High-temperature carbonisation — What is the principal effect of free moisture present in coal during high-temperature carbonisation in coke ovens?

Introduction:
Moisture content in the coal charge influences heat transfer, volatile evolution, and secondary reactions during coke oven operation. Understanding its role aids in by-product recovery quality and oven scheduling.

Given Data / Assumptions:

• Scenario: high-temperature carbonisation (metallurgical coke making).
• Presence of free (non-bound) moisture in the coal charge.
• Interest: effect on volatile products and process time.

Concept / Approach:
As the charge heats, free moisture evaporates, absorbing latent heat and locally lowering peak temperatures where volatiles emerge. This moderating effect can reduce thermal cracking of vapours at hot surfaces (coke and oven walls), helping preserve valuable by-products (e.g., tar fractions). However, the need to drive off this moisture typically increases, not decreases, overall coking time.

Step-by-Step Solution:
Recognise evaporation of free moisture absorbs heat (endothermic), moderating local temperatures.Lower local temperature → less secondary cracking of volatiles → protection of products.Time impact: moisture removal adds a drying stage, lengthening coking time rather than reducing it.

Verification / Alternative check:
Operationally, higher-moisture coal increases heat duty for drying and can extend push times; by-product composition often shifts toward heavier fractions due to reduced cracking.

Why Other Options Are Wrong:

• (a) Opposite of practice; drying time adds to the cycle.
• (c) Moisture tends to suppress dusting/agglomerate fines during charging, not increase loss.
• (e) Coke strength is governed mainly by rank, maceral balance, ash chemistry, and HTC conditions; mere moisture does not “temper” to strengthen.
• (d) Not applicable since (b) is correct.

Common Pitfalls:
Confusing effects on by-product cracking with effects on oven throughput; assuming moisture is always detrimental to by-products.

Final Answer:
It helps protect volatile products from cracking by moderating temperatures near hot coke and oven walls