Magnetohydrodynamic System

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Magnetohydrodynamic (MHD) Power Generation

Magnetohydrodynamic power generation provides a way of generatingelectricity directly from a fast moving stream of ionised gases without theneed for any moving mechanical parts - no turbines and no rotarygenerators. Several MHD projects were initiated in the 1960s butovercoming the technical challenges of making a practical system provedvery expensive. Interest consequently waned in favour of nuclear powerwhich since that time has seemed a more attractive option.MHD power generation has also been studied as a method for extractingelectrical power from nuclear reactors and also from more conventionalfuel combustion systems

Working PrincipleThe MHD generator can be considered to be a fluid dynamo. This issimilar to a mechanical dynamo in which the motion of a metal conductorthrough a magnetic field creates a current in the conductor except that inthe MHD generator the metal conductor is replaced by conducting gasplasma.

When a conductor moves through a magnetic field it creates an electricalfield perpendicular to the magnetic field and the direction of movement ofthe conductor. This is the principle, discovered by Michael Faraday,behind the conventional rotary electricity generator. Dutch physicist

Antoon Lorentz provided the mathematical theory to quantify its effects.
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The flow (motion) of the conducting plasma through a magnetic fieldcauses a voltage to be generated (and an associated current to flow)across the plasma, perpendicular to both the plasma flow and themagnetic field according to Fleming's Right Hand RuleLorentz Law describing the effects of a charged particle moving in aconstant magnetic field can be stated as

F = Q v BWhereF is the force acting on the charged particleQ is charge of particlev is velocity of particle

B is magnetic fieldThe MHD System

The MHD generator needs a high temperature gas source, which could bethe coolant from a nuclear reactor or more likely high temperaturecombustion gases generated by burning fossil fuels, including coal, in acombustion chamber. The diagram below shows possible system

components.

The expansion nozzle reduces the gas pressure and consequentlyincreases the plasma speed (Bernoulli's Law) through the generator duct
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to increase the power output. Unfortunately, at the same time, thepressure drop causes the plasma temperature to fall (Gay-Lussac's Law)which also increases the plasma resistance, so a compromise betweenBernoulli and Gay-Lussac must be found.The exhaust heat from the working fluid is used to drive a compressor toincrease the fuel combustion rate but much of the heat will be wastedunless it can be used in another process.The PlasmaThe prime system requirement is creating and managing the conductinggas plasma since the system depends on the plasma having a highelectrical conductivity. Suitable working fluids are gases derived from

combustion, noble gases, and alkali metal vapours.

The Gas Plasma

To achieve high conductivity, the gas must beionised, detaching the electrons from the atomsor molecules leaving positively charged ions of

the gas. The plasma flows through themagnetic field at high speed, in some designs,more than the speed of sound, the flow of thecharged particles providing the necessarymoving electrical conductor.Methods of Ionising the Gas

Various methods for ionising the gas areavailable, all of which depend on impartingsufficient energy to the gas. It may beaccomplished by heating or irradiating the gaswith X rays or Gamma rays. It has also beenproposed to use the coolant gases such as

Note that 90%

conductivity can beachieved with a fairly lowdegree of ionisation ofonly about 1%. (Note alsologarithmic scale)
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helium and carbon dioxide employed in somenuclear reactors as the plasma fuel for directMHD electricity generation rather thanextracting the heat energy of the gas through

heat exchangers to raise steam to drive turbinegenerators. Seed materials such as Potassiumcarbonate or Cesium are often added in smallamounts, typically about 1% of the total massflow to increase the ionisation and improve theconductivity, particularly of combustion gasplasmas.

ContainmentSince the plasma temperature is typically over1000 C, the duct containing the plasma mustbe constructed from non-conducting materialscapable of withstanding these hightemperatures. The electrodes must of coursebe conducting as well as heat resistant.

The Faraday CurrentA powerful electromagnet provides the magnetic field through which theplasma flows, and perpendicular to this field are installed the twoelectrodes on opposite sides of the plasma across which the electricaloutput voltage is generated. The current flowing across the plasmabetween these electrodes is called the Faraday current. This provides the

main electrical output of the MHD generator.The Hall Effect CurrentThe very high Faraday output current which flows across the plasma ductinto the load itself reacts with the applied magnetic field creating a HallEffect current perpendicular to the Faraday current, in other words, acurrent along the axis of the plasma, resulting in lost energy. The total
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current generated will be the vector sum of the transverse (Faraday) andaxial (Hall effect) current components. Unless it can be captured in someway, the Hall effect current will constitute an energy loss.Various configurations of electrodes have been devised to capture boththe Faraday and Hall effect components of the current in order to improvethe overall MHD conversion efficiency.One such method is to split the electrode pair into a series of segmentsphysically side by side (parallel) but insulated from each other, with thesegmented electrode pairs connected in series to achieve a higher voltagebut with a lower current. Instead of the electrodes being directly oppositeeach other, perpendicular to the plasma stream, they are skewed at a

