Basics of Steam Generation

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

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

Introduction The world energy consumption has doubled in the last thirty years and it keeps on increasing with about 1,5 % per year. While the earth's oil and gas reserves are expected to deplete after roughly one hundred years, the coal reserves will last for almost five hundred years into the future. In Finland, 50 % of the electrical power produced, is produced in steam power plants. But there are more reasons to why electricity generation based on steam power plant will continue to grow and why 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 Definition In a traditional context, a boiler is an enclosed container that provides a means for heat from combustion 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 that consumes the heat in the steam and turns it into work. A steam boiler fulfils the following statements: 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 auxiliary systems (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 the maximum 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 is dependent upon the type of boiler, the designed heat transfer surface and the fuels that power the combustion. The Basics of Steam Generation - 3

A simple boiler In order to describe the principles of a steam boiler, consider a very simple case, where the boiler simply is a container, partially filled with water (Figure 1). Combustion of fuel produce heat, which is transferred to the container and makes the water evaporate. The vapor or steam can escape through a pipe that is connected to the container and be transported elsewhere. Another pipe brings water (called feedwater ) to the container to replace the water that has evaporated and escaped. Since the pressure level in the boiler should be kept constant (in order to have stable process values), the mass of the steam that escapes has to be equal to the mass of the water that is added. If steam leaves the boiler faster than water is added, the pressure in the boiler falls. If water 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 is generated and pressure rises inside the boiler. If less fuel is combusted, less steam is generated and the pressure sinks. A simple power plant cycle The steam boiler provides steam to a heat consumer, usually to power an engine. In a steam power plant a steam turbine is used for extracting the heat from the steam and turning it into work. The turbine usually drives a generator that turns the work from the turbine into electricity. The steam, used by the turbine, can be recycled by cooling it until it condensates into water and then return it as feedwater to the boiler. The condenser, where the steam is condensed, is a heat exchanger that typically uses water from a nearby sea or a river to cool the steam. In a typical power plant the pressure, at which the steam is produced, is high. But when the steam has been used to drive the turbine, the pressure has dropped drastically. A pump is therefore needed to get the pressure back up. Since the work needed to compress a Figure 2: Rankine cycle fluid is about a hundred times less than the work 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). G The Basics of Steam Generation - 4

Carnot efficiency When considering any heat process or power cycle it is necessary to review the Carnot efficiency that comes from the second law of thermodynamics. The Carnot efficiency equation gives the maximum thermal efficiency of a system (Figure 3) undergoing a reversible power cycle while operating between two thermal reservoirs at temperatures T h and T c (temperature unit Kelvin). T T T H C C η max = = 1 (1) TH TH To give a practical example of the use of this theory on steam boilers, consider the Rankine cycle example presented in Figure 2. The temperature of the hot reservoir would then be the temperature of the steam produced in the boiler and the temperature of the cold reservoir would be the temperature of the cooling water drawn from a nearby river or lake (Figure 4). The formula in Equation 1 can then be used to get the theoretical maximum efficiency that we can get from the turbine. We can plot curve by of the maximum efficiency as a function of the steam exhaust temperature by keeping the cooling water temperature constant. If we suppose the temperature of the cooling water is around 20 C (293 K) on a warm summer day, we get a curve, which is presented in the figure: The bigger temperature difference, the higher thermal efficiency. Although no practical heat process is fully reversible, many processes can be calculated precisely enough by approximating them as reversible processes. Properties of water and steam Introduction Water is a useful and cheap medium to use as a working fluid. When water is boiled into steam its volume increases about 1,600 times, producing a force that is almost as explosive as gunpowder. The force produced by this Hot reservoir Qh (temperature Th) Wcycle = Qh - Qc Cold reservoir Qc (Temperature Tc) Figure 3: Carnot efficiency visualized Wp Hot reservoir Qh (temperature Th) Cold reservoir Qc (Temperature Tc) Figure 4: Carnot efficiency applied on the Rankine cycle.. 0,7 0,6 0,5 0,4 0,3 0,2 0,1 Carnot efficiency Wt 0 200 400 600 800 1000 Temperature [K] Figure 5: Carnot efficiency graph example. The Basics of Steam Generation - 5

