What are disadvantages of methane or ammonia compared to dihydrogen for metal ore reduction?

Metallurgy, such as steel-making processes, require reducing agents to get rid of the oxygen in metal ores. Using fossil coal for this is a large source of $\ce{CO2}$ globally. Recently, I have read a lot how green hydrogen is proposed as a replacement for the fossil coal as a reducing agent. I also believe that hydrogen has a few issues such as its adverse effect on some metals (embrittlement) and its difficult storage and transport.


Wouldn't other reducing agents that can be easily produced from renewable energies, such as $\ce{CH4}$ or $\ce{NH3}$, be much more useful replacements for coal in metallurgy? Or do these have some chemical disadvantages that make hydrogen a much better contender?


Exemplary references about direct iron ore reduction using methane:


* [Clean iron production through direct reduction of mineral iron carbonate with low-grade hydrogen sources; the effect of reduction feed gas composition on product and exit gas composition](https://www.sciencedirect.com/science/article/pii/S2666790821003050)
* [Kinetics of Iron Ore Reduction by Methane for Chemical Looping Combustion](https://pubs.acs.org/doi/10.1021/ef402142q)




Reactants in a solid state behave totally different from reactants in a gaseous state. And that is more than enough to limit the usefulness of hydrogen, or any hydrogen carrier such as methane or ammonia over carbon.


To start off easy; solid Carbon can be conveyored, dropped, siloed and charged with a hopper. It can be supplied with a dumper truck, shipped with an open bay vessel, and hoisted with a crane bucket. It does not constitute a mortal danger doing this. When it is inside the vessel you want it to be, it does not escape.


A solid substance has an activity (gamma) of 1, its behaviour is well understood and leads to an intrinsically stable operation. Gaseous reactants must be monitored and controlled carefully so as to maintain the right activity in the reaction zone. The vessel itself must perhaps be an autoclave for the right conditions. You could very well create Silicon by reduction with hydrogen, but unfortunately the thermodynamic equilibrium at atmospheric pressure is low yield; you would either need a two stage setup with gas transfer or an autoclave. For electrolytic reduction it is much the same. I wrote my thesis on titanium diboride anodes so as to get rid of the anthracite/graphite anodes; you could even gain efficiency by lowering the electrode separation distance substantially, but as se can see 20 years later it was and is still not economic to do so.


Not to mention the considerable added dangers when introducing hydrogen or ammonia to an already exciting situation.


It is usually, but not always, feasible. If it was easy or economic it would have been done already. Since we are talking about those that aren't, you can conclude that it is either a) impossible, b) so complicated it looks like option a or c) reduces yields or increases costs.


Finally even green hydrogen is no greener than the marginal green-ness of electricity. Which is very often a shade of absolutely pitch black.


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Chemists would not use either $\ce{CH4}$ either $\ce{NH3}$ in the first place.


Their true acting reducing agent is hydrogen again. But it would have to be formed first. The partial hydrogen pressure would be low, the reduction slow and inefficient.


$\ce{CH4}$ would still produce $\ce{CO2}$.


Additionally, iron and steel production count on carbon content in iron.


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