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Chapter 1 Basic Combustion

Chapter 1 Basic Combustion. Fuels and Combustion Theoretical and Actual Combustion Porcesses Enthalpy of Combustion. 1-1 Fuels and Combustion. Fuel≡Any material that can be burned (oxidized) to release energy. Hydrocarbon fuels , CnHm, eg. coal, gasoline, and natural gas.

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Chapter 1 Basic Combustion

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  1. Chapter 1 Basic Combustion • Fuels and Combustion • Theoretical and Actual Combustion Porcesses • Enthalpy of Combustion

  2. 1-1 Fuels and Combustion • Fuel≡Any material that can be burned (oxidized) to release energy. • Hydrocarbon fuels , CnHm, eg. coal, gasoline, and natural gas

  3. 1-1 Fuels and Combustion • Coal: major—carbon, minors—O2, H2, N2, S, moisture, and ash. • Most liquid hydrocarbon fuels are mixtures of many different hydrocarbons & are obtained from crude oil by distillation, eg., gasoline (octane,辛烷, C8H18 ), diesel fuel (dodecane, C12H26 ), methyl alcohol (CH3OH, methanol,甲醇) • Gaseous hydrocarbon fuel: natural gas (methane, CH4 )

  4. 1-1 Fuels and Combustion • Combustion≡A chemical reaction during which a fuel is burned (oxidized) and a large quantity of energy is released. • Air—the most often used oxidizer. • Dry air : 20.9% O2, 78.1% N2, 0.9% Ar, and small amount of CO2, He, Ne and H2, on a mole or a volume basis. ～ 21% O2, 79% N2 1 kmol O2 +3.76 kmol N2 =4.76 kmol air • N2 & H2O behave as inert gases at ordinary T. • At very high T N2 +O2 → NOx (NO & NO2) H2O → H2, O2 ,H, O, OH (dissociation)

  5. 1-1 Fuels and Combustion • Reactants Products combustion process • Conservation of mass principle Note: the total number of moles is not conserved. • Air-fuel ratio, AF ≡ mair/mfuel • H2O → H2, O2 ,H, O, OH (dissociation)

  6. 1-2 Theoretical and Actual Combustion Processes • Complete combustion ≡ C → CO2, H → H2O & S → SO2 • Incomplete combustion → Contain unburned fuel or components eg., C, H, CO, or OH ↑ insufficient oxygen, insufficient mixing, dissociation (at high T)(3T:Temperature, Time, Turbulence) • O2 is more strongly attracted to H2 → H2O, CO2, CO, C particles

  7. 1-2 Theoretical and Actual Combustion Processes • Stoichimetric or theoretical air ≡ the minimum amount of air needed for the complete combustion of a fuel • Stoichimetric or theoretical combustion ≡ A combustion process during which a fuel is burned completely with theoretical air, eg. Methane CH4+2(O2+3.76 N2) → CO2 + 2H2O + 7.52 N2 • Equivalence ratio, ψ≡AFactual/AFst ψ>1 → excess air; ψ<1 → deficient air

  8. 1-3 Enthalpy of Combustion • During a chemical reaction, assuming no nuclear reactions and disregarding any changes in kinetics and potential energies, ΔEsys= ΔEstate + ΔEchem • Enthalpy of reaction hR ≡ a property to represent the changes in chemical energy during a reaction hR ≡ Σ hprod,i -Σ hreact,i ≡ Hprod-Hreact @ a specific state (for both products and reactants) for a complete reaction

  9. 1-3 Enthalpy of Combustion • For combustion process, hR ≡ hC (enthalpy of combustion) ≡ the amount of heat released during a steady-flow combustion process when 1 kmol (or 1kg) of fuel is burned completely @ a specific T & P.

  10. 1-3 Enthalpy of Combustion • Heating value of the fuel, HV ≡ the amount of heat released when a fuel is burned completely in a steady-flow process and the products are returned to the state of the reactants. • HV depends on the phase of H2O in the products. HHV (higher heating value): H2O (liquid) in the products LHV (lower heating value):H2O (vapor) in the products HHV = LHV+ (Nhfg) H2O (kJ/kmol fuel) N: number of moles of H2O in the products hfg: enthalpy of vaporization of water @ a specific T.

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