STEAM POWER PLANT 79

(c) Recirculation burner capillary action. The fuel from the uppermost part of the
(d) Wick type burner wick is evaporated due to radiant heat from the flame and
2. Atomising fuel burners : the nearby heated surfaces. Air is admitted through holes
in the surrounding walls.
(a) Mechanical or oil pressure atomising burner
A wick burner is suitable for models or domestic
(b) Steam or high pressure air atomising burner
appliances.
(c) Low pressure air atomising burner.
2. Atomising fuel burners. Following are the
1. Vapourising oil burners. Following are the requirements of an automising fuel burner :
requirements of a vapourising/evaporation oil burner : (i) To automise the fuel into fine particles of equal
(i) To vapourise the fuel before ignition. size.
(ii) To mix the vapourised fuel thoroughly with (ii) To supply air in required quantity at proper
the air. places in the combustion chamber.
(iii) To minimise the soot formation. (iii) To give high combustion intensity.
(iv) To give high heat release by burning large (iv) To give high thermal efficiency.
quantity of oil per hour. (v) To operate without difficulty at varying loads.
(v) To allow for efficient combustion of fuel at (vi) To create necessary turbulence inside the
part load operation. combustion chamber for proper combustion of fuel.
(a) Atmospheric pressure atomising burner. (vii) To minimise soot formation and carbon deposit,
particularly on the burner nozzle.
This burner makes use of highly volatile liquid fuels such
as neptha, volatile gasoline etc. Here the fuel at low pressure (a) Mechanical atomising burners. A mechanical
is passed through a tube adjacent to the flame before being atomising oil burner consists of the following four principal
released through an orifice. While passing through the hot parts :
tube, most of the fuel is vapourised so that the fluid ejected (i) Atomiser (ii) Air register (iii) Diffuser (iv) Burner
from the orifice is more or less a vapour. The required throat opening.
quantity of primary air is supplied to burn the vapour (i) Atomiser. It breaks up the oil mechanically into
stream in a cylindrical tube. a fine uniform spray that will burn with minimum of excess
(b) Rotating cup burner. These burners are used air when projected into the furnace. The spray is produced
on low as well as medium capacity boilers. by using relatively high pressure to force oil at high velocity
In this type of burner, the fuel oil flows through a through small tangential passages of sprayer plate into a
tube in the hollow shaft of the burner and into the cup at chamber where it is rapidly rotated, centrifugal force in
the rotating oil causes it to break up into a thin layered,
the furnace end. An electric motor or an air turbine runs
mist like, hollow conical spray as it is released through the
the shaft and the cup at high speeds (3000 to 10000 r.p.m.).
orifice plate.
As a result of centrifugal force fuel is split into small
droplets. About 10 to 15 per cent of air is supplied as (ii) Air register. An air register is an integral part of
the oil-burner assembly. It consists of a number of
primary air. This air is supplied from a blower surrounding
overlapping vanes which deliver the air for combustion to
the cup. The shape of the flame is governed by the sharp
the furnace throat with the correct degree of spin.
edge of the cup and the position of air nozzle.
(iii) Diffuser. It is a shield in the form of a perforated
(c) Recirculating burner. The part of the
hollow metal cone mounted near the furnace end of the
combustion products may be recirculated in order to heat atomiser assembly. It stabilises the flame to prevent it from
up the incoming stream of fuel and air. Low ratio of the being blown away from the atomiser tip.
mass of recirculated combustion products to the mass of
(iv) Burner throat opening. It is circular and
unburnt fuel-air mixture results in less temperature rise
concentric with burner outlet. It is made of refractory. The
of the mixture, whereas, high ratio may extinguish the atomiser and diffuser assembly should be so positioned that
flame due to increased proportions of circulated products. the flame clears the throat opening sufficiently to avoid
An optimum ratio may be determined for different fuels striking. This burner has an insulated front and thus is
experimentally. designed to operate with preheated air.
In recirculation burner (utilising the above principle) (b) Steam atomising burners. Of various methods
circulation system is separated from the combustion by a of oil atomisation, that which employs steam is usually
solid wall. the most convenient. This method may, however, absorbs
(d) Wick burners. In this type of a burner a cotton some 4 to 5% of the total amount of steam generated. These
or asbestos wick is used which raises the liquid fuel by burners may be divided into two categories :
 80 POWER PLANT ENGINEERING

