Tribology in coal-fired power plants
Master Thesis
2005
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University of Cape Town
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Abstract
A series of alumina ceramics and silicon carbide (SiC) particulate composites were evaluated in terms of their erosive and abrasive wear behaviour under different conditions, with the aim of reducing wear damage in power plants. The alumina ceramics tested ranged in composition from 90% alumina to 97% alumina content. A nitride fired and an oxide fired SiC particulate composites were also tested for comparison. The impact angle, impact velocity, as well as particle size and type were varied for solid-partide erosion, whereas effects of the applied load, abrasive speed and type of abrasive were studied for abrasive wear. The target materials were also evaluated in terms of morphology and mechanical properties including hardness, flexural modulus and flexural strengths. The erosion rates of the tested alumina ceramics increase with an increase in the impact angle, reaching a maximum at 90°. The high purity 96% alumina dry-pressed body has the best erosion resistance at most impact angles, while the 92% alumina dry pressed body has the worst erosion resistance. The erosion rates also increased with an increase in particle impact velocity, resulting in a velocity exponent (n) value of 1.5. A decrease in the erosion rate was observed for both an increase in particle size range and a decrease in erodent partide hardness. At all angles of impact, solid partide erosion of the target materials is dominated by intergranular fracture and surfaces are typically characterized by erosion pits. The five alumina target materials also show a marked increase in erosion rates when the test temperature is increased from ambient to 150°C. The abrasive wear rates for the materials increased with both applied load and abrasive speed, owing to increased tribological stresses at the contacting asperities. There is also a general trend of increasing abrasion resistance with increasing alumina content. Severe wear, characterized by fracture and grain pullout, is the dominant mechanism of material removal during abrasive wear. This was accompanied by the formation of grooves on the wear surfaces. Although this study was successful in terms of material selection for wear damage reduction in power plants, it also highlighted significant factors and modifications that might need to be considered in future studies.
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Includes bibliographical references (pages 90-94).
Reference:
Moumakwa, D. 2005. Tribology in coal-fired power plants. University of Cape Town.