Furnace combustion control: to maintain a deliberately reducing atmosphere inside a process furnace, which flue-gas characteristic should be present?

Introduction / Context:
Industrial furnaces sometimes require a reducing atmosphere to prevent oxidation, scale formation, or to promote specific metallurgical reactions. Atmosphere control is verified via flue-gas analysis. Knowing which gas indicates a reducing condition is a core skill in furnace operation and combustion tuning.

Given Data / Assumptions:

• Fuel-fired furnace with controllable air-to-fuel ratio.
• Reducing atmosphere means oxygen potential is low; reducing species like CO or H2 can exist.
• Flue-gas sampling is representative of furnace conditions.

Concept / Approach:
When combustion air is sub-stoichiometric, incomplete combustion forms carbon monoxide (CO). Presence of CO signifies oxygen deficiency and a reducing environment. Conversely, measurable O2 in flue gas indicates excess air (oxidizing). High excess air further increases O2 and lowers CO, pushing conditions away from reducing.

Step-by-Step Solution:
1) Define reducing atmosphere: oxygen activity is low; reducing gases present (CO, H2).2) Identify indicator gas: CO presence suggests sub-stoichiometric combustion.3) Examine options: “no O2” may occur, but CO is the practical marker technicians use; “high excess air” is opposite of reducing.4) Conclude that CO in flue gas is the operational signature of a reducing furnace.

Verification / Alternative check:
Combustion curves (O2 vs. CO) show CO rises sharply as air is reduced below stoichiometric, confirming reducing conditions.

Why Other Options Are Wrong:
no O2 in flue gas: Possible, but absence alone is less reliable than positive CO presence; traces of O2 measurement error can mislead.high excess air: Produces oxidizing atmosphere, not reducing.none of these: Incorrect because CO is the standard indicator.

Common Pitfalls:
Relying only on O2 analyzers without CO measurement.Over-reducing, leading to efficiency loss and safety issues.

Final Answer:
CO in flue gas