Lessons from the past: materials-related issues in an ultra-supercritical boiler at Eddystone plant

Abstract

Electricity fuels the engine of technological progress, and so there is little dispute that the manner in which electrical power is generated in the coming years will affect all of our lives in ways both obvious and subtle. There will be economic effects that will dictate, for example, how much it costs to heat homes and run computers, and there will be environmental effects that will influence the quality of the resources available to sustain life. A dispute arises, however, in determining how best the power can be produced, since the relative sensitivity to the potential effects, be they economic or environmental, varies substantially among the many interested parties.Regardless of sensitivities, there is broad agreement that electric power must remain inexpensive, both to maintain the extraordinary standard of living achieved by the materially developed nations and, at least as importantly, to sustain the progress of developing nations toward equitable levels of prosperity. With abundant, inexpensive power as a primary goal for future generators, certain conclusions inevitably follow. Among these is the fact that for many parts of the world, including both the US and China, coal will remain an essential fuel in the production of electric power because it is plentiful, relatively cheap, and its supply secure.The reference to China is particularly significant, because China is in the midst of a massive construction effort that will add tens of thousands of megawatts of new coal-fired generating capacity in the next ten years, and there is every reason to expect that other developing countries will soon follow suit. There is enormous potential, therefore, to minimize the impact of this massive influx of capacity on the global environment, through an improvement in the overall efficiency of coal-fired boilers.The task for the worldwide power industry, therefore, is clear: improve the efficiency of coal-fired power plants to levels consistent with the best existing engineering technology. But what are those levels? The existing fleet of sub-critical coal-fired steam plants typically operate at efficiencies in the range of 32–34% (based on HHV). Recent units built in Europe and Japan have been designed to run at steam conditions that achieve a significant improvement in efficiency, approaching 40% (HHV). Major research efforts in both the US and Europe have made substantial progress toward the development of coal-fired ultra-supercritical steam generators that will operate at efficiencies approaching 50% based on HHV. A component test facility designed to evaluate operation of major steam generator components at 700°C (1292°F) will soon be operational in Germany, and the first demonstration plant incorporating advanced ultra-supercritical technology could be constructed as soon as the year 2010.With this ambitious schedule in mind, the question posed by those who will be responsible for the operation of these new ultra-supercritical power plants is obvious: will they perform reliably under the conditions of flexible operation that are likely to be imposed by modern de-regulated markets? The real intent of this question is more subtle: do the materials and methods of construction exist to build these advanced coal-fired boilers. In large part, the answer to that question lies in a US power plant located not far from the Philadelphia airport. The plant is Exelon’s (formerly the Philadelphia Electric Company) Eddystone plant. The steam generator for Unit 1 at that plant, which was designed and built by Combustion Engineering (now ALSTOM Power, Inc.) in the 1950s, has operated successfully for more than 44 years at steam conditions more advanced than any other coal-fired unit in operation today. This paper will briefly review salient features of the operating history of Eddystone 1, focussing on the materials-related problems that forced a modest retreat from the original and unprecedented design conditions, but emphasizing, as well, the record of many years of reliable operation at very aggressive steam conditions. It will then discuss a few of the more significant materials-related issues involved in operating a steam generator at Eddystone-like conditions as a basis for arguing that there exists today the materials and manufacturing understanding necessary to construct an advanced ultra-supercritical unit that will operate efficiently and reliably.