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Comment by jarvist | Hacker News Reader

I'm not sure if this article hammers home the key point: extracting carbon from the air always takes more energy that was gained by burning the fossil fuel to make that carbon. This is inevitable due to the entropy cost of de-mixing CO2 from the air, and from the limited efficiencies of thermal power generation (~33-40% for a coal power plant, perhaps 20% in an internal combustion engine), and limited efficiency of the carbon capture technology.

So it only makes sense to spend energy on sucking CO2 out of the atmosphere, once you've already de-carbonised everything which you possibly can.

Maybe it has a future role (>2050? >2100?) to start using excess low-carbon energy to take CO2 levels back to their pre-industrial level, or to constantly offset hard-to-decarbonise technologies such as air travel.

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Not objectively true, as plants and even rocks grab CO2 out of the air pretty efficiently. From a thermodynamics standpoint, you can burn fossil fuels at 50-70% efficient (50% is not uncommon), and the actual theoretical lower bound of CO2 direct air capture is ~20 kJ/mol,[0] or about 1.7MJ/kg of elemental carbon, compared to 33MJ/kg for burning elemental carbon (same as high grade coal).

(Burning methane is even better as it releases much more energy per unit CO2 i.e. ~860kJ-per-mole-of-CO2 vs anthracite's ~400kJ.)

And as others have said, rocks actually are able to exothermally absorb CO2 permanently. It's possible to speed this up by grinding rocks to a powder, placing in water with a catalyst.

...none of this is to say I support relying on direct air capture and just go ahead and burn fossil fuels. We need to keep that in the ground and transfer the whole civilization to non-fossil fuels, otherwise if some populist jerks wer elected, they could just order the expensive direct air capture program to stop (can't do that if you're already transitioned to clean energy and all your wellheads and drills and coal mining equipment are rusted or melted down for scrap).

Source: [0] https://www.researchgate.net/publication/257177020_The_therm...

> Burning methane is even better

Actually, burning is probably the best way to deal with any excess methane. Methane has a significantly higher warming potential than CO2 does. So if there's any methane we're venting into the atmosphere, we should burn it to convert it into CO2. Bonus points if you can use the generated energy to capture the CO2 as well.

From what I can recall, landfills produce more methane than any other process. How about we build a carbon capture next to every landfill, divert the methane and burn it to run the DAC?

Landfills do collect and burn methane. Those pipe like structures you see sticking out of a completed landfill are part of the process.

https://archive.epa.gov/climatechange/kids/solutions/technol...

TIL. Now we just need to execute the second part of his plan to capture the carbon from the energy produced

Or capture the methane, upgrade it and run generators/cars/busses of it. It's existing tech

https://en.wikipedia.org/wiki/Biogas

This scheme is used in real life in Czech Republic

Why bother for landfills?

If the landfill can generate electricity that would otherwise be generated, it will convert the CH4 more efficiently than a bus and you don't expand energy upgrading it.

You can power the bus with the already high quality CH4 you extracted from the ground that you would have otherwise burned in to generate the electricity.

Buses have different power requirements than cars; they need a lot more range because they run throughout the day.

I was reading somewhere that in buses in cold climates, heating is actually what eats up all the energy in an electric bus. ICE buses just use the waste heat from a combustion engine.

Or you just burn the trash directly.

This.

Releasing CO2 and NOx.

From what I can see, yes, methane is abstractly a worse greenhouse gas. However its absorption band is within that of water vapor. Since water vapor is plentiful around the blue marble, methane doesn’t really cause any problems.

This is incorrect, it’s well established that methane is a more potent GHG. See the reply here: https://news.ycombinator.com/item?id=21485738

What’s not taken into conservation with that comment is that methane at those altitudes does not impact much because the energy is radiate back to space.

You have doubled down on a casual assertion ("methane is not a significant greenhouse gas") because you don't understand radiative transfer in the upper atmosphere. It's OK, radiative transfer is complex. Your options are to accept this consensus, or do a lot more study than you have.

With solar prices hitting negative rates for some periods in a day capturing carbon and converting into burnable fuel will probably start making sense in the next few years.

