Maintaining an oxidising furnace atmosphere: To sustain an oxidising atmosphere during fired operation, a furnace should be operated with…

Introduction / Context:
Process metallurgy and heat treatment often require control over furnace atmosphere to influence reactions like oxidation, decarburization, or scaling. An oxidising atmosphere contains free oxygen that promotes oxide formation on metals or complete combustion of combustibles.

Given Data / Assumptions:

• We consider steady combustion of hydrocarbon or carbonaceous fuels.
• Excess air is the fraction of air supplied over the theoretical stoichiometric amount.
• Flue-gas indicators such as O2, CO, and CO2 reflect the achieved atmosphere.

Concept / Approach:
Operating with excess air ensures residual oxygen remains after the fuel is burned, producing an oxidising environment within the furnace. Fuel-rich conditions (deficient air) elevate CO and unburned hydrocarbons, which are reducing. While CO2 increases with complete combustion, CO2 alone is not the control lever; excess O2 is the essential criterion.

Step-by-Step Solution:
Target an O2 reading above zero in the flue gas.Achieve this by supplying more than stoichiometric air—i.e., excess air.Avoid elevated CO, which signifies reducing/oxygen-deficient combustion.

Verification / Alternative check:
Combustion tuning practice sets an O2 trim setpoint (e.g., a few percent O2 dry) to maintain an oxidising atmosphere with acceptable efficiency and safety.

Why Other Options Are Wrong:
Less excess air: promotes reducing conditions.More CO: characteristic of reducing/poorly mixed flames.“More CO2 at any cost”: CO2 level alone is not a guarantee of oxidising conditions; oxygen availability is decisive.Reduced air preheat: unrelated to oxidising/reducing balance.

Common Pitfalls:
Chasing high CO2 numbers without monitoring O2 can lead to unstable combustion; using an O2 trim ensures true oxidising operation.

Final Answer:
More of excess air (oxygen present above stoichiometric requirement)