b. fuels and combustion - lecture

7/29/2019 b. Fuels and Combustion - Lecture

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B. FUELS AND COMBUSTION - LECTURE

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1. DefinitionsFuel is composed of chemical elements which, in rapid chemical union with oxygen, produce combustion.

Combustion is that rapid chemical union with oxygen of an element whose exothermic heat of reaction is

sufficiently great and whose rate of reaction is sufficiently fast that useful quantities of heat are liberated at elevated

temperatures.

2. Classification of Fuels2.1Solid including coal, coke, peat, briquettes, wood, charcoal, and waste products2.2Liquid including petroleum and its derivatives, synthetic liquid fuels manufactured from natural gas and coal

shale oil, coal by-products (including tars and light oil), and alcohols.

2.3Gaseous including natural gas, manufactured and industrial by-product gases, and the propane and butane orliquefied petroleum (LP) gases that are stored and delivered as liquids under pressure but used in gaseous form.

3. Coal Classification3.1Classification by rank degree of metamorphism, or progressive alteration, in the natural series from lignite to

anthracite (lignite, subbituminous, semibituminous, bituminous, semianthracite, anthracite, superanthracite)

Probably the most universally applicable method of classification in which coals are arranged according to fixedcarbon content and calorific value, in Btu, calculated on the mineral-matter-free basis.

3.2Classification by grade quality determined by size designation, calorific value, ash, ash-softening temperatureand sulfur. The size designation is given first in accordance with the standard screen analysis method followed

by calorific value, and symbols representing ash, ash-softening temperature, and sulfur.

3.3Classification by type or variety determined by nature of the original plant material and subsequent thereof.4. Burners for Pulverized Coal

4.1Vertical firing with all the secondary air admitted around the burner nozzle so that it mixes quickly with coaprimary air mixture from the burner nozzle.

4.2 Impact firing a form of vertical firing, consists of burners located in an arch low in the furnace or in the sidewalls and directed toward the furnace door, with high velocities of both primary and secondary air. This type offiring is used exclusively in wet-bottom or slagging type.

4.3Horizontal firing employs a turbulent burner, which consists of a circular nozzle within a housing provided withadjustable valves, the unit being located in the front or rear wall.

4.4Corner or tangential firing is characterized by burners located in each corner of the furnace and directedtangent to a horizontal, imaginary circle in the middle of the furnace, thereby making the furnace the burner in

effect, since turbulence and intensive mixing occur where the streams met.

5. CokeCoke is the solid, infusible, cellular residue left after fusible bituminous coals are heated, in the absence of air

above temperatures at which active thermal decomposition of the coal occurs.

Pitch coke or petroleum coke are obtained by similar heating of coal-tar pitch and petroleum residues.

High temperature coke is made from coal at temperature ranging from 815 C to 1093 C.

Low temperature coke is formed at temperatures below 704 C. The residue, if made from a non-cooking coal, i

known as char.


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6. CharcoalCharcoal is produced by partial combustion of wood at about 400 C and with limited air.

7. Liquid FuelsFuel Oil is defined as any liquid or liquefiable petroleum products burned for the generation of heat in a furnace o

firebox, of the generation of power in an engine, exclusive of oils with a flash point below 37.7 C.

Four Classes of Fuel Oils in common uses

a. Residual oils which are topped crude petroleums or viscous residuum obtained in refinery operations.b. Distillate fuel oils which are distillates derived directly or indirectly from crude petroleum.c. Crude petroleums and weathered crude petroleums of relatively low commercial value.d. Blended fuels which are mixture of two or more of the preceding classes.Commercial Fuel Oil Specifications

a. Grade no. 1 a distillate oil intended for vaporizing pot-type burners and other burners requiring this grade ofuel.

b. Grade no. 2 a distillate oil for general purpose domestic heating in burners not requiring no. 1 fuel oil.c. Grade no. 4 an oil for burner installation not equipped with pre-heating facilities.d. Grade no. 5 a residual type oil for burner installation equipped with pre-heating facilities.e. Grade no. 6 an oil for burners equipped with pre-heaters permitting a high-viscosity fuel.

8. GasolineGasoline is defined as a refined petroleum naphtha which by its composition is suitable for use as a carburetant in

internal combustion engines.

Motor Gasoline is a mixture of hydrocarbons distilling in the range of 37.7 C to 204.4 C by the standard method of

test.