(i) The outside mix In case of inside mixing type burners steam and oil
(ii) The inside mix. are mixed inside the burner before the mixture is projected
In case of outside mixing [Fig. 3.33 (a)] type burners, in the furnace in either a flat spray or in a hollow cone.
oil is ejected through one side of the holes and is blasted by These burners provide high efficiency at the high firing rates
a high velocity jet of steam issuing from other holes. Mixing, and flexible flame shape. In this type of burner instead of
however, occurs outside the burner. steam high pressure air can also be used.
(c) Low pressure air atomising burners. They
Steam operate on the same principle as for burners described
earlier. In this case air pressure required ranges from 0.015
bar to 0.15 bar.
These are the simplest and most versatile atomising
Oil type of burners and usually give troublefree service for long
interrupted periods.
3.9.4.3. Gas burners
Gas burning claims the following advantages :
(i) It is much simpler as the fuel is ready for
Oil
combustion and requires no preparation.
(ii) Furnace temperature can be easily controlled.
Steam (iii) A long slow burning flame with uniform and
gradual heat liberation can be produced.
Oil
(iv) Cleanliness.
(v) High chimney is not required.
(vi) No ash removal is required.
For generation of steam, natural gas is invariably
used in the following cases :
Steam
(i) Gas producing areas.
(a)
(ii) Areas served by gas transmission lines.
(iii) Where coal is costlier.
Typical gas burners used are shown in Figs. 3.34
to 3.36.
Oil
Gas
Steam
Gas

Oil Air
Air

Air
Steam
Air
Air
Oil

Steam Fig. 3.34 Fig. 3.35

(b) Refer to Fig. 3.34. In this burner the mixing is poor
and a fairly long flame results.
Fig. 3.33. (a) Inside mixing (b) Outside mixing.
 STEAM POWER PLANT 81

Air The primary object of using the inert material is to

control the bed temperature, it accounts for 90% of the bed
volume. It is very necessary that the selection of an inert
material should be done judiciously as it remains with the
fuel in continuous motion and at high temperature to the
tune of 800°C. Moreover, the inert material should not
disintegrate coal, the parent material of the bed.

Flue gases

Walls

Steam
Gas
Ash over-
Fig. 3.36 flow Bubble

Refer to Fig. 3.35. This is a ring type burner in which Heat absorbing
a short flame is obtained. Water tubes
Refer to Fig. 3.36. This arrangement is used when Fuel and
both gas and air are under pressure. dolomite
In order to prevent the flame from turning back the Distributor
velocity of the gas should be more than the “rate of flame plate
propagation”.

3.10. FLUIDISED BED COMBUSTION (FBC) Air

A fluidised bed may be defined as the bed of solid particles Fig. 3.37. Basic FBC system.
behaving as a fluid. The principle of FBC-system is given
below : The cost economic shows that a saving of about 10%
in operating cost and 15% capital cost could be achieved
When a gas is passed through a packed bed of finely
for a unit rating of 120 MW and it may be still higher for
divided solid particles, it experiences a pressure drop across
bigger units.
the bed. At low gas velocities, this pressure drop is small
and does not disturb the particles. But if the gas velocity is Advantages :
increased further, a stage is reached, when particles are 1. As a result of better heat transfer, the unit size
suspended in the gas stream and the packed bed becomes a and hence the capital costs are reduced.
‘fluidised bed’. With further increase in gas velocity, the 2. It can respond rapidly to changes in load demand
bed becomes turbulent and rapid mixing of particles occurs. (since thermal equilibrium between air and coal particles
In general, the behavior of this mixture of solid particles in the bed is quickly established).
and gas is like a fluid. Burning of a fuel in such a state is 3. Low combustion temperatures (800 to 950°C)
known as a fluidised bed combustion. inhibits the formation of nitrogen oxides like nitric oxide
Fig. 3.37 shows the arrangement of the FBC system. and nitrogen dioxide.
On the distributor plate are fed the fuel and inert 4. Since combustion temperatures are low the
material dolomite and from its bottom air is supplied. The fouling and corrosion of tubes is reduced considerably.
high velocity of air keeps the solid feed material in 5. As it is not necessary to grind the coal very fine
suspending condition during burning. The generated heat as is done in pulverised fuel firing, therefore, the cost of
is rapidly transferred to the water passing through the coal crushing is reduced.
tubes immersed in the bed and generated steam is taken 6. Pollution is controlled and combustion of high-
out. During the burning sulphur dioxide formed is absorbed sulphur coal is possible.
by the dolomite and prevents its escape with the exhaust 7. FBC system can use solid, liquid or gaseous fuel
gases. The molten slag is tapped from the top surface of or mix as well as domestic and industrial waste. Any variety
the bed. of coal can be used successfully.
 82 POWER PLANT ENGINEERING