expansion is the source of power in all steam engines. It also makes the boiler a dangerous device that must be carefully treated. The theoretical amount of heat that can be transferred from the combustion process to the working fluid in a boiler is equivalent to the change in its total heat content from its state at entering to that at exiting the boiler. In order to be able to select and design steam- and power-generation equipment, it is necessary to thoroughly understand the properties of the working fluid steam, the use of steam tables and the use of superheat. These fundamentals of steam generation will be briefly reviewed in this chapter. When phase changes of the water is discussed, only the liquid-vapor and vapor-liquid phase changes are mentioned, since these are the phase changes that the entire boiler technology is based on. [2] Temperature [C] 180 160 140 120 100 80 60 40 Evaporation of water Phase change 20 0 500 1000 1500 2000 2500 3000 Net enthalpy of water [kj/kg water] Figure 6: Water evaporation plotted in a temperature-enthalpy graph. Boiling of water Water and steam are typically used as heat carriers in heating systems. Steam, the gas phase of water, results from adding sufficient heat to water to cause it to evaporate. This boiler process consists of three main steps: The first step is the adding of heat to the water that raises the temperature up to the boiling point of water, also called preheating. The second step is the continuing addition of heat to change the phase from water to steam, the actual evaporation. The third 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. In Figure 6, the left section represents the preheating, the middle section the evaporation, and the third section the superheating. When all the water has been evaporated, the steam is called dry saturated steam. If steam is heated beyond its saturation point, the temperature begins to rise again and the steam becomes superheated steam. Superheated steam is defined by its zero moisture content: It contains no water at all, only 100% steam. Evaporation During 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 430 kj/kg (sat. water) to 2690 kj/kg (sat. steam). When the water has reached the dry saturated steam condition, the steam contains a large amount of latent heat, corresponding to the heat that was led to the process under constant pressure and temperature. So despite pressure and temperature is the same for the liquid and the vapour, the amount of heat is much higher in vapour compared to the liquid. Superheating If the steam is heated beyond the dry saturated steam condition, the temperature begins to rise again and the properties of the steam start to resemble those of a perfect gas. Steam with higher The Basics of Steam Generation - 6

temperature than that of saturated steam is called superheated steam. It contains no moisture and cannot 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 temperature It is well known that water boils and evaporates at 100 C under atmospheric pressure. By higher pressure, water evaporates at higher temperature - e.g. a pressure of 10 bar equals an evaporation temperature of 184 C. The pressure and the corresponding temperature when a phase change occurs are called the saturation temperature and saturation pressure. During the evaporation process, pressure and temperature are constant, but if the vaporization occurs in a closed vessel, the expansion that occurs due to the phase change of water into steam causes the pressure to rise and thus the boiling temperature rises. From the diagram (Figure 7) we can se that when we exceed a certain pressure, 22,12 Mpa (the corresponding temperature is 374 C), the line stops. The reason is that the border between gas phase and liquid phase is blurred out at that pressure. That point, where the different phases cease to exist, is called the critical point of water. Basics of combustion Principles The process of combustion is a high speed, high temperature chemical reaction. It is the rapid union of an element or a compound with oxygen that results in the production of heat - essentially, it is a controlled explosion. Combustion occurs when the elements in a fuel combine with oxygen and produce heat. All fuels, whether they are solid, liquid or in gaseous form, consist primarily of compounds of carbon and hydrogen called hydrocarbons. Sulphur is also present in these fuels. Pressure [bar] 1000 100 10 0,1 0,01 22,12 MPa 1 0 100 200 300 400 Temperature [ C] Figure 7: Evaporation pressure as a function of evaporation temperature. Figure 8: A pulverized coal fired burner in action. The Basics of Steam Generation - 7

