1

ISAT 413 - Module IV:Combustion and Power Generation

Topic 3: Fossil Fuels and Boiler Efficiency

Fossil Fuels

Fluid-Moving Systems

Combustion Methods and Systems

Steam Generators

• Boiler Types and Classifications

• Primary Boiler Heat-Transfer Surfaces

• Secondary Boiler Heat-Transfer Surfaces

• Boiler Ratings and Performance

2

Fossil Fuels

The three general classes of fossil fuels are coal, oil, and natural gas.

Hydrocarbon Chemistry

• There are three major groups of hydrocarbon compounds ─ 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.

3

• The alkane hydrocarbons, also called paraffin series, are the saturated group of chain hydrocarbons. The general chemical formula for this group is CnH2n+2. such as 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 the hydrocarbons become less volatile. Figure below shows the chemical structure of n-Octane.

• The alkane hydrocarbons, also called paraffin series, are the saturated group of chain hydrocarbons. The general chemical formula for this group is CnH2n+2. such as 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 the hydrocarbons 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

HH

4

• The alkene hydrocarbons, also called olefin 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 alkyne hydrocarbons, also called acetylene series, have one triple bond in the hydrocarbon chain. The general formula for this group is CnH2(n-1), and some of the typical compounds are acetylene (C2H2), and ethylacetylene (C4H6) (right).

HC

H

H

CH C C

H

H

5

• The alicyclic hydrocarbons 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 typical compounds are cyclopropane (C3H6), cyclobutane (C4H8), (top), and cyclopentane (C5H10). • The aromatic hydrocarbons are composed of the basic benzene ring or rings. The ring is a six-atom carbon ring with double bonds between every other carbon atom. The general formula for this group is CnH2n-6, some of the typical compounds are benzene (C6H6) (bottom), 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

6

Standard FuelsStandard Fuels

• The 100-octane fuel standard for internal-combustion-engine is 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 “regular” gasolines range from 85 to 95.

• The 100-cetane fuel standard for compression-ignition or diesel fuels is n-hexadecane (C16H34), while 0-cetane fuel standard is alpha-methylnaphthalene (C11H10). The cetane ratings of most diesel fuels range between 30 and 60.

7

CoalCoal

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 AnalysesCoal Analyses

The two common coal analyses are the “proximate” analysis and the “ultimate” analysis.

8

Proximate AnalysisProximate AnalysisThe proximate analysis is the simplest coal analysis and gives the mass fractions of fixed carbon (FC), volatile 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 heated to 110oC for 20 min. The sample is then weighed again and the mass loss is divided by the original mass to obtain the moisture fraction.

The remaining sample is heated to 954oC in a closed container 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 the volatile matter in the sample.

9

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 fraction.

The mass fraction of fixed carbon is obtained by subtracting the moisture, volatile matter, and ash fractions from unity.

In addition to the FC, VM, M, and A, most proximate analyses list separately the sulfur mass fraction (S) and the higher heating value (HHV) of the coal.

10

Ultimate AnalysisUltimate Analysis

The ultimate coal analysis is 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.

11

Coal PropertiesCoal 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 sulfur content.

• If the coal is burned in 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 “weatherability” of a coal is a measure of its ability to withstand exposure to atmospheric conditions without excessive crumbling.

12

• The “grindability 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 face powder.

• The ash-softening temperature is an important consideration in the choice of coals for a particular power plant. The ash-softening temperature is the temperature 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 value of 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.

13

PetroleumPetroleumAlthough crude oil is a composition of many organic compounds, the ultimate analyses of all crude oils are fairly constant. The carbon mass fraction ranges from 84 to 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 light heating 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.

14

Petroleum PropertiesPetroleum Properties

The important properties of petroleum and petroleum products are the heating value, the specific gravity, the flash point, and the pour point.

• The specific gravity, s, of any liquid is the density of that liquid divided by the density of water at 15.6oC.

• The flash point of 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 point of 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.

15

Gaseous FuelsGaseous FuelsAlmost all gaseous fuels are either fossil fuels or byproducts of fossil fuels. These fuels can be divided into three general groups including natural gases, manufactured fuel gases, and byproduct fuel gases.

The composition of a fuel gas is commonly expressed in terms of the mole or volume fractions of the chemical compounds found in it.

The heating value of any fuel gas is commonly expressed in units of energy per unit volume (kJ/m3) but this value is directly proportional to the gas density, which in turn is directly proportional to the absolute pressure and inversely proportional to the absolute temperature.

