Fuels and combustion(2013)

FUELS AND COMBUSTION

Fuels and Combustion Types of Fuels Complete/Incomplete Combustion Oxidation of Carbon Oxidation of Hydrogen Oxidation of Sulfur Air composition Combustion with Air Theoretical Air Hydrocarbon fuels Combustion of Hydrocarbon Fuel

FUELS and

COMBUSTION

By. Engr. Yuri G. Melliza

Fuels and CombustionFuel: Substances composed of chemicalelements which in rapid chemical union with oxygen produced combustion.

Combustion: Is that rapid chemical unionwith oxygen of an element, whose exo-thermic heat of reaction is sufficientlygreat and whose rate of reaction is suf-ficiently fast whereby useful quantities ofheat are liberated at elevated tempera-ture.

TYPES OF FUELS Solid Fuels

ex: Wood, coal, charcoal Liquid Fuels

ex: gasoline, diesel, kerosene Gaseous Fuels

ex: LPG, Natural Gas, Methane Nuclear Fuels

ex: UraniumCombustible Elements1.Carbon (C) 3. Sulfur (S)2.Hydrogen (H2)

Complete Combustion: Occurs when all thecombustible elements has been fully

oxidized.Ex:

C + O2 CO2

Incomplete Combustion: Occurs when someof the combustible elements has not

been fully oxidized.Ex:

C + O2 CO

Common Combustion GasesGAS MOLECULAR

Weight (M)

C 12

H 1

H2 2

O 16

O2 32

N 14

N2 28

S 32

THE COMBUSTION CHEMISTRY Oxidation of Carbon

11 83

44 3612

32)1(12 1(16)1(12)

Basis Mass

1 11

Basis Mole

CO OC 2

2

Oxidation of Hydrogen

9 81

18 162

2)1(16 (32)1(2)

Basis Mass

1 1

Basis Mole

OH OH

2

1

21

2

22 21

Oxidation of Sulfur

2 11

64 3232

32)1(32 (32)1(32)

Basis Mass

1 11

Basis Mole

OS OS

1

22

Composition of AIRa. Percentages by Volume or

(by mole)O2 = 21%N2 = 79%

b. Percentages by MassO2 = 23%N2 = 77%

76321

79.

2

2

O of MoleN of Moles

Combustion with AirA. Combustion of Carbon with air

C + O2 + 3.76N2 CO2 + 3.76N2

Mole Basis:1 + 1 + 3.76 1+ 3.76

Mass Basis:1(12) + 1(32) + 3.76(28) 1(44) +3.76(28)12 + 32 + 3.76(28) 44 + 3.76(28) 3 + 8 + 3.76(7) 11+ 3.76(7)

kg of air per kg of Carbon:

C of kgair of kg

11.44=3

3.76(7)+8=

C of kgair of kg

B. Combustion of Hydrogen with airH2 + ½ O2 + ½ (3.76)N2 H2O +

½(3.76)N2 Mole Basis:

1 + ½ + ½(3.76) 1 + ½(3.76)Mass Basis:

1(2) + ½ (32) + ½(3.76)(28) 1(18) + ½(3.76)(28)

2 + 16 + 3.76(14) 18 + 3.76(14) 1 + 8 + 3.76(7) 9 + 3.76(7)

kg of air per kg of Hydrogen:

22 H of kgair of kg

34.32=1

3.76(7)+8=

H of kgair of kg

C. Combustion of Sulfur with airS + O2 + 3.76N2 SO2 + 3.76N2

Mole Basis:1 + 1 + 3.76 1 + 3.76N2

Mass Basis:1(32) + 1(32) + 3.76(28) 1(64) +

3.76(28) 32 + 32 + 105.28 64 + 105.28

kg of air per kg of Sulfur:

S of kgair of kg

4.29=32105.2832

=S of kgair of kg

Theoretical AirIt is the minimum amount of air required to oxidize the reactants or the combustible elements found in the fuel. With theoretical air no O2 is found in products.

Excess AirIt is an amount of air in excess of the theoretical requirements in order to influence complete combustion. With excess air O2 is present in the products.

HYDROCARBON FUELSFuels containing the element s Carbon and Hydrogen. Chemical Formula: CnHm

Family Formula Structure Saturated

Paraffin CnH2n+2 Chain Yes

Olefin CnH2n Chain No

Diolefin CnH2n-2 Chain No

Naphthene CnH2n Ring Yes

Aromatic

Benzene CnH2n-6 Ring No

Naphthalene CnH2n-12 Ring No

Alcohols Note: Alcohols are not pure hydrocarbon, because one of its hydrogen atom is replace by an OH radical. Sometimes it is used as fuel in an ICE.

Methanol CH3OH

Ethanol C2H5OH

Saturated Hydrocarbon: All the carbon atoms are joined by a single bond.Unsaturated Hydrocarbon: It has two or more adjacent Carbon atoms joined by adouble or triple bond.Isomers: Two hydrocarbons with the samenumber of carbon and hydrogen atoms butat different structures.

