Coal combustion on grates – operating and ash factors that accentuate clinkering On a furnace grate burning coal, which condition most strongly promotes clinker formation (sintered ash masses) and aggravates clinkering trouble?

Introduction / Context:
Clinkering is the formation of hard, fused ash masses on the grate. It disrupts air distribution, reduces combustion efficiency, and increases maintenance. Clinkering depends on ash chemistry (fusion temperature) and on-bed operating temperature, which is governed by air supply and bed management.

Given Data / Assumptions:

• Coal burned on a grate with primary air through the bed.
• Ash contains oxides (e.g., Fe, Ca, Na, K, Si, Al) influencing fusion temperature.
• Operating choices include bed thickness, air preheat, and draught intensity.

Concept / Approach:
Clinkering worsens as the ash approaches or exceeds its softening temperature. This occurs when the local bed temperature is high due to excessive thickness (poor cooling/air distribution) and preheated primary air (raising adiabatic flame temperature near the grate). While ash rich in iron or alkalis can lower ash softening temperature, the single operating condition that directly accentuates clinkering is a hot, thick bed with preheated air.

Step-by-Step Solution:

Verification / Alternative check:
Plant practice: Reducing bed thickness or air preheat, improving air distribution, and blending high-iron/alkali coals with high-fusion-ash coals mitigates clinkers.

Why Other Options Are Wrong:

• Low forced draught and low temperature: Tend to reduce the risk of ash fusion.
• Low-reactivity char with Fe/S: Lowers fusion temp but is a fuel property; the question asks which condition accentuates on the grate—operationally (c) is decisive.
• All of the above: Incorrect because (a) does not accentuate.

Common Pitfalls:
Confusing slagging in furnaces (radiant zone) with clinkering on grates; though related, the controlling temperatures and air paths differ.

Final Answer:
Thick fire bed combined with preheated primary air