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I experienced in 1960s the beginning of a large consumption of primary energy in the world and thought complete consumption of world primary energy in the near future. I was thinking in 1970s we can supply hydrogen to the world by producing hydrogen by seawater electrolysis using electricity of solar cell on rafts on sea. However, I understood that we have no technologies of combustion, storage and transportation of hydrogen, and hence hydrogen cannot be a major fuel in the world. In the late 1980s, we began to study “carbon dioxide recycling” by conversion of carbon dioxide to organic fuel through the reaction of carbon dioxide with hydrogen using a unique technology available at our Institute for Materials Research, Tohoku University. We found effective catalysts on which more than 99% pure methane was rapidly formed by the reaction of CO2 + 4H2 → CH4 + 2H2O. We reported internationally this result in 1993. We constructed a prototype plant of carbon dioxide recycling by methanation, on the roof floor of our Institute in 1996. The plant consisted of a solar cell for power generation, a water electrolyzer to form hydrogen and oxygen (4H2O = 4H2 + 2O2), a carbon dioxide methanation unit (CO2 + 4H2 = CH4 + 2H2O) and a methane combustion unit (CH4 + 2O2 = CO2+ 2H2O). The exhaust gas consists of carbon dioxide and steam. After removal of steam by cooling, the remaining exhaust gas was only CO2. The remaining CO2 was sent back to the carbon dioxide methanation unit. The methane production rate on this plant was 0.1 m3/h. If we burn methane with air the exhaust gas contains nitrogen which is 8 times as much as carbon dioxide. It is economically impossible to separate CO2 from N2, and the exhaust gas of methane combustion with air has to be released in air. In contrast, in our plant, methane combustion was carried out using oxygen formed by water electrolysis for production of hydrogen. After cooling the exhaust gas to remove steam, the remaining exhaust gas consists only of carbon dioxide, and the carbon dioxide is sent back to the carbon dioxide methanation plant for carbon dioxide recycling. This consists of perfect decarbonization systems. In 2003, we constructed a pilot plant for industrialization which formed methane at 1 m3/h. In those days the world did not show interest very much except for our friend in the world, because cheap LNG was easily available. My colleagues working at an industrial company tried the industrial development of this technology, in a case including collaboration with a foreign company. My colleagues constructed some industrial plants with the aid of government funds. Audi operates the methanation plant named fuel “e-gas” to supply to LNG car since 2013. Then, my colleagues considered that the technology of carbon dioxide methanation should not be monopolized by a company but should be used in the world, and started CCR workshop (Carbon Capture and Reuse) inviting about 50 companies in 2016. Since then, joint efforts of public and private sectors to use the technology of carbon dioxide methanation have been performed by member companies of the CCR workshop. Now in Japan the synthesized methane is named “e-methane” and carbon dioxide methanation is regarded as the key technology for “carbon neutral”. Recently, in Japan, refrains of renewable energy power generation are requested, because of no storage systems of surplus electricitry. In the prototype plant which we constructed in 1966 the replacement of the methane combustion unit by a LNG power generator we can cover the electricity shortage using the surplus green electricity. We store the surplus electricity in the form of methane. The shortage of green electricity generation and an increase in electricity consumption can be supplemented by methane power generation. Methane combustion is not done with air but with oxygen generated by water electrolysis for hydrogen production. After methane power generation the exhaust gas consists only of steam and carbon dioxide. After removal of steam by cooling, the remaining carbon dioxide is sent back to the carbon dioxide methanation unit. The amount of oxygen needed to methane power generation is exactly the same amount of oxygen formed by water electrolysis. Thus, in four units in the plant not only carbon dioxide but also water can be recycled if it is needed. In this manor, when the methane combustion unit is replaced by the methane power generator in our prototype pant combination, we can supply stable high quality electricity by covering the shortage of green electricity and the increase in electricity consumption. This is one of the key technologies for decarbonization.

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Prof. Dongsheng Wen is theChair of the Institute of Thermodynamics, Technical University of Munich. He received BEng in Aeronautics from Beihang University, MSc in Thermophysics from Tsinghua University and DPhil in Engineering Science from the University of Oxford. Prior to his current position, he was the Chair Professor at the University of Leeds and Beihang University, and had he worked in various academic positions at the Queen Mary University of London. Prof. Wen’ research is focused on heat: namely, how heat is produced, transported, stored and utilized across different scales via nanomaterials / nano-surfaces with targeted applications in energy engineering. Supported by over 50 research projects, including two prestigious ERC grants (consolidator + advanced), his research has produced over 20 patents, ~350 referred journal publications, with total citation of >24000 and current H-index = 70. He is a member of the Academia of Europaea (MAE), an elected Fellow of the Royal Society of Chemistry (FRSC) and the Energy Institute (FEI), the Editor-in-Chief of Advances in Aerodynamics, and Associate Editor of Applied Thermal Engineering.

Jing Sun received her Ph. D degree from the University of Southern California in 1989 and her master's and bachelor's degrees from the University of Science and Technology of China in 1984 and 1982, respectively. From 1989 to 1993, she was an assistant professor in the Electrical and Computer Engineering Department at Wayne State University. She joined Ford Research Laboratory in 1993, where she worked on advanced powertrain system controls. After spending almost ten years in the industry, she returned to academia in 2003. She joined the University of Michigan, where she is the Michael G. Parsons Collegiate Professor in the Naval Architecture and Marine Engineering Department, with joint appointments in the Electrical Engineering and Computer Science Department and Mechanical Engineering Department at the same university. She holds 45 U.S. patents and has published over 300 archived journal and conference papers. She is a Fellow of NAI (the National Academy of Inventors), IEEE (Institute of Electrical and Electronics Engineers), IFAC (International Federation of Automatic Control), and SNAME (the Society of Naval Architecture and Marine Engineering). She is a recipient of the 2003 IEEE Control System Technology Award.