Basics of Steam Generation

8/12/2019 Basics of Steam Generation

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Helsinki University of Technology Department of Mechanical Engineering

Energy Engineering and Environmental Protection Publications

Steam Boiler Technology eBook

Espoo 2002

Basics of Steam Generation

Sebastian Teir

Helsinki University of Technology

Department of Mechanical Engineering

Energy Engineering and Environmental Protection


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The Basics of Steam Generation - 2

Table of contentsIntroduction..........................................................................................................................................3Basics of boilers and boiler processes..................................................................................................3

Definition .........................................................................................................................................3A simple boiler.................................................................................................................................4A simple power plant cycle..............................................................................................................4Carnot efficiency ..............................................................................................................................5Properties of water and steam ..........................................................................................................5

Introduction..................................................................................................................................5Boiling of water ...........................................................................................................................6Effect of pressure on evaporation temperature ............................................................................7

Basics of combustion .......................................................................................................................7Principles......................................................................................................................................7Products of combustion................................................................................................................8Types of combustion....................................................................................................................8Combustion of solid fuels ............................................................................................................8Combustion of coal ......................................................................................................................8

Main types of a modern boiler .........................................................................................................9Heat exchanger boiler model .........................................................................................................10

General .......................................................................................................................................10Heat exchanger basics ................................................................................................................10T-Q diagram...............................................................................................................................11Heat recovery steam generator model........................................................................................12Heat exchanger model of furnace-equipped boilers ..................................................................13

References ......................................................................................................................................15


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IntroductionThe world energy consumption has doubled in the last thirty years and it keeps on increasing withabout 1,5 % per year. While the earth's oil and gas reserves are expected to deplete after roughlyone hundred years, the coal reserves will last for almost five hundred years into the future. InFinland, 50 % of the electrical power produced, is produced in steam power plants. But there aremore reasons to why electricity generation based on steam power plant will continue to grow andwhy there still will be a demand for steam boilers in the future:

The cost of the produced electricity is low The technology has been used for many decades and is reliable and available Wind and solar power are still expensive compared to steam power The environmental impact of coal powered steam plants have under the past decade been

heavily diminished thanks to improved SO x and NO x reduction technology The paper industry uses steam boilers as a vital utility to recycle chemicals and derive

electricity from black liquor (pulping waste) Waste and biofuels can effectively be combusted in a boiler

Basics of boilers and boiler processes

DefinitionIn a traditional context, a boiler is an enclosed container that provides a means for heat fromcombustion to be transferred into the working media (usually water) until it becomes heated or a gas(steam). One could simply say that a boiler is as a heat exchanger between fire and water. The

boiler is the part of a steam power plant process that produces the steam and thus provides the heat.

The steam or hot water under pressure can then be used for transferring the heat to a process thatconsumes the heat in the steam and turns it into work. A steam boiler fulfils the followingstatements:

It is part of a type of heat engine or process Heat is generated through combustion (burning) It has a working fluid, a.k.a. heat carrier that transfers the generated heat away from the

boiler The heating media and working fluid are separated by walls

In an industrial/technical context, the concept steam boiler (also referred to as steam generator)includes the whole complex system for producing steam for use e. g. in a turbine or in industrial

process. It includes all the different phases of heat transfer from flames to water/steam mixture(economizer, boiler, superheater, reheater and air preheater). It also includes different auxiliarysystems (e. g. fuel feeding, water treatment, flue gas channels including stack). [1]

The heat is generated in the furnace part of the boiler, where fuel is combusted. The fuel used in a boiler contains either chemically bonded energy (like coal, waste and biofuels) or nuclear energy. Nuclear energy will not be covered in this material. A boiler must be designed to absorb themaximum amount of heat released in the process of combustion. This heat is transferred to the

boiler water through radiation, conduction and convection. The relative percentage of each isdependent upon the type of boiler, the designed heat transfer surface and the fuels that power thecombustion.


