Heating value, adiabatic flame temperature, air factor
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1 Heating value, adiabatic flame temperature, air factor
2 Background heating value In a boiler fuel is burned (oxidized) to flue gas components. In this process, (chemical) energy is released and bound to the flue gas components. Heat is then removed from the flue gas to heat up water/steam. The fuel heating value refers to the amount of energy released from the fuel during oxidation (MJ/kg). For solid fuels, the HV is determined experimentally. For fuels with well defined chemical structure, the heating value can be calculated from thermodynamic data.
3 HV of CH 4 from thermodynamic data 1 CH 4 (g) + 2 O 2 (g) 1 CO 2 (g) + 2 H 2 O(g) Tabulating data for heat of reaction (HV) for each reaction would require own table for each reaction (~infinite number) Instead the concept of state variable is utilized, and formation enthalpies for reaction components are tabulated, and can be used to calculate the heat of reaction (HV) for any reaction
4 HV of CH 4 from thermodynamic data 1 CH 4 (g) + 2 O 2 (g) 1 CO 2 (g) + 2 H 2 O(g) Elements Elements at their reference state; most stable state at P=1 bar, T=25 C 1 C, 4 H, 4 O C(s) 2 H 2 (g) 2 O 2 (g)
5 JANAF tables ( )
6 HV of CH 4 from thermodynamic data 1 CH 4 (g) + 2 O 2 (g) 1 CO 2 (g) + 2 H 2 O(g) Elements 1 C, 4 H, 4 O Elements at their reference state; most stable state at P=1 bar, T=25 C C(s) 2 H 2 (g) 2 O 2 (g) Agreement (definition) that ΔH f = 0
7 JANAF tables ( )
8 HV of CH 4 from thermodynamic data 1 CH 4 (g) + 2 O 2 (g) 1 CO 2 (g) + 2 H 2 O(g) Elements 1 C, 4 H, 4 O Tabulated ΔH f give the change in enthalpy when formed from elements at their reference state Elements At their reference state; most stable state at P=1 bar, T=25 C C(s) 2 H 2 (g) 2 O 2 (g) Agreement (definition) that ΔH f = 0
9 HV of CH 4 from thermodynamic data 1 CH 4 (g) + 2 O 2 (g) 1 CO 2 (g) + 2 H 2 O(g) Elements 1 C, 4 H, 4 O 1 C, 4 H, 4 O Elements At their reference state; most stable state at P=1 bar, T=25 C C(s) 2 H 2 (g) 2 O 2 (g) ΔH f = 0 C(s) 2 H 2 (g) 2 O 2 (g)
10 JANAF tables ( )
11 HV of CH 4 from thermodynamic data 1 CH 4 (g) + 2 O 2 (g) 1 CO 2 (g) + 2 H 2 O(g) Tabulated ΔH f give the change in enthalpy when formed from elements at their reference state Elements 1 C, 4 H, 4 O 1 C, 4 H, 4 O Elements At their reference state; most stable state at P=1 bar, T=25 C C(s) 2 H 2 (g) 2 O 2 (g) ΔH f = 0 C(s) 2 H 2 (g) 2 O 2 (g)
12 HV of CH 4 from thermodynamic data Going either way results in same change in state: state 1 to state 2 1 CH 4 (g) + 2 O 2 (g) 1 CO 2 (g) + 2 H 2 O(g) Tabulated ΔH f give the change in enthalpy when formed from elements at their reference state Elements 1 C, 4 H, 4 O 1 C, 4 H, 4 O Elements At their reference state; most stable state at P=1 bar, T=25 C C(s) 2 H 2 (g) 2 O 2 (g) ΔH f = 0 C(s) 2 H 2 (g) 2 O 2 (g)
13 Tabulated ΔH f give the change in enthalpy when formed from elements at their reference state HV of CH 4 from thermodynamic data Going either way results in same change in state: state 1 to state 2 1 CH 4 (g) + 2 O 2 (g) 1 CO 2 (g) + 2 H 2 O(g) Reactants ΔH f Note the direction of ΔH f and path to get from state 1 to state 2 Products ΔH f C(s) 2 H 2 (g) 2 O 2 (g) C(s) 2 H 2 (g) 2 O 2 (g)
14 HV of CH 4 from thermodynamic data ΔH f (kj/mol): HV from ΔH f at 25 C 1 CH 4 (g) + 2 O 2 (g) 1 CO 2 (g) + 2 H 2 O(g)
15 HV of CH 4 from thermodynamic data HV from ΔH f at 25 C 1 CH 4 (g) + 2 O 2 (g) 1 CO 2 (g) + 2 H 2 O(g) ΔH f (kj/mol): ΔH f (kj): 1 x x 0 1 x x ΔH f RXN (kj): -(1 x x 0) +(1 x x ) ΔH f RXN (kj): (1 x x ) - (1 x x 0) =
16 HV of CH 4 from thermodynamic data HV from ΔH f at 25 C 1 CH 4 (g) + 2 O 2 (g) 1 CO 2 (g) + 2 H 2 O(g) ΔH f (kj/mol): ΔH f (kj): 1 x x 0 1 x x ΔH f RXN (kj): -(1 x x 0) +(1 x x ) ΔH f RXN (kj): (1 x x ) - (1 x x 0) = For each mole CH 4 that is oxidized, kj energy needs to be removed from system to maintain temperature 25 C. HV for CH 4 : kj/mol at 25 C
17 Background adiabatic flame temperature In a boiler fuel is burned (oxidized) to flue gas components. In this process, (chemical) energy is released and bound to the flue gas components. Heat is then removed from the flue gas to heat up water/steam. Adiabatic (flame) temperature refers to the theoretical, limiting situation where no heat is removed from flue gas; instead all energy released from combustion goes to heat up flue gas components (CO 2, H 2 O, O 2, N 2,...)
