1. Reacting Mixtures and Combustion
Chapter 13
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Reacting Mixtures and Combustion

2. Fundamentals Chemical Equations: Modeling Air:
79% Nitrogen, 21% Oxygen
Only the Oxygen reacts: Nitrogen considered inert
Mair = kg/kmol or lb/lbmol (Tables A-1)
Air Fuel Ratio:

3. Fundamentals Common fuels modeled as simple hydrocarbons:
Natural Gas  Methane (CH4)
Gasoline  Octane (C8H18)
Diesel  Dodecane (C12H26)
Chemical Equations:
Stoichiometric Coefficients (Four equations and four unknowns)
Theoretical Air:
The minimum amount of air that provides the necessary oxygen
for complete combustion (i.e. For one mole of octane the
theoretical air is 59.5 moles)

4. Fundamentals Percent Excess Air:
The percent of air supplied that is in excess of the theoretical air
Example: Combustion of Octane with 50% excess air (or 150% theoretical)
Stoichiometric Coefficients (Two equations and two unknowns)

5. Quiz
One kg/min of methane is burned in a combustor with 25% excess air.
The temperature and pressure of the air and fuel are 25oC and 101kPa
respectively. The design velocity for each intake is 15 m/s. Determine the
diameter of the air intake line in meters.

6. Quiz
One kg/min of methane is burned in a combustor with 25% excess air.
The temperature and pressure of the air and fuel are 25oC and 101kPa
respectively. The design velocity for each intake is 15 m/s. Determine the
diameter of the air intake line in meters.
Theoretical Air
CH4+a(0.21O2+0.79N2)  bCO2+cH2O+dN2
C) 1*1 = b* b=1
1*4 = c* c=2
O) a*2*0.21 = b*2+c*1 a=9.524
Theoretical air = kmol(air)/kmol(fuel)

7. Quiz
One kg/min of methane is burned in a combustor with 25% excess air.
The temperature and pressure of the air and fuel are 25oC and 101kPa
respectively. The design velocity for each intake is 15 m/s. Determine the
diameter of the air intake line in meters.
Actual Mass flow rate of air
Actual Air = Theoretical * (1+%excess)
= 9.524*1.25
= 11.9 kmol(air)/kmol(fuel)

8. Quiz
One kg/min of methane is burned in a combustor with 25% excess air.
The temperature and pressure of the air and fuel are 25oC and 101kPa
respectively. The design velocity for each intake is 15 m/s. Determine the
diameter of the air intake line in meters.
Diameter of air intake

9. Enthalpy: Reacting Systems
Tabular enthalpies inadequate due to arbitrary reference datums
Standard Reference State (Stable Elements): Tref = K, pref = 1 atm
First Law:

10. Enthalpy: Reacting Systems
Standard Reference
State
Tref = 25 oC
Pref = 1 atm
First Law:

11. Energy Balances: Steady State

12. Energy Balances: Steady State

13. Energy Balances: Steady State

14. Closed System Energy Balance

15. Fuel Enthalpies Enthalpy of Combustion
For Example: A Control Volume at Steady State
LHV (Lower Heating Value): The enthalpy of combustion when the
reactants and products are at the standard reference state and the
water formed by combustion is a gas
HHV (Higher Heating Value): …water formed by combustion is a liquid

16. Adiabatic Flame Temperature
When no power produced, and combustion carried out adiabatically, Tp reaches a theoretical maximum.
When using tables, requires iteration to determine!

17. Adiabatic Flame Temperature

18. Fuel Cells
Solid Oxide Fuel Cell
Proton Exchange Membrane Fuel Cell

19. Third Law of Thermodynamics
The absolute entropy of a pure-crystalline substance at the absolute zero of temperature is zero.
Clip art courtesy of MS Office 2000
Ideal Gas

20. Entropy Balances Control Volumes at Steady State, Reacting System
Closed, Reacting System

21. Chemical Exergy Thermo-mechanical Exergy found in Chapter 7
For a Hydrocarbon: CaHb

22. Chemical Exergy For Carbon Monoxide: CO For Water: H2O For N2, O2, CO2
For mixture of gas phases of Ideal Gases at T0, p0

23. Exergy Summary