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A simple boilerIn order to describe the principles of a steam

boiler, consider a very simple case, where the boiler simply is a container, partially filled withwater ( Figure 1) . Combustion of fuel produceheat, which is transferred to the container andmakes the water evaporate. The vapor or steamcan escape through a pipe that is connected tothe container and be transported elsewhere.Another pipe brings water (called feedwater)to the container to replace the water that hasevaporated and escaped.

Since the pressure level in the boiler should bekept constant (in order to have stable processvalues), the mass of the steam that escapes hasto be equal to the mass of the water that isadded. If steam leaves the boiler faster thanwater is added, the pressure in the boiler falls. Ifwater is added faster than it is evaporated, the

pressure rises. Figure 1: Simplified boiler drawing.

If more fuel is combusted, more heat is generated and transferred to the water. Thus, more steam isgenerated and pressure rises inside the boiler. If less fuel is combusted, less steam is generated andthe pressure sinks.

A simple power plant cycleThe steam boiler provides steam to a heatconsumer, usually to power an engine. In asteam power plant a steam turbine is used forextracting the heat from the steam and turning itinto work. The turbine usually drives a generatorthat turns the work from the turbine intoelectricity. The steam, used by the turbine, can

be recycled by cooling it until it condensatesinto water and then return it as feedwater to the

boiler. The condenser, where the steam iscondensed, is a heat exchanger that typicallyuses water from a nearby sea or a river to coolthe steam. In a typical power plant the pressure,at which the steam is produced, is high. Butwhen the steam has been used to drive theturbine, the pressure has dropped drastically. A

pump is therefore needed to get the pressure back up. Since the work needed to compress afluid is about a hundred times less than the work

G

Figure 2: Rankine cycle

needed to compress a gas, the pump is located after the condenser. The cycle that the described process forms, is called a Rankine cycle and is the basis of most modern steam power plant processes ( Figure 2) .


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Carnot efficiencyWhen considering any heat process or powercycle it is necessary to review the Carnotefficiency that comes from the second law ofthermodynamics. The Carnot efficiencyequation gives the maximum thermal efficiencyof a system ( Figure 3) undergoing a reversible

power cycle while operating between twothermal reservoirs at temperatures T h and T c (temperature unit Kelvin).

H

C

H

C H

T T

T T T

=

= 1max (1)

To give a practical example of the use of this

theory on steam boilers, consider the Rankinecycle example presented in Figure 2. Thetemperature of the hot reservoir would then bethe temperature of the steam produced in the

boiler and the temperature of the cold reservoirwould be the temperature of the cooling waterdrawn from a nearby river or lake ( Figure 4) .The formula in Equation 1 can then be used toget the theoretical maximum efficiency that wecan get from the turbine.

We can plot curve by of the maximumefficiency as a function of the steam exhausttemperature by keeping the cooling watertemperature constant. If we suppose thetemperature of the cooling water is around 20C(293 K) on a warm summer day, we get a curve,which is presented in the figure: The biggertemperature difference, the higher thermalefficiency.

Although no practical heat process is fullyreversible, many processes can be calculated

precisely enough by approximating them asreversible processes.

Properties of water and steam

IntroductionWater is a useful and cheap medium to use as aworking fluid. When water is boiled into steamits volume increases about 1,600 times,

producing a force that is almost as explosive asgunpowder. The force produced by this

Hot reservoir Qh(temperature Th)

Cold reservoir Qc(Temperature Tc)

Wcycle=

Qh - Qc

Figure 3: Carnot efficiency visualized

Hot reservoir Qh(temperature Th)

Cold reservoir Qc(Temperature Tc)

Wp Wt

Figure 4: Carnot efficiency applied on the Rankine cycle.

.Carnot efficiency

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

200 400 600 800 1000

Temperature [K]

Figure 5: Carnot efficiency graph example.


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expansion is the source of power in all steam engines. It also makes the boiler a dangerous devicethat must be carefully treated.

