Raising furnace temperature with lean fuel gas—what will NOT help? When firing a furnace with a lean, low-calorific-value gas, a higher furnace temperature cannot be achieved by which of the following actions?

Introduction / Context:
Lean fuel gases (e.g., blast furnace gas) have low heating value and high inert content, which depress adiabatic flame temperature. Operators use several strategies to raise furnace temperature. Understanding which tactics are effective avoids wasted effort and unintended cooling.

Given Data / Assumptions:

• Fuel is low in calorific value; excess air reasonably controlled.
• Furnace is draft-controlled via stack dampers or ID/FD fans.
• Objective: increase flame/furnace temperature.

Concept / Approach:
Preheating the fuel gas or the combustion air raises reactant enthalpy, directly increasing flame temperature. Oxygen enrichment reduces nitrogen ballast, increasing adiabatic flame temperature. In contrast, increasing draft generally increases cold-air infiltration and flue-gas throughput, which tends to cool the furnace and increase stack losses, not raise its temperature.

Step-by-Step Solution:

Verification / Alternative check:
Heat balance shows furnace temperature rises with reactant preheat and O2 enrichment, but falls with increased excess air and infiltration caused by excessive draft.

Why Other Options Are Wrong:

• Preheating fuel or air: proven methods to raise flame temperature.
• Oxygen enrichment: increases theoretical flame temperature by reducing nitrogen dilution.

Common Pitfalls:
Confusing stack draft control for combustion intensity; excessive draft often lowers residence time and raises stack temperature, degrading efficiency.

Final Answer:
Increasing the draft in the furnace