Adiabatic Flame Temperature
Adiabatic flame temperature (AFT) is the maximum theoretical temperature of complete combustion with no heat loss at constant volume and pressure. It helps to determine the performance of combustion systems, including efficiency, material selection, and pollutant emissions. In actual practice, some heat is always lost (e.g., through radiation, conduction to the walls of a combustion chamber), so the actual flame temperature is always lower than the adiabatic flame temperature. Adiabatic flame temperature is affected by the fuel-air ratio, the initial temperature of reactants, and the pressure.
The calculation of AFT assumes complete combustion that means all the fuel reacts perfectly with the oxidizer to form stable products. Hydrocarbons are converted into carbon dioxide and water upon complete combustion; however, in actual practice, at very high temperatures, some dissociation of products can occur. Carbon dioxide dissociates into carbon monoxide and oxygen and water into hydrogen and oxygen, which absorbs energy and lowers the actual flame temperature.
It is also assumed that no work is done by or on the system. This implies constant volume or constant pressure conditions, depending on the specific type of AFT being calculated. Constant volume AFT is typically higher than constant pressure AFT because no energy is used to expand the system.
The calculation of Adiabatic flame temperature is based on the First Law of Thermodynamics. It states that the total enthalpy of the reactants at their initial temperature must be equal to the total enthalpy of the products at the adiabatic flame temperature.
ΔHreactants = ΔHproducts.
ΔHreactants is the sum of the enthalpy of formation of the reactants at their initial temperature.
ΔHproducts is the sum of the enthalpy of formation of the products plus the heat required to raise their temperature from a reference temperature to the adiabatic flame temperature (sensible enthalpy change).
Adiabatic Flame Temperature of Common Fuels
| Fuels | Temperature in °C | Temperature in K |
|---|---|---|
| Propane | 1980 (in air) | 2253.15 (in air) |
| Butane | 1970 (in air) | 2243.15 (in air) |
| Wood | 1980 (in air) | 2253.15 (in air) |
| Kerosene | 2093 (in air) | 2366.15 (in air) |
| Gasoline | 2138 (in air) | 2411.15 (in air) |
| Anthracite | 2180 (in air) | 2453.15 (in air) |
| Lithium | 2438 (in oxygen) | 2711.15 (in oxygen) |
| Acetylene | 3480 (in oxygen) | 3753.15 (in oxygen) |
| Aluminium | 3732 (in oxygen) | 4005.15 (in oxygen) |
| Zirconium | 4005 (in oxygen) | 4278.15 (in oxygen) |
Example
Let's consider the combustion of methane (CH4) in air.
Case-1: Stoichiometric Combustion in Air
The stoichiometric equation for complete combustion of methane.
CH4 + 2(O2 + 3.76N2) → CO2 + 2H2O + 7.52N2
For methane burning stoichiometrically in air at 1 atm, the calculated adiabatic flame temperature is typically around 2200-2300 K (approximately 1927-2027°C), assuming no dissociation. If dissociation is considered (which is more accurate at these high temperatures), the temperature will be slightly lower.
Case-2: Combustion of Methane in Pure Oxygen
The balanced stoichiometric combustion reaction for methane in pure oxygen is
CH4 + 2O2 → CO2 + 2H2O
Without the inert nitrogen absorbing heat, the adiabatic flame temperature will be significantly higher. For methane in pure oxygen, the AFT can exceed 3000 K (approximately 2727°C). This illustrates the diluent effect of nitrogen in air.
Case-3: Effect of Excess Air
Consider burning methane with 50% excess air. The reaction becomes:
CH4 + 3(O2 + 3.76N2 ) → CO2 + 2H2O + O2 + 11.28N2
The adiabatic flame temperature will be lower than the stoichiometric case (around 1800-2000 K for methane with 50% excess air).
