FUELS AND COMBUSTION 501dharm
\M-therm\Th11-1.pm5Typical values of ∆Hf for different substances at 25°C (298 K) in kJ/mole are given below :
S. No. Substance Formula State ∆Hf- Oxygen O gas 249143
O 2 gas zero - Water H 2 O liquid – 285765
vapour – 241783 - Carbon C gas 714852
diamond 1900
graphite zero - Carbon monoxide CO gas – 111508
- Carbon dioxide CO 2 gas – 393443
- Methane CH 4 gas – 74855
- Methyl alcohol CH 3 OH vapour – 240532
- Ethyl alcohol C 2 H 5 OH vapour – 281102
- Ethane C 2 H 6 gas – 83870
- Ethene C 2 H 4 gas 51780
- Propane C 3 H 8 gas – 102900
- Butane C 4 H 10 gas – 125000
- Octane C 8 H 18 liquid – 247600
11.18.Calorific or Heating Values of Fuels
The “calorific value or heating value” of the fuel is defined as the energy liberated by the
complete oxidation of a unit mass or volume of a fuel. It is expressed in kJ/kg for solid and liquid
fuels and kJ/m^3 for gases.
If a fuel contains hydrogen water will be formed as one of the products of combustion. If this
water is condensed, a large amount of heat will be released than if the water exists in the vapour
phase. For this reason two heating values are defined ; the higher or gross heating value and the
lower or net heating value.
The higher heating value, HHV, is obtained when the water formed by combustion is
completely condensed.
The lower heating value, LHV, is obtained when the water formed by combustion exists
completely in the vapour phase.
Thus : (HHV)p = (LHV)p + m hfg ...(11.26)
(HHV)v = (LHV)v + m(ug – uf) ...(11.27)
where m = Mass of water formed by combustion,
hfg = Enthalpy of vaporisation of water, kJ/kg,
ug = Specific internal energy of vapour, kJ/kg, and
uf = Specific internal energy of liquid, kJ/kg.
In almost all practical cases, the water vapour in the products is vapour, the lower value is
the one which usually applies.
11.19.Determination of Calorific or Heating Values
The calorific value of fuels can be determined either from chemical analysis or in the labo-
ratory.
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