Products of combustion When the hydrogen and oxygen combine, intense heat and water vapor is formed. When carbon and oxygen 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 chemical reactions 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 the combustion 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 before it can combine with oxygen to form CO 2, passes out the vent as visible smoke. The intense yellow color of an oil flame is largely caused by incandescent carbon particles. As we mentioned in the introduction to this segment, combustion can never be 100% efficient. All fuels contain some moisture 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 theoretical amount 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 of air above the theoretical amount of air needed to burn the fuel. Complete combustion is always our goal. With complete combustion, the fuel is burned at the highest combustion efficiency with low pollution. Incomplete Combustion occurs when all the fuel is not burned, which results in the formation of soot and smoke. Combustion of solid fuels Solid fuels can be divided into high grade; coal and low grade; peat and bark. The most typical firing methods are grate firing, cyclone firing, pulverized firing and fluidized bed firing, as described below. Pulverized firing has been used in industrial and utility boilers from 60 MWt to 6000 MWt. Grate firing (Figure 9) has been used to fire biofuels from 5 MWt to 600 MWt and cyclone firing has been used in small scale 3-6 MWt. Figure 9: Stoker or grate firing. Combustion of coal Oil 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 is combusted in the bottom part of the combustor with low-velocity air. Stoker boilers also employ this type of combustion. Large-capacity pulverized coal fired boilers for power plants usually employ entrained bed combustion. In fluidized bed combustion, fuel is introduced into the fluidized bed and combusted. Main types of a modern boiler In a modern boiler, there are two main types of boilers when considering the heat transfer means from flue gases to feed water: Fire tube boilers and water tube boilers. In a fire tube boiler the flue gases from the furnace are conducted to flue passages, which consist of several parallel-connected tubes. The tubes run through the boiler vessel, which contains the feedwater. The tubes are thus surrounded by water. The heat from the flue gases is transferred from the tubes to the water in the container, 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 chamber 2. Flue gas collection chamber 3. Open furnace 4. Flame tube 5. Burner seat 6. Manhole 7. Fire tubes 8. Water space 9. Steam space 10. Outlet and circulation 11. Flue gas out 12. Blow-out hatch 13. Main hatch 14. Cleaning hatch 15. Main steam outlet 16. Level control assembly 17. Feedwater inlet 18. Utility steam outlet 19. Safety valve assembly 20. Feet 21. Inslulation Figure 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 the opposite of a fire tube boiler. The water circulates in many parallel-connected tubes. The tubes are situated in the flue gas channel, and are heated by the flue gases, which are led from the furnace through the flue gas passage. In a modern boiler, the tubes, where water circulates, are welded together and form the furnace walls. Therefore the water tubes are directly exposed to radiation and gases from the combustion (Figure 12). Similarly to the fire tube boiler, the water tube boiler received its name from having "water in the tubes". A modern utility boiler is usually a water tube boiler, because a fire tube boiler is limited in capacity and only feasible in small systems. Figure 12: Simplified drawing describing the water tube boiler principle. /4/ Heat exchanger boiler model General If a modern water tube boiler utilizes a furnace, the furnace and the evaporator is usually the same construction the inner furnace walls consists solely of boiler tubes, conducting feed water, which absorbs the combustion heat and evaporates. flue gas process steam In process engineering a boiler is modelled as a network of heat exchangers, which symbolizes the transfer of heat from the flue gas to the steam/water in boiler pipes. For instance, the furnace, abstracted as a heat exchanger (Figure 13), consists of the following streams: the fuel (at storage temperature), combustion air (at outdoors temperature) and feedwater as input streams. The output streams are the flue gas from the combustion of the fuelair mixture, and the steam. feed water air fuel Figure 13: Furnace heat exchanger model. Heat exchanger basics The task of a heat exchanger is to transfer the heat from one flow of medium (fluid/gas stream) to another - without any physical contact, i.e. without actually mixing the two media. When speaking about the two streams that interact (exchange heat) in a heat exchanger we usually talk about the hot stream 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 gas stream 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 heat exchangers: Parallel-flow and counter-flow. In a parallel flow heat exchanger the fluids flow in the same direction and in a counter flow heat exchanger the fluids flow in the opposite direction. Combinations of these types (like cross-flow exchangers and more complicated ones, like boilers) can usually be approximately calculated according to the counter-flow type. hot stream cold stream T-Q diagram A useful tool for designing a heat exchanger is the T-Q diagram. The diagram consists of two axes: Temperature (T) and transferred heat (Q). The hot stream and the cold stream are represented in the diagram by two lines on top of each other. If the exchanger is of parallelflow type, the lines proceed in the same direction (Figure 15). If the exchanger is a counter-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 heat transfer has caused. Since the heat strays from a higher temperature to a lower (according to the second law of thermodynamics) the wanted heat transfer happens by itself if and only if the hot stream is always hotter than the cold stream. That's why the streams must never cross. Since no material has an infinite heat transfer rate, the pinch temperature (Tpinch) of the heat exchanger defines the minimum allowed temperature difference between the two flows. If the streams cross, the lines must be horizontally adjusted (that is, external heating and cooling must be supplied) in order to correspond with the pinch temperature (Figure 17). Figure 14: A heat exchanger (also furnace). T1 T2 t2 t1 T hot stream cold stream Figure 15: T-Q diagram of a parallel-flow type heat exchanger. T1 T2 t2 t1 T deltaq Figure 16: T-Q diagram of a counter-flow type heat exchanger. Q Q The Basics of Steam Generation - 11

