Select the option that is NOT a commercially used primary feedstock for producing ammonia synthesis gas (H2 + N2 route).

Introduction / Context:
Ammonia synthesis gas (syngas for the Haber–Bosch loop) is produced by reforming or gasifying carbonaceous feedstocks to make hydrogen, combined with nitrogen from air separation or the secondary reformer. Recognising standard commercial feedstocks is essential for evaluating plant siting and economics.

Given Data / Assumptions:

• Commercial routes: steam methane reforming of natural gas, naphtha reforming, coal/coke oven gas routes.
• “Feedstock” here means the principal carbon/hydrogen source entering reforming/gasification.
• Electrolysis-based “green hydrogen” exists but is distinct from using water as a reformer feedstock in conventional plants.

Concept / Approach:
Natural gas and naphtha are common feedstocks for reforming; coal and coke oven gas are used where gas or liquids are scarce. “Tar” is a by-product contaminant from gasification, not a standard primary feedstock for ammonia syngas. Water supplies steam or is split by electrolysis to make hydrogen, but as a direct reforming feedstock in the conventional sense it is not the carbon source; thus the “not commercially used feed-stock” answer focuses on tar.

Step-by-Step Solution:

Verification / Alternative check:
Industry statistics show the majority of ammonia capacity based on natural gas; significant capacities exist on naphtha and coal/coke oven gas. “Tar” appears in gas cleanup sections as a contaminant to be cracked or removed, not as a purchased feed.

Why Other Options Are Wrong:

• Naphtha and Coal/coke oven gas: established commercial feedstocks.
• Natural gas: the dominant global feedstock.
• Water: while used for steam and electrolysis, the question targets primary feedstock; tar remains the best “not used” choice.

Common Pitfalls:
Confusing gasification by-products with feedstocks; tar must be minimized, not fed.

Final Answer:
Tar