Question

Adiabatic flame temperature is maximum when

• A. the reaction incomplete
• B. no heat loss and complete combustion
• C. the heat loss is maximum
• D. excess air is high

Hint:

To maximize flame temperature:
1. Prevent heat losses (insulate the combustion chamber).
2. Burn fuel completely.
3. Avoid excess air, as the extra inert Nitrogen ($N_2$) acts as a heat sink and lowers the temperature.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the conditions under which the adiabatic flame temperature of a combustion system reaches its absolute maximum value.

Step 2: Detailed Explanation:

• Adiabatic System Concept:
An adiabatic process is one in which there is no heat transfer between the system and its surroundings ($Q = 0$).
Therefore, "no heat loss" is a fundamental requirement to achieve the adiabatic condition. If any heat is lost to the surroundings through radiation, conduction, or convection, the temperature of the products will drop.

• Complete Combustion Concept:
To maximize the thermal energy released, all the chemical energy stored in the fuel must be completely converted into heat. This requires complete combustion (all carbon oxidized to $CO_2$, all hydrogen to $H_2O$, etc.).
Incomplete combustion leaves unburnt fuel or partially oxidized species (like $CO$), which means less energy is released, resulting in a lower flame temperature.

• Effect of Excess Air:
If excess air is introduced, the nitrogen and unused oxygen in the excess air must be heated up. This acts as a thermal sink, absorbing heat and significantly lowering the final flame temperature.
Thus, maximum temperature is achieved under stoichiometric conditions with complete combustion and zero heat loss.

Step 3: Final Answer:
The adiabatic flame temperature is maximum when there is no heat loss and complete combustion.