Safety and fouling — Why are fuel gases that contain significant hydrocarbons (e.g., coke oven gas) generally not preheated before combustion?

Introduction:
Preheating gaseous fuels can improve combustion efficiency, but gases rich in hydrocarbons (like coke oven gas) bring risks. Understanding thermal cracking and fouling mechanisms is crucial for reliable firing systems.

Given Data / Assumptions:

• Fuel contains light hydrocarbons and higher unsaturates/aromatics.
• Preheating involves high wall temperatures in recuperators or heaters.
• Downstream equipment is sensitive to fouling.

Concept / Approach:
Hydrocarbon cracking is a temperature-driven process where larger molecules decompose into smaller fragments and elemental carbon (soot) on hot metal surfaces. This carbon deposits inside preheaters and exchangers, raising pressure drop, impeding heat transfer, and potentially causing hot spots. Therefore, practice avoids preheating hydrocarbon-rich fuels; instead, mixing with air at ambient or using limited preheat under controlled conditions may be preferred.

Step-by-Step Solution:
Recognise composition: hydrocarbon-rich fuel → cracking prone.Assess thermal environment: hot surfaces → soot formation risk.Conclude main reason: fouling/choking from carbon deposition.

Verification / Alternative check:
Plant experience shows increased maintenance and exchanger plugging when preheating such fuels; design guidelines often prescribe limits or recommend on-burner steam injection to mitigate soot in some cases.

Why Other Options Are Wrong:

• (b) and (c) are incorrect; preheating does not inherently reduce calorific value or flame temperature before burning.
• (d) While explosion risks exist with any fuel handling, the principal, routine reason to avoid preheat is cracking/soot fouling.
• (e) Sulphur removal is unrelated to simple preheating.

Common Pitfalls:
Assuming preheating is universally beneficial; ignoring material balance of carbon deposition that progressively blocks passages.

Final Answer:
Because hydrocarbons can crack on hot surfaces, depositing carbon soot that fouls and chokes heat-transfer passages