Page 99 - Materials Chemistry, Second Edition

3.2 Energy Analysis 83

Table 3.4 Specific heat values of some fuels Boustead, 1992.

Fuel Higher heating value Lower heating value
−1
−1
(HHV) (MJ kg ) (LHV) (MJ kg )
Gasoline 45.85 42.95
Propane 50.00 46.95
Methane 53.42 48.16
Natural gas Approximately 51.50 46.10 a
Diesel (fuel oil) 42.90 40.50
Coal 30.60 29.65

a Average value from Table 3.3 converted at density of 0.78 kg m .
−3
Because HHV and LHV values of fossil energy carriers vary with deposits, if the
exact values are not known, averages are used for a mass-to-energy conversion. The
best-known average is
−1
< LHV(Coal) >= 29.3MJ kg
This defines the ‘kilograms of coal equivalent (kgce),’ which is not approved
officially but is often used in statistics. It signifies an energy unit or LHV of
−1
1 kg of average coal. This was formerly approximated as 7000 kcal kg ,thus
(1 cal = 4.184 J) 63)
−1
−1
−1
1kgce = 7000[kcalkg ]× 4.184[kJkcal ]= 29288[kJkg ]
≈ 29.3MJkg −1
Thus the accuracy of this specification is only apparent and is caused by the
conversion of the obsolete unit Calorie to the SI unit Joule.
In analogy to kgce, the unit ‘tonnes of coal equivalent (tce)’ is used: 1 tce = 29.3 GJ.
The advantage of these units is their descriptiveness by providing a mass equivalent
in (kg) or (t) for energy.
Concluding the discussion on lower versus higher heating values, it should
be pointed out that the guideline to CED (VDI 4600), contrary to the Boustead
recommendation, uses the LHV as data for the energy content of fuels. Additional
use of HHV is not excluded but should explicitly be noted.
Corresponding to VDI 4600, the overall efficiency of supply of fuels (g )is
OES
defined according to the following Equation 3.9:
g OES = LHV (3.9)
CED
s
CED = cumulative energy demand for the supply of an energy carrier, in an LCA,
s
calculated per functional unit

63) Calorie is a badly defined unit and should not be used any more. The conversion factor 4.184
(precise) refers to a ‘thermochemical’ or ‘defined’ calorie.