For a furnace fired with a low-calorific-value (lean) fuel gas, which measures can effectively raise flame temperature and overall furnace thermal performance?

Introduction / Context:
Lean fuel gases (low heating value) yield lower adiabatic flame temperatures. Furnace engineers compensate by improving reactant temperatures and oxidant quality to recover temperature head, improve heat transfer, and cut stack losses.

Given Data / Assumptions:

• Fuel is a low LHV gas (e.g., coke oven gas off-spec, producer gas).
• Goal: increase flame/furnace temperature.
• Standard combustion and heat recovery strategies are available.

Concept / Approach:
Adiabatic flame temperature rises when reactant sensible enthalpy increases or when oxidant contains more O2 and less ballast N2. Air or fuel preheat elevates inlet temperatures; oxygen enrichment reduces nitrogen dilution and increases reaction temperature.

Step-by-Step Solution:
Preheat combustion air: raises oxidant enthalpy, boosting flame temperature.Oxygen enrichment: increases O2 mole fraction, reduces sensible heat carried by N2, raising adiabatic temperature.Preheat fuel gas: adds sensible heat directly to fuel stream, improving ignition and flame stability.

Verification / Alternative check:
Simple adiabatic energy balance shows higher inlet temperatures or higher O2 fraction increase flame temperature at constant LHV.

Why Other Options Are Wrong:
All listed methods are valid; selecting only one ignores cumulative benefits achievable via combined heat recovery and enrichment.

Common Pitfalls:
Over-enriching with oxygen without revising burner design can cause hotspots and refractory damage; always match enrichment and preheat with burner/furnace limits.

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