CACHE Modules on Energy in the Curriculum
Hydrogen as a Fuel
Module Title: Manual Calculations of Adiabatic Flame Temperature:
Hydrogen vs. Conventional Fuels
Module Authors: Neelima Borate and Jeff Naber
Author Affiliation: Michigan Technological University
Course: Combustion and Air Pollution
Text Reference: Thermodynamics: An Engineering Approach by Yunus A. Cengel and Michael
A. Boles
Concepts: Adiabatic flame temperature, stoichiometric amount of air, enthalpy of formation
Problem Motivation
This module may be used as a continuation of the first Combustion and Air Pollution Module. In
that module, we calculated adiabatic flame temperature using EES with built-in fluid property
values (e.g. enthalpy of a species). In this module, we will calculate adiabatic flame temperature
manually with the enthalpy values from the tables provided at the end of the module.
1st Draft
N. S. Borate
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Oct 12, 2010
Problem Information
Example Problem Statement:
Hydrogen is burned with a stoichiometric amount of air during an adiabatic steady-flow
combustion process. Both the fuel and the air enter the combustion chamber at 25° C and 1 atm.
Calculate the exit temperature of the product gases, assuming complete combustion.
Solution:
As seen before, for combustion chamber under adiabatic conditions and with no work
interactions we can write,
HPHR
i.e.
 N P  h 0f (h h 0 )   N R  h 0f (h h 0 ) 

P

R
Where HR is the enthalpy of the reactant stream and HP is the enthalpy of the product stream.
0
0
Also, hf is the heat of formation at 25 oC and 1 atm pressure and hf is the enthalpy at a certain
state (R = reactants, 25 oC; (R = reactants, unknown temperature).
Here the fuel and the air enter at 25° C (i.e. 298 K) to the combustion chamber.
The combustion equation for H2 with stoichiometric air is
1
1
H 2   O 2  3.76 N 2   H 2O +  3.76  N 2
2
2
Now from the tables 2.1 to 2.6,
Species
h 0f
H2
O2
N2
H2O(g)
kJ/kmol
0
0
0
-241820
1st Draft
h 298
kJ/kmol
8669
9904
N. S. Borate
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Oct 12, 2010
R = N H2  h 0f (h 298K h 0 )   N O2 h 0f (h h 0 )   N N2 h 0f (h h 0 )  

 H2

 O2

 N2
= N H2  h 0f (h 298K  h 298K )   N O2  h 0f (h 298K  h 298K )   N N2 h 0f (h 298K  h 298K ) 

 H2

 O2

 N2
 00

Now,
P NH2O h 0f (h h 0 )   N N2 h 0f (h h 0 ) 

 H2 O

 N2




1
   h TH2O 9904   *(3.76) *   h TN2 8669 

 2


  h TH2O 1.88h TN2 16297.72kJ
h TH2O 1.88h TN2 268021.72
But HP = HR
So,
h TH2O 1.88h TN2 268021.72  0
h TH2O 1.88h TN2  268021.72kJ
From this equation the adiabatic temperature is obtained with trial and error method. Initial guess
value can be find out by dividing right hand side of the above equation by the total number of
268021.72
moles i.e.
i.e. 93063.1 kJ. This value corresponds to about 2750 K for N2. But
(1  1.88)
because of higher specific heat of H2O, adiabatic temperature should be less than 2750 K in this
case. So, by trial and error find out two temperatures at which the enthalpies ( h TH2O  1.88h TN2 )
are just above and below the required enthalpy value (here it is 268021.72 kJ).
At 2500 K,
h TH2O  1.88h TN2  108868  1.88*82981
 264872.28kJ
At 2550 K,
h TH2O  1.88h TN2  111565  1.88*84814
 271015.32kJ
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N. S. Borate
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Oct 12, 2010
By interpolation,
Temp (K)
Enthalpy (kJ)
2500
T
2550
264872.28
268021.72
271015.32
2550T
271015.32 268021.72

25502500 271015.32 264872.28
Solving this,
T  2525.63 K
This is the adiabatic flame temperature for hydrogen when both the fuel and the air enter the
combustion chamber at 25° C.
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Oct 12, 2010
Home Problem Statement:
Liquid gasoline (C8H18) at 25° C is burned steadily with the stoichiometric amount of air at 25
°C and 1 atm. Both the fuel and the air enter the combustion chamber at 25 °C and 1 atm.
Calculate the exit temperature of the product gases, assuming combustion is complete and
adiabatic.
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Table 2.1
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Table 2.2
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Table 2.3
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Table 2.4
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Table 2.5
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Table 2.6
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