8. Combustion temperature can be controlled 3.11.1. Ash Handling Equipment

accurately.
A good ash handling plant should have the following
9. The system can be readily designed for operation characteristics :
at raised combustion pressure, owing to the simplicity of
1. It should have enough capacity to cope with the
arrangement, small size of the plant and reduced likelihood
volume of ash that may be produced in a station.
of corrosion or erosion of gas turbine blades.
2. It should be able to handle large clinkers, boiler
10. The combustion in conventional system becomes
refuse, soot etc. with little personal attention of the
unstable when the ash exceeds 48% but even 70% ash
workmen.
containing coal can be efficiently burned in FBC.
3. It should be able to handle hot and wet ash
11. The large quantity of bed material acts as a
effectively and with good speed.
thermal storage which reduces the effect of any fluctuation
in fuel feed ratio. 4. It should be possible to minimise the corrosive
or abrasive action of ashes and dust nuisance should not
exist.
3.11. ASH HANDLING
5. The plant should not cost much.
A huge quantity of ash is produced in central stations, 6. The operation charges should be minimum
sometimes being as much as 10 to 20% of the total quantity possible.
of coal burnt in a day. Hundreds of tonnes of ash may have
7. The operation of the plant should be noiseless
to be handled every day in large power stations and
as much as possible.
mechanical devices become indispensable. A station using
low grade fuel has to deal with large quantities of ash. 8. The plant should be able to operate effectively
Handling of ash includes : under all variable load conditions.
(i) Its removal from the furnace. 9. In case of addition of units, it should need
(ii) Loading on the conveyers and delivery to the fill minimum changes in original layout of plant.
or dump from where it can be disposed off by sale or 10. The plant should have high rate of handling.
otherwise. The commonly used equipment for ash handling in
Handling of ash is a problem because ash coming large and medium size plants may comprise of :
out of the furnace is too hot, it is dusty and irritating to (i) Bucket elevator
handle and is accompanied by some poisonous gas. Ash (ii) Bucket conveyor
needs to be quenched before handling due to following (iii) Belt conveyor
reasons :
(iv) Pneumatic conveyor
(i) Quenching reduces corrosion action of the ash.
(v) Hydraulic sluicing equipment
(ii) It reduces the dust accompanying the ash.
(iii) It reduces temperature of the ash. (vi) Trollies or rail cars etc.
(iv) Ash forms clinkers by fusing in large lumps and Fig. 3.38 shows the outline of ash disposal
by quenching clinkers will disintegrate. equipment.

Remaining fly ash

Steam
generator Fly ash removal (Partial)
Fly ash 1. Stock sprays
suspended 2. Electrical precipitation
in gas stream
3. Wet baffles
All or bulk 4. Traps and centrifugal separators
Molten ash
of ash to 5. Special bladed fan
furnace hearth or
1. Continuous flow ash hopper Soot and fly ash
2. Periodically
tapped Solid ash
Conveying system 1. Hand raking
Conveying 2. Gravity dump
1. Water sluicing 3. Water jets
1. Ash dump system 2. Pivoted bucket conveyor
2. R.R. Car discharge to 3. Pneumatic conveyor
1. Hydraulic fill 4. Steam jet conveyor
3. Borge 2. Settling tank 5. Ash cars and carts
4. Motor truck 3. Dry ash pit 6. Wheel borrows
4. Ash bunker

Fig. 3.38. Outline of ash disposal equipment.

