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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:

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    $\begingroup$ Challenges of technology, where the solution is missing for decades, are very seldom solved by a single Q/A $\endgroup$
    – Poutnik
    Commented Jul 20, 2023 at 6:04
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    $\begingroup$ @Poutnik I am not solving any technological challenges but I am asking a simple chemistry question: "What are the disadvantages of CH4 and NH3 compared to H2 as a reducing agent in metal making?" If you feel deterred by the background I put in the question body, either ignore it or delete it, please. I edited the title to emphasize that question. $\endgroup$
    – tobalt
    Commented Jul 20, 2023 at 6:32
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    $\begingroup$ Aside from chemical disadvantages, using ammonia sounds like an energetically expensive solution, like using ethanol from corn as car fuel. $\endgroup$
    – Buck Thorn
    Commented Jul 20, 2023 at 7:45
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    $\begingroup$ Well, it would help to define "chemical disadvantage". Ultimately you might reduce this to a question of cost (capital), which might include environmental costs, or complexity, which might include factors such as safety, time to build a plant, practical limits to the scale of the process etc. $\endgroup$
    – Buck Thorn
    Commented Jul 20, 2023 at 9:33
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    $\begingroup$ @BuckThorn I am not after things like storage or transportation in this chemistry question. It is really about trying to reduce a piece of metal ore with CH4 or NH3 as opposed to C or H2. Does that work worse, i.e. requires higher temps or is slower, i.e. ultimately less economical? $\endgroup$
    – tobalt
    Commented Jul 20, 2023 at 9:56

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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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    $\begingroup$ The actual reduction agent in a classical blast furnace is CO. You're right with the rest of what you say. ;) $\endgroup$
    – Karl
    Commented Jul 20, 2023 at 21:30
  • $\begingroup$ Would you consider removing the last two irrelevant paragraphs and maybe instead elaborate why solid reactants (I guess) are favorable in terms of reaction economics to gaseous ones? $\endgroup$
    – tobalt
    Commented Jul 22, 2023 at 7:07
  • $\begingroup$ @tobalt the second last paragraph is a direct answer to your why not, and the last one is my personal quest to educate the servile masses in using the word 'green' sparingly. But I will elucidate once I am back on my laptop, writing equations is too arduous on a mobile device. $\endgroup$
    – Stian
    Commented Jul 22, 2023 at 13:58
  • $\begingroup$ @Stian Ok maybe "irrelevant" was the wrong term. What I mean is that these paragraphs contain no substance because they tell trivial truths. Thanks for now adding this substance :) I think, however, that you try to make a point in case of Carbon vs any of those replacements. But the question is about CH4/NH3 vs H2. $\endgroup$
    – tobalt
    Commented Jul 22, 2023 at 17:07
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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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  • $\begingroup$ Why would they not use CH4 or NH3 is the main question. The partial pressure of "hydrogen" at the same gas pressure is higher in both CH4 and NH3 than in H2, having 4 and 3 hydrogen atoms per molecule, instead of 2 if I am not mistaken. And the production of CO2 from renewably generated CH4 is of course exactly a net zero. The C alloy content in these metals is not really an argument pro-hydrogen either is it ? In summary, I don't see how this answer adresses the question in its current state. $\endgroup$
    – tobalt
    Commented Jul 20, 2023 at 9:14
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    $\begingroup$ You are really mistaken. Review what the partial pressure is. Sure, C content is against the whole H2/CH4/NH3 agenda. All that would be energetically and technologically very challenging. I did say chemists would not use CH4 nor NH3 (but H2 or C or CO), as you have asked a "simple chemistry question". $\endgroup$
    – Poutnik
    Commented Jul 20, 2023 at 9:21
  • $\begingroup$ Just to make sure I understand correctly: Does it mean that CH4 and NH3 aren't really reducing agents in molecular form, but only CO, C or H2 are? And that only the tiny amount of elemental H2 that thermally cracks from e.g. CH4 would work as an active reducing agent? $\endgroup$
    – tobalt
    Commented Jul 20, 2023 at 9:53
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    $\begingroup$ In a summary, you are correct. Additionally, NH3 and CH4 would still keep disadvantages of H2, regarding metallurgic aspects. For some other metals than iron, hydrogen is already used, especially if carbon is not applicable. $\endgroup$
    – Poutnik
    Commented Jul 20, 2023 at 9:54
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    $\begingroup$ At temperatures where H2 effectively reduces iron oxide, NH3 falls apart into its base components. Which means there is no point in making NH3 in the first place. $\endgroup$
    – Karl
    Commented Jul 20, 2023 at 21:36

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