Steam Boiler Technology (2003)

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The Basics of Steam Generation

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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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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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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

should be kept constant (in order to have Figure 4: Simplified boiler drawing.

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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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 be recycled by cooling it until it condensates into water and then return it as feedwater to the boiler. The condenser, where the steam is condensed, is a heat exchanger that typically uses water from a nearby sea or a river to cool the steam. In a typical power plant the pressure, at which the steam is produced, is high. But when the steam has been used to drive the turbine, the pressure has dropped drastically. A pump is therefore needed to get the pressure back up. Since the work needed to compress a fluid is about a hundred times less than the work

G

Figure 5: Rankine cycle

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

H C H C H T T T T T − = − = 1 max η (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

Hot reservoir Qh (temperature Th) Cold reservoir Qc (Temperature Tc) Wcycle = Qh - Qc

Figure 6: Carnot efficiency visualized .

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efficiency.

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

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.

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.

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-

Carnot efficiency 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 200 400 600 800 1000 Temperature [K]

Figure 7: Carnot efficiency graph example.

Hot reservoir Qh (temperature Th)

Cold reservoir Qc (Temperature Tc)

Wp Wt

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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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.

Evaporation

During the evaporation the enthalpy rises drastically. If water is evaporated at atmospheric pressure from saturated liquid to saturated vapour, the enthalpy rise needed is 2260 kJ/kg, from 430 kJ/kg (sat.

Evaporation of water 20 40 60 80 100 120 140 160 180 0 500 1000 1500 2000 2500 3000

Net enthalpy of water [kJ/kg water]

T emp erat u re [ C ] Phase change

Figure 9: Water evaporation plotted in a temperature-enthalpy graph.

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 22,12 MPa 0.01 0.1 1 10 100 1000 0 100 200 300 400 Tem perature [°C] P ress u re [ b ar ]

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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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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1. Turning chamber

2. Flue gas collection chamber 3. Open furnace 4. Flame tube 5. Burner seat 6. Manhole 7. Fire tubes 8. Water space 9. Steam space 10. Outlet and circulation 11. Flue gas out

12. Blow-out hatch 13. Main hatch 14. Cleaning hatch 15. Main steam outlet

16. Level control assembly 17. Feedwater inlet 18. Utility steam outlet 19. Safety valve assembly 20. Feet

21. Inslulation

Figure 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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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.

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 fuel-air mixture, and the steam.

Heat exchanger basics

The task of a heat exchanger is to transfer the heat from one flow of medium (fluid/gas stream)

air

flue gas

feed water

process steam

fuel

Figure 15: Furnace heat exchanger model.

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 parallel-flow 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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boiler), the lines points in the opposite direction (Figure 18). The length of the lines on the Q-axis shows the transferred heat rate and the T-axis the rise/drop in temperature that the heat transfer has caused.

Since the heat strays from a higher temperature to a lower (according to the second law of thermodynamics) the wanted heat transfer happens by itself if and only if the hot stream is always hotter than the cold stream. That 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 Q T1 T2 hot stream t2 t1 cold stream

Figure 17: T-Q diagram of a parallel-flow type heat exchanger. T Q T1 T2 t2 t1 deltaQ

Figure 18: T-Q diagram of a counter-flow type heat exchanger. T Q Tpinch T1 T2 external heating required t1 t1 external cooling required

Figure 19: Adjusted streams.

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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 Evaporator Superheater water saturated water saturated steam

Figure 20: Heat exchanger model of the HRSG.

T

Q

Eco

Eva

Sup

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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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 preheater

Air in

Air out

Figure 22: Furnace equipped boiler with air preheater.

T

Q

Eco

Eva Sup

Air

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

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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

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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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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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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

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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

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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]

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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].

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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].

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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 boiler-technology 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.

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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.

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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]

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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

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Modern Boiler Types and Applications

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STEAM BOILER TECHNOLOGY – Modern Boiler Types and Applications

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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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

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: • Removal of moisture - brown part

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

Air Fu_el Radia tion f rom w alls co_n ve cti_on R ad ia tio n a_n d

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.

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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 self-sustained 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. 80-90% 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 Air Preheater Pulverized Coal Air Air Overfire Air Main Combustion Zone Coal Reburn Burners Cyclone Burner Secondary Air Water Molten Slag Coal Primary Air Slag to Disposal Burnout Zone Reburn Zone Stack Electrostatic Precipitator

Dry Waste To Disposal

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

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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).

Economizer

Bottom Ash To Disposal

Electrostatic Precipitator Low-NOX Cell Burner System Windbox Stack Boiler Secondary Air Port Secondary Air Port Windbox Coal and Air Coal and Air Low-NOX Cell Burner System

Fly Ash To Disposal Ash

Figure 6: PCF Boiler schematics. [4]

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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.

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

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]

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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 TURBULENT CIRCULATING

PARTICLE MASS FLOW ENTRAINMENT VELOCITY MIN FLUID VELOCITY VELOCITY (LOG) ∆p (LOG)

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

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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.

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

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

SALA-HEBY ENERGI AB SWEDEN

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

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.

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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]

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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]

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STEAM BOILER TECHNOLOGY – Modern Boiler Types and Applications 1 Inlet Duct 2 Distribution grid 3 HP Superheater 1 4 Burner 5 Split Superheater 6 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

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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 furnace with grate 3 post combustion 4 boiler 8 wet scrubber 1 9 wet scrubber 2 10 SCR DENOx 11 dioxin removal 12 stack 5 electrostatic precipitator 6 economizer (not typically here) 7 draft fan

13 bottom ash conveyor

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.

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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]

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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]