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Comment by bloppe | Hacker News Reader

Is hydrogen really more renewable though? It releases energy when combined with Oxygen to make water, and it requires energy to split it from water (electrolysis). In this sense, it's not fundamentally different from fuel, which releases energy and carbon when burned, but requires energy (sunlight) and carbon (CO2) to re-create via photosynthesis. The involvement of carbon is a difference, but either way it's a 2-way street; we can't use either of them forever without reversing the process at some point.

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The main engine of natural hydrogen production is now thought to be a set of high-temperature reactions between water and iron-rich minerals such as olivine, which dominate Earth’s mantle. One common reaction is called serpentinization, because it converts olivine into another kind of mineral called serpentinite. In the process, the iron oxidizes, grabbing oxygen atoms from water molecules and releasing hydrogen.

So we might run out of olivine at some point, but it's going to be a long time.

But then we burn all that hydrogen and use up all our oxygen. CO2 may be a greenhouse gas, but plants pull the oxygen back off and return it to the air.

Indeed. The amount of ferrous iron in the Earth far exceeds the capacity of the atmosphere's oxygen to oxidize. The Earth is mostly reduced, with a very thin layer of oxidation on top.

I think that the 21% of the air that is oxygen is unlikely to be used up any time soon? Maybe i'm wrong...

You're right. We would need to burn the equivalent of about 1000 times the total biomass of the earth to convert the majority of the oxygen in the air into carbon dioxide (or water in this process).

I'm moderately interested in how much hydrogen we could burn without noticeably adding to sea-level rise. Probably quite a bit.

Plants aren't doing that job all that effectively, or the CO2 levels wouldn't be going up.

The entire history of industrial civilization has added 140 parts per million of CO2 to the atmosphere. For CO2, that's too much. But Oxygen is 21% of the atmosphere, or 210,000 parts per million. I think we'll be ok for a while.

>> Plants aren't doing that job all that effectively, or the CO2 levels wouldn't be going up.

Sure, but they DO reduce the level over time and they also grow faster with the higher concentration. The CO increase is less than all the CO2 we've produced with industry.

It's not ok to say there is 21 percent oxygen and this proposed process won't put a dent in it. One might have thought that about burning hydrocarbons. The question of how long can we "burn rocks" still stands. Is it 10k years or 10M or something else?

Plants do a great job pulling out CO₂ -- that's why CO₂ levels fall in the northern hemisphere's spring and early summer (see https://gml.noaa.gov/ccgg/trends/ ).

Unfortunately, insects and fungi are just as good at eating plant matter and exhaling the CO₂ back again (aided somewhat by fires) so it goes right back in to the air again.

Plants also respirate C02 when they use the sugars they generate from photosynthesis. It's why you can grow plants in a completely sealed terrarium. Some do better than others- they generally don't prefer a perfect balance, but they can survive in it.

Yes my understanding is that before there were fungi that could deal with the fallen trees they just stacked up on the ground and eventually became coal while the earth turned into snowball earth for a few million years. Sucking up CO2 has been too much of a good thing under some past circumstances.

I imagine so. The main trouble with the biofuels is that you'd have to get everything else right for it to truly be neutral impact. (i.e. not depleting the soil, making the chemical fertilizer, reactions to convert biomass into ethanol, etc.)

Electrolyzing water with wind/solar/nuclear would seem to result in far fewer steps that could go wrong.

The article talks about how they believe natural processes are constantly refilling the reservoirs. So in this context renewable means potentially not a limited source.

The story is about natural hydrogen produced through water-rock reactions.

This means it can be extracted 'for free' like natural gas and is naturally continually renewed.

Water reservoirs are naturally renewed. You’re still rate limited, as most irrigation zones around the world have found out.

Definitely is not renewable if you feel you need to drill for it.

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