Steam Boiler Technology (2003)

The Basics of Steam Generation Sebastian Teir

STEAM BOILER TECHNOLOGY – The Basics of Steam Generation

Table of contents Table of contents..................................................................................................................................2 Introduction..........................................................................................................................................3 Basics of boilers and boiler processes..................................................................................................5 A simple boiler.................................................................................................................................5 A simple power plant cycle..............................................................................................................6 Carnot efficiency..............................................................................................................................6 Properties of water and steam ..........................................................................................................7 Boiling of water ...........................................................................................................................7 Effect of pressure on evaporation temperature ............................................................................8 Basics of combustion .......................................................................................................................9 Products of combustion................................................................................................................9 Types of combustion....................................................................................................................9 Combustion of solid fuels ..........................................................................................................10 Combustion of coal ....................................................................................................................10 Main types of a modern boiler .......................................................................................................10 Heat exchanger boiler model .........................................................................................................12 Heat exchanger basics................................................................................................................12 T-Q diagram...............................................................................................................................12 Heat recovery steam generator model........................................................................................14 Heat exchanger model of furnace-equipped boilers ..................................................................15 References......................................................................................................................................16

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STEAM BOILER TECHNOLOGY – The Basics of Steam Generation

Introduction The world energy consumption has doubled in the last thirty years and it keeps on increasing with about 1.5% per year (Figure 1). While the earth's oil and gas reserves are expected to deplete after less than a hundred years, the coal reserves will last for almost five hundred years into the future (taking into account estimations of fossil fuel reserves that have not yet been found) (Figure 2). 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 world-wide dependency upon fossil fuels for power production (Figure 1, Figure 2, and Figure 3) 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 SOx and NOx 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 [1]

Coal Hydroelectricity Nuclear energy Natural gas

Oil

*Prior to 1994 Combustible Renewables & Waste final consumption has been estimated based on TPES. **Other includes geothermal, solar, wind, heat, etc.

Figure 1: Evolution from 1977 to 2002 of world primary energy consumption by fuel (Mtoe) [2]

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STEAM BOILER TECHNOLOGY – The Basics of Steam Generation

Coal

Gas

Oil

Figure 2: The world’s reserves-to-production ratio for fossil fuels. [2]

Coal Hydroelectricity Nuclear energy Natural gas

Oil

Figure 3: Regional primary energy consumption pattern 2002. [2]

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STEAM BOILER TECHNOLOGY – The Basics of Steam Generation

Basics of boilers and boiler processes 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). [3] 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.

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 4). 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 Figure 4: Simplified boiler drawing. 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

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STEAM BOILER TECHNOLOGY – The Basics of Steam Generation

water is added faster than it is evaporated, the pressure rises. 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 G 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 Figure 5: Rankine cycle back up. Since the work needed to compress a 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 5).

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 6) undergoing a reversible power cycle while operating between two thermal reservoirs at temperatures Th and Tc (temperature unit Kelvin).

η max =

TH − TC T =1− C TH TH

Hot reservoir Qh (temperature Th)

Wcycle = Qh - Qc

(1)

The maximum efficiency as a function of the steam exhaust temperature can be plotted by keeping the cooling water temperature constant. Assuming the temperature of the cooling water is around 20°C (a warm summer day), the curve gets the shape presented in Figure 7. Larger temperature difference leads to a higher thermal

Cold reservoir Qc (Temperature Tc)

Figure 6: Carnot efficiency visualized .

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STEAM BOILER TECHNOLOGY – The Basics of Steam Generation

efficiency. Although no practical heat process is fully reversible, many processes can be calculated precisely enough by approximating them as reversible processes.

Carnot efficiency 0,7 0,6 0,5 0,4

To give a practical example of the use of this theory on steam boilers, consider the Rankine cycle example presented in Figure 5. 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 8). The formula in Equation 1 can then be used to calculate the theoretical maximum thermal efficiency of the process.

0,3 0,2 0,1 0 200

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-

600

800

1000

Temperature [K]

Figure 7: Carnot efficiency graph example.

Properties of water and steam 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 expansion is the source of power in all steam engines. It also makes the boiler a dangerous device that must be carefully treated.

400

Hot reservoir Qh (temperature Th)

Wp

Wt

Cold reservoir Qc (Temperature Tc)

Figure 8: Carnot efficiency applied on the Rankine cycle.

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. [4] 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.

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STEAM BOILER TECHNOLOGY – The Basics of Steam Generation

Evaporation of water Phase change 180 160 140 Temperature [C]

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 9, 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.

120 100 80 60 40 20 0

500

1000

1500

2000

2500

3000

Net enthalpy of water [kJ/kg water]

Evaporation During the evaporation the enthalpy rises Figure 9: Water evaporation plotted in a drastically. If water is evaporated at temperature-enthalpy graph. 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 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.

When 22,12 Mpa is exceeded (the corresponding temperature is 374°C), the line stops (Figure 10). 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.

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STEAM BOILER TECHNOLOGY – The Basics of Steam Generation

1000 22,12 MPa

Pressure [bar]

100

10

1 0

100

200

300

400

0.1

0.01 Tem perature [°C]

Figure 10: Evaporation pressure as a function of evaporation temperature.

Basics of combustion Combustion can be defined as the complete, rapid exothermic oxidation of a fuel with sufficient amount of oxygen or air with the objective of producing heat, steam and/or electricity. The process of combustion occurs with a high speed and at a high temperature. 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 (natural gas, coal fuel oil, wood, etc.), which are converted in the combustion process to carbon dioxide (CO2) and steam. Sulphur, nitrogen, and various other components are also present in these fuels. 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. 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 CO2 and passes out the vent. The final gaseous product of combustion is called a flue gas. As mentioned in the introduction to this segment, combustion can never be 100% efficient. All fuels contain moisture. Other fuel components may form by-products, such as ash, and gaseous pollutants that need emission control equipment. [5] 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 stated earlier, 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. Solid fuels, such as coal, peat

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STEAM BOILER TECHNOLOGY – The Basics of Steam Generation

or biomass, are typically fired at air factors 1.1 – 1.5, i.e. 110-150% of the oxygen needed for perfect combustion. •

Incomplete Combustion occurs when part of 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. Pulverized firing has been used in industrial and utility boilers from 60 MWt to 6000 MWt. Grate firing (Figure 11) 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 11:Photo of 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:

• • •

Fixed bed combustion (grate boilers, Figure 11) Fluidized bed combustion (Figure 12) 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. [4]

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 (Figure 13) 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 12: Photo of fluidized bed combustion.

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STEAM BOILER TECHNOLOGY – The Basics of Steam Generation

1. Turning chamber 2. Flue gas collection chamber 3. Open furnace 4. Flame tube 5. Burner seat 6. Manhole 7. Fire tubes

8. 9. 10. 11. 12. 13. 14. 15.

Water space Steam space Outlet and circulation Flue gas out Blow-out hatch Main hatch Cleaning hatch Main steam outlet

16. 17. 18. 19. 20. 21.

Level control assembly Feedwater inlet Utility steam outlet Safety valve assembly Feet Inslulation

Figure 13: Schematic of a Höyrytys TTK fire tube steam boiler [6]. 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 14). 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. The various designs of water tube boilers are discussed further in “Steam/water circulation design”

Figure 14: Simplified drawing describing the water tube boiler principle. [7]

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STEAM BOILER TECHNOLOGY – The Basics of Steam Generation

Heat exchanger boiler model 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 15), 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 15: 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. The two interacting streams in a heat exchanger are referred to as the hot stream and the cold stream (Figure 16). 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). 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. 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 17). If the exchanger is a counter-flow (or cross-flow-combination, like a

hot stream

cold stream

Figure 16: A heat exchanger model.

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STEAM BOILER TECHNOLOGY – The Basics of Steam Generation

boiler), the lines points in the opposite direction (Figure 18). The length of the lines on the Qaxis shows the transferred heat rate and the Taxis 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 is 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 19).

T T1

hot stream

T2 t2

t1

cold stream

Q

Figure 17: T-Q diagram of a parallel-flow type heat exchanger. T T1

T2 t2

t1

deltaQ

Q

Figure 18: T-Q diagram of a counter-flow type heat exchanger.

T

t1 T1 Tpinch T2

t1

Q external heating required

external cooling required

Figure 19: Adjusted streams.

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STEAM BOILER TECHNOLOGY – The Basics of Steam Generation

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 9), the heat exchanger model is usually divided into at least three units.

The heat exchanger unit, where the evaporation occurs is called 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 will not 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 superheater tube material. Today's economically feasible material can take temperatures of 550-600 °C.

Economizer

water

Evaporator saturated water saturated steam

Superheater

Figure 20: Heat exchanger model of the HRSG.

T

Sup

Eva

Eco

Q Figure 21: T-Q diagram of the HRSG model in Figure 20.

The result is a heat exchanger cascade of a HRSG (with a single pressure level), which can be found in Figure 20. The T-Q diagram of the model is visualized in Figure 21.

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STEAM BOILER TECHNOLOGY – The Basics of Steam Generation

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 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 22. The T-Q diagram of the model is visualized in Figure 23

Air out

T

Eco Eva Sup Air

Air in

Air preheater

Q Figure 23: T-Q diagram of the heat exchanger model in Figure 22.

Figure 22: Furnace equipped boiler with air preheater.

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STEAM BOILER TECHNOLOGY – The Basics of Steam Generation

References 1.

Vakkilainen E. Lecture slides and material on steam boiler technology, 2001

2.

BP statistical review of world energy 2003. Web page, read September 2003. http://www.bp.com/centres/energy/primary.asp

3.

Ahonen V. “Höyrytekniikka II”. Otakustantamo, Espoo. 1978.

4.

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

5.

Zevenhoven R., Kilpinen P. Control of pollutants in flue gases and fuel gases. Energy Engineering and Environmental Protection Publications TKK-ENY-4, Espoo 2002. ISBN 951-22-5527-8.

6.

Höyrytys Oy. Web page, viewed at 8.9.2003. http://www.hoyrytys.fi/vaporworks/hoyrykattilat/ttk_kattila.htm

7.

American Heritage® Dictionary of the English Language: Fourth Edition. Web page, viewed at 10.8.2002. http://www.bartleby.com

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The History of Steam Generation Sebastian Teir

STEAM BOILER TECHNOLOGY – The History of Steam Generation

Table of contents Table of contents................................................................................................................................18 Introduction........................................................................................................................................19 Early boilers .......................................................................................................................................19 Newcomen’s boiler ........................................................................................................................20 Wagon boiler..................................................................................................................................21 Cylindrical boiler ...........................................................................................................................21 The development of modern boiler technology .................................................................................22 Fairbarn’s fire tube boiler ..............................................................................................................22 Wilcox’ water tube boiler ..............................................................................................................22 Steam drum boiler..........................................................................................................................24 Tube walled furnace.......................................................................................................................24 Once-through boiler .......................................................................................................................25 Supercritical boiler.........................................................................................................................26 Graphs and timelines of development in boiler technology ..............................................................26 Steam boilers and safety ....................................................................................................................27 References..........................................................................................................................................29

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STEAM BOILER TECHNOLOGY – The History of Steam Generation

Introduction Steam was early used to get mechanical power. Among the relics of ancient Egyptian civilization over 2000 years old records are found of the use of hot air for opening and closing temple doors (Figure 1). About the same time, mathematician Heron of Alexandria experimented with steam power and constructed among other things a rudimentary rotary steam engine. It was a spinning ball whose rotation was driven by steam jets coming from two nozzles on the ball. Although the inventor only considered it a toy, used for teaching physics to his students, it is the first known device to transform steam into rotary motion and thus the world's first reaction turbine (Figure 2). Hero’s experiments and theories can be found in his book, The Pneumatics [1]. Strangely enough, steam wasn't seriously considered a useful force until 1600 years later, when two British inventors began to turn steam power into practical devices - Thomas Savery in 1698 and Thomas Newcomen in 1705. James Watt further improved on their inventions, patenting several designs that earned him the title of father of the modern steam engine. Applications of steam power grew during the 1700s, when steam engines began to find use powering stationery machinery such as pumps and mills, and its usages expanded with time into vehicles such as tractors, ships, trains, cars and farm/industrial machinery. The age of steam lasted almost 200 years, until the internal combustion engine and the electricity took over. Even so, efficient steam turbines are still used today for submarine torpedo propulsion and for naval propulsion systems. But more importantly, steam power is still the most common means for generating electricity. [2] [3] [4] [5] [6] [7]

Figure 1: Machine that uses steam to open temple doors. [1]

Figure 2: Heron's steam engine. [1]

Early boilers Furnaces were developed originally from a need to fire pottery (4000 B.C.) and to smelt copper (3000 B.C.). Closely associated with furnaces are boilers, that were first used for warming water and are of Roman and Greek origin. Early boilers were recovered from the ruins of Pompeii.

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STEAM BOILER TECHNOLOGY – The History of Steam Generation

In 1698, Thomas Savery developed a steam-driven water pump. As the steam condensed, a vacuum was created causing the water to be drawn into the cylinder. The boiler continued to be refined and developed for use during the Industrial Revolution.

Newcomen’s boiler The era of first boilers for industrial use stems from England in the 1700 - 1800. The first use of boilers was pumping water out from mines. These boilers had a very low efficiency, but since there was no lack of fuel supply the boilers replaced the horse driven pumps. One of the first successful boilers was Thomas Newcomen's boiler (Figure 3). It was the first example of steam driven machine capable of extended period of operation. This type of boiler was called shell boiler. The steam was produced at atmospheric pressure. The boiler was made from copper, using rivets and bent metal sheets (Figure 4). In 1800, iron replaced copper in order to make the boiler last for increased pressures. Later the cylindrical design was replaced by the wagon-type design for increased capacities.

Figure 3: Newcomen's boiler, 1 - shell over the boiling water, 2 - steam valve, 3 - steam pipe, 4 - float for water level, 5 - grate doors. [2]

Figure 4: Different kinds of riveting techniques. Riveting was used as the main manufacturing method of boilers until the 1950's. Riveting is today used when manufacturing aircraft aluminium structures. [2]

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STEAM BOILER TECHNOLOGY – The History of Steam Generation

Wagon boiler When James Watt made some critical improvements to the Newcomen steam engine by separating the condenser from the cylinder and thus improving the efficiency substantially, the steam engine became in demand and provided a rapid growth of boilers. The earliest steam boilers were usually spheres or sections of spheres, heated entirely from the outside (Figure 5). Watt introduced the use of the wagon boiler (shaped like the top of a covered wagon), which is still being used with low pressures.

Cylindrical boiler Watt and Newcomen steam engines all operated at pressures only slightly above atmospheric pressure. In 1800 the American inventor Oliver Evans built a high-pressure steam engine utilizing a horizontal cylindrical boiler. Evans's boiler consisted of two cylindrical shells, one inside the other with water occupying the region between them. The fire grate was housed inside the inner cylinder, so flue gas flowed through the smaller cylinder and thus heated the water, permitting a rapid increase in steam pressure.

Figure 5: Wagon boiler. [2]

Figure 6: Cylindrical boiler. As can be seen from the picture (Figure 6), the flue gas passes also around the cylindrical boiler. One of the advantages of the cylindrical boiler is that it has a larger heat transfer surface per unit of working fluid. Therefore cylindrical boiler can be built cheaper than the earlier boilers. The pressure (and thus the temperature) can also be increased with the cylindrical design. Simultaneously but 21

STEAM BOILER TECHNOLOGY – The History of Steam Generation

independently, the British engineer Richard Trevithick developed a similar boiler, which was used in the world's first practical steam locomotive that he invented in 1801. The cylindrical boiler was later expanded to contain several passes and eventually formed the fire tube boiler.

The development of modern boiler technology The steam boiler became ever more important towards the end of the last century. The industry and transportation methods had become heavily dependant of steam power. Inventive engineers were set to work to develop increasingly new boiler types. There was room for improvement as efficiency and safety of many boilers frequently left a lot to be desired. Again and again there were boiler explosions with catastrophic consequences. Hundreds of workers died. In the USA in 1880, for instance, 170 notified boiler explosions are recorded involving 259 dead and 555 injured. The principles of the boiler technologies introduced in this chapter are still in use today.

Fairbarn’s fire tube boiler The first major improvement over Evans and Trevithick's boilers was the fire-tube Lancashire Boiler (Figure 7) , patented in 1845 by the British engineer Sir William Fairbairn, in which hot combustion gases were passed through tubes inserted into the water container, increasing the surface area through which heat could be transferred. The saturated steam was led out from the top. The main use was to run steam engines for motive power: It was used to power steamboats, railroad engines and run industrial machinery via belt drives. Fire-tube boilers were limited in capacity and pressure and were also, sometimes, dangerously explosive.

Figure 7: Cast iron fire tube boiler.

