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Why does power generation decrease as gas cools in a coal-fired MHD duct?

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magnetohydrodynamic power generator Comparison of the operating principles of (A) a turbogenerator and (B) an MHD generator.

In a coal-fired magnetohydrodynamic (MHD) generator, power generation relies on the electrical conductivity of the ionized gas (plasma) as it passes through a magnetic field.12

As the gas expands along the duct, its temperature decreases. Because the gas's electrical conductivity is directly tied to its temperature, this cooling causes the conductivity to drop significantly. Since the power output per unit volume of the channel is proportional to the gas conductivity, the reduction in temperature leads to a corresponding decrease in power generation.32

Thermal ionization is generally no longer effective once the gas temperature falls to approximately 2,500 K. To maintain efficiency, coal-fired MHD systems are typically designed as part of a binary cycle, where the hot gas is first used in the MHD generator ("topping") before being directed to a conventional steam plant for further energy extraction.3