Turning plastic and CO2 into fuel

We’re often covering the topic of climate change here on the Naked Scientists with justifiable cause for concern. But this next item could be a reason to celebrate. Cambridge chemist Erwin Reisner has developed a system that can use sunlight to transform plastic waste and greenhouse gases into sustainable fuels and other valuable chemicals…

Erwin - My laboratory really develops technologies that can use sunlight to convert waste into useful products. So about 10 years ago, we started work on using the greenhouse gas, carbon dioxide and developing systems that can convert this into fuels. And more recently, maybe five years ago, we started work on converting plastic waste into useful products and chemicals as well. And now for the first time we managed to combine these two approaches where we have a single solar powered reactor where we can feed in plastic waste and carbon dioxide and really make useful fuels and products.

Chris - Talk us through the nuts and bolts of this then. So the solar bit of it is the energy input because nothing happens. There's no free lunch in the universe is there? You've got to supply energy to make things happen. So that's powering the process. But what is the process?

Erwin - That's correct. We only use sunlight, the most abundant energy form on our planet. And the process is really a reactor that consists of two compartments. In one compartment we feed in the plastic waste. In our case, for example, we use plastic bottles, which is PET waste, that can convert PET to glycolic acid. And in the second compartment, we bubble carbon dioxide and convert it into fuel components, which are known as syngas, carbon monoxide, and formic acid. This sounds very technical, but it's a very interesting intermediate chemical that we can then further convert to interesting fuels that can be useful.

Chris - How does the sunlight work then? Is it converted into electricity that powers that process or is it the heat or is it ultraviolet to split some of the things apart and give them energy? How do you use the sunlight?

Erwin - That's an excellent question. So we use solar energy directly to drive the chemical reactions. We do not really produce intermittent electricity. This is an integrated approach where the sunlight is directly used to drive the chemistry.

Chris - In essence then you need a sunny day. But what is there some kind of reacting surface where the sun comes in and does something to that surface that makes it able to modify these chemicals in this way?

Erwin - Yes. So a sunny day helps a lot. It also helps to have a close and sunnier climate.

Chris - Not gonna have a lot of luck with that at the moment.

Erwin - In the UK, it's still okay. <laugh>. So essentially yes, we do need an area where we take the light, where we harvest the solar energy and then the chemistry takes place, but the light harvesting and the chemical conversion happens at the same place.

Chris - How would this work practically then? Would you imagine that you've got the equivalent of what we see for a solar farm would be a chemical recycling farm in your hands?

Erwin - This is our ultimate dream to build a solar power recycling upcycling facility. And this would mean we will certainly have large areas where we need to collect sunlight. This could either be a hundreds of square meters or square kilometers array of solar reactors, or we take solar energy and sunlight and concentrate on smaller reactors that we can do the chemistry with.

Chris - One has to take into account when doing any of these kinds of analyses, the sort of circular economy, which is that the plastic doesn't get itself to the recycling plant. The CO2 doesn't harvest itself from the atmosphere and feed itself into your system. So when you do those sorts of calculations, does this still make sense? Is it viable? Is it actually a carbon saving? We're not burning loads of diesel to get the plastic to your plant to do this?

Erwin - That's a very important point. So for us, this is very early stage technology. This is really just what we would call a proof of principle to say this actually is feasible and viable. To address exactly your point, we work quite closely with recap, that's the Peterborough and Cambridge Waste Recycling facility. And we have been at Amey, the waste treatment facility, for example, in Waterbeach to collect samples and even inspect the facilities to see where, how we could integrate this technology with existing waste recycling. So we would do this technology onsite where we accumulate the waste. And in addition we have to think about carbon capture as well. So we are also working on technologies where we can really take carbon dioxide, either from industry or even capture from air and use this as an input to drive the reactor.

Chris - When you say use plastic, presumably you're not just chucking plastic bags in there. So do you have to put the plastic into some kind of configuration first so that the process can consume it?

Erwin - So, yes and no. So ideally a strength of our system is we can use quite a broad range of plastics. So when we talk about plastics, it's not one product. We really talk about hundreds of different products.

Chris - Well you preempted my question, because I was thinking when we look at what goes in the bin. Even in the recycling bin, it's a whole mixed bag of plastics isn't it?

Erwin - So with our systems, we can convert a good range of plastics, which are known as condensation polymers. So this is PET, PLA, and other types of plastics. And the process also works with biomass, which is actually really attractive because if we have contaminated food waste, for example, we can use this straightaway in our process and do not need to worry too much about, purifying the waste stream process

Chris - So it's not going to gum up the waste stream.

Erwin - So it's a very versatile, robust process.

Chris - Yeah. And what's the secret ingredient that makes this happen? What is the surface over which there must be some kind of catalytic process that's driving this. What's the secret to that?

Erwin - So exactly. So the solar energy provides the energy and then we need the catalyst, these materials that can really accelerate these chemical reactions. So in this case, we try to develop surfaces that can really then take the waste streams, rearrange bonds, and make this interesting product. So in our case, this would be based on copper for example, or cobalt, which are quite abundant materials that are possibly scalable.

Chris - I mean expensive, still, because everything's become expensive, hasn't it? But certainly that you're not talking about really rare materials which many of these other processes have to use.

Erwin - Yeah. So this is one of our core concerns. We try to avoid precious metals and expensive components that could not be scaled in the longer term future.

Chris - So just to finish, what are the figures looking like? I mean how much could you convert with a process like this? Is it meaningful? As in, we know the world makes millions of tons of plastic every year. Can you deal with it with this?

Erwin - Yeah so we hope with our work to inspire and to show what's technologically possible. To really make this a meaningful process this will mean multimillion investments from companies, government, stakeholders to really get to the scale stability and efficiencies that are being needed. So for now, to give you a scale, we work in our laboratory on a milligram maximum gram scale. But as you said, of course we have produced probably 10 billion tons of plastic since the fifties. So there's still some scale up that is required in the next couple of years.