DISCIPLINA MIEEA 2018

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2 DISCIPLINA MIEEA 2018

3 Technologies of combustion

4 Combustion definition Combustion is essentially burning, fuels react with oxygen to release energy 4

5 Combustion use in the world No Combustion Combustion Combustion 5

6 + Q out Fuel T in Air Combustion Chamber Products T out 6

7 + Air composition 21% O2 and 79% N2 (by volume) / λ= # $ # $ % excess air coefficient λ<1 No sufficient air; fuel is not completely burned λ=1 Exact amount air, fuel is completely burned λ>1 Excess air; fuel is completely burned φ= & λ equivalence ratio # $ e(%)= ' # $ % # $ % *100%= &'φ = excess air in % φ 7

8 P#2 A combustion chamber burns propane, C3H8 with excess air. Dry analysis (excuding water) of combustion products was: 2%O2, 12.4% CO2 and 85.6% N2. Determine: a) The excess air. b) The coefficient of air excess. c) The equivalence ratio. 8

9 + Q out Fuel T in Air Combustion Chamber Products T out 9

10 1st Law thermodynamics conservation of energy Closed system Open system 10

11 Open system ( Massa que entra) ( Massa que sai) = ( Variação total massa sistema) Regime permanente/estacionário e m& e m& s = s dm dt vc = m& e m& A = área secção normal à direcção escoamento e s s dmvc, = 0 dt ρ= massa volumica kg/m 3 v n = velocidade normal a A m/s vc Caudal mássico (kg/s) m& = ρv da n A 11

12 Open system 1ªLei Conservação energia E = EP + EC + U = Q W 12

13 Open system Usando definição de entalpia 13

14 Closed system Open system Change internal energy (U), kinetic energy, potential energy Mass inside the system boundary (control volume) of internal combustion engine is constant, mair+mfuel 14

15 (( + q cv ) + *, ) + - (J) =0 -q cv =h products -h reactants (J/kg) =0 regime estacionário 15

16 16 M R R RT pv Z K kmol kj R = = =. / R T c T c RT T u T h h T u u RT pv v p + = + = = = = ) ( ) ( ) ( ) ( ) ( Constante universal dos gases Factor de compressibilidade kj/kg.k Z=1 é gas perfeito = = ) ( ) ( ) ( ) ( ) ( ) ( T T p T T v dt T c T h T h dt T c T u T u Thermochemistry

17 Temperatura e pressão reduzidas T p R R = = T T c p p c 17

18 Heat release 18

19 Heat release 19

20 -q cv =h products -h reactants (J/kg) Enthalpy of reaction = h R q cv = Heat of reaction 20

21 Equation HESS LAW total enthalpy change during the complete course of a chemical reaction is the same whether the reaction is made in one step or in several steps 21

22 HEATING VALUE AND ENTHALPY OF REACTION q cv Constant pressure 25 ο C 150 ο C LHV = qcv heat of vaporization of water (2257 kj/kg) in the reaction products is not recovered 22

23 + T ref = 25 ο C ( K) Standard reference state P ref = 1 atm ( kpa = 1 bar) h ref = =u+pv (J/kg) (J/kg) 23

24 CO 2 24

25 CO 2 25

26 H 2 O 26

27 H 2 O 27

28 N 2 28

29 N 2 29

30 P#3 Determine the reference enthalpy of combustion of methane, CH 4, with air with λ=1. what is the lower and higher heating value for methane combustion? 30

31 Fuel Chemical LHV Density LHV CO 2 (kg/l) CO 2 (g/mj) formula (MJ/kg) (kg/l) (MJ/L) Gasolina C n H 1.87n Gasóleo C n H 1.8n Etanol C 2 H 6 O Biodiesel C n H 2n O Gás CH natural (kg/m 3 )* (MJ/m 3 ) (kg/m 3 ) 55.0 GPL C 3 H (2 kg/m 3 )*

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34 P#4 34

35 P#5 Determine the upper and lower heating values at 298 K of gaseous n-decane, C10H22, per molof fuel and per kg of fuel, and (A/F)st (molar and mass basis). If the enthalpy of vaporization of n-decade is 359 kj/kgfuelat 298 K, what are the upper and lower heating values of liquid n-decane? 35

36 Constant pressure Adiabatic flame temperature = T ad Ti T ad H=0 for constant pressure 36

37 Constant volume Adiabatic flame temperature = T ad 37

38 Q out Fuel T in Air Combustion Chamber Products T out Maximum heat release, max Q out : T out =T in Maximum flame temperature, T ad : Hreag(T in )=Hprod (T ad )(constant pressure, e.g. Diesel engine, gas turbine, furnace) Hreag(T in )=Hprod (T ad )-R(n prod T ad -n reag T in )(constant volume, e.g. gasoline engine) 38

39 P#6 Estimate the constant-pressure adiabatic flame temperature for the combustion of a stoichiometric CH4-air mixture. The pressure is 1 atm and the initial reactant temperature is 298 K. Make the necessary assumptions. 39

40 P#7 Estimate the constant-volume adiabatic flame temperature for the combustion of a stoichiometric CH4-air mixture. The initial pressure is 1 atm and the initial reactant temperature is 298 K. Make the necessary assumptions. 40

41 Thanks

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