Wilcox’ water tube boiler The water tube boiler (Figure 8 and Figure 9) was patented in 1867 by the American inventors George Herman Babcock and Stephen Wilcox. The boiler had larger heating surfaces, allowed better water circulation, and, most noteworthy, reduced the risk of explosion drastically. In the water-tube boiler, water flowed through tubes heated externally by combustion gases through radiation and convection and steam was collected above in a drum. The large number of tubes and use of cross gas flow increases the heat transfer rate. Boilers of this type could be built with larger heat transfer surface per unit of working fluid than the previous design. Due to the higher rate of

22

STEAM BOILER TECHNOLOGY – The History of Steam Generation

heat transfer cooler flue gases could be used. Tubes could be made inexpensively and with higher quality than plate. [9] The water-tube boiler became the standard for all large boilers as they allowed for higher pressures than earlier boilers as well. Their first use was to run the largest steam machines but it quickly became the boiler type of choice for a steam turbine. Wilcox and Babcock founded in 1867 the first boiler-making company in Providence. This company exists still today and one of its former subsidiaries delivers boilers in Europe under the name Babcock Borsig. [10] Figure 8: Wilcox’ water tube boiler. [11]

Figure 9: A drawing of a Wilcox' water tube boiler. Bent tubes in a tight bundle receive heat from flue gas mainly convectively. The tubes are in a tilted position in order to achieve a natural circulation of water/steam. The furnace is usually made of bricks. [2]

23

STEAM BOILER TECHNOLOGY – The History of Steam Generation

Steam drum boiler The next step was the emergence of the drum boiler, which introduced a steam drum for separating steam from water (Figure 10). This coincided with the spreading of a new tube manufacturing technology, forming. This allowed cheap and reliable joint between the drum and a tube. Except from being easier to manufacture, the drum boiler was also beneficial by providing better control of the water quality by having a mud drum. Some early designs incorporated a number of steam drums, as in the picture. A boiler with two drums became quickly a standard. The limitation of a tube shell is its thickness required to withstand pressure. If larger units were required multiple boilers needed to be operated. In late 1800 some ten water tube boilers could be connected to a single steam engine or a turbine. With the new design much larger boilers could be built.

Figure 10: Multi drum boiler of Stirling type. [2]

Tube walled furnace The demand for even bigger boiler unit sizes to drive steam turbines required larger furnace volume, which eventually led to the development of the tube walled furnace (Figure 11). The tube walled furnace finally integrated the earlier separated combustion and heat transfer into the same space by building heat transferring tubes into the furnace. This meant high savings and started rapid unit size increase. About 1955 the first fully welded furnace (membrane wall) was developed. In a modern tube walled furnace the inside of the furnace wall is completely covered of heat transferring water tubes, welded together side by side. Since the water tubes are in the furnace the heat is being transferred mainly by radiation from the combustion process. A utility boiler is a boiler that is part of an industrial process. Welding forms today the basis of all modern steam boiler manufacture. The first applications of welding to boiler manufacturing were in the 1930's (Figure 12).

Figure 11: Early boiler with tube walled furnace [2].

24

STEAM BOILER TECHNOLOGY – The History of Steam Generation

Figure 12: Different methods of welding boiler tubes [2].

Once-through boiler In order to be able to increase the current unit size and efficiency of boilers, the restriction of natural circulation boilers needed to be overcome. The idea of a once through boiler, were no steam drum would be used and thus no circulation of non-vaporized water would take place, was not new. Patents for once through boiler concepts date from as early as 1824. The first significant commercial application of a once through boiler was not made until 1923, when the Czechoslovakian inventor Mark Benson provided a small 1.3 kg/s once through boiler for English Electric Co. The unit was designed to operate at critical steam pressure, but due to frequent tube failures, the pressure had to be dropped. The once through boiler uses smaller diameter and thinner walled tubes than the natural circulation boiler. In addition, the once through boiler eliminates the need for thick steel plate for the steam drum. Due to limited material availability in Europe, the once through philosophy was followed during the 1930's and 1940's, while the United States continued to rely on natural circulation boiler design. [12]

Figure 13: Benson type once through boiler with tilted tube wall. [2].

25

STEAM BOILER TECHNOLOGY – The History of Steam Generation

Supercritical boiler The era following the Second World War brought on rapid economic development in the United States and the desire for more efficient power plant operation increased. Improvements in both boiler tube metallurgy and water chemistry technologies in combination with once through boilertechnology made a power plant, operating at supercritical water pressure, possible.

Figure 14: The world's first supercritical power plant, built by Babcock&Wilcox and General Electric, started operating at 125 MW in 1957 with a main steam condition of 31 MPa and 621°C [12].

Graphs and timelines of development in boiler technology To conclude the chapter on the history of boiler technology up to date, we start with presenting a timeline on how the unit sizes of boilers have changed throughout history (Figure 15). The development of the main steam temperature in steam boilers increased until the 70's. The limiting factor for raising steam temperature is the tube materials. Although there are power plants running at main steam temperatures over 600°C, there are yet no good, economical materials that can take temperatures above 550°C available (Figure 16). The development of the main steam pressure increased also steadily until the 70's (Figure 17). The peak that can be spotted about 1930 comes from the early trials of once through boilers, cause the first once through boilers were run at critical steam pressures but later lowered since the tube material available couldn't take the high pressures. The pressure was stabilized in the 70's in order to correspond with steam temperature about 540-550°C.

26

STEAM BOILER TECHNOLOGY – The History of Steam Generation

Figure 15: Development of unit size. [2]

Figure 16: Graph presenting the development of the main steam temperature of boiler. [2]

Figure 17: Graph presenting the development of the main steam pressure of boilers. [2]

Steam boilers and safety The safety--or lack of safety--of steam was an important part of its history. The boilers, which contained the steam, were prone to explode. This occurred for a variety of reasons: undetected corrosion or furring of the heated surfaces, clumsy repairs, or failure to keep the water up to the required level, so causing firebox plates to overheat. As early as 1803 a safety device, a lead plug, was invented. The plug was designed to melt if the firebox crown became overheated and release steam before worse damage was done. However, this device was not adopted widely.

27

STEAM BOILER TECHNOLOGY – The History of Steam Generation

After an 1854 explosion in England that killed ten people, the Boiler Insurance and Steam Power Company was started. Not until 1882, though, was safety legislation introduced in Britain. In the United States there was no government regulation at all. Following the action of safety legislation in England, the number of lives lost in England from boiler accidents fell from 35 in 1883 to 24 in 1900 and to 14 in 1905. During a comparable time period in the United States, 383 people were killed in boiler accidents. The problem of safety with steam engines was eventually reduced by the introduction of new forms of power, including the steam turbine. However, boiler accidents remain a fact of life even today, and continue to cause fatalities. [5]

28

STEAM BOILER TECHNOLOGY – The History of Steam Generation

References 1.

Woodcroft B. (translator and editor) The pneumatics of Hero of Alexandria. London 1851. Online book, read September 2003. http://www.history.rochester.edu/steam/hero/

2.

Vakkilainen E. Lecture slides and material on steam boiler technology. 2001

3.

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

4.

Two thousand years of steam (Steam Boat Days). Web page, read autumn 2001. http://www.ulster.net/%7Ehrmm/steamboats/steam1.html

5.

Dreams of Steam: The History of Steam Power. Web page, read autumn 2001. http://www.moah.org/exhibits/archives/steam.html

6.

The Growth of the Steam Engine. Web page, read September 2001. http://www.history.rochester.edu/steam/thurston/1878/Chapter1.html

7.

Great Old Steam Pictures. Web page, read September 2001. http://www.bigtoy.com/photo/old_steam.html

8.

Steamboats.com. A Short History of Steam Engines. Web page, read September 2003. http://steamboats.com/engineroom4.html

9.

About.com. Inventors: Babcock & Wilcox. Web page, read September 2003. http://inventors.about.com/library/inventors/blbabcock_wilcox.htm

10.

Boiler - Water Tube Type. Web page, read September 2001. http://www.shomepower.com/dict/b/boiler_water_tube_type.htm

11.

Babcock & Wilcox. Printed brochure. http://www.babcock.com/

12.

Babcock & Wilcox. Supercritical (Once Through) Boiler Technology. PDF-file, read October 2001. http://www.babcock.com/pgg/tt/pdf/BR-1658.pdf

29

Modern Boiler Types and Applications Sebastian Teir

STEAM BOILER TECHNOLOGY – Modern Boiler Types and Applications

Table of contents Table of contents................................................................................................................................32 Introduction........................................................................................................................................33 Grate furnace boilers..........................................................................................................................33 Cyclone firing ....................................................................................................................................34 Pulverized coal fired (PCF) boilers....................................................................................................35 Fuel characteristics of coal.............................................................................................................35 Burners and layout .........................................................................................................................36 Oil and gas fired boilers .....................................................................................................................36 Fluidized bed boilers..........................................................................................................................37 Principles........................................................................................................................................38 Main types......................................................................................................................................38 Heat recovery steam generators (HRSG)...........................................................................................40 HRSGs in power plants..................................................................................................................41 Refuse boilers.....................................................................................................................................42 Recovery boilers ................................................................................................................................43 Bio-energy boilers..............................................................................................................................44 Packaged boilers ................................................................................................................................45 Scandinavian steam generator suppliers ............................................................................................46 References..........................................................................................................................................47

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STEAM BOILER TECHNOLOGY – Modern Boiler Types and Applications

Introduction Steam boilers can be classified by their combustion method, by their application or by their type of steam/water circulation. In this chapter the following boiler types will be presented and briefly described, to give the reader a perspective of the various types and uses of various steam boilers: • • • • • • • •

Grate furnace boilers Cyclone boilers Pulverized coal fired (PCF) boilers Oil and gas fired boilers Heat recovery steam generators (HRSG) Refuse boilers Recovery boilers Packaged boilers

Grate furnace boilers

• • •

Removal of moisture - brown part Pyrolysis (thermal decomposition) and combustion of volatile matter - yellow part Combustion of char - red part

Fu e

l R

n tio a i ad

m fr o

lls wa

Air

d n an iatio Rad tion vec con

Grate firing has been the most commonly used firing method for combusting solid fuels in small and medium-sized furnaces (15 kW - 30 MW) since the beginning of the industrialization. New furnace technology (especially fluidized bed technology) has practically superseded the use of grate furnaces in unit sizes over 5 MW. Waste is usually burned in grate furnaces. There is also still a lot of grate furnace boilers burning biofuels in operation. Since solid fuels are very different there are also many types of grate furnaces. The principle of grate firing is still very similar for all grate furnaces (except for household furnaces). Combustion of solid fuels in a grate furnace, which is pictured in Figure 1, follows the same phases as any combustion method:

Figure 1: Drawing of the combustion process in a sloping grate furnace.

When considering a single fuel particle, these phases occur in sequence. When considering a furnace we have naturally particles in different phases at the same time in different parts of the furnace. The grate furnace is made up a grate that can be horizontal, sloping (Figure 2) or conical (Figure 3). The grate can consist of a conveyor chain that transports the fuel forward. Alternatively some parts of the grate can be mechanically movable or the whole grate can be fixed. In the later case the fuel is transported by its own weight (sloping grate). The fuel is supplied in the furnace from the hopper and moved forward (horizontal grate) or downward (sloping grate) sequentially within the furnace. 33

STEAM BOILER TECHNOLOGY – Modern Boiler Types and Applications

The primary combustion air is supplied from underneath the fire bed, by which the air makes efficient contact with the fuel, when blowing through the bed, to dry, ignite and burn it. The secondary (and sometimes tertiary) combustion air is supplied above the bed, in order to burn combustible gases that have been released from the bed. The fuel is subjected to selfsustained burning in the furnace and is discharged as ash. The ash has a relatively high content of combustible matter. [1]

Cyclone firing The cyclone furnace chambers are mounted outside the main boiler shell, which will have a narrow base, together with an arrangement for slag removal (Figure 4). Primary combustion air carries the particles into the furnace in which the relatively large coal/char particles are retained in the cyclone while the air passes through them, promoting reaction. Secondary air is injected tangentially into the cyclone. This creates a strong swirl, throwing the larger particles towards the furnace walls. Tertiary air enters the centre of the burner, along the cyclone axis, and directly into the central vortex. It is used to control the vortex vacuum, and hence the position of the main combustion zone which is the primary source of radiant heat. An increase in tertiary air moves that zone towards the furnace exit and the main boiler. [3] Cyclone-fired boilers are used for coals with a low ash fusion temperature, which are difficult to use with a PCF boiler. 8090% of the ash leaves the bottom of the boiler as a molten slag, thus reducing the load of fly ash passing through the heat transfer sections to the precipitator or fabric filter to just 10-20% of that present. As with PCF boilers, the combustion chamber is close to atmospheric pressure, simplifying the passage of coal and air through the plant. [3]

Figure 2: Sloped grate furnace.

Figure 3: BioGrateTM - a rotating conical grate. [2] Boiler

Burnout Zone

Overfire Air Coal Reburn Burners Air

Secondary Air

Reburn Zone

Pulverized Coal

Air Preheater

Electrostatic Precipitator

Stack Coal

Air Primary Air Cyclone Burner Main Combustion Zone

Dry Waste To Disposal

Water Molten Slag

Slag to Disposal

Figure 4: Schematics of a 100 MW coal fuelled boiler with a cyclone burner. [4] 34

STEAM BOILER TECHNOLOGY – Modern Boiler Types and Applications

Cyclone firing can be divided into horizontal and vertical arrangements based on the axis of the cylinder. Cyclone firing can also be dry or molten based on ash behaviour in the cyclone. Based on cooling media the cyclones are either water-cooled or air-cooled (a.k.a. air cooled). Cyclone firing has successfully been used to fire brown coal in Germany. Peat has been fired in cyclones at Russia and Finland. Compared with the flame of a conventional burner, the high-intensity, high-velocity cyclonic flames transfer heat more effectively to the boiler's water-filled tubes, resulting in the unusual combination of a compact boiler size and high efficiency. The worst drawbacks of cyclone firing are a narrow operating range and problems with the removal of ash. The combustion temperature in a cyclone is relatively high compared to other firing methods, which results in a high rate of thermal NOx formation. [1]

Pulverized coal fired (PCF) boilers Coal-fired water tube boiler systems generate approximately 38% of the electric power generation worldwide and will continue to be major contributors in the future. Pulverized coal fired boilers, which are the most popular utility boilers today, have a high efficiency but a costly SOx and NOx control. Almost any kind of coal can be reduced to powder and burned like a gas in a PCF-boiler, using burners (Figure 5). The PCF technology has enabled the increase of boiler unit size from 100 MW in the 1950's to far over 1000 MW. New pulverized coal-fired systems routinely installed today generate power at net thermal cycle efficiencies ranging from 40 to 47% lower heating value, LHV, (corresponding to

Figure 5: PCF-burner. [5]

34 to 37% higher heating value, HHV) while removing up to 97% of the combined, uncontrolled air pollution emissions (SOx and NOx). [7]

Fuel characteristics of coal Coal is a heterogeneous substance in terms of its organic and inorganic content. Since only organic particles can be combusted, the inorganic particles remain as ash and slag and increase the need for particle filters of the fluegas and the tear and wear of furnace tubes. Pulverizing coal before feeding it to the furnace has the benefit that the inorganic particles can be separated from the organic before the furnace. Still, coal contains a lot of ash, part of which can be collected in the furnace. In order to be able to remove ash the furnace easier, the bottom of the furnace is shaped like a 'V' (Figure 6).

Boiler

Economizer Electrostatic Precipitator

Windbox

Ash Secondary Air Port

Coal and Air

Low-NOX Cell Burner System

Stack

Secondary Air Port

Coal and Air Windbox Low-NOX Cell Burner System

Fly Ash To Disposal

Bottom Ash To Disposal

Figure 6: PCF Boiler schematics. [4]

35

STEAM BOILER TECHNOLOGY – Modern Boiler Types and Applications

Burners and layout Another benefit from pulverizing coal before combustion is that the coal air mixture can be fed to the boiler through jet burners, as in oil and gas boilers. A finer particle is faster combusted and thus the combustion is more complete the finer the coal is pulverized and formation of soot and carbon monoxides in the flue gas is also reduced. The size of a coal grain after the coal grinder is less than 150 mm. Figure 8 shows various arrangement options of burners.

