Stack (chimney) heat loss comparison at 600°C flue gas temperature: For complete combustion, which fuel–oxidant combination typically gives the highest percentage stack loss?

Introduction / Context:
Stack loss (sensible heat carried out by flue gases) depends on flue gas temperature, specific heat, and mass of dry gas per unit heat released. Low-calorific fuels requiring large combustion air volumes yield higher stack loss percentages for the same exit temperature.

Given Data / Assumptions:

• All cases at 600°C flue gas temperature and complete combustion.
• Air contains substantial nitrogen diluent; oxygen firing avoids nitrogen ballast.
• Blast furnace gas (BFG) has low calorific value and high inert content (notably CO2 and N2).

Concept / Approach:
Percent stack loss increases with the amount of inert gas heated per unit of fuel energy. BFG burned with air generates large flue gas volumes (fuel inerts + air nitrogen), so the sensible heat fraction lost is large relative to the small heat input. Furnace oil has high calorific value and requires less air per unit energy; oxygen firing further reduces flue gas volume by eliminating nitrogen ballast.

Step-by-Step Solution:

Verification / Alternative check:
Heat balance calculations using typical ultimate analyses confirm that BFG has higher kg of flue gas per MJ; oxygen enrichment reduces losses markedly.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing absolute heat loss with percentage. Although hotter gas increases absolute loss, the percentage depends strongly on gas mass per heat input.

Final Answer:
blast furnace gas with air.