 STEAM POWER PLANT 83

3.11.2. Ash Handling Systems The hot ash released from the boiler furnaces is
made to fall over the belt conveyor after cooling it through
The modern ash-handling systems are mainly classified
water seal. This cooled ash is transported to an ash bunker
into four groups : through the belt conveyor. From ash bunker the ash is
1. Mechanical handling system removed to the dumping site through trucks.
2. Hydraulic system
3. Pneumatic system 2. Hydraulic system
4. Steam jet system. In this system ash is carried with the flow of water
with high velocity through a channel and finally dumped
1. Mechanical handling system
in the sump. This system is subdivided as follows :
Fig. 3.39 shows a mechanical handling system. This
system is generally employed for low capacity power plants (a) Low pressure system
using coal as fuel. (b) High pressure system.
Boiler furnaces (a) Low pressure system. Refer to Fig. 3.40. In
this system a trough or drain is provided below the boilers
Ash Ash Ash and the water is made to flow through the trough. The ash
directly falls into the troughs and is carried by water to
Ash sumps. In the sump the ash and water are made to pass
Belt conveyor
Water trough bunker through a screen so that water is separated from ash ; this
Control water is pumped back to the trough for reuse and ash is
valve removed to the dumping yard.
Truck

Fig. 3.39. Mechanical handling system.

Ash
sump Water

Sumps are used Water troughs Boilers

alternately for
settling out

Ash
sump Water
One stand by trough

Water
to permit repairs

Sump Boilers Water troughs

Water

Fig. 3.40. Low pressure system.

The ash carrying capacity of this system is 50 tonnes/ the top and on the sides. The top nozzles quench the ash
hour and distance covered is 500 metres. while the side ones provide the driving force for the ash.
(b) High pressure system. Refer to Fig. 3.41. The The cooled ash is carried to the sump through the trough.
hoppers below the boilers are fitted with water nozzles at The water is again separated from ash and recirculated.
 84 POWER PLANT ENGINEERING

The ash carrying capacity of this system is as large 7. Its ash carrying capacity is considerably large,
as 120 tonnes per hour and the distance covered is as large hence suitable for large thermal power plants.
as 1000 metres.
3. Pneumatic system
Nozzle Furnace Fig. 3.42 shows the schematic of a pneumatic ash
handling system. This system can handle abrasive ash as
High Ash
well as fine dusty materials such as fly-ash and soot. It is
pressure preferable for the boiler plants from which ash and soot
water Stoker must be transported some far off distance for final disposal.
The exhauster provided at the discharge end creates
High pressure
Nozzle
water
a high velocity stream which picks up ash and dust from
High all discharge points and then these are carried in the
pressure Nozzle conveyor pipe to the point of delivery. Large ash particles
water Cast iron walls
are generally crushed to small sizes through mobile
Trough crushing units which are fed from the furnace ash hopper
carrying
Main sump and discharge into the conveyor pipe which terminates into
water and ash a separator at the delivery end.
The separator working on the cyclone principle
Fig. 3.41. High pressure system. removes dust and ash which pass out into the ash hopper
at the bottom while clean air is discharged from the top.
Advantages of hydraulic system :
The exhauster may be mechanical or it may use
1. The system is clean and healthy.
steam jet or water jet for its operation. When a mechanical
2. It can also be used to handle stream of molten exhauster is used it is usually essential to use a filter or
ash. washer to ensure that the exhauster handles clear air. Such
3. Working parts do not come into contact with the type of exhauster may be used in a large station as the
ash. power requirements are less. Steam exhauster may be used
4. It is dustless and totally closed. in small and medium size stations. Where large quantities
5. It can discharge the ash at a considerable of water are easily and cheaply available water exhauster
distance (1000 m) from the power plant. is preferred.
6. The unhealthy aspects of ordinary ash basement The ash carrying capacity of this system varies from
work is eliminated. 25 to 15 tonnes per hour.

Secondary ash
Boiler Boiler separator

Crushers

Filter

Air from Air and Primary ash

separator h
atmosphere ash pipe As
Ash
hopper Exhauster
Ash carrying
truck

Fig. 3.42. Pneumatic or vacuum extraction ash handling system.

Advantages : 3. There is no chance of ash freezing or sticking in
1. No spillage and rehandling. the storage bin and material can be discharged freely by
2. High flexibility. gravity.
 STEAM POWER PLANT 85