Think we could frack with corn syrup? Inject that carbon back into the ground?

Not corn syrup. That takes way too much CO2 and water to produce. Not even corn itself I suspect.

It's partly the problem alleged marshland and forest sequestration schemes face, alongside the assumption they're going to grow well in the changed climate.

(Limited experimental data suggests this works up to some 500 ppm as long as there's excess water and minerals. If the conditions are bad you lose capture potential. And what do you do with the methane releases from these systems? It's much more diffuse than from landfills.)

As long as there's still fossil fuel powered transportation, it would make more sense to use that excess power to charge batteries, since that converts electricity into motion more efficiently than taking the detour through synthetic burnable fuel.

Destroying fossil fuel powered transportation then replacing them with electric would actually produce more carbon. All new transportation should be electric but the already running transportation if can be run on captured carbon would result net carbon decrease.

'Populist jerks' meaning people who are democratic?

You're assuming perfect elasticity of energy supply which there absolutely is not.

There is quite a mismatch between solar and wind production and electricity usage patters which is why storage is such a hot issue. Already there have been brief periods where 100% of energy production in a few regions was renewable. Those will certainly only grow in frequency and duration.

You don't start working on this technology when everything else is ready for it, you start as quickly as possible so the tech is ready when the rest of the world is ready for it.

I think that you're thinking of the cost of turning the CO2 (and water) back into fuel, which can be argued along thermodynamic and irreversibility lines to show that unscrambling that egg always requires more energy than obtained from combusting. However, I don't think there is a similar thermodynamic argument on the energy costs of mere capture. It's not a priori impossible that a fractional distillation column with a clever configuration of heat exchangers, for example, could capture all the carbon released by the fossil-fueled plant that powered the column in the first place (and more).

Arguing that mere capture would require more energy than is released would be akin to arguing that cleaning the fireplace would require more energy than was released by the fire, on the mistaken grounds that ash removal took as much energy as regrowing the wood that was burned. Both the ash and the CO2 are in a lower energy state, so there's thermodynamic room to maneuver.

> So it only makes sense to spend energy on sucking CO2 out of the atmosphere, once you've already de-carbonised everything which you possibly can.

Not necessarily. I'm thinking of cold polar winters with no wind and no sun, people burning fossil fuels to stay warm. Meanwhile in deserts solar panels powering carbon capture machines to reverse the greenhouse gases back again. Maybe even the carbon capture creates synthetic coal which can be shipped to the cold regions.

Yeah, you can basically use the atmosphere as a chemical battery.

Also, a better example than heating homes with coal is aircraft. Long distance air plane travel is so far only doable via fossil fuels due to their energy density. And while you can run a electric line to a house and heat it with electricity, this is not possible for long range aircraft.

So the idea is you use hydrocarbons to run aircraft and then capture the CO2 produced and turn it into more hydrocarbons to run your airplanes. This of course requires clean energy source to work, but is perfectly doable. :)

> it only makes sense to spend energy on sucking CO2 out of the atmosphere, once you've already de-carbonised everything which you possibly can.

Except possibly when energy prices are negative due to wind and solar intermittency - although that is also a signal that you need more storage in the grid, additional storage may not be economic for some reason.

Some captured CO2 might be used with cheap electricity to make carbon neutral fuel - which is of course a form of stored energy itself. A pilot project in Germany is doing this[1]:

> The green hydrogen will be combined with carbon dioxide captured from the cement plant to produce synthetic methanol, which would then be refined into carbon-neutral synthetic kerosene (ie, aviation fuel), for use at the nearby Hamburg airport. The airport is hoping that 5% of all fuel used there will be carbon-neutral by 2025.

[1] https://www.rechargenews.com/transition/offshore-wind-to-pow...

Honestly, I suspect that long-term storage will be inefficient for a long time.

For example, batteries exceed 90% efficiency for storage, but they are only cost effective for day-to-day storage. (Charge when the sun is out, discharge at night.)

In contrast, charge during long summer days, and discharge during shorter winter days will probably be lower efficiency at first. For example, generating hydrogen by electrolysis is about 75% efficient (https://en.wikipedia.org/wiki/Hydrogen_production#Electrolys...), and then pressurizing it will also consume energy.

