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