Benefits of using preheated combustion air in furnaces Which combined effects typically result from preheating the combustion air supplied to a furnace?

Introduction / Context:
Preheating combustion air is a classic energy-efficiency measure in furnace practice. By raising the air temperature before it enters the burner, less fuel is required to reach target flame and stock temperatures.

Given Data / Assumptions:

• Air preheat accomplished via regenerators or recuperators.
• Materials processed are sensitive to oxidation/scale at surface temperature.

Concept / Approach:
Hotter combustion air increases adiabatic flame temperature and reduces the fuel needed per unit of heat delivered to the stock. Faster heating and shorter time at high temperature can lower oxidation and thus reduce scaling on steel. Overall, preheated air improves thermal efficiency and lowers specific fuel consumption.

Step-by-Step Solution:

Verification / Alternative check:
Energy balances show reduced sensible heating duty for the air stream; industrial data report lower fuel-per-ton and improved productivity with air preheat.

Why Other Options Are Wrong:

Common Pitfalls:
Excessive air preheat without control can raise NOx; balancing efficiency with emissions is essential.

Final Answer:
all (a), (b) and (c).