16

Gaseous Fuels Heating ValuesGaseous 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, HHVv is obtained from the following equation:

Where (HHVv)i and Vi are the volumetric high heating value and the volumetric fraction of the i th gaseous component, respectively. The following equation can be used to convert the volumetric higher heating value at the reference pressure and temperature to some other pressure and temperature:

i

rT,rP,i

ni

ivrT,rPv VHHV

1

mixture of HHV

17

P

T

MW

R

P

RT

m

Vv u

A volumetric heating value HHVv at some temperature T and pressure P can be converted into a gravimetric heating value HHVm by multiplying the volumetric value by the specific volume v of the gas at the same pressure and temperature:

The specific volume of a gas mixture can be determined from the molecular weight (MW) of the gas and the ideal-gas equation of state, as follows:

T,PT,Pvm vHHVHHV

where Ru is the universal gas constant.

18

Example IV-3.1Example IV-3.1

Calculate the higher heating value, in kJ/m3 and 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

:atm 1 and 20 At

MW

CO for HHV

HC for HHV

CH for HHV

:atm 1 and C20 At

o

2

62

4

o

19

kg

kJ,

kg

m.kg/m,vHHVHHV

kg

m.

kPa.

K.

kmol

kg.

K.kmol

m.kPa.

P

T

MW

Rv

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

:atm 3 and C10 At o

20

Typical Fuel GasesTypical Fuel Gases

• There are two types of natural gas ─ that produced from the decay of organic matter and that which has been trapped deep in the earth’s crust since the earth was formed.

• Natural gas has the highest gravimetric heating value of all fossil fuels, about 55,000 kJ/kg, or 37,000 kJ/m3 at 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 GasNatural Gas

21

• 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 GasesManufactured Fuel Gases

22

• Coke-oven gas is an excellent fuel gas with a high heating value. The gas is essentially composed of the volatile 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 waste from large cattle feed lots, to generate the gas. Sewage gas is almost pure methane, which is produced in the decay process.

Byproduct Fuel GasesByproduct Fuel Gases

23

Fluid-Moving Systems Fluid-Moving SystemsTwo 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 efficiency mech, which is a measure of the machine’s ability to transmit mechanical work to the fluid flowing through the device. The mechanical efficiency for fluid-moving systems is given by:

input workactual

input workidealmech

For a primer mover, such as a turbine, the mechanical efficiency is:

output workideal

output workactualmech

24

The specific speed of a given pump is defined as the angular velocity, in r/min, of a geometrically similar pump, reduced in size, which will produce a volumetric flow rate of 1 gal/min against a total pressure rise of 1 lb/in2. The specific speed of a given pump can be determined from a known volumetric flow rate of Q gal/min over a pressure rise of P lb/in2 at an angular velocity of N r/min:

413

21

/

/

sP

NQN

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 commonly used for this purpose and the performance of these systems is usually expressed in terms of the specific speed Ns of the pump.

Boiler Feed PumpsBoiler Feed Pumps

25

• A condition that should be avoided during the operation of any liquid pump. This condition is called cavitation. Cavitation occurs when the liquid pressure on the surface of the 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 action can 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.

CavitationCavitation

26

There are two general types of air compressors — positive displacement air compressors and dynamic air 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 SystemsCombustion Air Systems

27

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 laws apply, that is, for geometrically similar centrifugal machines, operating at the same efficiencies, the pressure rise P across the device, the volumetric flow rate Q through the device, and the input power requirements P are related by the following equations:

Where is the fluid density, N is the angular velocity, and D is the diameter of the impeller.

5343

222

31 DNkPQkPDNkPNDkQ ; ;

28

The air required for combustion can be supplied by a natural-draft system, by a mechanical-draft system, or as is usually the case, by a combination of these two systems.

• In the natural-draft system, the air flow is produced by a driving force which is established by the difference in the specific weights of the stack exhaust gas and the atmospheric air. The driving force is equal to the product of the specific-weight difference and the effective height of the stack or chimney.

• Increasing the stack height not only increases the gas-flow rate but it improves the dispersion of exhaust products. Stack heights from 700 to 1200 ft (210 to 365 m) are fairly common.

Draft SystemsDraft Systems

29

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 SystemsVolumetric Flow Rate of Forced-Draft Systems

016189728

1

..P

TR

F

AQ

Q

u

D.G.AFD

FD

rate fuel

fan d-f for rateflow Volumetric

30

Example IV-3.2Example IV-3.2

A 600-MWe power 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 power plant. (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%.

385008863

34123412.

a

:Solution

th

rate heat plant of efficiency thermal The

Btu/kW 3412Btu/kW rate heat The

th

h

31

ton/h 248.7tonne/h 225.6kg/h225,600

kW600,000

fuel ofHHV burned-as

power thermalrate Coal

kJ/kg. 2605 refuse ofHHV :analysis Refuse kJ/kg. 33,160 HHV

S, 5.2% ,N 1.3% ,O 9.3% ,H 5.6% C, 78.6% :analysis ultimate Coal

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

rultb

rr

ult

r

rC

Rr

32

min

kg,..

