Ironmaking — In a blast furnace, introducing water vapour (moisture) in the hot blast primarily has which effect?

Introduction:
Blast furnace operation depends on gas composition and temperature in the raceway and stack. Moisture in the blast participates in endothermic reactions that alter the gas mixture, influencing both flame temperature and reducing power delivered to the burden. Understanding these trade-offs is important for control and fuel optimization.

Given Data / Assumptions:

• Hot blast with some water vapour enters the tuyere/raceway region.
• Available carbon (coke) reacts with oxidants and steam.
• Primary interest: effect on reducing potential of ascending gas.

Concept / Approach:

Water vapour reacts with hot coke via the water–gas reaction: C + H2O → CO + H2 (endothermic). The generated CO and H2 are strong reducing agents for iron oxides. Although added moisture lowers flame temperature (due to endothermicity and dilution), it increases the fraction of H2 and CO in the top gas at a given oxygen input, thus enhancing the overall reducing potential of the gas mixture acting on the burden.

Step-by-Step Solution:

Verification / Alternative check:

Operational data show hydrogen enrichment increases indirect reduction rates and can reduce coke rate; moisture addition is one route to generate H2 in situ, with careful control to avoid excessive temperature drop.

Why Other Options Are Wrong:

B: Steam lowers flame temperature, it does not increase it. C: The gas composition and thermals do change. D: Hydrogen largely remains in the gas; it does not significantly increase hydrogen content in the tapped hot metal. E: Opposite of the observed effect; CO/H2 formation increases reducing power.

Common Pitfalls:

Ignoring that stronger reducing gases can coexist with lower raceway temperature; both effects occur and must be balanced in practice.

Final Answer:

Increases the reducing potential of the gas