4. The dustless operation is possible as the 3. Due to abrasive action of ash the pipes undergo
materials are handled totally in an enclosed conduit. greater wear (and to reduce this wearing action the pipes
5. The cost of plant per tonne of ash discharged is are lined with nickel alloy).
less in comparison to other systems.
3.12. DUST COLLECTION
Disadvantages :
1. There is a large amount of wear in the pipe work 3.12.1. Introduction
necessitating high maintenance charges.
2. More noisy than other systems. The products of combustion of coal-fed fires contain
particles of solid matter floating in suspension. This may
4. Steam jet system be smoke or dust. If smoke, the indication is that combustion
In this case steam at sufficiently high velocity is conditions are faulty, and the proper remedy is in the design
passed through a pipe and dry solid materials of and management of the furnace. If dust, the particles are
considerable size are carried along with it. In a high mainly fine ash particles called “Fly-ash” intermixed with
pressure steam jet system a jet of high pressure steam is some quantity of carbon-ash material called “cinder”.
passed in the direction of ash travel through a conveying Pulverised coal and spreader stoker firing units are the
pipe in which the ash from the boiler ash hopper is fed. principle types causing difficulty from this source. Other
The ash is deposited in the ash hopper. stokers may produce minor quantities of dust but generally
This system can remove economically the ash not enough to demand special gas cleaning equipment. The
through a horizontal distance of 200 m and through a two mentioned are troublesome because coal is burned in
vertical distance of 30 m.
suspension—in a turbulent furnace atmosphere and every
Advantages : opportunity is offered for the gas to pick up the smaller
1. Less space requirement. particles and sweep them along with it.
2. Less capital cost in comparison to other systems. The size of the dust particles is measured in microns.
3. Auxiliary drive is not required. The micron is one millionth of a metre. As an indication of
4. It is possible to place the equipment in awkward the scale of this measure, the diameter of a human hair is
position too. approximately 80 microns. Typical classification of particles
by name is given in Fig. 3.43, but the limits shown are, for
Disadvantages : the most part, arbitrary. A critical characteristic of dust is
1. Noisy operation. its “Settling Velocity” in still air. This is proportional to the
2. This system necessitates continuous operation product of the square of micron size and mass density.
since its capacity is limited to about 7 tonnes per hour.
100
50 Spreader
Size – Microns

Smoke Dust
stoker
Cinder
10
Range – Electrostatic 5
Mechanical
Pulv. coal
1
01 1 1 10 100 1000 1 10 100
(a) Particle size – Microns (b) Per cent smaller than

Fig. 3.43. Typical particle sizes : (a) Flue gas particles and ranges of collecting equipment.
(b) Typical distribution of particle size in products of combustion.

3.12.2. Removal of Smoke (ii) A smoky atmosphere is unhealthy.

Smoke is produced due to the incomplete combustion of (iii) Smoke corrodes the metals, darkens the paints
fuels. Smoke particles are less than 1 micron in size. The and gives lower standards of cleanliness.
smoke disposal to the atmosphere is not desirable due to In order to check the nuisance of smoke the coal
the following reasons : should be completely burnt in the furnace. The presence of
(i) Smoke is produced due to incomplete combustion dense smoke indicates poor furnace conditions and a loss
of coal. This will create a big economic loss due loss of in efficiency and capacity of a boiler plant.
heating value of coal.
 NON-CONVENTIONAL POWER GENERATION AND DIRECT ENERGY CONVERSION 529