9. KeroseneKerosene is defined as a petroleum distillate having a flash point not below 22.8 C as determined by the Abetester and suitable as an illuminant when burned in a wick lamp.

10.Coal TarCoal Tar is a product of the destructive distillation of bituminous coal carried out at high temperature.

11.Liquefied Petroleum Gases (LPG)Liquefied Petroleum Gases (LPG) are mixtures of hydrocarbons liquefied under pressure for efficien

transportation, storage, and use. They are generally composed of ethylene, propane, propylene, butane, isobutene

and butylenes. Commercially, they are classed as propane, propane-butane mixtures, and butane. They are odorless

colorless, and non-toxic.

12.Diesel Fuel OilsRefiners grade fuels classified according to methods of production.

a. Distillate fuels are produced by distillation of crudes.b. Residual fuels are those left after the distillation process.c. Blended fuels are mixtures of straight distillate fuels with cracked fuel stocks.


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Cracked stocks are residual of fuels which have been treated thermally or catalytically to obtain yields of lighter

grade fuels or gasoline.

Lightest grade distillates classed as kerosene or No. 1 fuel oil, may have an initial boiling point of 176.6 C and end

point of 260 C.

Heaviest grades of distillates classed as No. 3 or 4 fuel oil, may have an initial boiling point of 232 C to 260 C and

end point of 343 C to 371 C.

Residual fuels, No. 4 or No. 5 are suitable only for the slower-speed diesel.

13.Gaseous FuelsGaseous fuels are commonly used in industry, whether distributed by public utilities or produced in isolated plants

are composed of one or more simple gases in varying proportions.

14.Diesel Lubricating OilsCrude oils are frequently described as paraffinic, napththenic, or mixed based according to the physica

characteristics of the crude.

Two broad types of oil

a. Straight oils are produced entirely from the crudes chosen through elimination of undesired constituents bysuitable refining processes.

b. Additive oils are produced by adding to straight mineral oils certain oil-soluble compounds that enhance thelubricating oil properties for use in a diesel engine.

Additives are used principally to inhibit or slow down oxidation, to increase film strength, to keep solids in finely

divided state and to hold them in suspension, to improve the viscosity index, to lower the pour point, to decrease

friction and wear under extreme pressure conditions, to reduce foaming, and as rust or corrosion inhibitors.

SAE Three Types of Lubricating Oils

a. Regular type suitable for moderate operating conditions.b. Premium type having oxidation stability and bearing corrosion preventive properties making it generally

suitable for more severe service than regular duty type.

c. Heavy duty type has oxidation stability, being corrosion-preventive properties, and detergent-dispersancharacteristics for use under heavy-duty service conditions.

SAE Numbers are a means of coordinating and standardizing the products of oil companies and the

recommendations by the oil companies. The system of SAE motor classification is a system based entirely on

viscosity and is totally unrelated the other qualities of a lubricating oil.

15.Specific GravitySpecific Gravity a dimensionless parameter, it is the ratio of the mass of a unit volume of fuel to the mass of the

same volume of a standard substance at a specified temperature.

waterofdensity

fuelliquidofdensitySG =

airofdensity

fuelgaseousofdensitySG =


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In reporting SG data the 15.6 C or 60 F standard is common, that is, the oil is at 15.6 C or 60 F and is referred to the

density of water taken at 15.6 C or 60 F. Specific gravity at other temperature with correction factor,

( )[ ]615000701615

...

= tSGSGCt o

in SI units

( )[ ]6000040160

= tSGSGFt

.o

in English units

American Petroleum Institute Gravity Unit, oAPI

- Is the accepted standard by the petroleum and oil industry, it was drawn up to correct vales measured byincorrectly calibrated hydrometers.

5131615

5141.

.

.=

CatSGAPI

o

o

Baume Gravity Unit,oBaume or

oBe

- Another standard commonly associated with brine.130

615

140=

CatSGBaume

o

o

.

16.ViscosityViscosity is measure of resistance to flow.

Absolute Viscosity is defined as that unit force required to move one layer of a fluid at unit relative velocity to

another layer of the fluid which is at unit distance from the first.

Kinematic Viscosity is defined as the ratio of absolute viscosity divided by density.

