Effects of Coal Type on Growth and Shedding of Ash Deposit in Pulverized Coal Combustors

Abstract

Removal of ash deposits formed on boiler tubes or furnace walls is required to inhibit the slagging and fouling problems. The effect of coal type on the growth and gravity shedding of ash deposits was evaluated by ash deposition experiments and scanning electron microscope (SEM) analysis of the cross-sectional structures of the ash deposits. Three types of pulverized coal—bituminous, subbituminous, and lignite coals—were tested in the experiments. The behavior of ash deposition on a water-cooled tube inserted at an inner furnace temperature of 1673 K was evaluated by video camera observation, temporal variation of the tube surface temperature, and chemical equilibrium calculations. For the subbituminous and lignite coals, the cycle periods of ash shedding were longer than that of the bituminous coal because of the formation of a wider ash deposit. The distributions of particle packing fraction (PPF), the particle sizes, and the chemical compositions of the ash particles within the deposits were analyzed by an SEM and a computer-controlled SEM. A powder layer was formed near the tube, and the PPF in the deposit gradually increased with the growth of the deposit because of ash coalescence. The effects of the PPF on the ash shedding were larger than that of the chemical compositions. Therefore, the deposit detachment occurred within the powder layer that became low strength due to the low PPF. Additionally, the thickness of the residual deposit that remained on the tube after ash shedding depended on the thickness of the powder layer in the deposit. The subbituminous and lignite coals have formed a deposit with a thin powder layer because ash coalescence progressed near the tube surface. Therefore, their residual deposits have been thin compared with the bituminous coal. The findings of this study can be put to practical use in the design of pulverized coal-fired boilers.