Question

High excess air in a furnace combustion process generally

• A. Increases furnace efficiency
• B. Decreases furnace efficiency due to stack losses
• C. Increases flame temperature
• D. Reduces fuel consumption

Hint:

Excess Air Rules of Thumb:
• Too little air \(\implies\) Incomplete combustion (Smoke, soot, and CO formation; dangerous and wasteful).
• Too much air \(\implies\) High stack losses (Heating unnecessary nitrogen that carries heat right out the chimney, dropping efficiency).

Answer 1

The Correct Option is B

Concept: In industrial combustion, excess air is the volume of air introduced into the burner assembly beyond the minimum theoretical stoichiometric requirement needed for complete fuel oxidation. While a small amount of excess air (typically 5–15%) is necessary to ensure complete combustion and prevent the formation of toxic unburnt carbon monoxide (CO), adding excessive air levels acts as a thermal ballast that degrades performance. Detailed Thermodynamic Impact:
• Introduction of Non-Reacting Mass: Air contains approximately 79% Nitrogen (\(\text{N}_2\)) and 21% Oxygen (\(\text{O}_2\)) by volume. Any excess air introduced does not take part in chemical heat release reactions; instead, it must be heated from its ambient inlet state up to the furnace's internal operating flame temperature.
• Suppression of Flame Temperature: Because a portion of the combustion energy is consumed to heat this inert surplus mass, the adiabatic flame temperature drops. This is governed by: \[ Q_{\text{released}} = \sum m_i c_{p,i} (T_{\text{flame}} - T_{\text{inlet}}) \] As the total mass flow (\(m_i\)) increases due to excess air, \(T_{\text{flame}}\) decreases.
• Amplification of Stack Losses: This heated surplus air eventually exits the furnace system via the exhaust flue or stack at elevated process temperatures. The energy carried away by this non-reacting gas represents a major thermal waste known as stack loss: \[ Q_{stack} = m_{excess air} \cdot c_{p, air} \cdot (T_{stack} - T_{ambient}) Evaluating the alternative options:
• Option (1) is incorrect: High excess air significantly degrades furnace efficiency by increasing exhaust stack losses.
• Option (3) is incorrect: The extra mass dilutes the concentrated chemical energy release, causing flame temperatures to fall rather than rise.
• Option (4) is incorrect: To compensate for the drop in net heat transfer rate to the stock caused by lower temperatures and high stack losses, fuel consumption must increase to sustain the process. Therefore, Option (2) is the correct option.