,

F

Am

.

...../...

.

NCO%CO%/CN%.

F

A

%....N%

...P

P.

..PP

c

D.G.Adf

ult,b

A.G.D

airdry

v

x,Co@satv

460388039043550160

600225

1

7680

11014010130781375058098803322

7680

3322

98807504947813100

89460930714

6220

7891150

222

2

050

rate coalfan d-f for rateflow mass Gas

coal air/kg kg9.803

orsat from

airdry O/kgHkg0.043550.89440.622

lb/in0.8946

0.5;humidity relative :air cmbustion the in Moisture

2

2

33

min

ft,,

.

.

.

ft

inatm.

atm

in/lbf.

RR.lbmol

lbf.ft

coallbm

airlbm.

h

minton

lbm

h

ton.

..P

TR

F

AV

V

in

u

D.G.Adf

df

3

2

22

0003711

0218

043550

7928

1

144930714

4601221545

803960

20007248

02187928

1

rate coal

fan d-f for rateflow volumetric Gas

34

Combustion Methods and Systems Combustion Methods and Systems

Gaseous fuels, including natural gas, are the easiest fossil 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 atmospheric gas burner: The momentum of the incoming gas is used to draw the primary air into the burner in a process called aspiration.

• The refractory gas 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-mix burner: The fuel gas is introduced from nozzles mounted at the angle in a rotating “spider” burner.

Gas-Fired SystemsGas-Fired Systems

35

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 SystemsOil-Fired Systems

• Vaporization or gasification: The vaporization technique is particularly well suited for the light fuel oils.

• Atomization of the oil droplets can be accomplished with the use of high-pressure air or steam, or the liquid oil film can be torn apart by centrifugal force. Figure at right shows a common rotary-cup (mechanical atomization) burner.

36

The stoker furnace is one of the oldest type of coal furnaces and is still in use, today. Its somewhat limited capacity does not lend this type of furnace to large power applications but it is widely used in industrial plants where relatively limited amount of process steam are desired.

Coal-Combustion SystemsCoal-Combustion Systems

Stoker FurnaceStoker Furnace

The cyclone furnace is a combination system that employs a number of independent combustion chambers (as many as 16 in a large power boiler) all of which feed hot exhaust gas into a large, water-cooled boiler.

Cyclone FurnaceCyclone Furnace

37

The pulverized-coal furnace burns finely powdered coal and air in a gaseous torch. This combustion system can produce much higher capacities than the stoker furnaces, 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 NOx emissions, it can burn all ranks of coal from 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 FurnacePulverized-Coal Furnace

38

The fluidized-bed furnace is a radically new type of combustion system that has been under development and testing during the last 30 years. In this unit, crushed coal 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 the direct 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 SystemFluidized-Bed Combustion System

39

Steam Generators Steam Generators• The steam generator or boiler is 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 boiler complex includes 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.

40

The heat transfer sections of a large boiler include the primary heat transfer surfaces (the evaporator, superheater, and the reheater) and the secondary 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.

41

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 flow paths. These two general classifications are the fire- tube boilers and the water-tube boilers.• The common fire-tube boiler is essentially composed of a water-filled pressure vessel containing a number of tubes which 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 fire-tube system, the high-pressure water is placed on the 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 ClassificationsBoiler Types and Classifications

42

The fire-tube steam generator is 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.

43

• 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 tube bundles 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 from the 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 the U.S. is the universal-pressure or Benson boiler.

44

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 he evaporator tubes, through a transition section, and finally through a convection superheater, where it is exhausted to the turbine at a pressure around 24 Mpa (3500 psia).

CS ─ convection superheater

E ─ economizer

FP ─ feed pump

O ─ steam to service

T ─ tube evaporating sections

TS ─ transition section

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 evaporative surface 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.

• Superheater sections 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 SurfacesPrimary Boiler Heat-Transfer Surfaces

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 is desirable 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 preheater transfers thermal energy from the exhaust gas to the cold combustion air.

Secondary Boiler Heat-Transfer SurfacesSecondary Boiler Heat-Transfer Surfaces

47

There are two broad classes of air preheater, the regenerative heaters and the recuperative heaters.• The recuperative 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 regenerative air 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 the combustion 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.

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).

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 efficiency commonly ranges from 70 to 90%.

• There are two ways to calculate the boiler efficiency, the direct method and the indirect method.

