7 October 2015
Solar fuels-researcher Gerard van Rooij and his team work to convert the greenhouse gas CO2 into new, clean fuels. In a paper for the Faraday conference of the Royal Society of Chemistry, the collaboration between FOM Institute DIFFER, RU Nijmegen and TU Eindhoven demonstrate how they can use a plasma to break up CO2 at higher efficiencies than are possible in thermal conversion. With an energy efficiency of 50%, they have set the first step to recycle CO2 into a sustainable energy carrier.
Publication: Taming microwave plasma to beat thermodynamics in CO2 dissociation
G. J. van Rooij et.al, Faraday Discussions
Solar fuels: fuel without the fossil
The end goal of solar fuels research is to have all the benefits of natural gas or other fuels, without dependence on fossil sources and without net emission of CO2. Our present energy infrastructure relies for 80% on the use of such energy rich and long lasting chemical fuels. Converting sustainable solar or wind power into artificial fuels would create a truly circular energy economy: the holy grail of modern energy research.
To create fuels from CO2, the first step is to break up the molecule and create carbon monoxide. This 'reverse combustion' always costs energy; the challenges is efficiency. Simply turning up the heat until the molecules collide hard enough to break up works, but is inefficient. The fragments will keep their high temperature, energy which is then lost to the outside..
Vibrate to pieces
Plasma physicist Gerard van Rooij sought a different route to break up CO2 and uses a specialty of DIFFER: partly ionising the CO2 to turn it into a charged gas - plasma. In this state some of the light electrons release from the much heavier atoms in the CO2 and fly around at great speeds.
Van Rooij: "In a plasma you can influence the electrons and the molecules separately. We are looking for the uncharted non-equilibrium regime where the electrons move around at great speeds, but the molecules themselves are practically standing still." Collisions with the fast electrons will induce vibrations in the CO2 molecules which will then stepwise cause them to vibrate themselves apart. Because the heavy molecules themselves are barely moving, this route wastes less energy than heating the entire mixture. The key is to keep the molecules cold and the electrons hot, but not too hot: then they will blast the molecules to pieces in one collision and waste energy again.
Past the theoretical limit
In the theoretical energy efficiency limit for simply heating the entire CO2 gas (the 'equilibrium reaction'), 42% of the energy is available to break up the CO2. In the Faraday paper, the researchers show that their plasma technique is already edging past that: their best results reach almost 50% energy efficiency.
The team are not yet content with the results, because their measeurements show that hard collisions in their plasma are the main mechanism to dissociate CO2. Fast electrons which cause the CO2 to gradually vibrate themselves to pieces are not yet the main process in the plasma..
Van Rooij: "In future research we will move further into the non-equilibrium regime, for instance by adding impurities to the plasma. These will enable the plasma to form at lower energies, so that less energy is wasted in liberating the electrons." Optimal use of the non-equilibrium conditions would enable efficiencies up to 90%.
Creating fuels with Fischer-Tropsch
To turn the CO that flows out of the plasma reactor into fuels, additional processing is needed. In the chemical industry, the Fischer-Tropsch process is a well-known technique to turn syngas (CO and H2) into fuels. "Starting from 80% energy efficiency in the plasma step we can compete with the benchmark in this field, namely CO production via electrolysis", says DIFFER's director Richard van de Sanden: "And not just because of the efficiency. Electrolysis of CO2 works at high temperatures, so if you want to quickly respond to peaks in sustainable energy production, the equipment always has to stand-by at that temperature. In contrast, we can create a plasma within a second from a cold start - that brings flexibility to the table."
Gerard van Rooij: "The end goal of this research is to be able to take CO2 and water from the atmosphere and use sustainable energy to turn them into fuels. When we use the fuel later on, the energy we stored and the original CO2 and water are released again and the cycle starts anew."