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Comment by webkike | Hacker News Reader

Could most uses of natural gas be replaced with hydrogen gas?

Sort of. You can burn it certainly, but it's extremely hard to ship - it leaks out of almost anything, and damages most metals.

For example you can transports natural gas in plastic pipelines - can't do that with hydrogen since it will leak right out.

Homes often have cast iron pipes for natural gas - can't put hydrogen in those because you will make the metal brittle.

There are metals that can handle it, but it would be unrealistic to replace everything.

There's a reason why different gasses are used in different applications. Combustiblity, pressure required for liquefaction, flashpoint, safety and so on.

There's a lot of overlap between these in some areas like home heating and BBQs, but each application requires some engineering to make it work with any particular fuel.

The energy storage density of hydrocarbons make them more practical as a fuel in situations where portability is a concern. For example: You could have a hydrogen BBQ, but the tank would probably have to be much larger for the same amount of burn time.

> You could have a hydrogen BBQ, but the tank would probably have to be much larger for the same amount of burn time.

Isn't the opposite true? I mean, hydrogen at 700 bar has a specific energy of 142 MJ/kg while hydrocarbons in general have less than 60 MJ/kg.

Yup but propane is 20x denser than hydrogen, so even if you have 2x energy per weight, you have 0.1x energy per volume. Or something like that.

Gotcha, thanks!

No, hydrogen gas is less dense than hydrocarbons (at the same pressure) since it has a lower molar mass (volume is inversely proportional to the molar mass).

Worst case you can convert the hydrogen to natural gas.

There are a lot of processes that use methane for the hydrogen though.

As soon as we can convert CO2 into hydrocarbons using solar or wind power in a way that's cost-competitive with conventional extraction we'll have turned an important corner in developing as a society.

I wonder if that is more of a technological problem or more of a social problem (recognizing the costs of conventional extraction).

If you can discover a way to cost-effectively synthesize hydrocarbons (CH chains) from CO2 and H2O, which contain all of the necessary components, you're pretty much guaranteed a Nobel prize.

So far the process to do this involves a lot of messy intermediate steps, like hundreds of millions of years and enormous pressure in the mantle of the Earth.

There are pilot projects:

http://www.audi.com/corporate/en/corporate-responsibility/we...

(mentioned on https://en.wikipedia.org/wiki/Sabatier_reaction)

Who knows what the economics really are, but they don't sound dire.

Why not avoid making the CO2 in the first place? There's a reason Reduce comes first in the "Reduce, Reuse, Recycle" mantra.

That's essentially what they mean.

We need energy in portable forms (and have other uses for hydrocarbons). Getting that energy from clean sources and storing it with atmospheric CO2 is better than burning fossil sources of energy.

Yeah, if you close the loop you absolutely eliminate the problem of carbon buildup in the atmosphere.

Growing sugarcane does a pretty good job of converting CO2 into hydrocarbons using solar power.

But to be cost-competitive with fracked methane, I'm pretty sure we're going to need some heavily genetically-engineered strains of algae, growing in artificial forests of pipes.

The irony is that algae can be used to clean up fracking wastewater, so it makes methane production cheaper as it competes.

Sugarcane and switchgrass might be able to close the loop, but these take a lot of mechanical processing and land to do so, which could introduce additional costs.

The promise of syntetic fossil fuels from things like water and CO2 is that you don't need a lot of land, you just need energy, and that energy can be renewable.

I'm not sure if it's a factor, but hydrogen gas is relatively difficult to store. It tends to seep through the walls of any container.

IIRC one "infrastructure-scale" battery idea is compressing hydrogen into salt domes. While I can't speak to the intersection of the article and this idea, people can clearly store hydrogen in massive amounts.

Unlike natural gas (which we can mine from the earth), hydrogen can't be an energy source because there's no place you can mine it. It can only be a medium of transmitting energy: you need some other source of energy in order to create hydrogen, and then you can transport it somewhere else and use it there.

So the question is what is that source of energy, and whether we could simply connect the source directly to where the energy is needed and cut out the middleman H2.

I think they are thinking about things like fertilizer, where natural gas is a major feedstock.

I think the idea would be to build plants that burn hydrogen to operate a steam turbine and transmit the electricity.

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