"But the breeze on the river, and the moon between the mountains, the ears get it for the sound, the eyes meet it and become color, take it inexhaustible, inexhaustible. It is the endless collection of the creator". Although the predecessor of Su Shi who said this sentence was very insightful, he never dreamed that the breeze could not only cook for "no rice", but also make "golden oil".
Not long ago, Chinese scientists achieved the first full synthesis from carbon dioxide in the air to starch molecules in the laboratory, providing a promising strategy to deal with the food crisis and climate change.
By no coincidence, a research team at the Swiss Federal Institute of Technology Zurich (German: Eidgenössische Technische Hochschule Zürich, ETH) has recently designed a device to produce liquid hydrocarbons or methanol fuel directly from sunlight and air, providing another bright path to absorb and utilize carbon dioxide.
The results have now been published in the top academic journal Nature. This device reportedly operates under everyday conditions and is capable of producing 32 mL of methanol in a 7-hour working day. Doesn't that sound amazing? Let's find out next.
The gleaming golden experimental equipment on the roof of ETH Zurich's Machine Lab building is the star of today's story. It looks extraordinary on the outside, simple and atmospheric, and also has a sun umbrella, a very stylish look. Is it as streamlined as the surface inside? It does not seem so, but a bit complicated.
But the avenue is simple, and the truth is often simple. To give you a clearer and quicker understanding of how it works, here is a simple process flow diagram of the device to produce air fuel for reference.
We all know that there is an important law of nature, and that is the conservation of mass. Matter remains the same type of atoms during a chemical reaction, the number does not increase or decrease, but only recombination occurs, transforming from one type of connection to another, just like a class redividing its groups after switching seats, disrupting and reorganizing, but the people in the class remain the same.
If we want to get methanol or other liquid hydrocarbon fuels, then the raw materials used to prepare them should also contain the same elements, namely - carbon, hydrogen and oxygen. Air is a mixture, which contains nitrogen, oxygen, rare gases, carbon dioxide and other substances. The volume fraction of carbon dioxide is about 0.04%, water vapor and other impurities about 0.002%, which is a significant content and contains the desired elements, which provides the possibility of liquid fuel production. After collection and purification by air capture devices, it is possible to obtain relatively pure carbon dioxide (98% purity) and water (less than 0.2 ppm, ppm means parts per million), and then the next task is to convert the carbon dioxide and water into fuel.
Since direct conversion is difficult, an expedient solution is to first prepare them into synthesis gas, i.e. hydrogen and carbon monoxide, which is the raw gas for the preparation of many chemical materials. The method used in this experimental setup is to use solar energy to drive a redox reaction between carbon dioxide and water vapor and cerium trioxide, where 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 thermally reduced to oxygen and cerium trioxide by heat absorption, which facilitates recycling again. The syngas carbon monoxide and hydrogen next enter the reaction plant to produce the destination product liquid hydrocarbon or methanol, also known as air fuel.
This route to prepare liquid fuels from air sounds reasonable, but does it work in practice? First let's look at the yield. The researchers found that the unit operated for seven hours a day under normal operating conditions and obtained a total of 96.2 liters of syngas through 17 consecutive redox cycles, which could be further processed in the unit to methanol.
The one-way molar conversion of syngas measured at the plant was 27% and the purity of the methanol produced was 65%.
The remaining unconverted syngas was converted in 6 cycles for a final total molar conversion of 85%. The amount of pure methanol obtained for a 7-hour day of operation is 32 mL. The heat of combustion for this yield is about the same as the electricity consumed by a 9-watt fluorescent lamp with 15 hours of lighting. Of course this device is not limited to methanol production, and by choosing a specific synthesis process, other hydrocarbon fuels can be customized as well.
The researchers envision that if the results are put to commercial use, they will create huge benefits. For example, a commercial-scale solar fuel plant could use 10 heliostat fields, and assuming that each field collects 100 megawatts of solar thermal energy and that the overall efficiency of the system is η 10%, it could produce 95,000 liters of kerosene per day, enough to fuel an Airbus A350 with 325 passengers for a round trip from London to New York.
So what is the quality of this fuel? Let's compare it to conventional aviation fuel. The current conventional way of producing aviation kerosene is by hydrocracking heavy oil, the products of which inevitably carry air pollutants such as sulfur-containing compounds, nitrogen-containing compounds, thick-ringed aromatics, heavy metals, etc. Combustion tests have shown that the jet fuel produced by this solar redox unit has significantly reduced harmful emissions, which is a unique advantage. In addition, oil is a non-renewable energy source, while air can be obtained continuously, which is also more promising in the long run.
The story doesn't end there. In this solar redox device, carbon dioxide and water are converted into liquid fuel by solar energy, which in turn produces carbon dioxide and water when the liquid fuel is put into use. From a material point of view, carbon emissions and consumption are equal, which is why the researchers call it a "carbon neutral milestone". From the energy perspective, most of the energy in the fuel preparation process comes from solar energy, and the subsequent combustion of the fuel can be converted into other forms of energy as needed, so it is equivalent to the indirect use of clean energy.
In addition, the researchers calculated that, based on the current performance of solar fuel systems, an air capture unit capturing 100,000 tons of CO2/year would require a footprint of approximately 4,500 square meters. Assuming an overall system efficiency η of 10%, such a solar fuel plant would produce about 34 million liters of fuel per year. In comparison, with global aviation kerosene consumption of 414 billion liters in 2019, the total footprint of all solar power plants to fully meet global demand would be about 45,000 square kilometers, or just 0.5% of the Sahara desert.
In this way, solar fuel systems, with their easy availability of raw materials, environmental friendliness and small footprint, seem easy to scale up, but in reality they face challenges. The high initial investment cost of solar thermochemical fuels, which typically cost no more than $1 per liter of conventional jet fuel compared to more than $10 per liter of solar jet fuel, does not provide an advantage in the short term.
In view of this, the researchers have two considerations, one is to call for policy support to create a short-term market for the first generation of commercial solar fuel power plants, it is critical to take this step; the second is to upgrade themselves, through the scale effect and process optimization, mass production of key components and lean to reduce costs, so as to improve market competitiveness.
Decarbonization is a long-term theme, and it is not something for a certain region or group to think about, but a common problem for the whole human race. From a conservation of mass perspective, carbon, while not disappearing, can be transformed into a more beneficial form of existence. So far, we don't know how much conversion potential carbon dioxide holds and how many possible uses there are for it. It all depends on the human imagination, which is a source of innovation and change.
From Drop-in Fuels from Sunlight and Air.