T t1 T1 Tpinch T2 t1 Q external heating required external cooling required Figure 17: Adjusted streams. Heat recovery steam generator model To 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 a furnace the HRSG extracts heat from flue gases originating from fuel combusted in an external unit. Since the HRSG only deals with two streams (flue gases as the hot stream and steam/water as the 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 usually divided into at least three units. We start with the heat exchanger unit, where the evaporation occurs the evaporator. Assuming that water enters the evaporator as saturated water and exits as saturated steam, the heat transferred from the flue gas is the required heat to change the phase of water into steam. The phase change occurs (water boils) at a constant temperature, and therefore the steam/water stream temperature won t change in the evaporator. In order to preheat the water for the evaporator, another heat exchanger unit is needed. This unit is called economizer, and is a cross-flow type of heat exchanger. It is placed after the evaporator in the flue 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 superheater heats 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 placed first in the flue gas stream. The maximum temperature of the boiler is limited by the properties of The 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 found in Figure 18. The T-Q diagram of the model is visualized in Figure 19. Economizer water saturated water Evaporator T Sup Eva Eco saturated steam Superheater Figure 18: Heat exchanger model of the HRSG. Figure 19: T-Q diagram of the HRSG model in Figure 18. Q Heat exchanger model of furnace-equipped boilers The 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 heat transfer surfaces on the flue gas side. In a boiler with a furnace, adequate cooling has to be maintained and material temperature should not 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 part of the boiler. Since the furnace is inside the boiler, high flue gas temperatures (over 1000 C) are obtained. After the flue gas has given off heat for the steam production, it is still quite hot. In order to cool down the 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. The Basics of Steam Generation - 13

The result is a heat exchanger model of a furnace-equipped boiler (e.g. PCF-boiler, grate boiler or oil/gas boiler), which can be found in Figure 20. The T-Q diagram of the model is visualized in Figure 21 Air out T Eco Eva Sup Air Air in Air preheater Figure 21: T-Q diagram of the heat exchanger model in Figure 20. Q Figure 20: Furnace equipped boiler with air preheater. The Basics of Steam Generation - 14

References 1. 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, 2001 4. American Heritage Dictionary of the English Language: Fourth Edition, http://www.bartleby.com The Basics of Steam Generation - 15