MHD Generator

MAGNETO HYDRO DYNAMIC (MHD) POWER GENERATION INTRODUCTION: The MHD (magneto hydro dynamic) generator or dynamo transforms thermal energy or kinetic energy directly into electricity. MHD power generation has also been studied as a method for extracting electrical power from nuclear reactors and also from more conventional fuel combustion systems. MHD generators are different from traditional electric generators in that they can operate at high temperatures without moving parts. MHD was developed because the exhaust of a plasma MHD generator is a flame, still able to heat the boilers of a steam power plant. So high-temperature MHD was developed as a topping cycle to increase the efficiency of electric generation, especially when burning coal or natural gas. It has also been applied to pump liquid metals and for quiet submarine engines. The basic concept underlying the mechanical and fluid dynamos is the same. The fluid dynamo, however, uses the motion of fluid or plasma to generate the currents which generate the electrical energy. The mechanical dynamo, in contrast, uses the motion of mechanical devices to accomplish this. The functional difference between an MHD generator and an MHD dynamo is the path the charged particles follow. MHD generators are now practical for fossil fuels, but have been overtaken by other, less expensive technologies, such as combined cycles in which a gas turbine's or molten carbonate fuel cell's exhaust heats steam for steam turbine. The unique value of MHD is that it permits an older single-cycle fossil-fuel power plant to be upgraded to high efficiency. Natural MHD dynamos are an active area of research in plasma physics and are of great interest to the geophysics and astrophysics communities. From their perspective the earth is a global MHD dynamo and with the aid of the particles on the solar wind produces the aurora borealis. The differently charged electromagnetic layers produced by the dynamo effect on the earth's geomagnetic field enable the appearance of the aurora borealis. As power is extracted from the plasma of the solar wind, the particles slow and are drawn down along the field lines in a brilliant display over the poles.

WORKING PRINCIPLE The MHD generator can be considered to be a fluid dynamo. This is similar to a mechanical dynamo in which the motion of a metal conductor through a magnetic field creates a current in the conductor except that in the MHD generator the metal conductor is replaced by conducting gas plasma. When a conductor moves through a magnetic field it creates an electrical field perpendicular to the magnetic field and the direction of movement of the 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.

The flow (motion) of the conducting plasma through a magnetic field causes a voltage to be generated (and an associated current to flow) across the plasma , perpendicular to both the plasma flow and the magnetic field according to Fleming's Right Hand Rule. The Lorentz Force Law describes the effects of a charged particle moving in a constant magnetic field. The simplest form of this law is given by the vector equation.

Where, F is the force acting on the particle, Q is charge of particle, v is velocity of particle, B is magnetic field. The vector F is perpendicular to both v and B according to the Right hand rule.

THE MHD SYSTEM The MHD systems can be classified broadly as follows: 1. Open cycle systems 2. Closed cycle systems. Closed cycle system can be sub classified as: 1. Seeded inert gas systems 2. Liquid metal system OPEN-CYCLE SYSTEMS: In this system, fuel used may be oil through an oil tank or gasified coal through a coal gasification plant. The fuel (coal, oil or natural gas) is burnt in the combustor or combustion chamber. The hot gases from combustor is then seeded with a small amount of an ionized alkali metal (cesium or potassium), to increase the electrical conductivity of the gas. The seed material, generally potassium carbonate, is injected into the combustion chamber, the potassium is then ionized by the hot combustion gases at temperature of roughly (2300 to 2700oC). To attain such high temperatures, the compressed air used to burn the coal (or other fuel) in the combustion chamber, must be preheated to at least 110 oc. A lower preheat temperature would be adequate if the air were enriched in oxygen. An alternative is to use compressed oxygen alone for combustion of the fuel, little or no preheating is then required. The additional cost of the oxygen might be balanced by saving on the pre-heater. The hot pressurized working fluid leaving the combustor flows through a convergent-divergent nozzle similar to a rocket nozzle. In passing through the nozzle, the random motion energy of the molecules in the hot gas is largely converted into directed, mass motion energy. Thus the gas emerges from the nozzle and enters the MHD generator unit at high velocity.

