Page 98 - Materials Chemistry, Second Edition

82 3 Life Cycle Inventory Analysis

E supplied by combustion with oxygen according to Equation 3.7:
therm
( m ) m
C H + n + O → nCO + H O + E (3.7)
n m 4 2 2 2 2 therm
for example, for methane (n = 1, m = 4):

CH + 2O → 1CO + 2H O +Δ (3.8)
4
2
2
2
ΔH =−857 kJ mol −160)
◦
The reaction enthalpy (ΔH) on the right side of Equation 3.8 refers to 25 C
and liquid water as final product. The minus sign of the reaction enthalpy in
Equation 3.8 corresponds to a convention for exothermic reactions, which thus
deliver energy (enthalpy). In endothermic reactions (taking up energy from the
surroundings), this enthalpy has a positive sign. This convention is valid in physical
chemistry. In technology this rule is not applied.
Coals and crude oils, the most important non-gaseous fossil energy carriers, are
chemically badly defined mixtures that may also contain, besides C and H, different
elements. This explains the ranges of heating values in Table 3.3. Furthermore, the
use of molar units does not make sense. Technical heat values of solid and liquid
energy carriers usually refer to a mass unit.
For the determination of the LHV, the reactants must be present before and after
◦
combustion at 25 C (298.1 K). The water formed during this combustion process
is considered to be in the vapour state, which is usual in technical processes (in
◦
spite of water being liquid at 25 C, the final temperature in the definition of the
heat value).
Thermodynamically more meaningful is the higher heating value (HHV), which
61)
◦
is also defined at 25 C as starting and final temperature; the water formed here
is, however, in the liquid state. The HHV is usually (as the name suggests) higher
than the LHV, because during condensation the heat of vapourisation is set free as
condensation heat and adds to the total measured enthalpy.
For LCAs, according to Boustead, 62) the thermodynamically more correct HHV
value is preferable. In praxis, however, the LHV values as provided in technology are
more easily available. The chemical composition of the energy carrier (the amount
of hydrogen must be known), which varies according to the origin of the fossil fuel,
or the amount of water formed during incineration has to be known for conversion
to occur. The difference between the numerical values of LHV and HHV amounts
to a maximum of 10% if the fuel is rich in hydrogen like methane; and it nearly
disappears for fuels poor in hydrogen, for example, (hard) coal (Table 3.4). The
vapourisation enthalpy of water needed for the conversion amounts to 2.45 kJ g −1
H O. The sign is positive for evaporation and negative for condensation. The latter
2
implies that the input energy during evaporation (with constant pressure it is called
enthalpy) will again be released during condensation.

60) Per mol means per formula (i.e. not per kilogram or per standard cubic metre).
61) Other names include gross energy, upper heating value or gross caloriferic or higher caloriferic
value (HCV).
62) Boustead, 1992.