Air turns into fuel? This technology may hold the code to energy

Produced by: Science Popularization China

Produced by: Hexian

Producer: Computer Network Information Center, Chinese Academy of Sciences

“Only the fresh breeze on the river and the bright moon in the mountains, which are heard by the ears as sound and seen by the eyes as color, are inexhaustible and inexhaustible. They are the endless treasure of the Creator.” Although Su Shi, who said this, was very insightful, he never dreamed that the fresh breeze could not only “cook without rice” but also be a “panacea”.

Not long ago, Chinese scientists achieved the first full synthesis of starch molecules from carbon dioxide in the air in the laboratory, providing a promising strategy for addressing food crises and climate change.

Coincidentally, a research team at the Swiss Federal Institute of Technology in Zurich (German: Eidgenössische Technische Hochschule Zürich, ETH for short) recently designed a device that uses sunlight and air to directly produce liquid hydrocarbons or methanol fuels, providing another bright path for the absorption and utilization of carbon dioxide.

This achievement has been published in the top academic journal Nature. It is reported that this device can produce 32 ml of methanol in 7 hours of working time under normal conditions. Doesn't it sound amazing? Let's find out.

Image source: Nature

The Secret of Air to Fuel:

This set of golden experimental equipment located on the roof of the Robot Laboratory Building of the Swiss Federal Institute of Technology is the protagonist of today's story. It looks extraordinary, simple and elegant, and even has a parasol, which looks very stylish. Is its heart as simple as the surface? It doesn't look like it, but a little complicated.

Process flow chart of the experimental device (Image source: Nature)

But the truth is always simple. In order to let you understand its working principle more clearly and quickly, here is a simple flow chart of the device producing air fuel for reference:

Image source: Made by the author

We all know that there is an important law in nature, that is, the conservation of mass. In the process of chemical reaction, the type of atoms of a substance does not change, and the number does not increase or decrease. It just recombines and transforms from one connection mode to another, just like a class is reshuffled and reorganized after changing seats, but the people in the class do not change.

If we want to get methanol or other liquid hydrocarbon fuels, the raw materials for their preparation should also contain the same elements, namely carbon, hydrogen and oxygen. Air is a mixture, containing nitrogen, oxygen, rare gases, carbon dioxide and other substances. The volume fraction of carbon dioxide is about 0.04%, and water vapor and other impurities account for about 0.002%. The content is considerable and contains the desired elements, which makes it possible to produce liquid fuels. After being collected and purified by an air capture device, relatively pure carbon dioxide (purity 98%) and water (pollutants are less than 0.2ppm, ppm means parts per million) can be obtained. The next task is to convert carbon dioxide and water into fuel.

Direct conversion is difficult, so a stopgap measure is to first prepare them into synthesis gas, that is, hydrogen and carbon monoxide, which is the raw gas for preparing many chemical raw materials. The method used in this experimental device is to use solar energy to drive carbon dioxide and water vapor to undergo redox reactions with cerium trioxide. Carbon dioxide and water are reduced to carbon monoxide and hydrogen, respectively, while cerium trioxide is oxidized to cerium dioxide. The oxidation product cerium dioxide can also be reduced to oxygen and cerium trioxide through endothermic heat, which is convenient for recycling. The synthesis gas carbon monoxide and hydrogen will then enter the reaction equipment to generate the target product liquid hydrocarbons or methanol, which is air fuel.

The actual running effect:

This route of preparing liquid fuel from air sounds reasonable, but is it actually feasible? First, let's look at the output. The researchers found that the device operated for 7 hours a day under normal working conditions, and through 17 consecutive redox cycles, a total of 96.2 liters of synthesis gas was obtained, which can be further processed into methanol in the device.

The single-pass molar conversion rate of synthesis gas measured by the device is 27%, and the purity of the produced methanol is 65%.

After the remaining unconverted synthesis gas is converted through six cycles, the final total molar conversion rate is 85%. After running for 7 hours a day, the amount of pure methanol obtained is 32 ml. The combustion heat of this output is about the same as the electricity consumed by a 9-watt fluorescent lamp for 15 hours. Of course, this equipment is not only capable of producing methanol. By selecting a specific synthesis process, other hydrocarbon fuels can also be customized.

The researchers envision that if this achievement is put into commercial application, it will create huge benefits. For example, a commercial-scale solar fuel plant can use 10 heliostat fields. Assuming that each heliostat field collects 100 megawatts of solar radiation heat energy and the overall efficiency of the system is 10%, it can produce 95,000 liters of kerosene per day, enough to provide fuel for an Airbus A350 carrying 325 passengers from London to New York and back.

Image source: Science Popularization China

So what is the quality of these fuels? Let's compare them with conventional aviation fuel. The conventional way to produce aviation kerosene is heavy oil hydrocracking, and the products will inevitably carry air pollutants, such as sulfur compounds, nitrogen compounds, polycyclic aromatic hydrocarbons, heavy metals, etc. Combustion tests have shown that the emissions of harmful substances in jet fuel produced by the solar redox device are significantly reduced, which is a unique advantage. In addition, oil is a non-renewable energy source, while air can be obtained continuously, which is more promising in the long run.

Image source: whbyhsh.com

The meaning of tossing around:

The story doesn't end there. In this solar redox device, carbon dioxide and water are converted into liquid fuel under the action of solar energy, and when the liquid fuel is put into use, carbon dioxide and water are generated again. From a material perspective, carbon emissions and consumption are equal, so researchers call it a "milestone in carbon neutrality." From an energy perspective, most of the energy in the fuel preparation process comes from solar energy, and the subsequent fuel combustion can be converted into other forms of energy as needed, so it is equivalent to indirect use of clean energy.

Image source: veer gallery

In addition, the researchers calculated that based on the performance of the current solar fuel system, an air capture device would require an area of ​​approximately 4,500 square meters to capture 100,000 tons of carbon dioxide per year. Assuming the overall efficiency η of the system is 10%, such a solar fuel plant would produce about 34 million liters of fuel per year. In comparison, global aviation kerosene consumption in 2019 was 414 billion liters. To fully meet global demand, the total area of ​​all solar power plants would be approximately 45,000 square kilometers, equivalent to only 0.5% of the area of ​​the Sahara Desert.

Image source: veer gallery

It seems that solar fuel systems are easy to promote because of their readily available raw materials, environmental friendliness, and small footprint. However, they are actually facing challenges. The initial investment cost of solar thermochemical fuels is very high. Conventional jet fuel usually costs no more than $1 per liter, while solar jet fuel costs more than $10 per liter, so it is not advantageous in the short term.

In view of this, the researchers have two considerations. First, they call for policy support to create a short-term market for the first generation of commercial solar fuel power plants. This step is critical. Second, they call for self-improvement, through economies of scale and process optimization, large-scale production of key components and continuous improvement to reduce costs, thereby improving market competitiveness.

Decarbonization is a long-term theme. It is not something that a certain region or group has to think about, but a problem faced by all of humanity. From the perspective of mass conservation, although carbon will not disappear, it can be transformed into a more beneficial form of existence. So far, we do not know how much transformation potential carbon dioxide has and how many possible uses it has. It all depends on human imagination, which is the source of innovation and change.

References:

Schäppi, R., Rutz, D., Dähler, F. et al. Drop-in Fuels from Sunlight and Air. Nature (2021).