This means it may be more cost efficient to extract CO2 from the atmosphere as "storage" and then burn natural gas... At least until we find a better way to store energy than hydrogen!

There is a well known relationship summarized in the Sherwood plot.

https://www.pnas.org/content/pnas/108/51/20428/F1.large.jpg

Your basic intuition is correct in that the more dilute something is, the more costly it is to extract/purify it.

A properly engineered DAC system will be much less expensive than the number they found.

https://www.cell.com/joule/fulltext/S2542-4351(18)30225-3

That paper only looks at the energy/cost balance of the plant, not the chemical feedstock. Production of that feedstock is energy intensive, the recycling is not 100% efficient per the paper (i.e. it will constantly have to be replenished), and the industrial infrastructure to produce those chemicals at the necessary scale to materially reduce atmospheric carbon simply doesn't exist.

Any cost model for large-scale DAC must include the capital and operational costs of new feedstock factories to keep the DAC supplied at scale. It won't be cheap. The pilots can ignore this because they rely on the excess capacity of existing industrial infrastructure.

Not at all -- the cost of makeup streams for chemicals are included in the O&M cost (table 2).

> extracting carbon from the air always takes more energy that was gained by burning the fossil fuel to make that carbon.

This sounds true at first glance, but is it always? It seems like it's obviously true for simple reactions like burning methane and converting it back to methane since the energy you get from it is equal to or less than the energy required to turn it back into methane, but might there be some other chemical reactions where you can turn it into something less energetic than the molecule it came from?

Yes, there are rocks that will "extract" carbon from the air on their own if crushed into sand. They are even reasonably abundant, so it seems like they would be a feasible tool for carbon capture. It would basically amount to speeding up the natural carbon cycle, which normally keeps carbon dioxide concentrations relatively stable in the very long run.

The rocks are called olivine, and are very common. They indeed look very promising for CO2 absorption.

https://www.betterworldsolutions.eu/is-olivine-the-solution-...

It's not true at all. Turning the CO2 back into fuel would take as much energy as burning it released, but simply capturing the CO2 from the air need not.

> Maybe it has a future role (>2050? >2100?)

It absolutely must, or else it's basically already too late: https://www.vox.com/2016/10/4/13118594/2-degrees-no-more-fos...

> This image should terrify you. It should be on billboards.

> As you can see, in either scenario, global emissions must peak and begin declining immediately. For a medium chance to avoid 1.5 degrees, the world has to zero out net carbon emissions by 2050 or so — for a good chance of avoiding 2 degrees, by around 2065.

> After that, emissions have to go negative. Humanity has to start burying a lot more carbon than it throws up into the atmosphere.... Thus far, most demonstration plants of any size attaching CCS to fossil fuel facilities have been over-budget disasters. What if we can’t rely on it? What if it never pans out?

> If we really want to avoid 1.5 degrees, and we can’t rely on large-scale carbon sequestration, then the global community has to zero out its carbon emissions by 2026.

(emphasis mine)

My dad sees articles like this and he'll just say that maybe the damage needed to prevent warming is worse than the damage that will be done by the warming.

The cost to ordinary people would actually be relatively minor. You buy an electric car instead of a gasoline one. The average new car price in the US is ~$36,000. A new Nissan Leaf starts below $30,000, Model 3 only a little more than that. Replace coal with alternatives. We can do at least as well as nuclear, which is barely any more expensive than coal has been. Best case is that storage gets a lot cheaper than it is and then the price of electricity goes down from the historical norm.

Capture could cost as little as zero. Renewables inherently have to build over-capacity to compensate for intermittency, but that means there are days when they generate more power than there is demand. Use the rest for carbon capture and it's free.

The real "cost" to fixing it, and the big lobby against it, is that the oil and coal companies would take a nosedive. Literally trillions of dollars in lost revenue. It moves to alternatives. It isn't economic damage, it's displacement. But the specific people who own the oil and coal reserves stand to lose their shirts, and these are not people lacking in political influence. Everything you hear about the high cost -- it's only really a cost to them. To you it's just an upgrade to electric.

