Prediction of the Ash Deposition Characteristics of Blended Coals in a 500 MWe Tangentially Fired Boiler

A computational simulation was performed to predict ash deposition in an industrial scale coal-fired boiler. The single-rate model and the improved nth order model were employed for devolatilization and char reaction with the model constants obtained from PC Coal LAB.(1) Ash particles were deposited on the wall by inertial impaction with the capture efficiency in terms of the estimated viscosity. The models for char combustion and ash deposition were implemented as new user defined functions in ANSYS-FLUENT.(2) The results showed that the deposition of blended coals was in good agreement with measurements under the influence of the surrounding flow field in a pilot scale furnace. The same combustion and ash deposition models were applied to the 500 MWe tangentially fired boiler burning blended coals. Ash deposition results for the boiler showed the same tendency as the measured data in terms of the wall fouling resistance. A larger deposition was predicted at the heat exchangers for sub-bituminous coal than for bituminous coal. However, there was no significant influence of the coal types in the furnace region. A parametric investigation was performed on ash deposition with respect to injection location and tilt angle of the burners. Deposition in the ash hopper was affected by the injection location but not by the coal types. The results showed a maximum ash deposition at the heat exchanger section for sub-bituminous coal from the lower burner set. Ash deposition increased in the ash hopper for the downward tilt angle of burners, whereas the maximum deposition occurred at the heat exchangers for the upward tilt angle.