Basic Combustion Basic Combustion

Fuels and CombustionFuels and Combustion

Theoretical and Actual Combustion Theoretical and Actual Combustion PorcessesPorcesses

Enthalpy of CombustionEnthalpy of Combustion

1-1 Fuels and Combustion1-1 Fuels and Combustion Fuel≡Any material that can be burned (oxidized) to Fuel≡Any material that can be burned (oxidized) to

release energy.release energy.

Hydrocarbon fuels , CnHm, eg. coal, gasoline, and Hydrocarbon fuels , CnHm, eg. coal, gasoline, and natural gasnatural gas

1-1 Fuels and Combustion1-1 Fuels and Combustion Coal: major—carbon, minors—OCoal: major—carbon, minors—O22, H, H22, N, N22, S, , S,

moisture, and ash.moisture, and ash.

Most liquid hydrocarbon fuels are mixtures of many Most liquid hydrocarbon fuels are mixtures of many different hydrocarbons & are obtained from crude different hydrocarbons & are obtained from crude oil by distillation, eg., gasoline (octane,oil by distillation, eg., gasoline (octane, 辛烷辛烷 , , CC88HH18 18 ), diesel fuel (dodecane, ), diesel fuel (dodecane, CC1212HH26 26 ), methyl ), methyl

alcohol (Calcohol (CHH33OH, methanol,OH, methanol,甲醇甲醇 ) )

Gaseous hydrocarbon fuel: natural gas (methane, Gaseous hydrocarbon fuel: natural gas (methane, CCHH4 4 ))

1-1 Fuels and Combustion1-1 Fuels and Combustion CombustionCombustion≡A chemical reaction during which a fuel is ≡A chemical reaction during which a fuel is

burned (oxidized) and a large quantity of energy is burned (oxidized) and a large quantity of energy is released. released.

Oxidant

Oxidant

Fuel

Reactants W Q

H2O

CO2

Products

1-1 Fuels and Combustion1-1 Fuels and Combustion

Air—the most often used oxidizer. Air—the most often used oxidizer. 1.1. Dry air : 20.9% ODry air : 20.9% O22, 78.1% N, 78.1% N22, 0.9% Ar, and small amount , 0.9% Ar, and small amount

of COof CO22, He, Ne and H, He, Ne and H22, on a mole or a volume basis. , on a mole or a volume basis. ～ ～ 21% O21% O22, 79% N, 79% N22

1 kmol O1 kmol O22 +3.76 kmol N +3.76 kmol N22 =4.76 kmol air =4.76 kmol air

2.2. NN2 2 & H& H22O behave as inert gases at ordinary T.O behave as inert gases at ordinary T.

3.3. At very high TAt very high T

NN2 2 +O+O2 2 → NO→ NOx x (NO & NO (NO & NO22) )

HH22O → HO → H22, O, O22 ,H, O, OH (dissociation) ,H, O, OH (dissociation)

1-1 Fuels and Combustion1-1 Fuels and Combustion Reactants Products Reactants Products

combustion processcombustion process

Conservation of mass principleConservation 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 Air-fuel ratio, AF ≡ ≡ mmairair//mmfuelfuel

1.1. HH22O → HO → H22, O, O22 ,H, O, OH (dissociation) ,H, O, OH (dissociation)

1-2 Theoretical and Actual 1-2 Theoretical and Actual Combustion ProcessesCombustion Processes

Complete combustion ≡ C Complete combustion ≡ C → CO→ CO22, H , H → H→ H22OO & &

S S → SO→ SO22

Incomplete combustion Incomplete combustion → Contain unburned → Contain unburned fuel or components eg., C, H, CO, or OHfuel or components eg., C, H, CO, or OH

↑ ↑ insufficient oxygen, insufficient mixing, insufficient oxygen, insufficient mixing,

dissociation (at high T)(3T:Temperature, Time, dissociation (at high T)(3T:Temperature, Time, Turbulence)Turbulence)

OO22 is more strongly attracted to is more strongly attracted to HH2 2 → H→ H22O, O, COCO22, CO, C particles, CO, C particles

1-2 Theoretical and Actual 1-2 Theoretical and Actual Combustion ProcessesCombustion Processes

Stoichimetric or theoretical air ≡ the minimum amount Stoichimetric or theoretical air ≡ the minimum amount of air needed for the complete combustion of a fuelof air needed for the complete combustion of a fuel

Stoichimetric or theoretical combustion ≡ A combustion Stoichimetric or theoretical combustion ≡ A combustion process during which a fuel is burned completely with process during which a fuel is burned completely with theoretical air, eg. Methanetheoretical air, eg. Methane

CHCH44+2(O+2(O22+3.76 N+3.76 N22) → CO) → CO22 + 2H + 2H22O + 7.52 NO + 7.52 N22

Equivalence ratio, ψ≡Equivalence ratio, ψ≡AFAFactualactual//AFAFstst

ψ>1 ψ>1 → excess air; → excess air; ψ<1 ψ<1 → deficient air→ deficient air

1-3 Enthalpy of Combustion1-3 Enthalpy of Combustion

During a chemical reaction, assuming no nuclear During a chemical reaction, assuming no nuclear reactions and disregarding any changes in reactions and disregarding any changes in kinetics and potential energies,kinetics and potential energies,

ΔEΔEsyssys= = ΔEΔEstatestate + + ΔEΔEchemchem Enthalpy of reaction hEnthalpy of reaction hRR ≡ a property to represent ≡ a property to represent

the changes in chemical energy during a reactionthe changes in chemical energy during a reaction

hhRR ≡ Σ h ≡ Σ hprod,iprod,i -Σ h -Σ hreact,i react,i ≡ H≡ Hprodprod-H-Hreact react @ a specific @ a specific state (for both products and reactants) for a state (for both products and reactants) for a complete reactioncomplete reaction

1-3 Enthalpy of Combustion1-3 Enthalpy of Combustion

For combustion process, For combustion process,

hhRR ≡ h ≡ hCC (enthalpy of combustion) (enthalpy of combustion)

≡ ≡ the amount of heat released during athe amount of heat released during a

steady-flow combustion process when steady-flow combustion process when

1 kmol (or 1kg) of fuel is burned completely1 kmol (or 1kg) of fuel is burned completely

@ a specific T & P.@ a specific T & P.

1-3 Enthalpy of Combustion1-3 Enthalpy of Combustion

Heating value of the fuel, Heating value of the fuel, HVHV ≡ the amount of heat ≡ the amount of heat released when a fuel is burned completely in areleased when a fuel is burned completely in a

steady-flow process and the products are returned to the steady-flow process and the products are returned to the state of the reactants.state of the reactants.

HVHV depends on the phase of depends on the phase of HH22O in the products.O in the products.

HHVHHV (higher heating value): (higher heating value): HH22O (liquid) in the productsO (liquid) in the products

LHVLHV (lower heating value):H (lower heating value):H22O (vapor) in the productsO (vapor) in the products

HHVHHV = = LHVLHV + (+ (NhNhfgfg) ) H2OH2O (kJ/kmol fuel) (kJ/kmol fuel)

NN: number of moles of : number of moles of HH22OO in the products in the products

hhfgfg: enthalpy of vaporization of water @ a specific T.: enthalpy of vaporization of water @ a specific T.

1-3 Enthalpy of Combustion1-3 Enthalpy of Combustion

1-3 Enthalpy of Combustion1-3 Enthalpy of Combustion

3T: long enough 3T: long enough TimeTime for mixing for mixing

high enough high enough TemperatureTemperature

strong enough strong enough TurbulenceTurbulence