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Centre for Green Energy Technology
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
(iii)Enthalpy of combustion systems
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
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.
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
ENTHALPY OF COMBUSTION SYSTEM
Heat transfer from the combustion gases takes place at
fairly constant pressure not far from the atmospheric.
Q - heat absorbed by surrounding
-ΔH – enthalpy decrease of the system
Enthalpy change is the change of heat content of system at
Knowledge of enthalpy is in relation to a reference
state(0ᴼC,760 mm) is sufficient.
Q = -ΔH
Enthalpy of a gas at temperature tᴼC is,
ΔHt = Cp(0-t) × t1 ..............................(3)
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)
For mixture of gases,
(Cp(0-t))mix = Xa × (Cp(0-t))a + Xb ×(Cp(o-t))b ..........(5)
(Cp(0-t))a and (Cp(0-t))b - mean specific heat of components of a
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.
EQUILIBRIUM CONSTANTS OF COMBUSTION
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
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
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
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.
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
The theoretical flame temp. is not a tangible concept while others are.
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
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
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
Hence maximum adiabatic flame temperature is
realised when fuel is slightly in excess of
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.
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)
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
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
Cp wg,(0-t) – Mean sp. heat of combustion products between t and 0ᴼC.
tf = CN+ ΔHfuel + AΔHair + Qdiss – Qloss + Vt Cp wg,(0-t)
VCp wg,(0-tf) ....(10)
Flame temp. has significance as it governs thermal
efficiency of transference of heat from flames to heating
η =(Tf – Ts)/Tf = 1-Ts/Tf ..................(12)
• Hence higher the Tf value ,greater is the efficiency.
Fuel &Combustion, S. Sarkar (3rd edition ,Universities
Thermodynamics for Chemists, S. Glasstone
Basic Thermo-chemistry, I.L.Levine