INTRODUCTION
Magneto hydrodynamics (MHD) (magneto
fluid dynamics or hydro magnetics) is the academic
discipline which studies the dynamics of electrically
conducting fluids. Examples of such fluids include plasmas, liquid metals,
and salt water. The word magneto hydro dynamics (MHD) is derived
from magneto- meaning magnetic field,
and hydro- meaning liquid, and -dynamics meaning movement. The
field of MHD was initiated by Hannes Alfvén , for which he received
the Nobel Prize in Physics in 1970
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80 % of total electricity produced in the world
is hydal, while remaining 20% is produced from nuclear, thermal, solar,
geothermal energy and from magneto hydro dynamic (mhd) generator.
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MHD power generation is a new system of electric
power generation which is said to be of high efficiency and low pollution. In
advanced countries MHD generators are widely used but in developing countries
like INDIA, it is still under construction, this construction work in in
progress at TRICHI in TAMIL NADU, under the joint efforts of BARC (Bhabha
atomic research center), Associated cement corporation (ACC) and Russian
technologists.
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As its name implies, magneto hydro dynamics
(MHD) is concerned with the flow of a conducting fluid in the presence of
magnetic and electric field. The fluid
may be gas at elevated temperatures or
liquid metals like sodium or potassium- SEEDING.
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An MHD generator is a device for converting heat
energy of a fuel directly into electrical energy without conventional electric
generator.
o
In this system. An MHD converter system is a
heat engine in which heat taken up at a higher temperature is partly converted
into useful work and the remainder is rejected at a temperature. Like all heat
engines, the thermal efficiency of an MHD converter is increased by supplying
the heat at the highest practical temperature and rejecting it at the lowest
practical temperature.
o
The output of the MHD is supplied to the
conventional Thermal Plants.
PRINCIPLES OF MHD POWER GENERATION
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When an electric conductor moves across a
magnetic field, a voltage is induced in it which produces an electric current.
o
This is the principle of the conventional
generator where the conductors consist of copper strips.
o
In MHD generator, the solid conductors are
replaced by a gaseous conductor, an ionized gas. If such a gas is passed at a
high velocity through a powerful magnetic field, a current is generated and can
be extracted by placing electrodes in suitable position in the stream.
o
The principle can be explained as follows. An
electric conductor moving through a magnetic field experiences a retarding
force as well as an induced electric field and current.
This effect is a result
of FARADAYS LAWS OF ELECTRO MAGNETIC INDUCTION.
o
The induced EMF is given by
Eind
= u x B
where u = velocity of the conductor.
B = magnetic field intensity.
o
The induced current is given by,
Jind = C x Eind
where
C = electric conductivity
The
retarding force on the conductor is the Lorentz force given by Find
= Jind X B
The electro magnetic induction principle
is not limited to solid conductors. The movement of a conducting fluid through
a magnetic field can also generate electrical energy.
When a fluid is used for the energy
conversion technique, it is called MAGNETO HYDRO DYNAMIC (MHD), energy
conversion.
The flow direction is right angles to the
magnetic fields
direction.
An electromotive force (or electric voltage) is induced in the direction at
right angles to both flow and field directions, as shown in the next slide.
The conducting flow fluid is forced
between the plates with a kinetic energy and pressure differential sufficient
to over come the magnetic induction force Find.
The end view drawing illustrates the
construction of the flow channel.
An ionized gas is employed as the
conducting fluid.
Ionization is produced either by thermal
means I.e. by an elevated temperature or by seeding with substance like cesium
or potassium vapors which ionizes at relatively low temperatures.
The atoms of seed element split off
electrons. The presence of the negatively charged electrons makes the gas an
electrical conductor.
VARIOUS MHD SYSTEMS
The MHD systems are broadly classified
into two types.
OPEN
CYCLE SYSTEM
CLOSED
CYCLE SYSTEM
Liquid
metal system
OPEN CYCLE SYSTEM
The fuel used maybe 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 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’ c to
2700’c.
To attain such high temperatures, the
compressed air is used to burn the coal in the combustion chamber, must be
adequate to at least 1100’c. A lower preheat temperature would be adequate if
the air is enriched in oxygen. An alternative is used to compress oxygen alone
for combustion of fuel, little or no preheating is then required. The
additional cost of oxygen might be balanced by saving on the preheater.
The hot pressurized working fluid living
in the combustor flows through a convergent divergent nozzle. In passing
through the nozzle, the random motion energy of the molecules in the hot gas is
largely converted into directed, mass of energy. Thus , the gas emerges from
the nozzle and enters the MHD generator unit at a high velocity.
The MHD generator is a divergent channel
made of a heat resistant alloy with external water cooling. The hot gas expands
through the rocket like generator surrounded by powerful magnet. During motion
of the gas the +ve and –ve ions move to the electrodes and constitute an
electric current.
