So I was just thinking about how my city is annoyingly polluted. In the absence of successful collective action to reduce the sulfur content of diesel fuels, and to require proper maintenance of motors, it seems like it would be possible at least to handle the problem at a more local level. Like a per-apartment level.
You could seal up your apartment or house, but that has a couple of different problems. The biggest one is that when the amount of CO₂ in the air rises to about 1% you will start to feel that it’s a bit hard to breathe, and when it reaches 10%, you will die — even though there is still plenty of oxygen left (barring a Biosphere-II-type surprise).
So this kind of thing is a problem that has been dealt with in a lot of different contexts over the years, including scuba diving, nuclear submarines, the Space Shuttle, the ISS, firefighting, mine rescue, and carbon capture for power-plant flue gas.
The scuba-diving approach, which is also sometimes used in submarines, is to pass the gas over a base that reacts with the CO₂; they use hydroxides of calcium, magnesium, sodium, and lithium, which transform from hydroxides into carbonates. The trouble with these is that you have to keep replacing the hydroxide, which is kind of undesirable. You can buy calcium hydroxide at the hardware store here for use as paint (whitewash), for about US$3 per kilogram, although it’s mixed with an unspecified quantity of calcium carbonate, and some other random shit. A kilo of CaOH sucks up about 1.2 kilos of carbon dioxide. So you’d be using about five kilos a week.
On the plus side, it’s super low tech. You can just paint it on the wall and let it suck up the CO₂ from there over the next few days. It’ll also suck it up just sitting in the bucket, but that will take many years. (Which suggests that you could buy, say, ten tons of it and just let it sit in a crate.)
You can regenerate calcium carbonate back into calcium hydroxide by roasting it, but you have to get it up to 850° or so, which is pretty hot. Magnesium carbonate is more promising: it starts to decompose back into magnesium oxide at just 350°, and if you don’t heat it past 700° or so, it remains deeply eager to suck the CO₂ back out of your air. So you could imagine some kind of solar kiln on your roof to bake out the carbon dioxide you’d exhaled over the last day or two. But you’re still having to deal with powders, which probably means batch rather than continuous-flow. And they’re caustic powders, so if you trip carrying a bucket of this shit, you’re going to the hospital. (Magnesium oxide gets a lot more stable if you heat it further, apparently.)
(The ISS uses a similar system, but uses zeolite molecular sieves rather than alkali.)
The US nuclear submarine fleet chose a different option, one used for flue-gas treatment: they use an aqueous solution of ethanolamine. Ethanolamine, like magnesium hydroxide or calcium hydroxide, is eager to suck up CO₂ from your air; unlike them, it does so while remaining happily liquid in an aqueous solution, so you can pump it around. Even better, you can persuade it to give up the CO₂ by heating it to merely 120° or so, which is a much easier thing to do. On the downside, it’s inflammable and toxic. Its cousin diethanolamine works too, and is much less inflammable and toxic. Also on the downside, to get it to absorb the CO₂, you need to compress the gas to several atmospheres, at least 5 but ideally more like 200 atmospheres.
But that seems like the kind of thing you could reasonably have in your house. I mean, your refrigerator is already compressing its R-134a refrigerant to about ten atmospheres, which I think is enough to persuade diethanolamine to take up the CO₂. So you could very reasonably install an “air conditioner” with a similar-size compressor motor, compressing air. (You might be able to allow the air to re-expand to atmospheric pressure through a series of turbines alternating with heat exchangers in order to offset some of the energy use of the compressor.) It would need to pump through about 100 m³ of air per day per person in order to suck up 1kg of CO₂ per day at a 1% concentration. That’s a bit more than a liter per second, or 115mℓ per second at the cooled compression output.
I don’t know how much the liquid flow needs to be. Presumably several milliliters per second. Most of the temperature difference in the liquid can be managed with a countercurrent heat exchanger, so that it doesn’t represent an ongoing energy waste, in particular the part where the hot liquid coming back into your house has to shed heat into your space to get back down to room temperature.
We can estimate the energy usage of this contraption. Isothermically compressing 1.15ℓ down to 115mℓ involves squeezing, say, 9cm out of a 10cm cylinder with 115cm² surface area, against a pressure that rises linearly from 0 to 10 bar; that’s 518J, so the compressor needs 518W. Some of this (let’s say 90%) can in theory be provided by decompressing the sweetened air through turbines, so you might need 52W. And if you’re heating up, say, 10mℓ of something water-like from 40° to 120° each second, that’s 80 cal/s, or 330 W, of which you can probably economize 90% with a countercurrent heat exchanger, getting you down to 33 W. And you probably need another 10 W or so for the water pump. Total, about 95W dissipated, out of 950W flowing hither and thither.
Now, of this 95W, some fraction is presumably the actual unavoidable Carnot loss from pumping CO₂ from a (potentially) low-CO₂ environment to a (potentially) high-CO₂ environment (although in fact we’re proposing here to pump it from your 10000ppm CO₂ sealed neurotic bunker to the 400ppm outside world). But that is probably a small amount compared to the 10% losses I’ve assumed in compressing-decompressing and heating-cooling cycles.
