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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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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.
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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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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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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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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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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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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
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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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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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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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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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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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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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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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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
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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
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%.%,
.,
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
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