Question

Consider a complete combustion process during which the reactants enter the combustion chamber at \(20^{\circ} \mathrm{C}\) and the products leave at \(700^{\circ} \mathrm{C}\). Combustion is achieved with \((a) 100\) percent theoretical air, \((b) 200\) percent theoretical air, and ( \(c\) ) the chemically correct amount of pure oxygen. For which case will the amount of heat transfer be the lowest? Explain.

Short answer

Answer: The lowest amount of heat transfer occurs in cases (a) and (c) - when combustion occurs with 100% theoretical air or the chemically correct amount of pure oxygen.

Step-by-step solution

Understand the impact of excess air

Theoretical air refers to the amount of air needed for complete combustion to occur. Different cases have different theoretical air amount. The important fact to note is that as we add excess air (air beyond the theoretical amount), the amount of heat transfer increases due to a higher mass flow rate of products that must be heated up.

Analyze case (a)

In this case, the combustion process occurs with the exact amount of air needed for complete combustion (100% theoretical air). Therefore, there is no excess air, and the heat transfer will be comparatively lower.

Analyze case (b)

In this case, the combustion process occurs with twice the amount of air needed for complete combustion (200% theoretical air). This results in a higher mass flow rate of products, which increases the amount of heat transfer.

Analyze case (c)

In this case, the combustion process occurs with the chemically correct amount of pure oxygen. This means no excess nitrogen is introduced into the combustion chamber, so the total mass flow rate of the products remains the same as in case (a).

Compare the heat transfer in each case

From the analysis of the three cases, we can conclude that the amount of heat transfer is lowest in cases (a) and (c) - when combustion occurs with 100% theoretical air or the chemically correct amount of pure oxygen. In both cases, there is no excess air, while in case (b), with 200% theoretical air, we have excess air that results in a higher mass flow rate of products and higher heat transfer.