H H H H H C C C CH H H H H

Chain structure Saturated

H H HC C=C C H H H H H

Chain Structure Unsaturated

Ring structure Saturated H H H C H C C H C H H H

Theoretical Air: It is the minimum or theoretical amount of air required to oxidized the reactants. With theoretical air no O2 is found in the products. Excess Air: It is an amount of air in excess of the theo-retical air required to influence complete combustion. With excess air O2 is found in the products.Combustion of Hydrocarbon Fuel(CnHm)

A. Combustion with 100% theoretical air CnHm + aO2 + a(3.76)N2 bCO2 + cH2O + a(3.76)N2

fuel

air

t kg kg

m12n

)a(3.76)(28a(32)FA

fuel

air

a kg kg

m12n

)a(3.76)(28a(32)e)(1

FA

B. Combustion with excess air e CnHm +(1+e) aO2 + (1+e)a(3.76)N2 bCO2 + cH2O + dO2 +

(1+e)a(3.76)N2

Actual Air – Fuel Ratio

fuel

air

ta kg kg

FA

e)(1FA

Where: e – excess air in decimalNote: Sometimes excess air is expressible in terms of theoretical air. Example: 25% excess air = 125% theoretical air

Orsat Analysis: Orsat analysis gives the volumetric or molal analysis of the PRODUCTS on a DRY BASIS, (no amount of H2O given).

Proximate Analysis: Proximate analysis gives the amount of Fixed Carbon, Volatiles, Ash and Moisture, in percent by mass. Volatiles are those compounds that evaporates at low temperature when the solid fuel is heated.00

ULTIMATE ANALYSIS: Ultimate analysis gives the amount of C, H, O, N, S in percentages by mass, and sometimes the amount of moisture and ash are given.

SOLID FUELSComponents of Solid Fuels:

1. Carbon (C) 2. Hydrogen (H2)3. Oxygen (O2)4. Nitrogen (N2)5. Sulfur (S)6. Moisture (M)7. Ash (A)

A. Combustion with 100% theoretical airaC + bH2 + cO2 + dN2 + eS + fH2O + gO2 +

g(3.76)N2 hCO2 + iH2O + jSO2 + kN2

B.Combustion with excess air x: aC + bH2 + cO2 + dN2 + eS + fH2O + (1+x)gO2

+(1+x)g(3.76)N2 hCO2 + iH2O + jSO2 + lO2 + mN2

WHERE: a, b, c, d, e, f, g, h, I, j, k, x are the number of moles of the elements.x – excess air in decimal

fuel kgair kg

18f32e28d32c2b12a

3.76(28)g32gFA

t

Theoretical air-fuel ratio:

Actual air-fuel ratio:

fuel kgair kg

18f32e28d32c2b12a

3.76(28)g32gx)(1

a

F

A

MASS FLOW RATE OF FLUE GAS (Products)

Air +Fuel Products

A. Without considering Ash loss

1

F

Amm Fg

B. Considering Ash loss

lossAsh 1

F

Amm Fg

Heating Value

Heating Value - is the energy released by fuel when it is completely burned and the products of combustion are cooled to the original fuel temperature.Higher Heating Value (HHV) - is the heating value obtained when the water in the products is liquid.Lower Heating Value (LHV) - is the heating value obtained when the water in the products is vapor.

For Solid Fuels with the presence of Fuel’sULTIMATE ANALYSIS

kg

KJ S9304

8

OH212,144C820,33HHV 2

2

where: C, H2, O2, and S are in decimals from the ultimate analysis

HHV = 31 405C + 141 647H KJ/kgHHV = 43 385 + 93(Be - 10) KJ/kg

For Liquid Fuels

where: Be - degrees Baume

For Coal and Oils with the absence of Ultimate Analysis

fuel of kg

air of Kg

3041

HHV

F

A

t

For Gasoline

kgKJ )API(93639,38LHV

kgKJ )API(93160,41HHV

kgKJ )API(93035,39LHV

kgKJ )API(93943,41HHV

For Kerosene

For Fuel Oils

Institute Petroleum AmericanAPI

kgKJ )API(6.139105,38LHV

kgKJ )API(6.139130,41HHV

For Fuel Oils (From Bureau of Standard Formula)

).t(.St@S 561500070API131.5

141.5S

HHV = 51,716 – 8,793.8 (S)2 KJ/kgLHV = HHV - QL KJ/kg

QL = 2442.7(9H2) KJ/kg

H2 = 0.26 - 0.15(S) kg of H2/ kg of

fuel

WhereS - specific gravity of fuel oil at 15.56 CH2 - hydrogen content of fuel oilQL - heat required to evaporate and superheat the water vapor formed bythe combustion of hydrogen in the fuelS @ t - specific gravity of fuel oil at any temperature tOxygen Bomb Calorimeter - instrument used in measuring heating value of solid and liquid fuels.Gas Calorimeter - instrument used for measuring heating value of gaseous fuels.

Properties of Fuels and Lubricantsa)Viscosity - a measure of the resistance to flow that a lubricant offers when it is subjected to shear stress.b) Absolute Viscosity - viscosity which is determined by direct measurement of shear resistance.c) Kinematics Viscosity - the ratio of the absolute viscosity to the densityd) Viscosity Index - the rate at which viscosity changes with temperature.e) Flash Point - the temperature at which the vapor above a volatile liquid forms a combustible mixture with air.f) Fire Point - The temperature at which oil gives off vapor that burns continuously when ignited.

g) Pour Point - the temperature at which oil will no longer pour freely.h) Dropping Point - the temperature at which grease melts.i) Condradson Number(carbon residue) - the percentage amount by mass of the carbonaceous residue remaining after destructive distillation.j) Octane Number - a number that provides a measure of the ability of a fuel to resist knocking when it is burnt in a gasoline engine. It is the percentage by volume of iso-octane in a blend with normal heptane that matches the knocking behavior of the fuel.

k) Cetane Number - a number that provides a measure of the ignition characteristics of a diesel fuel when it is burnt in a standard diesel engine. It is the

percentage of cetane in the standard fuel.

Prepared By: ENGR YURI G. MELLIZA, RME