Coal Generator

Lore & Background

MHD generators have been developed for use in combined cycle power plants to increase the efficiency of electric generation, especially when burning coal or natural gas. The hot exhaust gas from an MHD generator can heat the boilers of a steam power plant, increasing overall efficiency. Practical MHD generators have been developed for fossil fuels, but these were overtaken by less expensive combined cycles in which the exhaust of a gas turbine or molten carbonate fuel cell heats steam to power a steam turbine. Natural MHD dynamos are an active area of research in plasma physics and are of great interest to the geophysics and astrophysics communities since the magnetic fields of the Earth and Sun are produced by these natural dynamos.

In Their Own Story

In a conventional thermal power plant, like a coal-fired power station, the energy created by chemical reactions is absorbed in a working fluid, usually water. The coal burns in an open chamber surrounded by tubes carrying water; the heat from combustion is absorbed by the water which boils into steam. The steam is sent into a steam turbine which extracts energy by turning it into rotational motion, then turns an electrical generator. The efficiency of this Rankine cycle is a function of the temperature difference between the inlet to the boiler and the outlet to the turbine. For many practical reasons, coal plants generally extract about 35% of the heat energy from the coal. MHD generators can extract more energy from the fuel source than turbine-generator systems by skipping the step where heat is transferred to another working fluid, using the hot exhaust directly as the working fluid. In the case of a coal plant, the exhaust is directed through a nozzle that increases its velocity, essentially a rocket nozzle, and then directs it through a magnetic system that directly generates electricity.

Reader's Guide

The Lorentz Force Law describes the effects of a charged particle moving in a constant magnetic field: F = Q(v × B), where F is the force acting on the particle, Q is the charge, v is the velocity, and B is the magnetic field. For a large power station to approach operational efficiency, steps must be taken to increase the electrical conductivity of the conductive substance. Heating a gas to its plasma state, or adding other easily ionizable substances like the salts of alkali metals, can help accomplish this. Three MHD generator designs exist: the Faraday generator, the Hall generator, and the disc generator. The Faraday generator consists of a wedge-shaped pipe or tube of non-conductive material; when an electrically conductive fluid flows through the tube in the presence of a significant perpendicular magnetic field, a voltage is induced. The Hall generator uses the Hall effect to create a current that flows with the fluid, with arrays of short, segmented electrodes on the sides of the duct. The disc generator currently holds the efficiency and energy density records for MHD generation, with fluid flowing between the center of a disc and a duct wrapped around the edge, using a pair of circular Helmholtz coils for the magnetic excitation field.

Did You Know?

• MHD generators operate without moving parts like turbines, so there is no limit on the upper temperature at which they can operate.
• They have the highest known theoretical thermodynamic efficiency of any electrical generation method.
• The hot exhaust gas from an MHD generator can heat the boilers of a steam power plant, increasing overall efficiency.
• Practical MHD generators for fossil fuels were overtaken by less expensive combined cycles using gas turbines or molten carbonate fuel cells.
• Natural MHD dynamos produce the magnetic fields of the Earth and Sun.