413 topic iv-3 (fossil fuels and boiler efficiency)

8/13/2019 413 Topic IV-3 (Fossil Fuels and Boiler Efficiency)

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ISAT 413 - Module IV:

Combustion and Power Generation

Topic 3: Fossil Fuels and Boiler Efficiency

Fossil Fuels

Fluid-Moving SystemsCombustion Methods and Systems

Steam Generators

Boiler Types and Classifications

Primary Boiler Heat-Transfer Surfaces

Secondary Boiler Heat-Transfer Surfaces

Boiler Ratings and Performance


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Fossil Fuels

The three general classes of fossil fuels are coal, oil,

and natural gas.

Hydrocarbon Chemistry

There are three major groups of hydrocarboncompounds the aliphatic hydrocarbons, the alicyclic

hydrocarbons, and the aromatic hydrocarbons.

The aliphatic or chain hydrocarbons are further

divided into three subgroups the alkane, the alkene,and the alkyne hydrocarbons.


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The alkanehydrocarbons, also called paraff in series,

are the saturated group of chain hydrocarbons. The

general chemical formula for this group is CnH2n+2. suchas Methane (CH4), Ethane (C2H6), Propane (C3H8), Butane

(C4H10), Pentane (C5H12), Hexane (C6H14), Heptane

(C7H16), Octane (C8H18), Nonane (C9H20), Decane (C10H22),

etc. As the number of atoms in the alkane molecules

increase, the hydrogen fraction decreases and thehydrocarbons become less volatile. Figure below shows

the chemical structure of n-Octane.

HC

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

H


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The alkenehydrocarbons, also called olef in series,

have one double bond between two of the carbon atoms

in the chain. The general formula for this group is CnH2n,

and some of the typical compounds are ethylene (C2H4),propylene (C3H6) (left), butene (C4H8), pentene (C5H10),

and hexene (C6H12).

HC

H

H

C

H

H

C

H

The alkynehydrocarbons, also called acetylene series,

have one triple bond in the hydrocarbon chain. Thegeneral formula for this group is CnH2(n-1), and some of

the typical compounds are acetylene (C2H2), and

ethylacetylene (C4H6) (r ight).

HC

H

H

CH C C

H

H


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The alicyclichydrocarbons are composed of saturated

carbon-atom rings and have a general formula that is

identical to that of the alkene subgroup of aliphatic

hydrocarbons, i.e., CnH2n, some of the typicalcompounds are cyclopropane (C3H6), cyclobutane (C4H8),

(top), and cyclopentane (C5H10).

The aromatichydrocarbons are

composed of the basic benzenering or rings. The ring is a six-atom

carbon ring with double bonds

between every other carbon atom.

The general formula for this groupis CnH2n-6, some of the typical

compounds are benzene (C6H6)

(bot tom), toluene (C7H8), xylene

(C8H10), and naphthalene (C10H8).

HC

H

H

C

H

H

HC C

H

H

HC

H

H

C

HC C

H

H

C

C


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Standard Fuels

The 100-octane fuel standard for in ternal-combust ion-

engineis 2,2,4-trimethylpentane, C8H18(isooctane),while 0-octane fuel standard is n-heptane, C7H16. The

unknown fuel is burned in the engine and the

compression ratio is slowly increased until a certain

knock or detonation reading is obtained from a

vibration detector. The octane ratings of most regulargasolines range from 85 to 95.

The 100-cetane fuel standard for compression- ign i t ion

ordiesel fuelsis n-hexadecane (C16H34), while 0-cetane

fuel standard is alpha-methylnaphthalene (C11H10). Thecetane ratings of most diesel fuels range between 30

and 60.


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Coal

American Society for Testing Materials (ASTM) has

developed a method that ranks coal into four

classifications:

Class I coals: Anthracitic coals, the oldest.

Class II coals: Bituminous coals.

Class III coals: Subbituminous coals.Class IV coals: Lignitic coals.

Coal Analyses

The two common coal analyses are the proximateanalysis and the ul t imate analysis.


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Proximate Analysis

The prox imate analysisis the simplest coal analysis

and gives the mass fractions of f ixed carbon(FC),

vo lat ile matter(VM), ash(A ), and moisture (M) in the

coal.

This analysis can be determined by simply weighing,

heating, and burning a small sample of powdered coal.

The coal sample is carefully weighed and then heatedto 110oC for 20 min. The sample is then weighed again

and the mass loss is divided by the original mass to

obtain the moistu re fract ion.

