Reacting Mixtures and Combustion

1
Chapter 13
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• Reacting Mixtures and Combustion

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Fundamentals
Chemical Equations
Modeling Air

• 79 Nitrogen, 21 Oxygen
• Only the Oxygen reacts Nitrogen considered
  inert
• Mair 28.97 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

• CH4a(0.21O20.79N2) ? bCO2cH2OdN2
• C) 11 b1 b1
• 14 c2 c2
• O) a20.21 b2c1 a9.524
• Theoretical air 9.524 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
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
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Enthalpy Reacting Systems
Tabular enthalpies inadequate due to arbitrary
reference datums
Standard Reference State (Stable Elements) Tref
298.15 K, pref 1 atm
First Law
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Enthalpy Reacting Systems
Standard Reference State Tref 25 oC Pref 1
atm
First Law
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Energy Balances Steady State
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Energy Balances Steady State
13
Energy Balances Steady State
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Closed System Energy Balance
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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
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Adiabatic Flame Temperature
When no power produced, and combustion carried
out adiabatically, Tp reaches a theoretical
maximum.
When using tables, requires iteration to
determine!
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Adiabatic Flame Temperature
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Fuel Cells
Solid Oxide Fuel Cell
Proton Exchange Membrane Fuel Cell
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Third Law of Thermodynamics
The absolute entropy of a pure-crystalline
substance at the absolute zero of temperature is
zero.
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Ideal Gas
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Entropy Balances
Control Volumes at Steady State, Reacting System
Closed, Reacting System
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Chemical Exergy
Thermo-mechanical Exergy found in Chapter 7
For a Hydrocarbon CaHb
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Chemical Exergy
For Carbon Monoxide CO
For Water H2O
For N2, O2, CO2
For mixture of gas phases of Ideal Gases at T0, p0
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Exergy Summary