Numerical study of the erosion within the pulverised-fuel mill-duct system of the Loy Yang B lignite fuelled power station

Abstract

The quality of coal from a natural source can vary significantly as it is processed from the ground. Elevated sand loadings within the raw coal can significantly influence wear due to erosion within the mill-duct system of a lignite fuelled power station. A previous study [1] by the current authors investigated the gas and particle mass flow within the mill-duct system of a real-life operating power station both numerically and experimentally. This work extends the previous study by considering the wear of the mill-duct system caused by coal and varying levels of sand under normal operating conditions. At elevated levels of sand loading, the wear significantly increased in comparison with the coal only flow. Considering varying levels of sand loadings, the erosion wear at the highest sand loading case (30% by mass) produced slightly less than three times the total wear of the 10% sand loading case. This non-linear relationship can be attributed to the re-acceleration of the heavier sand particles after particle–wall collisions. The erosion patterns found within the swirl vane regions of the mill-duct confirm the findings of other researchers in the field of particulate roping. The findings suggested that the particle ropes twist around the circumference of duct when a series of bends is encountered. These findings were evident in the erosion distribution comparing the swirl vanes in the upper and lower legs of the mill-duct system. These differing erosion distributions were attributed to the rotational motion of the secondary gas flows and the difference in bend-to-swirl vane distance of the two legs. The predicted erosion wear on the upper leg swirl vanes was greater than those of the lower leg even though the particle mass flow was biased toward the lower leg. The upper and lower leg swirl vane geometry imparts an anti-clockwise rotation to the flow while the secondary flow created by the trifurcation geometry was anti-clockwise within the lower leg and clockwise within the upper leg. Thus the anti-clockwise motion created within the lower leg geometry actually aided the swirl vane motion, minimising the wear. The upper leg swirl vanes reversed the secondary flow resulting in a greater number of particle/swirl vane collisions, leading to a higher degree of wear.