The remaining sample is heated to 954oC in a closedcontainer for 7 min. The sample is then reweighed and

the resulting mass loss in this heating process is

divided by the original mass to obtain the fraction of

thevo lat ile matter

in the sample.


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The sample is then heated to 732oC in an open crucible

until it is completely burned. The residue is then

weighed and the final weight is divided by the original

weight to obtain the ash fract ion.

The mass fraction of f ixed carbonis obtained by

subtracting the moisture, volatile matter, and ash

fractions from unity.

In addition to the FC, VM, M, and A, most proximateanalyses list separately the su l fur mass fract ion(S)

and the higher heat ing value(HHV) of the coal.


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Ultimate Analysis

The ult im ate coal analys isis a laboratory analysis that

lists the mass fractions of carbon, C, hydrogen (H2

),

oxygen (O2), nitrogen (N2), and sulfur (S) in the coal

along with the higher heating value.

Most ultimate analyses include the moisture and ash

separately, but some analyses include the moisture as

part of the hydrogen and oxygen mass fractions.

The ultimate analysis is required to determine the

combustion-air requirements for a given combustion

system and this, in turn, is used to size the draft

system for the furnace.These calculations should be based on the as-burned,

ultimate coal analysis, if possible.


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Coal Properties

There are a number of properties that should be

considered when selecting a coal for a given application.

Among these are its sulfur content, its burning

characteristics, its weatherability, its ash-softening

temperature, its grindability index, and its energy

content.

It is desirable to use a coal with a low sul fur con tent.

If the coal i s bu rnedin a stationary bed with little

agitation, the coal should be a free-burning coal, not a

caking coal; caking coals must be mechanically agitated

when they are burned to break up the fused-coal masses.The weatherabil i ty of a coal is a measure of its ability

to withstand exposure to atmospheric conditions without

excessive crumbling.


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The gr indabi l i ty index is another important property

that should be considered when selecting a coal. This is

particularly true for the common pulverized-coal power

system where the coal is ground up finer than facepowder.

The ash-so ftening temperatureis an important

consideration in the choice of coals for a particular

power plant. The ash-softening temperature is thetemperature where the ash becomes very plastic,

somewhat below the melting point of the ash. Slagging

occurs as ash deposits build up on the heat-transfer

surfaces.The energy content or heating valueof a coal is a very

important property. The heating value represents the

amount of chemical energy in a given mass or volume of

fuel. HHV = LHV + hfg,fuel

.


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Petroleum

Although crud e oi lis a composition of many organic

compounds, the ultimate analyses of all crude oils are

fairly constant. The carbon mass fraction ranges from 84to 87%, the hydrogen mass fraction ranges from 11 to

16%, the sum of oxygen and nitrogen mass fractions

range from 0 to 7%, and the sulfur mass fraction ranges

from 0 to 4%.There are six grades of commercial fuel oil. No. 1 is the

lightest, least viscous, for vaporizing burners. No. 2 is a

distillate oil and is the general-purpose domestic heating

oil. No. 3 is no longer available. No. 4 is a relatively lightheating oil. No. 5 is a heavy, viscous, commercial-grade

heating oil, and No. 6, or bunker-C oil, is the heaviest

and most viscous of the residual fuel oils. Both Nos. 5

and 6 oils require heating before they can be pumped.


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Petroleum Properties

The important properties of petroleum and petroleum

products are the heating value, the specific gravity, the

flash point, and the pour point.

The speci f ic gravi ty,s,of any liquid is the density of

that liquid divided by the density of water at 15.6oC.

The f lash pointof a liquid fuel is the minimum fluid

temperature at which the vapors coming from a free

surface of the liquid will just ignite, producing a flash.

The pour po in tof a liquid fuel is the lowest fluid

temperature at which an oil or oil product will flow under

standard conditions.

The combustion of crude-oil products has some ash,

sulfur, and vanadium oxidizes (V2O5) problems. They are

expensive to remove.


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Gaseous Fuels

Almost all gaseous fuelsare either fossil fuels or

byproducts of fossil fuels. These fuels can be divided

into three general groups including natural gases,manu factured fuel gases, and byp rodu ct fuel gases.

The composition of a fuel gasis commonly expressed in

terms of the moleor vo lumefractions of the chemical

compounds found in it.The heating value of any fuel gas is commonly expressed

in units of energy per uni t vo lume(kJ/m3) but this value

is directly proportional to the gas density, which in turn is

directly proportional to the absolute pressure andinversely proportional to the absolute temperature.


