| Burning high-alkali content coals frequently causes severe fouling and slagging problems,which greatly threat the efficiency and safety of boilers.In this thesis,the fouling of high-alkali coal ash on single tube and superheater tube row / flat plate structure is predicted by numerical simulation.The main contents are listed as follows:In order to investigate the fouling of the high-alkali ash,a set of numerical models including fly ash particle transport models(inertial force,turbulent eddy effect,thermophoretic force),adhesion and erosion model and deposit growth model have been integrated.To consider the effect of molten alkali surface on the ash adhesion,the condensation of alkali vapor on the ash particles has been included in this work.Moreover,for better estimating the high-alkali ash erosion degree,a new deposit erosion model based on the energy dissipation at impaction and the alkali surface energy has been deduced.For validating the developed high-alkali ash deposition model,single tube ash deposition on the field test(Re=3935)and the experiment test(Re=304)conditions have been individually predicted.The results show that at high flue gas flow velocity,the erosion of the large ash particles is serious due to the large impaction velocity of these particles.Therefore,the windward deposit consists most of the small particle size ash particles(<10μm).Due to the relative high leeward impaction efficiency of sticky small particles,obvious leeward deposition has been observed.Compared with the corresponding experimental results,the numerical model can obtain proper prediction results for the fouling characteristics of high-alkali ash at tube windward and leeward surface.Then the fouling characteristics of the high-alkali ash on the superheater have been predicted.The results show that the overall average deposition efficiency of both the inline tubes and the staggered tubes increases with the decrease of the flue gas velocity.Fouling on the third to the sixth row is relatively serious,and this area needs more sootblowing treatment.Most parts in the furnace are at a high flue gas flow velocity,the staggered arrangement which possesses higher erosion efficiency has a lower fouling tendency than the inline arrangement.In combustion and fouling simulation of large furnace,the heat exchanging area is always simplified to plane structure to simplify the calculation and make the iteration easier to converge.Therefore,the corresponding high-alkali ash fouling under the corresponding plane structure is simulated and predicted,and the corresponding relationship with the deposition efficiency of the row structure is established to correct the fouling prediction results of the large furnace heat exchanger.Then,the average deposition efficiency of the plane and the tube row structure is fitted with the change of the flue gas flow velocity and the longitudinal pitch.The fitting prediction formulas with high correlation coefficients have been employed to optimize the tube row structural design and predict ash deposition efficiency on the superheater region with non-uniform flue gas velocity. |