But we still have to actually do it. A carbon tax so that people do all buy electric cars and stop burning coal. It's a thing we can do but not a thing we have done yet.

The point of the Vox article is that just switching from gas to electric isn't going to be enough. The argument is that without large-scale carbon capture we will not hit the 1.5C goal.

Large-scale carbon capture is still untested. Reforestation is the only well established large-scale carbon capture but is very land intensive.

Spekboom (Portulacaria afra) is said to sequester up 10 times the carbon of a rain forest by area.

This is largely due to a special mechanism that continues photosynthesis at night.

And it grows best in semi-arid regions.

Where people don't live or farm.

interesting, the Portulaca Oleracea is edible, I wonder if it has the same property and whether we should plant a ton of it.

Wikipedia sais it does:

"P. oleracea is one of very few plants able to utilize both CAM and C4 photosynthesis pathways, for a long time believed to be incompatible with each other despite biochemical similarities. P. oleracea will switch from C4 to CAM pathways during times of drought and there is transcription regulation and physiological evidence for C4-CAM hybrid photosynthesis during mild drought."

Your dad is right.

If the wild alarmists are correct that the only way to prevent destructive climate change is to immediately dismantle all human economies and return to a life more like that of the middle ages, maybe we should just forget the whole thing and invest in sunscreen.

>It absolutely must, or else it's basically already too late:

This is solution bias though. "We need it to work therefore it will"

The universe doesn't owe us anything. DAC has been added to these scenarios as a sort of balancing element to make the models add up but that has no bearing on whether DAC is possible at scale.

> So it only makes sense to spend energy on sucking CO2 out of the atmosphere, once you've already de-carbonised everything which you possibly can.

The actual point is much narrower than that. It doesn't make sense to spend energy on sucking CO2 out of the atmosphere, unless:

1) You use an energy source that produces less carbon than the amount you're taking out; and

2) You're not displacing demand that must then be satisfied by an energy source that produces more CO2 than you're taking out.

In other words, it doesn't make sense to burn coal to capture CO2, but it doesn't also make sense to use nuclear power to capture CO2 if that means that some other demand will have to be satisfied with coal instead of nuclear power.

But it goes too far to say that the above criteria only hold "once you've already de-carbonised everything which you possibly can."

For example, if you build a surplus of renewable power (which we have in some cases, where instantaneous generation exceeds what the grid can use at that moment), it makes sense to use that to suck out CO2, even if, for example, you haven't replaced ICE vehicles with electric vehicles.

As a practical matter, we need carbon capture much earlier than 2050 or 2100 to keep below 1.5C or 2C. For example, China plans to ban EVs by 2040: https://www.forbes.com/sites/energyinnovation/2018/05/30/chi.... Even if they stick to that plan, they will put out hundreds of millions of ICE vehicles on the road until that time. Even by 2025, official goals will leave 60% of new Chinese vehicles as ICE.

If you look at the mitigation pathways for 1.5C, they require the whole world hitting net zero emissions by 2050 and often going net negative soon after that: https://www.ipcc.ch/site/assets/uploads/sites/2/2019/05/SR15...

That will not happen. Indians and Chinese aren't going to stop building high-speed rail (which puts out a huge amount of carbon due to the concrete used), skyscrapers, etc. Nigeria's population is going to balloon to 400 million. We're going to need to build a large number of nukes and offshore wind to capture all that new CO2.

> if you build a surplus of renewable power … it makes sense to use that to suck out CO2

Although it's unclear that resources to build carbon capture infrastructure wouldn't be better spent on (say) energy storage.

> China plans to ban EVs by 2040

I think they plan to require EVs by 2040...

Here's my vote for planting more trees and especially letting older forests grow and thrive.

Sure, but does it matter? If we decide that's a better use for the energy, then we spend it that way. And can't we capture any carbon generated through that process instead of releasing it as further emissions?

Cheaper if you are planting trees/plants or algea for say diesel production

Exactly!

But even so, meaningful collective global action is seeming less and less likely. So I can imagine that some with surplus greenhouse-neutral energy could capture CO2 to offset emissions by others.

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