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82 3 Basics of Gas Combustion

where C p;i is a function of temperature, it can be computed using

C p;i ðTÞ¼ a i þ b i T ð3:56Þ

In this equation, the coefficients a i , and b i , are available in Table A.4, the
approximate thermodynamic data for species of combustion interest [2, 7, 9, 16].
The integration of Eq. (3.55) then leads to

1
o 2 2
h i ðTÞ¼ h ðT) þ a i ðT T 298K Þþ b i ðT T Þ ð3:57Þ
f ;i 298K
2
o
The enthalpy of formation, h , of a compound is defined as the change of
f
enthalpy that accompanies the formation of 1 mol of compound in its standard state
from its constituent elements in their standard states. This definition indicates that
the products and the reactants are at the standard state. The standard state is referred
to as atmospheric pressure (101.325 Pa) and a temperature of 298 K.
Values for standard heat of formation for different species are tabulated in Table
A.4 (selected from Burcat [2]). More can be found in Perry’s Chemical Engineers’
handbook or similar publications. Table A.4 shows the enthalpy of formation of an
element in its natural state at the standard condition is zero. Enthalpy of formation
can also be determined from their heating values that will be introduced in
Sect. 3.4.3.

3.4.3 Heating Values

When a fuel undergoes combustion with oxygen, energy is released as heat. The
maximum amount of heat is released when the combustion is stoichiometric, where
all the hydrogen and carbon contained in the fuel is converted to CO 2 and H 2 O.
This maximum energy from 1 mol of fuel is called the heat of combustion or the
heating value. Depending on the state of water in the combustion products, the heat
of combustion for fuels is expressed as the higher heating value (HHV) or lower
heating value (LHV). HHV is used when the water in the products is in the liquid
state (h H 2 O = h l ). LHV is used when the water in the products is in the vapor state
(h H 2 O = h g ), and the energy required to vaporize the water is not considered as heat
of combustion.
The conversion between mole based LHV and HHV is thus

n H 2 O
LHV ¼ HHV h fg ð3:58Þ
n fuel
where h fg is the latent heat of vaporation of water (J/mole), and n H 2 O =n fuel gives the
mole amount of water produced by 1 mol of fuel.