Unit 2
Nuclear, Diesel, Gas Power Plant
Atomic physics
● the smallest particle of an element which takes part in chemical reaction is known as an atom.
● The complex structure of an atom can be classified into electrons and nucleus. The nucleus consists of proton and neutron.
● Atom has diameter
K L M N
Electron 2 8 18 32
Mass number:- sum of total number of protons and neutrons in nucleus.
A=N+Z
N=number of electron
Z=atomic number
Atomic mass unit:-
The mass of the atom is expressed in term of the mass of the electrons.
Isotopes:-
In any atom, the number of electron=number of proton. This is independent of neutrons in nucleus. Atom having different number of neutrons then the number of protons are known as isotopes.
Radioactivity:-
The phenomenon of spontaneous emission of powerful radiations exhibits by heavy element is called radioactivity.
Example:- uranium, polonium, radium, radon, ionium, thorium, actinium & mesothorium.
Radioactive element emitted
1) Alpha rays or alpha particles
2) Beta ( rays or beta particles
3) Gamma ( rays or photons
● Natural radioactivity
● Artificial radioactivity
Nuclear radiation:-
In nuclear power technology there are only five types of radiation of interest
1) Gamma rays (or photons):- electromagnetic radiation
2) Neutrons:- uncharged particles, mass approximately 1
3) Photons:- +1 charged particles, mass approximately 1
4) Alpha particles:- helium nuclei, charge +2, mass.
5) Beta particle:- electron (charge -1), proton (+1), Mars very small
Nuclear reactions:-
During a nuclear reaction, the change in the mass of particle represents the release or an absorption of energy.
If the total mass of the particle after the reaction is reduced, the process releases the energy, consequently, the increase in the mass of the resultant particle will cause the absorption of energy.
A neutron is written as:
Electron is written as:
Example:- when is bombarded with the protons possesses high energy it is converted to
q- release or absorption of energy
2) when is bombarded with high energy protons it is transformed to
Transformation may adopt
1) Elastic scattering
2) Inelastic scattering
3) Capture
4) Fission:-
In the process that occurs when a neutron collide with the nucleus of certain of the heavier items causing the original nucleus to split into two or more unequal fragments which carry of most of energy of fission as kinetic energy.
Materials
I) Structural
1) Aluminium
2) Stainless steel
3) Nickle alloy
4) Zirconium
5) Magnesium
II) Fuel
1) Uranium
2) Uranium ceramics
3) Thorium
4) Thorium oxide
III) Coolent
1) Water
2) Liquid metals
3) Sodium, potassium
4) Mercury
5) Helium
6) Nitrogen
IV) Control
1) Boron steel
2) Cadmium
3) Somorium oxide
V) Moderator reflector
1) Water
2) Heavy water
3) Beryllium
4) Graphite
VI) Shielding
1) Water
2) Cement and concrete
3) Iron
4) Lead
5) Boron
6) Bismuth
Site selection
1) Distance from load centre
2) Distance from populated area
3) Availability of water
4) Land use
5) Waste disposal
6) Accessibility site
7) Safeguard against earthquakes
Plant layout of nuclear power plant
Nuclear reactor
Definition:- nuclear reactor is an apparatus in which nuclear fission is produced in the form of a controlled self sustaining chain reaction. In other words it is controlled chain reaction system.
Mechanism of heat production:-
Most of the energy is imported to the two fusion fragment into which the nucleus divides causing them to move at high speed. However, because they have taken birth in a dance mass of metal, they are rapidly slowed down and brought to rest by colliding with other atoms of the metal. In so doing, their energy is converted into heat in much the same way as energy given up by slowing motor can be converted into heat in the brake lining. In this way, the mass of uranium metal gets heated up.
1) Reactor core:- the reactor core is that part of a nuclear power plant where fission chain reaction is made to occur and where fission energy is liberated in the form of heat for operating power conversion equipment.
Diameter -0.5 to 15m.
2) Reflector:- a reflector is usually placed round the core to reflect back some of the neutrons that leak out from the surface of the core. It is generally made of the same material as the moderator.
3) Control mechanism:- it is an essential part of a reactor and serve the following purpose.
a) For starting the reactor
b) For maintaining operating level at steady state.
c) For shutting the reactor down under normal or emergency condition.
The control system is necessary to prevent the chain reaction from becoming violent and damaging the reactor. The number of neutron keep on increasing and temperature also increase rapidly.
