Hydrogen for ammonia synthesis (Haber–Bosch): among common commercial routes, which production method is generally the costliest per unit of hydrogen produced?

Introduction / Context:
Ammonia plants require large volumes of hydrogen, traditionally sourced from fossil feedstocks. Selection of the hydrogen route depends on feedstock price, capital cost, and energy efficiency. This question checks your awareness of relative cost among standard industrial options, ignoring special cases like stranded renewable electricity or by-product hydrogen.

Given Data / Assumptions:

• Conventional large-scale ammonia production.
• Typical energy prices; no special subsidies.
• Comparison across standard technologies: coke oven gas separation, steam reforming (naphtha or natural gas), and water electrolysis.

Concept / Approach:
Steam methane reforming (SMR) of natural gas is the dominant, lower-cost route at scale. Steam reforming of naphtha is similar but typically costlier where naphtha is pricier than pipeline gas. Recovery from coke oven gas can be economical at integrated steel plants. Electrolysis of water, while clean at point of use, requires substantial electricity; without very low-cost power, its levelized hydrogen cost is usually highest among these choices.

Step-by-Step Solution:

Verification / Alternative check:
Benchmark reports show hydrogen from SMR has historically lower cost per kilogram than electrolysis at typical grid prices; electrolysis becomes competitive only with very cheap renewable power.

Why Other Options Are Wrong:

• Coke oven gas: often an available by-product stream; separation can be cost-effective.
• Steam reforming of naphtha or natural gas: established, typically cheaper at scale.
• Partial oxidation of heavy oil: also typically cheaper than electrolysis for H2 per unit energy.

Common Pitfalls:
Assuming the “greenest” route is the cheapest; in practice, electricity price is decisive for electrolysis economics.

Final Answer:
Electrolysis of water.