If the gas entering the MHD duct at about 3000 ° C could be expanded to the ambient temperature of 30 ° C, the Carnot efficiency would have reached 90%. Unfortunately, the MHD power output is restricted because by the time the gas temperature falls to 2000 ° C the electrical conductivity becomes very low with the electrons combining with ions to form neutral atoms, and the generator then ceases to operate satisfactorily. Therefore, the MHD generator is used as a topping unit and the MHD exhaust at about 2000 ° C is utilized in raising steam to drive turbine and generate electricity in a conventional steam power
In the closed cycle scheme, helium (or argon) gas seeded with cesium is heated in a nuclear reactor, passed into the MHD duct and then into the steam generating system (Fig. 3.34). A gas turbine plant can also be used as a bottoming unit (Fig 3.35).
The material has to stand up to temperatures above 2200 ° C and the corrosive atmospheres of alkali-seeded gases.
The duct wall will also need to be an electrical insulator at these temperatures.
Materials used are magnesium oxide, strontium zirconate and hafnia. Electrodes in the dc MHD generator perform the same function as brushes in a conventional dc generator. Tungsten or carbon electrodes have been used.
Electrodes are often segmented to reduce energy losses due to Hall effect To reduce the power consumption of these electromagnets,cryogenic or superconducting coils at liquid helium temperatures have been suggested.
The only fuel which has better characteristics than coal is char, which contains almost no hydrogen and, in general, results in a 25% increase in the performance