Identify the furnace that is NOT directly fired (i.e., the charge is not in direct contact with flame or combustion products).

Introduction / Context:
Directly fired furnaces expose the charge to flame or hot combustion gases, while indirectly fired units heat through walls or heat exchangers. Correct classification guides materials selection, flue-gas handling, and product quality control.

Given Data / Assumptions:

• By-product recovery coke ovens heat coal in narrow chambers separated by flues.
• Open-hearth and many calcination/sintering units are directly fired.

Concept / Approach:
In a by-product coke oven, coal is carbonized with heat supplied through refractory walls from adjacent combustion flues. The charge does not contact flame or combustion products, making it indirectly heated (not directly fired).

Step-by-Step Solution:
Define direct vs. indirect firing based on exposure to combustion products.Analyze each option’s process path.Select the unit where heat crosses walls to the charge: the by-product coke oven.

Verification / Alternative check:
Typical battery diagrams show separate flues and charging/withdrawal sides with heat transmission through silica or silica-alumina walls.

Why Other Options Are Wrong:
Calcination kiln, sintering furnace, open-hearth: all commonly involve direct exposure to hot combustion gases.

Common Pitfalls:
Generalizing from one sintering variant; some processes can be indirectly heated, but the classic, widely referenced non-direct case is the by-product coke oven.

Final Answer:
By-product coke oven