Hacker News Reader

Comment by SoftTalker | Hacker News Reader

Houseplants. Lots of houseplants.

More seriously, a house air filter with CO2 scrubber is not really something that would be difficult to make.

IIRC you need ~300 plants to offset every one human in a house

Yes, it's impractically high: we exhale about 1kg of carbon daily, so your plants collectively need to be getting about 1kg heavier daily to absorb that.

Don’t the plants exhale oxygen? So they won’t be getting 1kg heavier, closer to 300g, but I don’t know the actual chemistry.

That's just dry mass and plants are mostly water.

Yeah it was sort of a joke, but having houseplants will make some difference. Not that they would be practical to absorb all the CO2 in the air. On the downside, keeping indoor plants healthy does take some ongoing time and effort and isn't everyone's cup of tea.

It often makes a negative difference because bacteria in the soil produce a lot of co2.

It's an order of magnitude worse than that, plants are mostly water, not carbon. They'll gain tens of kilos per day.

If you can feed off your house plants alone, then you have a chance of closing the carbon loop. This was tried (e.g. biosphere2) and it's extremely hard even at the industrial scale.

> They'll gain tens of kilos per day.

Feed me Seymour.

People don’t need to eat 10’s of kilos of plants per day which should suggest your off by an order of magnitude.

Chemistry means converting 1kg of CO2 > O2 would mean removing ~273 grams of carbon. Hydrogen and Wet vs dry weight more than offsets this, but you’re at closer to 2kg than tens of kilos.

This still assumes an air tight system where people never leave home, so plants can make a difference long before they are a 1:1 replacement.

Living on Earth surface all that's necessary is to bring in outdoor air and reduce it to say 300ppm, currently that's only a 120ppm gap.

I don't know what portion of a typical plants mass is carbon, and your 273 grams of carbon number is right (the 1kg number is CO2), but I don't think "people don't need to eat 10's of kilos of plants per day" actually justifies your claim about their density or invalidates GPs claim as to how much plant-mass you need to offset what you eat.

We don't just eat random (average) parts of plants, we eat selected ones, primarily things like fruits and bulbs that plants store energy in. If you just tried to eat, say, lettuce... at 2000 calories per day you would need roughly 14kgs [1, 2]. Which is surprisingly close to GPs 10s of kg number all things considered.

Either way, growing 2kg of plant/day or 20kg of plant/day... seems impractical to me. There's also the issue that if you equalize CO2 levels with the outdoors during the day as you open doors and windows, you're going to make night worse as both you and the plants output CO2 during the night.

[1] Lettuce is 14 calories / 100g - https://www.fatsecret.com/calories-nutrition/generic/lettuce...

[2] Math here: https://www.wolframalpha.com/input?i=2000+calories+%2F+%2814...

> We don’t just eat random (average) parts of plants, we eat selected ones

We can’t digest cellulose which throws off the calculations based on what calories are available to us. The question was the minimums not the worst case. Obviously most carbon sequestered by trees isn’t available to us a calories.

In a closed environment simply eating plants wouldn’t result in the correct carbon balance long term. We would need to breakdown our waste via microbes, fungi, or burning.

About 1kg of carbon dioxide, not carbon. 1kg of CO2 is only about 0.27kg carbon.

this ignores "system balance" and assumes "closed system" no? In other words, no one says that plants alone will compensate.. but instead they tip a system in a more favorable direction.. Plus some people really respond to house plants in lots of complimentary positive ways.

More like 700 according to this

https://medium.com/@candidegardening/how-many-plants-would-i...

When plants remove CO2 and release O2, they have to do something with that carbon atom. They can use it to make sap or fruit, but mostly they use it to build more plant.

A plant that isn't growing, like, a LOT, can't possibly be removing much CO2.

Most of a tree is CO₂ that the plant has taken out of thin air.

This only clicked for me when I started wondering why you never have to refill the soil for a houseplant even when it grows.

Richard Feynman talked about this in his talk about “What is Fire?”

https://www.youtube.com/watch?v=N1pIYI5JQLE

The soil itself is also storing CO2, in a variety of different less than intuitive processes. The most obvious is when parts of the plant die and breakdown into the soil itself.

I would love that, but how? There is no available residential CO2 scrubbers.

Bringing in external air (and filtering it, if it's polluted) will be way cheaper than scrubbing CO2 at current levels.