slight angle from perpendicular to be in line with the vector sum of theFaraday and Hall effect currents, as shown in the diagram below, thusallowing the maximum energy to be extracted from the plasma.Hall Effect - When a fixed conductor carrying an electric current is placedin an external magnetic field perpendicular to the current there is voltagedrop across the conductor at right angles to the current which isproportional to the magnetic field. Used to measure magnetic field

strength.


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Power OutputThe output power is proportional to the cross sectional area and the flowrate of the ionised plasma. The conductive substance is also cooled andslowed in this process. MHD generators typically reduce the temperatureof the conductive substance from plasma temperatures to just over 1000C.

An MHD generator produces a direct current output which needs anexpensive high power inverter to convert the output into alternating currentfor connection to the grid.EfficiencyTypical efficiencies of MHD generators are around 10 to 20 percent mainly

due to the heat lost through the high temperature exhaust.This limits the MHD's potential applications as a standalone device butthey were originally designed to be used in combination with other energyconverters in hybrid applications where the output gases (flames) areused as the energy source to raise steam in a steam turbine plant. Totalplant efficiencies of 65% could be possible in such arrangements.Experience

Demonstration plants with capacities of 50 MW or more have been built inseveral countries but MHD generators are expensive. Typical use couldbe in peak shaving applications but they are less efficient than combined-cycle gas turbines which means there are very few installations and MHDis currently not considered for mainstream commercial power generation.magnetohydrodynamic power generator, any of a class of devices thatgenerate electric power by means of the interaction of a moving fluid(usually an ionized gas orplasma) and a magnetic field.Magnetohydrodynamic (MHD) power plants offer the potential for large-scale electrical power generation with reduced impact on the environment.Since 1970, several countries have undertaken MHD research programswith a particular emphasis on the use ofcoal as a fuel. MHD generatorsare also attractive for the production of large electrical power pulses.
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The underlying principle of MHD power generation is elegantly simple.Typically, an electrically conducting gas is produced at high pressure bycombustion of a fossil fuel. The gas is then directed through a magneticfield, resulting in an electromotive force within it in accordance withFaradays law of induction. The MHD system constitutes a heat engine,involving an expansion of the gas from high to low pressure in a mannersimilar to that employed in a conventional gas turbogenerator. In theturbogenerator, the gas interacts with blade surfaces to drive the turbineand the attached electric generator. In the MHD system, the kinetic energyof the gas is converted directly to electric energy as it is allowed toexpand.

Interest in MHD power generation was originally stimulated by theobservation that the interaction of a plasma with a magnetic field couldoccur at much higher temperatures than were possible in a rotatingmechanical turbine. The limiting performance from the point of view ofefficiency in heat engines was established early in the 19th century by theFrench engineerSadi Carnot. The Carnot cycle, which establishes themaximum theoretical efficiency of a heat engine, is obtained from the

difference between the hot source temperature and the cold sinktemperature, divided by the source temperature. For example, if thesource temperature is 3,000 K and the sink temperature 300 K, themaximum theoretical efficiency would be 90 percent. Allowing for theinefficiencies introduced by finite heat transfer rates and componentinefficiencies in real heat engines, a system employing an MHD generatoroffers the potential of an ultimate efficiency in the range of 60 to 65percent. This is much better than the 35 to 40 percent efficiency that canbe achieved in a modern conventional plant. In addition, MHD generatorsproduce fewer pollutants than conventional plants. However, the higherconstruction costs of MHD systems have limited their adoption.
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Principles of operation

In an MHD generator the hot gas is accelerated by a nozzle and injectedinto a channel. A powerful magnetic field is set up across the channel. Inaccordance with Faradays law of induction, an electric field is establishedthat acts in a direction perpendicular to both the gas flow and themagnetic field. The walls of the channel parallel to the magnetic fieldserve as electrodes and enable the generator to provide an electriccurrent to an external circuit.The power output of an MHD generator for each cubic metre of its channelvolume is proportional to the product of the gas conductivity, the square ofthe gas velocity, and the square of the strength of the magnetic field

through which the gas passes. For MHD generators to operatecompetitively with good performance and reasonable physical dimensions,the electrical conductivity of the plasma must be in a temperature rangeabove about 1,500 C. The turbine blades of a gas-turbine power systemare unable to operate at such temperatures.An adequate value ofelectrical conductivity10 to 50 siemens per metrecan be achieved ifan additive, typically about 1 percent by mass, is injected into the hot gas.