3. Utilization of only part of the photon energy for (ii) The manufacturing cost is low (possibly avoiding
creation of electron hole pairs. the need for single crystal growth).
4. Incomplete collection of electron-hole pairs. (iii) High power-to-weight ratios.
5. A voltage factor. (iv) Low array costs, because the number of
6. A curve factor related to the operating unit at connections needed will be greatly reduced.
maximum power. The example of this type of cell is cadmium sulphide
(CdS) cells. CdS cells having areas of 50 cm2 have been
7. Additional delegation of the curve due to internal
made by evaporating the semiconductor on to a flexible
series resistance.
substrate such as kapton, a metallized plastic substrate.
Fabrication of Cells : A barrier layer of copper sulphide is then deposited on top
of the CdS. Power to weight ratios of 200 watts/kg are
A. Silicon cells
claimed for such cells. These cells have low efficiency and
Silicon cells are most widely used. Next to oxygen, instability.
silicon is the most abundant element on earth, the pure
silicon used in cell manufacture is extracted from sand Advantages and disadvantages of Photovoltaic
which is mostly silicon dioxide (SiO2). The silicon required solar energy conversion
for solar cell use, because of its high purity, is expensive. Advantages :
The fabrication of silicon cells include the following (i) There are no moving parts.
steps : (ii) Solar cells are easy to operate and need little
(i) The pure silicon is placed in an induction furnace maintenance.
where boron is added to melt. This turns the (iii) They have longer life.
crystal resulting from the melt into P-type (iv) They are highly reliable.
material. (v) They do not create pollution problem.
(ii) A small seed of single crystal silicon is dipped (vi) Their energy source is unlimited.
into the melt and withdrawn at a rate slower (vii) They can be fabricated easily.
than 10 cm per hour, the resulting inset looks
(viii) They have high power to weight ratio.
like a medium sized carrot. The rate of growth
and other conditions are adjusted so that the (ix) They can be used with or without sun tracking,
making possible a wide range of application
crystal that is pulled is a single crystal.
possibilities.
(iii) Wafers are then sliced from the grown crystal
(x) They have ability to function unattended for long
by the use of a diamond cutting wheel. The slices
periods as evident in space programme.
are then lapped, generally by hand, to remove
the saw marks and strained regions. Disadvantages :
(iv) After a fine lap the slabs are etched in (i) The cost of a solar cell is quite high.
hydrofluoric acid or nitric acid to complete the (ii) The output of a solar cell is not constant, it varies
first phase of preparation of the cells. We now with the time of day and weather.
have thin slices of P-type silicon with a carefully (iii) Amount of power generated is small.
finished surface.
(v) The wafers are then sealed in a quartz tube 10.7.4. Magnetohydrodynamics (MHD) System
partly filled with phosphorous pentoxide and the Introduction. Magnetohydrodynamics (MHD), as the
arrangement is placed in a diffusion furnace name implies, is concerned with the flow of a conducting
where temperature is carefully controlled ; this fluid in the presence of magnetic and electric field. The
process causes the phosphorous to diffuse into fluid may be gas at elevated temperature or liquid like
the P-type silicon to a depth of about 10–4 cm to sodium or potassium.
10–5 cm. MHD generator is a device which converts heat
(vi) The cells are then etched in a concentrated acid energy of a fuel directly into electrical energy without a
to remove unwanted coatings that formed during conventional electric generator. MHD converter system is
manufacture. Wax or Teflon masking tape is a heat engine whose efficiency, like all heat engine, is
used to protect the surfaces not to be etched. increased by supplying the heat at the highest practical
temperature and rejecting it at the lowest practical
B. Thin film solar cells temperature. MHD generation looks the most promising
These cells have the following advantages : of the direct conversion techniques for the large scale
(i) The material cost is low. production of electric power.
 530 POWER PLANT ENGINEERING

Principle of MHD Power Generation :

Faraday’s law of electromagnetic induction states
N N
that when a conductor and a magnetic field move in respect
to each other, an electric voltage is induced in the conductor.
The conductor need not be a solid—it may be a gas or liquid.
Gas Gas
The magnetohydrodynamic (MHD) generator uses this
flow flow
principle by forcing a high-pressure high temperature S S
combustion gas through a strong magnetic field.
Fig. 10.33 shows the comparison between a Turbogenerator MHD generator
turbogenerator and the MHD generator. Fig. 10.33. Comparison between the conventional
MHD systems turbogenerator and the MHD generator.

The broad classification of the MHD systems is as Open Cycle MHD systems
follows : Fig. 10.34 shows an open cycle MHD system. Here
1. Open cycle systems the fuel (such an oil, coal, natural gas) is burnt in the
2. Closed cycle systems combustion chamber, air required for combustion is
supplied from air preheater. The hot gases produced by the
(i) Seeded inert gas systems combustion chamber are then seeded with a small amount
(ii) Liquid metal systems. of an ionized alkali metal (cesium or potassium) to increase

Inverter
A.C. D.C. Electrode
Stack power supply
Seed Air D.C. Magnet
recovery supply Nozzle
Fuel
Steam Air Combus-
generator preheater tor
Removal Hot
of N2 and S gases Hot air
G
Steam
turbine
Generator
Make up
seed

Fig. 10.34. Open cycle MHD system.