The theoretical amount of heat that can betransferred from the combustion process to

the working fluid in a boiler is equivalentto the change in its total heat content fromits state at entering to that at exiting the

boiler. In order to be able to select anddesign steam- and power-generationequipment, it is necessary to thoroughlyunderstand the properties of the workingfluid steam, the use of steam tables and theuse of superheat. These fundamentals ofsteam generation will be briefly reviewedin this chapter. When phase changes of thewater is discussed, only the liquid-vaporand vapor-liquid phase changes arementioned, since these are the phasechanges that the entire boiler technology is

based on. [2]

Evaporation of water

20

40

60

80

100

120

140

160

180

0 500 1000 1500 2000 2500 3000

Net enthalpy of water [kJ/kg water]

T e m p e r a

t u r e

[ C ]

Phase change

Figure 6: Water evaporation plotted in a

temperature-enthalpy graph.

Boiling of waterWater and steam are typically used as heat carriers in heating systems. Steam, the gas phase ofwater, results from adding sufficient heat to water to cause it to evaporate. This boiler processconsists of three main steps: The first step is the adding of heat to the water that raises thetemperature up to the boiling point of water, also called preheating. The second step is thecontinuing addition of heat to change the phase from water to steam, the actual evaporation. Thethird step is the heating of steam beyond the boiling temperature of water, known as superheating.The first step and the third steps are the part where heat addition causes a temperature rise but no

phase change, and the second step is the part where the heat addition only causes a phase change. InFigure 6, the left section represents the preheating, the middle section the evaporation, and the thirdsection the superheating. When all the water has been evaporated, the steam is called dry saturatedsteam. If steam is heated beyond its saturation point, the temperature begins to rise again and thesteam becomes superheated steam. Superheated steam is defined by its zero moisture content: It

contains no water at all, only 100% steam.EvaporationDuring the evaporation the enthalpy rises drastically. If we evaporate the water at atmospheric

pressure from saturated liquid to saturated vapour, the enthalpy rise needed is 2260 kJ/kg, from 430kJ/kg (sat. water) to 2690 kJ/kg (sat. steam). When the water has reached the dry saturated steamcondition, the steam contains a large amount of latent heat, corresponding to the heat that was led tothe process under constant pressure and temperature. So despite pressure and temperature is thesame for the liquid and the vapour, the amount of heat is much higher in vapour compared to theliquid.

Superheating

If the steam is heated beyond the dry saturated steam condition, the temperature begins to rise againand the properties of the steam start to resemble those of a perfect gas. Steam with higher


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temperature than that of saturated steam is called superheated steam. It contains no moisture andcannot condense until its temperature has been lowered to that of saturated steam at the same

pressure. Superheating the steam is particularly useful for eliminating condensation in steam lines,decreasing the moisture in the turbine exhaust and increasing the efficiency (i.e. Carnot efficiency)of the power plant.

Effect of pressure on evaporation temperatureIt is well known that water boils andevaporates at 100C under atmospheric

pressure. By higher pressure, waterevaporates at higher temperature - e.g. a

pressure of 10 bar equals an evaporationtemperature of 184C. The pressure and thecorresponding temperature when a phasechange occurs are called the saturationtemperature and saturation pressure. Duringthe evaporation process, pressure andtemperature are constant, but if thevaporization occurs in a closed vessel, theexpansion that occurs due to the phase changeof water into steam causes the pressure to riseand thus the boiling temperature rises.

From the diagram ( Figure 7) we can se thatwhen we exceed a certain pressure, 22,12Mpa (the corresponding temperature is374C), the line stops. The reason is that the

border between gas phase and liquid phase is blurred out at that pressure. That point, wherethe different phases cease to exist, is calledthe critical point of water.

22,12 MPa

0,01

0,1

1

10

100

1000

0 100 200 300 400

Temperature [C]

P r e s s u r e

[ b a r ]

Figure 7: Evaporation pressure as a function ofevaporation temperature.