The MHD generator is a divergent channel (or duct) made of a heat-resistant alloy (e.g inconel) with external water cooling. The hot gas expands through the rocket like generator surrounded by powerful magnet. During the motion of the gas the positive and negative ions move to the electrodes and constitute an electric current. The magnetic field direction, which is at right angles to the fluid flow, would be perpendicular to the plane of paper. A number of oppositely located electrode pairs are inserted in the channel to conduct the electric current generated to an external load. The electrodes pair may be connected in various ways. An MHD generator, unlike a conventional generator, produced direct current. This can be converted into commonly used alternating current by means of an inverter. The arrangement of the electrode connections is determined by the need to reduce losses arising from the Hall Effect, the magnetic field acts on the MHD generated (faraday) current and produces a voltage in the flow direction of the working fluid rather than at right to it. The resulting current in an external load is then called the hall current. Various electrode connection schemes have been proposed to utilize the Faraday current. A better, but more complicated, alternative is to connect each electrode pair across a separate load. Another possibility is to utilize the Hall current only; each electrode pair is short circuited outside the generator, and the load is connected between the electrodes as the two ends of the MHD generator. As the working fluid travels along the MHD generator and its energy is converted into electricity, its temperature falls. When the gas temperature reaches about 1900 oc, the extent of ionization of the potassium is insufficient to maintain an adequate electrical conductivity. This places a lower limit on the useful operating temperature of the MHD systems. The large residual heat available from the hot discharge working gas can then be utilized in several ways. For example, it conserves to preheat the combustion air by way of a heat exchanger similar to the regenerator in a gas turbine. At this stage, some 25 to 35 percent of the heat energy in the working fluid should have been converted into electrical energy. The still hot gas leaving the air pre-heater would be used in waste heat (heat exchanger) boiler to produce steam for operating a turbine generator.

In this way, another 25 to 30 percent of the initial heat should be recovered as electrical energy in a combined cycle system. The seed material is recovered for successive use in seed recovery apparatus. Prior to the discharge of the working gas (as flue gas) from the steam boiler to the atmosphere the fly ash, as is usually done, it may have to be treated for recovery of the seed material which is mixed with ash. Unless the sulfur in the coal has been removed (e.g. in a fluidized bed combustor), the original potassium carbonate seed will have been converted into potassium sulphate. This must be extracted from the fly ash and reconverted by chemical reactions into potassium carbonate. The MHD generator needs a high temperature gas source, which could be the coolant from a nuclear reactor or more likely high temperature combustion gases generated by burning fossil fuels, including coal, in a combustion chamber. The diagram below shows possible system components.

The expansion nozzle reduces the gas pressure and consequently increases the plasma speed (Bernoulli's Law) through the generator duct to increase the power output. Unfortunately, at the same time, the pressure drop causes the plasma temperature to fall (Gay-Lussac's Law) which also increases the plasma resistance, so a compromise between Bernoulli and Gay-Lussac must be found.

The exhaust heat from the working fluid is used to drive a compressor to increase the fuel combustion rate but much of the heat will be wasted unless it can be used in another process. CLOSED SYSTEM:

Fig. Closed cycle MHD Generators Seeded inert gas system 

The carrier gas (argon/helium) operates in a form of Brayton cycle

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The gas is compressed & heat is supplied by the source at constant pressure

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The compressed gas expands in a MHD generator and its pressure & temperature fall

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Heat is removed from the gas by cooler, then the gas is recompressed & returned for reheating

Liquid metal system 

When a liquid metal provides the electrical conductivity, an inert gas is a convenient carrier

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The carrier gas is pressurized &heated by passage through a primary heat exchanger within the combustion chamber

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Hot gas is incorporated into liquid metal to form the working fluid

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Working fluid is introduced into the MHD generator through a nozzle

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The carrier gas provide the required high directed velocity of the electrical conductor(i.e. liquid metal)

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After that the liquid metal is separated from the carrier gas

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Part of the heat remaining in the gas is transferred to water in secondary heat exchanger to produce steam

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Finally the gas is recompressed & returned for reheating.

POWER OUTPUT The output power is proportional to the cross sectional area and the flow rate of the ionised plasma. The conductive substance is also cooled and slowed in this process. MHD generators typically reduce the temperature of the conductive substance from plasma temperatures to just over 1000 °C. An MHD generator produces a direct current output which needs an expensive high power inverter to convert the output into alternating current for connection to the grid. Maximum power per unit volume

Pmax = (σ u2 B2 )/ 4 σ – conductivity of the gas, mho/m u - average gas velocity, m/s B – magnetic field of flux density, Wb/m EFFICIENCY Typical 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 stand alone device but they were originally designed to be used in combination with other energy converters in hybrid applications where the output gases (flames) are used as the energy source to raise steam in a steam turbine plant. Total plant efficiencies of 65% could be possible in such arrangements. MATERIALS FOR MHD GENERATORS Owing to the high temperature of plasma (2700°C), refracting materials are required in several parts of generators like electrodes, channel or duct wall. Due to stringent environmental conditions selection of materials is very limited. Following are the important factors to be considered for selection of materials. 1. Thermal shock resistance 2. Electrical conductivity 3. Corrosion resistance 4. Erosion resistance 5. Oxidation reduction resistance 6. Melting point of materials