18 Adiabatic flame temperature T init T adiabatic > T init CH 4 (g) CO 2 (g) H 2 O (g) O 2 (g) N 2 (g) O 2 (g) N 2 (g) Above desribes situation of complete combustion (all CH 4 to CO 2 and H 2 O). Tad applies also in situations of incomplete combustion (deficiency of O 2 ). Tad can also be calculated considering equilibrium/dissociation due to high T (even if enough O 2, at high temperature not all C is fully oxidized to CO 2 )
19 How to calculate adiabatic flame temperature? T init T adiabatic > T init CH 4 (g) CO 2 (g) H 2 O (g) N 2 (g) O 2 (g) N 2 (g) O 2 (g) Data needed 1) Energy released from oxidation (HV) 2) Enthalpy of flue gas components at different temperatures (Cp) 3) T init E = m Cp (T ad T init )
20 Adiabatic flame temperature CH 4 JANAF tables ( ) Cp (T T ref ) Cp (T T ref ) Data needed 1) Energy released from oxidation (HV) 2) Enthalpy of flue gas components at different temperatures (Cp) 3) T init E = m Cp (T ad T init )
21 H-Href(kJ/mol) Adiabatic flame temperature CH CH 4 + 2O 2 1CO 2 + 2H 2 O CO2 H2O T ( C) Data needed 1) Energy released from oxidation (HV) 2) Enthalpy of flue gas components at different temperatures (Cp) 3) T init E = m Cp (T ad T init )
22 H-Href(kJ) Adiabatic flame temperature CH CH 4 + 2O 2 1CO 2 + 2H 2 O Considering stoichiometry 1CO 2 + 2H 2 O CO2 H2O CO + H2O = flue gas T ( C) Data needed 1) Energy released from oxidation (HV) 2) Enthalpy of flue gas components at different temperatures (Cp) 3) T init E = m Cp (T ad T init )
23 H-Href(kJ) Adiabatic flame temperature CH CH 4 + 2O 2 1CO 2 + 2H 2 O Change in enthalpy due to heat of reaction (HV) CO2 H2O 400 CO + H2O = flue gas 300 Heat of reaction 200 Tad T ( C) Data needed 1) Energy released from oxidation (HV) 2) Enthalpy of flue gas components at different temperatures (Cp) 3) T init E = m Cp (T ad T init )
24 H-Href(kJ) Adiabatic flame temperature CH CH 4 + 2O 2 1CO 2 + 2H 2 O Change in enthalpy due to heat of reaction (HV) HV calculated at T init CO2 H2O CO + H2O = flue gas 400 Heat of reaction 200 Increase in temperature (T init to T ad ) Tad Tinit T ( C) Data needed 1) Energy released from oxidation (HV) 2) Enthalpy of flue gas components at different temperatures (Cp) 3) T init E = m Cp (T ad T init )
25 Air factor (λ) = Air factor (λ) 1CH 4 + 2O 2 1CO 2 + 2H 2 O 1CH 4 + 1O 2? 1CH 4 + 3O 2? Air (oxidizer) Fuel Stoichiometric air (oxidizer) Fuel Air-to-fuel ratio in e.g. a boiler or a boiler section = Air (oxidizer) Stoichiometric air (oxidizer) Air-to-fuel ratio at which just enough oxygen to oxidize fuel completetly; see top-most reaction equation
26 Air factor (λ) λ 1, oxidizing conditions λ < 1, reducing conditions Air factor (λ) = Air (oxidizer) Fuel Stoichiometric air (oxidizer) Fuel = Air (oxidizer) Stoichiometric air (oxidizer)
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