1. LECTURE UNIT NO. 7
FUELS AND COMBUSTION
I. Fuel – a compound containing carbon and hydrogen in elemental form or compounds which, in
rapid chemical union with oxygen, produce combustion
Classification of fuels
1. Solid fuels
a.) coal (steam power plant fuel)
b.) bagasse, wood barks and ipil – ipil (dendro thermal fuel)
c.) fuel from garbage (biomass)
d.) waste products from industrial and agricultural operations
e.) coke (blast furnace fuel)
2. Liquid fuels
a.) gasoline (C8H18) – octane
b.) distilled fuel oil
- diesoline (C16H32)
c.) blended fuel oil
- diesel fuel oil (C12H26) – dodecane
d.) alcohols (CxHyOz)
- ethyl and methyl alcohols
e.) alco-gas (green gasoline)
- blend: 70% gasoline + 30% anhydrous alcohol
f.) light heating oils
g.) kerosene
h.) jet fuel
i.) liquefied petroleum gas (LPG)
- propane + butane + odorizer at high pressure
3. Gaseous fuel
a.) natural gas
- Methane (CH4)
- Ethane (C2H6)
- Propane (C3H8)
- Butane (C4H10)
b.) coke – oven gas
c.) blast furnace gas
d.) water gas
e.) enriched water gas, carbureted water gas
f.) producer gas
g.) biogas (gas emitted from animal waste)
4. Nuclear fuels
a.) natural – U238 (natural uranium)
b.) prepared – U235 (enriched uranium)
Basic desired fuel properties
1. It must have a high heating or calorific value
2. Easy handling of fuel
Four basic conditions that must be considered to burn a fuel completely (complete combustion)
1. maintain a high flame temperature enough to ignite incoming air-fuel mixture
2. sufficient time to complete the burning process of the combustible elements in the fuel
3. correct air-fuel ratio
4. thorough mixing of air and fuel
II. Combustion – a chemical reaction in which fuel combines wth oxygen liberating large quantities
of heat
Methods of reporting fuel analysis
A.) Volumetric or Molar Analysis – represents the percent or fraction of its constituents by volume
or moles with respect to the total volume of the mixture measured at the same temperature and
pressure
Illustration: consider 1 m3 of fuel

2.  For Gas X
Note:
According to Avogadro’s Law in any mixture of gasses, the ratio of the volumes of each gas is
proportional to their corresponding mol ratios at the same temperature and pressure.
(% by volume of gas X) % VX = VX x 100%= nX x 100%
VT nT
 For Gas Y
(% by volume of gas Y) % VY = VY x 100%= nY x 100%
VT nT
Illustration: consider 1m3 of dry air
 For N2
(% by volume of N2) % VN2 = VN2 x 100%
VT
% nN2 = nN2 x 100%
nT
 For O2
(% by volume of O2) % VO2 = VO2 x 100%
VT
% nO2 = nO2 x 100%
nT
From Avogadro’s Law (mol proportion of nN2 ):
nO2
Therefore:
That for every mol of O2 there are 3.76 mols of N2
B.) Gravemetric or Ultimate Analysis – represent the percent or fraction of its constituents by
weight or mass with respect to the total mass of the mixture
Illustration: consider 1 kgm of fuel
 For Gas X
(% by weight of gas X) % GX = mX x 100%
mT

3.  For Gas Y
(% by weight of gas Y) % GY = mY x 100%
mT
Illustration: consider 1 kg of dry air
 For N2
(% by weight of N2) % GN2 = mN2 x 100%
mT
 For O2
(% by weight of O2) % GO2 = mO2 x 100%
mT
Seatwork
1. Convert: Gravemetric or Ultimate Analysis to Volumetric or Molal Analysis of fuel
%GC = 80%
%GH2 = 20%
2. Covert: Volumetric or Molal Analysis to Gravemetric or Ultimate Analysis of Fuel
% VC = 40%
% VH2 = 60%
COMBUSTION EQUATION
Fuels + Moist or Humid Air Products of Combustion
Flue Gas for ECPP
Exhaust Gas for ICPP
Fuels + Dry air + Water Vapor Products of Combustion
Note: That for every mol of O2 there are 3.76 mols of N2
Complete Combustion (100% air or Oxygen) or Stoichemetric Air
With Excess Air

4. Example 1: Combustion of Octane with 10% Excess Air
Octane: C8H18
Molal Equation
Balancing
Balanced Molal Equation:
Converting to Mass
Applying Conservation of Mass
Reactants
Products
1. Solving for Actual Air-Fuel ratio
2. Solving for theoretical Air-Fuel ratio

5. 3. Solving for the theoretical mass of air
4. Solving for the amount of excess air
5. Solving for the dilution coefficient
6. Solving for the equivalence ratio
7. Gravemetric or Ultimate analysis of the products of combustion
8. Volumetric or Molal Analysis of the products of combustion

6. 9. Gravemetric ot Ultimate Analysis of the dry products of combustion
10. Volumetric or Molal Analysis of the dry products of combustion
11. Solving for the Gas Constant of the products of Combustion
12. Solving for the Dew point (condensation) temperature of water vapor (moisture) in the products of
combustion

7. TERMS
 Anthracite – a clean, dense, hard coal, comparatively difficult to ignite, but which burns
uniformly and smokelessly with a short flame
 Ash – noncombustible, solid mineral matter
 Bituminous coal – a coal that varies in composition but which generally has a higher volatile
content than anthracite, starts easily, and burns freely with a long flame. Smoke and soot
are possible if this coal is improperly fired
 Coke – the carbonaceous residue containing ash and sulfur created by burning coal with
little or no air. The lighter coal constituents are vaporized, while heavier hydrocarbons crack
and form carbon
 Fixed carbon – The combustible residue (not all of which is carbon) remaining in coal when
all volatile matter has been driven off
 Flash point – the temperature at which an oil spontaneously ignites in the presence of
sparks or flame
 Flue gas – the hot gases resulting from combustion, consisting primarily of nitrogen, carbon
dioxide, water and small quantities of carbon monoxide and sulfur dioxide. If water vapor is
removed, it is known as dry stack gas.
 Heating value – the heat generated during complete combustion of fuel
 Ignition temperature – the temperature at which more heat is generated by the combustion
reaction then is lost to the surroundings, after which combustion becomes self-sustaining
 Lignite – a coal of woody structure, very high in moisture content with low heating value. It
normally ignites slowly due to moisture content, breaks apart when burning, and burns with
little smoke or soot
 Moisture – water content of fuels. Bed moisture of coal indicates moisture level when
removed from earth. As received moisture indicates moisture level of a coal before dried or
burned
 Primary air – air that is mixed with a fuel to initiate and sustain the combustion reaction
 Secondary air – air which combines with the flue gas
 SSU viscosity – kinematic viscosity determined on a Saybolt viscometer
 Stack gas – see “ flue gas”
 Stoichiometric combustion – a reaction in which all fuel is burned with the theoretically
correct amount of air. Neither unburned fuel nor free oxygen is present in the stack gases.
CO2 is at a maximum value in the combustion products
 Sulfur – a common constituent of lower grade fuels. Organic sulfur is combined with
hydrogen and carbon in compounds. Pyritic sulfur is elemental.
 Volatile matter – that portion of fuel which is driven off as a gas or a vapor when the fuel is
heated according to standard tests. It consists mainly of organic gasses such as methane.