Units of viscosity:

Absolute viscosity,

1 reyn = 1 kb-sec / in2

1 poise 1 dyne-se/cm2 = 0.1 Pa-sec

Kinematic Viscosity,

1 stoke = 1 cm2/sec = 0.0001 m2/sec

Centipoises and centistokes are more commonly used.

Saybolt viscosimeter measures the time required for a given quantity of oil at standard temperature to flow

through a specified tube.

SSU (Saybolt Second Universal) is obtained by timing the interval required for 60 cc of oil to flow through tube or

pass through a standard orifice.

For 30 to 45 SSU at 37.8 C, Centistokes = 0.308(SSU 26)

Or scentistokeSSU

SSU180

22.0 =

SSF (Saybolt Second Furol) unit used for very viscous liquids using a relatively large orifice.

62 SSF = 600 SSU


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17.Other PropertiesFlash point is the temperature at which oil gives off vapor that burns temporarily when ignited.

Flash point is the temperature to which oil must be heated to give off sufficient vapor to form an inflammable

mixture with air.

Flash point is the temperature at which ignition of the fuel vapors rising above the heated oil will occur when

exposed to an open flame.

Fire point is the temperature at which oil gives off vapor that burns continuously when ignited.

Pour point is the temperature at which oil will no longer pour freely or the temperature at which oil will solidify.

Dropping point is the temperature at which grease melts.

Cloud point is the temperature at which the paraffin elements separate from oil.

Conradson number (carbon residue) is the carbonaceous residue remaining after destructive distillation, expressed

in percentage by weight of the original sample.

Viscosity index indicates the relative change in viscosity of an oil for a given temperature change.

Octane number the ignition quality rating of gasoline, which is the percentage by volume of iso-octane in a mixture

of iso-octane and heptanes that matches the gasoline in anti-knock quality.

Cetane number the ignition quality rating of diesel, which is the percent of cetane in the standard fuel.

Aniline point is that temperature where equal parts if oil and aniline will dissolve in each other.

Volatility is the ability of a liquid fuel to change into vapor which is manifested in the temperature range at which

various portions of the fuel are vaporized.

18.Composition of Fuelsa. Paraffins, CnH2n+2 saturated hydrocarbons, very stable in charactersb. Olefins, CnH2n unsaturated hydrocarbons, characterized by the presence of a double bond between carbon

atoms.

c. Diiolefins, CnH2n-2 less saturated than olefins, characterized by the presence of two double bonds.19.Analysis of Composition

19.1 Proximate analysis is made by heating the coal until it decomposes successively into three of the fourcomplex items of proximate analysis. The fourth is found by the difference. A typical proximate analysis of

coal determines the percentage of moisture, volatile matter, fixed carbon, and ash.

a. Moisture is determined by subjecting a 1-g sample of the coal to a temperature of 220 F to 230 Ffor a period of exactly 1 hr.

b. Volatile matter consists of hydrogen and certain hydrogen-carbon compounds that can beremoved from the coal merely by heating it.

c. Ash is performed by heating the sample of coal used in the moisture determination to atemperature of 1290 F to 1380 F in an uncovered crucible, with good air circulation, until the coal is

completely burned.


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d. Fixed Carbon is the difference between 100 % and the sum of the percentages of moisture, ash,and volatile matter.

19.2 Ultimate analysis analysis of composition of fuel which gives, on mass basis, the relative amounts ofcarbon, hydrogen, oxygen, nitrogen, sulfur, ash, and moisture.

20.Basis of Reporting Analysisa. As received or as firedb. Dry or moisture freec. Moisture and ash free or combustibled. Moisture, ash, and sulfur free

21.Heating Values of Fuels or Calorific Valuea. Higher heating value (gross calorific value), HHV is the heating value obtained when the water in the products

of combustion is in the liquid state.

b. Lower heating value (net calorific value), LHV is the heating value obtained when the water in the products ofcombustion is in the vapor state.

22.Methods of Determining Heating Values22.1 Laboratory experiment

22.1.1 Bomb calorimeter for solid and liquid fuels22.1.2 Gas calorimeter for gaseous fuels

22.2 Empirical formulas22.2.1 Dulongs formula for solid fuels of known ultimate analysis.

kgkJSO

HHHV 304,98

212,144820,33 +

+=

lbBtuSO

HHHV 40508

000,62600,14 +

+=

22.2.2 ASME Formula for petroleum products( ) kgkJAPIHHV o6.139130,41 +=

( ) lbBtuAPIHHV o60680,17 += 22.2.3 Bureau of Standard formula

( ) kgkJSGHHV 28.793,8716,51 =( ) lbBtuSGHHV 23780230,22 =

Difference between higher and lower heating values

HHV LHV= 9H2(2442) in SI units

HHV- LHV= 9H2(1050) in English unitsWhere:

9H2= lbs or kg of water formed per lb or kg of fuel burned.