7. Density Requirements for materials in MHD generators: 1. Electrodes – For high conductivity, high temperature is required. Electrically conductive and structurally stable at high temperature. 2. Duct liner: ♣ Cooler than electrodes ♣ Must be electrical insulator ♣ Must be thermal insulator 3. Magnet – Stronger than permanent magnets or ferromagnetic cores can provide. Few materials have high melting points. For insulators – Oxides, Nitrides, Zirconates, etc., for conductors – Refractory metals, Nitrides Borides and carbides. Field coils: Permanent magnet can produce only 0.4 Wb/m2. Electromagnets can produce only 1 Wb/m2 but massive and lossy. In magnetic field below a certain temperature metals, compounds and alloys exhibit reduction in resistance to zero-super conductivity. Electrode materials: Electrodes with higher electrical conductivity contribute towards better charge transfer across the electrodes/plasma interface. Most of the initial channel wall designs involved water cooled metallic electrodes due to the oxidizing condition. But the cold surface creates cold boundary layer which generally promotes high thermal losses, low conductivity and leads to considerable voltage drop in this region.

The alkali seed condensation and cold slag

condensation degrade the properties of electrode.

Important aspects of any practical electrode system for MHD generator are high reliability and long life of operation (10,000 hrs) and efficient in transferring current densities of 1-5 amp/cm2. Electrode material should be stable chemically, thermally and mechanically. There are several potential ceramic materials for the electrode system that operate near 1500-2000°C. The carbides (SiC, ZrC, MbC) borides (ZrB 2, TiB6, LaB6), silicides (MO, Si2, Ws1), chromites (Sr, Mg. doped), oxides and their solid solutions (ZrO 2-CeO2, CeO2) and graphite as electrically conducting refractory materials. Most promising of these, however stabilized zirconia.

Materials for channel or Duct wall: Construction of duct through which to pass the very hot, sometimes very corrosive gas is second major problem. Materials to withstand temperatures of the order of 3000°C are hard to find. Search of suitable ceramics or coatings of ceramics is continuing. Steam cooled walls which are made of insulated laminations of very thin nickel or stainless steel have been tried. Apart from high temperature, the duct wall has to withstand the stresses encountered in use and also the thermal shock in the event of accidental shut down of the gas supply. The duct material has to be electrically insulating at the operating temperatures. The electrodes which are required to emit a copious stream of electrons throughout their life time are required to withstand more rigorous environment than the duct. Water cooled copper electrodes have been used with some success. High temperature coupled with corrosion by alkali seeded plasma and erosion by high velocity plasma constitute severe environments experienced by structural materials in MHD generators. Ceramic materials, inherently, offer superior chemical stability towards oxidation and corrosion by alkali seed materials. However, thermal shock of ceramic is necessary to be taken into account. The density of ceramic should be high in order to reduce the penetration of

slag, seed and gas diffusion which enhance degradation of insulation and electrode material. At the same time density should not be high to jeopardize the thermal shock resistance. Resistivity and dielectric breakdown strength of ceramic should also be taken into account. Dielectric breakdown strength of Al2O3 is 105 – 106 V/cm at room temperature. This value decreases at high temperature; it drops from 106 V/cm to 104 V/cm at 1400°C. Al2O3MgO, MgAl2O3 are potential channel wall materials. However, Al2O3 reacts with potassium seed at high temperatures to give potassium Beta alumina (K Al11O17), on the other hand, MgO, stands seed and slag corrosion. It also has good thermal shock resistance. Hence use of MgO as duct wall material is favorable.

ADVANTAGES: 1. The conversion efficiency of an MHD system can be around 50 percent as compared to less than 40 percent for the most efficient steam plants. Still higher thermal efficiencies (60-65%) are expected in future, with the improvements in experience and technology. 2. Large amount of power is generated. 3. It has no moving parts, so more reliable. 4. The closed cycle system produces power free of pollution. 5. It has ability to reach the full power level as soon as started. 6. The size of the plant (m2/kW) is considerably smaller than conventional fossil fuel plants. 7. Although the costs can not be predicted very accurately, yet it has been reported that capital costs of MHD plants will be competitive with those of conventional steam plants. 8. It has been estimated that the overall operational, costs in an MHD plant would be about 20% less than in conventional steam plants.

9. Direct conversion of heat into electricity permits to eliminate the gas turbine (compared with a steam power plant). This elimination reduces losses of energy. 10. These systems permit better fuel utilization. The reduced fuel consumption would offer additional economic and special benefits and would also lead to conservation of energy resources. 11. It is possible to utilize MHD for peak power generations and emergency service (upto 100 hours per year). It has been estimated the MHD equipment for much duties is simpler, has the capability of generating in large units and has the ability to make rapid start to full load. REFERENCES  G.D.Rai, Non-conventional energy sources, Khanna publishers, Delhi, 2002.

 Hugo K. Messerle, "Magnetohydrodynamic Power Generation", 1994, John Wiley, Chichester, Part of the UNESCO Energy Engineering Series.  G.J. Womac, "MHD Power Generation", 1969, Chapman and Hall, London.