of coal combustion.