2442 kJ/kg or 1050 Btu/lb latent heat of vaporization of water.

Also H2 = 26-15(SG), percent by weight.

23.Fuel Production Processa. Fractional distillation the primary method of crude oil refining.


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b. Thermal cracking changing heavy oil into gasoline by means of high pressure, high temperature and longerexposure time.

c. Catalytic cracking subjects oil to high pressure and high temperature in the presence of a catalyst; permitaccurate control of the compounds formed and produces a gasoline of higher octane number than the one

produced in thermal cracking.

d.

Hydrogenation process of catalytic cracking in a hydrogen atmosphere; obtained are more saturated productsthan those from cracking process alone.

e. Isomerization process by which the atoms of carbon and hydrogen in normal hydrocarbons are rearranged toproduce a more complex structure of higher anti-knock value.

f. Polymerization makes use of high pressure, high temperature and a catalyst to combine light and volatile gasesinto gasoline.

g. Alkylation process of combining an isoparaffin usually iso-butane, with an olefin, usually butane or propane, toform a large isoparaffin molecule, usually iso-octane or iso-heptane, having a very high octane number.

h. Reforming used to obtain fuels with substantially higher than 100 octane number; currently used to processabout forty percent of motor gasoline.

i. Hydrodesulfurization process of adding hydrogen to unsaturated hydrocarbons and reducing the sulfurcontent of the resulting fuel oil.

24.CombustionCombustion a chemical reaction between fuel and oxygen (air) which is accompanied by heat and light.

25.Composition of Air and Molecular Weightsa. Composition by weight

76.8 % nitrogen, 23.2 % oxygen

Or 76.8 / 23.2 = 3.3 lb of nitrogen per lb of oxygen.

b. Composition by volume79.0 % nitrogen, 21.0 % oxygen

Or 79.0/21.0 = 3.76 moles of nitrogen per moles of oxygen

c. Molecular weightsAir = 28.97 kg/kgmoleC = 12 kg/kgmole

H2 = 2 kg/kgmole

O2 = 32 kg/kgmole

N2 = 28 kg/kgmole

S = 32 kg/kgmole

26.Air Fuel RatioTheoretical air-fuel ratio, Wta is the exact theoretical amount, as determined from the combustion reaction, of ai

needed to burn a unit amount of fuel, kg air per kg fuel or lb air per lb fuel.

SOHCWta 3248

36345311 22 ... ++=

where:

Wta= theoretical air, lb per lb fuel

C= carbon, lb per lb fuel

H2= hydrogen, lb per lb fuel

O2 = oxygen, lb per lb fuel

S = sulfur, lb per lb fuel


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Actual air-fuel ratio, Waa is determined by the presence of excess air which is defined as the amount of air supplied

over and above the theoretical air.

( ) taaa WeW += 1

ta

taaa

W

WWe

=

where e is the excess air in decimal.

27.Typical Combustion ReactionFuel + Air = Product of Combustion

( ) ( ) ( )22222

25076350250763250 NmnOmHnCONmnOmnHC mn ...... +++++++

( )( ) ( )

mn

mn

mn

mnWta

+

+=

+

++=

12

25028137

12

2876332250 ....

28.Classification of combustion reactiona. Combustion reaction with chemically-correct or stoichiometric condition general chemical formula of the fuel is

CnHm.

b. Combustion reaction with greater amount of theoretical air, or having a fuel-lean mixture.c. Combustion reaction with lesser amount of theoretical air, or having a fuel-rich mixture.

29.Equivalence ratio for a given mass of air, .aa

ta

W

W=

Note:

= 1, for stoichiometric mixture.

< 1, for fuel-lean mixture.

> 1, for fuel-rich mixture.

30.Orsat AnalyzerOrsat analyzer is a convenient portable apparatus for determining the volumetric percentage ofCO2, O2, and CO in

the dry flue gas.

31.Dry Flue Gases from Actual Combustion( ) abdg

CCOCO

OCOW

+

++=

2

22

3

7004

Boiler test code formula corrected to account for the SO2.

