Combined MHD-Steam Power Plant

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 of the generator.