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Energy Equilibrium Combustion Thermodynamics
Prof. Asish K Sarangi
Dept. of Energy Science and Engineering
IIT BOMBAY
Prof. Asish Kumar Sarangi,
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Dept.for
of Energy
Energy.Science
Department
and Engineering,
of Energy Science
IIT Bombay.
and Engineering
Email – asish.sarangi@iitb.ac.in
Learning objectives
• Determine the adiabatic flame temperature
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Reactions for Energy. Department of Energy Science and Engineering
Adiabatic flame temperature
If a fuel-air mixture burns adiabatically at constant pressure, the
absolute enthalpy of the reactants at the initial state equals the
absolute enthalpy of the products at the final state.
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Reactions for Energy. Department of Energy Science and Engineering
Adiabatic flame temperature
Example 5: Determine the constant pressure adiabatic flame temperature for the
combustion of a stoichiometric methane-air mixture. The pressure is 1 atm and the
initial reactant temperature is 298 K. Use the following assumptions.
a. Complete combustion i.e., the product mixture consists of only CO2, H2O and N2.
b. The product mixture enthalpy is estimated using constant specific heats
evaluated at 1200 K, where Tad is guessed to be 2100 K.
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Reactions for Energy. Department of Energy Science and Engineering
Adiabatic flame temperature (5)
Dissociation of products at high temperature at ambient
pressure:
Example: The space shuttle burns H2 and O2 in the main engine. To
estimate the maximum flame temperature, let’s consider combustion of 1
mol of gaseous H2 with ½ mol of gaseous O2 at STP. Determine the adiabatic
flame temperature.
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Equilibrium composition of products
Example 3: In fuel rich combustion products mixtures, equilibrium
between the species CO2, H2O, CO and H2 is often assumed to determine the
burned gas composition. For Φ=1.2, for C8H18-air combustion products,
determine the mole fractions of the product species at 1700 K.
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Reactions for Energy. Department of Energy Science and Engineering
Background on equilibrium calculation
Thermodynamics of combustion shows that if we know the
initial state of a reactive gas mixture, the final state of the
mixture after chemical equilibrium can be determined.
But the chemical equilibrium calculation does not tell us
- How does the mixture get from initial state to final state?
- How long does it take to complete the reaction?
Example: The equilibrium constant of the reaction at 298 K is
1040
H2+O2
H2O
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Next Lecture
▪ Chemical kinetics
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Reactions for Energy. Department of Energy Science and Engineering