Basics of steam generation

Helsinki University of Technology Department of Mechanical EngineeringEnergy Engineering and Environmental Protection PublicationsSteam Boiler Technology eBookEspoo 2002Basics of Steam GenerationSebastian TeirHelsinki University of TechnologyDepartment of Mechanical EngineeringEnergy Engineering and Environmental Protection

Table of contentsIntroduction..........................................................................................................................................3Basics of boilers and boiler processes..................................................................................................3Definition .........................................................................................................................................3A simple boiler.................................................................................................................................4A simple power plant cycle..............................................................................................................4Carnot efficiency..............................................................................................................................5Properties of water and steam ..........................................................................................................5Introduction..................................................................................................................................5Boiling of water ...........................................................................................................................6Effect of pressure on evaporation temperature ............................................................................7Basics of combustion .......................................................................................................................7Principles......................................................................................................................................7Products of combustion................................................................................................................8Types of combustion....................................................................................................................8Combustion of solid fuels ............................................................................................................8Combustion of coal ......................................................................................................................8Main types of a modern boiler .........................................................................................................9Heat exchanger boiler model .........................................................................................................10General.......................................................................................................................................10Heat exchanger basics................................................................................................................10T-Q diagram...............................................................................................................................11Heat recovery steam generator model........................................................................................12Heat exchanger model of furnace-equipped boilers ..................................................................13References......................................................................................................................................15The Basics of Steam Generation - 2

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 beenheavily 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 deriveelectricity from black liquor (pulping waste)• Waste and biofuels can effectively be combusted in a boilerBasics of boilers and boiler processesDefinitionIn 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. Theboiler 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 theboiler• The heating media and working fluid are separated by wallsIn 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 industrialprocess. 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 aboiler 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 theboiler 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.The Basics of Steam Generation - 3

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 tothe working fluid in a boiler is equivalentto the change in its total heat content fromits state at entering to that at exiting theboiler. 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 isbased on. [2]Temperature [C]180160140120100806040Evaporation of waterPhase change200 500 1000 1500 2000 2500 3000Net enthalpy of water [kJ/kg water]Figure 6: Water evaporation plotted in atemperature-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 nophase 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: Itcontains no water at all, only 100% steam.EvaporationDuring the evaporation the enthalpy rises drastically. If we evaporate the water at atmosphericpressure 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.SuperheatingIf 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 higherThe Basics of Steam Generation - 6

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 samepressure. 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 100°C under atmosphericpressure. By higher pressure, waterevaporates at higher temperature - e.g. apressure of 10 bar equals an evaporationtemperature of 184°C. 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 is374°C), the line stops. The reason is that theborder between gas phase and liquid phase isblurred out at that pressure. That point, wherethe different phases cease to exist, is calledthe critical point of water.Basics of combustionPrinciplesThe process of combustion is a high speed, hightemperature chemical reaction. It is the rapidunion 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.Pressure [bar]1000100100,10,0122,12 MPa10 100 200 300 400Temperature [°C]Figure 7: Evaporation pressure as a function ofevaporation temperature.Figure 8: A pulverized coal fired burner inaction.The Basics of Steam Generation - 7

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 areformed. 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 combustionThere 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:The Basics of Steam Generation - 8

• 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 firedboilers 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 boilerswhen considering the heat transfer means from fluegases to feed water: Fire tube boilers and water tubeboilers.In a fire tube boiler the flue gases from the furnace areconducted to flue passages, which consist of severalparallel-connected tubes. The tubes run through theboiler 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 easyway to remember the principle is to say that a fire tubeboiler has "fire in the tubes".Figure 10: Fluidized bed combustion.1. Turning chamber2. Flue gas collectionchamber3. Open furnace4. Flame tube5. Burner seat6. Manhole7. Fire tubes8. Water space9. Steam space10. Outlet and circulation11. Flue gas out12. Blow-out hatch13. Main hatch14. Cleaning hatch15. Main steam outlet16. Level control assembly17. Feedwater inlet18. Utility steam outlet19. Safety valve assembly20. Feet21. InslulationFigure 11: Schematic of a Höyrytys TTK fire tube steam boiler [Höyrytys].The Basics of Steam Generation - 9

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 thefurnace 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 tubeboiler, because a fire tube boiler is limited incapacity and only feasible in small systems.Figure 12: Simplified drawing describing thewater tube boiler principle. /4/Heat exchanger boiler modelGeneralIf 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.flue gasprocess steamIn 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 followingstreams: 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 fuelairmixture, and the steam.feed waterairfuelFigure 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 givesaway 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).The Basics of Steam Generation - 10

There are two different main types of heatexchangers: Parallel-flow and counter-flow. In aparallel flow heat exchanger the fluids flow inthe same direction and in a counter flow heatexchanger the fluids flow in the oppositedirection. Combinations of these types (likecross-flow exchangers and more complicatedones, like boilers) can usually be approximatelycalculated according to the counter-flow type.hot streamcold streamT-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 parallelflowtype, the lines proceed in the samedirection (Figure 15). If the exchanger is acounter-flow (or cross-flow-combination, like aboiler), 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).Figure 14: A heat exchanger (also furnace).T1T2t2t1Thot streamcold streamFigure 15: T-Q diagram of a parallel-flow typeheat exchanger.T1T2t2t1TdeltaQFigure 16: T-Q diagram of a counter-flow typeheat exchanger.QQThe Basics of Steam Generation - 11

Tt1T1TpinchT2t1Qexternal heatingrequiredexternal coolingrequiredFigure 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 temperaturewon’t 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 maximumtemperature. 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 ofThe Basics of Steam Generation - 12

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.EconomizerwatersaturatedwaterEvaporatorTSupEvaEcosaturatedsteamSuperheaterFigure 18: Heat exchanger model of the HRSG.Figure 19: T-Q diagram of the HRSG model inFigure 18.QHeat 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 theflue 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 600°C. 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 1000°C) 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 thecombustion air. The heat exchanger used for this purpose is called an air preheater.The Basics of Steam Generation - 13

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 21Air outTEcoEva Sup AirAir inAir preheaterFigure 21: T-Q diagram of the heat exchangermodel in Figure 20.QFigure 20: Furnace equipped boiler with airpreheater.The Basics of Steam Generation - 14

References1. Ahonen, V. “Höyrytekniikka 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, 20014. American Heritage ® Dictionary of the English Language: Fourth Edition,http://www.bartleby.comThe Basics of Steam Generation - 15