Figure 7: Schematics of a Low-NOx burner. [4]

Two broadly different boiler layouts are used. One is the traditional two-pass layout where there is a furnace chamber, topped by some heat transfer tubing to reduce the FEGT. The flue gases then turn through 180°, and pass downwards through the main heat transfer and economiser sections. The other design is to use a tower boiler, where virtually all the heat transfer sections are mounted vertically above each other, over the combustion chamber. [4]

Oil and gas fired boilers Oil and natural gas have some common properties: Both contain practically no moisture or ash and both produce the same amount of flue gas when combusted. They also burn in a gaseous condition with almost a homogenous flame and can therefore be burnt in similar burners with very little air surplus (Figure 9 and Figure 10). Thus, oil and gas can be combusted in the same types of boilers. The radiation differences in the flue gases of oil and gas are too high in order to use both fuels in the same boiler. Combusting oil and gas with the same burner can cause flue gas temperature differences up to 100°C. The construction of an oil and gas boiler is similar to a PCF-boiler, with the

Figure 8: PCF-boiler with horizontal coal firing with two-pass layout. [4]

Figure 9: Photo of a flame from a burner combusting oil. [7]

36

STEAM BOILER TECHNOLOGY – Modern Boiler Types and Applications

exception of the bottom of the furnace, which can be horizontal thanks to the low ash content of oil and gas (Figure 11). Horizontal wall firing (all burners attached to the front wall) is the most affordable alternative for oil and gas burners. [1]

Figure 10: Photo of a flame from a burner combusting gas. [7]

Figure 11: Oil/gas Boiler with horizontal wall firing. [6]

Fluidized bed boilers Fluidized bed combustion was not used for energy production until the 1970's, although it had been used before in many other industrial applications. Fluidized bed combustion has become very common during the last decades. One of the reasons is that a boiler using this type of combustion allows many different types of fuels, also lower quality fuels, to be used in the same boiler with high combustion efficiency. Furthermore, the combustion temperature in a fluidized bed boiler is low, which directly induce lower NOx emissions. Fluidized bed combustion also allows a cheap SOx reduction method by allowing injection of lime directly into the furnace.

FIXED BED

BUBBLING

MIN FLUID VELOCITY

TURBULENT

ENTRAINMENT VELOCITY

CIRCULATING

PARTICLE MASS FLOW

∆p (LOG)

VELOCITY (LOG)

Figure 12: Regimes of fluidized bed systems [8].

37

STEAM BOILER TECHNOLOGY – Modern Boiler Types and Applications

Principles The principle of a fluidized bed boiler is based on a layer of sand or a sand-like media, where the fuel is introduced into and combusted. The combustion air blows through the sand layer from an opening in the bottom of the boiler. Depending on the velocity of the combustion air, the layer gets different types of fluid-like behaviour, as listed and described in Figure 12. This type of combustion has the following merits: • • • •

Fuel flexibility; even low-grade coal such as sludge or refuse can be burned High combustion efficiency Low NOx emission Control of SOx emission by desulfurization during combustion; this is achieved by employing limestone as a bed material or injecting limestone into the bed. • Wide range of acceptable fuel particle sizes; pulverizing the fuel is unnecessary • Relatively small installation, because flue gas desulfurization and pulverizing facilities are not required

Main types There are two main types of fluidized bed combustion boilers: Bubbling fluidized bed (BFB) and circulating fluidized bed (CFB) boilers.

BUBBLING FLUIDIZED BED BOILER 30.8 MWth, 11.9 kg/s, 80 bar, 480 °C

In the bubbling type, because the velocity of the air is low, the medium particles are not carried above the bed. The combustion in this type of boiler is generated in the bed. Figure 13 and Figure 14 show examples of BFB boilers. The CFB mode of fluidization is characterized by a high slip velocity between the gas and solids and by intensive solids mixing. High slip velocity between the gas and solids, encourages high mass transfer rates that enhance the rates of the oxidation (combustion) and desulfurization reactions, critical to the application of CFBs to power generation. The intensive mixing of solids insures adequate mixing of fuel and combustion products with combustion air and flue gas emissions reduction reagents. In the circulating type (Figure 15), the velocity of air is high, so the medium sized particles are carried out of the combustor. The carried particles are captured by a cyclone installed in the outlet of combustor.

©PIIRTEK OY #8420

SALA-HEBY ENERGI AB SWEDEN

Figure 13: Example of a BFB boiler. [9]

Combustion is generated in the whole combustor with intensive movement of particles. Particles are continuously captured by the cyclone and sent back to the bottom part of the combustor to combust unburned particles. This contributes to full combustion.

38

STEAM BOILER TECHNOLOGY – Modern Boiler Types and Applications

Figure 14: BFB boiler used in a CHP power plant, [10] The CFB boiler (Figure 16) has the following advantages over the BFB Boiler: • •

Higher combustion efficiency Lower consumption of limestone as a bed material • Lower NOx emission • Quicker response to load changes The main advantage of BFB boilers is a much larger flexibility in fuel quality than CFB boilers. BFB boilers have typically a power output lower than 100 MW and CFB boilers range from 100 MW to 500 MW. In recent years, many CFB boilers have been installed because of the need for highly efficient, environmental-friendly facilities. Figure 15: Cutaway of a CFB furnace and cyclone. [11]

39

STEAM BOILER TECHNOLOGY – Modern Boiler Types and Applications

Figure 16: A CFB boiler schematics. [9]

Heat recovery steam generators (HRSG) As the name implies, heat recovery steam generators (HRSGs) are boilers where heat, generated in different processes, is recovered and used to generate steam or boil water. The main purpose of these boilers are to cool down flue gases produced by metallurgical or chemical processes, so that the flue gases can be either further processed or released without causing harm. The steam generated is only a useful by-product. Therefore extra burners are seldom used in ordinary HRSGs. HRSGs are usually a link in a long process chain, which puts extremely high demands on the reliability and adaptability of these boilers. Already a small leakage can cause the loss of the production for a week. Problems occurring in these boilers are more diverse and more difficult to control than problems in an ordinary direct heated boiler. Figure 17 shows an example of a HRSG with horizontal layout. Figure 18 explains the different parts of the same HRSG.

Figure 17: A HRSG with horizontal layout. [12]

40

STEAM BOILER TECHNOLOGY – Modern Boiler Types and Applications

1 2 3 4 5 6

Inlet Duct Distribution grid HP Superheater 1 Burner Split Superheater HP Superheater 2

7 CO Catalyst 8 HP Steam Drum 9 Top Supports 10 SCR Catalyst 11 LP Steam Drum 12 HRSG Casing

13 Deareator 14 Stack 15 Preheater 16 DA Evaporator 17 HP/IP Economizer

18 IP Evaporator 19 IP Superheater 20 HP Economizer 21 Ammonia Injection Grid 22 HP Evaporator

Figure 18: Various parts of the HRSG in Figure 17 explained. [12]

HRSGs in power plants Gas turbines and diesel engines are nowadays commonly used in generating electricity in power plants. The temperature of the flue gases from gas turbines is usually over 400°C, which means that a lot of heat is released into the environment and the gas turbine plant works on a low efficiency. The efficiency of the power plant can be improved significantly by connecting a heat recovery boiler (HRSG) to it, which uses the heat in the flue gases to generate steam. This type of combination power generation processes is called a combined cycle (Figure 19).

Figure 19: Simplified combined cycle, utilizing a HRSG. [12]

Since the flue gases of a gas turbine are very clean, tubes can be tightly seated or rib tubes can be used to improve the heat transfer coefficient. These boilers are usually natural circulation boilers. If the life span of the power plant is long enough, the boiler is usually fitted with an economizer. If more electrical power output is wanted, but the temperature of the flue gas is insufficient, the boiler

41

STEAM BOILER TECHNOLOGY – Modern Boiler Types and Applications

can be equipped with an extra burner (that burns the same fuel as the gas turbine) in order to increase the flue gas temperature and thus generate steam with a higher temperature.

Refuse boilers The standard refuse (or waste) recovery boiler incinerates solid or liquid waste products. This boiler type is not to be mixed with the recovery boilers used in pulp and paper industry. Therefore, we will always refer to refuse boilers when talking about waste and recovery boilers when we mean the specific chemical recovery process used in the pulp and paper industry. The combustion of waste differs radically compared to other fuels mostly due to the varying properties of waste. Also, the goal when combusting waste is not to produce energy, but to reduce the volume and weight of the waste and to make it more inert before dumping it on a refuse tip.

1 storage bin

2 3 4 13

furnace with grate post combustion boiler bottom ash conveyor

5 electrostatic precipitator 6 economizer (not typically here) 7 draft fan

8 9 10 11 12

wet scrubber 1 wet scrubber 2 SCR DENOx dioxin removal stack

Figure 20: Municipal Solid Waste Incineration plant. Waste is burned in many ways, but the main method is to combust it in a grate boiler with a mechanical grate (Figure 20). Other ways to burn waste is to use a fixed grate furnace, a fluidized bed for sludge or rotary kilns for chemical and problematic waste. Waste is usually “mass burned”, i.e. it is burned in the shape it was delivered with minimal preparation and separation. The main preparation processes are grinding and crushing of the waste and removal of large objects (like refrigerators). Waste has to be thoroughly combusted, so that harmful and toxic components are degraded and dissolved. Waste can be refined into fuel, by separating as much of the inert and inorganic material as possible. This is called refuse derived fuel (RDF) and can be used as the primary fuel in fluidized bed boilers or burned as a secondary fuel with other fuels. RDF is becoming more common nowadays.

42

STEAM BOILER TECHNOLOGY – Modern Boiler Types and Applications

Recovery boilers All paper is produced from one raw material: pulp. One of the most common methods used to produce pulp is the Kraft process, which consists of two related processes. The first is a pulping process, in which wood is chemically converted to pulp. The second is a chemical recovery process, in which chemicals used in pulping are returned to the pulping process to be used again. The waste liquid, from where chemicals are to be recovered, is called black liquor. The largest piece of equipment in power and recovery operations is the recovery boiler (Figure 21). It serves two main purposes. The first is to "recover" chemicals in the black liquor through the combustion process (reduction) to be recycled to the pulping process. Secondly, the boiler burns the organic materials in the black liquor and produces process steam and supplies high pressure steam for other process components. Black liquor is injected into the recovery boiler from a height of six meters (Figure 22). The combustion air is injected at three different zones in the boiler. The burning black liquor forms a pile of smelt at the bottom of the boiler, where complicated reactions take place. The smelt is drained from the boiler and is dissolved to form green liquor. The green liquor is then causticized with lime to form white liquor for cooking the wood chips. The residual lime mud is burnt in a rotary kiln to recover the lime. Energy released by the volatilization of the liquor particles in the recovery boiler yields a heat output that is absorbed by water in the boiler tubes and steam drum. Steam produced by the boiler is utilized primarily to satisfy heating requirements, and to co-generate the electricity needed to operate the various pieces of machinery in the plant.

Figure 21: Recovery boiler schematics. [13]

Figure 22: Schematics of the black liquor spraying in the furnace of a recovery boiler. The pile on the bottom is the smelt. [13]

43

STEAM BOILER TECHNOLOGY – Modern Boiler Types and Applications

Bio-energy boilers Renewable energy production is becoming a worldwide priority as countries strive for sustainable growth and better living conditions. Many countries (e.g. EU) have already set demanding targets to increase electricity production using bio-energy resources and have introduced attractive incentives to accelerate this process. Bio-energy solutions are based on a local fuel supply and thus provide price stability, a secure supply of heat and power, and also local employment. Biofuels are increasingly becoming locally traded commodities, which will further secure fuel price stability and availability. At the same time, green certificates and emission trading offer new opportunities for financing bio-energy projects. Figure 23, Figure 24, and Figure 25 shows examples of biofuel combusting boiler applications.

Figure 23: Firetube hot water boiler in a Wärtsilä 8 MWth thermal plant, combusting biofuels. [14]

Figure 24: Wärtsilä CHP plant using a water tube boiler connected to the furnace. [14]

44

STEAM BOILER TECHNOLOGY – Modern Boiler Types and Applications

Boilers combusting biofuels can be used to produce only electricity, but they are mostly used in combined heat and power (CHP) plants and district heating plants. These boilers are designed to operate on a wide variety of biofuels, including extremely wet fuels such as wood residues, wood chips, bark and sawdust. Smaller boilers use grate firing technology for biofuel combustion, while larger plants use fluidized bed combustion technology. Smaller grate fired plants for thermal heat production, (300

1400->1000

Superheaters

300->600

1000->600

Economizer

105->290

600->300

Air preheater

20->200

300->150

The heat transfer in the furnace results in a phase change of the working fluid (water to steam or fluid to gas). The small water/steam temperature rise is due to the fact that the water enters the furnace slightly sub-cooled (not saturated). These temperatures are only examples. They can be at various levels at different types of boiler, but the heat load graph look practically the same. The heat load graph, constructed from the table above, can be found in Figure 3. [1]

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STEAM BOILER TECHNOLOGY – Heat Exchangers in Steam Boilers

1600 Flue gas stream Water/steam stream

1400

Air stream 1200

Temperature [°C]

1000

800

600 Air preheater 400

200 Furnace

Superheater

Economizer

0 0%

10 %

20 %

30 %

40 %

50 %

60 %

70 %

80 %

90 %

100 %

Share of heat load [%]

Figure 3: Example of a heat load graph for a furnace equipped boiler.

Furnace The furnace is the part of the boiler where the combustion of the fuel takes place. The main role of the boiler furnace is to burn the fuel as completely and stably as possible. Leaving unburned material will decrease the heat efficiency and increase the emissions. Combustion must be performed in an environmentally sustainable way. The emissions from the furnace must be as low as possible. The furnace walls of a modern boiler consist of vertical tubes that function as the evaporator part of the steam/water cycle in the boiler. The boiler Figure 4: Inside a recovery boiler furnace. [2] roof is usually also part of the evaporator as well as the flue gas channel walls in the economizer and the air preheater parts of the boiler. Figure 4 shows a photograph from the inside of a recovery boiler furnace. Adequate furnace cooling is vital for the boiler. However, when burning very wet fuels as wood chips, some parts of the furnace should not be cooled in order not to remove too much heat from furnace. Thus a part of the furnace of boilers using such fuels consists of a refractory material, which reflects the heat of combustion to the incoming wet fuel.

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If the flue gas temperature after furnace is too high, the smelting of ash can occur such problems as ash deposition on superheater tubes. High temperature corrosion of superheater tubes can appear as well. Figure 5 presents an example of a temperature distribution in a two-pass boiler.

Membrane wall Nowadays, the furnace is generally constructed as a gas-tight membrane wall. The membrane wall construction consists of tubes, which have been welded together separated by a flat iron strip, called the membranes. The membranes act as fins to increase the heat transfer. They also form a continuous rigid and pressure tight construction for the furnace. The most common furnace tube used is a finned carbon steel tube that forms the membrane wall. A drawing visualizing a typical membrane tube wall can be found in Figure 6.

Convection evaporators In boilers with low steam pressure, the share of the heat needed for evaporation is bigger than when considering a high-pressure boiler. Thus the furnace-wall evaporator cannot provide enough heat for evaporation process in low-pressure boilers. Convection evaporators supply the supplementary heat needed for complete evaporation. They are normally placed after the superheater stage in boiler process. Convection evaporators can cause local tube overheat problems with partial loads. Boiler bank A boiler bank is a convection evaporator that uses two drums: one on the top of the evaporator tubes, and another in the bottom. A boiler bank is usually used in parallel with the natural circulation based evaporator/furnace, as in Figure 7. Boiler banks are less common nowadays and are nowadays typically used in low pressure and small boilers.

Figure 5: Furnace temperature distribution. Gas tight modern tube wall

Insulation wool

Outer wall

Figure 6: Modern gas-tight membrane tube wall construction. Unfinned wall tubes are welded together with metal strips.

Figure 7: Boiler generating bank (marked with green colour). 106

STEAM BOILER TECHNOLOGY – Heat Exchangers in Steam Boilers

Economizer After the feedwater pump, the water has the required pressure and temperature to enter the boiler. The pressurized water is introduced into the boiler through the economizers. The economizers are heat exchangers, usually in the form of tube packages. The purpose of economizers is to cool down the flue gases leaving the superheater zone, thus increasing the boiler efficiency. The limiting factor for cooling is the risk of low temperature corrosion, i.e. dew point of water. Economizers are placed after the superheater zone in the flue gas channel. They are usually constructed as a package of tubes fastened on the walls of the flue gas channel.

Figure 8: Economizer tube from a recovery boiler. [2]

Flue gases are cooled down with feedwater, which gets preheated up to its saturation temperature. In order to prevent the feedwater from boiling before it has entered the furnace/evaporator, the temperature of the feedwater exiting the economizer is usually regulated with a safety margin below its saturation temperature (about 10°C). The heated water is then led to the steam drum. The economizer shown in Figure 8 consists of two long-flow, vertical sections. Each economizer section is comprised of straight vertical finned tubes, which are connected in parallel to one another. The tubes are connected at the top and bottom to larger headers. This kind of vertical tube packages is typical for chemical recovery boilers. Other boilers use packages of horizontal tubes. The bundles are placed in the second pass of the boiler, behind the superheaters. Here, the water is utilizing the heat of the flue gases that is left from the superheaters, before the flue gases leave the boiler. The flue gas temperature should always stay above the dew point of the gases to prevent corrosion of the precipitators and ducts.

