Metallurgical problems and opportunities in coal-fired steam power plants

Abstract

External constraints on the generation of electricity in steam power plants that determine metallurgical research and development include 1) the source of fuel, 2) environmental controls, 3) the economy of scale, 4) efficiency of energy conversion, and 5) reliability of the power generating equipment. These are reviewed in the current time frame. Economic and political factors dictate that coal and nuclear fission are the major fuels for generation of electricity until the end of the century. Environmental constraints on SO2 emission dominate a great deal of current materials research and development. Only a small percentage of available coal is low enough in sulfur to meet new source standards. Conversion of coal to a clean gaseous, liquid, or solid fuel is replete with difficult metallurgical problems, chiefly in high temperature corrosion. To compete with stack gas desulfurization it is necessary to burn the clean converted coal in more thermally efficient combined cycles, which requires development of high temperature industrial gas turbines. The fabrication problems of advanced air or water cooling of metal hot components are traded off against the brittle design problems of ceramic components. Removal of sulfur in the combustion stage in fluidized beds containing an SO2 acceptor has many attractive features, but potential problems exist in hot corrosion of the in-bed tubes and erosioncorrosion-fouling of expander turbines. The economy of scale has increased power plant size to about 1200 MW, where further increases seem to have stalled because of metallurgical problems. Improvements in thermal efficiency from high steam temperatures and pressures also have stalled, and even retreated, because of metallurgical barriers. The main current effort in materials research and development is aimed at improved reliability rather than efficiency. Two metallurgical opportunities to improve reliability are described in detail: titanium low pressure turbine blades and large rotor forgings.