Modeling Of Ash Formation And Its Characteristics During Pulverized Biomass Combustion

Partial or total of biomass replace of coal combustion is a feasible technology to reduce CO₂ emission rapidly,economically and efficiently.The characteristics of biomass different from coal can lead to ash related problems such as slagging,ash deposition,corrosion and particulate emission of boiler heating surface during pulverized biomass combustion,which depend on ash formation and its characteristics during combustion.Therefore,it is very important and necessary to understand and describe the ash formation process and its characteristics during pulverized biomass combustion.In this study,aiming at the model description and prediction of ash formation process and characteristics of biomass combustion,a mathematical model was established to describe the particle size distribution（PSDs）of fly ash particles during biomass combustion based on the mechanism of“fragmentation-coalescence”.Based on one-dimensional plug flow reactor model,the mechanism mathematical model of homogeneous nucleation,heterogeneous condensation and collision condensation was also established.Finally,the integrated sub-models were used to describe the evolution of ash formation process during pulverized biomass combustion,especially the characteristics of particle concentration,size and composition.The influence of main mechanism processes and interactions on ash formation process and its characteristics was studied.Based on the“fragmentation-coalescence”mechanism,a mathematical model for the particle size distribution of fly ash particles generated during pulverized biomass combustion was established and tested,and the effects of particle shape,stochastic of crushing and size fraction on the PSDs of fly ash were studied.It is found that both uniform and stochastic crushing models can well predict the PSDs of fly ash particles generated by biomass combustion with wide sieving size range.Particle shape has a great influence on the size distribution of fly ash,which reflects the importance of reasonable description of biomass powder shape and particle fragmentation.The stochastic crushing model may be closer to the reality prediction when predicting the PSDs after narrow sieving biomass powder combustion.Based on the one-dimensional plug flow model,the sub-model of alkali vapor generating submicron particles by homogeneous nucleation was established and tested,and the effects of alkali chloride vapor concentration,the flue gas cooling rate and the concentration of seed particle on the behavior of homogeneous nucleation of submicron particles were studied.It is found that the model considering Tolman effect is reasonable and accurate.The initial alkali vapor concentration is the dominant factor determining the nucleation temperature.KCl vapor begins to nucleate at around 680℃,while Na Cl vapor at around 668℃,slightly lower than KCl,which is mainly related to the physical properties of K and Na,but there is little difference.Alkali vapors concentration and flue gas cooling rate have great influence on the initial nucleation of submicron particles and the particle size and number distribution of reactor outlet.The initial vapor concentration plays a dominate role in the resulting sub-micron particle mass concentration.The concentration of seed particles in flue gas affect the nucleation behavior of alkali metal chlorides.The mean critical seed particle concentration of KCl vapor is about 9×10⁴ cm^-3.Increasing the cooling rate and reducing the concentration of alkali chlorine gas will reduce the critical concentration of seed particles inhibiting homogenous nucleation.Based on one-dimensional plug flow model and a simple kinetic mechanism model,the sulfation reaction in flue gas and its subsequent ash formation formation process were described.The effects of alkali vapor sulfation reaction,gas phase composition of flue gas and seed particles on the characteristics and composition of submicron particles were studied.It is found that the established model can be used to predict the sulfate reaction of alkali vapors and the characteristics of submicron particles.The mass concentration of submicron particles increases but the particle size decreases after sulfated.K₂SO₄ vapor begins to nucleate at about 820℃,while Na₂SO₄vapor at about 750℃,slightly lower than K₂SO₄.Gas phase component has important effects on the characteristics of submicron particles,such as mass concentration distribution and particle composition,eg.increasing the concentration of SO₂ and KCl,will increase submicron particles mass concentration,and increasing the initial concentration of SO₂ and O₂ will increase the S content in particulate matter and correspondingly decrease the Cl content;while increasing the KCl concentration will decrease the S content in the particulate matter.Seed particles promote heterogeneous condensation of alkali metal chloride and alkali sulfate gas,but have little inhibitory effect on homogeneous nucleation of alkali sulfate gas.The mathematical model was established and verified by integrating the mechanism process sub-models of ash formation and the experimental results in the literature to describe the ash formation process and behavior.The evolution of ash formation and its characteristics caused by alkali vapor ash formation and heterogeneous condensation in the ash formation process of pulverized biomass combustion were emphatically studied.The effects of biomass feeding rate and flue gas cooling rate on the nucleation characteristics of alkali vapors,the collected PM₁₀formation and particle size distribution of elements（Na,K and Cl）,as well as ash formation characteristics under the influence of actual pulverized fuel（PF）boiler combustion conditions and sulfate reaction were numerically studied.The research shows that the established model can reasonably describe the ash formation process and characteristics during pulverized biomass combustion.It is predicted that with the decrease of cooling rate,the fine mode particles size distribution of PM₁₀ and its elements（Na,K and Cl）gradually moves to the large size direction,the peak width becomes narrow but the peak concentration increases.With the increasing of the biomass feeding rate,the initial nucleation temperature and the initial nucleation particle size increase;with the flue gas cooling rate increasing,the initial nucleation number concentration increases,while the initial nucleation particle size decreases.Ash-forming elements（Na,K and Cl）are mainly concentrated in PM₁,but they are rarely distributed in PM_1-10,and their condensation amount on coarse fly ash is increased by biomass feeding rate and decreased by cooling rate.The results of ash formation characteristics obtained from the laboratory condition with ultra-high flue gas cooling rate and minimal biomass feeding rate have a large deviation compared with that of the actual biomass combustion.Sulphation can convert corrosive chlorides into low corrosive sulfate particles without necessarily reducing ash deposition.