chapter 3: stationary combustion systems

CHAPTER 3: STATIONARY COMBUSTION SYSTEMS

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Dr. Anwar Abu-Zarifa . Islamic University Gaza . Department of Industrial Engineering

At the present time, most of the world’s electricity is producedusing combustion technologies that convert fossil fuels toelectricity.

These technologies have an application both in reducing fossilfuel consumption and CO2 emissions in the short to mediumterm, and as part of large-scale combustion-sequestrationsystems in the future.

These technologies are situated in electrical power plants, andconsist of three main components: (1) a means of converting fuel to heat, either a combustion

chamber for gasfired systems or a boiler for systems that use wateras the working fluid.

(2) a turbine for converting heat energy to mechanical energy. (3) a generator for converting mechanical energy to electrical

energy.

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Since fossil fuels are the leading resource for electricitygeneration, the majority of all of the world’s electricity isgenerated in these facilities.

By nuclear and hydro power plants use these threecomponents in some measure, although in nuclear powerthe heat source for boiling the working fluid is the nuclearreaction, and in hydro power there is no fuel conversioncomponent.

At present there exists a strong motivation to develop andinstall an efficient new generation of combustion-basedgenerating systems.

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Figure 6-1. Schematic of components of coal-fired electric plant, with conversion of coal to electricity via boiler, turbine, and generator

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Fundamentals of Combustion Cycle Calculation Any discussion of combustion cycles begins with a review

of the underlying thermodynamics. Recall that both thequantity of energy available, or enthalpy, and the quality ofthe energy, or entropy, are important for the evaluation ofa thermodynamic cycle.

According to the first law of thermodynamics, energy isconserved in thermal processes. In an energy conversionprocess with no losses, all energy not retained by theworking fluid would be transferred to the application, forexample, mechanical motion of the turbine.

In practical energy equipment, of course, losses will occur,for example, through heat transfer into and out ofmaterials that physically contain the working fluid.

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According to the second law of thermodynamics, acombustion cycle can only return from its initial state ofentropy back to that state with an entropy of equal orgreater value.

A process in which entropy is conserved is calledisentropic.

The Carnot cycle is the most efficient cycle operatingbetween two specified temperature limits but it is nota suitable model for power cycles.

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THE CARNOT VAPOR CYCLE

T-s diagram of two Carnot vapor cycles.

The Carnot cycle is the most efficient cycle operating between two specified temperature limits but it is not a suitable model for power cycles. Because:Process 1-2 Limiting the heat transfer processes to two-phase systems severely limits the maximum temperature that can be used in the cycle (374°C for water)Process 2-3 The turbine cannot handle steam with a high moisture content because of the impingement of liquid droplets on the turbine blades causing erosion and wear.Process 4-1 It is not practical to design a compressor that handles two phases.The cycle in (b) is not suitable since it requires isentropic compression to extremely high pressures and isothermal heat transfer at variable pressures.

1-2 isothermal heat addition in a boiler 2-3 isentropic expansion in a turbine 3-4 isothermal heat rejection in a condenser4-1 isentropic compression in a compressor


The Carnot limit provides a useful benchmark forevaluating the performance of a given thermodynamiccycle.

Let TH and TL be the high and low temperatures of athermodynamic process, for example, the incoming andoutgoing temperature of a working fluid passing through aturbine.

The Carnot efficiency ηCarnot is then defined as:

Thermal efficiency ηth:

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Quiz 1 A Carnot engine operates between 300°C and 40 °C. What

is the efficiency of the engine?

A. 87%B. 65%C. 45%D. 30%

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Dr. Anwar Abu-Zarifa . Islamic University Gaza . Department of Industrial Engineering 10


Rankine Vapor Cycle The Rankine vapor cycle is the basis for a widely used

combustion cycle that uses coal, fuel oil, or other fuels tocompress and heat water to vapor, and then expand the vaporthrough a turbine in order to convert heat to mechanicalenergy.

It is named after the Scottish engineer William J.M. Rankine,who first developed the cycle in 1859.

The following stages occur between states in the cycle: 1-2 Compression of the fluid using a pump 2-3 Heating of the compressed fluid to the inlet temperature of

turbine, including increasing temperature to boiling point, andphase change from liquid to vapor

3-4 Expansion of the vapor in the turbine 4-1 Condensation of the vapor in a condenser

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Figure 6-2. Schematic of components in simple Rankine device

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Figure 6-3. Temperature-entropy diagram for the ideal Rankine cycle

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The net work for the cycle and the thermalefficiency: Wnet=Wturbine-Wpump or Qin-Qout

Thermal efficiency ηth =Wnet/Qin

A steam power plant (Rankine cycle)

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EXAMPLE The Simple Ideal Rankine Cycle

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Consider a steam power plant operating on the simple ideal Rankine cycle.Steam enters the turbine at 3 MPa and 350°C and is condensed in t thecondenser at a pressure of 75 kPa. Determine the thermal efficiency of thiscycle ?