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Gaseous Fuels Heating Values

T

T

P

P r

rrT,rPvT,Pv

HHVHHV

If the volumetric heating values of the gas components at

some reference pressure Pr

and reference temperature Tr

are known, the volumetric heating vale of the gas

mixture, HHVvis obtained from the following equation:

Where (HHVv)iand Viare the volumetric high heating

value and the volumetric fraction of thei thgaseous

component, respectively. The following equation can be

used to convert the volumetric higher heating value atthe reference pressure and temperature to some other

pressure and temperature:

irT,rP,i

ni

ivrT,rPv

VHHV

1

mixtureofHHV


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P

T

MW

R

P

RT

m

V

v u

A volumetric heating value HHVvat some temperature T

and pressure Pcan be converted into a gravimetric

heating value HHVmby multiplying the volumetric value

by the specific volume vof the gas at the same pressureand temperature:

The specific volume of a gas mixture can be determinedfrom the molecular weight (MW) of the gas and the ideal-

gas equation of state, as follows:

T,PT,Pvm vHHVHHV

where Ruis the universal gas constant.


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Example IV-3.1

Calculate the higher heating value, in kJ/m3and kJ/kg,

at 10oC and 3 atm for gas mixture with the following

composition: 94.3% CH4, 4.2% C2H6, and 1.5% CO2.

3

3

3

3

6453700150910640420030379430

0521701440150071300420043169430

0

91064

03037

m

kJ,.,.,.

C

kmol/kg.......

m/kJ

m/kJ,

m/kJ,

:Solution

mixturev

v

v

v

HHV

:atm1and20At

MW

COforHHV

HCforHHV

CHforHHV

:atm1andC20At

o

2

62

4

o


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kg

kJ,

kg

m.kg/m,vHHVHHV

kg

m

.kPa.

K.

kmol

kg.

K.kmol

m.kPa.

P

T

MW

R

v

m

kJ,

.

.,

T

T

P

P

kg

kJ,

kg

m.kg/m,vHHVHHV

kg

m.

kPa.

K.

kmol

kg.

K.kmol

m.kPa.

P

T

MW

Rv

vm

u

r

rrT,rPvmixturev

vm

u

1205345430920116

454303251013

15283

0517

3148

92011615283

1293

1

364537

12053411164537

4111325101

15293

0517

3148

33

3

3

3

33

3

3

HHVHHV

:atm3andC10At o


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Typical Fuel Gases

There are two types of natural gas that produced fromthe decay of organic matter and that which has been

trapped deep in the earths crust since the earth was

formed.

Natural gas has the highest gravimetric heating value ofall fossil fuels, about 55,000 kJ/kg, or 37,000 kJ/m3at 1 atm

and 20oC.

Natural gas is commonly sold in units of therms( 1 therm

= 100,000 Btu)

Natural gas can be converted to liquified natural gas

(LNG) at -127oC. Some companies use large underground

cavities, including domed, sealed aquifers to store LNG.

Natural Gas


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Liquified petroleum gas (LPG), sometimes called refinery

gas, is composed of the light distillates of petroleum,primarily propane and butane.

Water gas is a manufactured fuel gas that is produced by

alternately passing steam and air through a bed of

incandescent coke.

There are many proposed processes for producing

high-Btu and medium-Btu fuel gases from coal. The

high-Btu gas is commonly called synthetic natural gas or

simply SNG.

There are several fuel gases are called producer gas,which are produced normally by burning low-grade coal

seams in the ground (in situ) with insufficient air for

complete combustion.

Manufactured Fuel Gases


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Coke-oven gas is an excellent fuel gas with a high

heating value. The gas is essentially composed of thevolatile matter of a caking coal. The gas is a byproduct of

the industry that supplies coke to the steel industry.

Blast-furnace gas was a low-quality fuel gas resulting

from the steel industry. It was produced by burning natural

gas or other fuel with insufficient air.

Sewage gas has been used as a heating fuel in several

cities in the eastern U.S. since colonial times. Most of the

interest in sewage gas involves the utilization of animal

and vegetable wastes (biomass), particularly the wastefrom large cattle feed lots, to generate the gas. Sewage

gas is almost pure methane, which is produced in the

decay process.