Superheater The superheater is a heat exchanger that overheats (superheats) the saturated steam. By superheating saturated steam, the temperature of the steam is increased beyond the temperature of the saturated steam, and thus the efficiency of the energy production process can be raised. Superheated steam is also used in facilities that don't produce electricity. The benefits of using superheated steam are: • • •

Zero moisture content No condensate in steam pipes Higher energy production efficiency

The superheater normally consists of tubes conducting steam, which are heated by flue gases passing outside the tubes. The tubes are usually connected in parallel using headers, with steam entering from one header and exiting in another header. There can be several superheater units in

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STEAM BOILER TECHNOLOGY – Heat Exchangers in Steam Boilers

the same boiler, as well as reheaters, which is a superheater for heating external steam (steam already used in a process outside the boiler). [2]

Types of superheater surfaces Superheaters can be divided into convection based and radiation based superheaters. Radiation superheaters Radiation based superheaters are used to gain higher steam temperatures and the heat is mainly transferred by radiation. These superheaters have to be placed within reach of the flame radiation. Thus radiant superheaters are usually integrated as tubes in the boiler-walls or built as panels hanging from the boiler roof. The radiation superheater is located in the top of the furnace, where the main means of heat transfer is radiation. Convection superheaters Convection superheaters are the most common superheaters in steam boilers. Convection based superheaters are used with relatively low steam temperature, and the heat from the flue gases is mainly transferred by convection. They are Figure 9: Panel superheaters in production. [2] placed after the furnace protected from the corrosive radiation of the flames. This type of superheater can also be protected from radiation by a couple of rows of evaporator tubes. Convection based superheaters can hang from the boiler roof or they can be placed in the second pass of the boiler (in a two-pass design), and are called back-pass superheaters. Panel superheater The panel superheater (shown in Figure 9 and Figure 10) functions on both radiation and convention heat transfer, depending on its location in the boiler. It consists of tubes that are tightly bundled in thin panel walls, which hang from the roof in the exhaust of the furnace. The distance between the panels is usually about 300-500 mm. The tubes are laid out according to the inline arrangement. This kind of superheater can be located e.g. first in the flue gas stream after furnace in which coal with low heating value is burned (brown coal). The panel superheater is resistant to fouling and can withstand high heat flux.

Figure 10: Panel superheaters installed. [2]

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Wing wall superheater The wing wall superheater is a kind of panel superheater that extends from a furnace (Figure 11). The bank of tubes, which are welded together, is usually built in the front wall of boiler. It has become popular especially in CFB applications. The tube is often made of carbon steel. The wing wall superheater receives heat mainly through radiation. Radiation superheaters

Panel superheater

Back-pass superheater Wing wall superheater

Convection superheater

Figure 11: Arrangement of various types of superheater units. Back-pass superheater set Convection superheaters, located in the flue gas channel (Figure 11 and Figure 12) where the flue gas starts flowing downwards, are called back-pass superheaters. In large CFB, coal and oil boilers horizontal tube arrangements are commonly used. Back-pass superheater tubes hang from the back-pass roof.

Reheater A reheater is basically a superheater that superheats steam exiting the high-pressure stage of a turbine. The reheated steam is then sent to the low-pressure stage of the turbine. By reheating steam between high-pressure and lowpressure turbine it is possible to increase the electrical efficiency of the power plant cycle beyond 40%. The reheat cycle is used in large

Figure 12: Back-pass superheater. [1]

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STEAM BOILER TECHNOLOGY – Heat Exchangers in Steam Boilers

power boilers since it is feasible economically only in larger power plants. Reheater design is very much similar to superheater design because both operate at high temperature conditions. The effect of the reheater in a T-S diagram is plotted in Figure 13.

B

T

A

Connections of superheater elements Considering the steam flow, superheater elements are usually connected in series, e.g. first convection stage and then radiant stage. When looking in the direction of the flue gas flow, the radiant stage is placed before the convectional stage of the superheaters. The steam temperature that can be reached with convection type superheaters is significantly lower than that reached with radiant type superheaters. Thus, boilers having high live steam temperature use radiant type superheaters as final superheater.

D

C

S

Figure 13: The reheater (line C-D) in a power plant cycle, plotted in a T-S diagram for steam/water.

The small amount of saturated water still remaining in steam evaporates in the first superheater section. This makes solid impurities of boiler water stick on inner surface superheater tubes and thus decreases the heat transfer coefficient of the tubes. Superheater stages are therefore placed in counter-current order, i.e. the first superheater stage is situated at the lowest flue gas temperature.

Superheated Steam OUT Reheated Steam OUT Feedwater IN Reheater IN

Saturated Steam IN

Reheater I Superheater I Reheater II Superheater III Superheater II

Figure 14: Connection of superheater and reheater stages. 110

STEAM BOILER TECHNOLOGY – Heat Exchangers in Steam Boilers

However, the superheater situated at the hottest spot within the boiler (normally convective superheater) is not usually the final superheater because of the possible overheating problems. Thus, the convective superheater is connected in forward-current order in relation to flue gas flow to provide enough cooling for superheater tubing (Figure 14) The superheater banks are connected to proceeding banks by interconnecting piping, i.e. pipes connect each ends of an outlet header to the opposite ends of the next superheater's inlet Figure 15: Cross-connections of superheater headers, as shown in Figure 15. This cross-over headers. [2] of steam flow assures even distribution of steam circulation through the entire superheater system and minimized temperature variations from one side of the boiler to the other.

Air preheater Air preheaters have two important functions in a steam boiler: they cool the gases before they pass to the atmosphere (thereby increasing the efficiency), and they raise the temperature of the incoming combustion air (thereby drying solid fuel faster). The heated air from air preheaters is also used for transporting the fuel in PCF boilers and fluidized bed boilers. Air preheaters can be of a regenerative or recuperative type. [3]

Figure 16: Heat transfer surfaces of the rotor. [4]

Regenerative air preheaters In regenerative air preheaters no media for heat transfer is used - they use the heat accumulation capacity of a slowly rotating rotor for transferring the heat. The rotor is alternately heated in the flue gas stream and cooled in the air stream, heat-storage being provided by the mass of the packs consisting of closely spaced metal sheets (Figure 16), 0.5-0.75 mm thick, which absorb and give off heat on both sides. The rotor is divided into pie-shaped 'baskets' of theses metal sheets, which in turn pick up heat from flue gases and release it into the combustion air, as shown in the drawing in Figure 17.

Figure 17: The heat-transfer principle of a regenerative air preheater. [4]

Regenerative air preheaters occupy little space; about 1/4 or 1/6 of the space required by recuperative air preheaters and can be produced cheaply. Without exaggeration it can be claimed that they have rendered possible the low flue-gas exit temperatures achieved today. Their reduced tendency to dew point corrosion should also be stressed, in particular where sulphur-containing fuels are used. Moreover, any sheet metal packs that have become corroded can be replaced easily 111

STEAM BOILER TECHNOLOGY – Heat Exchangers in Steam Boilers

and quickly. They can also be cleaned easily by playing a jet of steam over the gaps in the packs of sheet metal. The Ljungstrom air preheater (Figure 18) has acquired exceptional importance; since the last war it has found wide acceptance in Europe. The Rothmühle air preheater (Figure 19) is another type of regenerative air preheater, where the duct rotates around the battery of plates, which is fixed. The problem of regenerative air preheaters is the gas leakage from one side to another. This can cause fires due to air leakage if flue gases contain high amount of combustibles (due to poor combustion).

Figure 18: A photograph of a Ljungstrom air preheater. [4]

Recuperative air preheaters In a recuperative air heater the heat from a hightemperature flowing fluid (flue gas) passes through a heat transfer surface to cooler air. The heating medium is completely separated at all times from the air being heated. The recuperative principle implies the transfer of heat through the separation partition, with the cool side continuously recuperating the heat conducted from the hot side. Thus, the advantage of recuperative air preheaters in general is the lack of leakage because the sealing is easier to implement here than in the Figure 19: Rotmühle air preheater. [1] regenerative type. The separating surface may be composed of tubes or plates. The rate of flow is determined by temperature differential, metal conductivity, gas film conductivity, conductivity of soot, and ash and corrosion deposits. The cumulative effect of these factors may be large. There are two types of recuperative heat exchangers: tubular and plate preheaters. Tubular recuperative air preheater Tubular air preheater is comprised of a nest of long, straight steel or cast-iron tubes expanded into tube sheets at both ends, and an enclosing casing provided with inlet and outlet openings. If the tubes are placed vertically, the flue gases pass through or around them (Figure 20). If the tubes are placed horizontally, the flue gases only pass around them (Figure 21). The design, which usually provides a counter-flow arrangement, may consist of a single pass or multiple passes with either splitter (parallel to tubes) or deflecting (cross-tube) baffling. Traditionally the tubes were made of cast iron for good corrosion resistance. Thus the whole preheater was heavy and needed massive foundations.

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Flue gas Air

Flue gas Air

Figure 21: Two-pass (horizontal) air preheater design. Figure 20: Straight (vertical) air preheater design. Plate recuperative air preheater A newer, alternative design is the plate-frame type recuperative air preheater. It offers the same heat transfer capacity with reduced unit weight and size. Plate air preheater consists of a series of thin, flat, parallel plates assembled into a series of thin, narrow compartments or passages, all suitably cased. Flue gas and air pass through alternate spaces in counter-flow directions. The plate air preheater may be arranged more compactly than the tubular type. Because of cleaning difficulties, however, its use is diminishing.

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References 1. Vakkilainen E. Lecture slides and material on steam boiler technology, 2001 2. Andritz. Recovery Boiler Operation Manual, Ahlstrom Machinery Corporation 1999. CDrom. http://www.andritz.com/ 3. Combustion Engineering. Combustion: Fossil power systems. 3rd ed. Windsor. 1981. 4. Alstom. Air preheater company web page, read September 2003. http://www.airpreheatercompany.com/airpreheaters.asp

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Boiler Calculations Sebastian Teir, Antto Kulla

STEAM BOILER TECHNOLOGY – Boiler Calculations

Table of contents Table of contents..............................................................................................................................116 Steam/water diagrams used in boiler calculations ...........................................................................117 Temperature-heat (T-Q) diagram.................................................................................................117 Temperature-entropy (T-s) diagram.............................................................................................118 Application of the T-s diagram ................................................................................................119 Pressure-enthalpy (p-h) diagram..................................................................................................120 Enthalpy-entropy (Mollier, h-s) diagram .....................................................................................121 Determination of steam/water parameters .......................................................................................122 Given parameters .........................................................................................................................122 Pressure losses..............................................................................................................................122 Procedure for determination of specific enthalpies and mass flow rates.....................................122 Superheaters and reheaters...........................................................................................................123 Spray water group mass flow.......................................................................................................124 Calculations of heat load..............................................................................................................125 Evaporator................................................................................................................................125 Superheater...............................................................................................................................125 Reheater ...................................................................................................................................125 Economizer ..............................................................................................................................126 Air preheater ............................................................................................................................126 Determination of boiler efficiency...................................................................................................126 Standards......................................................................................................................................126 Major heat losses..........................................................................................................................126 Heat loss with unburned combustible gases ............................................................................126 Heat loss due to unburned solid fuel........................................................................................127 Heat loss due to wasted heat in flue gases ...............................................................................127 Heat loss due to wasted heat in ashes ......................................................................................127 Losses due to heat transfer (radiation) to the environment......................................................128 Losses of blowdown, sootblowing and atomizing steam.........................................................128 Internal power consumption.........................................................................................................128 Calculating boiler efficiency........................................................................................................129 Direct method...........................................................................................................................129 Indirect method ........................................................................................................................129 References........................................................................................................................................130

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Steam/water diagrams used in boiler calculations Temperature-heat (T-Q) diagram The T-Q diagram is a useful tool for designing heat exchangers. It can also be used to present the heat transfer characteristics of an existing heat exchanger or heat exchanger network. The T-Q diagram consists of two axes: The current stream temperature on the y-axis and the amount of heat transferred on the x-axis. Sometimes the streams are marked with arrowheads to clarify the direction of the streams, but these are not necessary: since heat cannot move from the colder stream to the hotter stream according to the second law of thermodynamics, the directions of the streams are explicitly determined: The hot stream transfers its heat to the cold stream, thus the flow direction of the hot stream is towards lower temperature and the flow direction of the cold stream is towards higher temperatures. For the same reason, the hot stream is always above the cold stream in the T-Q diagram (Figure 1).

Figure 1: Examples of T-Q diagrams for a parallel flow heat exchanger (left), and a counter (or cross) flow heat exchanger (middle). The hot stream is marked with red color and the cold with blue color. 1600 Flue gas stream Water/steam stream

1400

Air stream 1200

1000 Temperature [°C]

When designing or reviewing heat exchanger networks, the T-Q diagram gets useful. The T-Q diagram is therefore applied when designing boilers; especially the heat exchanger surface arrangement can be clearly visualized with a T-Q diagram (Figure 2).

800

600 Air preheater 400

200 Furnace

Superheater

Economizer

0 0%

10 %

20 %

30 %

40 %

50 %

60 %

70 %

80 %

90 %

100 %

Share of heat load [%]

Figure 2: Example of a T-Q diagram representing the heat surfaces in a furnace equipped boiler.

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STEAM BOILER TECHNOLOGY – Boiler Calculations

tant p = cons

v = constan

Temperature

nt sta n co

v= p = constant

Liquid-vapour region X = 0,2

X = 0,9

va po ur

d

Sa t

ur a

ted

liq

uid

p = constant

e at

The enclosed region in the middle is the region where water is a mixture of vapor and liquid. Steam that contains water in any form, either as

Critical point

tur Sa

The left border, up to the critical point, is the border where the liquid is saturated (Figure 3). That is, the water is still liquid and contains no steam. But if we go further right (increase the entropy), steam bubbles starts to form in the water. In other words, saturated water starts to boil when heat is added and entropy is increased.

t

The T-s diagram represents the various phases of steam/water with temperature as a function of the specific entropy. It is often used to visualize steam power processes. The T-s diagram is also commonly used for displaying reversible processes (or real processes simplified as reversible processes), which in the Ts diagram appear as closed curves (loop).

p = cons tant

Temperature-entropy (T-s) diagram

Entropy

Figure 3: Simplified T-s diagram of steam/water.

minute droplets, mist or fog, is called wet steam. The quantity called ‘x’ in the diagram represents the amount (percentage by weight) of dry vapor in the wet steam mixture. This quantity is called the quality of steam. For instance, if there is 10% moisture in the steam, the quality of the steam is 90% or 0.9. The temperature of wet steam is the same as dry saturated steam at the same pressure. The right border, down from the critical point, is the line where steam is saturated. When steam is heated beyond that border, steam is called superheated. Water boils under constant temperature and pressure, so a horizontal line inside the enclosed region represents a vaporization process in the T-s diagram. The steam/water heating process in the boiler represented by the diagram in figure 2 can also be drawn in a T-s diagram (Figure 4), if the boiler pressure is assumed to be e.g. 10 MPa.

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Figure 4: Detailed T-s diagram of the PCF boiler steam/water heating process from figure 2 (note: color of the steam/water process line is changed from blue to red). Application of the T-s diagram Consider the simple steam power plant based on the Rankine cycle, as visualized in (Figure 5). The plant consists of a steam boiler (superheater, evaporator and economizer), turbine with generator, condenser and a feed pump. The Rankine cycle consists of the following processes:

Sup erheate r

1

Turbine & Generator

G

6 Evapo rator/ Furn ace 2 Econo mizer

5 Pump

1-2:

Expansion of high-pressure steam in the turbine 4 (isentropic) 2-3: Condensation of low- pressure Figure 5: Rankine cycle steam in the condenser (isobaric and isothermal) 3-4: Compression of water in the feed pump (isentropic) 4-5: Heating of water in the economizer at a high pressure (isobaric) 5-6: Evaporation of water in the evaporator at a high pressure (isobaric) 6-1: Heating of steam in the superheater at a high pressure (isobaric)

3

Con denser

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STEAM BOILER TECHNOLOGY – Boiler Calculations

Pressure-enthalpy (p-h) diagram

1

q Temperature

The process can be visualized by drawing the process into a T-s diagram ( Figure 6). Since the process is assumed to be isentropic, the expansion and compression lines are strictly vertical. If the losses in the turbine and pump were considered, the vertical lines would be slightly tilted so that entropy increases. [1]

w

w

6

5 4 3

q

2

x=1

x=0

Another tool used in boiler calculation is the pressure-enthalpy diagram for steam/water Entropy (Figure 7). With the p-h diagram it is easy to visualize the partial shares of the total heat Figure 6: T-s diagram of the Rankine cycle in load on different heat exchanger surfaces in Figure 5. the boiler: drawing the steam heating process in the boiler onto the p-h diagram will give a horizontal line (if we simplify the process and set pressure losses to zero). Figure 7 shows the same boiler steam/water process from Figure 4, drawn in the steam/water p-h diagram.

