Numerical Simulation Of The Coupling Deposition Of Alkali Vapor And Fly Ash Particles

The traditional coal industry has been criticized for a long time because of its environmental pollution.Promotion of efficient and clean use of coal is an inevitable trend.Zhundong coal has the advantages of low combustion temperature,high burnout rate,low content of polluting elements(such as heavy metals,etc.)and low mining cost,which makes it in line with the development direction of clean utilization of coal,and can be used as a good power coal and clean raw materials.However,the combustion of Zhundong lignite with high content of alkali metal will produce a lot of fly ash and alkali vapor with low melting temperature.In the operation of the boiler,it will not only cause scaling on the heat transfer surface,blockage of flue gas channel and reduction of heat transfer rate,but also cause corrosion to the superheater tube.In this paper,the movement and deposition distribution of fly ash particles carried by high temperature flue gas and the condensation of alkali vapor carried by high temperature flue gas in HRSG are simulated.In Fluent,the user-defined functions are used to link the melting component model and direct condensation model of alkali vapor to simulate the dynamic deposition process of fly ash particles under different conditions,and the unsteady calculation method is used to update the deposition surface temperature.This paper mainly focuses on the analysis of the influence of alkali vapor concentration,fly ash particle size,inlet flue gas velocity,furnace temperature and different tube types for alkali vapor and fly ash particles transportation,collision,deposition distribution,heat transfer performance and the influence of resistance characteristics,and at the same time predict the impact of different stages of the main ash deposition mechanism(i.e.inertial impact,thermophoretic force and direct condensation of alkali vapor)on the deposition process.In addition to the effects of flue gas velocity,furnace temperature and tube type on the growth of sediment,the impact and adhesion behavior of particles with the growth of sediment are also studied.The results show that the collision efficiency of fly ash particles increases with the increase of the Stokes number,small and medium particles are deposited on the windward and leeward sides of the circular tube under the action of thermophoretic force and inertial collision,and the large particles are basically deposited on the windward side,as the Stokes number increases,the adhesion efficiency of the particles increases accordingly.When the flue gas flow velocity on the inlet side increases,the collision efficiency of small particles generally increases with the flue gas inlet velocity.The collision efficiency of medium and large particles increases with the increase of flue gas inlet velocity.When the furnace temperature increases,the collision efficiency of fly ash particles decreases and the adhesion efficiency increases.And the axial length ratio of the elliptical tube increases During the process,both the collision efficiency and the deposition efficiency decrease.Inertial collision is the main deposition mechanism,and the alkali vapor condensation mainly exists in the early stage of the deposition stage.The comprehensive performance of heat exchange flow is evaluated by analyzing the Nusselt number(Nu)and pressure drop(?p)obtained by numerical calculation under different working conditions.With the increase of the flue gas flow rate on the inlet side,Nu and ?p both show an increasing trend,and the comprehensive performance of the heat exchange flow of the heat exchange tube is improved.When the furnace temperature increases,Nu decreases and ?p increases,heat transfer The comprehensive performance of the heat exchange flow of the tube has decreased.The comprehensive performance of Nu,?p and heat exchange flow of the high-temperature flue gas passing over the surface of the round tube are larger than that of the elliptical tube.