Experimental And Numerical Simulation Of Fly Ash Deposition Of High Alkali Coal

In Chinese energy system,high-alkali coal represented by Zhundong coal has the advantages of low ignition point,high heating value and low ash content.The estimated reserves are as high as 390billion tons,which is an important energy reserve.However,the alkali metals such as Na and K that are enriched result in severe deposition phenomenon of the heated area during the combustion of high-alkali coal,restricting the use and promotion of high-alkali coal.In this context,this paper has carried out experimental research and numerical simulation of the high-alkali coal fly ash particle deposition process.The effects of operating parameters such as flue gas flow rate,particle size,flue gas temperature and wall temperature on the collection rate of fly ash particles,ash expansion angle and ash thickness are studied.The ash accumulation experiment of high-alkali coal was carried out on a small fluidized bed test bench,and the ash accumulation characteristics under different working conditions were obtained through sampling pipe.The results of the experiment found that the ash accumulation characteristics of potassium silicate particles,a coal ash mold compound,are similar to those of ash deposition;fly ash particle collection rate,ash spreading angle,and maximum ash thickness are reduced with the increase of flue gas flow rate and particle size,and increased with the increase of flue gas/wall temperature difference;the ash deposition characteristic parameters obtained from the combustion of Zhundong coal under oxy-fuel combustion conditions increase with the increase of the O₂/CO₂ ratio,but they are all lower than the air combustion atmosphere;the deposition characteristics of potassium silicate particles are generally close to the fouling characteristics of O₂/CO₂=21/79 oxy-fuel combustion conditions.In the aspect of numerical simulation,a viscous particle collision and adhesion model based on the CFD-DEM method was built,and the ash accumulation process of the ash accumulation section of the small fluidized bed experimental platform was calculated.The simulation results showed that the fouling is formed on the windward side of the sampling tube,and its geometric shape has high symmetry,and the thickness of the fouling is high in the center along the axial direction of the tube wall and low at both ends;as the flue gas flow rate and particle size increase,the collision rate of fly ash particles increases,and the adhesion rate and capture rate decrease;the influence of flue gas temperature and wall temperature on the collision rate and capture rate of large particles is not obvious;the collision rate and adhesion rate of small particles increase with the increase of flue gas temperature and wall temperature,and the increase in adhesion rate is more obvious;the influence law of the flue gas flow rate and other working conditions parameters on the fouling characteristics obtained by the numerical simulation is close to the results of the verification experiment.In order to analyze the microstructure of the formed ash deposit,the collision and adhesion characteristics of fly ash particles on different walls,and the wear of fly ash particles on the existing ash deposit,a numerical simulation of the ash deposit process under the flow field around a cylinder is carried out.The simulation results show that the mass proportion of small and medium-sized particles in the ash deposit has increased compared to the entrance,while the proportion of large-sized particles has decreased;as the thickness of the ash deposit increases,the average particle size of fly ash particles in the ash deposit increases,and the structure becomes looser;on a smooth wall,the collision frequency of particles and the relative normal velocity during collision increase with the increase of particle size,and decrease with the increase of the circumferential angle corresponding to the collision position;the tangential relative velocity during collision increases with the increase of the circumferential angle corresponding to the collision position,but it has no obvious relationship with the particle size;on the existing ash accumulation wall,the collision frequency distribution of fly ash particles has not changed significantly compared to the smooth wall,but the particle capture rate has been significantly improved;the wear rate of fly ash particles to the existing ash increases with the increase of the flue gas flow rate and the circumferential angle corresponding to the collision position,but the wear rate of the existing ash on the wall generally decreases with the increase of the circumferential angle.