The erosion of candidate materials for valves in coal gasification systems

Abstract

The erosion of ashlock hopper valves in coal gasification systems and the maintenance downtime resulting from this erosion are a cause of considerable concern for the coal conversion industry. In order to alleviate this problem a test rig was designed and built to simulate service conditions of temperature, pressure differential and valve geometry. The macro- and microscopic appearance of the worn surfaces closely resembled that of the valves themselves. The fine ash, moving at sonic velocities, caused rapid erosion of metallic alloys and the wide range of hardness values obtained in heat-treated steels did not influence their rate of erosion. Numerous coated and surface-treated alloys, hard cermets and ceramic materials have been tested and ranked according to erosion resistance. The eroded surfaces and cross-sections have been closely studied in order to detect the extent of plastic flow, microfracture and thermal degradation. Surface treatments such as carburizing of steels provided only a small improvement in resistance; cobalt, nickel and Co-Cr-based cemented carbides together with sialon-type ceramics provided good resistance. Low binder contents and small carbide particle size were found to improve the erosion resistance of the cermets. The paper will discuss the performance and erosion mode of the various materials in terms of their mechanical and thermal properties.