Hydrogen production estimates will vary by more than a factor of 10 depending on what one chooses to include in the tally.
Hydrogen is produced deliberately (AKA, on-purpose) and recovered from various industrial processes as a byproduct. On-purpose production may be consumed by the same company (AKA, captive) or sold to another company (AKA, merchant). Similarly, byproduct hydrogen may be consumed by the same company or sold to another company.
The largest captive producers are associated with ammonia and methanol plants, followed by oil refineries. Some may question the inclusion of hydrogen associated with methanol production and any other chemical process starting with syngas generation because, unlike ammonia production, hydrogen is only produced as part of a mixture of gases.
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