Page 107 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

3.4 Thermodynamics of Combustion System 81

When the final temperature of the products is the same as the initial temperature
of the reactants T 1 = T 2 = T, Eq. (3.52) can be used to define enthalpy of reaction
(DH R ), which is defined as the heat per unit fuel (J/mole) released from a com-
bustion. Mathematically, it is described as
" #
Q 1 X X
DH R TðÞ ¼ ¼ p i h i TðÞ r i h i TðÞ ð3:53Þ
n f n f
P R
Again, the unit of enthalpy of reaction is J/(mole of fuel). It is temperature
dependent. For the reaction of aA þ bB ! cC þ dD that takes place under an
isothermal steady state, its enthalpy of reaction is the enthalpy difference between
the products and the reactants.

3.4.2 Enthalpy Scale for Reacting System

For a reacting system the working fluid changes molecularly from reactants to
products while undergoing a combustion process. The enthalpy of a substance at
condition of (P, T)is h(P, T). The enthalpy of a substance at standard condition is
defined as that at 1 atm and 298 K taking the symbol of h(STP). So,
hP; TÞ ¼ h STPÞ þ hP; TÞ h STPފ:
ð
ð
ð
½
ð
The enthalpy of every element in its natural state at the standard condition is
zero. The enthalpy of all other substances at STP is simply the heat of formation of
the substance, since it is formed from its corresponding elements. For example,
1
= 2O 2 ðgÞþ H 2 ðgÞ! H 2 OðlÞ
1
Recall that at standard condition (1 atm and 298 K), q ¼ h H 2 OðlÞ = 2h O 2 ðgÞ
h H 2 ðgÞ h o , i.e., h H 2 OðlÞ ¼ h o . The enthalpy of the ith component in a
f ;H 2 OðlÞ f ;H 2 OðlÞ
mixture is

ð
ð
½
ð
ð
hP; TÞ ¼hSTPÞ þ hP; TÞ h STPފ
ð3:54Þ
o
¼h f ;i þ h i P; TÞ h i STPފ
½
ð
ð
where h o is the heat of formation of substance i, and the term in the bracket is
f ;i
sensible enthalpy. Therefore, the enthalpy of the ith component in a mixture is
Z T
o
h i ðTÞ¼ h þ C p;i dT ð3:55Þ
f ;i
298 K