ETH Zurich, Department of Mechanical and Process Engineering
"The role of CCUS in a net-zero-CO2-emissions world"
Carbon Capture, Utilization and Storage systems play a central role in political debates, technological efforts and scientific developments around climate change, as a consequence of two factors. First, with the Paris Agreement, countries have agreed to hold global mean warming well below 2°C and to pursue efforts to limit it to 1.5°C. Second, scientists have shown that any limit to global mean warming implies a maximum cumulative amount of greenhouse gas (GHG) emissions, the so called carbon budget. Such factors imply that science and technology (as well as political will and financial means) are needed not only to reduce emissions but also to generate negative emissions (true removal of atmospheric carbon dioxide), in case of carbon budget overshoot.
In order to assess the central role of CCUS we here advocate, and follow, a simplified system analysis approach, rather than a detailed technical, environmental, and/or economic analysis. This approach is based on the framework of a net-zero-CO2–emissions world. We have chosen this framework because CCUS technologies that are developed now are likely to be deployed in a net-zero-CO2 world; leading scientists estimate in fact that we may have to reach net-zero-CO2-emissions already by 2050 to fulfil the targets of the Paris agreement. Moreover, the net-zero framework drastically simplifies the analysis: every carbon atom released to the atmosphere must be taken back; every fossil carbon atom produced must be returned to the subsurface. Finally, the simplified and somewhat idealised approach proposed here allows drawing very insightful conclusions about key features of CCUS sub-systems and provides a sanity-check on proposed CCUS schemes and a rather valuable preliminary framework before carrying out a full-fledged LCA.
In this context, the reuse of CO2 as a feedstock to produce fuels or chemicals has greatly appealed to scientists, industry, and policy makers, as a way to mitigate climate change, and to contribute to a circular economy. Since the progress of CO2 capture and permanent underground storage (CCS) appears to be slower than expected, CO2 capture and utilisation (CCU) has been advocated as a valid alternative to mitigate CO2 emissions from power plants and industry. At the same time, the emergence of larger shares of intermittent renewables in the power system has provided a thrust to search for means of long-term, seasonal storage and long-range transport of renewable energy. The production of hydrogen from green electricity could provide such a means, through its combination with captured CO2 to form carbon-based fuels.
While the debate has thus far focused on the conversion step of CO2 into usable products, a more system-oriented view is required to truly understand its merits and drawbacks. Using the net-zero-CO2–emissions framework, we analyse the efficiencies, land use, and storage requirements of CO2-based fuel systems. We used seasonal, renewable electricity storage with chemical energy carriers as the starting point in a comparative assessment of CO2-based methane and methanol and carbon-free hydrogen and ammonia. The resulting loss of primary renewable power that drives each system is high: converting power into a synthesized fuel and combusting it to reproduce power on demand leads to an efficiency loss of up to 75% which goes up to 92% when they are used for propulsion. Compared to hydrogen, the other energy carriers suffer from increased system complexity and consequently lower efficiency. We use exergy analysis to investigate the improvement potential of all systems. This work highlights clear challenges for the use of chemical energy carriers for seasonal energy storage, although viable alternatives are yet unavailable.
"Design and optimization of adsorbents and cycles in CO2 capture by adsorption"
The talk will address the processes to capture carbon dioxide not only out of the flue gas resulting from a combustion process, either in a power plant or in an industrial process, but also either from industrial mixtures such as those in hydrogen production or from air (direct air capture). While the state of the art technology is based on scrubbing using aqueous solutions, typically of amines, adsorption based processes can be considered as a valuable alternative. As it happens, adsorption based processes have advantages and disadvantages with respect to absorption based processes.
In both cases, the availability of a suitable sorbent material, either in solution or in solid form, plays a crucial role. Admittedly, the virtually infinite possibilities offered by the introduction of MOF (metal organic framework) adsorbents have generated a lot of excitement about the potential of adsorption-based processes. Recent work on new diamine-appended MOFs seems to be particularly important in this context (see McDonald et al., Nature, 2015, 519, 303–308).
Another feature of adsorption based processes is the potential complexity of the process cycle, indeed in all cases, i.e. Pressure Swing Adsorption (PSA), Temperature Swing Adsorption (TSA), Vacuum Swing Adsorption (VSA), or combinations thereof. The more specialized the adsorption behaviour of new materials, the more complex the design of the adsorption cycle. In the case of the MOFs mentioned above, it was shown that they have high CO2/N2 selectivity, while exhibiting a CO2 adsorption isotherm with a temperature-dependent sharp step and CO2 sorption hardly affected by the presence of water. These features call for a model-based ad hoc design of a TSA cycle in order to exploit the full potential of the step-like isotherm (see Hefti et al., Faraday Discuss., 2016, 192, 153-179).
The talk will explore the motivation for the capture of carbon dioxide in the context of measures to mitigate climate change, and the potential of adsorption-based carbon dioxide capture processes. The process engineering challenges associated to the design of such processes will be discussed, while highlighting the potential of the design of both new materials and new cycles and underlining the need for a holistic process optimization that considers material and cycles in a synergistic manner.
Marco Mazzotti, an Italian and Swiss citizen born in 1960, married, with two children, has been professor of process engineering at ETH Zurich since May 1997 (associate until March 2001 and Full Professor thereafter). He holds a Laurea (MSc, 1984) and a Ph.D. (1993), both in Chemical Engineering and from the Politecnico di Milano, Italy. Before joining ETH Zurich, he had worked five years in industry (1985-1990), and had been Assistant Professor at the Politecnico di Milano (1994–1997).
He was coordinating lead author of the IPCC Special Report on CCS (2002-2005), President of the International Adsorption Society (2010–2013), and chairman of the Board of the Energy Science Center of the ETH Zurich (2011-2017). He is Chairman of the Working Party on Crystallization of the EFCE (since 1.07.2014) and one of the six Executive Editors of Chemical Engineering Science (since 1.1.2012). He was a contributor to the Nobel Peace Prize for 2007 awarded to the Intergovernmental Panel on Climate Change (IPCC). He was the recipient of an honorary doctorate from the Otto von Guericke University Magdeburg, Germany (2014). He has been awarded a European Research Council Advanced Grant towards "Studying secondary nucleation for the intensification of continuous crystallization" (2018-2023).
He has published more than 300 papers in refereed international journals and books and 6 book chapters, and has delivered more than 120 invited lectures. His ISI h-factor is 49 (August 23rd, 2018), with more than 8900 citations, of which about 7500 not self-citations. 40 doctoral students have graduated with him and 18 doctoral students are currently advised by him.
He was the chair of the 9th International Conference on Fundamentals of Adsorption FOA9 (Taormina, I, May 20–25, 2007), and of the 18th International Symposium on Industrial Crystallization (Zurich, CH, September 15–16, 2011). He will be the chair of the 2019 Gordon Research Conference on Carbon, Capture, Utilization and Storage (Les Diablerets, CH, May 5-10, 2019).