Figure 7: Detailed p-h diagram of the PCF boiler steam/water heating process (red line) from Figure 4.

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ant p = const

p = cons

T = constant

va p ou r

T = constant

Sat u ra ted

The most frequently used tool for determining steam properties is probably the enthalpyentropy (h-s) diagram, also called Mollier diagram (Figure 8). If two properties of the steam state are known (like pressure and temperature), the rest of the properties for steam (enthalpy, entropy, specific volume and moisture content) can be read from the diagram. A more detailed h-s diagram can be found in Figure 9. Since the diagram is very large, the diagram is usually found as two versions, consisting of zoomed portions of the original: one for steam properties (Figure 8) and another for water properties.

tant

Enthalpy-entropy (Mollier, h-s) diagram

Critical point

X = 0,9 X=0

6

,90

Liquid-vapour region

Figure 8: Mollier (h-s) diagram, simplified version.

Figure 9: Large-scale Mollier h-s diagram for steam.

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STEAM BOILER TECHNOLOGY – Boiler Calculations

Determination of steam/water parameters Given parameters Normally in a steam boiler design assignment the parameters describing the live (output) steam, e.g. mass flow, pressure and temperature are given. If the steam boiler to be designed has a reheat cycle, also reheat pressure and temperature are given. Reheat steam mass flow can be given as well. These parameters are used to determine the rest of the steam/water parameters. [2]

Pressure losses The pressure losses in the heat exchanger units of the boiler are estimated according to the following approximations: • •

• •

Economizer: the pressure loss is 5-10% of the pressure of the feedwater entering the economizer. Evaporator: ƒ Once through boilers: in once-through boilers the pressure loss of the evaporator is between 5 and 30%. ƒ Forced and natural circulation boilers: the pressure drop in the evaporator part of drumbased boilers does not affect the pressure loss of the main steam/water flow through the boiler. This means that saturated steam leaving the steam drum has the same pressure as the feedwater entering the steam drum. The pressure loss of the evaporator has to be overcome using the driving force (natural circulation) or circulation pump (forced circulation). Superheater: the total pressure drop of all superheater packages is less than 10% of the pressure of the superheated steam. Reheater: the pressure drop in the reheater is about 5% of the pressure of reheated steam

Pressure losses of connection tubes between different heat transfer surfaces (e.g. between evaporator and superheater) can be neglected in these calculations.

Procedure for determination of specific enthalpies and mass flow rates 1. The specific enthalpy of the superheated steam can be determined with an h-s diagram if both the temperature and the pressure of the steam are known. Thus, the specific enthalpies for live (superheated) steam and reheated steam can be calculated. 2. The total pressure loss of the superheater stages should be chosen. Thus, the pressure in steam drum (drum-type boilers) or pressure after evaporator (once-through boilers) can be calculated by adding the pressure loss over the superheater stages to the pressure of the superheated steam. 3. Specific enthalpy of saturated water and steam (in the steam drum) can be read from an h-s diagram or steam tables, as the pressure in the steam drum is known. In once-through boilers the determination of specific enthalpy after the evaporator is based on the temperature. The reason for this is the unclear state of supercritical steam after the evaporator in once-through circulation. The temperature after the evaporator in once-through boilers is typically between 400 and 450°C.

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4. For removal of salts and minerals concentrated in the steam drum, a part of the water in steam drum is removed as blowdown water from the bottom of the steam drum. Normally the mass flow rate of blowdown is 1-3% of the mass flow rate of feedwater coming into steam drum. 5. In principle, the feedwater coming into steam drum should be saturated water. To prevent the feedwater from boiling in the transportation pipes, the temperature of the feedwater reaching the steam drum is 15-30°C below saturation temperature. This temperature difference is called the approach temperature. The feedwater is then called subcooled (in contrast to supercooled). When the temperature in the steam drum and the value of the approach temperature are known, the temperature after the economizer can be determined. The water pressure after the economizer can be assumed to be equal to the pressure in the steam drum and specific enthalpy after the economizer can then be read from a h-s diagram. In once through boilers the pressure after the economizer can be calculated by adding the pressure loss in the evaporator to the pressure after evaporator. The temperature after the evaporator is normally between 300 and 350°C (can be chosen as a unique value for the boiler). Knowing the pressure and the temperature, the specific enthalpy after the evaporator can be defined. 6. The pressure before the economizer can be calculated by adding the pressure loss in the economizer to the feedwater pressure after economizer. The feedwater temperature might be stated in the boiler design assignment. If it is not given, it should be chosen from the range of 200-250°C. The mass flow rate before the economizer is the blowdown mass flow rate added to the mass flow rate from the steam drum to the superheaters.

Superheaters and reheaters Reheating takes usually place in two stages. The pressure before the reheater is the reheated steam pressure added on the pressure loss in the reheater. The steam goes through a highpressure turbine before it enters the reheater. In the high-pressure turbine, the specific enthalpy of steam decreases according to the isentropic efficiency of the turbine. Isentropic efficiency is normally between 0.85 and 0.95. A part of the low-pressure steam coming from highpressure turbine continues to the high-pressure feedwater heater (closed-type feedwater heater). However, the mass flow rate of reheated steam is still 85-90% of that of the live steam.

t °C

535 505 475 435 410

354

I

II

III

Heat load

Superheating and reahiting is often applied in three stages having spray water groups Figure 10: An example of the heat load share of superheater stages. between each other to regulate steam temperature when necessary. Spray water group dimensioning is usually based on a steam temperature decrease of 15-40°C by water spraying. Spray water originates normally from the feedwater line before the economizer. Thus the pressure difference is the pressure loss of the

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STEAM BOILER TECHNOLOGY – Boiler Calculations

heat transfer surfaces between the economizer inlet and the location of the spray water nozzle. An example of a possible heat load share between the superheater stages is shown in Figure 10. Pressure loss in superheaters can be divided into equal partial pressure losses corresponding to each superheater stage. Pressure loss of the spray nozzles can be neglected. Temperature rise over all superheaters can be divided into quite similar parts along the same principle.

Spray water group mass flow Normally the mass flow rate of superheated steam (live steam) is known. Thus, mass flow rate calculations start usually by calculating the mass flow rate of spray water to the last spray water group (which is in this example between the second and third superheater stages). The mass flow rates can be solved with energy and mass balance equations. With the equations below (equation 1), the mass flow rate of steam after second superheater stage and mass flow rate of spray water to the last spray water group can be calculated. The mass flow rate of spray water to the first spray water group can be calculated along the same procedure: m& SHII + m& SPRAYII = m& SHIII m& SHII ⋅ hSHII , 2 + m& SPRAYII ⋅ hSPRAY = m& SHIII ⋅ hSHIII ,1

(1)

where m& SHII is the mass flow rate of steam after second superheater stage [kg/s], m& SPRAYII the mass flow rate of spray water to second spray water group, m& SHIII the mass flow rate of superheated steam (live steam), hSHII , 2 the specific enthalpy of steam after second superheater stage [kJ/kg], hSPRAY the specific enthalpy of spray water (feedwater), and hSHIII ,1 the specific enthalpy of steam before third superheater stage. Figure 11 shows a flow chart with the symbols visualized of the boiler arrangement used in this calculation model. HP Steam OUT

HP Steam OUT

Reheat IN

SPRAYII 2

SHI

Coal IN

1

2

SHIII

1

Flue Gas OUT

SPRAYI

2

1

RH

EVAP

2

1

SHII

2

ECO

1 2 Ash OUT

1

Air IN

APH

Feedwater IN

Figure 11: Flow chart of the PCF boiler arrangement used in this heat load calculation model.

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Calculations of heat load When the steam parameters and mass flows have been determined, the heat load of the heat exchanger units can be calculated. The heat load is the heat transferred by a heat exchanger (calculated in kW). Evaporator The heat load of the evaporator part of the boiler can be calculated as:

φ EVAP = m& SH (h′′ − hECO 2 ) + m& BD (h′ − hECO 2 )

(2)

where m& SH is the mass flow of steam before superheater [kg/s], h ′′ the specific enthalpy of saturated steam at steam drum pressure [kJ/kg], hECO 2 the specific enthalpy after economizer m& BD the mass flow of blowdown water from steam drum, and h ′ the specific enthalpy of saturated water at steam drum pressure [kg/s]. Superheater Normally superheating takes place in three or four stages in a big boiler. This calculation example is based on three stage superheating. The heat load of the first superheater stage is

φSHI = m& SH (hSHI , 2 − h′′)

(3)

where hSHI , 2 is the specific enthalpy of steam after the first superheater stage. In the second superheater stage the heat load added can be calculated as:

φSHII = m& SHII (hSHII , 2 − hSHII ,1 )

(4)

where m& SHII is the mass flow of steam before the second superheater [kg/s], hSHII , 2 the specific enthalpy of steam after the second superheater stage [kJ/kg], and hSHII ,1 the specific enthalpy of steam before the second superheater stage. Similarly, the heat load added in third superheater stage can be calculated as:

φSHIII = m& SHIII (hSHIII , 2 − hSHIII ,1 )

(5)

wher m& SHIII = Mass flow of steam before third superheater [kg/s], hSHIII , 2 the specific enthalpy of steam after third superheater stage [kJ/kg], and hSHIII ,1 the specific enthalpy of steam before third superheater stage [kJ/kg]. Reheater The heat load of the reheater stage can be calculated as:

φ RH = m& RH (hRH 2 − hRH 1 )

(6)

where m& RH is the mass flow rate of steam in the reheater [kg/s], hRH 2 the specific enthalpy of steam after the reheater [kJ/kg] , and hRH 1 the specific enthalpy of steam before the reheater.

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Economizer The heat load of the economizer can be calculated as:

φ ECO = m& ECO (hECO 2 − hECO1 )

(7)

where m& ECO is the mass flow rate of feedwater in the economizer [kg/s], hECO 2 the specific enthalpy of feedwater after the economizer [kJ/kg], and hECO1 the specific enthalpy of feedwater before the economizer. Air preheater In order to calculate the heat load for the air preheater, we need to know the combustion air mass flow, the temperature of the flue gases and the incoming air. The combustion air fed into air preheater, is taken from upper part of the boiler room. The temperature of the combustion air before the air preheater is therefore between 25 and 40°C (in Finnish conditions). The flue gases exiting the boiler are usually kept above 130-150°C in order to prevent corrosion. The enthalpies can be taken from tables:

φ APH = m& FUEL ⋅

m& AIR ⋅ (hAPH 2 − hAPH 1 ) m& FUEL

where m& FUEL is the mass flow rate of fuel fed into the boiler [kg/s],

(8) m& AIR the mass flow rate of m& FUEL

combustion air divided by the mass flow rate of fuel fed into the boiler, h APH 1 the specific enthalpy of combustion air before the air preheater [kJ/kg], and h APH 2 the specific enthalpy of combustion air after the air preheater.

Determination of boiler efficiency Standards There are two main standards used for definition of boiler efficiency. Of those, the German DIN 1942 standard employs the lower heating value (LHV) of a fuel and is widely used in Europe. The American ASME standard is based on higher heating value (HHV). However, this chapter calculates the efficiency according to the DIN 1942 standard. [2] It should be marked that with the DIN standard it is possible to reach boiler efficiencies over 100%, if the condensation heat of the flue gases is recovered.

Major heat losses Heat loss with unburned combustible gases The typical unburned combustible gases are carbon monoxide (CO) and hydrogen (H2). In large boilers usually only carbon monoxide can be found in significant amounts in flue gases. Assuming that flue gases contain only these two gases, the losses [kW] can be calculated as:

φ L1 = m& CO ⋅ H l ,CO + m& H ⋅ H l , H 2

2

(9)

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STEAM BOILER TECHNOLOGY – Boiler Calculations

where m& CO is the mass flow of carbon monoxide [kg/s], m& H 2 the mass flow of hydrogen, H l ,CO the lower heating value (LHV) of carbon monoxide (10.12 MJ/kg), and H l ,H 2 the lower heating value (LHV) of hydrogen (119.5 MJ/kg). If a relevant amount of some other flue gas compound can be found in the flue gases, it should be added to the equation. Heat loss due to unburned solid fuel Unburned fuel can exit the furnace as well as bottom ash or fly ash. The heating value of ashes can be measured in a specific laboratory test. The losses [kW] of unburned solid fuels can be calculated as:

φ L 2 = m& ubs ⋅ H l ,ubs

(10)

where m& ubs is the total mass flow of unburned solid fuel (bottom ash and fly ash in total) [kg/s], and

H l ,ubs the lower heating value (LHV) of unburned solid fuel (fly ash and bottom ash in total) [kJ/kg]. Some estimates of the losses with unburned solid fuels are presented in Table 1: Table 1: Estimates of losses with unburned solid fuel. [2] Boiler type

Heat loss per heat input of fuel

Oil fired boiler

0,2 - 0,5 %

Coal fired boiler, dry ash removal

3%

Coal fired boiler, molten ash removal

about 2 %

Grate boiler

4-6 %

Heat loss due to wasted heat in flue gases Flue gases leave the furnace in high temperature and thus they carry significant amount of energy away from boiler process. The heat loss due to wasted heat in flue gases is much larger than any other loss; therefore this is the most dominating factor affecting the boiler efficiency. To decrease flue gas losses, flue gas exit temperature should be decreased. However, the acid dew point of flue gases restricts the flue gas temperature to about 130-150°C for sulfur containing fuels. The losses caused by the sensible heat of flue gases can be calculated as:

φ L 3 = m& fuel ⋅ ∑ i

m& i ⋅ hi m& fuel

(11)

where m& fuel is the fuel mass flow [kg/s], m& i the mass flow of a flue gas component, and hi the specific enthalpy of a flue gas component (e.g. CO2) [kJ/kg]. Heat loss due to wasted heat in ashes Ash can exit the furnace either as bottom ash from bottom of the furnace or as fly ash with flue gases. The losses related to the sensible heat of ash can be calculated as:

φ L 4 = m& ba ⋅ c p ,ba ⋅ ∆Tba + m& fa ⋅ c p , fa ⋅ ∆T fa

(12)

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STEAM BOILER TECHNOLOGY – Boiler Calculations

where m& ba is the mass flow of the bottom ash [kg/s], c p ,ba the specific heat of the bottom ash [kJ/(kgK)], ∆Tba the temperature difference between the bottom ash temperature and the reference temperature [°C], m& fa the mass flow of fly ash, c p , fa the specific heat of fly ash, ∆T fa the temperature difference between the fly ash temperature and the reference temperature [°C]. Usually the reference temperature is 25°C. In recovery boilers the bottom ash is removed as molten ash in temperature of about 700-800°C. In addition, the amount of bottom ash divided by the amount of fuel is about 40%. The loss of sensible heat of ash is therefore of great importance in recovery boilers. Losses due to heat transfer (radiation) to the environment The main form of heat transfer from boiler to boiler room is radiation. It is proportional to the outer surface area of the boiler and is usually 200-300 W/(m2K) for a well-insulated boiler having its outer surface temperature below 55°C. Another possibility to determine the heat transfer losses to the environment is to use a table from the DIN 1942 standard, presented in Table 2.

Table 2: Estimations of heat transfer losses by radiation. [2] Mass flow rate of steam [t/h]

Combustion method 10

20

40

60

80

100

200

400

600

800

-

1,3

1,0

0,9

0,75

0,7

0,55

0,4

0,35

0,3

Grate

1,5

1,1

0,9

0,7

-

-

-

-

-

-

Oil/gas fired boiler

1,3

0,9

0,7

0,6

0,55

0,4

0,3

0,25

0,2

0,2

Pulverized firing Loss [%]

Losses of blowdown, sootblowing and atomizing steam Blowdown water from the steam drum and sootblowing steam (used to remove soot from heat exchanger surfaces within the boiler) use a part of the steam produced by the boiler. This lowers the boiler efficiency. In addition, steam is sometimes also used to atomize fuel in the burners. The losses can be calculated as:

φ L 6 = m& bd ⋅ h′ + m& sb ⋅ hsb + m& atomizing ⋅ hatomizing

(13)

m& bd is the mass flow of blowdown water [kg/s], h ′ is the specific enthalpy of saturated water (blowdown water from steam drum) [kJ/kg], m& sb is the mass flow of sootblowing steam, hsb is the specific enthalpy of steam used for sootblowing (when leaving the boiler), m& atomizing is the mass flow of atomizing steam, and hatomizing the specific enthalpy of steam used for atomizing the fuel (when leaving the boiler) [kJ/kg].

