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Mohamed Koubaa blog

Making silicon carbide in the desert

Carbon Sequestration

In the modern era - CO₂ is getting more plentiful in the atmosphere, leading to catastrophic climate change. But what can be done about this? Of course, there are multiple vectors to come at this issue. One of these - which this post will delve into - is carbon capture. (Now, I am by no means claiming that carbon capture is more important than reducing emissions or any other vector)

Carbon capture simply means to pull carbon out of the atmosphere and capture it somewhere so that it will not simply return to the atmosphere. There are many promising candidates for these carbon jails. First, the most obvious is in living things, after all, we are all made of carbon. This is where much of the earth's carbon is currently captured. Our animals, forests, plankton, and even little diatoms have a huge capacity to sequester carbon.

I think we can and should all be planting trees. It is a remarkably low-tech, distributed, and cheap portion of the overall solution if a solution exists. With modern drones and autonomous vehicle technology, the scale of tree planting can even be increased significantly. That being said, trees need to be planted on land, which itself can be thought of as a finite and scarce resource. The fact is, land use is increasing in many places of the world. Ultimately, I think vertical farming and lab-grown meat has the potential to significantly reduce the land use required by humans to be self-sustaining at an eventual population of 10 billion, which is the projected asymptote for human population increase (citation needed). I am all for giving land back to apex predators and large mammals, to restore the ecosystem to something close to pre-human expansion.

Short of planting trees, what other organic carbon jails could be grown by human activity? One other possibility is dropping large structures onto the ocean floor, encouraging ecosystems to develop around them. This happens with shipwrecks. It remains unclear (to me) if this can be done at a scale that can significantly curb climate change.

Aside from organic carbon jails, there must also exist inorganic carbon jails. For instance, some companies are commercializing inserting carbon dioxide into underground cavities where it will hopefully remain forever. Another option is of course as a raw material in objects that we humans find useful. We are already doing this for organic chemistry compounds and industrial materials. The opportunity here is that often the industrial materials are using carbon extracted from the earth. We could just as well use carbon extracted from the atmosphere if it can be done economically.

Silicon carbide

Silicon carbide is an extremely useful substance, with uses in many industries ranging from automotive to semiconductors. While not extremely cheap, its manufacturing process is remarkably simple. And I believe it is worth exploring whether it can be done using carbon from the atmosphere.

First let's discuss the proposed chemical process. The chemical process to produce both silicon carbide and graphite is the Acheson process. Essentially, silicon dioxide is mixed with carbon in its elemental form, such as powdered coke. Then, in a (usually electric) furnace, the mixture is heated to 1700-2500 °C. There are four chemical reactions involved:

C + SiO₂ → SiO + CO
SiO₂ + CO → SiO + CO₂
C + CO₂ → 2CO
SiO + 2C → SiC + CO

Silicon dioxide might seem like some exotic substance, but its literally just sand. As for coke, this ingredient is slightly more controversial. Usually, coke is produced by burning coal, which is clearly not environment friendly.

The carbon capture here is going to be essentially to suck CO₂ out of the atmosphere and converting it to its to elemental carbon, which will be substituted for coke in the Acheson process. The process to convert CO₂ into elemental carbon is known as the Bosch Reaction. Really there are two reactions:

CO₂ + H₂ -> CO + H₂O
CO + H -> C + H₂O

Basically, using hydrogen, CO₂ can be converted to elemental carbon. In the process, water is also produced. Elemental hydrogen is a by-product of the petrochemical industry and is cheap. It can also be produced by the following reaction:

CH₄ -> C + 2 H₂

So why am I talking about these almost trivially basic chemical reactions? The reason is that there exists and increasingly economically viable way to produce silicon carbide at industrial scale in a carbon-negative manner. Silica can be found abundantly in deserts, where coincidentally solar power is also highly cost-effective.

My proposal is simply to design a Silicon Carbide manufacturing facility in the desert using sand and solar power. As an added plus, water and heat are byproducts which can be used to provide the facility with water and reduce the solar energy required to power the Acheson process. The only piece of this puzzle which must be supplied is hydrogen, unless a naturally occurring natural gas reservoir happens to be in the neighborhood.

Scale

You may be thinking that this will not sequester enough carbon to make a difference, and you're probably right. But just tree planting is critical despite the fact that any individual tree does not sequester enough carbon to make a difference, small to medium scale carbon negative industrial enterprises can also be on the critical path to net-zero or negative emissions. The fact is, industry will continue whether it is carbon positive or negative, and to design carbon negative industrial applications that are cost-competitive with traditional methods will be necessary. In fact, if governments begin to place a price on carbon emissions, they may just as well place bounties on carbon capture, meaning a cost-competitive carbon-negative operation may become highly profitable.

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