Chapter 13

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# Chapter 13 - PowerPoint PPT Presentation

##### Chapter 13

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1. Chapter 13 Reacting Mixtures and Combustion Photo courtesy of www.freefoto.com

2. 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 • CH4+a(0.21O2+0.79N2)  bCO2+cH2O+dN2 • C) 1*1 = b*1 b=1 • 1*4 = c*2 c=2 • O) a*2*0.21 = b*2+c*1 a=9.524 • 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 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 = 298.15 K, pref = 1 atm First Law:

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

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!