CYAN DECARBONIZER Q&A
Welcome to our FAQ!
Where did Cyan come from and who invented it?
The Cyan idea was conceived by Dahl Winters of DeepScience Ltd in late November, 2020. She built and later open-sourced Cyan; it is now the first OSHWA-certified direct air carbon capture unit.
What community is contributing to the Cyan project and who can join it?
The OpenAir Collective is the headquarters of the Cyan project. Anyone is welcome to join OpenAir . and contribute to the progress of Cyan.
What is the ultimate goal of the Cyan project? E.g., is it to be an educational resource so people can make their own boxes?
The goal of Cyan is to introduce small-scale carbon removal devices so that people feel more comfortable with the idea of carbon removal and are more likely to support the production and use of larger units.
Is there an intention to sell Cyan kits, or maybe completed Cyan units for people that don't have time to build their own?
We might sell kits and completed boxes in the future, especially if the complexity of these grows, but for now the intention is to have people build their own.
Is there a goal of OpenAir that open-source businesses might arise using the Cyan (or Violet) technology?
OpenAir wants to promote the growth of carbon removal and direct air carbon capture, so to the extent that open-source businesses might arise, that would seem consistent with our goals. However, there is no current goal to commercialize the Cyan/Violet technology, we just want to get the technology out there for as many people to use and improve.
Technical
What is the mechanism by which humidifying the air improves performance? Is it that the $CO_2$ forms a carbonic acid mist, which reacts with the $Ca(OH)_2$? Or another mechanism?
The mechanism in which we are exploiting is, there must be a thin layer of water on the surface of each grain of $Ca(OH)_2$ for the $CO_2$ to react with and form carbonic acid. The carbonic acid ($H_2CO_3$) can then react with $Ca(OH)_2$ to form $CaCO_3$ and $H_2O$. This is an acid-base reaction and also a double displacement reaction. Humidifying the air increases the opportunity for this thin layer of water to form.
Would moistening the $Ca(OH)_2$ accomplish the same thing without having to humidify the air?
Moistening the $Ca(OH)_2$ would certainly help the thin layer of water to form faster, but too much moisture impedes the airflow of $CO_2$ between grains of $Ca(OH)_2$. If all the pores are blocked with water, then we have the equivalent of an aqueous solution and $CO_2$ is less soluble in water than in air.
Are there any studies (theoretical or experimental) of the ideal humidity level?
There have been experimental studies of the ideal humidity level from concrete carbonation. Concrete contains calcium hydroxide and the ideal humidity has been between 50-75%
What is the purpose in bubbling the air through the water in the bottom of the Cyan? If the air were humidified another way, would there still be any reason to bubble it? And in that case, would the water in the bottom of the box still be needed?
The air bubbling through water serves two purposes - to continuously add $CO_2$ to the system for removal, and to get water onto the filter paper at a slow enough rate that the water is wicked across the filter paper and $Ca(OH)_2$ powder to help form the necessary thin layer of water across each $Ca(OH)_2$ grain. If the air were humidified another way, some mechanism would still be necessary to quickly wet the powder since air humidification is not fast enough (it takes concrete years for its calcium hydroxide to carbonate). The water at the bottom of the box is only there to wet the filter paper through bubbling.
I saw a reference to an experiment using an ultrasonic humidifier; was that successful?
The ultrasonic humidifier was successful but it was not as energy efficient and required a longer time to thoroughly wet the material to near saturation. With larger quantities of material it might be useful, but for just 10 grams of $Ca(OH)_2$ the emissions due to a power consumption of 24W (24V at 1A) was more than the amount of carbon dioxide that would be removed.
In the standard Cyan build, is there any significance to the dimensions of the specific hummus container used? Or to the exact depth of the water?
There is no significance to the dimensions of the hummus container used or to the exact depth of the water. The hummus container merely provides something for the filter paper containing the input material to rest on while it is being slowly wetted from below from the air bubbling and consequently the water being disturbed. If it is possible to wet the filter paper through a different means, as through an atomizer, there quite possibly would be no need for standing water at the bottom.
