Emerging power technologies and oxide scale spallation

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Abstract
There is now a shift towards combined cycle for large scale power generation. In these plants, the main spallation issue will be that of combustor can alloys. Units using reheat will add a new dimension to this problem. Spallation of highly cooled turbine blading as a possible issue should not be neglected, however. Combined cycle will be linked in the longer term to coal gasification processes. Where the gasifiers are of the entrained or fluidised bed type, HC1 induced spallation of downstream heat exchangers is a neglected aspect of much experimental work. This form of attack is likely to occur over a wide range of temperatures. In the UK and Northern Europe CHP (Combined Heat and Power), using natural gas as a fuel, will take an increasing share of the market. The most advanced concepts will use recuperative gas turbines, as these have many advantages at low to medium outputs. Again, scale spallation will be an important design consideration. Household or Domestic CHP could well be a new entry into the field of environmentally beneficial, localised power generation. Many of the prime movers under consideration for this sector will require a high temperature air preheater. Here the need to avoid excessive oxidations and scale spallation will be paramount. The paper calls for more efforts on alloys other than chromia formers; alumina formers offer much better high temperature behaviour, apart from their spallation behaviour. There is also a strong economic case for work on alloys reliant on silica. Although these are normally viewed as having poor spallation resistance, they are used in quantity by the Automotive Industry in cyclic applications. A brief review is given of spallation and related phenomena on alumina forming ODS alloys intended for advanced indirect fired gas turbine cycles. Emphasis is given to the statistically variable nature of spallation, and to the fact that, in practice, spallation failures involve more than one mechanism.

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