The arrangement of the electrode
connection is determined by the need to reduce the losses arising from the Hall
effect. By this effect, the magnetic field acts on the MHD-generated current
and produces a voltage in flow direction of the working fluid.
CLOSED CYCLE SYSTEM
Two general types of closed cycle MHD
generators are being investigated.
Electrical conductivity is maintained in
the working fluid by ionization of a seeded material, as in open cycle system.
A liquid metal provides the conductivity.
The carrier is usually a chemical inert
gas, all through a liquid carrier is been used with a liquid metal conductor.
The working fluid is circulated in a closed loop and is heated by the
combustion gases using a heat exchanger. Hence the heat sources and the working
fluid are independent. The working fluid is helium or argon with cesium
seeding.
SEEDED INERT GAS SYSTEM
In a closed cycle system the carrier gas
operates in the form of Brayton cycle. In a closed cycle system the gas is
compressed and heat is supplied by the source, at essentially constant
pressure, the compressed gas then expands in the MHD generator, and its
pressure and temperature fall. After leaving this generator heat is removed
from the gas by a cooler, this is the heat rejection stage of the cycle.
Finally the gas is recompressed and returned for reheating.
The complete system has three distinct
but interlocking loops. On the left is the external heating loop. Coal is
gasified and the gas is burnt in the combustor to provide heat. In the primary
heat exchanger, this heat is transferred to a carrier gas argon or helium of
the MHD cycle. The combustion products after passing through the air preheated
and purifier are discharged to atmosphere.
Because the combustion system is separate
from the working fluid, so also are the
ash and flue gases. Hence the problem of extracting the seed material from fly
ash does not arise. The fuel gases are used to preheat the incoming combustion
air and then treated for fly ash and
sulfur dioxide removal, if necessary prior to discharge through a stack to the
atmosphere.
The loop in the center is the MHD loop.
The hot argon gas is seeding with cesium and resulting working fluid is passed
through the MHD generator at high speed. The dc power out of MHD generator is
converted in ac by the inverter and is then fed to the grid.
LIQUID METAL SYSTEM
When a liquid metal provides the
electrical conductivity, it is called a liquid metal MHD system.
An inert gas is a convenient carrier
The carrier gas is pressurized and heated
by passage through a heat exchanger within combustion chamber. The hot gas is
then incorporated into the liquid metal usually hot sodium to form the working
fluid. The latter then consists of gas bubbles uniformly dispersed in an
approximately equal volume of liquid sodium.
The working fluid is introduced into the
MHD generator through a nozzle in the usual ways. The carrier gas then provides
the required high direct velocity of the electrical conductor.
After passage through the generator, the
liquid metal is separated from the carrier gas. Part of the heat exchanger to
produce steam for operating a turbine generator. Finally the carrier gas is
cooled, compressed and returned to the combustion chamber for reheating and
mixing with the recovered liquid metal. The working fluid temperature is
usually around 800’c as the boiling point of sodium even under moderate
pressure is below 900’c.
At lower operating temp, the other MHD
conversion systems may be advantageous
from the material standpoint, but the maximum thermal efficiency is lower. A
possible compromise might be to use liquid lithium, with a boiling point near
1300’c as the electrical conductor lithium is much more expensive than sodium,
but losses in a closed system are less.
ADVANTAGES
The conversion efficiency of a MHD system
can be around 50% much higher compared to the most efficient steam plants.
Still higher efficiencies are expected in future, around 60 – 65 %, with the
improvements in experience and technology.
Large amount of power is generated.
It has no moving parts, so more reliable.
The closed cycle system produces power,
free of pollution.
It has ability to reach the full power
level as soon as started.
The size if the plant is considerably
smaller than conventional fossil fuel plants.
Although the cost cannot be predicted
very accurately, yet it has been reported that capital costs of MHD plants will
be competitive to conventional steam plants.
It has been estimated that the overall
operational costs in a plant would be about 20% less than conventional steam
plants.
Direct conversion of heat into
electricity permits to eliminate the turbine (compared with a gas turbine power
plant) or both the boiler and the turbine (compared with a steam power plant)
elimination reduces losses of energy.
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.
It is possible to use MHD for peak power
generations and emergency service. It has been estimated that MHD equipment for
such duties is simpler, has capability of generating in large units and has the
ability to make rapid start to full load.
FUTURE PROSPECTS
It
is estimated that by 2020, almost 70 % of the total electricity
generated in the world will be from MHD generators.
Research and development is widely being
done on MHD by different countries of the world.
Nations involved:
USA
Former USSR
Japan
India
China
Yugoslavia
Australia
Italy
Poland
