A circular carbon economy – where carbon released from the Earth is captured and repurposed – is a promising concept, and CO2 conversion to sustainable products has been at the forefront of research.
Dr Gary Grim at the National Renewable Energy Laboratory, USA, provides a detailed comparison of available conversion processes with direct or indirect use of renewable electricity.
Read more in Research Outreach
Read the original research: doi.org/10.1039/C9EE02410G
Image credit: Adobe Stock / Anastasiia
Transcript:
Hello and welcome to Research Pod! Thank you for listening and joining us today.
In this episode we look at the work of Dr Gary Grim from the National Renewable Energy Laboratory in the USA, who has been working on quantifying the feasibility of CO2 conversion. A circular carbon economy – where carbon released from the Earth is captured and repurposed – is a promising concept, and CO2 conversion to sustainable products has been at the forefront of research. Grim provides a detailed comparison of available conversion processes with direct or indirect use of renewable electricity to propose an informed approach towards a circular carbon economy.
The continuous release of carbon dioxide, CO2, into the atmosphere is detrimental to the climate and life as we know it. Since the Industrial Revolution and human introduction to energy and products derived from fossil fuels, CO2 emissions have gone up dramatically, which has led to the challenging situation we are facing today: climate change and its severe effects on our lives. Such effects include unpredictable and harsh weather conditions, extinction of certain animal and plant species, ongoing or imminent inhabitability of certain areas leading to millions of climate refugees, and price increases of key commodities, including food.
For the past few years, many research initiatives have been devoted to identifying ways to reduce CO2 emissions or to capture and convert CO2 into valuable chemicals. Dr Gary Grim, Staff Scientist at the National Renewable Energy Laboratory in the USA, has been examining the different ways of producing sustainable fuels and products via CO2 conversion. This technology will ultimately help us close the carbon loop by repurposing existing carbon instead of releasing more fossil-fuel-derived carbon into the atmosphere, saturating and exceeding its capacity to balance it out through natural processes such as photosynthesis.
A circular carbon economy relies on the conversion of existing carbon – in the form of CO2, biomass, or other carbon-bearing substances – into other chemicals that act as ‘raw materials’ for various processes. If we want to ensure a liveable planet for us and future generations, we need to invest in research initiatives around the conversion of existing and continuously produced CO2 to other chemicals with high demand and avoid depleting resources that could be used for other purposes, such as farmland and crops. This strategy supports the repurposing of CO2 which, as it stands, is damaging to the environment, and it relieves the pressure of providing adequate biomass quantities towards energy conversion, which in itself might be unsustainable. The aim should be to identify pathways towards the improvement of CO2 capturing and conversion technologies and their implementation from research to large scale.
A lot of work has been done and is underway in the field of CO2 capture and conversion. Researchers are looking into ways of capturing CO2 directly from emission sources such as refineries and from the environment, ways of storing CO2, and – last but certainly not least – how to convert CO2 into useful chemicals. There is quite a range of products that can be derived by CO2 conversion through single reactions or multiple reactions, including well-known simple chemicals such as carbon monoxide, methane, and ethanol, all the way to more complex carboxylic acids and hydrocarbon fuels. The latter category is exceptionally interesting as hydrocarbon fuels can be used as sustainable aviation fuels, an alternative to current aviation fuels such as kerosene or jet fuel derived directly from petroleum.
Several conversion technologies have been suggested and tested, at least on a small scale. The technologies most widely examined use electricity in some form, either to directly break down CO2 or indirectly by supporting the production of chemicals such as hydrogen that can break down CO2 through a subsequent process. As the chemical structure of CO2 is disturbed in the presence of these energy carriers, synthesis of new chemicals can begin under the appropriate conditions. Each conversion process and combination of parameters can lead to different chemicals, which is an exciting and encouraging prospect. However, a lot of fine-tuning is required before we can confidently say that a particular process and combination of parameters is suitable for large-scale application and production of certain chemicals. In addition to technical performance, the cost relevant to raw materials and operation, the location, and the research and development process also needs to be considered, making commercialisation a particularly challenging endeavour.