4) Moderator:-
Function
a) To slow down the neutron from the high velocities.
b) To slow down the neutrons but not absorb them:
Materials:-
5) Coolants:-The function of a coolant is to remove the intense heat produced in the reactor and to bring out for being utilized.
Classification of nuclear reactor-
Nuclear reactors may be classified in several ways i.e on basis of their applications, type of fission, fuel used, state of fuel, fuel cycle, arrangement of fissile and fertile materials, are arrangement of fuel and moderator, moderator material, cooling system employed, coolant used etc.
- According to the application the reactors are classified as
- Research and development reactors- The reactors are used for testing new reactor designs and research
- Production- These reactors are used for converting fertile materials into fissile materials
- Power- These reactors are used for generation of electrical energy
2. According to the type of fission the reactors are classified as fast reactors, slow reactors and intermediate reactors.
3. According to the type of fuel use the reactors may be classified as.
- Natural Uranium
- Enriched Uranium
- Plutonium
4. According to the state of fuel the reactors may be classified as
- Solid
- Liquid
5. According to the fuel cycle the reactors may be classified as
- Burner thermal reactor- Such reactors a design for generating heat only without any recovery of converted fertile material
- Converter reactor- Such a reactor convert fertile materials into fissile material different from the one initially fed into the reactor core.
- Breeder reactor- Such reactors convert fertile material into fissile material which is similar to one initially supplied to the reactor core.
6. As per arrangement of fissile and fertile material the reactors may be classified as
- One reason ( fissile and fertile material mixed)
- Two reason ( fissile and fertile material separate)
7. According to the arrangement of fuel and moderator the reactors may be classified as
- Homogeneous
- Heterogeneous
8. On the basis of moderator material used the reactors may be classified as
- Heavy water
- Graphite
- Ordinary water
- Beryllium
- Organic reactors
9. On the basis of coolant used the directors may be classified as
- Gas
- Water
- Heavy water
- Liquid metal reactor
10. On the basis of cooling system employed the reactors may be classified as
- Direct
- Indirect reactors
Nuclear waste disposal-
- Solid radioactive wastes arise from use filters, sludge from the cooling ponds, pieces of discarded fuel element cans, splitters etc.
- This along with discarded items of plant such as control rods have to be stored on site in shielded concrete vaults.
- There are many ways for disposing of the solid fission products. The storing in shielded storage vaults consists in fixing the solid waste in boro-silicate glass and then storage of this class in leak tight capsules.
- These capsules can then be stored in deep salt mines or in deep wells drilled in the stable ocean floor.
- Deep salt mines are suggested because the presence of the salt rockets indicates that there has been no ground water in the vicinity for thousands of years. Sometimes, suitable containers are filled with radioactive waste and sunk to the bottom of Seas and oceans. However, this method does not completely prevent the radioactivity from leading into the water.
- Another way of disposal is the separation and transmutation of the long lived isotopes to short lived or stable products following neutron absorption in a breeder of fusion reactor.
- The past of firing these long live products into the sun or into a long-term stable orbit is also being considered.
- It is safe enough to store radioactive waste underground in liquid form in suitable tanks or in reduction to clinker.
- Gaseous effluents are filtered for discharging into atmosphere.
Diesel power plant
Main component and its working:-
2 to 50 MW
1) Engine-This is the main component of the power plant which develops power. The diesel engines employed for diesel electric power plants may be for 2 stroke engines. In a four stroke engine the complete cycle operation is performed in 4 strokes namely suction, compression, expansion and exhaust strokes and two revolutions of the engine.
In the case of two stroke diesel engine all the four operations are completed within two strokes of the piston for during one revolution of the crank shaft- the power and exhaust operations are completed during the travel of the Piston in backward direction while intake and compression operations are completed during the forward travel of the piston.
2) Air intake system-Air intake system is provided to supply necessary air to engine for fuel combustion. Air requirements of large diesel electric power plants are considerable (about 4 to 8 meter cube per kWh). The air filters are provided to remove dust and other suspended impurities from the ear to be supplied to the engine. The supercharger is usually employed to increase the pressure of intake air above atmospheric one in order to develop an increased the power output.
3) Exhaust system-This system is provided to discharge the engine exhaust to the atmosphere outside the building.