( )

( )SSC

COCO

NCOOCOW

abdg 8

5

8

3

3

7811

2

222+

+

+

+++=

where:

CO2, O2, CO, and N2 are volumetric Orsat analysis

Cab and S are decimal fractions by weight.

32.Weight of dry refuse from the coal analysisr

rC

AW

=

1


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where:

Wr= dry refuse per lb coal as fired, lb

A = ash in coal, lb

Cr= combustible In 1 lb of refuse.

33.Carbon Actually BurnedAWCC rab +=

Or

60014,

rrab

HVWCC =

where:

Cab= carbon actually burned per lb of fuel, lb

C= carbon in 1 lb of fuel, lb

HVr= heating value of the dry refuse, Btu per lb.

34.Carbon burned to CO due to incomplete combustion.abi C

COCOCOC +

=

2

where Ciis the pounds of carbon the CO per pound of fuel burned.

35.Air Actually Used During Combustion2

2

28

8 NSCO

HWW abdgaa

+=

Values ofH2, O2, S, and N2 are obtained from the ultimate analysis of the fuel and all values are expressed as

decimals.

36.Boiler Heat BalanceConsist of percentage energy absorbed by boiler fluid, energy loss due to dry flue gases, energy loss due to moisturein fuel, energy loss due to evaporating and superheating moisture formed by combustion of hydrogen, energy loss

due to incomplete combustion of carbon to CO, energy loss due to combustible in the refuse, and energy loss due to

radiation and unaccounted for totaling to higher heating value as 100%.

a. Energy absorbed by boiler fluid.The useful output of the steam generator is the heat transferred to the fluid.

( )

f

w

W

hhWQ 121

=

in which

Ww= weight of fluid flowing through the boiler during the test, lbh1 and h2 = fluid enthalpies entering and leaving the boiler, respectively, Btu per lb

Wf= weight of fuel burned during test, lb

Q1 expressed as a percentage of the higher heating value of the fuel is the boiler efficiency.


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b. Energy loss due to dry flue gas.This loss is the greatest of any of the boiler losses for a properly operated unit.

agdg ttWQ = 2402 .

in which

0.24 = specific heat of the flue gas at constant pressure, Btu per lb per deg F.

tg = temperature of the gas leaving the boiler, F

ta = temperature of the air entering the boiler, F

c. Energy loss due to evaporating and superheating moisture in fuel.Moisture entering the boiler with the fuel leaves as a superheated vapor in the same way as does moisture

from the combustion of hydrogen.

FtwhenttMQ gfgf 57546010893 += ,.

whereMf= moisture in fuel, lb per lb of fuel

tf= temperature of fuel, F

d. Energy loss due to evaporating and superheating moisture formed by combustion of hydrogen.This loss is higher for gaseous fuels containing relatively large percentages of hydrogen than for the average low

hydrogen coal.

ffhhHQ = 24 9

where:

h2 = weight of hydrogen in the fuel, lb per lb fuel

h = enthalpy of superheated vapor, Btu per lbhff= enthalpy of liquid at the incoming fuel temperature

or

FtwhenttHQ gfg 57546010899 24 += ,.

The proper value of H2 to be used in the equation is the amount of hydrogen in the fuel that is available for

combustion. To obtain the value ofH2, deduct from the value ofH2 in ultimate analysis one ninth of the weight

of moisture from the proximate analysis.

e. Energy loss due to incomplete combustion.Products formed by incomplete combustion may be mixed with oxygen and burned again with a further release

of energy.

lbBtuCOCO

COCCQ abi

+

==

2

51601016010 ,,

f. Energy loss due to unconsumed carbon.All combustible in the refuse may be assumed to be carbon, since the other combustible parts of coal would

probably be distilled out of the fuel before live embers would drop into ash pit.


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( ) lbBtuCCQ ab= 600146 ,

or

rrHVWQ =6

g. Unaccounted-for and radiation loss.This loss is due to radiation, incomplete combustion resulting in hydrogen and hydrocarbons in the flue gas, and

unaccounted-for losses.

6543217QQQQQQHHVQ =

h. Boiler Heat Balance TabulationItem Energy, Btu per lb fuel Percentage

Q1

Q2

Q3

Q4

Q5

Q6

Q7

HHV 100%

- End -


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b. fuels and combustion - lecture

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