High-ash coals – reasons for undesirability in boilers and furnaces Why is a high ash content in coal generally undesirable for combustion and metallurgical operations?

Introduction / Context:
Ash is the incombustible residue of coal. While all coals contain some mineral matter, higher ash percentages degrade performance, raise maintenance costs, and impair product quality in metallurgical furnaces. Understanding the multiple adverse effects explains why fuel buyers specify maximum ash limits.

Given Data / Assumptions:

• Coal is fired in pulverised-fuel boilers or on grates, or used to make coke.
• Refractory-lined furnaces are subject to thermal cycling.
• Ash composition can lead to low-melting eutectics.

Concept / Approach:
High ash increases erosive wear in mills, ducts, and burners. When ash becomes molten or sticky, it can infiltrate refractory linings and, upon cooling, cause spalling due to different expansion coefficients. In metallurgical units, ash elevates slag volume, ties up sulphur/phosphorus, and can deposit on heat-transfer surfaces (slagging/fouling), reducing efficiency and increasing outages. Hence, all listed drawbacks are valid and collectively justify the undesirability of high-ash fuels.

Step-by-Step Solution:

Verification / Alternative check:
Plant data show higher ash coals correlate with higher mill wear rates and boiler tube fouling indices; metallurgical plants report increased slag handling and flux consumption.

Why Other Options Are Wrong:

• Any single statement alone is incomplete; all mechanisms contribute.

Common Pitfalls:
Assuming ash is inert and harmless; its chemistry (iron/alkalis) and melting behavior are critical.

Final Answer:
All of the above