Harnessing sunlight to convert CO2 to fuels

The utilisation of carbon dioxide has a part to play not just in reducing emissions, but by taking advantage of a waste product to produce fuel and feedstocks. Solistra, a spin-out from the University of Toronto, Canada, has developed a photoreactor to convert CO2 emissions into downstream products. I spoke to Alexandra Tavasoli, Co-founder and CEO of Solistra and PhD Candidate at the University of Toronto’s Solar Fuels Center, about how her interest in sustainability led her to chemical engineering and how Solistra will have an important role in decarbonising industries.

Tavasoli first became interested in sustainability while in highschool, inspired by the documentary An Inconvenient Truth. She spent the last couple of years of highschool trying to figure out how to turn environmentalism into a career, such as whether to study political science to do policy or pure science to do research. She turned to engineering when she realised how important it is in understanding how clean technologies are implemented. “I decided to study chemical engineering to learn those skills because I felt that it’s just so fundamental to the field and how we make decisions about energy infrastructure, chemical infrastructure, supply chains, and things like that.”

She obtained a Bachelor’s degree in chemical engineering from the University of Toronto and after she graduated washired by the MaRS Discovery District, which offers strategic partnerships for clean tech startups in Canada. “I spent about six months there learning about the clean tech startup stage in Canada and the goals and challenges that exist in taking new technologies from academic labs to full commercialisation.” She then joined Geoff Ozin’s Solar Fuels Group to do her PhD on the technology that eventually was used to spin-out Solistra. The project involves trying to get CO₂ to react using sunlight, therefore using less energy and producing fewer greenhouse gas emissions than is required with conventional fossil fuel methods. “The opportunity to use sunlight to reduce the energetic barrier of converting greenhouse gases into usable chemicals, and also the idea of using waste gases to make traditional petrochemicals, was a really attractive solution to me.”

Solistra was then spun out of the lab via a Canadian Government initiative called Women in Cleantech, designed to help women to develop early-stage, potentially disruptive technologies. Tavasoli is one of the six finalists of the challenge and each finalist is awarded C$115,000/y (US$87,000) for two-and-a-half years and the chance to win C$1m in the final phase of the competition in 2021. The funding enabled them to hire a small team and build a pilot plant version of their lab-scale reactor.

Using a photocatalyst for CO2 reactions

“Solistra’s core technology takes carbon dioxide and methane gas and converts it into synthesis gas which is a mixture of carbon monoxide and hydrogen that has several industrial uses,” said Tavasoli.

The majority of syngas is produced through steam methane reforming, but it can also be produced via dry reforming in the absence of water vapour. Dry reforming is more suitable for producing syngas for use in the Fischer-Tropsch process to potentially produce liquid fuels, and it also makes it possible to use CO₂ emissions as a feedstock. However, conventional dry reforming is an energy-intensive process, requiring temperatures of around 800^oC, with the heat provided by fossil fuel combustion, which makes it costly and produces emissions.

Using a photocatalyst means the reaction can be activated using solar energy, thus negating the need for fossil fuels to produce a high-temperature reaction. This is known as solar dry reforming and has the added advantage that the photocatalyst is more stable compared to conventional catalysts, as it is resilient to coking, a process that deposits carbon on the surface of the catalyst and makes it inactive. The photocatalyst designed by Solistra is also environmentally friendly, as the processing steps in the catalyst synthesis and coating manufacture are aqueous-based and take place at low temperatures.

The catalyst can also work with artificial light. “This allows us to take advantage of the positive economics associated with being on-line for 24 hours per day,” said Tavasoli. “Further, using light during the night hours is still beneficial, as it allows the process to continuously operate at low temperatures, which reduces capital costs as we can use materials of construction with lower temperature ratings.”

Tavasoli said that one of the two main challenges they faced is designing the photoreactor. “We have had to do a lot of modelling to develop proprietary catalyst architectures to be able to optimise the light access to the catalyst surface through the diameter of the reactor.”

The second challenge was the availability of upstream and downstream units for use at a small scale. “While we have built this reactor system, practical auxiliary components like gas compression, pressure swing adsorption, and other unit operations that we need to use to turn this into a real chemical process, are difficult to source economically at small scales as their conventional implementation benefits economically from being at really large scale.”

Biogas to hydrogen

Currently the main focus for Solistra’s technology is converting biogas or landfill gas to hydrogen. This could be used to produce on-site hydrogen for hydrogen fuelling stations, particularly as producing hydrogen via electrolysis may not be feasible in remote or desert areas that are not on the electrical grid, and may not have access to fresh water. “Being able to use waste from these communities or farms to produce hydrogen on-site without using water or electricity, that could be a really good setup for the hydrogen fuelling stations to expand in a way they haven’t been able to,” said Tavasoli.

They are currently setting up their commercial-scale pilot and looking for a suitable site for converting biogas to hydrogen. “Following that we will be looking to sell those units to owners of hydrogen fuelling stations,” said Tavasoli. “Right now we’re working on hydrogen, we hope to expand into other chemicals in the future because it’s a widely applicable technology.”

The Solistra units can be standalone or used as a modular reactor system, and can also potentially be integrated into existing facilities. Tavasoli said that the technology could be used to retrofit existing steam methane reformers to recycle the CO₂ and combine that with natural gas existing on-site to add to syngas capacity. “However, since we’re still in the process of scaling our technology we need to develop our system to one more scale higher than our pilot before we can add to those petrochemical scale operations. But that’s definitely something we are interested in doing and we see a lot of value in that because you can’t just abandon all of the existing petrochemical infrastructure. That in itself is quite wasteful and so retrofitting them would be a really good idea.”

She added that breaking into the petrochemicals sector will be difficult as the supply chains have been built up over a century and it has had a lot of investment, therefore it will be a big challenge to make it sustainable, and be competitive with the economies of scale afforded by this deep supply chain integration.