Internal power consumption The power plant itself consumes a part of the electricity produced. This is due to the various auxilary equipments required, like feedwater pumps, circulation pumps and air/flue gas blowers. In forced circulation boilers the share of electricity consumed by the circulation pump is about 0.5% of the electricity produced by the plant. The power consumption of the flue gas fan and the air blower are 0.75 – 1% each.. The largest power consumer is the feed water pump (about 2%). 128

STEAM BOILER TECHNOLOGY – Boiler Calculations

Normally the internal power consumption is about 5% of the electricity produced by the power plant. Since the power used is electrical (and taken from the grid), the internal power consumption share is reduced from the final boiler efficiency in boiler calculations.

Calculating boiler efficiency There are two different means of calculating the boiler efficiency: The direct method and the indirect method. Direct method In the direct method, the boiler efficiency is directly defined by the exploitable heat output from the boiler and by the fuel power of the boiler:

η=

φoutput φinput

(14)

where φ output is the exploitable heat output from boiler, and φ input the fuel power of the boiler. The direct method can be used for steam boilers where it is possible to measure the fuel heat input accurately. Indirect method Indirect method determines the efficiency of a boiler by the sum of the major losses and by the fuel power of the boiler:

η =1−

φlosses φinput

(15)

where φ losses is the sum of the major losses within the boiler, and φ input is the fuel power of the boiler. The indirect method provides a better understanding of the effect of individual losses on the boiler efficiency and is used for boilers where the fuel heat flow is difficult to measure (eg. Biomass and peat fired steam boilers).

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STEAM BOILER TECHNOLOGY – Boiler Calculations

References 1.

Khartchenko N. V. Advanced energy systems. Taylor & Francis 1998, U.S. ISBN 1-56032-611-5

2.

DIN 1942 standard. "Abnahmeversuche an Dampferzeugern".

130

Thermal Design of Heat Exchangers Sebastian Teir, Anne Jokivuori

STEAM BOILER TECHNOLOGY – Thermal Design of Heat Exchangers

Table of contents Table of contents..............................................................................................................................132 General design issues .......................................................................................................................133 Heat transfer modes .....................................................................................................................133 Conduction ...............................................................................................................................133 Convection ...............................................................................................................................133 Radiation ..................................................................................................................................134 Pressure losses..............................................................................................................................134 Definition .................................................................................................................................134 Gas side pressure drop for inline tube arrangement.................................................................135 Gas side pressure drop for staggered tube arrangement ..........................................................135 Choice of tube surface..................................................................................................................136 Sizing of heat transfer surfaces ....................................................................................................136 Furnace design .................................................................................................................................137 Furnace strain level ......................................................................................................................138 Tube wall design ..........................................................................................................................139 Load characteristics......................................................................................................................140 Fuel type effect on furnace size ...................................................................................................140 Typical furnace outlet temperatures.............................................................................................140 Furnace air levels .........................................................................................................................141 CFB furnace design......................................................................................................................142 BFB furnace design......................................................................................................................143 Heat recovery steam generator (HRSG) design...........................................................................144 Furnace dimensioning, stirred reactor..........................................................................................146 Superheater design ...........................................................................................................................146 Design velocity ............................................................................................................................147 Design spacing .............................................................................................................................147 Tube arrangement ........................................................................................................................147 Economizer design...........................................................................................................................149 Design method .............................................................................................................................149 Air preheater design .........................................................................................................................151 References........................................................................................................................................152

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STEAM BOILER TECHNOLOGY – Thermal Design of Heat Exchangers

General design issues Heat transfer modes Conduction Conduction is the transfer of heat from one part of a body at a higher temperature to another part of the same body at a lower temperature, or from one body at a higher temperature to another body in physical contact with it at a lower temperature. The conduction process takes place at the molecular level and involves the transfer of energy from the more energetic molecules to those with a lower energy level. Heat power [W] by conduction is: Φ = λA

t1 − t 2 s

(1)

Heat power depends on the heat transfer area (A), temperature difference (t1-t2), thermal conductivity of material (λ) and the thickness of separating wall (s). The thermal conductivity is a property of the material; metals conduct well heat whereas gases not. An example of thermal conductivities in various materials is shown in Table 1. [1] Table 1: Thermal conductivities for various materials. Material

Thermal conductivity [W/(m*K)]

Copper

370

Aluminium

210

Steel

45

Stainless steel

20

Insulations

0,03-0,1

Convection Convection is heat transfer between a moving fluid or gas and a fixed solid. Convection can be natural or forced: if a pump, a blower, a fan, or some similar device induces the fluid motion, the process is called forced convection. If the fluid motion occurs as a result of the density difference produced by the temperature difference, the process is called free or natural convection. Heat power by convection can be calculated as: Φ = α c A(t1 − t 2 )

(2)

The heat transfer coefficient αc varies much depending on e.g. flow velocity, type of fluid motion and pressure. Heat transfer coefficients of liquids are much higher than those of gases, as can be seen in the comparison presented in Table 2.

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Table 2: Convection heat transfer coefficients for various fluids. Fluid

Heat transfer coefficient [W/(m2K)]

Steady water

100-500

Water flow

500-10000

Water boiling

1000-60000

Steady air

3-15

Air flow

10-100

Radiation Radiation, or more correctly thermal radiation, is electromagnetic radiation emitted by a body by virtue of its temperature and at the expense of its internal energy. All heated solids and liquids, as well as some gases, emit thermal radiation. The importance of radiation heat transfer will increase, when the temperature becomes higher. Radiation heat transfer is the main heat transfer mode for the furnace and radiation superheaters. Emitted heat by radiation can be calculated as:

Φ r = ε fwσA(T f4 − Tw4 )

(3)

where εfw is the view factor between the flame and the water walls:

ε fw =

1

εf

+

1 1

εw

(4) −1

where εf is the emissivity of the flame (typically 0.35-0.85), εw the emissivity of the water walls (typically 0.6), σ the Stefan-Boltzmann constant (5.6787*10-8 W/m2K4), A the effective water wall surface (m2), Tf the average gas temperature in the furnace and Tw the average water wall surface temperature surrounding the flame. Radiation heat can also be expressed as Φ = α rad A(t1 − t 2 )

(5)

where αrad is the radiation heat transfer coefficient.

Pressure losses Definition The difference between pressure gage readings in parts of a system operating with a positive pressure relative to that of the atmosphere is generally called pressure drop. The pressure drop on the gas side is equal to the friction losses, according to VDI Wärmeatlas [1]:

∆p gs = ∆p f

(6) 134

STEAM BOILER TECHNOLOGY – Thermal Design of Heat Exchangers

Gas side pressure drop for inline tube arrangement For inline tube arrangement the pressure drop coefficient for heat transfer surface with horizontal tubes is: ∆p = n rζ r ∆p d

(7)

where nr is the number of tube rows in the heat transfer unit, ∆pd dynamic pressure calculated at the gas side using the mean temperature and the smallest area. The single row pressure drop ξr for inline tube arrangement is calculated as

ζ r = ζ l + ζ t (1 − e

−

Re−1000 2000

)

(8)

where 0.5

ζl =

280π (( s l − 0.6) 2 + 0.75) 1 .6

(4 s t s l − π ) s t Re

0.94 0.6 ⎤ ⎡ (1 ) ⎥ ⎢ sl 0.47(s t /s l -1.5) ⎢(0.22 + 1.2 ⎥ + 0.03( s t − 1)( sl − 1) ζ t = 10 (s t - 0.85) 1.3 ⎥ ⎢ ⎢⎣ ⎥⎦

(9)

(10)

where ζ l is the laminar part of the pressure drop coefficient, ζ t is the turbulent part of the pressure drop coefficient, s t is the dimensionless transverse pitch (s t = S t / d o ), s l is the dimensionless longitudinal pitch ( s l = S l / d o ) and Re is the Reynolds number, calculated at the gas side mean temperature and smallest area.

Gas side pressure drop for staggered tube arrangement The single row pressure drop ζ r for staggered tubes is calculated similarly to inline tube arrangement, with the following exceptions: Re − 200 − ⎞ ⎛ ⎜ ζ r = ζ l + ζ t ⎜1 − e 1000 ⎟⎟ ⎠ ⎝

(

)

2 0.5 280π ⎛⎜ s l − 0.6 + 0.75 ⎞⎟ ⎠ ⎝ ζl = 1.6 (4st sl − π )c Re where c = st ; s l ≥ 2s t - 1/2 2 s c = ( t ) 2 + s l ; sl < 2st − 1 / 2 2

(11)

(12)

(13)

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STEAM BOILER TECHNOLOGY – Thermal Design of Heat Exchangers

3

⎞ ⎛ ⎛s ⎞ ⎛s ⎞ 1 .2 ⎟ + 0.4⎜ l − 1⎟ − 0.01⎜ t − 1⎟ ζ t = 2.5 + ⎜⎜ 1.08 ⎟ ⎜ ⎟ ⎜ ⎟ ⎝ st ⎠ ⎝ sl ⎠ ⎝ (s t − 0.85 ) ⎠

3

(14)

Choice of tube surface Surfaces used in tubular heat transfer units can be finned or unfinned (smooth surface). Heat transfer properties can be improved using finned tubes, because the fins enlarge the tubular heat transfer area. The tubes in the economizer are usually finned, because the heat transfer properties of the flue gas side are not as good as on the water side. Economizers are made of cast iron or steel tubes. Cast iron tubes are easily equipped with fins, but also steel tubes can be equipped with fins. Finned tubes are more difficult to clean than unfinned tubes, thus economizers with unfinned steel tubes are used in boilers burning fuels with a high ash content. Figure 1, Figure 2, and Figure 3 provide some examples on finned steel tubes. Spiral finned tubes are often used in heat recovery steam generators. By bending fins heat transfer properties can also be improved. Steel tube with aluminium fins endures better in corrosive conditions. Compound composition conists of a cast iron tube equipped with fins and steel tube inside. A compound composistion endures higher pressure.

Figure 1: Spiral finned tubes.

In air preheaters finned steel tubes are not used, since the heat transfer properties are practically the same on both air and flue gas sides. When cast iron tubes are used, heat transfer surfaces are usually finned on both sides to improve the heat transfer. Superheaters and furnaces use unfinned tubes.

Sizing of heat transfer surfaces

Figure 2: Finned tubes. When sizing the heat transfer surface of a heat exchanger the heat power to be transferred and stream temperatures of inlets and outlets have to be known. The heat power is proportional to the area of the heat exchanger, heat transfer coefficient and temperature difference (between the streams):

Φ = kA∆Tlm

(15)

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STEAM BOILER TECHNOLOGY – Thermal Design of Heat Exchangers

The mean logarithmic temperature difference in equation 15 can be calculated as: ∆Tlm =

∆Tmax − ∆Tmin ∆T ln max ∆Tmin

(16)

where ∆Tmax is the largest temperature difference and ∆Tmin the smallest temperature difference:

∆Tmax = th1-tc2 ∆Tmin = th2-tc1

(17)

where the inlet and outlet temperatures are explained in Figure 4. The heat surface area can be calculated from equation 15, when temperatures and the heat transfer coefficient have been determined, which is the capability of the heat exchanger to transfer heat between two fluids.

Figure 3: Parallel finned tube.

Figure 4: Heat exchanger stream descriptions (for a cross-flow heat exchanger), used in equation 17.

Furnace design The main parameters for the furnace sizing are furnace dimensions (height, depth, width and configuration), furnace wall construction and desired furnace outlet temperature. The heat transfer surface area of furnace consists of sides, base and beak, which is an "L"-formed bending of the evaporator tubes that protect the superheaters from radiation. Most of utility and industrial boiler furnaces have a rectangular shape. A large number of package boilers have a cylindrical furnace. Furnace bottom for typical PCF boiler is double inclined or v-form, as shown in Figure 5. Flat bottom is more typical for grate and fluidized bed boilers. The ratio of height and width varies 1-5 for boilers with two-pass layout. The larger the boiler is, the larger is also the ratio. The largest boilers have a width of 20 m and a height of 100 m. The fuel and vaporization efficiency determines the size of the furnace. To be able to dimension furnaces the overall mass balance, heat balance and heat transfer must be specified. 137

STEAM BOILER TECHNOLOGY – Thermal Design of Heat Exchangers

The overall furnace (gas side) mass balance is

m& fg = m& air + ∑ m& fi − m& ash + m& sootb (18) where the streams are described in Figure 6. ∑ m& fi is the sum of all the fuel streams into the

h

boiler and m& sootb is the sootblowing steam. The furnace heat balance can be specified similarly:

Φ fur = Φ net − Φ loss − Φ exit

εw α dg + ε w − α dg ε w

− α dg Tw ) + α c ⋅ Aeff ⋅ (Tg − Tw )

V

(19)

where the heat fluxes are shown in Figure 7. If the gas side temperatures and emissivities are known, the furnace heat flux absorbed by the furnace walls can be expressed as

Φ fur = Aeff ⋅ σ ⋅

A

4

⋅ (ε dg Tg

b1

b2

Figure 5: Furnace dimensions. The painted areas are the total effective furnace heat transfer area.

(20)

4

m& fg

where Aeff is the effective heat transfer surface, σ the Stefan-Boltzmann constant, ε w and ε dg the emissivity of the wall and the (dusty) gas respectively, α dg the absorbability of the (dusty) gas, α c the convective heat transfer coefficient, and Tg and Tw the temperature of the gas and wall respectively. The effect of convective term is usually fairly small, often less than 10%.

Furnace strain level The furnace is preliminarily dimensioned with a suitable strain level. The volume (marked with a “V” in Figure 5) strain level is calculated as the following: qV =

Φ fuel b1b2 h

m& air

∑ m&

m& sootb

fi

m& ash Figure 6: Fuel/flue gas side mass balance. (21)

where Φ pa is the heat released from the fuel in the furnace and other variables furnace dimensions according to Figure 5. The strain level depends largely on different fuels. Reference values on strain levels from different fuels are presented in Table 3. The area strain level is calculated as the heat power in the furnace per base area of the furnace (marked with an “A” in Figure 5): 138

STEAM BOILER TECHNOLOGY – Thermal Design of Heat Exchangers

loss

Φ fuel

qF =

exit

(22)

b1b2

Table 3: Strain level effects of various fuels. Fuel

Strain [kW/m3]

Coal

145-185

Peat

~175

Oil, natural gas

290-690

fur

net

level

Figure 7: Furnace heat balance. If the electric power of power plant is known, strain levels for the volume and base area can be chosen from the graphs in Figure 8, and thereby the physical dimensions of the furnace can be determined.

6

0,25

[MW/m3]

[MW/m2]

5

0,20

4

0,15

3

0,10

2

0,05 0

200

400

600 MWe

0

200

400

600

MWe

Figure 8: Charts for selecting strain levels of the furnace. The effective heat transfer surface area of the furnace, consisting of sides, base and beak, can be calculated as following (assuming the beak adds 0.4*base area): EPRS ≈ 2lb1 + 2lb2

(23)

The first two terms forms the effective projected radiant surface (EPRS), which is a widely used concept.

Tube wall design When the size of the furnace has been dimensioned, the tube size and material can be chosen and the wall thickness can be calculated according to the SFS 3273, DIN or another applicable standard. Then input velocity of water to furnace is chosen and number of necessary tubes is calculated. The diameter of an evaporator tube is usually 30-80 mm and the wall thickness can be calculated from the following equation:

139

STEAM BOILER TECHNOLOGY – Thermal Design of Heat Exchangers

s=

du ⋅ p + C1 + C 2 ⎞ ⎛ σl − p ⎟ ⋅ν + 2 ⋅ p ⎜2⋅ n ⎠ ⎝

(24)

where du is the outside diameter of tube, p the design pressure, σ l the design strength, n a safety factor (usually 1.5), ν the strength factor (usually 1.0), C1 an additional thickness, (normally 10% of the wall thickness) and C2 an additional thickness considering corrosion.

Load characteristics When designing a steam-generating unit it is necessary to determine the following load characteristics: 1. Minimum, normal and maximum load 2. Time duration of each load rate 3. Load factor 4. Nature of the load (constant or fluctuating) The load factor is the actual energy produced by a power plant during a given period, given as a percentage (share) of the maximum energy that could have been produced at full capacity during the same period. The design will determine the boiler's ability to carry a normal load at a high efficiency as well as to meet maximum demand and rapid load changes. It will also determine the standby losses and the rapidity with which the unit can be brought up to full steaming capacity. In smaller boiler sizes it is possible to select a standardized unit that will meet the requirements; larger units are almost always custom designed.

