Why theoretical flue-gas compositions are rarely achieved In practical furnace operation, why is zero O2 (or theoretical maximum CO2) in flue gas seldom attained?

Introduction / Context:
Combustion control targets stoichiometric or slightly lean/rich conditions. In theory, perfect stoichiometry produces zero residual oxygen and a “theoretical” CO2 percentage in dry flue gas. Real-world furnaces rarely hit this because of mixing and leakage limitations.

Given Data / Assumptions:

• Conventional burners and practical furnace seals.
• Non-ideal mixing at realistic velocities.
• Some air leakage at doors, joints, and viewports.

Concept / Approach:
Achieving zero O2 requires perfect instantaneous mixing and zero excess air. In practice, to ensure complete combustion and avoid CO, a margin of excess air is introduced. Additionally, infiltration (air leaking into a negative-pressure furnace) raises stack O2 and lowers CO2 below the theoretical maximum. Hence, imperfect mixing and leakage are the principal reasons for not reaching the theoretical composition.

Step-by-Step Solution:

Verification / Alternative check:
Portable flue-gas analyser trends versus damper settings confirm O2 does not reach zero and CO2 lags theoretical unless sealing and mixing are ideal.

Why Other Options Are Wrong:

• Non-preheated air: Affects efficiency, not whether zero O2 is attainable.
• Pulverised fuels: Can actually enhance mixing; not the reason for persistent O2.
• Excessive positive draft: Tends to exfiltrate gases; most furnaces run negative, making infiltration the issue.
• Refractory heat losses: Influence efficiency, not flue-gas stoichiometry.

Common Pitfalls:
Assuming CO-free means stoichiometric; typically it means some excess air, which by definition yields non-zero O2.

Final Answer:
Imperfect fuel–air mixing and infiltration of air.