Basics, Types and Electricity Generation

1. Magneto Hydro Dynamics
(MHD Power Plant)
By:
Mohammad Ahmad
Assistant Professor
REC Bijnor

2. Introduction
 Magneto Hydro Dynamics (MHD) is also known as Magneto Fluid
Dynamics or Magneto Hydro Magnetics
Magneto - Magnetic field
Hydro – Liquid
Dynamics - Movement
 Magneto hydro dynamics (MHD) is the study of an electrically
conducting fluid in the presence of magnetic field.
 Examples of such fluids include plasmas, liquid metals, and
salt water.
 It is the method of power generation with non-moving parts.

3.  An MHD generator is a device for converting heat energy of a fuel
directly into electrical energy without a conventional electric
generator.
 Faraday law of electromagnetic induction.
 An electric conductor moving through a magnetic field experiences a
retarding force as well as an induced electric field and current. This
current can be extracted by placing electrodes in suitable position in
the stream.
 Faraday passed Mercury through the magnetic field and found
that a conducting fluid can be used to generate electricity.
 It is a new system of electric power generation which is said to be
of high efficiency.
 Also it causes low pollution.

5. FARADAY’S EXPERIMENT
 In the year 1832, he placed two metal plates across river THAMES
from waterloo bridge and connects a 960 feet long copper wire
along with a galvanometer.
 The water of the river is salty, it has certain amount of conductivity.
 The conducting fluid interacts with the earth magnetic field, to
produce a small current in the galvanometer through electrodes
connected across the banks.
 Conventional electromagnetic power converters use the same
principle.
 However, instead of using solid metal, a CONDUCTING Fluid is
employed in MHD generation.

7. Working of MHD
 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.
 This effect is a result of FARADAYS LAWS OF ELECTRO MAGNETIC
INDUCTION.
 The induced EMF is given by
Eind = u x B
(Direction can be find by Fleming Right hand rule)
where u = velocity of the conductor, B = magnetic field intensity.
 The induced current is given by,
Iind = C x Eind
where C = electric conductivity
 The retarding force on the conductor is the Lorentz force given by
Find = Iind X B (=B.i.l.sinθ)

9.  The power of an MHD generator depends upon the conductivity, fluid
speed and magnetic field density.
 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.
 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
vapours 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.

10. Types of MHD
The MHD system are broadly classified into two types.
OPEN CYCLE SYSTEM
The working fluid is used only once.
CLOSED CYCLE SYSTEM
 The working fluid is continuously re-circulated.
 This is mainly divided into 2 main system
1. Seeded Inert gas system
2. Liquid Metal system

11. OPEN CYCLE MHD SYSTEM
 The 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 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.

12. Open cycle MHD plant

13.  To attain such high temperatures, the compressed air is used to burn
the coal in the combustion chamber, must be adequate to at least
1100ºC.
 The hot pressurized working fluid living in the combustor flows
through a convergent divergent nozzle.
 Thus, the gas emerges from the nozzle and enters the MHD
generator unit at a high velocity.
 In passing through the nozzle, the random motion energy of the
molecules in the hot gas is largely converted into directed, mass
of energy.
 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.

14.  During motion of the gas the +ve and –ve ions move to the
electrodes and constitute an electric current.
 MHD generates DC power, and an inverter is utilised to convert it into
AC power.
 The exhaust of an MHD channel is first used to preheat the air
intake for combustor and then to raise the steam in a waste-heat
steam generator.
 The flue gases are released to the atmosphere through a chimney
after seed recovery and removal of pollutants.
 The recovered seed material, normally Aqueous potassium
carbonate, is recycled after mixing the additional quantity to make
up the loss of seed.

15. CLOSED CYCLE SYSTEM
1. Seeded Inert Gas System
 When the electrical conductivity is maintained in the working fluid by
ionization of a seeded material like Cesium metal, Potassium etc. is called
seeded inert gas system.
 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.

