Numerical Investigation On Unsteady Evolution Behavior Of High-alkali Coal Deposition Structure And Heat Transfer Characteristics

Burning high-alkali content coals frequently causes severe fouling and slagging problems,which greatly threat the efficiency and safety of boilers.The deposit structure and heat transfer characteristics of high-alkali coal ash are the important factors affecting the adhesion of coal ash.In this paper,the numerical simulation method is applied to verify the deposition characteristics on single tube of high-alkali coal ash under different working conditions,and predicts the deposition characteristics on the superheater tube row.The main contents are listed as follows:In order to investigate the process of deposit of the high-alkali ash,a set of models including fly ash particle transport models(inertial force,turbulent eddy effect,thermophoretic force),porosity model,emissivity model,thermal conductivity model,adhesion and erosion model and deposit growth model have been integrated.And considering that the growth of the deposit layer will affect the flow field and temperature field,a dynamic grid model is introduced.In the model verification and sensitivity analysis,it is verified that the sharp drop of the surface emissivity of the deposit is one of the reasons for the sharp drop of the heat flux and the sharp increasement of the surface temperature in the initial stage of deposit.When the gas velocity is low,the melt model and the high-alkali model are precise enough;when the gas velocity is high,the high-alkali model is more precise.In order to compare the thermal conductivity of deposit predicting by thermal conductivity model and experimental result,A thermal conductivity model is integrated.And results showed that the two have a high degree of consistency.Based on above,this paper discussed the influence of dynamic mesh on flow field and temperature field.It was found that the growth of deposit would increase the gas velocity and the impaction rate of subsequent incoming particles.Therefore,the erosivity of particle rises and the probability of adhesion increases.Finally,the dynamic simulation method established above was applied to investigate the fouling on superheater tube row and predicts the deposition characteristics of tube row in the initial stage.The deposition height of first tube row is highest,and the second one is smallest;the morphology of each tube row is predicted,and the reasons for formation of the deposit morphology are analyzed.As a result,the deposition of first tube row is mainly concentrated on the stagnation point,the deposition of the rest of tube row are mainly distributed on both sides.