thermal power engineering
DESCRIPTION
basics of thermal power engineeringTRANSCRIPT
Thermal Power Engineering
Course Content
• Energy Sources and Scenario: Conventional and Non-conventional
• Review of Power Plant Cycles – Reheat – Regenerative – Supercritical – Coupled and Combined – Cogeneration Plants, Exergy analysis of plant cycles.
• Solid fuels and combustion, Fluidized bed combustion
• Analysis and sizing of power plant components: Steam generator, Condenser, cooling tower and other heat exchangers
• Principle of Nuclear Energy – Nuclear Power Plants
• Power Plant Economics – Recent tends in Power Production
Why does a power plant trip?
The supply frequency of a power plant is dependent on the load.
In contrast, the grid has a constant frequency.
Due to a difference in frequency between the plant and the grid, during some part of the waveform, power flows from the grid to the plant.
This is undesirable as this power is converted into heat inside the alternator.
The temperature of the alternator windings should not exceed, say, 100oC. Otherwise the coated insulation on the alternator windings ‘burns’ off.
Why does a power plant trip?
As the insulation burns off, short circuiting of the windings
occurs and the number of turns of the coils decreases.
With decrease in the number of turns the resistance to the
current flow decreases.
This causes more heat to be generated.
As a result, the number of turns decreases even further.
The vicious cycle continues until the entire generator is lost!
Why does a power plant trip?
• Alternators are very expensive.
• From placement of purchase order of an alternator to delivery, it could take many months.
• It will be very uneconomical to keep the other elements of the plant idle during this period.
• The control systems have, typically, up to 15 seconds to match the grid frequency and restore the system.
Otherwise the system trips
BLACK OUT or BROWN OUT: In blackout entire grid fails while in brownout control system fails and we get poor quality of electricity.
At what speed do turbines run?
• You might know that steam turbines and gas
turbines run typically at about 30,000 rpm.
• That is the speed at which they are most efficient.
• But do you know at what speed the steam turbine
works in a power plant?
Steam turbines in power plants run at 3000 rpm.
Let us see how this is chosen.
At what speed do turbines run?
We need a supply frequency of 50Hz.
In USA they use 60 Hz.
f = pn / 120.
Here p = 2 (pole machine) and
f = 50 Hz gives
n = 3000 rpm
n’ is the speed of the electrical machine and not of the steam
turbine.
Then why do we choose to run the turbine at 3000 rpm despite
the turbine efficiency being lower at that speed?
At what speed do turbines run?
Let us assume that we choose to run the turbine at 30,000 rpm.
Then we need to reduce the speed from this to 3000 rpm of the alternator shaft using a gearbox.
The speed reduction factor is 10.
• For such a high value of the speed reduction factor, the frictional losses will be high.
• Also imagine the size of the gear box!
• So the overall efficiency of turbine + transmission is higher with the turbine running at 3000 rpm than with 30,000 rpm.
Steam Power Plant • Steam Power Plant – bulk energy converted from fuel to electricity.
Since the fluid is undergoing a cyclic process, there will be be no net change in its internal energy over the cycle
Therefore 0dE
Q W
in out T PQ Q W W
1net in out outT Pcycle
in in in in
W Q Q QW W
Q Q Q Q
Review of Power Plant Cycles
The efficiency of the vapor power cycle would thus be
Rankine Cycle(Ideal) William John M. Rankine(1820-1872) was a Professor of Civil Engg. At Glasgow
University.
The Rankine cycle is a vapour-and-liquid cycle.
Rankine Cycle
B
1! Super heater
Boiler/Evaporator
Economizer
Steam Generator
1
2
1!
4 B
3
Rankine Cycle(Ideal)
3
4
1
2
B 1!
4 3
1
2
1!
B
1-2 or 1!-2! Adiabatic reversible expansion through turbine. The exhaust vapour at 2 or 2! is usually in two-phase region. 2-3 or 2!-3 Constant temperature and being a two-phase mixing process, Constant pressure heat rejection in the condenser. 3-4 Adiabatic reversible compression by a pump of saturated liquid at the condenser pressure, 3, to a sub-cooled liquid at the steam generator pressure 4. 4-1 or 4–1! Constant pressure heat addition in the steam generator. 4-B represent an economizer B-1!
represent the boiler or evaporator 1!-1 represent the super heater
Some Parameter in Rankine Cycle(Ideal) Work Ratio
Steam Rate The capacity of a steam plant is often expressed in terms of Steam Rate(SR)
or
Specific Steam Consumption(SSC).
