Question

11\. For a given fuel, how would the adiabatic flame temperature vary if the percent of theoretical air were increased? Why?

Short answer

Increasing the percent of theoretical air reduces the adiabatic flame temperature because the excess air absorbs heat.

Step-by-step solution

Understanding Adiabatic Flame Temperature

The adiabatic flame temperature is the temperature reached by a flame when no heat is lost to the surroundings. It depends on the heat released during combustion and the specific heat capacities of the products.

Theoretical Air Definition

Theoretical air is the exact amount of air needed to completely combust a given amount of fuel. It provides enough oxygen for the fuel to react stoichiometrically, which means no excess or deficiency of either reactant.

Effect of Increasing Percent of Theoretical Air

Increasing the percent of theoretical air means providing more air than the stoichiometric amount. This introduces excess air into the combustion process.

Impact on Adiabatic Flame Temperature

When the percent of theoretical air is increased, the mixture becomes leaner. The excess air absorbs some of the heat released during combustion, resulting in a lower adiabatic flame temperature. Simply put, the temperature decreases because the additional air requires energy to heat up.

Reason for Temperature Change

The additional air introduced acts as a heat sink. More air means more nitrogen and oxygen needing to be heated, distributing the energy over a larger mass, leading to a lower overall temperature.

Conclusion

Thus, increasing the percent of theoretical air in the combustion process reduces the adiabatic flame temperature because the excess air absorbs some of the heat.

Key concepts

Theoretical Air

The term 'theoretical air' refers to the precise amount of air required to completely combust a given amount of fuel. In combustion, achieving a perfect balance between fuel and air is essential. This balance is known as the stoichiometric ratio. When this exact ratio is provided, there is just enough oxygen to react with the fuel, resulting in complete combustion without any excess or deficiency of reactants.
Fuel and oxygen combine in the combustion process to produce heat, water, and carbon dioxide. Ensuring the correct ratio is vital for maximizing fuel efficiency and minimizing pollutants. Excess air or deficient air can both lead to incomplete combustion, which results in reduced efficiency and increased emissions.
Understanding theoretical air is crucial for controlling the combustion process in engines, industrial burners, and heating systems. It helps in optimizing fuel usage and maintaining desired temperature levels in various applications.

Combustion Process

The combustion process is a chemical reaction where a fuel reacts with an oxidizing agent, usually oxygen, to produce heat and light. This process can be categorized into complete and incomplete combustion.
In complete combustion, the fuel fully reacts with the oxygen, resulting in maximal energy release and producing mainly water and carbon dioxide. For complete combustion to occur, a precise mixture of fuel and theoretical air is required.
In practical applications, however, often some excess air is supplied to ensure complete combustion and account for variations in the fuel-air mixture. When excess air is introduced beyond the theoretical requirement, it leads to what is called a 'lean' mixture. Excess air can absorb part of the released heat, which impacts the temperature and efficiency of the combustion.
Regular and controlled combustion is essential for various industries and applications, from car engines to power plants. Effective combustion helps in reducing fuel consumption and emissions, making the process more environmentally friendly.

Heat Absorption

Heat absorption in the context of combustion involves the energy produced during the reaction being distributed among the combustion products. When more air than the theoretical amount (excess air) is introduced into the combustion process, this excess air absorbs some of the heat produced.
In a mixture with 100% theoretical air, all the heat produced is used to heat the combustion products and get the highest adiabatic flame temperature. However, when the percent of theoretical air increases, the additional air acts as a heat sink. This means that more nitrogen and oxygen from the air need to be heated, distributing the energy over a larger mass, which lowers the overall temperature.
This concept is important in practical applications where controlling the temperature is necessary. For instance, in industrial furnaces, excess air may be intentionally used to control temperatures and prevent damage to the equipment.
A balance needs to be struck, as too much excess air will lower the efficiency of the combustion process, while too little can lead to incomplete combustion and higher pollutant emissions.