Can the hummus container be replaced with anything (e.g. a wire shelf) that holds the $Ca(OH)_2$ above the surface of the water?
Yes.
What is the purpose of using a coffee filter instead of just scattering the material on the upturned lid?
The coffee filter is something that can be weighed beforehand and that also wicks water to slowly wet the $Ca(OH)_2$ powder. Scattering the material on the upturned lid would not allow for easy removal (the powder sticks to the lid) and there is no easy way to slowly wet the powder.
You mentioned that the aquarium pump can be shut off when the air is humid enough. In that case, how does more air get into the container?
The aquarium pump does not have to be run continuously; it can be shut off for intervals of time while the $CO_2$ already in the container is taken up by the $Ca(OH)_2$. The container is not air-tight so $CO_2$ is always diffusing in and out.
Is the drying action of the fans an essential part of the carbon capture process, or is it a temporary R&D addition to the design to allow us to measure the effectiveness by weighing?
A significant amount of $CO_2$ (as much as 50%) is taken up during the drying process and that $CO_2$ uptake is aided by greater airflow. It improves the likelihood that a $CO_2$ molecule will react with the wet $Ca(OH)_2$. However, the fan is not deemed absolutely essential to the carbon capture process. $CO_2$ will still be taken up, just at a slower rate.
The current design captures about 2g of $CO_2$ for each 10g of $Ca(OH)_2$. How does this compare to the theoretical (stoichiometric) maximum amount that can be captured?
The theoretical maximum amount that can be captured is $ 5.94g CO_2 / 10g Ca(OH)_2$ (and for $Mg(OH)_2$, it is $ 7.55g CO_2/10g Mg(OH)_2$). We are at best achieving about a third of the theoretical maximum.
If current performance is short of the theoretical, is that just because not enough surface area of the powder is exposed to the air?
There are a number of factors involved. Increases in temperature and pressure generally improve reaction rates. However, performance was increased by a factor of two just by manually disturbing the surface of the powder once, halfway during the experimental run. This leads us to believe that there is a surface area limitation. Mechanically disturbing the layer of powder in an intermittent manner should help greatly.
Would efficiency be expected to improve if the air is made turbulent?
We believe we are not seeing laminar airflow in our system given the fluid properties and speeds of the air. The air is likely already turbulent, and especially so when the fan is being used for drying. If the air is made even more turbulent, efficiency could possibly improve, but this has not been tested.
Has there been any confirmation of the accuracy of the weighing method for measuring captured $CO_2$?
The weighing method is called gravimetric analysis and is widely used in chemistry for measuring the amount of a compound based on its mass. It suffers from the ability of product to be lost through container transfers and through evaporation. Of higher accuracy would be thermogravimetric analysis, but this is expensive and time consuming.
What other materials besides $Ca(OH)_2$ have been tried, and with what success?
$Mg(OH)_2$ has been tried with similar and possibly slightly higher levels of carbon removal. It is also a more friendly material in terms of pH.
Has anyone tried to calculate the carbon footprint of the whole device (plastic, etc.)?
Not completely, but it has been partially attempted. The carbon footprint of the energy consumption and the transportation of input material has been taken into account in the Cyan Supporting System Calculations.xlsx document available from our GitHub repository. In a separate document, estimates of the energy consumption used in producing $Mg(OH)_2$ have also been done, namely to determine the degree of impact of various manufacturing processes on Cyan's ability to achieve negative emissions.
Would it work to dissolve the X ($OH$) in solution, bubble the air through it, and let the X ($CO_3$) precipitate out?
This would indeed work; aqueous solutions are used in industrial carbon removal (Carbon Engineering for example uses an aqueous solution of $KOH$). The difficulty here is in controlling leakage and evaporation effects and in filtering out the precipitate to weigh it and calculate the amount of $CO_2$ captured. Inevitably, some precipitate sticks to the inner walls of the device, causing product losses prior to weighing.