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Mercury vapour turbine

A Mercury vapour turbine has been used, in conjunction with a steam turbine, for generating electricity. This example of combined cycle generation does not seem to have been widely adopted, probably because of high capital cost and the obvious toxic hazard of leaking mercury vapour.

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The mercury cycle offered higher efficiencies because it could put energy into the Rankine Cycle at higher temperature. Metallugical developments have allowed steam only plants to increase in efficiency without the need for mercury.

Historical example

The following is quoted from the Electrical Year Book, 1937:

The advantage of operating a mercury-vapour turbine in conjunction with a steam power plant lies in the fact that the complete cycle can be worked over a very wide range of temperature without employing any abnormal pressure. The exhaust from the mercury turbine is used to raise steam for the steam turbine. The Hartford Electric Light Co. (U.S.A.) has a 10,000kW turbo-generator driven by mercury vapour, which reaches the turbine at 70 lb. per sq. in. (gauge), 880°F. The mercury vapour is condensed at 445°F and raises 129,000 lb. steam per hr. at 280 lb. per sq. in. pressure. The latter is superheated to 735°F and passed to the steam turbines. During 4 months continuous operation, this plant averaged about 0.715 lb. of coal per kWh of net output, about 43% of the output being from the mercury turbine generator and 57% from the steam plant. On maintained full-load the heat output averages 9800 B.Th.U. per net kWh. It is believed that maintenance costs will be lower than in ordinary steam plant. The back-pressure on the mercury turbine is fixed by the steam boiler pressure; only a small vacuum pump is needed, as there is no air or other gas in the mercury system.