It is defined as the rate of steam flow(kg/s) required to produce unit shaft output(1kW)
Steam rate(SR) = 1/Wnet ( kg/kWs)
Heat Rate The heat rate is heat input (kJ/s) required to produce unit shaft output(1kW)
1 1Heat rate
T P
q kJ
w w kWs
Net Work
Gross Work
netR
T
WW
W
Performance Improvement
Mean Temperature of Heat Addition
• In the Rankine cycle, heat is added reversibly at a constant pressure but at infinite
temperatures. If Tm1 is the mean temperature of heat addition.
• The area under 4-1 is equal to the area under 5-6, the heat added is
q1 = h1-h4=Tm1 (s1-s4)
Therefore, mean temp. of heat addition
Since heat rejected q2 = h2-h3=T2(s1-s4)
Therefore,
The lower is the T2 for a given Tm1
lower is the condenser pressure, the higher will be the efficiency of the Rankine
cycle. But, lowest practical temp. of heat rejection is To (surroundings).
The sat. pressure corresponding to this temp.To is the minimum pressure to which
steam can be expanded in turbine. This being fixed by the ambient conditions,
The higher the Tm1 of heat addition the higher will be the cycle efficiency.
1 41
1 4
m
h hT
s s
2 2 1 4 2
1 1 4
( )1 1 1
( )m m
q T s s T
q T s s T
1
6
2 3
5
4
1!
B
Tm1
1( ) onlymf T
Effect of Superheat
Increasing the temperature of the steam entering the turbine (superheat)
• Increase in the heat input requirement
• Increase in the net work output
• The liquid content at the exit of the turbine is decreased
• The maximum possible temperature is limited by the properties of the materials used.
p
P P p
S S TC T
T T C
gas water
T T
S S
a
e
1!!
1!
1
2!! 2 3
4
If hot flue gas is the primary fluid or heat source for steam generation in the power cycle, the use of super heat also reduces the thermal irreversibility. Since Now (Cp)water > (Cp)gas Therefore Fossil fuel steam generators as well as gas cooled and liquid metal cooled nuclear power plant employ superheat.
Effect of Inlet Pressure Increasing the boiler pressure
• The heat addition process involves a constant pressure phase change.
Tsat ∝ Psat
• The liquid content at the exit of the turbine is increased.
Reheating of Steam
Ideal reheat Rankine cycle
Two-stage expansion and heat addition (reheat)processes.
The steam is resuperheated (or reheated) at constant pressure in boiler(2-3) and the remaining
expansion(3-4) of steam is carried out in the low pressure(L.P.) turbine.
For 1kg of steam q1 = h1-h6+h3-h2 q2 = h4-h5
WT = h1-h2+h3-h4 Wp = h6-h5
The efficiency is given by
1
2
3
6
4 5
8 7
1 2 3 4 6 5
1 1 6 3 2
( ) ( )
( )
T pW W h h h h h h
q h h h h
Ideal Regenerative Cycle
Adopting the Stirling cycle procedure to a Rankine Cycle.
The Condensate after leaving the pump circulates around the turbine casing so that heat is
transferred from the vapor expanding in the turbine to the condensate circulating around it.
It is assumed that this heat transfer process is reversible, i.e. at each point the temperature of
vapor is only infinitesimally higher than the the temperature of the liquid.
The process 1-2 thus represents reversible expansion of steam in the turbine with reversible
heat rejection to the surrounding liquid heated reversibly in the process 4s-5.
Regenerative Feedwater Heating
A compromise that would reduce rather than eliminate the economizer irreversibility is
accomplished by the use of feedwater heating.
Feedwater heating involves normal adiabatic expansion in the turbine.
The compressed liquid at 4 is heated in a number of finite steps, rather then continuously, by
vapor bled form the turbine at selected stages.
Heating of the liquid takes place in heat exchanges called Feedwater Heaters.
Modern large steam power plants use between 5 and 8 Feedwater heating stages.
Three type of feedwater heaters in use:
1. Open or direct-contact type
2. Closed type with drains cascaded backward
3. Closed type with drains pumped forwarded (additional pump called drip pump is needed)
Regeneration Cycle with Open Feedwater Heater
For 1kg of steam at turbine inlet, m1kg of steam is extracted at pressure P2 to adiabatically in
the open feedwater heater.
The remaining steam (1-m1)kg expands reversibly to the condenser pressure P3.
The heat and work transfer quantity of the cycle are
WT = 1(h1-h2)+(1-m1)(h2-h3) Wp = (1-m1)(h5- h4)+1(h7 - h6)
q1 = 1(h1- h7) q2 = (1-m1)(h3- h4)
The efficiency η = (q1-q2)/q1 = (WT-Wp)/q1
1
2
3 4
1! B
7
6 5
1
2
3
4 5
6
7 1kg
m1
1-m1
1kg
m1
1-m1
Regeneration Cycle with Closed-Type Feedwater Heaters
Closed heaters are Shell-and-tube heat exchangers where the feedwater flows
through the tubes and the extracted steam condenses outside the tubes in the
shell.