4) Fuel system-Fuel transfer pumps are required to transfer fuel from delivery point to storage tanks and from storage tanks to daily consumption tanks and then to engine. Strainers are provided to remove the suspended impurities and thus clean fuel supply to the engine. Heaters are required to heat the oil especially during winter season.
a) Fuel injection system
b) Fuel pump
5) Cooling system-It is known that the heat generated by the burning of fuel in the engine cylinder is partially converted into useful work. The remainder is wasted as heat in the outgoing exhaust gases and in hitting the engine, and if not removed main disintegrate the lubricating oil film on the cylinder walls and damaged the cylinder liners, heads, walls, piston and rings.
Temperature 35-2750°C
a) Air cooling
b) Liquid cooling
6) Lubrication system-This system is provided for lubricating the moving parts, removing the heat from the cylinders and the bearings, helping the piston ring to seal the gases in the cylinders and for carrying away the solid that particles from the rubbing parts.
7) Engine starting system-This system is provided to rotate the engine initially, while starting, until the firing starts and the unit runs under its own power.
a) Auxiliary engine
b) Used electric motor
c) Compressed air system:- 17 bar
Diesel plant efficiency:-
It is the ratio of indicated work done to energy supplied by the fuel.
I.P. :- total power developed by combustion of fuel in the combustion chamber is called indicated power.
:- mass of the fuel in kg/sec
C:- calorific value of fuel
1,4800kJ/kg
Heat balance sheet
The performance of an engine is generally given by heat balance sheet.
Item | KJ | Percent |
Heat supplied by fuel= | - | - |
Heat absorb in I.P.= | - | - |
Heat taken away by cooling tower= | - | - |
Heat carried away by exhaust gases= | - | - |
Heat unaccounted for different | - | - |
Choice of diesel power plant
1) Peak load plant
2) Mobile plant
3) Standby unit
4) Emergency unit
5) Nursery station
6) Starting station
7) Central station
8) Industrial concern where power requirement small se 500kW.
Site selection of diesel power plant
The following factors should be considered while selecting a site for diesel electric power plant
- Distance from the load centre- The site should be as near to the load centre as possible in order to avoid transmission cost and losses.
- Availability of land- The land should be available at cheap rate to keep the capital cost of the plant to the reasonable one.
- Availability of fuel-The fuel should be easily available and at reasonable rate.
- Availability of transportation facilities- The transportation facilities should be available.
- Availability of water- the water should be available in sufficient quantity for cooling purposes
- Distance from populated area- the site should be away from thickly populated area because of noise and nuisance caused from exhaust.
- Type of land- the land should be of high bearing capacity to withstand the load of the plant and also vibration transmitted to the foundations from compressor and diesel engines.
Gas power plant
Gas turbine electric power plant is a power plant in which a gas turbine is used as the prime mover for the generation of electrical energy.
In steam power plant the products of combustion do not form the working medium. These are utilized to produce and intermediate flood, called the steam, which is is expanded in the turbine. In case the in intermediate step of converting water to steam by means of gases is eliminated, the arrangement would be simple and less wasteful. This principle is employed in gas turbine electric power plants where the gas is directly expanded in the turbine. The gas turbine is different from the diesel engine in this respect that the combustion region is external to the prime mover.
Simple gas turbine power plant
In a gas turbine, the working medium is either a mixture of combustion products and air or heated air at a certain pressure and higher initial temperature.
The principle of of a gas turbine power plant is that 88 Turbo compressor compressors working medium to a high pressure and then it is burnt in the combustion chamber where the combustion takes place at a constant pressure and temperature of the working medium is raised.
The high pressure and high temperature working medium is then expanded in a gas turbine coupled to the generator. The overall efficiency of a gas turbine as a prime war is limited due to the fact that a large portion of the power developed by the turbine is used in driving the compressor and also so by the temperature safely attainable.
A simple gas turbine power plant essentially consist of the reaction type non condensing turbine, a compressor mounted on the same shaft and coupled to the turbine, the combustion chamber, an alternator coupled to the turbine itself and auxiliary is such as starting motor, auxiliary lubrication pumps, oil system, fuel system and duct system etc.
Gas cycle
Method to improve thermal efficiency:-
1) Intercooling
Intercooling means the removal of the heat from compress the ear between the stages of compression. This necessitates the use of compressors with two stages viz low pressure and high pressure compressor. The intercooler is heat exchanger which is the partly compressed air in order to reduce volume and increased density. the inter cooling results in improvement of thermal efficiency, air rate and work ratio. By use of intercooling the size of turbine and compressor for the same output is reduced.
