Lecture 12
Flame temperature
Contents
What is a flame temperature?
Method of calculation
Specific heat values
Procedure to calculate flame temperature
Importance of flame temperature
Key words: Flame temperature, furnace, blast furnace
What is a flame temperature?
It is the temperature attained by the products of combustion, when fuel is burnt with air or oxygen. Flame temperature is difficult to measure when combustion of a mixture of fuel + air occurs inside the furnace.
But flame temperature may be calculated
Method of calculation
Heat balance is done to calculate the flame temperature
[Sensible heat in air + fuel] + [Heat of combustion]‐[Heat absorbed due to dissociation of products of combustion] = (Sensible heat in POC) + [Heat losses from flame to surrounding] (1)
Assumptions
1) Combustion is adiabatic, i.e. no loss of heat.
2) There occurs complete combustion i.e. products of combustion are CO2, H2O, SO2 etc. 3) Dissociation of POC is neglected.
4) Combustion occurs at constant pressure so that ∆H CP dT Under the above conditions, flame temperature is called “Theoretical maximum adiabatic flame temperature” (TMAFT)
When dissociation of POC is considered, flame temperature is “Adiabatic flame temperature”.
Thus TMAFT or AFT is > Actual flame temperature.
For adiabatic combustion and without dissociation of POC and from assumptions 2 and 4 one can write eq. 1as:
∑ HT H for fuel and air ∑ ∆H for all combustion reaction = ∑ HT H for all combustion products. (2)
T ∑ H H ∑ C dT (3) T P .
T is temperature at which reactants are heated. If T = 298K ten sensible heat of reactants is zero.
T ∑ H H ∑ C dT (4) POC T POC P POC
Where Tf is the flame temperature and
CP ∑ n CP (5) n1 = moles of specie of specific heat CP i= No. of specie in a mixture, for example in air we have O2 and N 2 and in POC we have CO2, HO 2 and N2 etc.
Specific heat values
Mostly POC consist of CO2, CO, H 2O, N 2 and O, 2 H. 2 Below are given CP values in cal/mol
. C CO 10.55 2.16 10 T (6) P T
. C CO 6.79 0.98 10 T (7) P T
. C H O v 7.17 2.56 10 T (8) P T
CP N 6.66 1.02 10 T (9)
. C O 7.16 1 10 T (10) P T
. C H 6.52 0.78 10 T (11) P T
For quick estimation of AFT one may use the overage CP values of POC. Some of the values are
C 12.5 C 7.61 P CO P O
C 7.67 C 7.73 P CO P H O
C 7.12 C 6.72 P N P H
However, the calculated value of flame temperature using average specific heat values will be greater than when equations 6‐11 are used.
Procedure to calculate flame temperature
i) First calculate amounts of air and POC for given quantity of fuel. ii) If air and fuel are preheated, calculate sensible heat of air and fuel. iii) Determine heat of combustion from heat of formations of POC. Assume complete combustion. iv) If CO in the products of combustion is known, than subtract the heat of combustion of CO from heat of combustion value. v) Perform heat balance, as shown in equation 2.
Flame temperature importance
Flame temperature can be used to
i. Characterize the quality of fuel. Higher is TMAFT/AFT higher is the quality of fuel. ii. Determine suitability of fuel for an application. For a given fuel and amount of air TMAFT/AFT must be greater than furnace temperature so that heat transfer between POC and furnace can occur. iii. Determine extent of substitution of a lower quality fuel to conserve high quality fuel. For example, what % of producer gas can be substituted for natural gas fired furnace. iv) In other processes the reaction temperature is important. In this case we may call adiabatic reaction temperature. (ART). In thermit welding of steel sections, the ART of a mixture of steel shot.
Fe2 O3 and aluminum powder governs the extent of heating and size and location of heat affected
zone. The reaction 2Al+Fe2 O3 =2 Fe +Al2 O3 is exothermic and generates high temperature rapidly.
In the operations of blast furnace and shaft furnace with air or air +fuel injection, the reaction temperature or AFT is important
References: O.P.Gupta: elements of fuels, furnaces and refractories