Basic Combustion Fuels and Combustion Fuels and Combustion Theoretical and Actual Combustion Porcesses Theoretical and Actual Combustion Porcesses Enthalpy of Combustion Enthalpy of Combustion

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

1-1 Fuels and Combustion Coal: major—carbon, minors—O 2, H 2, N 2, S, moisture, and ash. Coal: major—carbon, minors—O 2, H 2, N 2, 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, C 8 H 18 ), diesel fuel (dodecane, C 12 H 26 ), methyl alcohol (CH 3 OH, methanol, 甲醇 ) Most liquid hydrocarbon fuels are mixtures of many different hydrocarbons & are obtained from crude oil by distillation, eg., gasoline (octane, 辛烷, C 8 H 18 ), diesel fuel (dodecane, C 12 H 26 ), methyl alcohol (CH 3 OH, methanol, 甲醇 ) Gaseous hydrocarbon fuel: natural gas (methane, CH 4 ) Gaseous hydrocarbon fuel: natural gas (methane, CH 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. Combustion ≡A chemical reaction during which a fuel is burned (oxidized) and a large quantity of energy is released. Oxidant Fuel Reactants W Q H2OH2O CO 2 Products

1-1 Fuels and Combustion Air—the most often used oxidizer. Air—the most often used oxidizer. 1.Dry air : 20.9% O 2, 78.1% N 2, 0.9% Ar, and small amount of CO 2, He, Ne and H 2, on a mole or a volume basis. ～ 21% O 2, 79% N 2 1 kmol O kmol N 2 =4.76 kmol air 1 kmol O kmol N 2 =4.76 kmol air 2.N 2 & H 2 O behave as inert gases at ordinary T. 3.At very high T N 2 +O 2 → NO x (NO & NO 2 ) N 2 +O 2 → NO x (NO & NO 2 ) H 2 O → H 2, O 2,H, O, OH (dissociation) H 2 O → H 2, O 2,H, O, OH (dissociation)

1-1 Fuels and Combustion Reactants Products Reactants Products combustion process combustion process Conservation of mass principle Conservation of mass principle Note: the total number of moles is not conserved. Note: the total number of moles is not conserved. Air-fuel ratio, AF ≡ m air /m fuel Air-fuel ratio, AF ≡ m air /m fuel 1.H 2 O → H 2, O 2,H, O, OH (dissociation)

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

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 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 Stoichimetric or theoretical combustion ≡ A combustion process during which a fuel is burned completely with theoretical air, eg. Methane CH 4 +2(O N 2 ) → CO 2 + 2H 2 O N 2 CH 4 +2(O N 2 ) → CO 2 + 2H 2 O N 2 Equivalence ratio, ψ≡AF actual /AF st Equivalence ratio, ψ≡AF actual /AF st ψ>1 → excess air; ψ 1 → excess air; ψ<1 → deficient air

1-3 Enthalpy of Combustion During a chemical reaction, assuming no nuclear reactions and disregarding any changes in kinetics and potential energies, During a chemical reaction, assuming no nuclear reactions and disregarding any changes in kinetics and potential energies, ΔE sys = ΔE state + ΔE chem ΔE sys = ΔE state + ΔE chem Enthalpy of reaction h R ≡ a property to represent the changes in chemical energy during a reaction Enthalpy of reaction h R ≡ a property to represent the changes in chemical energy during a reaction h R ≡ Σ h prod,i -Σ h react,i ≡ H prod -H a specific state (for both products and reactants) for a complete reaction h R ≡ Σ h prod,i -Σ h react,i ≡ H prod -H a specific state (for both products and reactants) for a complete reaction

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

1-3 Enthalpy of Combustion Heating value of the fuel, HV ≡ the amount of heat released when a fuel is burned completely in a 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. steady-flow process and the products are returned to the state of the reactants. HV depends on the phase of H 2 O in the products. HV depends on the phase of H 2 O in the products. HHV (higher heating value): H 2 O (liquid) in the products LHV (lower heating value):H 2 O (vapor) in the products HHV = LHV + (Nh fg ) H2O (kJ/kmol fuel) HHV = LHV + (Nh fg ) H2O (kJ/kmol fuel) N: number of moles of H 2 O in the products N: number of moles of H 2 O in the products h fg : enthalpy of vaporization of a specific T. h fg : enthalpy of vaporization of a specific T.

1-3 Enthalpy of Combustion

3T: long enough Time for mixing 3T: long enough Time for mixing high enough Temperature high enough Temperature strong enough Turbulence strong enough Turbulence