2) Reheating
In reheating the combustion gases are not expanded in one turbine only but in two turbines. The Exhaust of the high pressure turbines is reheated in a reheater and then expanded in a low pressure turbine. Reheating improves the output from the turbine due to multiple heating in the same way a as intercooling improves the performance of the compressor. However the cost of additional fuel may be heavy e unless a heat exchanger is also employed.
3) Regeneration
Regenerator is usually capsule and tube construction. The exhaust gases are made to flow inside the nest of tubes while air flows outside the tubes in the shell in the counter flow and heated up by the heat given out by the exhaust gases.
Thus the Regenerator utilizes the heat of exhaust gases to heat the compressed air before it is sent to the combustion chamber, reduces the fuel consumption of the plant and improves the cycle thermal efficiency.
Open loop cycle power plant
Simple gas turbine P.P.
Closed loop cycle power plant
In a closed cycle plant the medium is heated externally and is continuously circulated through the compressor, heat exchangers, intercoolers, reheaters and gas recoolers. the pressure ratios, the temperature and their velocities in the system remain practically constant.
Advantages of closed cycle Power Plant
- The risk of corrosion and abrasion of the interiors of the turbines is eliminated as the turbines are kept free from the combustion products.
- Since the working medium is heated externally the fuel is not mix with it, it any fuel can be used.
- The working medium is at a relatively high internal pressure. So, so special big volume is left and the dimensions of the compressor and turbine can be reduced and the maximum unit capacity can be increased.
- A working medium with physical properties superior to those the of air may be used such as Helium, argon, hydrogen, neon etc.
- There is improvement in the rate of heat transmission.
- There is a reduction in fluid friction loss due to higher Reynolds number.
Gas turbine materials
● Turbine rotor are made of austenitic steel with 12% to 18% chromium, 8% to 12% nickel and small percentage of Titanium, molybdenum & tungsten.
● Uniform coating -: silicon carbide or silicon nitride -1500°C
Gas power plant layout
1) LP compressor
2) LP turbine
3) Starting motor
4) Generator
5) HP compressor
6) Combination chamber
7) Heat exchanger
Combined cycle power plants
The gas turbine power plants are mainly used for supplying peak loads another types of power plants eg. In steam and hydroelectric power plants. The heat content of gas turbine exhaust is quite substantial. The temperature of gas turbine exhaust is about 540 degree celsius for and is roughly 16%. Essentially a combined gas turbine steam turbine cycle aims at improving the overall plant efficiency by using the heat of exhaust gases from the gas turbine as a heat source for steam plant cycle.
There are three arrangements of combine cycles usually employed As given below
- Use of exhaust gases of gas turbine power plant for heating of feed water
- Use of exhaust gases from gas turbine as combustion air in steam boiler
- Use of gases from a supercharged boiler for expansion in the gas turbine
Advantages of combined cycle operation
- Saving in exhaust heat of the gas turbine and therefore, increase in its heat rate
- Reduction of stack emissions.
- Reduction in space requirement in comparison with conventional generating units of a given capacity.
- Reduction in the requirements of condenser water by 60% as compared to a fossil fuel plant of given capacity.
Concept of heat to power ratio
The most important parameter in cogeneration is ‘heat to power ratio’ which determines the proportion of generated heat to electrical power in a single cogeneration system.
New technologies with higher power-to-heat ratios include combined cycle (gas and steam) cogeneration plants, block-type thermal power stations, or fuel cells. For those, the power-to-heat ratio is around 1.0 and higher. This compares to ratios for existing plants of between 0.3 and 0.7, e.g., for steam power plants with back-pressure turbines.
Reference Books:
- [R1] Arora and Domkundwar, “A Course in Power Plant Engineering”, DhapatRai Publication.
- [R2] Dr. S. P. Sukhatme, “Solar Energy”, Tata McGraw Hill Publication.
- [R3] Mukund Patel, “Wind and Solar Power Plants”, CRC Press.
- [R4] Gilbert Masters John, “Renewable Energy”, Wiley and sons’ publications.
- [R5] Robert Foster, Majid Ghassemi, Alma Cota “Solar Energy” CRC Press