Byproduct Fuel Gases


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Fluid-Moving Systems

Two basic fluid moving systems are employed in almost

all steam-generator systems. These are the pumps

needed to supply the working fluid to the steam

generator and the air compressors or fans needed to

supply combustion air to the furnace. An important

parameter for these systems is the mechanical eff ic ienc y

mech, which is a measure of the machines ability totransmit mechanical work to the fluid flowing through the

device. The mechanical efficiency for fluid-moving

systems is given by:

inputworkactual

inputworkidealmechh

For a primer mover, such as a turbine, the mechanical

efficiency is:

outputworkideal

outputworkactualmechh


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The speci f ic speed of a given pump is defined as theangu lar veloc i ty, in r/min , of a geometr ically sim i lar

pump, reduced in s ize, wh ich w i l l prod uce a volumetr ic

f low rate of 1 gal/m in against a total pressu re rise of 1

lb/in

2

. The specific speed of a given pump can bedetermined from a known volumetric flow rate of Q

gal/min over a pressure rise of Plb/in2at an angular

velocity of Nr/min:

413

21

/

/

s

P

NQN

D

The boiler feed pump supplies high-pressure liquid

water to the boiler and commonly operates over a wide

range of pressures. The centrifugal pump is commonlyused for this purpose and the performance of these

systems is usually expressed in terms of the speci f ic

speed Nsof the pump.

Boiler Feed Pumps


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A condition that should be avoided during the operation of

any liquid pump. This condition is called cavitat ion.

Cavitation occurs when the liquid pressure on the surface ofthe impeller falls below the vapor pressure of the liquid. This

causes vapor bubbles to form on the surface of the impeller

and these bubbles collapse as they move into a region of

higher pressure. The sudden collapse of these bubbles

causes severe impact loads on the impeller and this actioncan cause severe erosion of the impeller surface. Not only

can cavitation physically damage the pump but it also

drastically lowers the mechanical efficiency of the pump and

makes it noisy.

Cavitation can be alleviated by increasing the fluid pressure

at the pump inlet. This pressure, minus the vapor pressure of

the liquid, is called the net positive suction head or NPSH,

which is commonly specified by the pump manufacturer.

Cavitation


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There are two general types of air compressors

posi t ive displacementair compressors and dynamicair

compressors.

In the positive-displacement compressor, the impeller

or piston forcibly displaces the air volume to compress

it. Common positive-displacement air compressors are

the reciprocating and rotary compressors.In the dynamic air compressor, the high-velocity

impeller transfers momentum from the impeller to the

air. The two categories of dynamic air compressors are

the axial-flow (gas turbine) and centrifugal (fossil-fuel)compressors.

Combustion-air fans (centrifugal) usually have very

high flow rates but total pressure rises of less than 15

to 20 kPa (2 to 3 psia).

Combustion Air Systems


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Since the pressure across any fan is relatively small,

the air flow through the fan can be assumed to be

incompressible. The so-called fan or pump lawsapply,

that is, for geometrically similar centrifugal machines,operating at the same efficiencies, the pressure rise P

across the device, the volumetric flow rate Qthrough

the device, and the input power requirements Pare

related by the following equations:

Where ris the fluid density, Nis the angular velocity,and Dis the diameter of the impeller.

5343

222

31 DNkPQkPDNkPNDkQ rDrD ;;


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There are two basic types of mechanical-draft systems,

the forced-draft and the induced-draft systems.

In the induced-draft (i-d) system, the fan draws

combustion products from the combustion chamber

and discharge them into the stack.

In the forced-draft (f-d) system, the fan pumps only

combustion air into the furnace.

For the f-d fan, we should consider both the air and

the water vapor separately. The volumetric flow rate for

f-d fan can be calculated as:

Volumetric Flow Rate of Forced-Draft Systems

016189728

1

..P

TR

F

AQ

Q

u

D.G.AFD

FD

ratefuel

fand-fforrateflowVolumetric


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Example IV-3.2

A 600-MWepower plant burns Lafayette County ,

Missouri, coal with average moisture and ash fractions

of 14 and 11%, respectively. This plant operates with a

heat rate of 8863 Btu/kWh. An analysis of the refuse pit

gives a higher heating value of 2605 kJ/kg. An orsat

analysis of the flue gas gives 13.78% CO2, 4.9% O2, and

0.75%CO. Find (a) The thermal efficiency of the powerplant. (b) The coal rate. C) The capacity of the f-d fan, in

kg/min and ft3/min, if atmospheric conditions are 50oC,

0.93 atm, and a relative humidity of 50%.

