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##### Chapter 13

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**Chapter 13**Reacting Mixtures and Combustion Photo courtesy of www.freefoto.com**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:**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)**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)**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.**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)**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)**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**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:**Enthalpy: Reacting Systems**Standard Reference State Tref = 25 oC Pref = 1 atm First Law:**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**Adiabatic Flame Temperature**When no power produced, and combustion carried out adiabatically, Tp reaches a theoretical maximum. When using tables, requires iteration to determine!**Fuel Cells**Solid Oxide Fuel Cell Proton Exchange Membrane Fuel Cell**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**Entropy Balances**Control Volumes at Steady State, Reacting System Closed, Reacting System**Chemical Exergy**Thermo-mechanical Exergy found in Chapter 7 For a Hydrocarbon: CaHb**Chemical Exergy**For Carbon Monoxide: CO For Water: H2O For N2, O2, CO2 For mixture of gas phases of Ideal Gases at T0, p0