The heat released by condensation is transferred through the walls of the tubes.
Regeneration Cycle with Closed-Type Feedwater Heaters
Closed heaters are Shell-and-tube heat exchangers where the feedwater flows
through the tubes and the extracted steam condenses outside the tubes in the
shell.
The heat released by condensation is transferred through the walls of the tubes.
Regeneration Cycle with Feedwater Heaters and Reheating
Considering Irreversibilities
Placement of Feed water Heaters What are the pressures at which steam is to be bled form the turbine that will results in the
maximum increase in efficiency?
For one feedwater heater, three positions(1,2 and 3) may be considered with respect to cycle.
The heat transfers to the feed water are caused
by ΔTB1 and ΔT1C .
In position 3 the corresponding heat transfers
are the results of TB-T3 and T3-TC.
In both these cases one of these ΔT’s is very
large.
In position 2, both temp. differences are
minimum,
where TB-T2=T2-TC, Thus the optimum from
efficiency point view.
In general, for n feedwater heaters, the optimum temp. rise per heater would be given as:
ΔTB1 = (TB-TC)/n+1
Supercritical Pressure Cycle
In Fig. the feed water is pressurized at 8 to a pressure beyond the
critical pressure of the vapour(221.2 bar).
The feed water heating curve shoes a gradual charge in temp and
density but not in phase to the steam temperature at 1.
Such heating can be made to be closer to the hear source temperature
than a sub critical cycle with the steam temp that shows on abrupt
change in temp with in the two phase region.
Looking at the another way, the supercritical pressure cycle receivers
more of its heat at higher temperature than a sub critical cycle with
the same turbine inlet steam temperature.
Because of gradual change in the density, supercritical-pressure cycle
use once through steam generators instead of the most common
drum-type steam generators.
Cogeneration
Cogeneration is the simultaneous generation of electricity and steam
(or heat) in a single power point.
There are several industries such as paper mills, textile mills,
chemical factories, jute mills, sugar factories, rice mills and some
where saturated steam at the desired tem is required for heating
drying etc.
Apart from the process heat, the factory also needs power to drive
various machines for lighting and other purposes.
Cogeneration is not usually used by large utilities which tend to
produce electricity only.
Cogeneration
From an energy resource point of view, cogeneration is beneficial if it
saves primary energy when compared with separate generation of
electricity and steam (or heat).
The cogeneration plant efficiency ηco is given by
(1)
Where E = electricity generated
ΔHs = heat energy, or heat energy in process steam
(enthalpy of steam entering the process) - ( enthalpy of process
condensate returning to plant)
Q1 = heat added to plant
1
sco
E H
Q
Types of Cogeneration
There are two board categories of cogeneration:
1. The topping cycle
2. The bottoming cycle
1.The topping cycle:-in which primary heat at the higher temperature
end of the Ramkine cycle is used to generate high pressure and
temp steam and electricity in the usual manner.
Depending on process requirements, process steam at low pressure
and temperature is either
(a)Extracting from the turbine at an immediate stage, much as for feed
water heating, or
(b) taken at the turbine exhaust, in which case it is called a break
pressure turbine. process steam pressure requirement vary widely,
between 0.5 and 40 bar.
Types of Cogeneration
2. The bottoming cycle:- in which primary heat is used at high
temperature directly for process requirements .e.g the high-temp
cement Kiln.
The process low-grade (low temp. and availability) waste heat is then
used to generate electricity , obviously at low efficiency.
Only the topping cycle, therefore can provide true saving in primary
energy. Introduction, most process applications require low grade
steam. Such steam is conveniently produced in a topping cycle.
Binary vapor Cycles
In a binary vapor cycle two working fluids are used, one with high
temperature characteristics and another with good characteristics
at the lower-temp. end of the operating rang.
(Hg(topping)-H2O(bottoming) cycle,)
In an actual plant, the steam cycle is always a regenerative cycle with
feed water heating.
Coupled Cycles
1. The Hg-H2O cycle represents the two fluid cycles where 2 Rankine
cycles have been coupled in series.
2. If a SO2 cycle is added to it in the low temp. range, so that the heat
released during the condensation of steam is utilized in forming
SO2 vapor which expands in another turbine,
Then 3 fluid or tertiary cycle i.e. Hg-H2O-SO2 cycle
3. Similarly, other liquid metal, apart from mercury, Na or K may be
considered for a working fluid in the topping cycle.
4. Apart from SO2, other refrigerants( NH3, freons etc.) may
considered as working fluids for the bottoming cycle.
5. Na-Hg-steam tertiary cycle is shown in Fig.
Combined gas–steam power plant.