Plasma-assisted CO2 Recycling: from Earth to Mars

MIT PIs:
Ahmed F. Ghoniem -Ronald C. Crane (1972) Professor of Mechanical Engineering; Director, Center for Energy and Propulsion Research; Director, Reacting Gas Dynamics Laboratory

Carmen Guerra-Garcia – Atlantic Richfield Career Development Professor in Energy Studies; Assistant Professor of Aeronautics and Astronautics

PT PIs:
Vasco António Dinis Leitão Guerra – Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico

Abstract: The need to revise the technologies on which economy is based is no longer in doubt, as the growth in fossil fuel consumption has already led to planetary observable changes in the climate [1]. Moving away from the use of fossil resources has numerous benefits: besides averting disruptive shifts in climate, it will contribute to improving air, land, and water quality for a still-growing global population, as well as minimizing political instability. While the objective is clear, the solutions are not. Electricity generation from wind and solar energies has grown by a factor of 50 since 2000 [2]. Despite the enormous progress, it constitutes just over 1% of the global energy consumption.

The share of renewables can increase up to ~60% [2], but two main difficulties remain. On the one hand, few technologies can compete with the energy density provided by fossil fuels, as shown in figure 1 (see annexes). On the other hand, the intermittency and geographic nature of renewables calls for efficient storage of the energy surplus during peak production. A vast effort is under way to develop carbon capture, utilization and storage strategies. CO2 recycling aims at transforming carbon dioxide into high value-added products, such as hydrocarbons, acids, alcohols or oxygen. In this approach, CO2 is no longer seen as a pollutant, but as a raw material to be valorized. A major route is the production of liquid fuels using only green electricity, promoting the transition from fossil to solar fuels. CO2 conversion can also play a key role in human exploration beyond Earth, by enabling the production of fuel and breathable oxygen on Mars [3]. CO2 is abundant in the Martian atmosphere and can be converted in-situ into carbon monoxide (CO) and oxygen (O2). Both CO and O2 can be used in a propellant mixture, while O2 can be collected and made available for breathing. Further decomposition can be pursued to arrive at carbon, of use for manufacturing carbon structures and for the synthesis of different organic molecules.

Two crucial steps in the processes of recycling and utilizing CO2, both on Earth and on Mars, are the efficient dissociation of CO2 and the separation of the conversion products. Dissociation is a strongly endothermic process, difficult to activate efficiently by conventional thermal and catalytic methods, while separation remains energy intensive [4]. Nonthermal plasma technologies (NTP) are in an excellent position to solve these problems. NTP are highly reactive gas media sustained by electrical discharges, that offer unique ways to break the strong C=O bond by taking advantage of the energy stored in the internal degrees of freedom [4]. A combination of plasmas and ion-conducting membranes can enhance the oxygen permeability and stimulate CO2 conversion by product separation [5]. Besides their energy efficiency, plasma technologies are compact, scalable, selective, versatile (the same reactor can be used to produce different molecules), do not require the use of expensive materials, and can instantaneously start and stop operation, as required by a power supply from intermittent renewable energy sources. CREATOR consists of a thorough theoretical, modelling and simulation investigation, aiming at unveiling the mechanisms underlying plasma CO2 dissociation and the plasma-surface interactions relevant for product separation.

The final goal is to identify the optimal conditions for a plasma reactor to operate for both Terrestrial and Martian CO2 recycling applications. It builds on the Seed Project “Inverse design and Modeling of Plasma-Assisted CO2-conversion Technologies” (IMPACT) financed in the 2021 MIT-Portugal call. The Department of Aeronautics and Astronautics from the MIT joins the project as a Participating Institution. CREATOR focuses on nanosecond pulsed discharges ignited in pure CO2, operating both at high (Earth) and low (Mars) pressure, to be investigated at the MIT in the framework of IMPACT. The different working pressures imply a modification of the dominant energy transfer pathways, from direct electron impact processes to a plasma chemistry mediated by vibrationally and electronically excited states [4].

The proposed research follows three main axes: – investigation of the role of vibrationally and electronically excited states in the process of gas-phase CO2 dissociation at different pressures, based on a global (0D) model of the discharge; – study of plasma-surface interactions, assessing the influence of different species and surface processes on conversion, selectivity, and activation of membranes for product separation; – development of a 1D radial model, to describe accurately the transport of the active species to the membranes, their interaction, and self-consistently integrate the two former axes. By its end, the project will provide a coupled and integrated description of both volume and surface kinetics in CO2 plasmas, from conversion to separation, from Earth to Mars.