Recoverable heat from furnace flue gases primarily depends on which set of factors in an industrial heating operation?

Introduction / Context:
Heat recovery reduces fuel use and stack losses. The extent to which useful heat can be reclaimed from flue gases depends on furnace performance and flue-gas characteristics.

Given Data / Assumptions:

• Standard furnace with a recuperator/regenerator or waste-heat boiler.
• Objective: maximize useful heat recovered without condensation/corrosion issues unless designed for it.

Concept / Approach:
Recoverable heat ∝ mass flow rate * specific heat * temperature drop. Higher furnace efficiency helps by lowering excess air and optimizing heat transfer, while more flue-gas flow and a larger temperature drop across recovery equipment increase the available sensible heat.

Step-by-Step Solution:
Consider energy balance: Q_recovered = m_dot * Cp * (T_in - T_out).m_dot depends on combustion stoichiometry/excess air and load.T_in and T_out are set by furnace outlet and recovery approach temperature limits.

Verification / Alternative check:
Practical sizing of recuperators uses the same relationship; higher gas temperature and flow permit more recovery but are constrained by materials limits and fouling.

Why Other Options Are Wrong:
Each individual factor matters; omitting any one underestimates recoverable energy.

Common Pitfalls:
Ignoring acid dew-point corrosion when pushing T_out too low; designing without considering fouling or pressure drop.

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