38500

8863

34123412.

a

:Solution

th

rateheat

plantofefficiencythermalThe

Btu/kW3412Btu/kWrateheatThe

th

h

h

h


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ton/h248.7tonne/h225.6kg/h225,600

kW600,000

fuelofHHVburned-as

powerthermalrateCoal

kJ/kg.2605refuseofHHV:analysisRefusekJ/kg.33,160HHV

S,5.2%,N1.3%,O9.3%,H5.6%C,78.6%:analysisultimateCoal

e

222

11014011603338510

36001

5800095011014017860

0095011011950

1195092050

110

92050079500101

0795077832

2605

..kg/kJ,kJ/kJ.

h/ss.kW/kJ

coalkg/burnedCkg.....CCC

coalkg/Ckg...ARC

coalkg/Rkg..

.

R/A

AR

Rkg/Akg...R

C.

R

A

Rkg/Ckg.,HHV

HHV

R

C

b

ththe

ee

rul tb

rr

ul t

r

rC

Rr


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32

min

kg,..

,

F

Am

.

...../...

.

NCO%CO%/CN%.

F

A

%....N%

...P

P.

..PP

c

D.G.Adf

ul t,b

A.G.D

airdry

v

x,Co@satv

460388039043550160

600225

1

7680

11014010130781375058098803322

7680

3322

98807504947813100

89460930714

6220

7891150

222

2

050

ratecoalfand-fforrateflowmassGas

coalair/kgkg9.803

orsatfrom

airdryO/kgHkg0.043550.89440.622

lb/in0.8946

0.5;humidityrelative:aircmbustiontheinMoisture

2

2


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min

ft,,

.

.

.

ft

inatm.

atm

in/lbf.

RR.lbmol

lbf.ft

coallbm

airlbm.

h

min

ton

lbm

h

ton.

..P

TR

F

AV

V

in

u

D.G.A

df

df

3

2

22

0003711

0218

043550

7928

1

144930714

4601221545

8039

60

20007248

02187928

1

ratecoal

fand-fforrateflowvolumetricGas


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Combustion Methods and Systems

Gaseous fuels, including natural gas, are the easiestfossil fuels to burn. The fuel gas needs little or no

preparation before combustion. It must be simply

proportioned, mixed with air, and ignited. This can be

accomplished in the following ways:The atmospher icgas burner: The momentum of the

incoming gas is used to draw the primary air into the burner in

a process called aspiration.

The refractorygas burner: Commonly used in steam

generators. The combustion air is drawn in around the burner,

which has multiple gas jets that produce good mixing.

The fan-mixburner: The fuel gas is introduced from nozzles

mounted at the angle in a rotating spider burner.

Gas-Fired Systems


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Oil is somewhat more difficult to burn than natural gas

because the burner must prepare the fuel for combustion

as well as proportion it, mix it with air, and burn it. There

are several ways to prepare the fuel oil for combustion:

Oil-Fired Systems

Vaporization or gasification:

The vaporization technique is

particularly well suited for thelight fuel oils.

Atomization of the oil droplets

can be accomplished with the

use of high-pressure air orsteam, or the liquid oil film can

be torn apart by centrifugal force.

Figure at right shows a common

rotary-cup (mechanical

atomization) burner.


36/60


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The pu lver ized-coal furn aceburns finely powdered coal

and air in a gaseous torch. This combustion system can

produce much higher capacities than the stokerfurnaces, it gives fast response since there is little

unburned fuel in the combustion chamber, it reduces the

amount of excess air required for combustion and this

reduces the NOxemissions, it can burn all ranks of coalfrom anthracitic to lignitic, and it permits combination

firing (refers to the capacity of burning coal, oil, or

natural gas in the same burner). Normally, only one type

of fuel is burned at a time although two different fuels

can be simultaneously burned for short periods of time.

The pulverized-coal furnace finds widespread application

in coal-fired power plants.

Pulverized-Coal Furnace


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The f lu idized-bed furnaceis a radically new type of

combustion system that has been under development

and testing during the last 30 years. In this unit, crushedcoal and either crushed dolomite or limestone are mixed

in a bed that is then levitated by the combustion air

entering the bottom of the furnace. The boiler evaporator

tubes are immersed directly in the fluidized bed and thedirect contact between the burning coal particles and the

water tubes produces very high heat-transfer rates,

reducing the size of the unit. This arrangement (see Culp

text Figure 4.18 on your course pack) also produces very

low combustion temperatures, and traps the sulfur in the

furnace, thereby permitting the utilization of high-sulfur

coal.

