How Captured Carbon Can Be Used

The UK Government suddenly seems a fan of carbon capture - primarily, it seems, as an excuse to keep using carbon-generating fuels (i.e. oil and gas). This is not good.

However, carbon capture isn’t inherently bad. Atmospheric carbon has been growing for decades, due to the aforementioned oil and gas, and is continuing to do so. We’ve already passed the “oh shit” point, arguably several times. Carbon capture gives us a way to reduce atmospheric carbon levels and reduce the impacts of climate change.

Think of it this way: We have a messy flat, and it needs cleaning. The UK Government wants to hire a cleaner… While continuing to make a mess. The sensible option is to, yes, hire the cleaner, but also stop making a mess. And it is possible to live without making a mess, even if some of my former housemates might disagree.

Now we’ve hired the cleaner, what to we do with the mess? We could just throw everything away, but there are other options too - reuse it, repurpose it, store it, sell it. The same can be done with atmospheric carbon.

Carbon capture fundamentally involves taking CO2 out of the air and putting it somewhere else. There a number of ways of doing this, and I’ve categorised the methods into three groups, one which returns the CO2 to nature (usually in a solid form), and two which leaves us with the CO2 to do with as we wish.

Plants and trees use CO2 to photosynthesise and release oxygen (carbon is stored in their bodies). Wetlands and regenerative farming are similar (carbon is stored in the soil). As long as we don’t burn all the trees, I like this.

Burial involves converting CO2 into a solid form and burying it underground (similar to coal). Example forms of this are biochar (a form of charcoal which also can improve soil fertility) and mineralisation (dissolving CO2 into metal oxide rocks to create carbonates such as limestone). I like this.

Ocean sequestration involves various methods of having the oceans absorb more CO2, most commonly by encouraging the growth of CO2-absorbing marine life (e.g. seaweed, algae, plankton), which can either be used (ideally not burnt), or die and sink to the bottom of the ocean. Assuming we don’t destroy the ocean with crazy experiments trying to accelerate absorption, I like this.

CO2 injection involves pumping CO2 gas underground into geological formations. I guess this is okay, although it’s not my favourite.

Direct air capture (DAC) uses chemical processes to extract CO2 directly from the air. I like this a lot.

Point source capture captures CO2 from sources like power plant smokestacks, which prevents (some of) the CO2 entering the atmosphere. It’s creating CO2, then trying to catch it again. I don’t really like this.

Carbon scrubbing absorbs CO2 from other industrial gases and flue streams using chemical scrubs and filters. Similar to above, I don’t really like this.

If you are planning on CO2-producing activities, using these technologies is far superior than not. However, it’s much better to try to reduce the creation of new CO2 to start with.

And this is where the carbon capture scam comes in. Oil and gas companies (and, it seems, the UK Government) love carbon capture, because it allows them to continue with business as usual (making billions of dollars in annual profits) by burning fossil fuels and producing CO2, while greenwashing to suggest, because they use carbon capture tools, they’re now not emitting and they’re “green”. However, as mentioned above, this has flaws:

  1. Carbon capture is not 100% effective, so CO2 will be released into the atmosphere.
  2. “Oh no, the carbon capture machine broke.”
  3. Unless there is rigorous independent emissions testing and hefty fines, the system is open to fraud.
  4. It’s an excuse to continue supporting unsustainable, unrenewable energy sources, instead of moving into the twenty-first century.
  5. It continues the profitability of oil and gas exploration, which indirectly supports power plants not using carbon capture technologies, by keeping the supply high and price low.

Assuming we did one of the latter two options (and ideally DAC), we now have CO2. What do we do with it? Some good uses, some bad uses.

The oil and gas company favourites.

Enhanced Oil Recovery (EOR) involves injecting CO2 into oil fields helps mobilise more oil. This is the largest current industrial use of captured CO2. I don’t like this.

Fuel production combines captured CO2 with hydrogen to create synthetic hydrocarbon fuels which can then be burnt put the CO2 back into the atmosphere. The cycle continues. I don’t like this.

I quite like all of these.

Food and beverage: CO2 is used to carbonate drinks, package foods, and chill and preserve products.

Horticulture: CO2 injected into greenhouses boosts plant growth and crop yields.

Fire suppression: CO2 systems provide fire suppression capabilities through the displacement of oxygen.

Chemical production: CO2 is a raw material for producing chemicals like methanol, polymers, urea, and inorganic carbonates. This can even be polymerised to produce EVA pellets, which can be used to create foam to use in running shoes.

Concrete: Exposing concrete to CO2 accelerates curing and enhances strength. CO2 injection can supplement steam curing. CO2 can also be stored in concrete.

