In furnace operation, “overfire burning” refers to which characteristic phenomenon observed in the upper zones of the chamber?

Introduction / Context:
Incomplete combustion near burners can leave combustible species, notably carbon monoxide and unburned hydrocarbons. In some furnaces, additional air admitted higher in the chamber completes oxidation above the primary combustion zone—a practice known as overfire air or overfire burning. Understanding this concept is important for emissions control and heat-release distribution.

Given Data / Assumptions:

• Primary burners create a fuel-rich core or incomplete mixing near the flame root.
• Secondary (overfire) air ports are located higher along the gas path.
• Goal is to finish oxidation of CO and combustibles while managing temperature profiles and NOx.

Concept / Approach:
Overfire burning intentionally stages combustion. The first zone runs closer to stoichiometric or slightly fuel-rich to control peak temperatures and NOx. Further downstream, additional air promotes the oxidation of CO to CO2 and breakdown of hydrocarbons, shifting heat release away from the burner tile and providing better overall mixing. This is distinct from simply “supplying excess air” uniformly at the burners; the key is where the air is introduced and what it oxidizes in the upper zone.

Step-by-Step Solution:
Identify unburned species leaving primary zone (CO, VOCs).Recognize overfire air addition higher in the chamber.Define the phenomenon: completion of oxidation in the upper zone—overfire burning.

Verification / Alternative check:
Low-NOx furnace designs and many boiler firing systems employ overfire air (OFA) to reduce NOx and to complete burnout of CO at distance from the flame root.

Why Other Options Are Wrong:
Excess fuel: Leads to reducing conditions and high CO if not corrected; not the definition.Supply of excess air (generic): Too vague; the staged, upper-zone oxidation is the defining feature.None of these: Incorrect, because the staged upper-zone oxidation description is correct.

Common Pitfalls:

• Confusing uniform high excess air with staged overfire air.
• Ignoring impacts on heat-transfer distribution and refractory hot spots when moving heat release upstream or downstream.

Final Answer:
burning carbon monoxide and other incombustible in upper zone of furnace by supplying more air.