
10.4.3.1 Higher and lower calorific values
The enthalpy of reaction of CH
4
derived above (50,178 kJ/kg) is the negative of the lower calorific
value (LCV) because it is based on gaseous water (H
2
O(g)) in the products. If the water in the products
(exhaust) was condensed to a liquid then extra energy could be released by the process. In this case the
enthalpy of reaction would be
Q
p
25
¼ðDH
f
Þ
CO
2
þ 2ðDH
f
Þ
H
2
OðlÞ
ðDH
f
Þ
CH
4
¼890225 kJ
=
kmol
This gives an enthalpy of reaction per kilogram of CH
4
of 55,639 kJ/kg. This is the negative of
the higher calorific value.
Normally the LCV, or lower internal energy or enthalpy of reaction, is used in engine calculations ...