ABSTRACT

The Department of Energy (DOE), National Energy Technology Center (NETL), has initiated a strategic plan for the development of advanced technologies needed to design and build fossil fuel plants with very high efficiency and environmental performance. These plants, referred to as "Vision 21" by DOE, will produce electricity, chemicals, fuels, or a combination of these products, and possibly secondary products such as steam/heat for industrial use. Certain key components have been indentified as necessary for the success of a Vision 21 power plant, one of which is a high temperature heat exchanger. Thus the DOE has funded a project with the objective being to develop/produce an oxide dispersion strengthened (ODS) heat exchanger tube such that a full scale heat exchanger can be manufactured and the alloy MA956 has been chosen for the material in this study. The major tasks related to this objective are: (a) increasing the circumferential strength of a MA956 tube, (b) joining of the MA956 tube, (c) determining the tube bending limits of the MA956 alloy, and (d) determining the high temperature corrosion limits of the MA956 alloy in expected Vision 2 i power plant environments. Although all of these tasks are critical to the success of this project, this paper will only discuss the strength and corrosion properties of the MA956 alloy and work being performed in these two areas.

INTRODUCTION

Interest in increasing the efficiency of coal-fired power plants has led to the examination of alternatives to the steam boiler-Rankine cycle systems, for which increases in efficiency have been limited by both the slow progress in developing steam handling capabilities at temperatures above 565°C (1050°F) and the lack of easily-accessible sources of naturally-occurring low-temperature cooling water. Indirect-firing of gas turbines in open or closed cycles is one approach to linking the higher efficiencies possible via the Brayton cycle while still using coal as the fuel. An experimental program in the 1980's 1 demonstrated a coal-fired, low-emissions heat exchanger (fluidized-bed combustor) capable of heating air to 843°C (1550°F) in a metallic heat exchanger, and to 954°C (1750°F) or 1232°C (2250°F) with an additional ceramic heat exchanger. Current programs involving indirectly-fired gas turbine cycles are aimed at high cycle efficiencies, of the order of 47 percent based on the higher heating value (HHV) of the fuel, and involve open cycle systems in which air is heated to 760°C (1400°F) in a metallic heat exchanger, followed by further heating to 982°C (1800°F) in a natural gas-fired ceramic heat exchanger 24. A variant of this approach is one where part of the coal is pyrolyzed to produce the fuel gas used to fire the ceramic heat exchanger or the turbine with air entering the turbine heated to 1288°C (2350°F). Another program envisions using a coal-fired ceramic heat exchanger for the whole duty of heating air to 1200°C (2192°F) 5.

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