Blending practice in coking: Why is prime coking coal blended with medium or non-coking coal prior to carbonization in coke ovens?

Introduction:
Coal blending before carbonization is a critical operational step in coke making. Prime coking coals develop strong plastic layers and can swell markedly during heating, potentially exerting damaging pressures on oven walls if not moderated by suitable blend components.

Given Data / Assumptions:

• Coke ovens are refractory-lined chambers sensitive to mechanical stress.
• Prime coking coals have high fluidity and swelling indices.
• Medium or non-coking coals reduce excessive plasticity and adjust coke quality.

Concept / Approach:
Excessive swelling can cause high wall pressures and structural damage. Blending balances plastic layer development, controls expansion, and tailors coke CSR/CRI, porosity, and strength after reaction. The objective is oven safety and target coke properties, not to increase ash or because prime coking coal makes “unreactive” coke by itself.

Step-by-Step Solution:
Identify risk: prime coking coal → high swelling → wall pressure.Mitigation: blend with less coking coals to moderate plasticity.Outcome: safe operation and desired coke strength/reactivity.

Verification / Alternative check:
Operational guidelines use Swelling Index, Gieseler fluidity, and Audibert–Arnu dilatation to design blends that keep wall pressure within limits.

Why Other Options Are Wrong:

• (b) is incorrect; prime coking coal does not inherently make “unreactive” coke.
• (c) includes (b) and is therefore incorrect; (e) is undesirable—higher ash harms coke quality.

Common Pitfalls:
Equating “more coking ability” with “better” without considering mechanical pressures and oven integrity.

Final Answer:
to prevent excessive swelling and high oven wall pressure that could damage the coke oven