I recently ran into a web site discussing the use of mercury vapor as a working fluid in power plants. I felt this deserved a discussion along with an equally interesting technology - magnetohydrodynamic (MHD) generation. Both represent attempts to increase power generation efficiency long before widespread awareness of the necessity for energy resource conservation came about.
Mercury Turbines
Several commercial power plants utilizing a mercury topping cycle were built in the early 20th century. The advantage was that it provided a higher inlet temperature to the turbines, increasing efficiency. Because of limits on boiler construction, the temperature to which steam could be made was limited as the pressure became too great. Mercury vapor at a given temperature produces less pressure than water vapor, hence the advantage.
Once the mercury was expanded in the turbine, it was cooled in a steam boiler and the steam produced powered an additional turbine in a combined-cycle configuration, leading to quite efficient power generation.
Today, metallurgical advances have made possible boilers capable of withstanding the extreme pressures of steam at high temperature, allowing inlet temperatures to the turbines of well over 500°C in some cases, negating any and all advantages of mercury.
It can only be imagined what a nightmare mercury vapor flowing through a power plant at high temperature and pressure would be. As Douglas Self of the Museum of Retro Technology states, there are good reasons that the vast majority of heat engines today utilize either air or water as their working fluids.
These appear to have been abandoned due to the fact that natural gas (or gasified coal) combined cycle with a gas turbine is cheaper and more efficient. I think we can be happy about this.
MHD
MHD (magnetohydrodynamic) methods of electricity production were also tried in the mid 20th century. The extremely hot gases (plasma) from coal combustion are "seeded" with alkali metal salts which become ionized due to the temperature. Being conductive, the plasma is expanded through a nozzle in the presence of a magnetic field. Electrodes on the sides of the nozzle allow a direct current to be produced. This is inverted and delivered to the grid.
The MHD generators themselves are not terribly efficient, but they once again have the advantage of being a topping cycle - utilizing the extreme temperatures of combustion before producing steam to drive a traditional power plant. MHD also has no moving parts. According to Wikipedia, Westinghouse did research on MHD in the 1950s and 1960s, as did Russia, China, and others later on, but as of today there do not appear to be many if any plants operating. Most likely, coal is simply too cheap to counter the maintenance issues with MHD (fly ash and sulfurous gas at 5000 degrees Farenheit blasting through the thing at Mach 1.2 is, well, a bit demanding on maintenance to say the least! ) - to get more electrical capacity, more steam-only cycles are added with the accompanying increase in coal burn and streams of usable heat dumped to the environment accepted due to economics.
Burning the coal at extremely high temperatures (in excess of 2000°C) and pressure (several atmospheres) with pure oxygen and using an MHD topping cycle and steam bottoming cycle may be an efficient method of capturing carbon dioxide and greatly cutting nitrogen oxide emissions while generating large amounts of electricity (and little by-product heat). This would have to compete with IGCC (gasification) and other energy sources (renewable, nuclear).