Fluidized-Bed Combustion System


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Steam Generators

The steam generatoror boi leris a combination of

systems and equipment for the purpose of converting

chemical energy from fossil fuels into thermal energy

and transferring the resulting thermal energy to a

working fluid, usually water, for use in high-temperature

processes or for partial conversion to mechanical energy

in a turbine.In most modern large power plants, one boiler is used

to supply steam to one steam-turbine generator unit. The

bo i ler complexincludes the ductwork and air-handling

equipment, the fuel-handling and processing equipment,the furnace, the water supply and treatment system, the

steam drums and piping, the exhaust gas system, and

the pollution control systems including scrubber and

electrostatic precipitator or baghouse filter.


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The heat transfer sections of a large boiler include the

prim ary heat trans fer su rfaces(the evaporator,

superheater, and the reheater) and thesecondary heat

transfer surfaces(the air preheater and the economizer).An energy flow diagram for a typical large steam

generator is shown in the figure below.


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Steam boilers can be classified many ways but there are

actually two basic types of steam generators, depending

on the orientation of the water-steam and hot-gas flowpaths. These two general classifications are the f i re-tube

boilers and the water-tubeboilers.

The common f ire-tubeboiler is essentially composed of a

water-filled pressure vessel containing a number of tubeswhich are the passage-ways for the hot exhaust gas and

through which heat is transferred from the hot gas to the

water in the vessel. This system is the simplest and probably

the least expensive of all the steam generators.

In the f ire-tubesystem, the high-pressure water is placed onthe external surface of the tubes. Since most pressure-vessel

codes will limit the external pressure on a tube to half that for

internal pressure, the fire-tube systems are limited to relatively

low steam pressures.

Boiler Types and Classifications


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The f i re-tube steam generatoris commonly employed in

small industrial plants, and these systems can be

purchased in the form of complete operation package.

Figure below shows a typical two-pass, packaged, fire-tube steam generator.


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The water-tube boilers are best suited for high-

pressure, high-capacity steam generators. The high-

pressure water and the steam flows from tube headers or

drums through tubes in the furnace walls or in the tubebundles mounted in the exhaust gas duct.

The water-tube steam generators may be classified as either

natural-circulation systems or forced-circulation boilers.

In a natural-circulation boiler the saturated water flows fromthe steam drum high in the boiler, through the downcomer

tubes to the bottom or mud drum.

In a forced-circulation boiler, the fluid is pumped through the

evaporator section of the boiler.

The most widely used forced-circulation boiler system in theU.S. is the universal-pressure or Benson boi ler.


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Benson Boiler

In the Benson boiler, the

water is pumped to about

35 MPa (5000 psia) in the

main feed pump. The

compressed water is then

piped to the economizer

section, through heevaporator tubes, through a

transition section, and

finally through a convection

superheater, where it isexhausted to the turbine at

a pressure around 24 Mpa

(3500 psia).

CS convection superheater

E economizerFP feed pump

O steam to service

T tube evaporating sections

TS transition section


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45

The primary heat-transfer surfaces in the boiler include the

evaporator section, the superheater section, and the reheat

section if the power cycle employs reheat.The evaporat ivesurface is usually located in the hottest part of

the boiler near the combustion zone because the boiling water in

the tubes protects the tube material from excessive

temperatures.Superheatersections are the heat-transfer surfaces in which

heat is transferred to the saturated steam to increase its

temperature and available energy. Superheaters are particularly

important in the production of turbine steam to reduce the

moisture content of the steam as it passes through the turbine.The reheat section of a large boiler is that portion of the boiler

in which all of the steam exhausting from the high-pressure

turbine is returned for additional superheat before it is sent to

the intermediate-pressure turbine or turbine section.

Primary Boiler Heat-Transfer Surfaces


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46

The secondary heating surfaces recover heat from the

flue gas after it has passed over the primary heat-transfer

surfaces. In order to achieve a high boiler efficiency, it isdesirable to lower the temperature of the exhaust gas as

much as possible. There are two kinds of secondary

heat-transfer surfaces, the economizer and the air

preheater.The economizer(normally a cross-flow heat exchanger)

transfers heat from the flue gas to the incoming boiler

water. It has been estimated that an increase of 6 to 7oC

in the temperature of the feedwater produced from the

heat recovery in the economizer will increase the boiler

efficiency about 1%.

