[throwaway894345]

Why do fertilizer and steel require hydrogen in particular rather than just green-electricity-powered induction heating? Is the hydrogen meant to be a chemical component in the manufacture of fertilizer and steel rather than merely a heat fuel?

[pfdietz]

Hydrogen is needed for the Haber-Bosch process for making ammonia. For steel making, hydrogen would be used for reduction of iron ores to iron metal (currently, most of that is done with coke).

There's also a large market for using hydrogen to upgrade petroleum (hydrodesulfurization), but that market continuing to exist presumes some way of dealing with the CO2. Direct air capture, perhaps. There could also be markets developed to make synfuels from CO2 and hydrogen, or using hydrogen to upgrade biomass to get more fuel (hydrodeoxygenation).

There are various smaller markets using hydrogen. For example, making one of the precursors to polyurethane involves hydrogen as a reagent, as does manufacture of hydrogen peroxide.

[mattashii]

> hydrogen would be used for reduction of iron ores to iron metal

Wouldn't electric reduction cells like those used in aluminium production work as well? Why would you need an input of hydrogen?

[tuatoru]

Indeed, electrolytic smelting is around and is currently used to produce high purity iron.

There are a few engineering difficulties arising from the high temperatures required and the chemicals around.

And note that aluminum smelting also releases quite a lot of carbon dioxide for various reasons, one of them being consumption of the graphite electrodes.

1. https://en.wikipedia.org/wiki/Electrolytic_iron

2. https://www.newscientist.com/article/dn9878-electrolysis-may... (2006)

3. https://link.springer.com/article/10.1007/s10800-017-1143-5 (2018)

[pfdietz]

Hydrogen production via electrolysis has the property that it can be turned on and off nearly instantly. Aluminum electrolysis in molten cryolite must be kept running to maintain the temperature gradient between the molten electrolyte and the walls (if not maintained, either the walls overheat or the electrolyte freezes, ruining the cell.) The temperature for iron electrolysis in molten materials would be even higher.

One can imagine electrolyzing iron in aqueous solutions, but I understand this actually needs more energy than producing hydrogen and using that to reduce iron oxide. There is some electrolytic iron produced today, for applications that require very high purity (as high as 99.999%).

[Tagbert]

Fertilizer requires the production of ammonia NH4. That is one nitrogen bonded to 4 hydrogens. The nitrogen can come from the air but the hydrogen has to come from some other compound.

[jxf]

Dumb question: why can't the hydrogen also come from the water vapor in the air?

[pfdietz]

That would be highly endothermic. The energy needed to separate the hydrogen from the water has to come from somewhere. If you meant "why can't we electrolyse that water to get the hydrogen", well yes of course that could be done, and that's what's being talked about (using water from some other source, though; why from the air?) But the water doesn't just decompose on its own.

[throwaway894345]

You can, but doing so with renewable power would be under the rubric of "solving green hydrogen for industrial processes". In practice it's probably just easier to use renewably powered electrolysis to separate hydrogen and oxygen atoms from liquid water rather than try and use some chemical process to pull them out of the air.

[tuatoru]

Not dumb at all. Well, not exactly air, but that's how it's done. Air doesn't contain enough water, so you need to add some.

Currently nearly all ammonia is produced by "steam reformation" of methane in air (which is mainly nitrogen). Very hot steam, air, and methane are mixed. The carbon in the methane is released as carbon dioxide.

The idea is to take the methane out of the process.

[fifteenforty]

Correct. It’s not purely for heating.