(We could apply this same approach to removing CO₂ from the air at scale. Note, however, that this 95W is about the same wattage as the human being that hypothetically exhaled that kilogram per day of CO₂, so you end up using about the same energy to get the CO₂ out of the air with this approach that you originally got out of burning it — and that’s after the CO₂ has reached 10 000 ppm! So, while this is a viable approach to carbon dioxide removal in a world where we have a great deal more energy available than we ever used in fossil fuels, it is too expensive at the moment.)
You may be able to use photosynthesis, as in Biosphere II, to remove some of the CO₂ from the atmosphere. However, this will require significant indoor acreage; at normal CO₂ concentrations, you need about as much sunlight to turn your CO₂ back into carbohydrates as it took to turn it into the carbohydrates you ate. Beema bamboo, a thick-walled variety, supposedly produces 50 tons of dry biomass per acre per year (112 tonnes/ha/y), yielding 4000 kcal/kg. That’s about 6.6 W/m², which means that your 100-watt body would need 15m² of bamboo to consume the CO₂ it emits. Other high-yielding plants are similar — sugarcane commercially reaches 70 dry tonnes per hectare per year and experimentally has reached 98 dry tonnes/ha/y, which is 5.2 W/m². (Hardier plants like switchgrass and Miscanthus, which thrive without the subtropical conditions that beema and sugarcane demand, are down around 15 to 40 dry tonnes/ha/y).
Most crops, apparently, increase production substantially (varying from 25% to 200%) CO₂-supplemented in a greenhouse, but I don’t know if this applies to super-high-biomass-productivity crops like Beema bamboo and sugarcane. 1000 ppm is a common supplementation level. 10 000 ppm is high enough that it will likely cause problems for plants; Alberta’s official advice to greenhouse gardeners is to keep below 4500 ppm for the sake of plants, or 5000 ppm for the sake of humans; other sources suggest that over 1200ppm some crops start to “show undesirable growth responses”. You’d only reach that level if the plants fall behind your breathing, and since the curve levels off then, at that point the overall system could go unstable — you’d want to keep the concentration in the range where the plants can increase their photosynthesis to respond to elevated CO₂ levels.
One great difficulty with this approach, however, is that if you bring 15m² or 30m² of sunlight into your house to grow plants, you need to have a way to shed the kilowatts of heat you’re adding — roughly two orders of magnitude more than those dissipated by the diethanolamine system, which hopefully you can also manage to dissipate outside the house, at least in the summer.
(You may in fact be able to get by with 15m² of sunlight concentrated onto a significantly smaller area of plants.)
One approach to solving this problem is to keep the plants in a separately-insulated greenhouse, which can exchange air with your dwelling space via a heat exchanger. If you were to follow Alberta’s recomendations, you could perhaps send air to the greenhouse at 4000 ppm CO₂ and get it back at 1000 ppm, thus dropping 3000 ppm; the 1kg/day/person mentioned above then is about 4 liters per second or 8 cfm. You can get 10 cfm out of a 75mm-diameter 4-watt 3000 RPM fan that you might mount on your computer’s CPU. Another benefit of keeping the greenhouse at a different temperature is that CO₂-supplemented plants may prefer higher temperatures, like 35°, which is very uncomfortable for people; but you could keep it at those higher temperatures without bothering yourself.
If we figure that a city block is normally 100m×100m, and (to play it safe) that we need 30m² of sunlight to fix a person’s CO₂, a city block can house some 330 people.
Once we’ve taken care of the CO₂, there are other things in the air we need to take care of. A variety of artificial objects outgas pollutants like formaldehyde; in the absence of ventilation to vent these to the outside world, they could accumulate to potentially dangerous levels. One approach is to work hard not to allow things into your house that are likely to outgas pollutants, but that’s very difficult.
The approach taken in the ISS is to have a separate trace contaminant control system, which is just an activated-carbon air filter. The Mir space station thermally regenerated its activated carbon with a hot non-oxidizing gas; the ISS uses disposable activated carbon instead, plus an 0.5%-palladium-on-alumina catalytic converter to burn up methane and other things that get past the activated carbon, followed by LiOH sweetening.
Their activated carbon is impregnated with phosphoric acid in order to soak up ammonia off-gassed by human metabolic processes.
What about other carbonates or bicarbonates --- might they be easier to regenerate? They are more difficult, as it turns out. Sodium bicarbonate dehydrates to sodium carbonate at between 50° and 270°, at which temperature the decomposition is complete; Wikipedia says that it's fast at 200°; above 850° it releases carbon dioxide. Potassium carbonate (pearl ash or salt of wormwood) doesn't melt until 891°, rubidium carbonate doesn't decompose until 900°, and lithium carbonate doesn't decompose until 1300°.
See also Notes on a possible household air filter.