The air p reheatertransfers thermal energy from the

exhaust gas to the cold combustion air.

Secondary Boiler Heat-Transfer Surfaces


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47

There are twobroad classes of air p reheater, the

regenerative heaters and the recuperative heaters.

The recuperat ive heater is a plate-type or tubular heat

exchanger operating as either a counteflow or crossflow unit.A shot-cleaning system, rather than a soot-blower system, is

commonly used to clean the flue-gas side of these heat

exchangers.

The regenerativeair preheater, or Ljungstrum heater,

employs a large rotor assembly with approximately half of the

element mounted in the exhaust gas duct and the other half in

the supply air duct. The rotating element, which usually turns

2 to 4 r/min, contains many corrugated laminas that are

alternately heated by the flue gas and cooled by thecombustion air.

The air preheaters are useful in other ways than just

improving the overall efficiency of the unit, it reduces the time

required for fuel ignition, thereby improving fuel combustion.


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48

One problem associated with any coal-fired boiler

system, particularly a pulverized-coal system, is the ash

content of the flue gas and the resulting buildup of ash or

slag deposits on the heat-transfer surfaces of the boiler,both the primary and the secondary surfaces. It is

common practice in coal-fired boilers to incorporate

devices, called soot blowers, to remove the ash deposits

from the tubes (as shown in the figure below).


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49

Most of the modern steam generators are rated in terms

of steam capacity (usually lbm/h) along with the steam

outlet pressure and temperature.The figure of merit for operation of a boiler is the boiler

or steam-generator efficiency sg. This quantity is defined

as the fraction the input chemical energy that is

transferred to the working fluid. The boiler efficiencycommonly ranges from 70 to 90%.

There are two ways to calculate the boiler efficiency, the

direct method and the indirect method.

Boiler Rating and Performance


50/60


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51

It is assumed that the total fuel-input energy is either

transferred to the working fluid or is lost in a number of

ways. There are a total s ixboiler heat losses and all ofthem are calculated in terms of energy lost per unit mass

of fuel (kJ/kg). Using this system, the steam-generator

efficiency becomes:

Indirect Method to Calculate Boiler Efficiency

%HHV

HHV

%

fuel

fuel100

100

lossestotal

fuelofvalueheatinghigher

lossestotal-fuelvalueofheatinghigherEfficiencyBoiler sgh

1 The Dry Gas Loss (DGL)


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52

The dry-gas loss (DGL) is that portion of the boiler losses

that can be attributed to the combustion air supplied to

the steam generator.

1. The Dry-Gas Loss (DGL)

analyses.ultimateburned-asandrefusethefromdetermined

asfractionmasshydrogenandmoisture,refuse,theareand,,and

Ce,temperaturgas-flueoutlet

Ce,temperaturairinlet

air)ofasthesamebeto(assumedgasflueofheatspecific

fuel/kggasfluedryofkg,

where

DGL

o

o

2

2

2

00351

901

901

HMR

T

T

CkJ/kg..c

HMR.F

Aw

TTcHMR.F

A

TTcw

out,g

in,g

op

D.G.A

g

in,gout,gpD.G.A

in,gout,gpg

2 The Moisture Loss (ML)


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53

The moisture loss (ML) includes the loss due to

vaporizing the moisture in the fuel and the loss due to the

latent heat of the moisture produced from the combustionof the hydrogenin the fuel:

2. The Moisture Loss (ML)

in,gout,gws

oout,g

in,gout,gws

oout,g

in,gw

out,gs

ws

T.T..hh

T

T.T.hh

T

kg/kJTh

kg/kJ

Th

hhHM

1874926162492

300

187409322442

300

9 2

C,toequalorthanlessisIf

C,exceedsIf

,e,temperaturgasinlettheatwaterofenthalpyspecific

gas),fluetheinvaporwatertheofpressurepartialeapproximat(the

kPa7ofpressureaandatsteamdsuperheateofenthalpyspecific

where

ML

3 The Moisture in Combustion air Loss (MCAL)


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54

Another but much smaller moisture loss is the moisture-

in-combustion-air loss (MCAL), it is at least an order of

magnitude lower than the moisture and dry-gas losses formost fuels.