The feasibility of the circular carbon economy and the industrial applicability of promising processes for CO2 conversion have been the main focus of Grim and his collaborators over the past few years. In recent work, Grim performed a thorough analysis of the economic and technological factors relevant to bring a method from laboratory to commercial scale, such as technology readiness level, capital, and operating costs. He looked at several CO2 conversion methods and their products, specifically focusing on the production of 18 industrially relevant chemicals, including commonly used alcohols like methanol or ethanol, carboxylic acids such as formic, acetic, oxalic acid, and hydrocarbon fuels.
An important part of Grim’s technoeconomic analysis was to identify model input assumptions to the CO2 conversion system that captured the current state of the art for conversion but also accounted for the fluctuation of certain factors, thereby future-proofing his analyses. One such factor was the price of electricity. Production of electricity from renewable sources such as wind, solar, or hydroelectric – although achieved and viable – is not yet where it needs to be in terms of production capacity and unit cost to be viable for CO2 conversion processes at large scale alongside traditional electricity uses. Rather than using only the current price of electricity, Grim examined three possible pricing scenarios: the current, future, and optimal price.
Another factor was the cost of CO2 capture, which undoubtedly affects the production cost and minimum selling price of any of the sustainable products down the line. Grim provided a detailed analysis of the available methods that can capture CO2 either directly at the point of production or from the air, aiming to target legacy emissions and CO2 produced from low volume sources such as car exhaust. He explained each method’s pros and cons and how the final product cost is affected, showcasing that the method of CO2 capture and the cost associated with it can increase the minimum selling price of the final conversion product up to a staggering 250%.
A big part of Grim’s recent work has been devoted to the use of electricity as a power source, especially considering the technological advancements towards producing renewable electricity at scale and at a cost competitive to fossil fuel-derived power. Electricity can be used to break down CO2 and thus plays a key role in a circular carbon economy. Electrolysis is a technique that uses electricity to split up chemical bonds. The starting material is broken down to simpler forms with the use of electricity passing through a closed space, or cell, with an anode and cathode that facilitate oxidation – the loss of electrons – and reduction – the gain of electrons – respectively. A common example of electrolysis is the conversion of water, H2O, to oxygen, O2, and hydrogen, H2, but another emerging example is CO2 conversion to other products.
Besides the direct electrolysis of CO2, there are also processes that use renewable electricity to convert CO2 indirectly. An example of electricity used in combination with biology is electrolysis-assisted microorganism operation, or hydrogen produced by electrolysis to facilitate enzymatic reactions, representing direct and indirect use of electricity, respectively. Based on his analysis, Grim suggests that indirect methods currently have an edge across performance metrics, with the potential to scale up and deliver high volumes of products.
However, methods employing indirect electrolysis also face challenges such as low theoretical conversion rates, indicating a lower product-to-raw-material ratio, and a more limited number of possible products, which could impact the long-term viability of these methods. On the other hand, Grim shows that current projections indicate direct electrolysis could have tremendous potential on a long-term scale if R&D obstacles are overcome. Current advancements have demonstrated breakthroughs in CO2 conversion to relatively small molecules like CO, but when it comes to higher-value products, there are considerable limitations to be overcome.
Through carefully designed simulations and analysis, Grim was able to provide educated predictions for the development of direct conversion technologies, including year-by-year targets. His forecasting could be used to inform researchers and scientists on possible paths to commercialisation of direct electrolysis or other suitable methods for CO2 conversion to targeted chemicals over the next couple of decades.
In his work, Grim looked at the specifics of CO2 conversion via direct or indirect pathways to 18 different chemicals, as part of a wider power-to-X decarbonisation strategy to convert CO2 to liquid, power-to-liquid, or gas, power-to-gas, products. His analysis showed that there are several technologically exciting processes towards a circular carbon economy and that it is crucial to fully understand how each process works to convert CO2, and how operating and other factors affect its outcomes. It is imperative to also understand and work towards the goals and timelines required to enable and scale up decarbonisation; only then will we be able to identify economically viable solutions for CO2 conversion that could help us reach a circular carbon economy.
That’s all for this episode – thanks for listening, and stay subscribed to Research Pod for more of the latest science.
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