Industrial chemistry time! Let’s stipulate that the world’s chemical feedstock industries, on the whole, are not what you would describe as environmentally friendly. There are a lot of moving parts, and some of them are definitely better than others (in their use of energy, carbon emissions, and use of renewable resources as starting materials), but everything is short of the ideal. What’s the ideal? Well, how about not using natural gas or petroleum as carbon starting materials at all, but rather stripping the carbon dioxide back out of the current waste streams and using that as the feedstock?
Electrochemical activation and conversion of CO2 and water into hydrocarbons and oxygenates could potentially offer a sustainable route to produce many of the world’s most needed commodity chemicals (Fig. 1A). Coupling renewable sources of energy (solar, wind, hydroelectric) with electrochemical reduction of CO2 to chemicals, if done efficiently, could address the nondispatchable nature of renewables by providing storage in chemical bonds. Electrocatalysis also provides a route to transforming carbon resources into chemicals without the need to burn carbon fuels, assuming the CO2 is taken from air.
This article runs the numbers on how close we are to that, what it’ll take to get there, and what the strategies might be. The authors note that as things stand, the more easily realized goals (as is often the case) will make very little impact. For example, producing formic acid electrochemically from carbon dioxide is pretty efficient, but wouldn’t do much because overall formic acid production doesn’t amount to much, either. That could change if someone revs up an efficient fuel-cell system that uses it (for example), but that’s asking for several big developments at once.
At right is a diagram of how the industrial chemicals world might look if carbon dioxide capture and its electrochemical reduction continue to advance. Here are the terms you’re seeing: “HER” is electrolysis, and “CO2R” and “COR” are reduction of carbon dioxide and carbon monoxide. Syngas (synthesis gas), as most chemists will know, is a mixture of hydrogen and carbon monoxide, and it’s used in a variety of chemical processes – one of those being, as shown, the various types of Fischer-Tropsch reactors to make synthetic fuels. As it stands, syngas is generally produced by steam reforming of natural-gas methane. Hydrogen production via electrolysis, though, is already an industrial route that’s been gaining some share, although the great bulk of the world’s hydrogen still comes from that steam reforming, and recall that one use of it is to go into the Haber-Bosch process that came up here the other week.
The rest of it’s not too trivial, either. You’ll note ethylene (C2H4) in the middle, which gets turned into a lot of the world’s plastics and much else besides. Right now, most of that is produced by steam cracking of natural hydrocarbons (either petroleum naphtha or ethane), so making it from carbon dioxide and/or carbon monoxide would be quite a shift. The Sabatier process shown at the bottom would be used to make renewable natural gas (RNG) – that’s a method to reduce CO2 (or CO) directly down to methane. It’s been around for over a century, with many improvements along the way, but rises and falls in most parts of the world with the price and availability of natural methane (which currently is cheap, as shale gas). To the best of my knowledge, the only place to see it in operation in the US is up around Beulah, North Dakota. In the first half of the 20th century (before the development of the natural gas industry per se), gas was generally produced rather brutally from coal, and the only place in the US to see one of those plants (as a nonworking historic site) is in Seattle.
So the general idea (the whole upper-left portion of that graphic) is farewell to natural gas and to petroleum as chemical feedstocks. It’s a tall order, because those two tend to be cheaper – usually a lot cheaper – than the alternatives. The Fischer-Tropsch process for liquid fuels, for example, was famously used in Germany during World War II, since the country was well-stocked with coal but had basically zero petroleum. Later on, it was developed further in South Africa, for the same reasons, but you can see that it has a certain backs-to-the-wall aspect to it. Other gas-to-liquids processes are in the same category. Overall, whenever natural gas is easily available, no one runs the Sabatier process, and whenever petroleum is easily available, no one runs gas-to-liquids.
On the positive side, the products of carbon dioxide reduction would feed right into existing chemical infrastructure, so it’s basically the front end of the business that’s getting reworked, not the whole thing. There’s also (separately) a lot of work going into CO2 capture, raising the question in every case of what to do with it once you’ve captured it, and turning it back into something useful (and solid!) is an appealing idea.
There are government levers (subsidies, tax breaks, carbon use taxes and so on) that can change the economic landscape, but the paper estimates that you’d need electrochemical efficiencies of at least 60% and electricity available at 4 cents/kilowatt-hr or better to make these ideas profitable (with the usual 30-year-amortization assumption about the plants themselves). How close are we? Many of the processes are currently in the 40-50% efficiency range, and need further scale-up work: within sight, but not there yet. And renewable electricity costs vary a great deal by region. The best cases are getting down around that figure, though, and continuing to improve. One feature of electrochemical synthesis is that it would (as mentioned in the excerpt above) provide a use for the mismatched local excess electrical production that can happen with renewables – it’s storage of energy in chemical bonds as opposed to batteries, flywheels, or what have you. But on the other hand, running a chemical plant 24/7 is by far the most economical way to set things up, so the best solution would be coupling with some steadier source of electricity as well.
The paper goes on to look at the various route in the graphic above and evaluates them according to how feasible they look. Methanol and ethanol are available pretty cheaply now, and those could be tough markets to enter. But ethylene looks like a better bet, not least because it has to be separated from ethane at considerable expense under the current methods, and the electrochemical routes don’t produce ethane at all. There are also some hybrid approaches, like electrochemistry to make syngas followed by biological conversion to other products, that are being tried out right now.
All of this is going to depend on location (ideally next to big emitters of carbon dioxide, and within sight of large users of the products, although that’s not always going to work out), commodity prices, and regulation (taxation, penalties, incentives, etc). It’ll also depend on advances in catalysis and electrochemical engineering, cost of electricity generation, and if someone makes a breakthrough or two in carbon dioxide capture techniques that’ll be just fine, too. All of these are active fields, though, and well worth keeping an eye on. The good news is that it’s not a crazy idea.