Basics of combustion

PrinciplesThe process of combustion is a high speed, hightemperature chemical reaction. It is the rapid

union of an element or a compound with oxygenthat results in the production of heat -essentially, it is a controlled explosion.Combustion occurs when the elements in a fuelcombine with oxygen and produce heat. Allfuels, whether they are solid, liquid or ingaseous form, consist primarily of compoundsof carbon and hydrogen called hydrocarbons.Sulphur is also present in these fuels.

Figure 8: A pulverized coal fired burner inaction.


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Products of combustionWhen the hydrogen and oxygen combine, intense heat and water vapor is formed. When carbon andoxygen combine, intense heat and the compounds of carbon monoxide or carbon dioxide are

formed. When sulfur and oxygen combine, sulfur dioxide and heat are formed. These chemicalreactions take place in a furnace during the burning of fuel, provided there is sufficient air (oxygen)to completely burn the fuel. Very little of the released carbon is actually "consumed" in thecombustion reaction because flame temperature seldom reaches the vaporization point of carbon.Most of it combines with oxygen to form CO 2 and passes out the vent. Carbon, which cools beforeit can combine with oxygen to form CO 2, passes out the vent as visible smoke. The intense yellowcolor of an oil flame is largely caused by incandescent carbon particles. As we mentioned in theintroduction to this segment, combustion can never be 100% efficient. All fuels contain somemoisture and non-combustibles:

Top-quality coal has 20% noncombustibles.

Residual oil is 10% noncombustible.

Natural gas has 1 - 15% (depending on origin) of noncombustible gases like N 2 and CO 2.

Types of combustion There are three types of combustion:

Perfect Combustion is achieved when all the fuel is burned using only the theoreticalamount of air, but as we said before perfect combustion cannot be achieved in a boiler.

Complete Combustion is achieved when all the fuel is burned using the minimal amount ofair above the theoretical amount of air needed to burn the fuel. Complete combustion isalways our goal. With complete combustion, the fuel is burned at the highest combustionefficiency with low pollution.

Incomplete Combustion occurs when all the fuel is not burned, which results in theformation of soot and smoke.

Combustion of solid fuelsSolid fuels can be divided into high grade; coaland low grade; peat and bark. The most typicalfiring methods are grate firing, cyclone firing,

pulverized firing and fluidized bed firing, asdescribed below. Pulverized firing has been usedin industrial and utility boilers from 60 MWt to6000 MWt. Grate firing ( Figure 9) has beenused to fire biofuels from 5 MWt to 600 MWtand cyclone firing has been used in small scale3-6 MWt. Figure 9: Stoker or grate firing.

Combustion of coalOil and gas are always combusted with a burner, but there are three different ways to combust coal:


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Fluidized bed combustion Fixed bed combustion (grate boilers) Entrained bed combustion (pulverized coal combustion)

In fixed bed combustion, larger-sized coal iscombusted in the bottom part of the combustor withlow-velocity air. Stoker boilers also employ this typeof combustion. Large-capacity pulverized coal fired

boilers for power plants usually employ entrained bedcombustion. In fluidized bed combustion, fuel isintroduced into the fluidized bed and combusted.

Main types of a modern boilerIn a modern boiler, there are two main types of boilers

when considering the heat transfer means from fluegases to feed water: Fire tube boilers and water tube boilers.

In a fire tube boiler the flue gases from the furnace areconducted to flue passages, which consist of several

parallel-connected tubes. The tubes run through the boiler vessel, which contains the feedwater. The tubesare thus surrounded by water. The heat from the fluegases is transferred from the tubes to the water in thecontainer, thus the water is heated into steam. An easy

way to remember the principle is to say that a fire tube boiler has "fire in the tubes". Figure 10: Fluidized bed combustion.