Boiler Rating and PerformanceBoiler Rating and Performance

50

The total heat added to the working fluid in the economizer, evaporator, superheater, and reheater sections of the boiler is evaluated and this quantity is divided by the total fuel-input energy:

Direct Method to Calculate Boiler EfficiencyDirect Method to Calculate Boiler Efficiency

reheater. the to in and out steam of enthalpiesspecific the are and

rsuperheate the to in waterand out steam of enthalpiesspecific the are and

generator steam the to feed boiler of rateflow mass the is

reheater the to steam of rateflow mass the is

rate,flow mass fuel the is

where

energy input fuel total

fluid workingthe to addedenergy totalEfficiency Boiler sg

34

12

3412 100

100

hh

hh

m

m

m

%HHVm

hhmhhm

%

s

r

f

fuelf

rs

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 six boiler heat losses and all of them 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 EfficiencyIndirect Method to Calculate Boiler Efficiency

%HHV

HHV

%

fuel

fuel 100

100

losses total

fuel of value heating higher

losses total-fuel valueof heating higherEfficiency Boiler sg

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)1. The Dry-Gas Loss (DGL)

analyses. ultimate burned-as and refuse the from determined

as fraction mass hydrogen and moisture, refuse, the are and , , and

C e,temperatur gas-flue outlet

C e,temperatur air inlet

air) of as thesame be to (assumed gas flue of heatspecific

fuel /kggas fluedry of kg ,

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.Ag

in,gout,gpD.G.A

in,gout,gpg

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 combustion of the hydrogen in the fuel:

2. The Moisture Loss (ML)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, to equal or than less is If

C, exceeds If

, e,temperatur gas inlet the at waterofenthalpy specific

gas), flue the in vapor waterthe of pressure partial eapproximat (the

kPa 7 of pressure a and at steam dsuperheate ofenthalpy specific

where

ML

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 for most fuels.

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

at vapor waterthe of pressure saturation the is

humidity relative the is

and

CkJ/kg. 1.926 or vaor, waterof heatspecific the is

airdry /kgOH kg in air, entering the of ratiohumidity the

where

MCAL

o

2

6220

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 unburned carbon 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)4. The Unburned-Carbon Loss (UCL)

carbon of value heating higher the is

refuse the in fuel of mass unit per carbon unburned of mass the is

where

UCL

carbon

r

carbonr

HHV

C

HHVC

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 can be determined from the following equation:

5. The Incomplete-Combustion Loss (ICL)5. The Incomplete-Combustion Loss (ICL)

analysis orsat the fromdirectly value the is

analysis orsat the fromdirectly value the is

fuel of mass per burned carbon of mass the is

where

12.01ICL

2

22

236300128

CO%

CO%

C

kg/kJCO%CO%

CO%C

CO%CO%

HHVCCO%.

b

bCOb

57

The radiation and unaccounted loss (RL) cannot be explicitly calculated, but is estimated from the data presented in the Figure 4.31 below. The data from this graph 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

4.31 Figure from factor

6. The Radiation Loss (RL)6. The Radiation Loss (RL)

58

Example IV-3.3Example IV-3.3Using 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 that are water-cooled and the system is operating at 10% of full power during the boiler test.

fuelkgkJ

TTcHMRF

ADGL

CTcoalkgkgHkgkJHHV

coalkgkgCairdrykgOHkgcoalkgkgC

MWPRMCT

Solution

inoutpDGA

ooutfuel

br

eo

in

/.

......

.

.,/./,

,/.,/.,/.

,,.,.,

:

..

92427

50288003510420914011950018039

901

:(DGL) loss gas-Dry

2880420 and ;87024

58004355000950

6001195014050 3.2,-IV Example From

2

2

2

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

8863kW 600,000power input boiler (max.) Design

:(RL) loss Radiation

%CO%CO

%CO

:(ICL) loss combustion-Incomplete

:(UCL) loss carbon-Unburned

:(MCAL) loss air-combustion-in-Moisture

:(ML) loss Moisture

2

60

%.%,

.,

fuelkg/kJ.,.,

HHV

fuelkg/kJ.......

fuelkg/kJ.,.HHV.

...

h/MBtu..

h/MBtu.

sg

fuel

fuel

67710078024

922519efficiencygenerator Steam

9225191555478024

losses totalsteam thefer toheat trans Useful

1555464414707431171951147092427

RLICLUCLMCALMLDGLlosses Total

:balanceEnergy

6441780240178001780RL

017808100220

wallscooled- water3for factor correction

wallscooled 0for factor 4.31 Figure fromFactor

4425425410poweroutput boiler Actual

4254532080power output boiler (max.)Design

Then 80%. efficiencyboiler that theAssume