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CHAPTER THREE
3 FUEL AND COMBUSTION
3.1 Chemical composition of the fossil fuels
Fuel can be classified as solid, liquid and gaseous. Fuel consists of the following elements:
carbon, hydrogen, oxygen, water, sulphur, nitrogen and mineral salt.
3.1.1 Solid fuels
Important solid fuels include wood, peat, lignite or brown coal, bituminous coal and
anthracite. Others include wastes like sludges, baggasse and coffee husks, sawdust etc.
According to the elements present in the solid fuel, it can be further classified as
a). Raw mass (as received): C + H + O + W + S + N + A = 100%
b). Dry mass (d.m. or wf): C + H + O + S + N + A = 100 %
c). Dry and ash free mass (daf or waf): C + H + O + S + N = 100 %
d). Volatile matter: Cv + H + O + W + S + N + = 100 %
e). Char Fixed carbon + Ash
Conversion from one mass to another
Given mass Raw Wf Waf
Raw 100 100
1 100 W raw 100 ( Araw W raw )
Wf 100 W raw ) 100
100 1 100 Adry
Waf 100 ( Araw W raw ) 100 Adry
100 100 1
Table 3.1. Conversion from one mass to another
Effect of the elements on the combustion quality of the fuel
MPE 571E: POWER PLANT ENGINEERING
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The various elements of solid fuels have different effects on the combustion process of solid
fuels. Carbon and hydrogen are the main combustible component of solid fuel and the
higher their composition in the fuel the better. Oxygen supports combustion and has no
direct effect on the combustion process but fuels with high contents of oxygen exhibit low
calorific value. The present of moisture in solid fuel is undesirable because it lowers
calorific value of fuel, makes the fuel bulky and weighty and results into larger volume of
product of combustion. Sulphur and Nitrogen are also undesirable since their oxides are
considered pollutants and can also lead to corrosion. The presence of ash lowers the quality
of the fuels, limits the furnace temperature, and complicates the combustion process due to
the necessity for ash removal. Volatile matter determines the combustion characteristics of
the fuels like ignition, combustion rate and the type of smoke.
Proximate and Ultimate analysis
Bituminous Brown Peat Digested Digested Wood Unit
Coal Coal Sludge Sludge
Proximate Analysis
Water 6.0 36.3 20 13.0 78.5 wt.-% (raw)
Ash 35.4 17.7 4 45.3 24.8 1.5
Volatiles 28.3 48.0 65 51.0 wt.-% (wf)
Volatiles 43.8 58.3 68 92.7 wt.-% (waf)
LCV 19,400 13,864 9,724 1,935 kJ/kg (raw)
32,112 28,148 20,984 23,817 10,450 kJ/kg (waf)
Ultimate Analysis
C 81.6 70.8 45.8 53.0 54.3 48.5
H 5.8 5.2 6.7 7.8 7.9 6.0
O 9 17 46.2 31.3 31.7 29.8 wt.-% (waf)
N 1.3 1.4 1.58 6.5 5 8.8
S 2.3 5.6 3.1 1.4 1.1 1.7
Table 3.2: Elementary analyse of some fuels
3.1.2 Liquid fuels
Some of the most commonly used liquid fuels for power engineering are: gasoline or petrol,
paraffin, diesel oil and heavy oil. The characteristics are shown in Table 3.2. Liquid fuel
(industrial oil) is normally classified according to their sulphur contents as small sulphur
content oil (S less than 1 wt.-%), medium sulphur content oil (S= 1- 2 wt.-%) and high
sulphur content oil (S greater than 2.5 wt.- %)
MPE 571E: POWER PLANT ENGINEERING
CHAPTER THREE
3 FUEL AND COMBUSTION
3.1 Chemical composition of the fossil fuels
Fuel can be classified as solid, liquid and gaseous. Fuel consists of the following elements:
carbon, hydrogen, oxygen, water, sulphur, nitrogen and mineral salt.
3.1.1 Solid fuels
Important solid fuels include wood, peat, lignite or brown coal, bituminous coal and
anthracite. Others include wastes like sludges, baggasse and coffee husks, sawdust etc.
According to the elements present in the solid fuel, it can be further classified as
a). Raw mass (as received): C + H + O + W + S + N + A = 100%
b). Dry mass (d.m. or wf): C + H + O + S + N + A = 100 %
c). Dry and ash free mass (daf or waf): C + H + O + S + N = 100 %
d). Volatile matter: Cv + H + O + W + S + N + = 100 %
e). Char Fixed carbon + Ash
Conversion from one mass to another
Given mass Raw Wf Waf
Raw 100 100
1 100 W raw 100 ( Araw W raw )
Wf 100 W raw ) 100
100 1 100 Adry
Waf 100 ( Araw W raw ) 100 Adry
100 100 1
Table 3.1. Conversion from one mass to another
Effect of the elements on the combustion quality of the fuel
MPE 571E: POWER PLANT ENGINEERING
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The various elements of solid fuels have different effects on the combustion process of solid
fuels. Carbon and hydrogen are the main combustible component of solid fuel and the
higher their composition in the fuel the better. Oxygen supports combustion and has no
direct effect on the combustion process but fuels with high contents of oxygen exhibit low
calorific value. The present of moisture in solid fuel is undesirable because it lowers
calorific value of fuel, makes the fuel bulky and weighty and results into larger volume of
product of combustion. Sulphur and Nitrogen are also undesirable since their oxides are
considered pollutants and can also lead to corrosion. The presence of ash lowers the quality
of the fuels, limits the furnace temperature, and complicates the combustion process due to
the necessity for ash removal. Volatile matter determines the combustion characteristics of
the fuels like ignition, combustion rate and the type of smoke.
Proximate and Ultimate analysis
Bituminous Brown Peat Digested Digested Wood Unit
Coal Coal Sludge Sludge
Proximate Analysis
Water 6.0 36.3 20 13.0 78.5 wt.-% (raw)
Ash 35.4 17.7 4 45.3 24.8 1.5
Volatiles 28.3 48.0 65 51.0 wt.-% (wf)
Volatiles 43.8 58.3 68 92.7 wt.-% (waf)
LCV 19,400 13,864 9,724 1,935 kJ/kg (raw)
32,112 28,148 20,984 23,817 10,450 kJ/kg (waf)
Ultimate Analysis
C 81.6 70.8 45.8 53.0 54.3 48.5
H 5.8 5.2 6.7 7.8 7.9 6.0
O 9 17 46.2 31.3 31.7 29.8 wt.-% (waf)
N 1.3 1.4 1.58 6.5 5 8.8
S 2.3 5.6 3.1 1.4 1.1 1.7
Table 3.2: Elementary analyse of some fuels
3.1.2 Liquid fuels
Some of the most commonly used liquid fuels for power engineering are: gasoline or petrol,
paraffin, diesel oil and heavy oil. The characteristics are shown in Table 3.2. Liquid fuel
(industrial oil) is normally classified according to their sulphur contents as small sulphur
content oil (S less than 1 wt.-%), medium sulphur content oil (S= 1- 2 wt.-%) and high
sulphur content oil (S greater than 2.5 wt.- %)
MPE 571E: POWER PLANT ENGINEERING