1. Turning chamber2. Flue gas collection

chamber3. Open furnace4. Flame tube5. Burner seat6. Manhole7. Fire tubes

8. Water space9. Steam space10. Outlet and circulation11. Flue gas out12. Blow-out hatch13. Main hatch14. Cleaning hatch15. Main steam outlet

16. Level control assembly17. Feedwater inlet18. Utility steam outlet19. Safety valve assembly20. Feet21. Inslulation

Figure 11: Schematic of a Hyrytys TTK fire tube steam boiler [ Hyrytys ] .
http://www.hoyrytys.fi/http://www.hoyrytys.fi/http://www.hoyrytys.fi/


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In a water tube boiler, the conditions are theopposite of a fire tube boiler. The watercirculates in many parallel-connected tubes. Thetubes are situated in the flue gas channel, and areheated by the flue gases, which are led from the

furnace through the flue gas passage. In amodern boiler, the tubes, where water circulates,are welded together and form the furnace walls.Therefore the water tubes are directly exposed toradiation and gases from the combustion ( Figure12). Similarly to the fire tube boiler, the watertube boiler received its name from having "waterin the tubes".

A modern utility boiler is usually a water tube boiler, because a fire tube boiler is limited incapacity and only feasible in small systems. Figure 12: Simplified drawing describing the

water tube boiler principle. /4/

Heat exchanger boiler model

GeneralIf a modern water tube boiler utilizes a furnace,the furnace and the evaporator is usually thesame construction the inner furnace wallsconsists solely of boiler tubes, conducting feedwater, which absorbs the combustion heat andevaporates.

In process engineering a boiler is modelled as anetwork of heat exchangers, which symbolizesthe transfer of heat from the flue gas to thesteam/water in boiler pipes.

For instance, the furnace, abstracted as a heatexchanger ( Figure 13) , consists of the following

streams: the fuel (at storage temperature),combustion air (at outdoors temperature) andfeedwater as input streams. The output streamsare the flue gas from the combustion of the fuel-air mixture, and the steam.

air fuel

flue gas

feed water

process steam

Figure 13: Furnace heat exchanger model.

Heat exchanger basicsThe task of a heat exchanger is to transfer the heat from one flow of medium (fluid/gas stream) toanother - without any physical contact, i.e. without actually mixing the two media. When speakingabout the two streams that interact (exchange heat) in a heat exchanger we usually talk about the hotstream and the cold stream ( Figure 14) . The hot stream (a.k.a. heat source) is the stream that gives

away heat to the cold stream (a.k.a. heat sink) that absorbs the heat. Thus, in a boiler the flue gasstream is the hot stream (heat source) and the water/steam stream is the cold stream (heat sink).


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There are two different main types of heatexchangers: Parallel-flow and counter-flow. In a

parallel flow heat exchanger the fluids flow inthe same direction and in a counter flow heatexchanger the fluids flow in the opposite

direction. Combinations of these types (likecross-flow exchangers and more complicatedones, like boilers) can usually be approximatelycalculated according to the counter-flow type.

T-Q diagramA useful tool for designing a heat exchanger isthe T-Q diagram. The diagram consists of twoaxes: Temperature (T) and transferred heat (Q).The hot stream and the cold stream arerepresented in the diagram by two lines on topof each other. If the exchanger is of parallel-flow type, the lines proceed in the samedirection ( Figure 15) . If the exchanger is acounter-flow (or cross-flow-combination, like a

boiler), the lines points in the opposite direction(Figure 16) . The length of the lines on the Q-axis shows the transferred heat rate and the T-axis the rise/drop in temperature that the heattransfer has caused.

Since the heat strays from a higher temperatureto a lower (according to the second law ofthermodynamics) the wanted heat transferhappens by itself if and only if the hot stream isalways hotter than the cold stream. That's whythe streams must never cross. Since no materialhas an infinite heat transfer rate, the pinchtemperature (Tpinch) of the heat exchangerdefines the minimum allowed temperaturedifference between the two flows.

If the streams cross, the lines must behorizontally adjusted (that is, external heatingand cooling must be supplied) in order tocorrespond with the pinch temperature (Figure17).

hot stream

cold stream

Figure 14: A heat exchanger (also furnace).

T

Q

T1

T2t2

t1

hot stream

cold stream

Figure 15: T-Q diagram of a parallel-flow type

heat exchanger.