This additive is a readily ionizable alkali material, such as cesium,potassium carbonate, orsodium, and is referred to as the seed. Whilecesium has the lowest ionizing potential (3.894 electron volts), potassium(4.341 electron volts) is less costly. Even though the amount of seedmaterial is small, economic operation requires that a system be providedto recover as much of it as possible.

The hot gas with its seed is at a pressure of several million pascals.It isaccelerated by a nozzle to a speed that may be in the range of 1,000 to2,000 metres per second. The gas then enters the channel or duct, acrosswhich the magnetic field is applied. To produce a competitive MHDsystem, this magnetic field must have high intensity. Typically, asuperconducting magnet is employed to provide a magnetic field in therange of three to five teslas across the channel.An electromotive force
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acting in a direction perpendicular to both the flow and the field is set up,and the walls parallel to the magnetic field serve as electrodes to providecurrent to an external electric circuit. The remaining two walls of thechannel are electric insulators. Theoretically, an MHD system with a gasconductivity of 25 siemens per metre, an average magnetic field of threeteslas, and an average gas velocity of 1,000 metres per second is capableof generating electric power with a density of about 250 million watts percubic metre of channel volume.

A complicating feature of a plasma MHD generator is the occurrence of apronounced Hall effect. This results from the behaviour of electrons in thepresence of both magnetic and electric fields. Electrons in the plasma

have a much higher mobility than ions. When electric load current flowsacross the channel, the electrons in this current experience a forcedirected along the channel.This is the Hall Effectnamed for itsdiscoverer, the American physicist Edwin H. Hall. As a result of this effect,the electric current flows at an angle across the channel. An additionalelectric field, called the Hall field, is established along the axis of thechannel. This in turn requires that either the electrode walls in a typical

generator configuration be constructed to support this Hall field or that theHall field itself be used as the output to drive current through the electriccircuit external to the MHD system.

A number of generator configurations have been devised to accommodatethe Hall effect.In a Faraday generator, as shown in part A of the figure, the electrodewalls are segmented and insulated from each other to support the axialelectric field and the electric power is taken out in a series of loads.In the alternate configuration known as a Hall generator, the Faraday fieldacross each sector of the channel is short-circuited and the sectors areconnected in series. This allows the connection of a single electric loadbetween the ends of the channel. Consideration of the electric potentialsat different points in the channel leads to the observation that an
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equipotential runs diagonally across the insulator walls and that electrodesmay be appropriately staggered to match the equipotentials. The seriesconnection of these electrodes in this diagonal generatorpermits a singleelectric load to be used.Major types of MHD systems

Coal-fired MHD systems

The choice of type of MHD generator depends on the fuel to be used andthe application. The abundance ofcoal reserves throughout much of theworld has favoured the development of coal-fired MHD systems forelectric power production. Coal can be burned at a temperature high

enough to provide thermal ionization. However, as the gas expands alongthe duct or channel, its electrical conductivity drops along with itstemperature. Thus, power production with thermal ionization is essentiallyfinished when the temperature falls to about 2,200 C. To be economicallycompetitive, a coal-fired power station would have to combine an MHDgenerator with a conventional steam plant in what is termed a binarycycle. The hot gas is first passed through the MHD generator (a process

known as topping) and then on to the turbogenerator of a conventionalsteam plant (the bottoming phase). An MHD power plant employing suchan arrangement is known as an open-cycle, or once-through, system.Coal combustion as a source of heat has several advantages. Forexample, it results in coal slag, which under magnetohydrodynamicconditions is molten and provides a layer that covers all of the insulatorand electrode walls. The electrical conductivity of this layer is sufficient toprovide conduction between the gas and the electrode structure but not sohigh as to cause significant leakage of electric currents and consequentpower loss. The reduction in thermal losses to the walls because of theslag layer more than compensates for any electrical losses arising from itspresence. Also, the use of a seed material in conjunction with coal offersenvironmental benefits. In particular, the recombination chemistry that
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occurs in the duct of an MHD generator favours the formation ofpotassium sulfate in the combustion of high-sulfur coals, thereby reducingsulfur dioxide emissions to the atmosphere. The need to recover seedmaterial also ensures that a high level of particulate removal is built intoan MHD coal-fired plant. Finally, by careful design of the boiler and thecombustion controls, low levels of nitrogen oxide emissions can beachieved.

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Magnetohydrodynamic System