Fuel type effect on furnace size The most important item to consider when designing a utility or large industrial steam generator is the fuel the unit will burn. The furnace size, the equipment to prepare and burn the fuel, the amount of heating surface and its placement, the type and size of heat recovery equipment, and the flue gas treatment devices are all fuel dependent. The major differences among boilers that burn coal, biomass, oil or natural gas result from the ash in combustion products. Firing oil in the furnace produces relatively small amounts of ash. Natural gas produces no ash. For the same power output, due to larger volumetric flue gas flow, coal-burning boilers must have larger furnaces. The velocities of the combustion gases in the convection-based heat exchangers must be lower, due to the high ash content of coal. Figure 9 presents an example of the relative sizes of furnaces using three different fuels: natural gas, oil and coal. The power of the boiler is the same in all three cases. Peat, biomass and recovery boilers are even bigger than coal fired boilers.

Typical furnace outlet temperatures Furnace outlet temperature is the flue gas temperature after the radiation-based heat transfer surfaces before entering the convection-based heat transfer surfaces. The outlet temperature depends on the characteristics of the combusted fuel. If the temperature is too high, ash layers build up on the surface of the superheater tubes. This leads to poorer heat transfer, increased corrosion and it can even block flow paths.

140

STEAM BOILER TECHNOLOGY – Thermal Design of Heat Exchangers

Coal

Natural gas Oil

1,5*h 1,2*h

h

b1

b2 1,05*b1 1,06*b2

1,1*b1 1,12*b2

Figure 9: Boiler fuel type effect on furnace size. The following factors affect the choice of furnace outlet temperature: • • • •

Ash characteristics; the control of ash behaviour at superheaters is a key design parameter Fuel (gas and oil have low ash content and can have higher outlet temperatures) Choice of superheater material Desired superheating temperature

Table 4 presents some typical furnace outlet temperatures. Table 4: Typical furnace outlet temperatures on various boiler types. Fuel type

Furnace outlet temperature [°C]

Biomass, circulating fluidized bed

900 - 1000

Peat, pulverized firing

950 - 1000

Coal, high volatiles

950 - 1000

Recovery boiler

900- 1050

Biomass, fluidized bed

1050 - 1150

Natural gas

900- 1200

Oil

900- 1200

Furnace air levels The type of fuel determines the quantity of air required for combustion. It is necessary to provide air in excess of this quantity to assure complete combustion. The amount of this excess air is determined by the following factors: 1. Composition, properties, and condition of fuel when fired 2. Method of burning the combustible

141

STEAM BOILER TECHNOLOGY – Thermal Design of Heat Exchangers

3. Arrangement and proportions of the grate or furnace 4. Allowable furnace temperature 5. Turbulence and thoroughness of the mixing of combustion air and volatile gases Excess air reduces efficiency by lowering the furnace temperature and by absorbing heat that would otherwise be available for steam production. NOx is formed when nitrogen of air reacts with oxygen of air in high temperature, over 1400°C. NOx can be reduced decreasing temperature, decreasing air excess, or using low-NOx-burners. In using low-NOx-burner air will be fed into flame in two or three phases.

CFB furnace design When dimensioning a circulating fluidized bed (CFB) furnace the high content of sand has to be taken into consideration. This means that the temperature profile and thus the heat transfer near to the furnace wall differs from other types of furnaces. The furnace of a CFB (circulating fluidized bed) boiler contains a layer of granular solids, which have a diameter in the range of 0.1-0.3 mm. It includes sand or gravel, fresh or spent limestone and ash. The operating velocity of the flue gas stream in a CFB boiler is 3-10 m/s. The solids move through the furnace at much lower velocity than the gas; solids residence times in the order of minutes are obtained. The long residence times coupled with the small particle size produce high combustion efficiency and high SO2 removal with much lower limestone feed than in conventional furnaces. Figure 10 shows a flow chart of a typical CFB boiler. After the furnace flue gas moves through a cyclone (named compact separator in Figure 10), where solids are separated from the gas and are returned to the furnace. Flue gas from the cyclone discharge enters the convection back-pass in which the superheaters, reheaters, economizers and air preheaters are located. A dust collector separates the fly ash before the flue gas exits the plant. The combustion air from the fan pneumatically transports the solids for creating the circulating fluid. The design of the furnace in a CFB boiler depends on: • • •

required velocity of gas time of complete combustion of fuel heat required for vaporization.

The amount of cyclones also has an influence on the shape of furnace. Flue gas must flow to the cyclone fast enough (20 m/s), and the diameter of the cyclone must be below 8 m in order to get an efficient removal of solids. Circulating fluidized bed boilers have a number of unique features that make them more attractive than other solid fuel fired boilers. Fuel flexibility is one of the major attractive features of CFB boilers. A wide range of fuels can be burned in one specific boiler without any major change in the hardware. The combustion efficiency of a CFB boiler is high. It is generally in the range of 99,5 to 97,5 %. Sulphur capture in a CFB is very efficient, due to the possibility to inject sulphur absorbing limestone directly into the bed. A typical CFB boiler can capture 90 % of the sulphur dioxide. The low emission of nitrogen oxides is also a major attractive feature of CFB boilers. CFB furnace design is explained in detail in the chapter about CFB boiler design.

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Steam Outlet

Steam

Foster Wheeler CFB Flow Chart

Water

Steam Drum

Downcomer Water Wall

Fuel

Limestone

Compact Separator

Economizer

Air heater

Feed Water Inlet Dust Collector

Combustion Chamber

Fly Ash

compact.eng/comflow.ds4/0801/tap

Bottom Ash

Secondary Air Fan To Ash Silos

Induced Draft Fan

Primary Air Fan

Figure 10: Flow chart of a CFB boiler. [2]

BFB furnace design Bubbling fluidized bed (BFB) boilers use a low fluidizing velocity, so that the particles are held mainly in a bed, which have a depth of about 1 m and a definable surface. Sand is often used to improve bed stability, together with limestone for SO2 absorption. As the coal particles are combusted and become smaller, they are elutriated with the gases, and subsequently removed as fly ash. In-bed tubes are used to control the bed temperature and generate steam. The flue gases are normally cleaned using a cyclone, and then pass through further heat exchangers, raising steam temperature. In the furnace (Figure 11 and Figure 12) of a BFB boiler size of a grain of sand is about 1-3 mm and the operating velocity is 0.7-2 m/s. Fuel is fed onto the bed mechanically. Thanks to the large heat capacity of the bed, a BFB furnace is able to burn very moist fuel. Moist fuel will dry fast, when it is fed to the sand bed. Many

Figure 11: Inside a BFB boiler furnace. [4]

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STEAM BOILER TECHNOLOGY – Thermal Design of Heat Exchangers

different kinds of fuels can be combusted in a BFB furnace. The wall area covered by the bed is free from water tubes, in order to protect the tubes from excessive erosion (Figure 11). This is called a refractory lining. The temperature of a BFB furnace outlet is 700900°C, and the air factor is usually 1.1-1.4. Air is fed in several phases. The temperature of air varies from 20 to 400°C. The overall thermal efficiency of a BFB boiler power plant is around 30%. BFB furnaces with an atmospheric operational pressure are mainly used for boilers up to about 25 MWe, although there are a few larger plants where a BFB boiler has been used to retrofit an existing unit.

Heat recovery steam generator (HRSG) design Heat recovery steam generators (HRSGs) are used in power generation to recover heat from hot flue gases (500-600 °C), usually originating from a gas turbine or diesel engine. The HRSG consists of the same heat transfer surfaces as other boilers, except for the furnace. Since no Figure 12: BFB-boiler, Härnösand fuel is combusted in a HRSG, the HRSG have Energi&Miljö Ab. [3] (instead of a furnace) convention based evaporator surfaces, where water evaporates into steam. However, a HRSG can be equipped with a supplementary burner (as can be seen in Figure 13) for raising the flue gas temperature. A HRSG can have a horizontal or vertical layout, depending on the available space. When designing a HRSG, the following issues should be considered: • • •

the pinch-point of the evaporator and the approach temperature of the economizer the pressure drop of the flue gas side of the boiler optimization of the heating surfaces

The pinch-point (the smallest temperature difference between the two streams in a system of heat exchangers) is found in the evaporator, and is usually 6-10°C, which can be seen in Figure 14. To maximize the steam power of the boiler, the pinch-point must be chosen as small as possible. The approach temperature is the temperature difference of the saturation temperature in the evaporator and the output of the economizer. This is often 0-5°C. The pressure drop (usually 25-40 mbar) of the flue gas side has also an effect on the efficiency of power plant. The heat transfer of the HRSG is primarily convective. The flow velocity of the flue gas has an influence on the heat transfer coefficient.

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The evaporator of heat recovery boiler can be of natural or forced circulation type. The heat exchanger type of the evaporator can be any of parallel-flow, counter-flow or cross-flow. In parallel-flow arrangement the hot and cold fluids move in the same direction and in counter-flow heat exchanger fluids move in opposite direction.

Flue Gas OUT

Feedwater IN

Economizer

Evaporator

Heating surfaces of a heat recovery steam generator are usually heat transfer packages, which consist of spiral-finned tubes. The thickness of the fin is 1-2 mm, the height 8-16 mm and the fin distance 3.2-8 mm. Tube sizes vary a lot.

Superheater HP Steam OUT

Fuel IN

Supplementary burner

Flue Gas IN

Figure 13: Process scheme of single-pressure HRSG with a supplementary burner. 700 Flue gas stream Water/steam stream 600

Temperature [°C]

500

400

300

200

100 Superheater

Evaporator

Economizer

0 0%

10 %

20 %

30 %

40 %

50 %

60 %

70 %

80 %

90 %

100 %

Share of heat load [%]

Figure 14: Example of a heat load graph for a HRSG boiler.

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STEAM BOILER TECHNOLOGY – Thermal Design of Heat Exchangers

Furnace dimensioning, stirred reactor One of the most used furnace dimensioning methods is the stirred reactor model. The furnace is approximated as being filled with a homogenous three-atom gas and a dust mixture at a uniform temperature and pressure. At the furnace exit the temperature is decreased by a specified amount. The stirred reactor furnace dimensioning process is as follows: 1. Guess initial furnace dimensions; shape, height, width, depth 2. Guess furnace exit temperature, Texit 3. Calculate heat transfer using flue gas temperature Tfg = Texit+∆T 4. Calculate furnace exit temperature from heat balance with calculated heat transfer 5. If the mode does not converge, then return to step 2 6. If the calculated furnace exit temperature differs from the desired one, return to step 1 The typical values of ∆T to use for the different types of furnaces can be seen in Table 5. The stirred reactor model is not optimal for designing a recovery boiler furnace. Table 5: Typical values of ∆T for various types of furnaces. Boiler type

∆T [°C]

PCF (molten), coal

200 (100-300)

PCF (dry), coal

180 (100-250)

Grate firing, coal

130 (100-180)

PCF, lignite

120 (100-150)

Oil and gas

150 (100-200)

BFB

130 (100-150)

CFB

0

Superheater design The production of steam at higher temperature than the saturation temperature is called superheating. The temperature added to the saturation temperature is called the degree of superheat. Superheated steam has no moisture; hence it is less erosive and corrosive than wet saturated steam carrying droplets. In order to have a sustainable turbine operation, the steam cannot contain any moist at all. The design procedure for a superheater can be divided into the following steps: • • • • • • • •

Tube size and material are chosen. Wall thickness is calculated. Flow velocity in tube is chosen, number of tubes is calculated, tube construction and width of heat exchanger are chosen. Height of heat exchanger is calculated according to the chosen flue gas velocity. Internal heat transfer coefficient (for the inside, water side of the tube) is calculated. External heat transfer coefficient (for the outside, gas side of the tube) is calculated. Thermal resistance of dirt layer is calculated. Thermal resistance/tube length is calculated. Conductance is calculated 146

STEAM BOILER TECHNOLOGY – Thermal Design of Heat Exchangers

• • • • • •

Necessary tube length is calculated. Necessary number of passages is calculated. Assumed values are iterated. Main dimensions are calculated. Inside and outside pressure losses are calculated. Heat exchangers are drawn to the technical drawing of boiler.

Design velocity Superheaters transfer heat from flue gas to steam (gas phase of water). Heat transfer between two gases is not very effective compared to heat transfer from gas to fluid. For that reason, steam must flow fast enough (10-20 m/s) in order to give the superheater tubes enough cooling. Lower steam pressure weakens the heat transfer rate, so with lower pressures, steam must have a greater velocity (15-40 m/s). When flue gas is cooled, its volume decreases. In order to keep a constant flow rate of the flue gas, the cross-sectional flow area decreases as well. In the radiant superheater, the velocity of gas is very small (< 5 m/s). In the convection superheater, the velocity can be quite large (15-30 m/s). The maximum velocity depends on the fuel used. To limit pressure-part erosion from fly ash, the flue gas velocity must not exceed certain limits. Depending upon the ash quantity and abrasiveness, the design velocity is generally 16-18 m/s. A furnace that burns coals yielding a heavy loading of erosive ash (usually indicated by a high silica/aluminium content) may have a design velocity of approximately 15 m/s. Such velocities are based on the predicted average gas temperature entering the tube section, at the maximum continuous rating of the steam generator fired at normal excess-air percentage.

Design spacing Superheater of boiler consists of banks of tubes. A system of tubes is located in the path of the furnace gases in the top of furnace. Heat transfer in superheaters is based mainly on radiation, but in the primary superheaters convection often plays a major role. A superheater must be built so that it superheats approximately the same amount of steam from low to high loads. This can be achieved by a proper choice of radiative and convective superheating surfaces. Changing tube lengths between passes can control temperature differences. The outermost tube that receives the most radiative flux should be shorter than the rest of the tubes. Proper superheater arrangement also eliminates much of the problems with uneven or biased flue gas flow. Figure 15 and Figure 16 shows examples of the arrangement of superheater and reheater surfaces in the form of a process scheme.

Tube arrangement Tubes in superheaters can be arranged according to inline or staggered arrangement (Figure 17). Inline tube arrangement is preferred for fouling, PCF, bark and recovery boilers. Staggered arrangement is preferred for oil, gas and heat recovery steam generator. As free space with staggered arrangement is much smaller than with inline arrangement the reason for decreased fouling with inline is evident. The heat transfer for a staggered arrangement is better than for an inline arrangement. The superheater tube diameter is usually 30-50 mm. For convection heat surfaces the dimension ‘a’ (Figure 17) is 80-200 mm and ‘b’ is 60-150 mm. For radiation heat surfaces ‘a’ is over 500 mm and ‘b’ is approximately the same as the external tube diameter. 147

STEAM BOILER TECHNOLOGY – Thermal Design of Heat Exchangers

Superheated Steam OUT Feedwater IN

Saturated Steam IN

Superheater II Superheater III Superheater I

Figure 15: An example of superheater block arrangements.

Superhe ated Steam OUT Reheated Steam OUT Feedwater IN Reheater IN

Saturated Steam IN

Reheater I Superheater I Reheater II Superheater III Superheater II

Figure 16: An example of superheater and reheater block arrangements. The number of tubes in the superheater is calculated according to the average flow velocity and volume flow. In the convection superheater the width of the superheater is the same as the width of the furnace. When the number of tubes is known, all tubes are preliminarily placed next to each other in the flue gas channel. If the cross-sectional area of the flue gas pass between two tubes (dimension ‘a’ in Figure 17) becomes too small, the tubes have to be placed in two or more rows. 148

STEAM BOILER TECHNOLOGY – Thermal Design of Heat Exchangers

Inline

Clear lane

b

a

Direction of gas flow

b

Staggered

Figure 17: Inline and staggered tube arrangement.

Economizer design An economizer consists of an arrangement of tubes through which the feed water is passed immediately before entering the boiler. The combustion gases leaving the boiler convection surfaces pass over these tubes. As the entering feed water has a lower temperature than that of the boiler steam, the heat transfer is more effective at this point than in the convection surfaces of the boiler. This fact has prompted the present trend in boiler design to increase the economizer surface and proportionally decrease the evaporator heating surface. Economizers can be made of cast iron or steel tube. Finned tubes are used, unless the flue gases origins from fuels with high ash content.