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The quality of this mixture, x, is the ratio of the difference between the change in entropy from s4 to fluid


Brayton Gas Cycle

For a gaseous fuel such as natural gas, it is practical tocombust the gas directly in the combustion cycle, ratherthan using heat from the gas to convert water to vapor andthen expand the water vapor through a steam turbine.

For this purpose, engineers have developed the gasturbine, which is based on the Brayton cycle.

This cycle is named after the American engineer GeorgeBrayton, who in the 1870s developed the continuouscombustion process that underlies the combustiontechnique used in gas turbines today.

Note that unlike the Rankine cycle, there is no fourth steprequired to return the combustion products from theturbine to the compressor.

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Figure 6-4. Components in Brayton cycle

The following stages occur between states:1. 1-2 Air from the atmosphere is drawn in to the system and compressed to themaximum system pressure.2. 2-3 Fuel is injected into the compressed air, and the mixture is combusted atconstant pressure, heating it to the system maximum temperature.3. 3-4 The combustion products are expanded through a turbine, creating thework output in the form of the spinning turbine shaft.

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Figure 6-5. Temperature-entropy diagram for the ideal Brayton cycle

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Ideal Brayton Cycle Analysis 3 4 1 23 4 1 2

2 3 3 2th

pth

h h h hW WQ h h

C

3 4 1 2

p

T T T T

C

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11 4

3 23 2 32

2

2 3 1 4

11 1

1

;

TTTT T

T TT T TTT

P P P P

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In the Rankine cycle, the amount of work required topressurize the water is small, because liquid water isalmost entirely incompressible.

Air entering the Brayton cycle is highly compressible, andrequires more work in order to achieve the pressuresnecessary for combustion and expansion.

As with the Rankine cycle, calculating the cycle efficiencyrequires calculation of the enthalpy values at each stage ofthe cycle. Alternative approaches exist for calculatingenthalpies; here we use the relative pressure of the air orfuel-air mixture at each stage to calculate enthalpy.

The relative pressure is a constant parameter as a functionof temperature for a given gas, as found in the air tables.

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Combined Cycle

The remaining energy in the exhaust from the gas turbinecan be put to some other use in order to increase overallsystem output.

If the exhaust gas temperature is sufficiently high, oneinnovative application is to boil water for use in a Rankinecycle, thus effectively powering two cycles with the energyin the gas that is initially combusted.

This process is called a combined gas-vapor cycle, orsimply a combined cycle.

Oklahoma Gas & Electric in the United States firstinstalled a combined-cycle system at their Belle IsleStation plant in 1949, and as the cost of natural gas hasrisen, so has the interest in this technology in manycountries around the world.

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Schematic of combined cycle system components The combined cycle consists of thefollowing steps:1. Gas-air mixture is combusted andexpanded through a turbine, as in theconventional Brayton cycle.2. The exhaust is transferred to a heatexchanger where pressurized, unheatedwater is introduced at the other end. Heatis transferred from the gas to the water atconstant pressure so that the water reachesthe desired temperature for the vaporcycle.3. Steam exits the heat exchanger to asteam turbine, and gas exits the heatexchanger to the atmosphere.4. Steam is expanded through the turbine and returned to a condenser and pump, to be returned to the heat exchanger at high pressure.

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Figure 6-8. Combined cycle Temperature-entropy diagram

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Table 6-1. Thermal efficiency at design operating conditions for a selection of combined cycle power plants

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Cogeneration System, Combined Heat and Power (CHP)

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Figure 6-14. Schematic of Cornell combined heat and power project with gas turbine, steam turbine, and district heating system for campus buildings

Source: Cornell University Utilities & Energy Management. Reprinted with permission.

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List of largest power stations in the world

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http://en.wikipedia.org/wiki/List_of_largest_power_stations_in_the_world

Shoaiba power and desalination plant

Primary fuel fuel oil

Units operational 14

Capacity 5,600 MW


Class Seminars and Discussion

Presentation 2: Brayton Gas Cycle, Gas Turbine Technologies Presentation 3: Combined Heat and Power Plant(CHP) Presentation 4: Energy Software Tools

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chapter 3: stationary combustion systems

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