Indices For Predicting Slagging And Comprehensive Simulation Of Ash Deposition In Coal Combustion

In the process of coal combustion, the slagging and fouling on the water wall and heated surfaces are the serious problems for the pulverized coal fired boiler's efficient and safety operation. The prediction of slagging and fouling is significant for optimization for the coal type and operating conditions of the plant boiler. This paper focuses on the two basic methods to predict slagging (indices for predicting slagging and numerical simulation).At first, the paper proposes a method to predict the deposition characteristic of the coal with the coal ash viscosity calculated, on the basis of summarizing the different calculation models for the ash viscosity, Viscosity less than 10~4P is calculated by Kalmanovitch viscosity model, viscosity greater than 10~4P is calculated by Senior & Srinviasachar viscosity model. According the critical viscosity in the deposition process, the criterion for the coal ash viscosity to predict the slagging is that: if the coal ash viscosity is greater than 10~5 P, the coal is slight deposited; if the coal ash viscosity lies between 10~4P and 10~5 P, the deposition of coal ash is medium; if the coal ash viscosity is less than 10~4P, the deposition of coal ash is serious. This method is used to predict the slagging characteristic of 10 kinds of coals including Indonesian coal, Yitai coal etc. The result is more accurate than the other indices.The paper then set up a comprehensive numerical model for the process of slagging and fouling during the coal combustion, including three main sub-processes: the formation of fly ash, transportation and deposition. It uses some models including full coalescence ash formation model, partial coalescence ash formation model, stochastic separated flow transportation model, particle cloud transportation model, collision model based on the critical speed of the fly ash, adhesion model based on the critical viscosity. The numerical simulation based on the comprehensive model is carried out on the pulverized coal fired furnace built by the Imperial Institute of Technology. The result of the simulation can predict the slagging in the pulverized coal fired furnace reasonable. This paper compares the impact of different ash formation models and transportation models on the ash deposition. The result shows that: For the experimental coal, the size of fly ash formed by the partial coalescence model is smaller than the size of fly ash formed by the full coalescence model. when stochastic separated flow model is used to simulate the fly ash transportation, the difference of the ash deposition caused by different fly ash formation model is relatively small; the particle cloud model is more efficient than the stochastic separated flow model, but the particle cloud is lack of consideration for the transportation of the small particle near the wall, so there is a difference in the number and size distribution of particles deposited on the wall calculated by the two transportation model. The key results of deposition rate and composition distribution calculated by the two models are similar.