Question

A fuel is completely burned first with the stoichiometric amount of air and then with the stoichiometric amount of pure oxygen. For which case will the adiabatic flame temperature be higher?

Short answer

Answer: The adiabatic flame temperature will be higher for the case when the fuel is burned with the stoichiometric amount of pure oxygen. This is because, in this case, there are no inert gases present to absorb heat from the combustion process, allowing the heat to be used solely for increasing the combustion product temperature.

Step-by-step solution

Stoichiometric Combustion with Air

Combustion with air means that the fuel is burned with the presence of nitrogen (N2), oxygen (O2), trace amounts of carbon dioxide (CO2), and other trace gases. Air consists of approximately 21% oxygen and 78% nitrogen. Nitrogen is considered inert during the combustion process, meaning it doesn't participate in the reaction but absorbs some amount of heat. This leads to a lower flame temperature as some of the heat is used to heat up this inert gas.

Stoichiometric Combustion with Pure Oxygen

Combustion with pure oxygen means that the fuel is burned with only oxygen gas and no presence of nitrogen or other trace gases. Therefore, the heat released during combustion can be used solely to increase the temperature of the combustion products. As a result, a higher flame temperature is expected when burning with pure oxygen compared to burning with air.

Adiabatic Flame Temperature Comparison

The adiabatic flame temperature is the temperature achieved when combustion occurs under adiabatic conditions, meaning no heat is lost to the surroundings. Since the heat released during combustion with pure oxygen is not absorbed by any inert gases (unlike the case with air), we can expect that the adiabatic flame temperature will be higher when the fuel is burned with the stoichiometric amount of pure oxygen. So, the adiabatic flame temperature will be higher for the case when the fuel is burned with the stoichiometric amount of pure oxygen.

Key concepts

Stoichiometric Combustion

Understanding stoichiometric combustion is critical for unraveling the mystery behind adiabatic flame temperatures. Stoichiometric combustion refers to the ideal reaction where fuel is burned with the exact amount of oxygen needed for a complete combustion process. In simpler terms, it's like baking a cake with the precise amount of ingredients—no more, no less—leading to the perfect outcome without any leftovers.In the realm of chemistry, it's a balanced equation with all reactants being consumed to produce only carbon dioxide (CO₂) and water (H₂O) when dealing with hydrocarbon fuels. There are no excess reactants or remaining fuel, and this precise mixture is key to achieving the most efficient combustion possible. However, in real-world applications, we rarely utilize stoichiometric mixtures as it could lead to higher temperatures that might damage engines or reactors. But for the purpose of academic exercises and controlled environments, understanding stoichiometric combustion sets the stage for exploring variations like combustion with air or pure oxygen.

Combustion with Air

Combustion with air is the typical scenario we encounter in everyday life. When fuels such as gasoline burn in the engine of a car or when grilling on a barbecue, it's the air around us that provides the oxygen for combustion. Air, however, is not pure oxygen. It contains about 21% oxygen and 78% nitrogen, alongside minute amounts of other gases. During combustion, that nitrogen doesn't just sit idly by; it absorbs some of the heat without reacting, behaving as an excess baggage that shares the energy without contributing to the reaction. This process deceptively 'steals' heat and results in a lower adiabatic flame temperature as compared to combustion with pure oxygen.To put it more visually: imagine throwing a party where you've prepared food just for your invited guests (the oxygen), but uninvited guests (the nitrogen) show up and consume some of the food without adding to the fun—the overall satisfaction (flame temperature) decreases because of these party crashers.

Combustion with Pure Oxygen

On the other hand, combustion with pure oxygen is akin to hosting that party with an exclusive guest list—no uninvited guests allowed. Only having oxygen present, without the nitrogen or trace gases, means that the heat from the combustion is used more effectively. This exclusive arrangement leads to a higher flame temperature because all the energy released during combustion is solely devoted to heating the products of combustion (CO₂ and H₂O), without any loss to inert gases such as nitrogen.This scenario, albeit less common in everyday situations, is crucial in industrial processes where higher temperatures are necessary for certain reactions or for achieving a more complete burn of fuels. Whether it's for cutting metals with precision in welding or in specialized furnaces, using pure oxygen can significantly increase efficiency. Whenever we opt for combustion with pure oxygen, we deliver all the energy right to the intended 'guests,' enabling the highest possible adiabatic flame temperature.