Mechanistic Studies On The Formation Of Fly Ash Particles In Full Sizes During Pulverized Coal Combustion

Fly ash particles generated during pulverized coal combustion are not only the main source of particulate matter（PM）emissions from coal-fired power plants,but also an hidden danger causing severe slagging and fouling problems in coal boilers.The practice shows that the massive usage of high-alkali coals in recent years tends to arouse more serious slagging/fouling in boilers.With the intelligent upgrading and flexibility promotion of coal plants,a deep understanding of the fly ash formation process is of great significance to prevent the ash/slag on heat exchangers and control the PM emissions under variable loads.Based on the above background,this paper conducts mechanistic studies on the formation of fly ash in full sizes during pulverized coal combustion,aiming to provide key theoretical support for the combustion optimization and ash/slag control in coal plants.Firstly,the progressive formation of fine PM₁₀ during the combustion of high-alkali Dananhu coal with a Na content of～6%was investigated by using the Tsinghua 25 k W one-dimensional（1-D）furnace.Through analyzing the properties of PM samples from different sampling ports,the mass particle size distribution（PSD）of PM₁₀ and size partitioning of mineral elements at volatile combustion,char combustion,and char burnout were obtained.The role of alkali and alkaline earth metals（AAEMs）in the formation of ultrafine PM_0.1,submicron PM_0.1-1,and supermicron PM_1-10 in different coal combustion stages were intensively explored.Based on the experimental phenomenon of the massive existence of Na in ultrafine PM during the volatile combustion of high-alkali coal,we selected Zhundong coal with a higher Na content to study the ultrafine PM formation in the early coal combustion stage using an optically-accessible multi-element flat-flame burner（termed as Hencken burner）.With a time-scale analysis,a consistency on the characteristic time was discovered among coal devolatilization,Na phase transition,and ultrafine particle formation.Analogized with the multi-component metal flame synthesis process,the ultrafine PM formation in the early stage of coal combustion was interpreted by revealing a new"devolatilization-nucleation-coagulation"mechanism.Further,a prediction model based on the population balance theory was established for ultrafine PM formation.Besides,observations on the fragmentation characteristics during combustion were conducted on different coals.The main features of the coal fragmentation in different combustion stages were summarized.Based on the idea of hard-sphere model（HSM）,a three-dimensional（3-D）model close to the real transformation from included minerals to fly ash particles during coal combustion was developed.A sequence fragmentation model and a perimeter fragmentation model were applied to describe the effects of fragmentation during coal devolatilization and during char combustion on the transformation of included minerals,respectively.The calculation results were demonstrated that the fragmentation of Zhundong coal during devolatilization significantly increases the fraction of PM_2-10,and ultimately leads to the mass PSD of coarse ash particles presenting bi-modal shape.Finally,this paper developed a mechanistic model for predicting the formation of fly ash particles in all sizes during pulverized coal combustion.The model successfully predicted the mass PSDs of fly ash from traditional high-ash-fusion（HAF）bituminous,high-sodium Zhundong coal,and high-sodium Dananhu coal in the combustion experiments of 1-D furnace.With the model sensitivity analysis,the size fractions of fly ash particles contributed by mineral precursors,mineral fragmentation,included minerals,and excluded minerals were classified.The effects of particle coagulation,coal fineness,and condition variations on fly ash formation were quantitatively measured.