Definitely. Filtering particulates and tempering the outside air is the way to go in anything but most extreme situations. I suspect even designing a fallout shelter you would want to go that route.

An ERV costs about ~$1000 plus labour (you can install yourself).

This is true, but mostly because the cost of bring in external air is near $0 in most cases - just open a few windows! But there's a lot of absurdly cool ways to scrub CO2 that are not especially expensive, like the video-game level of serendipity+convenience that's the Sabatier Reaction [1]. Combine hydrogen + CO2 at 300C => methane + water.

It's already being used on the ISS to keep the air fresh. It'll also be a key component on Mars. There you have basically unlimited CO2 and unlimited hydrogen which yields unlimited methane and unlimited water. Carry out electrolysis on some of the water and you get hydrogen + oxygen. The hydrogen can be fed back into the original cycle, and so you end up left with methane + oxygen which is also known as rocket fuel, uncoincidentally the exact sort SpaceX's Starship is using.

Back on Earth you could use this reaction to create practically endless carbon neutral fuel so long as the CO2 was sourced from either the air or a regrown natural capture source like trees or what not. Rockets using this fuel would even have a negative carbon footprint since some of that fuel will get burnt outside the atmosphere. It all seriously feels like a video game where there's just this perfect, but just utterly contrived, rule allowing you to thrive on the next level of difficulty.

[1] - https://en.wikipedia.org/wiki/Sabatier_reaction

The reaction is most interesting, but there are three concerns:

(1) It presumably requires concentrated CO2. It's not trivial to concentrate CO2 from the air without the appropriate technology.

(2) Pure hydrogen can risk an explosion.

(3) What does one do with the methane? Also, it too can risk a fire.

For these reasons, this is not a reaction that one would seek to do at home.

The Sabatier reaction is completely safe. Here [1] is a video of a few kids doing it completely manually with a couple of syringes, a pipe, and a torch. Producing methane is one of the huge benefits. Methane is just a normal fuel - use it to generate electricity, for your oven/stove, heating, whatever. But at home if all you want is CO2 removal then just use some lye, soda lime, or a million other simple capture technologies. Not quite as sexy, but it'll get the job done.

<rant> There's no grand technologies required for any of these things. The only reason it's not done at scale is the same reason we can't have nice things in general. Governments are far more interested in war and cock measuring contests than actually solving problems. We spent literally trillions of dollars on Iraq and Afghanistan, and the latest wars will probably dwarf that. Just imagine all the crazy sci-fi scale of things we could have done with all that money. Alas, humanity seems to be running on loop, except now we have nukes, paired alongside what are likely some of the most idiotic 'leaders' in centuries. Want to know the answer to the Fermi Paradox? </rant>

[1] - https://www.youtube.com/watch?v=VdNPkW9I0fQ

Well said. The root cause as I see it is the nature of money used by the people. If money were gold or bitcoin or a similar limited commodity, with a sufficient layer of anonymization, also with global validity and use, then many of these problems would not happen because the governments just would not have so much money to waste. The governments use their guns to enforce that we use their money instead which is prone to heavy inflation and excessive taxation. Reasonable taxation can still exist even with gold/bitcoin via property taxes. We do not have financial freedom.

ERV/HRV systems exist and their principle of operation is super simple. Labour to install the piping costs more than the system itself.

Don't the activated carbon prefilters used in many residential air filters work to scrub CO2 at least a little?

Try calculating how much heavier the filter must become every day to make a difference

Not in the slightest. Those are for filtering volatile organics.

There will be. Mind you, I’m not too happy about a future in which nobody can go outside.

Instead of scrubbing CO2, just adding the other important components like N2 & O2 to reduce the percent of CO2 in the air would probably be easier

No, because CO2 is only 0.04% of the air, O2 is 21% and N2 is 78%.

Navies have been removing CO2 from submarines for decades. I think they use sodium hydroxide.

Manned space programs must remove CO2 air breathed by astronauts. They used lithium hydroxide for decades (including famously in Apollo 13) but ISTR the international space station has a more sophisticated process.

The ISS has a clever trick of using zeolite beds to absorb CO2 (a rock with pores that selectively hold onto CO2, and H2O but they dry the air first), and then venting it to space to get rid of the CO2 and recharge them.

I wonder whether they need to heat the zeolites to help them release the co2 or whether exposure to vacuum is enough.

If I recall correctly they do heat it (I dug up the documents on this awhile ago, just going from my memory right now).

Reply on news.ycombinator.com