16. Seeded Inert Gas System

17.  In the primary heat exchanger, this heat is transferred to a carrier
Inert gas argon or helium of the MHD cycle.
 The combustion products after passing through the air preheated
and purifier are discharged to atmosphere.
 The flue gases are used to preheat the incoming combustion air
and then treated for fly ash and sulphur dioxide removal, if necessary
prior to discharge through a stack to the atmosphere.
 The loop in the centre 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 residual heat of hot argon is utilised to generate additional power
through a waste heat boiler (secondary heat exchanger), turbine and
generator.
 The dc power output of MHD generator is converted in ac by the
inverter and is then fed to the grid.

18. 2. LIQUID METAL SYSTEM
 When a liquid metal provides the electrical conductivity to
working fluid, it is called a liquid metal MHD system.
 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 (Liquid metal) 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.
 The working fluid temperature is usually around 800ºC as the
boiling point of sodium even under moderate pressure is below 900ºC.
 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.

19. LIQUID METALSYSTEM

20.  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, as explained in seeded inert gas system.
 Finally the carrier gas is cooled, compressed and returned to the
combustion chamber for reheating and mixing with the recovered liquid
metal.
 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, to improve efficiency, might be to use liquid
lithium, with a higher boiling point near 1300ºC.
 The electrical conductor lithium is much more expensive than sodium, but
losses in a closed system are less.

21. APPLICATIONS OF MHD GENERATORS
1.Space applications
 Power supplies, space power system and space craft application.
2.Industrial Application
 Most industrial processes require a large amount of D.C. power like metal
smelting industries. This could be supplied by MHD at increased efficiency
and without noise and vibration.
3.Ship Propulsion
 These generators can be used in sub-marines.
4.Seismic studies
 These generators are good for pulse power operation and are used for
determining the earth quakes by the Russians known as Pamir Generator.
5.Emergency power supplies
 If the power grid fails an MHD generator could supply power for a hospital
within minutes of ignition. The MHD power generator can go from cold start
to full power within seconds.

22. EFFICIENCY COMPARISON
OF VARIOUS ENERGY SOURCES
S.No. ENERGY SOURCE Efficiency ή (%)
1 THERMO-ELECTRIC 10
2 PHOTOVOLTAIC 18
3 THERMIONIC 22
4 STEAM AND GAS TURBINE 30
5 INTERNAL COMBUSTION Engine 30
6 DIESEL ENGINE 40
7 FUEL CELLS 60
8 MHD 60
9 MHD ADVANCED CYCLE 70

23. EXISTING MHD FACILITIES
 Retrofit of MHD Plant to a 200 MW steam power plant
at Wales point north of Sydney, Australia.
 Retrofit a 230 MW thermal power plant in Brindisi, Italy.
 MHD Development corporation project at billings at
Montana, USA.
 Russia: U-25 facility at Ryzan, near Moscow.
 China Shanghai and Beijing 60 MW coal fired based
plant.

24. MHD in India
 In advanced countries, MHD generators are widely used.
 But in developing countries like India, it is still under
construction.
 This generator is situated at Trichi in Tamil Nadu under
the joint efforts of
 BARC (Bhabha Atomic Research Centre)
 BHEL
 Associated Cement Corporation(ACC)And
 Russian Technologists

25. Advantages of MHD
 The conversion efficiency of an MHD system can be around 50-60%
as compared to less than 40% for the most efficient steam plants.
 It has no moving parts, so more reliable, less maintenance.
 Lower emission of pollutants due to better pollution control.
 Quick power generation, it has ability to reach the full power level as
soon as started.
 Compact in size, the size of plant (m2 /kW) is considerably smaller
than conventional fossil fuel plants.
 The costs 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.

26. Disadvantages/ Limitations of MHD
 Very high operating temperature restricts the choice of material for
various equipments.
 Life of equipments is short due to high temperature stress.
 As the power available is in DC form, so it requires an inverter for
sending the power to the grid.
 There are technical limitations on enhancing the fluid conductivity
and the strength of magnetic field.
 The combustor, MHD generator, electrodes and air pre-heater are
exposed to corrosive combustion gases at high temperatures.