the electrical conductivity of the gas. The ionization of heated in the breeder reactor is passed through the nozzle
potassium (generally potassium carbonate is used as seed where its velocity is increased. The vapour formed due to
material) takes place due to gases produced at temperature nozzle action are separated in the separator and
of about 2300–2700°C by combustion. The hot pressurised condensed and then pumped back to the reactor as shown
working fluid so produced leaves the combustion chamber in Fig. 10.35. Then the liquid metal with high velocity is
and passes through a convergent divergent nozzle. The passed through MHD generator to produce D.C. power. The
gases coming out the nozzle at high velocity then enter the liquid potassium coming out of MHD generator is passed
MHD generator. The expansion of the hot gases take place through the heat exchanger (boiler) to use its remaining
in the generator surrounded by powerful magnents. The heat to run a turbine and then pumped back to the reactor.
MHD generator produces direct current. By using an This system entails many constructional and
inverter this direct current can be converted into operational difficulties.
alternating current. Advantages of MHD systems
Closed cycle MHD systems 1. More reliable since there are no moving parts.
A liquid metal closed cycle system is shown in 2. In MHD system the efficiency can be about 50%
Fig. 10.35. A liquid metal (potassium) is used as working (still higher expected) as compared to less than
fluid in this system. The liquid potassium after being 40% for most efficient steam plants.
 NON-CONVENTIONAL POWER GENERATION AND DIRECT ENERGY CONVERSION 531

Condenser 10.7.5. Electrostatic Mechanical Generators

Electrostatic mechanical generators convert mechanical
MHD Pump energy, usually mechanical potential energy of a fluid
Feed Separator Nozzle
Gen. directly into electrical energy.
water
Fig. 10.36 shows the principle of working of liquid
han t
exc Hea

Reac-
ger

Inverter tor drop electrostatic mechanical generator. In this, the

gravitational potential energy of water droplets is directly
Steam converted into electrical energy. The electric charge is
Generator transferred from one electrode to another by an insulated
Turbine
belt. All these electrostatic devices are having the
Pump
characteristic of fairly low currents and very high voltages.
Liquid potassium This is yet only a laboratory model and commercial power
generation has yet to be done.
Fig. 10.35. Closed cycle (liquid metal) system.
3. Power produced is free of pollution. Water
4. As soon as it is started it can reach the full power supply
level.
5. The size of plant is considerably smaller than
conventional fossil fuel plants.
6. Less overall operational cost. + –
7. The capital cost of MHD plants is comparable to
– +
those of conventional steam plants.
d+ d–
8. Better utilization of fuel.

and electrode
9. Suitable for peak power generation and

Catch basin
emergency service.
0+
Drawbacks of MHD system
1. MHD systems suffer from the reverse flow (short Positively Negatively
circuits) of electrons through the conducting fluids around charged charged
the ends of the magnetic field. This loss can be reduced by
(i) increasing aspect ratio (L/d) of the generator, (ii) by
permitting the magnetic field poles to extend beyond the Fig. 10.36. Liquid drop electrostatic mechanical generator.
end of electrodes, and (iii) by using insulated vans in the
10.7.6. Electro Gas-Dynamic Generators (EGD)
fluid ducts and at the inlet and outlet of the generator.
2. There will be high friction losses and heat transfer The EGD generator uses the potential energy of a high
losses. The friction loss may be as high as 12% of the input. pressure gas to carry electrons from a low potential
electrode to a high potential electrode, thereby doing work
3. The MHD system operates at very high
against an electric field. A schematic diagram of EGD is
temperatures to obtain high electrical conductivity. But
shown in Fig. 10.37.
the electrodes must be relatively at low temperatures and
Carona electrode at the entrance of the duct
hence the gas in the vicinity of the electrodes is cooler.
generates electrons. This ionised gas particles are carried
This increases the resistivity of the gas near the electrodes
down the duct with the neutral atoms and the ionized
and hence there will be a very large voltage drop across
particles are neutralised by the collector electrode, at the
the gas film. By adding the seed material, the resistivity
end of the insulated duct. The working fluid in these
can be reduced.
systems are commonly, either combustion gases produced
4. The MHD system needs very large magnets and by burning of fuel at high pressures or it is a pressurised
this is a major expense. reactor gas coolant. The maximum power output from EGD
5. Coal, when used as a fuel, poses the problem of is about 10 to 30 W per channel. Hence, several thousand
molten ash which may short circuit the electrodes. Hence channels are connected in series and parallel. The voltage
oil or natural gas are considered to be much better fuels produced is very high, of the order of 1,00,000 to 2,00,000 V.
for this system. This restriction on the use of fuel makes Thus, it needs very good high voltage insulators. (Beryllium
the operation more expensive. oxide, Beo, is generally used).