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The ash-related problems of fouling and slagging occur easily during high-alkali coal combustion in boilers. In this study, numerical calculations were performed to investigate the sodium migration mechanism and ash deposition on heating surfaces in a large-scale circulating fluidized bed (CFB) boiler during the burning of high-alkali coal. The calculation principle was based on a comprehensive three-dimensional CFB combustion model and a sodium migration model from our previous study. The computing object of this study was a 300 MW subcritical CFB boiler. By performing a simulation, the distributions of NaCl(g) and Na2SO4(g) along the furnace were displayed. The rates of sodium deposition via condensation, inertial impaction, and thermophoresis on the water walls and suspended panels were analyzed. Additionally, the net deposition rates of fly ash on the water walls and suspended panels were determined. Then, the tendency of fouling and slagging in different areas of the CFB furnace could be predicted and judged. Calculation results for the gas–solid flow, furnace temperature, and gaseous species agreed with the measured data.

Investigation of the release behaviors of alkali and alkaline earth metals is necessary to understand the mechanisms of ash-related problems such as fouling, slagging, and corrosion when burning fuel with high alkali contents. Using a 2-D circulating fluidized bed (CFB) combustion model combined with a sodium migration model, sodium transformation was predicted in a 30 kW CFB combustor. Simulation results from the 2-D model are in agreement with those from the 3-D model and experiments. Calculations and analysis of sodium transformation with different parameters, including furnace temperature, excess air, and secondary air ratio, were performed. The results show that a higher furnace temperature leads to a higher number of deposited sodium and ash particles on the probe surface. Increasing the amount of excess air or decreasing the secondary air ratio may reduce the amounts of deposited sodium and ash particles.