Erosion-resistant materials for critical areas of coal liquefaction and coal gasification systems

Abstract

Erosion has been encountered and has caused severe problems in a number of the prototype plants used to evaluate liquefaction and gasification processes. Analysis of the causes of these problems and of the available engineering systems solutions has shown that, in many cases, the rate of loss can be reduced to acceptable levels by careful attention to process control, component design, and materials selection. Nevertheless, some areas of recurring erosion problems are likely to remain, especially in the areas of raw material preparation and handling, and in effluent handling and disposal systems. Components such as valves and pumps are probably the most critically affected because of their importance to process control and safety, and because they are high cost items. Some areas of the associated piping, especially bends, and solids clean-up equipment such as cyclones and hydrocyclones may also be included in this category, but are more amenable to maintenance. Such critical areas can usually justify the application of the most erosion-resistant materials available, which are cermets or ceramics. These materials exhibit orders of magnitude better erosion resistance than the best metallic systems, but are also considerably more expensive and more difficult to apply. The WC-Co-base cermets have shown good performance as small inserts in valves in severe service in liquefaction plants. Laboratory studies in an erosion rig designed to simulate essential aspects of these service conditions have revealed relationships between erosion rates and materials properties such as hardness, fracture toughness, and microstructural parameters. It appears that a minimum erosion rate occurs at intermediate values of the materials properties, the reasons for which are discussed. Ceramic materials offer potentially greater erosion resistance and reduced costs, especially for larger sizes, compared to the WC-Co cermets. Rig test results confirm the potential for reduced erosion, but illustrate the profound effects of microstructure on the erosion behavior of a given ceramic. This has significant implications for both the manufacture of components and for techniques used to apply ceramics in, for instance, critical areas of valve and pump internals. Essentially, candidate ceramics exhibited one of two types of erosion behavior in the rig tests; ceramics within each group were found to show some relationship between hardness and erosion resistance. In practice, the application of ceramic materials will require revised handling and possibly operating procedures, and may require redesign of components to accommodate their low fracture toughness.