Describes thermodynamics of combustion and temperature-enthalpy diagram

1. Alfa Sharma
M.Tech(1st yr.)
Redg.No-13307002
Centre for Green Energy Technology
Pondicherry University
INDIA-6050143/23/2014 1

2. INTRODUCTION
 Major thermodynamic functions of combustion
processes which fundamentally influence the
utilisation of fuels in diverse appliances are given as;
(i)Heat of combustion
(ii)Equilibrium constant of reactions during
combustion
(iii)Enthalpy of combustion systems
(iv)Flame temperature
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3. HEAT OF COMBUSTION
 Also known as potential heat of fuel.
 Can be calculated by applying Hess’s law.
 Heat of combustion(Hc) of carbon depends on its
allotropic form.
 In physics and thermo chemistry,β-graphite is
used as a basis of heat of formation(ΔHf =0). But
in technical processes amorphous carbon like coke
carbon is the basis.
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4. Contd......
 Application of Hc :-
Calorific value can be determined easily from heat of
combustion. For example,
For carbon , CV = 97000/12 = 8083Kcal/kg
For any substance, GCV = ΔHc /22.4 .........(1)
for CH4 = 212798/ 22.4 = 9500 Kcal/Nm3
%η(efficiency of combustion) =
(potential heat in flue gas +potential heat in refuse)
potential heat in fuel ×100
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5. ENTHALPY OF COMBUSTION SYSTEM
 Heat transfer from the combustion gases takes place at
fairly constant pressure not far from the atmospheric.
...............(2)
Where,
Q - heat absorbed by surrounding
-ΔH – enthalpy decrease of the system
 Enthalpy change is the change of heat content of system at
constant pressure.
 Knowledge of enthalpy is in relation to a reference
state(0ᴼC,760 mm) is sufficient.
Q = -ΔH
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6. CONTD...............
 Enthalpy of a gas at temperature tᴼC is,
ΔHt = Cp(0-t) × t1 ..............................(3)
Where,
Cp(0-t) – mean specific heat between reference temperature
0ᴼC and the given temp. tᴼC expressed in Kcal/Nm3
ᴼC(volume basis) and Kcal/kg ᴼC(in weight basis)
Hence the enthalpy change between two temperatures t1 &t2
can be determined by,
ΔHt(1) – ΔHt(2) = ( Cp(0-t1)×t1) – ( Cp(0-t2)×t2) .............(4)
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7.  For mixture of gases,
(Cp(0-t))mix = Xa × (Cp(0-t))a + Xb ×(Cp(o-t))b ..........(5)
where,
(Cp(0-t))a and (Cp(0-t))b - mean specific heat of components of a
and b.
Xa and Xb - volume fraction or weight fraction of components.
Hence enthalpy of mixture of gases is given by,
(ΔH(0-t))mix = (Cp(0-t))mix × t ...................................(6)
 Specific heat and enthalpy of combustion gases are used in
calculation of flame temperature , heat loss with flue gas
and furnace efficiency.
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8. EQUILIBRIUM CONSTANTS OF COMBUSTION
REACTION
 Reactions like dissociation of water vapour, carbon dioxide
are endothermic in which a part of heat of combustion
gases into potential heat in form of Hc of combustible
components formed.
H2O ↔ H2 + ½ O2, ΔH = +57798Kcal
H2O↔ OH + ½ H2, ΔH = +67858 Kcal
CO2 ↔ CO + ½ O2, ΔH = +67636 kcal
H2 ↔ 2(H) , ΔH = +104178 Kcal
O2 ↔ 2(O) , ΔH = +118318 Kcal
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9.  Combustion of fuels is rendered incomplete at high
temperature has 3 effects;
(i) Combustion efficiency is lowered
(ii) Temp. of system falls
(iii)Dissociation is an increase in volume and no. of moles of
gases.
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10. Enthalpy- Temp Diagram
 On complete combustion,
ΔH(flue gas) = Σ(ΔH(theoretical flue gas) + ΔH(excess air)) ...(7)
 For a given fuel enthalpy of flue gas depends on function of two
variables i.e. temp.& excess air.
 At temp. above than 1600ᴼC endothermic effect of dissociation
reaction takes place which forms the basis of enthalpy-
temperature diagram(Ht- or It- diagram).
 Ht- diagram covers the temp. range of 100- 2500ᴼC and shows
relationship between enthalpy and temp. for different air content
of flue gas.
 It is useful in rapid workout of problems concerning heat loss
from flue gas and flame temperature.
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12. FLAME TEMPERATURE
 It is the average temperature attained by combustion products of a mixture
of fuel and oxidant.
 Classified into 4 types as;
(i)Theoretical flame temperature
(ii)Adiabatic flame temperature
(iii)Actual flame temperature
(iv)Maximum adiabatic flame temperature or Maximum flame
temperature
 The theoretical flame temp. is not a tangible concept while others are.
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13.  Theoretical flame temp. is the resultant temp. obtained when
combustion of fuel is complete and entire heat of combustion goes to
heat the products of combustion .
 But in reality combustion never completed at high temperature owing
to dissociation reactions.
 Adiabatic flame temp. Come into picture when endothermic effect of
dissociation reaction taken into account which is lower than theoretical
flame temp..
 Whereas actual flame temp. is the resultant average temp. of
combustion products as always some heat is losed to the surrounding
of the system.
 All the above types of flame temperature depends on composition of
fuel- oxidant mixture.
(a) Quantity of oxidant is low- incomplete combustion
(b) Quantity of oxidant is large – dilute the products and heat taken away
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14. Theoretical flame temp. has maximum value at
stoichiometric composition of fuel and oxidant.
With increase of temperature the degree of
dissociation markedly increases enhancing the
heat loss.
Hence maximum adiabatic flame temperature is
realised when fuel is slightly in excess of
stoichiometric composition.
Again flame temperature of a fuel is much higher
in oxygen than in air because of high N- content of
air takes away a significant quantity of heat.
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16. Flame Temp. Calculation
 By balancing a heat equation between fuel and air on one hand
and combustion products on the other hand as given below,
CN + ΔHfuel +AΔHair = VΔHwg + Qloss + Qdiss. ........(8)
Where,
CN - Net calorific value of fuel (in Kcal/Nm3)
ΔHfuel – Enthalpy of fuel above reference temperature (in Kcal/Nm3)
ΔHair – Enthalpy of air above reference temperature
ΔHwg - Enthalpy of combustion products above reference temperature
A – Air supplied
V – Combustion gases produced (in Nm3)
Qloss – Heat loss to the surrounding
Qdiss. - Heat loss by dissociation
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17. But,
ΔHwg = tf Cp wg,(0-tf) – tCp wg,(0-t) ...........(9)
tf – flame temp.
t - reference temp.(25ᴼC)
Cp wg,(0-tf) – Mean specific heat of combustion products between tf
and 0ᴼC
Cp wg,(0-t) – Mean sp. heat of combustion products between t and 0ᴼC.
Hence,
tf = CN+ ΔHfuel + AΔHair + Qdiss – Qloss + Vt Cp wg,(0-t)
VCp wg,(0-tf) ....(10)
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18.  Flame temp. has significance as it governs thermal
efficiency of transference of heat from flames to heating
surface.
η =(Tf – Ts)/Tf = 1-Ts/Tf ..................(12)
• Hence higher the Tf value ,greater is the efficiency.
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19. REFERENCES:-
Fuel &Combustion, S. Sarkar (3rd edition ,Universities
Press)
Thermodynamics for Chemists, S. Glasstone
Basic Thermo-chemistry, I.L.Levine
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