Fuel economy in industrial furnaces can be achieved by multiple measures. Which option correctly groups common, practical methods that simultaneously help lower fuel use?

Introduction / Context:
Furnace efficiency is improved by both combustion-side enhancements and enclosure/operation improvements. A holistic approach typically yields the best savings. This question validates understanding of three widely adopted strategies and whether they complement each other to reduce fuel consumption.

Given Data / Assumptions:

• Standard fuel-fired furnace with stack, burners, and access doors.
• Opportunities include oxygen enrichment, air preheating, and minimizing openings/air leakage.
• Safety and material limits are respected.

Concept / Approach:
Oxygen enrichment raises flame temperature and can reduce flue-gas mass (less N2), improving heat transfer and lowering stack loss. Air preheating recovers waste heat, reducing fuel needed to reach setpoint temperatures. Sealing doors and controlling openings reduces cold air infiltration and uncontrolled heat escape—both of which otherwise raise fuel use. Together, these measures target different loss mechanisms and are additive when applied correctly.

Step-by-Step Solution:
Identify each measure’s effect: hotter flames, recovered heat, reduced infiltration.Recognize synergy: less fuel per ton processed and improved throughput.Choose the combined option that includes all three.

Verification / Alternative check:
Industrial energy assessments rank air preheaters, controlled excess O2/enrichment, and better sealing as high-impact items. Case studies show measurable drops in specific fuel consumption after implementation.

Why Other Options Are Wrong:
Picking only one measure ignores other proven levers and leads to suboptimal savings.

Common Pitfalls:

• Over-enrichment without proper control can increase NOx or refractory stress.
• Excessive preheat risks acid dew-point corrosion in air preheaters if not managed.

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