3. The Moisture-in-Combustion-air Loss (MCAL)

in,gsat

satatm

sat

w,p

in,gout,gw.pD.G.A

TP

PP

P.

c

TTcF

A

atvaporwatertheofpressuresaturationtheis

humidityrelativetheis

and

CkJ/kg.1.926orvaor,waterofheatspecifictheis

airdry/kgOHkginair,enteringtheofratiohumiditythewhere

MCAL

o

2

6220

4 The Unburned Carbon Loss (UCL)


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55

The unburned-carbon loss (UCL) is the boiler loss

associated with the appearance of carbon in the refuse.

This loss is equal to the product of the mass of unburnedcarbon per unit mass of fuel in the refuse (Cr) and the

higher heating value of the carbon (HHV)carbon:

4. The Unburned-Carbon Loss (UCL)

carbonofvalueheatinghighertheis

refusetheinfuelofmassunitpercarbonunburnedofmasstheis

where

UCL

carbon

r

carbonr

HHV

C

HHVC

5 The Incomplete Combustion Loss (ICL)


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56

The incomplete-combustion loss (ICL) is the energy lost

as the result of the formation of carbon monoxide instead

of carbon dioxide in the combustion process. The ICL canbe determined from the following equation:

5. The Incomplete-Combustion Loss (ICL)

analysisorsatthefromdirectlyvaluetheis

analysisorsatthefromdirectlyvaluetheis

fuelofmassperburnedcarbonofmasstheiswhere

12.01ICL

2

22

236300128

CO%

CO%

C

kg/kJCO%CO%

CO%C

CO%CO%

HHVCCO%.

b

bCOb

6 The Radiation Loss (RL)


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57

The radiation and unaccounted loss (RL) cannot be

explicitly calculated, but is estimated from the data

presented in the Figu re 4.31 below. The data from thisgraph give the radiation loss as a function of the actual

steam output and the maximum design output, in MBtu/h,

as well as the number of cooled walls in the furnace.

fuelHHV

RL

4.31Figurefromfactor

6. The Radiation Loss (RL)

E l IV 3 3


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58

Example IV-3.3

Using the data from Example IV-3.2 perform an energy

balance for the system and calculate the boiler

efficiency. Assume that the boiler has three sides thatare water-cooled and the system is operating at 10% of

full power during the boiler test.

fuelkgkJ

TTcHMRF

ADGL

CTcoalkgkgHkgkJHHV

coalkgkgCai rdrykgOHkgcoalkgkgC

M WPRMCT

Solution

i nou tp

DGA

oou tfuel

br

eoi n

/.

......

.

.,/./,

,/.,/.,/.

,,.,.,

:

..

92427

50288003510420914011950018039

901

:(DGL)lossgas-Dry

2880420and;87024

58004355000950

60011950140503.2,-IVExampleFrom

2

2

2

:(ML)lossMoisture


59/60

59

h

MBtu

kWh

Btu

fuelkg/kJ...

..,C,ICL

fuelkg/kJ..,C,UCL

fuelkg/kJ....

TTcF

AMCAL

fuelkg/kJ.

.....

T.T..HM

hhHMML

b

r

inoutw,pD.G.A

inout

ws

5318

47077813750

7505806302363023

4311009507783277832

71955028892610435508039

11470

50187428892616249204209140

18749261624929

9

2

2

8863kW600,000powerinputboiler(max.)Design

:(RL)lossRadiation

%CO%CO%CO

:(ICL)losscombustion-Incomplete

:(UCL)losscarbon-Unburned

:(MCAL)lossair-combustion-in-Moisture

:(ML)lossMoisture

2


60/60

60

%.%,

.,

fuelkg/kJ.,.,

H H V

fuelkg/kJ.......

fuelkg/kJ.,.H H V.

...

h/MBtu..

h/MBtu.

sg

fuel

fuel

67710078024

922519efficiencygeneratorSteam

9225191555478024

lossestotalsteamthefer toheat transUseful

1555464414707431171951147092427

RLICLUCLMCALMLDGLlossesTotal

:balanceEnergy

6441780240178001780RL

017808100220

wallscooled-water3forfactorcorrection

wallscooled0forfactor4.31FigurefromFactor

4425425410poweroutputboilerActual

4254532080poweroutputboiler(max.)Design

Then80%.efficiencyboilerthat theAssume

h

413 topic iv-3 (fossil fuels and boiler efficiency)

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