Case-4: Effect of Initial Temperature
If the methane and air in case-1 were preheated to, say, 500 K (227°C) instead of 298 K, their initial enthalpy would be higher. This means the combustion energy would have to heat the products up from an already higher sensible energy level.
The adiabatic flame temperature would be higher than if starting from 298 K. This is why preheating air is a common energy-saving technique in industrial furnaces.
Factors Affecting Adiabatic Flame Temperature
Following factors can affect the adiabatic flame temperature.
Fuel Composition: Different fuels release different amounts of energy upon combustion. Fuels with higher heating values generally lead to higher adiabatic flame temperature. For example, hydrogen burning in air has a higher adiabatic flame temperature than methane in air due to hydrogen's higher energy density.
Initial Temperature of Reactants: Preheating the fuel and/or oxidizer increases their initial enthalpy that means less energy released from combustion is needed to raise their temperature to the reaction temperature, thus leading to a higher adiabatic flame temperature.
Pressure: Higher pressure can slightly increase adiabatic flame temperature, especially due to its influence on dissociation equilibrium. At very high temperatures, dissociation of products becomes more significant, which absorbs energy and effectively lowers the AFT. Increased pressure tends to suppress dissociation, thus slightly increasing adiabatic flame temperature.
Air vs. Pure Oxygen: Combustion in pure oxygen increases the adiabatic flame temperature than combustion in air because air contains approximately 79% inert nitrogen.
Humidity: The presence of moisture in the air can also lower the adiabatic flame temperature, as water vapor has a high specific heat and absorbs heat.
Stoichiometric Mixture (Equivalence Ratio = 1): The adiabatic flame temperature is maximized when enough oxidizer is supplied to completely burn all the fuel.
Lean Mixtures (Excess Air): If there is excess air, the extra nitrogen and oxygen absorb heat, leading to a lower adiabatic flame temperature.
Rich Mixtures (Excess Fuel): If there is insufficient air (incomplete combustion), not all the fuel burns, and unburned fuel also absorbs heat, resulting in a lower adiabatic flame temperature.
Test Your Knowledge (MCQs)
1. What is the primary assumption made when calculating Adiabatic Flame Temperature (AFT)?
A. Some heat is always lost to the surroundings.
B. Product dissociation significantly lowers the flame temperature.
C. All fuel reacts perfectly with the oxidizer to form stable products.
D. Work is done by the system during combustion.
View Answer
Option C is correct answer.
All fuel reacts perfectly with the oxidizer to form stable products.
2. Why is Constant Volume Adiabatic Flame Temperature typically higher than Constant Pressure Adiabatic Flame Temperature?
A. Constant volume processes involve less heat loss.
B. No energy is used to expand the system in a constant volume process.
C. Reactants are at a higher initial temperature in constant volume processes.
D. More dissociation occurs at constant pressure.
View Answer
Option B is correct answer.
No energy is used to expand the system in a constant volume process.
3. Which of the following factors will generally lead to a lower Adiabatic Flame Temperature?
A. Preheating the fuel and/or oxidizer.
B. Combustion in pure oxygen instead of air.
C. Burning a stoichiometric mixture of fuel and air.
D. The presence of excess air (lean mixture).
View Answer
Option D is correct answer.
The presence of excess air (lean mixture).
4. According to the First Law of Thermodynamics, what equality is fundamental to AFT calculation?
A. ΔUreactants = ΔUproducts
B. ΔHreactants = ΔHproducts
C. Q = W + ΔE
D. P1V1 = P2V2
View Answer
Option B is correct answer.
ΔHreactants = ΔHproducts.
5. What is the effect of product dissociation (e.g., CO2 → CO + O2) at very high temperatures on the actual flame temperature?
A. It increases the actual flame temperature by releasing more energy.
B. It has no significant effect on the flame temperature.
C. It absorbs energy, thus lowering the actual flame temperature.
D. It only occurs at very low pressures, so it's negligible.
View Answer
Option C is correct answer.
It absorbs energy, thus lowering the actual flame temperature.