Using only carbon dioxide, water and sunlight, a solar thermal tower in Spain produces carbon-neutral, sustainable versions of diesel and jet fuel. Built and tested by ETH Zurich researchers, it is a promising clean fuel project.

Why do we need Sustainable Aviation Fuel (SAF)?

Fossil fuels can be replaced by batteries or hydrogen in cars and trucks, but airplanes are more problematic. With more than 25,000 commercial aircraft in service today, and a lifespan of about 25 years, airlines are looking for carbon-neutral fuels to reduce emissions. It is a transitional but important step, until the clean aviation technology is ready and the entire global fleet can switch to something else.

Carbon-free fuels are a replacement for today’s kerosene Jet-A fuel. They are mixed with regular fuel and burned in jet engines normally, producing a normal amount of carbon emissions. The difference is that instead of pulling that carbon straight out of the ground, they collect it from other places. It ends up in the atmosphere again, but at least it does some useful work before it gets there, reducing the amount of conventional fuel we burn.

How is SAF currently made?

There are many ways to produce carbon neutral fuels, but not all are acceptable. For example, biofuels grown from specially grown corn crops they create their own emissions, from fertilizers and agricultural equipment, and use land that could otherwise produce food. Deforestation and the use of wood as biomass is also ruled out, for reasons that should be obvious, but the fact that there are rules about it suggests that even in the sustainability game there are still bad operators.

Disposal facilities for jet fuel are popping up here and there, taking municipal garbage or old cooking oil and use them as raw material to create synthetic gas, which can be refined into synthetic fuels. But the pyrolysis process usually involves a lot of energy—either dirty or clean energy that could be used elsewhere—and the feedstock is so random that the resulting fuels sometimes need an extra, energy-intensive cleanup step before they’re ready for use.

Another way is to it takes carbon directly from other emission sources and turns it into fuel. This can be done by using green electricity to power an electrolyser, then mixing the resulting hydrogen with carbon monoxide to create syngas, which can then be refined into fuels – but there are energy losses in each of these steps.

Which brings us to ETH Zurich’s new, much simpler design, which was built and tested at the IMDEA Energy Institute in Spain.

This pilot plant works on the concentration of solar thermal energy. 169 reflector panels that follow the sun, each measuring three square meters in area, redirects sunlight into a 16cm hole in the solar reactor at the top of the 15m high central tower. This reactor receives on average about 50kW of solar thermal energy.

Ta Built solar tower that produces jet fuel from carbon, water and sunlight in two steps. Water and pure carbon dioxide are fed into a ceria-based redox reaction, which simultaneously converts them into hydrogen and carbon monoxide or syngas. Since all this is done in one chamber, it is possible to adjust the amount of water and CO2 in order to manage the exact composition of the syngas live.

This syngas is fed to a Gas-to-Liquid (GtL) unit at the bottom of the tower, which provides a liquid phase containing 16% kerosene and 40% diesel, as well as a wax phase with 7% kerosene and 40% diesel – proving that a ceria-based ceramic solar reactor definitely produces syngas clean enough to convert into synthetic fuels.

How much fuel can be produced?

The researchers worked for nine days, six to eight cycles a day, weather permitting. Each cycle lasted an average of 53 minutes, and the total experimental time was 55 hours. Several cycles had to be stopped due to overheating, when reactor temperatures rose beyond the target 1,450°C to the critical temperature of 1,500°C.

In total, the experimental pilot plant produced about 5,191 liters of syngas during those nine days, but the researchers don’t say exactly how much kerosene and diesel became after processing the syngas.

They note that the overall efficiency of the system (measured by the energy content of the syngas as a percentage of the total solar input) was only about 4% in this implementation, but they see ways to increase this by over 20% by recovering and recycling more heat and changing the structure of the ceria.

“We are the first to demonstrate the entire thermochemical process chain of water and CO2 to kerosene in a fully integrated solar tower system,” said ETH Professor Aldo Steinfeld, corresponding author of the research paper. “This solar tower fuel plant was operated with a setup relevant to industrial deployment, setting a technological milestone towards the production of sustainable aviation fuels.“

“A fuel plant with a solar tower is described here represents a sustainable path towards the global implementation of solar fuel production“, the study states.

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Source: E2 Portal by www.e2.rs.

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