Mineral carbonation: CO2 can be chemically reacted with metal oxides to form stable mineral carbonates for manufacturing and construction (or burial, as discussed above).

CO2 is carbon + oxygen. Carbon is potentially more useful than CO2 (and oxygen is pretty useful too), so it could be good to split CO2 into its separate parts. There are a few ways of doing this:

Thermochemical decomposition involves heating CO2 to high temperatures (2000°C) to break the molecular bonds. This thermally decomposes CO2 into carbon and O2 gas. If using a catalytic approach, lower temperatures (350°C+) is required. This forms compounds from which pure carbon can be extracted.

Electrochemical reduction involves applying an electrical current through CO2 in an electrolytic cell. On the cathode surface, the CO2 is reduced into carbon and O2 gas is evolved at the anode.

Plasma decomposition involves inputting CO2 into electrically generated plasma which breaks down the molecule. Carbon is deposited on a substrate while O2 is liberated as a gas.

Photocatalytic reduction involves exposing CO2 to a photocatalyst material like titanium dioxide with sunlight. This energetically decomposes the CO2 into solid carbon and O2 gas.

Biological conversion involves bacteria enzymatically reducing CO2 into carbon-rich compounds as part of their metabolism.

Of these, which is best?

A key factor is energy use - the less energy the process uses, the better. And from this standpoint, we have a clear winner. Thermochemical decomposition is the most energy intensive, due to the high temperatures required. Electrochemical reduction and plasma decomposition also use quite a lot of energy, although less than thermochemical. Photocatalytic reduction uses far less than the previous three, and biological conversion uses almost none.

However, currently only thermochemical decomposition is commercially available; the rest are still in R&D stages. Scientific breakthroughs will be needed to utilise the less energy intensive methods repeatably at scale and with efficiency levels that make them commercial viable. So, realistically, for now we only have one option.

The complication is that it’s difficult to generate the temperatures required for thermochemical decomposition from renewable energy sources. Resistive heating from electricity struggles to produce such high temperatures. Concentrated solar power or nuclear-generated steam could be used, but then the CO2 processing facility would have to be co-located with the power station, reducing options. Currently fossil fuels are used to generate high heat for industrial processes… Which is not ideal.

But technology will keep improving, and in the future we will be able to converting CO2 to carbon in an efficient, environmentally-friendly way. What now?

Carbon is a very useful element. It’s the 15th most abundant element in the Earth’s crust, and makes up ~1/5 of the human body. Its atomic number is 1+5 (6).

Carbon can come in, or be transformed into, a number of physical forms, called allotropes. Some carbon allotropes are diamond, graphite, amorphous (including carbon black), and fullerenes (including nanotubes). Carbon atoms can also be bonded to form carbon fibres.

Creating the different allotypes are not necessarily straightforward, however. For example, conventional carbon black production involves the incomplete combustion of heavy petroleum oils in a reactor - not great. Alternatively, carbon black could be produced by the break down of CO2 thermally, electrochemically, or through solar thermochemistry, with he carbon black particle size and structure being controlled by process conditions like temperature and residence time - although at present cost, scale, and quality are still issues.

Let’s say we figure this out and have our allotropes. Some common uses for carbon include:

Steel production: Carbon is a key ingredient in steel as an alloying element. It controls hardness, strength and ductility.

Fibre production: Carbon fibre composites (i.e. carbon fibres with a resin), carbon nanotubes, and graphite fibres have structural applications (including my favourite, motorsport).

Refractories: Graphite can be used to line high temperature furnaces and crucibles due to refractory properties.

Lubricants: Graphite or carbon black can be used as a dry lubricant or additive in greases and oils.

Catalysts: Carbon materials can be used as catalyst supports and activators in chemical production processes.

Electrodes: Graphite can be used as electrodes in many electrical and industrial applications, like arc steel furnaces.

Batteries: The anode (negative electrode) in lithium-ion batteries is made from graphite carbon.

Electronics Fullerenes are being studied for use in transistors and solar cells due to their high electron mobility.

Medicine Fullerenes are being studied for use in cancer therapy and drug delivery.

Cleaning Fullerenes are being studied for their ability to absorb or break down pollutants.

Pigments: Carbon black can be used as a pigment and reinforcing agent in tires, inks, plastics and paints.

Polymers: Carbon black can be used for structural reinforcement and colour.

Plastic Although still in early stages of R&D, pure carbon can be used to create plastics through pyrolysis or chemical vapour deposition (CVD).

CO2 has a bad reputation, but fundamentally, it’s the building blocks of life. With so much floating around us all the time (and increasingly so!), there’s a huge opportunity to grab the excess out of the air and turn it into something useful, from fizzy drinks to structural fibres, pacemakers to paint.