T

Q

T1

T2

t2

t1

deltaQ

Figure 16: T-Q diagram of a counter-flow typeheat exchanger.


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T

Q

TpinchT1

T2

t1

t1

external heatingrequired

external coolingrequired

Figure 17: Adjusted streams.

Heat recovery steam generator modelTo give an example of the construction of a heat exchanger model, a heat recovery steam generator(HRSG) is constructed next as a heat exchanger cascade. The HRSG is basically a boiler without afurnace the HRSG extracts heat from flue gases originating from fuel combusted in an externalunit. Since the HRSG only deals with two streams (flue gases as the hot stream and steam/water asthe cold stream), it represents the simplest heat exchanger model of a modern boiler application.Since the heating of water occurs in three steps ( Figure 6) , the heat exchanger model is usuallydivided into at least three units.

We start with the heat exchanger unit, where the evaporation occurs the evaporator. Assumingthat water enters the evaporator as saturated water and exits as saturated steam, the heat transferredfrom the flue gas is the required heat to change the phase of water into steam. The phase changeoccurs (water boils) at a constant temperature, and therefore the steam/water stream temperaturewont change in the evaporator.

In order to preheat the water for the evaporator, another heat exchanger unit is needed. This unit iscalled economizer, and is a cross-flow type of heat exchanger. It is placed after the evaporator in theflue gas stream, since the evaporator requires higher flue gas temperature than the economizer.

The heat exchanger unit that superheats the saturated steam is called superheater. The superheaterheats the saturated steam beyond the saturation point until it reaches the designed maximum

temperature. It requires therefore the highest flue gas temperature to receive heat and is thus placedfirst in the flue gas stream. The maximum temperature of the boiler is limited by the properties of


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the superheater tube material. Today's economically feasible material can take temperatures of 550-600 C.

The result is a heat exchanger cascade of a HRSG (with a single pressure level), which can be foundin Figure 18. The T-Q diagram of the model is visualized in Figure 19.

Economizer

Evaporator

Superheater

water

saturatedwater

saturatedsteam

Figure 18: Heat exchanger model of the HRSG.

T

Q

EcoEvaSup

Figure 19: T-Q diagram of the HRSG model in Figure 18.

Heat exchanger model of furnace-equipped boilersThe order of the heat transfer units on the water/steam side is always economizer - evaporator -superheater (downstream order). The temperature levels and the temperature difference between the

flue gases and the working fluid usually limits the arrangement variation possibilities of the heattransfer surfaces on the flue gas side.

In a boiler with a furnace, adequate cooling has to be maintained and material temperature shouldnot exceed 600C. Thus the evaporator part of the water/steam cycle is placed in the furnace walls,since the heat of the evaporation provides enough cooling for the furnace, which is the hottest partof the boiler.

Since the furnace is inside the boiler, high flue gas temperatures (over 1000C) are obtained. Afterthe flue gas has given off heat for the steam production, it is still quite hot. In order to cool downthe flue gases further to gain higher boiler efficiency, flue gases can be used to preheat the

combustion air. The heat exchanger used for this purpose is called an air preheater.


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The result is a heat exchanger model of a furnace-equipped boiler (e.g. PCF-boiler, grate boiler oroil/gas boiler), which can be found in Figure 20. The T-Q diagram of the model is visualized inFigure 21

Air preheater

Air in

Air out

Figure 20: Furnace equipped boiler with air preheater.

T

Q

EcoEva Sup Air

Figure 21: T-Q diagram of the heat exchanger

model in Figure 20.


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References

1. Ahonen, V. Hyrytekniikka II. Otakustantamo, Espoo. 1978.

2. Combustion Engineering. Combustion: Fossil power systems . 3 rd ed. Windsor. 1981.

3. Esa Vakkilainen, lecture slides and material on steam boiler technology, 2001

4. American Heritage Dictionary of the English Language: Fourth Edition,http://www.bartleby.com
http://www.bartleby.com/http://www.bartleby.com/http://www.bartleby.com/

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Basics of Steam Generation

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