Design method The following variables will be chosen •

Inside and outside tube diameters di and do, from which we can calculate the wall thickness: d − di (25) δ = o 2 • Distance of tubes in direction of flow and in side direction: s1 and s2 (named ‘a’ and ‘b’ in Figure 17) • The size of flue gas channel: b1 and b2

The number of tubes in one row (counter-flow) can then be calculated as: M =

b2 s2

(26)

The cross-sectional area of the flue gas channel can then be calculated from equation 27. Afg = b1b2 – Mdob1

(27)

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STEAM BOILER TECHNOLOGY – Thermal Design of Heat Exchangers

Holes of flow-through area combined circle are: U = (M+1)*(2* b1-2*(s1- do))

(28)

The hydraulic diameter can then be calculated as: dh =

4 ⋅ A fg

(29)

U

Then s1/do, s2/do, C and m can be read from charts. [5] The average flue gas temperature of the economizer is: Tf =

T fg sup + T fgeco

(30)

2

The outside convection heat transfer coefficient is calculated from the following equation (turbulent gas flow): Nu =

α oc d h = C ⋅ Re m ⋅ Pr 0,31 λ fg

-> α oc =

λ fg dh

⋅ C ⋅ Re m ⋅ Pr 0,31

(31)

where λfg is the thermal conductivity of the flue gas, Pr is Prandtl number, of flue gas, αo the outside convectional heat transfer coefficient and Re Reynolds number, which can be calculated as: Re =

d h ⋅ w fg

(32)

ν

where wfg is the flue gas velocity in the flue gas channel, dh the hydraulic diameter of the channel (Equation 30) and ν the cinematic viscosity of flue gas. The needed tube surface area in the economizer can then be calculated as:

A=

G k

(33)

where G is the conductance (kW/K) and k the heat transfer coefficient, which can be calculated according to equation 35: d 1 1 δ = o + + k d iα i α o ⎛ δ ⎜⎜1 − ⎝ do

⎞ ⎟⎟ ⋅ λ ⎠

+ mdirt

(34)

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STEAM BOILER TECHNOLOGY – Thermal Design of Heat Exchangers

where di and do are the inside and the outside tube diameter [m] respectively, αi and αo the inside and outside heat transfer coefficient respectively, δ the tube wall thickness, λ the thermal conductivity and mdirt the heat transfer resistance of a tube with a dirt layer on its surface. The outside heat transfer coefficient is the sum of the outside radiative and convective heat transfer coefficients: αo = αoc + αrad

(35)

The surface area of one tube is: At = π* do*b1

(36)

The number of tube rows in depth direction is: N=

A At ⋅ M

(37)

And the depth of the economizer is: he = N* s1

(38)

Air preheater design Recuperative air preheater design is similar to other convective heat transfer surfaces. The tubes of air preheaters are larger than the tubes of superheaters and economizers: the diameter is about 50-80 mm. Wall thickness is sized according to the strength of the construction, because the pressure difference between air and flue gases is small. The flue gas velocity in the air preheater is 10-14 m/s in the tubular heat exchanger type, 9-13 m/s in the plate heat exchanger type, 10-11 m/s in a finned tube heat exchanger, and 13-15 m/s if both sides of the heat exchanger are finned. In a vertical tube heat exchanger flue gas flows inside tubes and number of tubes can be chosen according to the flue gas velocity and volume flow. By choosing suitable tube divisions, dimensions of horizontal cross section of heat exchanger can be calculated. Air is flowing horizontally outside tubes. By choosing air velocity height of heat exchanger can be calculated. According thermal sizing length of heat exchanger can be found. In horizontal tube heat exchanger air flows inside tubes and number of tubes can be chosen according to the air velocity and volume flow. Regenerative air preheaters are usually made of enamel coated ceramic elements. This is popular, because ceramics are non-combustibles and have a low low-temperature corrosion rate. Another option is metallic dimple elements. Metallic elements have higher efficiency, require lower height and have lower pressure drop. Problems are a possible high corrosion rate of metallic elements.

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STEAM BOILER TECHNOLOGY – Thermal Design of Heat Exchangers

References 1. VDI Wärmeatlas. 2. Pictures and schematics supplied by Foster Wheeler. http://www.fwc.com/ 3. Picture supplied by Härnösand Energi&Miljö Ab, Fortum. http://www.fortum.com 4. Photograph by Rintala T., Fortum. http://www.fortum.com 5. Alvarez H. Energiteknik del 1 and Energiteknik del 2. Studentlitteratur, Lund. 1990. p. 368 6. M. Huhtinen, A. Kettunen, P. Nurminen, H. Pakkanen, Höyrykattilatekniikka, Oy Edita Ab, Helsinki 1994, ISBN 951-37-1327-X 7. Opetusmoniste kevät 2000: Ene-47.110 Yleinen energiatekniikka, erä 1, HUT 8. Opetusmoniste kevät 2000: Ene-47.124 Höyrykattilatekniikka, erä 1, HUT 9. Opetusmoniste kevät 2000: Ene-47.124 Höyrykattilatekniikka, erä 2, HUT 10. V. Meuronen, 4115 Höyrykattiloiden suunnittelu, Opetusmoniste 1999, LTKK, ISBN 951764-382-9 11. Combustion Engineering. Combustion: Fossil power systems. 3rd ed. Windsor. 1981. 12. Vakkilainen E. Lecture slides and material on steam boiler technology. 2001.

152

Circulating Fluidized Bed Boilers Dianjun Zhang, Sebastian Teir

STEAM BOILER TECHNOLOGY – Circulating Fluidized Bed Boilers

Table of contents Table of contents..............................................................................................................................154 Introduction to fluidized bed boilers................................................................................................155 Fluidized bed principles ...............................................................................................................155 Basic principles of CFB boilers ...................................................................................................157 Characteristics of CFB systems ...................................................................................................159 The advantages of CFB boilers....................................................................................................160 Combustion in CFB boilers .............................................................................................................161 Fuel flexibility..............................................................................................................................162 Combustion zones in a CFB boiler ..............................................................................................162 Heat transfer in a CFB boiler ...........................................................................................................163 Bed to wall heat transfer ..............................................................................................................163 Bubbling bed to external heat surfaces ........................................................................................164 Heat transfer and part-load operation...........................................................................................164 Load control in CFB boiler ..........................................................................................................165 Emissions .........................................................................................................................................165 SO2 Emissions..............................................................................................................................165 NOx - emissions...........................................................................................................................166 Particulate matter (PM) emission.................................................................................................168 Carbon monoxide and hydrocarbons ...........................................................................................168 References........................................................................................................................................169

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STEAM BOILER TECHNOLOGY – Circulating Fluidized Bed Boilers

Introduction to fluidized bed boilers In order to control emission levels from coal combustion, advanced combustion technologies and pollutant capture technologies are utilized. Pulverized coal (PC) combustion with flue gas cleaning using a desulfurization plant, including bag-house filters for desulfurization and electrostatic precipitators for fly ash, is the commonly used technology. But one of the shortages of PC boilers is that high combustion temperature in the furnace causes high NOx formation. During the recent decades, fluidized bed combustion (FBC) has been developed and put into use rapidly due to its good features, such as SO2 removal during combustion, low NOx emissions and multi-fuel flexibility. The fluidization process was invented by Fritz Winkler in 1921. The process used for coal burning was developed and promoted by Douglas Elliott in 1960s. After that Lurgi of Germany and Alhlström Group in Finland developed FBC further. Foster Wheeler, Babcock & Wilcox, and Lurgi are currently the largest FBC boilers manufacturers. Fluidized bed boilers can be categorized into three main types, bubbling fluidized bed (BFB), atmospheric circulating fluidized bed (ACFB, commonly referred to as CFB), and pressurized circulating fluidized bed (PCFB). This chapter will focus on CFB boilers.

Fluidized bed principles Fluidization is a phenomenon where fine solids are transformed into a fluid-like state through contact with a fluid, either gas or liquid. Under the fluidized condition, gravitational forces on granular, solid particles are offset by the fluid drag on them. Thus, the particles remain in a semisuspended condition and take on many of the physical characteristics of a fluid. As the gas velocity increases through a bed of particles many changes occur in the gas/solid contact mode. At low velocities the gas is essentially flowing through a fixed bed of particles, while at high velocities the solids are entirely entrained in the gas stream. When comparing various combustion technologies, stoker-fired boilers operate with a fixed bed, while pulverized boilers operate with solids completely entrained. The furnace of a CFB boiler operates in a regime somewhere between these two extremes. The principle of fluid bed systems can also be explained by examining the relationship between differential gas pressure across a bed of particles and the superficial gas velocity through that bed (Figure 1). For a fixed bed, the log of differential pressure is proportional to the log of gas velocity and represents the frictional pressure drop of the gas through the bed. As the gas velocity increases beyond the minimum fluidization velocity, the bed begins to expand and the particles become fluidized. A distinct bed level is visible in the fluid bed. As the gas flow rate through the fixed bed increases, the pressure drop continues to rise until the superficial gas velocity reaches the critical minimum fluidization velocity, Umf.. At that point the gravitational forces are overcome by the buoyant drag forces on the particles and they become suspended (i.e., fluidized). The minimum fluidizing velocity depends on many factors including particle diameter, gas and particle density, particle shape, gas viscosity, and bed void fraction. The following formula calculates the minimum fluidizing velocity:

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STEAM BOILER TECHNOLOGY – Circulating Fluidized Bed Boilers

U mf =

µs d p ρs

⎡ ⎤ d p ρs (ρ g − ρ s )g − 33.7 ⎥ ⎢ 33.7 2 + 0.0408 2 µg ⎢⎣ ⎥⎦

(1)

where µg dp ρg ρp g

is dynamic viscosity is particle diameter is gas density is particle density is acceleration of gravity

FIXED BED

BUBBLING

MIN FLUID VELOCITY

TURBULENT

ENTRAINMENT VELOCITY

CIRCULATING

PARTICLE MASS FLOW

∆p (LOG)

VELOCITY (LOG)

Figure 1: Regimes of fluidized bed systems. [1] At velocities above Umf, the pressure drop through the bed remains constant and equals the weight of solids per unit area as the drag forces on the particles barely overcome gravitational forces. The following equation shows the pressure drop:

∆p = ( ρ g − ρ s )(1 − ε ) gH

(2)

where ρg ρp ε g

is gas density is particle density is ratio of empty volume in bed is acceleration of gravity.

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STEAM BOILER TECHNOLOGY – Circulating Fluidized Bed Boilers

As gas velocity is further increased above the minimum fluidization velocity, the differential pressure remains almost constant until the bed material begins to elutriate at the entrainment velocity of the so-called bubbling bed. The degree of turbulent mixing of the solids continues to increase between the minimum fluidization and the entrainment velocity. Beyond the entrainment velocity or the terminal velocity, the particles are carried out of the vessel and an inventory of particles can only be maintained by collecting and recirculating the entrained particles back to the vessel or by adding additional solid particles. The entrainment velocity marks the transition from a bubbling bed to a circulating bed. Beyond this velocity, the differential pressure becomes a function of velocity and solid recirculation rate. The terminal velocity for a fluidized bed can be calculated as Ut =

4 d p (ρ p − ρ g ) g ρ g Cd 3

(3)

where Cd is the drag coefficient. In the context of its use in power generation, the circulating fluidized bed may be defined as a high velocity gas-solid suspension where particles are elutriated by the fluidizing gas. The particles are recovered and returned to the base of the furnace at a rate high enough to cause a degree of solid refluxing that will insure a uniform temperature level in the furnace. The CFB mode of fluidization is characterized by a high slip velocity between the gas and solids and by intensive solids mixing. High slip velocity between the gas and solids, encourages high mass transfer rates, that enhance the rates of the oxidation (combustion) and desulfurization reactions, critical to the application of CFBs to power generation. The intensive mixing of solids insures adequate mixing of fuel and combustion products with combustion air and flue gas emissions reduction reagents. [1]

Basic principles of CFB boilers A Circulating Fluidized Bed (CFB) operates under a special fluid dynamic condition, in which the fine solids particles are transported and mixed through the furnace at a gas velocity exceeding the average terminal velocity of the particles. The major fraction of solids leaving the furnace is captured by a solids separator and recirculated back to the lower part of the furnace. The high recycle rate intensifies solids mixing and evens out combustion temperatures in the furnace. Figure 2 and Figure 3 shows a schematic diagram of a CFB boiler. The boiler can be divided into two sections. The first section consists of the furnace, solid separator, recycle device, and possible external heat exchanger surfaces. The second section of the boiler is called back-pass where the heat of the high temperature flue gas is absorbed by the reheater, superheater, economizer, and airpreheater, which are installed one after one in downstream order. Coal and limestone (sorbent for SO2 capture) is injected from the lower part of the furnace into the sand bed. The injected coal and limestone is fluidized by primary air (less than stoichiometrical amount) entering the furnace through an air distributor or grate in the furnace floor. Coal is heated by hot segregated particles in the bed above its ignition temperature so that it can be burnt. The sulfur in the coal reacts with limestone, thus lowering the possibility of SO2 formation and

157

STEAM BOILER TECHNOLOGY – Circulating Fluidized Bed Boilers

emissions from the furnace. Secondary air is injected at some height above the grate to complete the combustion. Bed solids are well mixed throughout the height of the furnace to ensure the uniform bed temperature in the range of 800-900°C. Some particles segregate and return to the bed before leaving the furnace, while some particles are captured in a gas-solid separator (e.g. cyclone) and are recycled back to the furnace (Figure 4). The separator is designed for a very high solid collection efficiency with nearly 100% efficiency for particles greater than 60 microns in diameter. Furnace and cyclone

Backpass

Cyclone Superheaters and reheaters

Furnace

Economizer

Secondary air supply Air preheater Recycling of solids

Primary air supply

INTREX Superheater

MÄLARENERGI AB VÄSTERÅS, SWEDEN

Figure 2: Shematics of a CFB boiler (157 MWth, 55.5/48 kg/s, 170/37 bar, 540/540 °C). [1] Finer dust that escapes from the separator is collected by bag-house filters or electrostatic precipitators (Figure 3), which are installed downstream after the boiler. The collected solids are returned to the combustion chamber via the loop seal, which provides a pressure seal between the positive pressure in the lower furnace and the negative draft in the solids separator. This prevents the furnace flue gas from short circuiting up the separator dipleg and collapsing the separator collection efficiency. The recirculation system has no moving parts and its operation has proven to be simple and reliable. By injecting small amounts of high pressure fluidizing air into the loop seal, the solids movement back to the lower furnace is maintained. Typically gravity / mechanical feeding of fuel directly into the combustor have proven satisfactory for meeting the desired level of efficient mixing.

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STEAM BOILER TECHNOLOGY – Circulating Fluidized Bed Boilers

Superheaters and reheaters

Furnace (water walls)

External heat transfer surfaces

Economizer

Air preheater

Electrostatic precipitator

Figure 3: CFB boiler in Rovaniemi, Finland (95.8 MWth, 38 kg/s, 115 bar, 535°C). [1]

Characteristics of CFB systems CFB systems operate in a fluid dynamic region between that of a Bubbling Fluidized Bed (BFB) and a transport reactor (pulverized combustion). This fluidization regime is characterized by high turbulence, solid mixing and the absence of a defined bed level. Instead of a well defined solids bed depth, the solids are distributed throughout the furnace with a steadily decreasing density from the bottom to the top of the furnace. CFB is characterized by: • • • •

High fluidizing velocity of 4.0-6.0 m/s. Dense bed region in lower furnace without a distinct bed level Water-cooled membrane walls (evaporator). Optional in-furnace heat transfer surfaces located above the dense lower bed

Figure 4: Cutaway of a CFB furnace and cyclone. [3]

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STEAM BOILER TECHNOLOGY – Circulating Fluidized Bed Boilers

• •

Solids separator to separate entrained particles from the flue gas stream and recycle them to the lower furnace. Aerated sealing device, loop seal, which permits return of collected solids back to the furnace

The advantages of CFB boilers Compared with PC boilers, CFB boilers have a number of unique features that make them more attractive in energy production. Table 1 compares different types of boilers with CFB boilers. Extensive fuel flexibility: In the furnace bed, fuel particles constitute less than 1-3% by weight of all bed solids. The rest are non-combustibles, such as sorbents, flue ash and sand. This feature makes CFB boilers flexible enough to use a wide range of fuels, coal (with ash content up to 40-60%), peat, bark, wood waste, and straw. High combustion efficiency: Normally the combustion efficiency of CFB boilers is 97.5-99.5%. The good result is due to the following factors: • • • •

Good gas-solid mixing. High burning rate. Long combustion zone (40m). The majority of unburned fuel particles are recycled back to the furnace and combusted.

Efficient sulfur removal: The long combustion zone in the furnace gives a long reaction time for the sorbents to react with SO2. The average residence time of gas in the combustion zone is 3-4 seconds. The furnace temperature of CFB boilers is also ideal for the capture of sulfur (850°C optimal). SO2 reacts with CaO in calcined sorbents and forms calcium sulfate. SO2 removal during combustion is much cheaper and simpler than flue gas desulfurization. Table 1: Comparison of boiler types.

1)

Items Height of bed of fuel burning zone (m) Superficial velocity m/s Excess air % Grate heat release rate MW/m2 Coal size mm Turndown ratio Combustion efficiency % NOx emission ppm SO2 capture in furnace %

Stoker boilers 0,2

BFB boilers 1-2

CFB boilers 15-40

PC boilers 27-45

1,2 20-30 0,5-1,5 32-6 4:1 85-90 400-600 None

1,5-2,5 20-25 0,5-1,5 6-0 3:1 90-96 300-400 80-90

4-8 10-20 3-5 6-0 3-4:1 95-99 50-200 80-90

4-6 15-30 4-6